Fundamentos de Sismicidad y Diseño de Prefabricado Sísmico

Basics of Seismicity and Precast Seismic Design STRUCTURE

SURFACE WAVES

SOIL BODY WAVES

FAULT RUPTURE

EQ GEOLOGIC STRATA

Contents 1. Preface........................................................................................ 4 2. Introduction to earthquakes...................................................5 3. Seismic analyses and response of buildings in an earthquake.................................................... 18 4. Eurocode 8............................................................................... 29 5. Basic principles in seismic design....................................... 33 6. Solutions for precast construction in seismic regions ..................................................................44

Bas ics of Seismicity an d P re cast Se i s m i c Des i g n, 201 3 First Ed ition, Printe d 20 13, M & P Pai n o, F i n l an d Co py rig ht: Peikko G ro u p Co rpo rat i o n an d E l e m a t ic O y A b

7. Glossary.................................................................................... 66

Th is b ooklet is pa r t of t h e pro j e c t I n d u st r i a l i ze d Building Constructi o n fo r C h i n a fu n de d by Pe i k ko G ro up an d Elema tic with s u ppo r t of Te kes, t h e F i n n i s h Fund ing Agency for Technol o gy an d I n n ovat i o n .

8. References................................................................................68

Tex t: L a sse Rajala / Swe co Layo ut: Jenni Erkintal o This document (68 pages) is only for persons who are professionals in designing of precast concrete structures. In all cases only the user is responsible for the implementation of information. The document authors shall not be liable for direct, consequential or incidental damages or for loss of profits or for any claim or demand by the user or any third party arising out of the use of this document.

4

Contents / Basics of Seismicity and Precast Seismic Design

Basics of Seismicity and Precast Seismic Design /

3

1.

Preface

2.

Introduction to earthquakes

An increasing number of the global population lives in seismic areas, which means that seismicity has a remarkable influence on the global construction business.

During the past decades great efforts have been made towards understanding the causes of earthquakes and defining the loading on buildings. Recent studies have created a better basis for the development and design of modern solutions and products for the construction business.

The target of this booklet is to provide the necessary basic understanding about earthquakes for everyone involved in the global construction business.

This booklet is part of the project Industrialized Building Construction for China, which is funded by Peikko Group and Elematic, with the support of Tekes, the Finnish Funding Agency for Technology and Innovation.

In any geographic areas, three main factors together determine seismic risks: the level of the seismic hazard, the number of people and amount of property exposed to seismic hazards, and how vulnerable people and property are to the hazards.

The main focus of the project is to provide solutions for housing in seismic areas so they are less vulnerable, and thus reduce the seismic risks. The main focus of the project is China, but the principles are applicable globally.

Chapter 7 contains a glossary with the most common terms concerning seismicity and precast seismic design.

4

Preface / Basics of Seismicity and Precast Seismic Design

In an earthquake situation, seismic waves occur from sudden movements in a rupture zone (active fault) in the Earth’s crust. Waves of different types and velocities travel in different paths before reaching a building’s location and subjecting the ground to various motions.

The ground moves rapidly back and forth and in all directions, usually mainly horizontally, but also vertically. An earthquake of average intensity lasts for approximately 10–20 seconds. For a typical earthquake of an approximate magnitude of 6 on the Richter scale, the amplitudes in the various directions of the horizontal plane can be about 10 cm. During an earthquake of a magnitude of 6.5 on the Richter scale, the ground displacements can reach 20 cm.

The magnitude of an earthquake is gradually reduced, but a number of further quakes normally occur. The first severe earthquake is called the mainshock and the following quakes are aftershocks. Before a mainshock, milder quakes sometimes occur as precursors. These quakes are called foreshocks.

Earthquakes occur throughout the world, but mostly along narrow belts, from a few tens to hundreds of kilometers wide, marking the boundaries of the tectonic plates. Ninety per cent of all earthquakes take place at plate boundaries and are the result of the constant movement of the plates against each other.

These plates are roughly divided into two main types: continental plates and oceanic plates.

Together they are responsible for all seismic and volcanic activity on rocky planets, in a process called plate tectonics.

There are several types of boundaries between tectonic plates, and each has its own pattern of earthquakes, or seismicity pattern. These are not mutually exclusive, and combinations of fault types are common.

Basics of Seismicity and Precast Seismic Design / Introduction to earthquakes

5

80˚ 150˚

FIGURE 1 global seismic hazard map /u/ /b/ /c/

120˚

90˚

60˚

30˚

0˚

30˚

60˚

90˚

120˚

150˚ LOW HAZARD

MODERATE HAZARD

HIGH HAZARD

VERY HIGH HAZARD

GLOBAL SEISMIC HAZARD MAP

Produced by the Global Seismic Hazard Assesment Program (GSHAP) a demonstration project of the UN/International Decade of Natural Disaster Reduction, conducted by the International Lithodphere Program.

Global map assembled by D.Giardini, G. Grünthal, K. Shedlook and P. Zhang 1999

150˚

120˚

90˚

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30˚

0˚

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LOW HAZARD

6

Introduction to earthquakes / Basics of Seismicity and Precast Seismic Design

Basics of Seismicity and Precast Seismic Design / Introduction to earthquakes

MODERATE HAZARD

HIGH HAZARD

VERY HIGH HAZARD

7

When ground moves rapidly back and forth, the foundations are forced to follow these movements.

Because of the mass of inertia the upper part of a building does not usually instantly follow these movements, causing strong vibrations in the structure and parallel and large internal forces. This often results in plastic deformation of the structure and, in extreme cases, collapse.

Figure 3 illustrates a simplified representation of a building during an earthquake. As the ground supporting the building is displaced, the foundations of the building are also moved with the ground.

However, the inertia of the building mass resists

this motion and causes the building to suffer a distortion. This distortion wave travels along the height of the structure. During seismic action the continued shaking of the base causes the building to undergo a complex series of oscillations.

When energy is released at the focus of an earthquake, longitudinal (P-waves) and transverse (S-waves), waves are simultaneously generated. The velocity of P-waves is greater than that of S-waves.

P- and S-waves travel through the interior of the Earth from the focus to the surface. This is why they are called body waves (primary (P) and secondary (S) waves).

figure 4 Motions caused by body waves (P and S) /m/ and /f/.

EPICENTER

SEISMIC WAVES

FAULT LINE

figure 2 Three basic fault types /e/.

FOCUS

FAULT LINE UNDERGROUND

BODY WAVES

NORMAL FAULT

STRIKE-SLIP FAULT

P WAVE

REVERSE FAULT

S WAVE

figure 3 The response of a building during an earthquake /c1/.

8

Introduction to earthquakes / Basics of Seismicity and Precast Seismic Design

Basics of Seismicity and Precast Seismic Design / Introduction to earthquakes

9

When the two types of body waves reach the surface of the Earth, they produce large amplitude motions in the ground surface. They decay at a much slower rate than body waves and hence result in maximum damage.

The earthquake ground motion quantity most commonly used in analytical studies is the timewise variation of the ground acceleration in the immediate vicinity of a structure. Because buildings are most sensitive to lateral distortions, it has been the practice in most instances to consider the structural response to the horizontal components of ground motion only.

The effects of the vertical components of ground motion have generally not been considered significant enough to merit special attention, except as they influence gravity loads.

When a structure responds elastically to ground motion during a severe earthquake, the maximum response accelerations may be several times the maximum ground acceleration.

The values for seismic design acceleration in the codes are notably local and the design value can be doubled within a 50 km distance.

The effects of an earthquake on a building are primarily determined by time histories of the

figure 5 Schematic of travel paths of observed surface and body waves /l/.

three ground motion parameters with their specific frequency contents: ground acceleration, ground velocity, and ground displacement.

The ground motion parameters due to an earthquake of a given magnitude may vary strongly.

They depend on several factors, such as distance, direction, depth, and mechanism of the fault zone in the epicenter, and also in particular the local soil characteristics. In comparison with rock, softer soils are particularly prone to substantial local amplification of seismic waves.

As for the response of a building to the ground motion, it depends on important structural

characteristics (own frequency, type of structure, ductility, etc.).

figure 6 Arrival of Seismic Waves at a Site /m/.

STRUCTURE

2700 KM SURFACE WAVES

SURFACE WAVES KATRINA CALIFORNIA

NEW ORLEANS 1100 KM

SOIL

BODY WAVES BODY WAVES

FAULT RUPTURE

10

Introduction to earthquakes / Basics of Seismicity and Precast Seismic Design

EQ GEOLOGIC STRATA

Basics of Seismicity and Precast Seismic Design / Introduction to earthquakes

11

ML = log10A(∆) –log10A0(∆), where A is the maximum trace amplitude for a given earthquake at a given distance as recorded by a Wood-Anderson instrument and A0 is that for a particular earthquake selected as reference.

ML in its original form is rarely used today, but it remains a very important magnitude scale because it is the first widely used size measure and all other magnitude scales are tied to it. ML is also useful for engineering, because many structures have natural periods close to that of Wood-Anderson instrument (0.8 s).

