Influence of Site-Specific Spectral Accelerations on the Performance of Lead Rubber Bearing Base-Iso

International Research Journal of Engineering and Technology (IRJET)

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Influence of Site-Specific Spectral Accelerations on the Performance of Lead Rubber Bearing Base-Isolated Structure Ma. Jan Nickole F. Estrada1 1Faculty, Civil Engineering Department, Tarlac State University, Tarlac City, Tarlac, Philippines ---------------------------------------------------------------------***--------------------------------------------------------------------predicted seismic performance of a lead rubber bearing Abstract – In highly seismic regions, the accurate

base-isolated building, providing insights for optimizing design.

characterization of seismic input is paramount for the design of earthquake-resistant structures. This study investigates the seismic performance of a four-storey reinforced concrete school building with lead rubber bearing base isolation system, specifically focusing on the effect of site-specific spectral accelerations on its response. The analysis, executed using Response Spectrum Analysis in SAP2000, used seismic parameters from both Uniform Building Code (UBC 97)’s conventional zonal approach and American Society of Civil Engineer 7 (ASCE/SEI 7)’s more advanced site-specific methodology. Key performance metrics, including storey drifts, floor accelerations, fundamental periods, and base shears, were analyzed. The findings demonstrate that the ASCE/SEI 7 code, incorporating a more site-specific approach, generally predicted lower and more favorable responses compared to UBC 97. There is a significant reduction in seismic demands on the superstructure: storey drifts, floor accelerations, and base shears decreased by 25% to 28%. The structure’s fundamental period increased by 3.8%.

2. METHODOLOGY A four-storey reinforced concrete school building situated on stiff soil at Tarlac, Philippines was considered for the study.

For UBC 97, the Seismic Zone is 4. For ASCE/SEI 7, by consulting local seismic hazard maps, the structure is subjected to a seismic hazard with a 0.5g peak ground acceleration. Lead rubber bearings were designed for the building adhering to the distinct provisions of UBC 97 and ASCE/SEI 7. The structure, including its LRB system, was then modeled in SAP2000, and its seismic performance was analyzed using Response Spectrum Analysis.

The modeling process was directly informed by the building’s structural and architectural plans. Each storey maintains a uniform height of 3.2 m, contributing to a total building height of 12.8 m. The building details are shown in Table 1.

Key Words: Site-Specific Spectral Accelerations, Lead Rubber Bearing, Base Isolation, UBC 97, ASCE/SEI 7, Response Spectrum Analysis

Table - 1: Building Model Details Parameter

Value

1. INTRODUCTION

Plan Dimensions

23.0 m x 9.5 m

Earthquakes pose a significant threat to structures [1], with conventional fixed-base designs often lead to damage or even collapse during earthquakes. To enhance the seismic performance of these structures, innovative systems like base isolation are employed [2]. The core concept behind base isolation is to place a flexible barrier between the superstructure and its foundation, effectively decoupling the building from the potentially damaging ground motions [3].

Lead rubber bearing is one of the commonly used types of base isolation devices [4]. The design of base isolation system hinges on the precise characterization of seismic input, traditionally guided by building codes. UBC 97 and ASCE/SEI 7 represent different generations of seismic design standards, with ASCE/SEI 7 being a more advanced document. A critical distinction between these codes lies in their approach to defining seismic input, particularly the emphasis on site-specific spectral accelerations in ASCE/SEI 7 [5]. Unlike the generalized seismic zones utilized in UBC 97, site-specific spectral accelerations provide a more refined representation of expected ground motions at a particular location. This study aims to quantify how these distinct code philosophies and parameters influence the

Number of Storeys

4

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Height of each Storey

3.2 m

Total Height of the Structure

12.8 m

Bays in X Direction

4 bays of 4.5 m length 1 bay of 5.0 m length

Bays in Y Direction

1 bay of 7.0 m length 1 bay of 2.5 m length

Slab Thickness

0.125 m

The different material properties used in the building models are shown in Table 2, and Table 3 presents the gravity loads used.

Table - 2: Material Properties

|

Property

Value

Concrete Strength, f’c

20.7 MPa

Unit Weight of Concrete

23.56 kN/cu.m

Yield Strength of Steel

275 MPa and 230 MPa

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International Research Journal of Engineering and Technology (IRJET)

e-ISSN: 2395-0056

Volume: 12 Issue: 07 | July 2025

p-ISSN: 2395-0072

www.irjet.net

The gathered seismic parameters informed the preliminary design of the LRB isolation system, where target properties were estimated and calculated. Then these are used to determine the LRB specific properties and dimensions. As a result, the following design parameters were obtained, as shown in Table 6.