When the magnitude is increased by 0.2, the energy is doubled; and when it is increased by 1.0, the energy is increased 32-fold. It is worth observing that the energy of an 8.3 magnitude earthquake, such as the 1964 Alaska earthquake, is not twice as large as that in a shock of magnitude 4.2, but the magnitude of 8.3 earthquake releases one million times as much energy as one of magnitude 4.2.

The magnitude of the earthquake does not have so strong an influence on the maximum (or peak) acceleration as was once thought. Measured peak accelerations during some earthquakes show that

figure 7 Principle of early seismographs /m/.

MAGNET

STRING

PENDULUM BOB PEN SUPPORT

ROTATING DRUM

CHART PAPER DIRECTION OF GROUND SHAKING RECORDED

earthquakes with magnitudes as low as 4.5 have given recorded peak accelerations of 0.6g. Such a large acceleration occurs usually as only two or three high frequency peaks carry little energy and thus has an insignificant effect on substantial structures. Hence the magnitude, or the peak

ground velocity, is a better description of the damage potential of an earthquake, with respect to the maximum acceleration.

Figure 8 The Richter Scale. In seismogram’s photosensitive paper one micron is 0,001 mm /j/.

RICHTER SCALE A GRAPHIC REPRESENTATION Compiled by V.J.Ansfeld

109 MICRONS OF AMPLIFIED MAXIMUM GROUND MOTION (Note Rapidly Changing Vertical Scale)

Magnitude and intensity Seismic waves are measured and recorded using a seismograph. To be useful for engineering it is critical to know the size of an earthquake. To measure these characteristics seismologists use two fundamentally different but equally important types of scales: the magnitude scale and the intensity scale.

The magnitude of an earthquake is related to the amount of energy released by the geological rupture causing it and is therefore a measure of the absolute size of the earthquake without reference to the distance from the epicenter.

The best known measure of earthquake magnitude is the Richter Scale, now referred to as the local magnitude (ML). Richter studied Californian earthquakes by using the same type of seismometer, a simple Wood-Anderson torsion instrument. In this instrument there was no pendulum as in early seismographs. The motion generated when this instrument was shaken by an earthquake came from the rotation of a small inertial mass affixed to a thin wire under high tension, hence the name torsion seismometer.

Largest Recorded (Offshore Chile, 1960)

8.9

Alaska, 1964 New Madrid, MO, 1812 GREAT

GREAT

108

8

EXAMPLE: SEISMOGRAM FROM STD. SEISMOGRAPH 100 KM FROM EPICENTER

107

104

10

105 104 102 101

MAJOR

STRONG

6 MODERATE

5

-1

0

-1

0

1

2

Great Devastation and Many Fatalities Possible * Loma Prieta, CA, 1989

7

MICRONS (= 1 CM)

TIME

6

San Francisco, 1906

3

4

Damage Begins * Fatalies Rare

SMALL

MINOR

NOT FELT 1

2

3

4

5

6

7

8

9

MAGNITUDE =

LOGARITHM (BASE 10) OF MAXIMUM AMPLITUDE MEASURED IN MICRONS **

12

Introduction to earthquakes / Basics of Seismicity and Precast Seismic Design

Basics of Seismicity and Precast Seismic Design / Introduction to earthquakes

13

Seismic moment characterizes the overall deformation at the fault. Seismic moment can be interpreted simply in terms of the ground deformation. The moment is proportional to the product of the area of dislocation and the displacement across the fault.

The intensity of an earthquake is a measurement of the observed damage at a particular location.

This intensity will vary with distance from the epicenter and depends on local ground conditions.

There are dozens of intensity scales around the world. The Modified Mercalli intensity scale used

in many places, was originally produced in 1931 to fit construction conditions in California. The 1956 version, produced by Charles Richter, corrected a few anomalies. The Modified Mercalli scale is still popular, with the latest New Zealand version appearing in 1996.

The scales usually have 5 to 15 levels. Most scales have 6 to 8 levels, indicating different degrees of damage. Even the Modified Mercalli scale, with 12 levels, has only 7 or 8 that relate to damage. Finer divisions seem unwarranted.

Some scales deal only with damage intensity, whereas others refer to both the magnitude of the hazard and the damage intensity or damage potential. Scales such as the Saffir-Simpson hurricane damage potential scale begin with hazard information about wind speeds and surge heights to suggest potential damage. Others, such as the

figure 9 Comparison of Earthquake Magnitude Scales /k/.

9 Ms MJMA

8

Fujita Tornado scale, begin with damage and infer the associated wind speeds. That is, some scales work from Hazard -> Vulnerability, others from Vulnerability -> Hazard.

The below figure draws a comparison between the various levels on six earthquake intensity scales. The MM - MSK comparison is based on Munich Re [1988]. The MM - People’s Republic of China (PRC) comparison is based on Krinitzsky [1993]. All other comparisons with MM are derived from Hopper [1984]. It is worth noting that comparisons drawn from other sources would have been different, emphasizing that there is no unanimous agreement about the relativities between scales. The Damage % is based on the midpoints of the suite of MM - loss curves produced by Cochrane and Schaad [1992].

mB ML

7

Table 1 A comparison between the various levels on six earthquake intensity scales /i/.

Ms MJMA MAGNITUDE

6 Ms MJMA

mb

DAMAGE %

mB ML

I

mB5

II

ML

mb

M

L

mb

M s

4 SCALE ML Ms mb mB MJMA

M s

M

L

3

3

4

7

8 14

Mw

2

SCALE 7 ML MOMENT MAGNITUDE, Mw Ms mb SCALE MAGNITUDE mB ML Local or Richter MJMA Surface wave Ms mb Short-period body wave m Long-period 5 6 8 body9wave 10 B 7 MJMA Japanese MOMENT MAGNITUDE, Mw Meteorological Agency

2

9

RossiForel

10

3

4

5

6

MAGNITUDE 8 9 10 Local or Richter Surface wave Short-period body wave Long-period body wave Japanese Meteorological Agency

Introduction to earthquakes / Basics of Seismicity and Precast Seismic Design

MAGNITUDE Local or Richter Surface wave Short-period body wave Long-period body wave Japanese Meteorological Agency

0

20 30

I

PRC

JMA

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40 50 70

I

Geoflan

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Modified Mercalli

V

III IV V VI VII VIII

VI

IX X XI

VIII

90 XII

XII

Basics of Seismicity and Precast Seismic Design / Introduction to earthquakes

XII

XII

15

figure 10 JMA seismic intensity scale /g/.

Table 2 Examples of recorded parameters of different earthquakes /l1/.

0 • Imperceptible to people.

1 • Felt slightly by some people keeping quiet in buildings 2 • Felt by many people keeping quiet in buildings 3 • Felt by most people in buildings 4 • Most people are startled.

• Hanging objects such as lamps swing significantly.

• Unstable ornaments may fall.

5 LOWER • Many people are frightened and feel the need to hold onto something stable.

• Dishes in cupboards and items on bookshelves may fall.

• Unsecured furniture may move and unstable furniture may topple over.

5 UPPER • Many people find it difficult to walk without holding onto something stable.

• Dishes in cupboards and items on bookshelves are more likely to fall.

• Unsecured furniture may topple over.

• Unreinforced concrete-block walls may collapse.

PGA *

Mag

Depth (km)

Fatalities Earthquake

PGA ** 2.70 g

2.99 g

9.0

30

> 15 000 11.3.2011 JP

2.20 g

-

6.3

5

181 Feb 2011 NZ

2.13 g

-

6.4

6

1 Jun 2011 NZ

-

4.36 g

6.9 / 7.2

8

12 2008 Japan

1.70 g

-

6.7

19

67 1994 USA

1.01 g

-

7.3

8

2 415 1999 Taiwan

0.8 g

-

6.8

16

6 434 1995 Kobe

0.78 g

-

8.8

23

521 2010 Chile

0.6 g

-

6.0

10

143 1999 Athens

* single direction (max recorded) ** vector sum (H1, H2, V) (max recorded)

6 LOWER • It is difficult to remain standing.

• Many unsecured furniture moves and may topple over. Doors may become wedged shut.

• Wall tiles and windows may sustain damage and fall.

• I wooden houses with low earthquake resistance, tiles may fall and buildings may lean or collapse.

6 UPPER • It is impossible to move without crawling. People may be thrown through the air.

• Most unsecured furniture moves and is more likely to topple over.

• Wooden houses with low earthquake resistance are more likely to lean or collapse.

• Large cracks may form and large landslides and massif collapses may be seen.

7 • Wooden houses with low earthquake resistance are even more likely to lean or collapse.

• Wooden houses with high earthquake resistance may lean in some cases.

• Reinforced-concrete buildings with low earthquake resistance are more likely to collapse.

16

Introduction to earthquakes / Basics of Seismicity and Precast Seismic Design

Basics of Seismicity and Precast Seismic Design / Introduction to earthquakes

17

3.

Seismic analyses and the response of buildings in a earthquake Response and resonance of a building during an earthquake During an earthquake, the ground under a structure moves rapidly back and forth causing accelerations to the base of the structure. If the structure is completely rigid, it experiences an acceleration equal to that of the ground, and a force of magnitude f = ma would be generated all over it, where m is the mass of the structure and a is the acceleration of the ground. Since structures are not rigid, their response acceleration differs from the ground acceleration, and, in general, it is different within the structure itself.

The difference between ground and structural acceleration depends on the dynamic properties of the structure. When a force is applied in a harmonic fashion, the dynamic displacement differs from the static displacement the structure would experience for a static application of force with the same amplitude. This difference varies with the ratio of the period of the structure to the period of the harmonically varying force. The amplification is maximized when these two periods coincide. This condition is called resonance.

figure 11 Height is the main determinant of fundamental period each object has its own fundamental period at which it will vibrate. The period is proportionate to the height of the building /k1/.

Rigid body structures would follow the ground acceleration in their whole, instant by instant.

Instead, in deformable structures, each point follows its own motion path: displacements, velocities and accelerations are different from point to point and vary during time. This motion is called the response of the structure to the excitation from the ground. The response is governed by the exciting motion, the stiffness of the structure, and the masses involved. It is proportional to the peak ground acceleration and, in the majority of cases, is amplified with respect to the ground motion (up to 5 times and more).

Damping is a measure of the dissipation of energy in the structure. Typically, a reinforced concrete building will have 1 to 2% damping prior to an earthquake. As cracking and structural and nonstructural damage develop during the earthquake, the damping increases to about 5%.

A description of the harmonic components in the earthquake is the Fourier spectrum of the harmonics. The response of the SDOF (single degree of freedom) oscillator to each of these harmonics can be evaluated as exemplified in the figure below.

The maximum response acceleration of a SDOF structure will depend on the natural period and the amount of damping present.

figure 12 Structural response, a) rigid structure, b) deformable structure /d1/.

40 STORY CITICORP

u

EQUIPMENT

F4=m4•a4 F3=m3•a3

BUILDINGS 10-20 STORY

F2=m2•a2 F1=m1•a1

4 STORY 1 STORY

0.05

18

0.1

0.5

1.0—2.0

7.0

Seismic analyses and response of buildings in an earthquake / Basics of Seismicity and Precast Seismic Design

ag

A)

ag

B)

Basics of Seismicity and Precast Seismic Design / Seismic analyses and response of buildings in an earthquake

19

The effect of a given earthquake on a wide range of elastically responding SDOF structures with different periods is represented by an elastic response spectrum.

A response spectrum is a plot of the peak response (displacement, velocity or acceleration) of a series of oscillators having different natural frequencies and which are forced into motion by the same base vibration or shock.

SPECTRAL RESPONSE ACCELARATION, Sa

figure 14 Design response spectrum /o/.

Response spectra can be computed from earthquake accelograms, either natural or artificial, by means of several computer programs. Freeware versions of software such as Seismisignal and USEE are available on the Internet at http://seismilinks.com and http://mae.ce.uiuc.edu/usee.

For the purpose of design, some form of smooth design spectrum is usually employed, obtained either as a regularized envelope of the spectra of individual ground motions.

To

Ts

1.0

PERIOD, T

figure 15 Envelope response spectrum for design /n/.

figure 13 Harmonic oscillator with damping and SDOF simulation on the right.

DESIGN ENVELOPE SPECTRUM FOR THE SPECIFIC LEVEL OF DAMPING

me

k

m

x

He

C

SPECTRAL ACCELARATION

F SPECTRA FOR 3 POSSIBLE MOTIONS FOR A GIVEN LEVEL OF DAMPING

STRUCTURAL PERIOD

20

Seismic analyses and response of buildings in an earthquake / Basics of Seismicity and Precast Seismic Design

Basics of Seismicity and Precast Seismic Design / Seismic analyses and response of buildings in an earthquake

21

Soil Soft soils generally have a tendency to increase shaking as by much as 2 to 6 times as compared to rock. Also, the period of the soil coinciding with the natural period of the building can greatly amplify the acceleration of the building and is therefore a design consideration.

Any amplification in building vibration is undesirable. A structural design should ensure that the building period does not coincide with the period of the supporting soil. Short, stiff (shortperiod) buildings sited on soft (long-period) ground

would be appropriate as well as tall buildings (longperiod) built on hard, stiff (short-period) soil.

Predominant periods of the ground motion at a firm ground site are typically in the range 0.2–0.4 s, while they can reach 2.0 s or more on soft ground.

Since building structures have fundamental periods of approximately 0.1N (where N is the number of storeys), it can be seen that resonant amplification may well take place.

A spectacular example of this was in Mexico City during the 1985 earthquake, which saw enormous damage in medium-rise buildings of 10–20 storeys,

figure 16 Tall buildings will undergo several modes of vibration, but for seismic purposes (except for very tall buildings) the fundamental period, or first mode, is usually the most significant /k1/.

which had periods matching the 2.0 s predominant period of the earthquake motions in the city centre, while adjacent low-rise buildings, with much shorter periods, were far less damaged. Similar site effects are observed in most damaging earthquakes.

Earthquake shaking tends to be greater in a building situated on soft ground than in one built over hard ground.

figure 17 Average acceleration spectral shapes (5% damping) for different local site conditions.

(After Seed et al., 1976) /d/.

4

SPECTRAL ACCELERATION

MAXIMUM GROUND ACCELERATION

SPECTRA FOR 5% DAMPING

FIRST MODE

SECOND MODE

THIRD MODE

SOFT TO MEDIUM CLAY AND SAND (TYPE 4)

3 DEEP COHESIONLESS SOILS (>250 FT) (TYPE 3) STIFF SOIL CONDITIONS (< 150 FT) (TYPE 2)

2

ROCK (TYPE 1)

1

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

PERIOD – SECONDS

22

Seismic analyses and response of buildings in an earthquake / Basics of Seismicity and Precast Seismic Design

Basics of Seismicity and Precast Seismic Design / Seismic analyses and response of buildings in an earthquake

23

In softer soil sites it is thus useful, if not necessary, to perform a site-response analysis, which consists of a dynamic analysis of the soil deposits under the building and produces a sitespecific estimate of the ground motion.

In normal cases, i.e. when the site soil conditions do not require special consideration, the effect of the soil deposit response to the excitation from the bedrock on the input motion to the building at the surface, is expressed by modifying the shape of the elastic response spectrum. This is usually done by means of a factor, denoted by S in the Eurocode 8, which amplifies the amplitude of the entire spectrum, and increases as the stiffness of the soil deposit decreases. Further, the soil

response, changes also the frequency content of the earthquake motion at the surface, and this is reflected by specifying different spectral shapes as a function of soil category.

Ductile and brittle response Data about earthquakes is rather uncertain. It is accepted just to avoid collapse if there is a risk of high intensity events. To keep a structure within an elastic state by high intensity events is unpractical, because such a criterion would require extremely high resistance. In such cases the resulting total horizontal force might be greater than the weight of the whole building.

figure 18 Example of ground acceleration during earthquake compared to gravitational acceleration, g /l1/.

The response to high intensity quakes is seen as involving inelastic deformations of selected structural elements in order to have plastic dissipation of energy. This reduces the accelerations and, consequently, the resistance demanded, in terms of force, also reduces.

In capacity design, distinct parts of the structure, such as member plastic hinges or rotating joints, are chosen and detailed for nonlinear behavior according to an identified mechanism of nonlinear response.

In precast concrete structures, the nonlinear locations will typically be flexural plastic hinges in members or rotating joints in or between members.

These nonlinear locations of the structure are given

adequate design strength and ductility capacity for the seismic demands. All other regions of the structure and other possible behavior modes are then provided with sufficient strength to ensure that nonlinear behavior occurs in the intended locations according to the chosen mechanism, and that undesirable failure modes do not occur.

A ductile structure or connection is one that can maintain its stability under repeated cyclical deflections considerably larger than its yield deflection. The ductile structure therefore resists the extreme earthquake not by mere strength, but by allowing plastic deformations to absorb the kinetic energy induced by the ground shaking. This strategy implies that considerable damage may

figure 19 Capacity design analogy (chain) /f1/.

DUCTILE LINK

ACCELARATION (g)

1

PGA F1

0.5

F1

0 -0.5 -1

BRITTLE LINKS

0

10

20

30

BRITTLE LINKS

40

TIME (SECONDS)

24

Seismic analyses and response of buildings in an earthquake / Basics of Seismicity and Precast Seismic Design

Basics of Seismicity and Precast Seismic Design / Seismic analyses and response of buildings in an earthquake

25

occur, possibly to the extent that the structure is not repairable. However, since such response is only envisaged in a rare extreme event, in principle, the strategy is justified. Moreover, the provision of ductility is the surest way of avoiding catastrophic collapse if an earthquake occurs which produces motions considerably larger than those allowed for in the design (Booth, 1994).

Modern earthquake codes take advantage of ductile yielding to reduce the level of seismic design force, typically to a level four to six times lower than

the strength required for the structure to remain elastic.

The design seismic force is related to the available structure ductility factor, i.e., to the ratio of the maximum displacement capacity to the displacement at yielding.

The reduction in the design force is achieved by dividing the elastic force by a force reduction factor, called the behavior factor, and denoted by q in Eurocode 8.

figure 20 Brittle and ductile behavior /l1/.

The design force depends on the local seismicity, the soil category, the importance of the structure and the fundamental period of vibration of the structure.

As an alternative to ductile structures, structures of limited ductility can be designed, with higher forces but with less stringent detailing requirements for ductility.

Torsion Seismic ground motions are predominantly translational, not rotational. However, where the centers of mass and stiffness in the floors of a building do not coincide, coupled lateral-torsional response occurs.

Structures with significant torsional eccentricity are found to have a much worse performance during earthquakes.

figure 21 Coupled lateral-torsional response /n/.

BRITTLE FRACTURE

STRESS

DUCTILE FRACTURE

SIMPLE SWAY OSCILLATIONS

SWAY TORSIONAL OSCILLATIONS

PLAN A

0

26

PLAN B

STRAIN

Seismic analyses and response of buildings in an earthquake / Basics of Seismicity and Precast Seismic Design

Basics of Seismicity and Precast Seismic Design / Seismic analyses and response of buildings in an earthquake

27

4.

Eurocode 8

Methods for analysis Traditional earthquake-resistant design methods of buildings are based on linear analysis methods, in conjunction with capacity design rules. Nonlinear analysis methods have also been the subject of extensive research and are now quite mature, though they are inherently more appropriate as a design verification tool rather than a tool for direct design.

Non-linear dynamic analysis has been recognized as one of the most effective ways of measuring the quantitative responses of complex systems composed of subsoil, foundations, substructures, superstructures, and interior equipment. The method, however, involves significantly larger computational effort than the corresponding RSA.

Linear methods include: • The equivalent static or seismic coefficient method • The modal response spectrum analysis method • The linear time-history analysis method Non-linear methods include: • The non-linear static (pushover) analysis method • The non-linear time-history analysis method A simple approach for evaluating seismic force is called the seismic coefficient method. This method is permitted in most codes of practice for regular, low to medium rise buildings and begins with an estimate of the peak earthquake load. f = kmg where f denotes the seismic force, m the mass, g the gravitational acceleration and k the seismic coefficient.

With the advent of powerful desktop computers, modal response spectrum analysis has become the norm in design practice. It involves calculating the main elastic modes of vibration of a structure.

The maximum responses in each mode are then calculated from a response spectrum and these are combined by appropriate methods to produce the overall maximum response.

Linear time-history analysis has an extremely wide scope of application, mainly with regard to bridge structures. Non-linear analysis under monotonically increasing forces produces a curve, commonly referred to as the pushover curve or capacity curve, which plots the total base shear force versus the top displacement. The non-linear analysis is performed until failure is attained.

28

Seismic analyses and response of buildings in an earthquake / Basics of Seismicity and Precast Seismic Design

Eurocode 8, denoted in general by EN 1998: Design of structures for earthquake resistance, applies to the design and construction of buildings and civil engineering works in seismic regions.

Eurocode 8 consists of six parts dealing with different types of constructions or subjects: EN1998-1: General rules, seismic actions, and rules for buildings EN1998-2: Bridges EN1998-3: Assessment and retrofitting of buildings EN1998-4: Silos, tanks, and pipelines EN1998-5: Foundations, retaining structures, and geotechnical aspects EN1998-6: Towers, masts, and chimneys Out of these parts, Part 1, Part 3, and Part 5 are those relevant to the design of buildings.

Eurocode 8 (briefly EC8) reflects the most modern knowledge on seismic engineering. Its declared purpose is to ensure that in the event of earthquakes: • Human lives are protected • Damage is limited • Structures important for civil protection remain operational For the sake of simplicity, the philosophy adopted by most seismic codes for the design of structures against seismic actions is based on only two performance levels—also in Eurocode 8:

A. Damage limitation or Damage Control level The structure is expected to experience limited structural and non-structural damage during frequent earthquakes. In this limit state, the structural members retain their strength and stiffness, negligible permanent deformations and drifts occur, and minimum repair is needed.

The corresponding seismic action is usually termed as the serviceability earthquake. The definition of this earthquake varies in different codes, but a reasonable probability of exceedance is 10% in 10 years (mean return period of 95 years).

According to the originally proposed PBSE matrix (Performance-Base Seismic Engineering), this level

Basics of Seismicity and Precast Seismic Design / Eurocode 8

would thus correspond to the above described Operational Limit State.

Compliance criteria for the damage limitation performance level are usually expressed in terms of deformation limits.

B. Collapse prevention level or no-collapse requirement For an earthquake with a small possibility of occurrence during the life of the structure, prevention of collapse and retention of structural integrity and sufficient residual structural capacity should be ensured. Although significant damage might happen, the structure should be able to bear the vertical loads and retain sufficient lateral strength and stiffness to protect life during aftershocks. The corresponding seismic action is referred to as the design earthquake. For structures of ordinary importance, it has a 10% probability of exceedance in 50 years (mean return period of 475 years). For structures of higher importance, a longer return period is assigned.

According to the originally proposed PBSE matrix, this level would thus correspond to the Near Collapse or Collapse Prevention Limit State.

The no-collapse performance level is considered the ultimate limit state against which the structure should be designed according to the EN 1990 on the basis of structural design.

As indicated above, the two performance levels are to be checked against two different levels of the seismic action, interrelated by the seismicity of the region. The definition of these levels of the seismic action for design purposes falls within the scope of the Nationally Determined Parameters. In fact, the random nature of the seismic events and the limited resources available to counter their effects are such as to make the attainment of the design objectives only partially possible and only measurable in probabilistic terms.

The levels of the seismic action described above are meant to be applied to ordinary structures and are considered the reference seismic action (which is anchored to the reference peak ground acceleration agR). However, EN 1998-1 foresees the possibility of differentiating the target reliabilities (of fulfilling the no-collapse and damage limitation requirements) for different types of buildings or other constructions, depending on its importance

29

figure 22 Seismic performance design objective matrix, recommended minimum seismic performance design objectives for buildings after Bertero and Bertero (2002) /n/.

EARTHQUAKE DESIGN LEVEL

EARTHQUAKE PERFORMANCE LEVEL Fully Operational

Operational

Life Safety

Near Collapse

Frequent (43 years)

UNACCEPTABLE PERFORMANCE (FOR NEW CONSTRUCTION)

Occasional (72 years)

BASIC OB

Rare (475 years)

JECTI

ESSENTIA SAFETY

a displacement-based design are also being introduced in the appendix part of Eurocode 8.

In fact, the damage and collapse (performance) level of structural members is more correctly related to their deformation and not to the forces induced in them during the seismic action. For this reason, and for the time being, according to most codes, a proper check of the actual displacement demand versus capacity shall at least be carried out, even when following a force-based design approach, before finalizing the design of the structure.

It is well known that a good seismic response of a building is much more easily achievable if its structural system possesses some characteristics that enable clear and simple structural response under the action of seismic event.

Table 3 Importance classes and recommended values for importance factors for buildings /h1/.

VE L

Importance class

Buildings

Importance factor γι (recommended value)

I

Buildings of minor importance for public safety, e.g. agricultural buildings, etc.

0.8

II

Ordinary buildings, not belonging in the other categories.

1.0

III

Buildings whose seismic resistance is of importance in view of the consequenses associated with a collapse, e.g. schools, assembly, halls, cultural institutions etc.

1.2

IV

Buildings whose integrity during earthquakes is of vital importance for civil protection, e.g. hospitals, fire stations, power plants, etc.

1.4

OB

JECTI

CRITICA

VE

Very Rare (970 years)

L

OB

JECTI VE

30

and consequences of failure.

The different levels of reliability are obtained by multiplying the reference seismic action by this importance factor γI which, in case of using linear analysis, may be applied directly to the action effects obtained with the reference seismic action.

Although EN 1998-1 (and also the other Parts of EN 1998) presents recommended values for the importance factors, this is a Nationally Determined Parameter, since it depends not only on the global policy for seismic safety of each country but also on the specific characteristics of its seismic hazard.

It is worth noting that in modern seismic codes, including Eurocode 8, compliance criteria for the nocollapse performance level have been typically only expressed in terms of forces (force-based seismic design). More recently, take the New Zealand Design Standard (NZS1170.5), for example, a displacementfocused approach has been introduced, and a maximum drift level of 2–2.5% is suggested for Ultimate Limit State design. Similar trends towards

Eurocode 8 / Basics of Seismicity and Precast Seismic Design

Basics of Seismicity and Precast Seismic Design / Eurocode 8

31

5.

Basic principles in seismic design The guiding principles for a good conceptual design referred in EC8 are: • • • • • •

structural simplicity uniformity, symmetry, and redundancy bi-directional resistance and stiffness torsional resistance and stiffness diaphragmatic behavior at the story level adequate foundations.

EC8 provides the following analysis options for the design of buildings and for the evaluation of their seismic performance: • linear static analyses (termed the lateral force method of analysis in EN 1998-1, but often in practice called equivalent static analysis • modal response spectrum analysis (also termed in practice linear dynamic analysis, with the risk of being confused with linear time-history analysis) • non-linear static analysis (commonly known as pushover analysis) • non-linear dynamic analysis (time-history or response-history analysis).

Concrete buildings should be classified into one of the following structural types according to their behavior under horizontal seismic actions: a) frame system b) dual system (frame or wall equivalent) c) ductile wall system (coupled or uncoupled) d) system of large lightly reinforced walls e) inverted pendulum system f) torsionally flexible system.

Chapter 5.11 in Eurocode 8 specifies the provisions for precast concrete structures. The following structural types, are covered by 5.11: • frame systems • wall systems • dual systems (mixed precast frames and precast or monolithic walls) • wall panel structures (cross wall structures) • cell structures (precast monolithic room cell systems).

Figure 24 Irregular Building Configurations /k1/.

Buildings with regular configuration have Shear Walls or Moment-Resistant Frames or Braced Frames and generally the following properties: • • • •

Symmetrical plans Uniform sections and elevations Direct load paths Maximum torsional resistance.

T-SHAPED PLAN

Buildings with irregular configuration have problematic stress concentrations and torsion and they differ from the regular definition above.

L-SHAPED PLAN Figure 23 Examples of preferred regular plan configurations /s/.

Earthquake resistant concrete buildings should normally be designed to provide energy dissipation capacity and overall ductile behavior. Overall ductile behavior is ensured if the ductility demand involves globally a large volume of the structure spread to different elements and locations of all its storeys.

To this end, ductile modes of failure (e.g. flexure) should precede brittle failure modes (e.g. shear) with sufficient reliability. Such concrete buildings are classified into two ductility classes: DCM (medium ductility) and DCH (high ductility), depending on their hysteretic dissipation capacity.

According to Eurocode 8, concrete buildings may alternatively be designed for low dissipation capacity and low ductility, by applying only the rules of EN 1992-1-1:2004 for the seismic design situation, and neglecting the specific provisions, provided the given requirements are met. For buildings which are not base-isolated, design with this alternative, termed ductility class L (low), is recommended only in low seismicity cases.

32

Building configuration Building configuration defines a building’s size and shape, and structural and nonstructural elements.

Building configuration determines the way seismic forces are distributed within the structure, their relative magnitude, and problematic design concerns.

U-SHAPED PLAN

OTHER COMPLEX SHAPES

Eurocode 8 / Basics of Seismicity and Precast Seismic Design

Basics of Seismicity and Precast Seismic Design / Basic principles in seismic design

33

Figure 25 Solutions which are not recommended for bracing or positioning shear walls /z/.

Experience based in the history of seismic engineering and recent experience in the behavior of reinforced concrete structures have shown that proper basic design concepts have proved to be the most important factors in achieving fundamental requirements in earthquake design.

Seismic Design Strategies and Devices In seismic design different principles can be used either alone or combined. Below are described some main design strategies to be used in seismic design.

Diaphragms: Floors and roofs can be used as rigid horizontal planes or diaphragms to transfer lateral forces to vertical resisting elements such as walls or frames.

Shear walls: Strategically located stiffened walls are shear walls and are capable of transferring lateral forces from floors and roofs to the foundation.

Braced frames: Vertical frames that transfer lateral loads from floors and roofs to foundations.

Like shear walls, Braced Frames are designed to take lateral loads but are used where shear walls are impractical.

Moment-Resistant Frames: Column/beam joints in moment-resistant frames are designed to take both shear and bending thereby eliminating the space limitations of solid shear walls or braced frames. The column/beam joints are carefully designed to be stiff yet to allow some deformation for energy dissipation taking advantage of the ductility of steel (reinforced concrete can be designed as a Moment-Resistant Frame as well).

Energy-Dissipating Devices: Making the building structure more resistive will increase shaking which may damage the contents or the function of the building. Energy-Dissipating Devices are used to minimize shaking. Energy will dissipate if ductile materials deform in a controlled way. An example is Eccentric Bracing whereby the controlled deformation of framing members dissipates energy.

However, this will not eliminate or reduce damage to building contents. A more direct solution is the use of energy dissipating devices that function like shock absorbers in a moving car. The period of the building will be lengthened and the building will ride out the shaking within a tolerable range.

34

Base Isolation: This seismic design strategy involves separating the building from the foundation and acts to absorb shock. As the ground moves, the building moves at a slower pace because the isolators dissipate a large part of the shock. The building must be designed to act as a unit or rigid box of appropriate height (to avoid overturning) and have flexible utility connections to accommodate movement at its base. Base Isolation is easiest to incorporate in the design of new construction.

Existing buildings may require alterations to be made more rigid to move as a unit with foundations separated from the superstructure to insert the Base Isolators. Additional space (a moat) must be provided for horizontal displacement (the whole building will move back and forth a whole foot or more). Base Isolation retrofit is a costly operation that is most commonly appropriate in high asset value facilities and may require partial or the full removal of building occupants during installation.

The experience of past earthquakes has confirmed the common sense expectation that buildings which are well tied together and have well-defined, continuous load paths from top to the foundation perform much better in earthquakes than structures lacking such features.

Buildings with irregular configuration should be avoided in buildings designed for medium or high seismic areas.

Basic principles in seismic design / Basics of Seismicity and Precast Seismic Design

AVOID BRACING OFFSET!

M

AVOID ASYMMETRICAL HORIZONTAL BRACING!

M

DISCONTINUITIES IN STIFFNESS AND RESISTANCE CAUSE PROBLEMS!

Basics of Seismicity and Precast Seismic Design / Basic principles in seismic design

35

Figure 27 Buildings which had soft storey in ground floor /x/.

The degree of symmetry has a significant influence on earthquake resistance. Regular structures get over the earthquakes much better than buildings with significant irregularity. The reason is that sudden changes in section cause stress concentrations and potential failure points.

The most common example is the soft storey

(or weak storey), often caused by architectural requirements for openness at ground-floor level.

The result is that deflections are concentrated at this level during an earthquake. Soft storeys have caused perhaps more collapses in earthquakes than any other feature.

Figure 26 Soft-storeys are a common risk during seismic actvity /z/.

AVOID SOFT-STOREY GROUND FLOORS!

AVOID SOFT-STOREY UPPER FLOORS!

36

Basic principles in seismic design / Basics of Seismicity and Precast Seismic Design

Basics of Seismicity and Precast Seismic Design / Basic principles in seismic design

37

Figure 29 Combination of rigid masonry and flexible frame should be absolutely avoided (although it still is very common in most seismic areas) /x/.

Reinforced concrete structural walls of rectangular cross-section constitute the most suitable bracing system against seismic actions for skeleton structures. The walls may be relatively short in the horizontal direction from 3 to 6 m or about 1/3 to 1/5 of the building height – they must, however, extend over the entire height of the building. In a zone of moderate seismicity, in most cases for low rise buildings two slender and capacity designed ductile walls in each major direction are sufficient.

Mixed structural systems with concrete or steel columns and structural masonry walls behave very unfavorably during earthquakes. The columns in combination with the slabs or beams form frames, which have a substantially smaller horizontal stiffness than the masonry walls. The earthquake actions are therefore carried to a large extent by the masonry walls.

REINFORCED CONCRETE FRAME

STRUCTURAL MASONRY WALL

Figure 28 Minimum bracing system for low-rise based on shear walls in low seismic areas /x/.

AVOID MIXED SYSTEMS OF COLUMNS AND STRUCTURAL MASONRY WALLS!

TWO SLENDER REINFORCED CONCRETE STRUCTURAL WALLS IN EACH PRINCIPAL DIRECTION! AVOID “BRACING” OF FRAMES WITH MASONRY INFILLS!

38

Basic principles in seismic design / Basics of Seismicity and Precast Seismic Design

Basics of Seismicity and Precast Seismic Design / Basic principles in seismic design

39

It is still a common opinion that filling in frame structures with masonry walls improves the behavior under horizontal loads including seismic actions. This is true only for small loads, and as long as the masonry remains largely intact. The combination of two very different and incompatible construction types performs poorly during earthquakes. The frame structure is relatively flexible and somewhat ductile, while unreinforced masonry is very stiff and fragile and may explode under the effect of only small deformations.

Pounding and hammering of adjacent buildings can cause substantial damage, if not collapse. The threat of collapse is greatest when the floor slabs of adjacent buildings are at different levels and hit against the columns of the neighboring building.

In such cases the joints must conform with the relevant design rules.

This implies the following: the joints must have a certain minimum width (specified in the building codes) and the joints must be empty (no contact points).

Figure 31 On some building sites there is a tendency to create recesses in the structure for services, air ducts etc., or even larger openings for other purposes, without consulting the civil engineer. These recesses and openings are often inserted into the formwork of reinforced concrete elements or even jack hammered after concreting. The repercussions are particularly serious when the openings are located in plastic zones. It is necessary to avoid this practice because it can lead to the premature failure of carefully designed critical structural elements and therefore to serious safety problems /x/.

Figure 30 In order to allow building wings oriented orthogonally to each other to oscillate independently, they should be separated by a sufficiently wide and compressible seismic joint /x/.

UNFAVOURABLE

BETTER

PROHIBITED! SEPARATE ADJACENT BUILDINGS

BY SEISMIC JOINTS! FAVOUR COMPACT PLAN CONFIGURATION!

NO OPENING OR RECESSES IN PLASTIC ZONES!

40

Basic principles in seismic design / Basics of Seismicity and Precast Seismic Design

Basics of Seismicity and Precast Seismic Design / Basic principles in seismic design

41

Figure 32 In certain soils, the local ground motion parameters and structural response may differ substantially from the values obtained with the design response spectrum of the building codes. Under such conditions, the ground is likely to experience strong vibrations even for a moderate earthquake (significant amplification of the ground shaking from the bedrock to the surface). In such cases, it is necessary to perform a site specific investigation, especially for important buildings /x/.

Nonstructural Damage Control Items which are not part of the structural system are considered as nonstructural, and include building elements such as: • • • • • •

N Mx

OVERSTRENGTH SECTIONAL FORCES

OVERSTRENGTH SECTIONAL FORCES

Vx

Exterior cladding and curtain walls Parapet walls Canopies and marquees Chimneys and stacks Suspended ceilings Mechanical and other equipment.

These items must be stabilized with bracing to prevent their damage or total destruction. Building machinery and equipment can be outfitted with seismic isolating devices, which are modified versions of the standard Vibration Isolators. Loss arising from non-structural damage can be a multiple of the structural losses.

Figure 33 The facade elements’ anchoring must therefore be designed and detailed not only for gravity loads but also for horizontal cyclic forces (tension / compression) /x/.

PROTECT FOUNDATIONS THROUGH CAPACITY DESIGN!

INSUFFICIENT

DEVELOP A SITE SPECIFIC RESPONSE SPECTRUM!

42

Basic principles in seismic design / Basics of Seismicity and Precast Seismic Design

DEVELOP A SITE SPECIFIC RE-

BETTER

ANCHOR and FACADE ELEMENTS AGAINST between The anchorages possible connections HORIZONTAL FORCES! the facade elements should be able to follow the expected deformations of the structure

Basics of Seismicity and Precast Seismic Design / Basic principles in seismic design

43

6.

Solutions for precast construction in seismic regions Precast and prestressed concrete has had significant and successful application in earthquake resisting structures in many parts of the world.

Experience of earthquakes and laboratory testing gives confidence that precast and prestressed concrete elements can be used very successfully in structures designed for earthquake resistance, providing careful attention is paid to design and construction.

It is a matter of extreme importance always to make sure that the precast solutions are properly designed and built having taken into account the local circumstances and seismic risk. The recent increased knowledge about seismicity and seismic design enables proper seismic precast design taking into consideration the necessary aspects.

A well-designed and well-constructed building has a reliable load path that transfers lateral forces through the structure to the foundation where the soil can resist them. Horizontal earthquake forces are usually resisted by either walls or frame elements. At the base of wall and frame elements, foundation components transfer the earthquake forces to the ground.

The diaphragms, walls, frames and foundations of a building are the key elements in the load path through the structure. The connections between these elements are also important components of the chain that makes up the horizontal and vertical load paths that transfer the horizontal forces. The earthquake resistance of a building is only as strong as the weakest link in the load path.

Precast concrete building structures are composed of some basic types of structural systems.

These systems can be combined in different ways to obtain an appropriate and effective structural concept that fulfills the needs of a specific building.

The most common precast systems are: • beam and column systems (beam elements, column elements, connections) • floor and roof systems (floor elements, roof elements, connections) • bearing wall systems (wall elements, connections) • facade systems (facade wall elements, connections)

44

Moment resisting frames are often chosen as the lateral force resisting systems in design because of their inherent space flexibility. Frames have advantages over wall systems that energy dissipation takes place in many regions and that they are highly redundant. The main challenge when using moment resisting frames as the lateral force resisting system is to size the beam and column elements to provide an effective control to earthquake induced inter-storey lateral displacements. This control is particularly important when the building incorporates parts not specifically designed to undergo large inter-storey lateral displacements.

Structural walls have long been recognized as a very efficient lateral force resisting system in lowand high-rise buildings. The large lateral stiffness of structural walls and their deformed shape when subject to lateral forces make them highly desirable for controlling earthquake induced interstorey lateral displacement demands, and hence, minimizing non-structural damage, in building structures. Structural walls combined with long span prestressed slab elements also provide space flexibility.

In seismic design, the solution for a lateral-forceresisting system is a matter of top priority, the two main principles for the design of a precast lateralforce-resisting system are: 1. Emulation of monolithic concrete construction.

2. Jointed precast, relying on unique properties of precast concrete.

Monolithic emulation of a precast structure can either be done by using moment frames or by using structural walls. The connections can either be strong or ductile.

The emulative approach has some limitations, which must be especially considered when choosing the precast system for medium or high seismic areas.

The emulative approach does not exploit the advantages of precast concrete. It also slows down the erection speed causing higher costs and makes the connections complex.

In areas with frequent seismic activity, it must be considered also that ductility means permanent

Solutions for precast construction in seismic regions / Basics of Seismicity and Precast Seismic Design

deformations after the earthquake and usually high costs of repairing (when feasible to repair).

To solve the problems of the emulative approach, especially in high seismic areas, there is the socalled new generation of seismic resisting systems.

Jointed ductile, hybrid, and controlled rocking systems are included among these.

In terms of the lateral force – lateral displacement response characteristics both moment-resisting frames and precast concrete wall systems can be classified into following two main groups and sub-groups:

• Linear • Non-linear

- equivalent monolithic

- jointed

- limited ductility

- hybrid

- rocking The main principles of these groups for frame and wall systems are presented in the following chapters.

Figure 34 Seismic design requirements for precast structures in ACI 318-02 /a/.

PRECAST SEISMIC SYSTEMS

JOINED PRECAST

MONOLITHIC EMULATION

MOMENT FRAMES

SPECIAL

STRONG CONNECTION

DUCTILE CONNECTION

STRUCTURAL WALLS

SPECIAL

DUCTILE CONNECTION

INTERMEDIATE

MOMENT FRAMES

SPECIAL

STRUCTURAL WALLS

SPECIAL

DUCTILE CONNECTION

STRUCTURAL WALL = SHEAR WALL

Basics of Seismicity and Precast Seismic Design / Solutions for precast construction in seismic regions

45

Shear wall systems Shear walls are elements that resists, in addition to gravity loads, in plane (as distinct from out of plane) lateral forces. They are like vertical cantilever beams and are typically quite deep.

Shear walls make up the most common lateralforce-resisting elements in the precast industry.

DISPLACEMENT

LINEAR

46

DISPLACEMENT

EQUIVALENT MONOLITHIC

DISPLACEMENT

LIMITED DUCTILITY

Solutions for precast construction in seismic regions / Basics of Seismicity and Precast Seismic Design

FORCE

FORCE

FORCE

FORCE

FORCE

Figure 35 Classification of precast concrete wall systems according to the lateral force â&#x20AC;&#x201C; lateral displacement response characteristics /i1/.

DISPLACEMENT

HYBRID

DISPLACEMENT

ROCKING

Basics of Seismicity and Precast Seismic Design / Solutions for precast construction in seismic regions

47

Building systems with precast concrete shear walls as part of the Lateral Force Resisting System (LFRS) are commonly used to provide safe, serviceable, and economical earthquake-resistant construction.

The horizontal stability of the walls is assumed by means of the cantilever action of the walls, along their height. In this respect, proper dimensioning of the connections between walls and between walls and slabs is needed for their strength, deformability, and ductility.

If two or more shear walls are connected with relatively rigid members, they are referred to as coupled shear walls.

In Figure 38 a hybrid wall system is described, where an energy dissipation device and unbounded prestressing tendons are combined to obtain a self-centering mechanism that eliminates residual displacements following an earthquake.

Figure 37 In plane response (cantilevering construction) of the shear wall under horizontal actions /s/.

Figure 36 Different configurations of large panels systems a) cross-wall system, b) spine wall system, c) two-way system /s/.

a)

b)

2

2

1

Figure 38 Hybrid wall system.

1

c) UNBONDED POST-TENSIONED TENDONS

2 1

1. wall panel 2. floor panel dashed lines indicate non bearing walls

48

Solutions for precast construction in seismic regions / Basics of Seismicity and Precast Seismic Design

ENERGY DISSPATION DEVICES

Basics of Seismicity and Precast Seismic Design / Solutions for precast construction in seismic regions

49

Generally, reinforced concrete precast frame systems may be divided into two main categories: 1. Systems, which are characterized by their hinged beam-column connections, while columns are fixed at the building base.

Frame systems Frames respond differently from shear walls to lateral forces. Frames consist of vertical elements (columns) and horizontal elements (beams and slabs).

2. Precast frame systems, in which precast members are connected together and with the foundations in a way that all connections are moment resisting. Such frame systems usually are designed to be able to meet the requirements of equivalent monolithic ones.

DISPLACEMENT

LINEAR

50

DISPLACEMENT

EQUIVALENT MONOLITHIC

DISPLACEMENT

LIMITED DUCTILITY

Solutions for precast construction in seismic regions / Basics of Seismicity and Precast Seismic Design

FORCE

FORCE

FORCE

FORCE

FORCE

Figure 39 Classification of precast concrete moment-resisting systems according to the lateral force â&#x20AC;&#x201C; lateral displacement response characteristics /i1/.

DISPLACEMENT

HYBRID

DISPLACEMENT

ROCKING

Basics of Seismicity and Precast Seismic Design / Solutions for precast construction in seismic regions

51

A frame resists being deformed by lateral forces due to the rigidity of the beam-column joints.

Because of this rigidity (the tendency of a joint to retain the initial 90-degree angles between beams and columns meeting at the joint), the beams and columns bend as shown in the figure below.

Precast prestressed concrete beams and deck members are usually the most economical when they can be designed and connected into a structure as simple span members. This is because: • Positive moment strengths are much easier and less expensive to achieve with pre tensioned members than negative moment strength at supports.

Figure 40 Response of frame to seismic forces /a1/.

h4

h3

h2

Therefore, it is desirable to design precast prestressed concrete structures with connections that allow lateral movement and rotation and to design the structure to achieve lateral stability through the use of floor and roof diaphragms and shear walls.

Figure 41 Different arrangement of precast elements in precast moment resisting frame construction /i1/.

5

f5 h5

• Connections that achieve continuity at the supports are usually complex and costly.

• The restraint to volume changes that occur in rigid connections may cause serious cracking and unsatisfactory performance, or, in extreme cases, even structural failure.

BEAM-COLUMN CORNER UNIT

4

f4

CONTINUOS BEAM UNIT

BEAM-COLUMN T-UNIT

BEAM-COLUMN CRUCIFORM UNIT

BEAM ELEMENT MIDSPAN CONNECTION

3

f3

BEAM-COLUMN DOUBLE CRUCIFORM UNIT

2

f2 1

f1

h1

HALF-PRECAST BEAM UNIT

GROUTED JOINT

COLUMN BAR SPACE

CAST-IN-PLACE TOPPING

COLUMN UNIT

BEAM SHELL UNIT COLUMN SHELL UNIT

L GROUTED JOINT

52

Solutions for precast construction in seismic regions / Basics of Seismicity and Precast Seismic Design

CAST-IN-PLACE CONCRETE

Basics of Seismicity and Precast Seismic Design / Solutions for precast construction in seismic regions

53

Connection details Joints can be classified in three main systems: Typical joint system: Dry joints with mechanical connectors generally composed of angles, plates, channel bars, anchors, fasteners, bolts, dowel bars etc. including joints completed in-situ with mortar for filling or fixing.

Figure 42 In hybrid frame systems mild steel reinforcement and unbonded pre stressing tendons are combined at the critical connection interface to obtain a centered – oriented response /s/.

Emulative joint system: Wet joints with rebar splices and cast-in-situ concrete restoring the monolithic continuity proper of cast-in-situ structures and leading usually to moment-resisting unions.

Mechanical joint system: Dry joints with bolted flanges or other steel fittings similar to those used in metallic constructions fixed at the end of the precast member.

The connections between precast units may be designed to act as ductile fuses in various ways.

MILD STEEL

UNBONDED LENGTH

M M UNBONDED TENDONS

54

BEAM COLUMN

Solutions for precast construction in seismic regions / Basics of Seismicity and Precast Seismic Design

Ductile dissipative behavior of the connection can be provided by the steel connectors, if correctly designed for a failure involving flexural or tension-compression modes and not shear modes or by other dissipative phenomena like friction.

For a ductile connection, in addition to a ductile connector, the criteria of capacity design shall be applied, under-proportioning the connector with respect to the lateral parts.

Also the geometric compatibility of deformations shall be checked (e.g. against the loss of bearing).

Non-ductile connections shall be opportunely overproportioned by capacity design with respect to the resistance of the critical dissipative regions of the structure or proportioned on the base of the action obtained from a structural analysis that does not account for any energy dissipation capacity.

The ductility of the connections may contribute or not to the global ductility of the structure depending on their position in the structural assembly and on their relative stiffness.

The main parameters which characterize the seismic behavior of the connection, as measured through monotonic and cyclic tests, refer to the six properties of: • strength: maximum value of the force which can be transferred between the parts • ductility: ultimate plastic deformation compared to the yielding limit • dissipation: specific energy dissipated through the load cycles related to the correspondent perfect elastic-plastic cycle • deformation: ultimate deformation at failure or functional limit • decay: strength loss through the load cycles compared to the force level • damage: residual deformation at unloading compared to the maximum displacement and/or details of rupture

Basics of Seismicity and Precast Seismic Design / Solutions for precast construction in seismic regions

55

Precast column connections Bolt connections

Figure 43 Precast column connections In the following figures the mechanical joint systems for columns by Peikko are shown.

These solutions are well applicable in seismic conditions.

Dowel connection Different precast beam column connections were tested in the Safecast project. According to the test results, it can be roughly expected that under cyclic loading the shear resistance of the connection will decrease by at least 50% in comparison to its shear resistance under monotonic loading.

Grouted sleeve splice One of the most popular and economical column splice details is the grouted splice sleeve.

Figure 44 Alternative methods for grouted sleeve splice.

GROUT INSERTED UNDER PRESSURE AT A UNTIL APPEARING AT B

B ALTERNATIVE GROUT TUBE WHERE GROUT IS POURED FROM TOP

SURFACE OF TUBE NOT TO BE SMOOTH

ANCHORAGE LENGTH

CONNECTION OF A COLUMN TO FOUNDATION, USING COLUMN SHOES AND ANCHOR BOLTS.

A 10

DRY PACK TO COMPLETE JOINT

DEFORMED BAR PROJECTING FROM LOWER COLUMN

RIGID BEAM TO COLUMN CONNECTION, USING BEAM SHOES AND ANCHOR BOLTS.

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COLUMN SPLICE CONNECTION, USING COLUMN SHOES AND ANCHOR BOLTS.

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Jointed connection systems The non linear jointed frame connections can have either limited ductility or they can be hybrid or rocking.

In the following figure is shown example of rocking connection.

Figure 45 Splice sleeve coupler /j1/.

Figure 46 Post-Tensioned Moment Resisting Connection and its reinforcement /s/.

Grouted sleeve coupler splice

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Solutions for precast construction in seismic regions / Basics of Seismicity and Precast Seismic Design

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Figure 48 Intersection of sandwich wall elements and partition wall element.

Wall to wall connection The connections between precast units may be designed to act as ductile fuses in various ways.

Numerous details are currently used to connect the wall panels at vertical joints. These connections can be divided in four groups: welded connections, bolted connections, monolithic joints and connections to adjacent columns.

Figures 47, 48 and 49 show vertical wall-to-wall connection details that make use of cast-in-situ concrete. Typically, these connections are made overstrong to ensure behaviour as if monolithic.

The wall panel interfaces are typically roughened and in some case incorporate shear keys,

Figure 47 Example of sandwich wall element corner connection with vertical cast cast-in-situ joint.

Figure 49 Intersection of partition wall elements.

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Solutions for precast construction in seismic regions / Basics of Seismicity and Precast Seismic Design

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61

As a rule, shear connections are provided with keyed interfaces between the precast panels and the in-situ concrete of the joint and well distributed reinforcement across the joint by means of loops.

In cases when the horizontal connections are partly under compression and partly under

tension (under the design seismic situation), and vertical post-tensioning is not applied to render the horizontal joint fully under compression, then the total tension force (corresponding to the tension zone of the joint) should be covered by continuous tensile vertical reinforcement fully anchored in the

Figure 50 Configuration of shear connections with indication of possible development of cracks for monolithic (a) and cyclic loading (b) and possible development of compression struds under cyclic loading (c), in all cases with the same type of reinforcement (a) /s/.

INSITU CONCRETE

INSITU CONCRETE

LONGITUDINAL REINFORCEMENT

A)

B1

B1

B2

OPEN CLOSED

UPPER PANEL

CRACKS

PRECAST

A

2

PRECAST

PRECAST

PRECAST

ÙA

A2

DIMENSIONS OF THE SPECIAL KEY ARE INDICATIVE

> 100 MM

OPEN CRACK

CLOSED CRACK

HORIZONTAL JOINT AREA

JOINT HEIGHT

LOWER PANEL

B2 A 1 B1 B)

MONOTONIC LOADING

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COMPRESSED ELEMENT

A1

B2

PRECAST

CO D MP RE TE IAG SS SHE NS ON ION A RC IO AL RAC N KS CR AC KS

PRECAST

ÙA

ho > lt:2 > 40 CM

B2

TRANSVERSAL REINFORCEMENT

Figure 51 Vertical section of a horizontal joint with special key /s/.

INSITU CONCRETE

B1

Us

Us

body of the upper and lower panel.

The continuity of this reinforcement should be secured by ductile welding within the horizontal joint or, preferably, within special keys provided for this purpose. Alternative solutions could be ductile overlapping of reinforcement in the panels or with protruding bars and ducts.

C)

POSSIBLE CRACK DEVELOPMENT POSSIBLE DEVELOPMENT OF COMPRESSION STRUDS UNDER CYCLIC LOADING

Solutions for precast construction in seismic regions / Basics of Seismicity and Precast Seismic Design

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Figure 53 Example of connection between sandwich wall and slab.

Wall to slab connection A variety of connection details are employed to connect precast concrete floors and the precast wall panels. These connections are generally designed to

transfer floor inertia forces to the wall panels and often detailed to avoid unseating or concrete pullout failure during earthquakes.

Figure 52 Horizontal shear wall joints with preformed metal duct filled with expansive grout.

Figure 54 Example of connection between partition wall and slab.

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7.

Glossary

Accelerograph An instrument that records the acceleration of the ground during an earthquake, also commonly called an accelerometer.

Accelerogram The recording of the acceleration of the ground during an earthquake.

Active fault A fault that is likely to have another earthquake sometime in the future. Faults are commonly considered to be active if they have moved one or more times in the last 10,000 years.

Aftershocks Aftershocks are earthquakes that follow the largest shock of an earthquake sequence. They are smaller than the mainshock and within 1–2 rupture lengths distance from the mainshock. Aftershocks can continue over a period of weeks, months, or years.

In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue.

Base shear Total horizontal seismic shear force at the base of a structure Behavior factor The ratio q=Fel/Fy is termed in Eurocode 8 the behavior factor (in North America the same quantity is termed the force reduction factor). Fel is the peak force that would develop if the SDOF system were linear-elastic and Fy is the yield force of the system.

Body wave A body wave is a seismic wave that moves through the interior of the Earth, as opposed to surface waves that travel near the Earth’s surface. P and S waves are body waves. See also Figure 6.

Coupled Structural element composed of two or more single walls, connected in a regular pattern by adequately ductile beams (coupling beams), able to reduce by at least 25% the sum of the base bending moments of the individual walls if working separately.

66

Crust The crust is the outermost major layer of the Earth, ranging from about 10 to 65 km in thickness worldwide. The uppermost 15–35 km of crust is brittle enough to produce earthquakes.

Foreshocks Foreshocks are relatively smaller earthquakes that precede the largest earthquake in a series, which is termed the mainshock. Not all mainshocks have foreshocks.

Natural frequency The natural frequency is the frequency at which a particular object or system vibrates when pushed by a single force or impulse, and not influenced by other external forces or by damping.

Diaphragm Relatively thin but stiff horizontal structural systems which transmit inplane lateral forces to, or between, vertical lateral force resisting elements.

The diaphragms tie the whole structure together.

Ground motion Ground motion is the movement of the Earth's surface from earthquakes or explosions. Ground motion is produced by waves that are generated by a sudden slip on a fault or sudden pressure at the explosive source and travel through the Earth and along its surface.

Performance-Based Seismic Engineering approach Identification of seismic hazards, selection of the performance levels and performance design objectives, determination of site suitability, conceptual design,numerical preliminary design, final design, acceptability checks during design, design review, specification of quality assurance during the construction and of monitoring of the maintenance and occupancy (function) during the life of the building.

Dual system Structural system in which support for the vertical loads is mainly provided by a spatial frame and resistance to lateral loads is contributed to in part by the frame system and in part by structural walls, coupled or uncoupled.

Ductility The ability of a member or structure to deform beyond its elastic limit without any significant loss of strength.

Ductility factor The ratio of the maximum displacement capacity to the displacement at yielding.

Elastic Indicates a return to initial state upon unloading, without residual deformations Energy dissipation Dispersion of the earthquake energy input into a structure by hysteresis, damping and other mechanisms.

Epicenter The point on the Earth’s surface directly above the hypocenter (or focus) is the epicenter.

Fault A fault is a fracture along which the blocks of crust on either side have moved relative to one another parallel to the fracture.

Hybrid system Combination of unbonded post-tensioning and dissipaters.

Intensity Intensity is the impact of ground-shaking on population, structures and the natural landscape; the impact will be greater nearer the site and less far away. There are many intensities for an earthquake, depending on where you are, unlike the magnitude, which is one number for each earthquake.

Inverted pendulum A pendulum which has its center of mass above its pivot point.

Lithosphere The lithosphere is the outer solid part of the Earth, including the crust and uppermost mantle.

Magnitude Magnitude is the amount of energy released from the source or focus of the earthquake.

Mainshock The mainshock is the largest earthquake in a sequence, sometimes preceded by one or more foreshocks, and almost always followed by many aftershocks.

RSA Standard response spectrum analysis.

Richter scale On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude of 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value /v/.

Rupture front The rupture front is the instantaneous boundary between the slipping and locked parts of a fault during an earthquake.

Shear Wall A wall designed to resist lateral forces parallel to the wall. A shear wall is normally vertical, although not necessarily so.

Focus, hypocenter The source of an earthquake within the crust is commonly termed the focus or hypocenter.

Glossary / Basics of Seismicity and Precast Seismic Design

Period The period is the time interval required for one full cycle of a wave.

Basics of Seismicity and Precast Seismic Design / Glossary

67

8.

References

SDOF system The simplest vibratory system can be described by a single mass connected to a spring (and possibly a dashpot). The mass is allowed to travel only along the spring elongation direction. Such systems are called Single Degree-of-Freedom (SDOF) systems and are shown in the Figure 14.

Stiffness Stiffness is a measure of a material’s resistance to deformation when stressed within its elastic range.

Tectonic plates The plates forming the Earth’s fractured crust, known also as the lithospheric plates.

/a/ Cleland, N and Ghosh S.K., 2007,

Seismic Design of Precast/Prestressed

Concrete Structures

/q/ New Zealand standard, NZS4203:1992

/f1/ Fundamentals of earthquake engineering, Amr

S. Elnashai, Luigi Di Sarno

Design Guidelines for Connections of Precast Structures under Seismic Actions, European Commission Joint Research Centre, Institute for the Protection and Security of the Citizen

/b/ www.universetoday.com/85320/

lithospheric-plates/

/r/

/c/ www.projectgroundswell.com/2010/03/03/

catastrophe-in-chile-vs-haiti-and-the-built-

environment/

/s/ Precast concrete buildings in seismic areas

- Practical aspects, draft November 2012,

FIB Commission 6

/h1/ Eurocode 8: Seismic Design of Buildings

Worked examples, JRC Scientific and Technical

Reports, 2011

/d/ Sen, Tapan K, 2009, Fundamentals of Seismic

Loading on Structures

/t/ A modal pushover analysis procedure to

estimate seismic demands for unsymmetric-

plan buildings Anil K. Chopra1 and Rakesh K.

Goel2

/i1/ Seismic design of precast concrete building

structures, FIB Bulletin 27, 2003

/u/ http://geology.about.com/od/seishazardmaps/

ss/World-Seismic-Hazard-Maps.htm

/k1/ http://www.wbdg.org/resources/seismic_design.

php /l1/ Earthquakes and seismic loading on structures,

Aalto University

/e/ http://soundwaves.usgs.gov/2009/11/

fieldwork2.html

/f/ http://earthquake.usgs.gov/learn/glossary/ /g/ http://www.jma.go.jp/jma/en/Activities/

earthquake.html

/v/ http://earthquake.usgs.gov/learn/topics/richter.

php

/h/ http://mohumes.wikispaces.com/

How+Does+An+Earthquake+Travel%3F

/x/ http://www.world-housing.net/whereport1view.

php?id=100105

/i/ http://www.riskfrontiers.com/nhq/nhq4-3tables.

htm

/y/ http://en.wikipedia.org/wiki/Response_

spectrum

/j/ http://www.sdgs.usd.edu/publications/maps/

earthquakes/rscale.htm

/z/ Seismic Conceptual Design of Buildings – Basic

principles for engineers, architects, building

owners, and authorities, Hugo Bachmann

/k/ http://www.fhwa.dot.gov/bridge/tunnel/pubs/

nhi09010/13.cfm /l/ http://www.acoustics.org/press/151st/Gerstoft.

html /m/ http://theconstructor.org/earthquake/how-the-

ground-shakes-during-earthquake/2648/ /n/ Seismic design manual for precast concrete

structures, Consolis Technology /o/ http://civil-engg-world.blogspot.fi/2012/04/

fema-356-earthquake-hazard-due-to.html

/g1/ Eurocode 8: Design of structures for

earthquake resistance - Part 1: General rules,

seismic actions and rules for buildings

/j1/ www.daytonsuperior.com

/a1/ http://inderc.blogspot.fi/2012/05/captive-and-

short-column-effects.html /b1/ Canterbury earthquakes, royal commission,

final report /c1/ Building structures illustrated – patterns,

systems and design, Francis D.K. Ching, Barry S.

Onouye, Douglas Zuberbuhler. /d1/ Structural connections for precast concrete

buildings, FIB Bulletin 43, 2008 /e1/ www.substech.com

/p/ Bungale S., Taranath S.E., Wind and Earthquake

Resistant Buildings, Structural Analysis and

Design

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Notes

Elematic Elematic, founded in 1959, supplies precast concrete machines and complete production plants for building construction worldwide.

Elematic is the leading one-stop-supplier of precast concrete technology. Our customers are in the precast business of floors, walls, frames and foundations, or even all of them. We offer precast solutions for all needs – to match today’s high standards for structural and architectural design.

With support available throughout the life cycle, we assist our customers to achieve maximum profitability regardless of the investment level. www.elematic.com

Peikko Group Peikko Group Corporation is a leading global supplier of concrete connections and composite structures. Peikko’s innovative solutions make the customers’ building process faster, easier and more reliable. Peikko also focuses on assisting architects and structural designers with sophisticated design tools and software.

Peikko has subsidiaries in over 30 countries in Asia-Pacific, Europe, the Middle East, and North America, with manufacturing operations in 9 countries. Peikko is a family-owned and run company with over 900 professionals. Peikko was founded in 1965 and is headquartered in Lahti, Finland. Further information on Peikko can be found on www.peikko.com.

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References / Basics of Seismicity and Precast Seismic Design