Table - 3: Loadings Load

Value

DL 150mm thick CHB Wall

2.73 kPa

DL 100mm thick CHB Wall

2.11 kPa

LL Classrooms

1.90 kPa

LL Toilets

2.40 kPa

LL Corridors above Stairs

3.80 kPa

Parameter

UBC 97

ASCE/SEI 7

LL Corridors on Ground

4.8 kPa

U1 Effective Stiffness (kN/m)

214217.39

197478.90

The seismic parameters as per UBC 97 and ASCE/SEI 7 used are presented in Tables 4 and 5.

U2 and U3 Effective Stiffness (kN/m)

5311.20

4896.20

Table - 4: Seismic Parameters (UBC 97)

U2 and U3 Stiffness (kN/m)

531.12

489.62

31

34

Table - 6: LRB Design Parameters

Parameter

Value

Seismic Zone Factor, Z

0.4

U2 and U3 Yield Strength (kN)

Soil Profile Type

SD

Bearing Diameter (m)

0.52

0.52

Seismic Source Type

A

LRB Height (m)

0.37

0.38

Distance from a Seismic Source

10 km

Near Source Factor, Na

1.0

Near Source Factor, Nv

1.2

Seismic Coefficient, Ca

0.44

Seismic Coefficient, Cv

0.768

Design Period, TD (s)

2.5

Effective Damping (%)

20

Damping Coefficient

1.5

The calculations yield a slightly stiffer lead rubber bearing for UBC 97.

3. RESULTS AND DISCUSSIONS The key performance data extracted from the SAP2000 response spectrum analysis include storey drifts (calculated from storey displacements), floor accelerations, fundamental period, and base reactions. These results are shown in chart 1 to chart – 6 to facilitate understanding and comparison.

From chart -1 and chart – 2, the peak storey drift obtained using ASCE/SEI 7 is 26% lower than that obtained using UBC 97.

Table - 5: Seismic Parameters (ASCE/SEI 7) Parameter

Value

Peak Ground Acceleration

0.5

Site Class

D

Risk Category

IV

Distance from a Seismic Source

10 km

Spectral Acceleration at 0.2s

1.271

Spectral Acceleration at 1.0s

0.425

Short Period Coefficient, Fa

1.0

Long Period Coefficient, Fv

1.875

Design Spectral Acceleration at 0.2s

0.847

Design Spectral Acceleration at 1.0s

0.531

Design Period, TM (s)

2.5

Effective Damping (%)

20

Damping Coefficient

1.5

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From chart – 3 and chart – 4, there is a 25% to 26% reduction in floor acceleration when ASCE/SEI 7 was used.

From chart – 5, ASCE/SEI 7 calculated a slightly longer period than UBC 97. The designed lead rubber bearing for UBC 97 is slightly stiffer than that designed using ASCE/SEI 7.

From chart – 6, the green bar is for UBC 97 and the blue bar is for ASCE/SEI 7. ASCE/SEI 7 resulted in a lower calculated base shear value compared to UBC 97. This represents a 28% reduction.

These differences show that variations in seismic parameters between the UBC 97 and ASCE/SEI 7 codes, lead to different storey displacements, floor accelerations, fundamental periods, and base reactions.

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International Research Journal of Engineering and Technology (IRJET)

e-ISSN: 2395-0056

Volume: 12 Issue: 07 | July 2025

p-ISSN: 2395-0072

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Chart - 4: Floor Acceleration in Y Direction Chart - 1: Storey Drift in X Direction

Chart - 5: Fundamental Period Chart - 2: Storey Drift in Y Direction

Chart - 6: Base Reaction in X and Y Directions

4. CONCLUSIONS

Chart - 3: Floor Acceleration in X Direction

This study investigated the effect of using sitespecific spectral accelerations on the performance of a lead rubber bearing base-isolated reinforced concrete building.

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International Research Journal of Engineering and Technology (IRJET)

e-ISSN: 2395-0056

Volume: 12 Issue: 07 | July 2025

p-ISSN: 2395-0072

www.irjet.net

The base-isolated building analyzed using ASCE/SEI 7 exhibited lower storey drifts, floor accelerations, and base shear values than when designed with UBC 97. Furthermore, ASCE/SEI 7 yielded a slightly longer fundamental period than UBC 97. These findings indicate that ASCE/SEI 7, with its more recent and refined site-specific approach, tends to result in designs or analyses that lead to different seismic demands and improved performance for base-isolated structure.

[8]

P. Mhatre, S. Mhatre, Ashwini P, S. Stephen, “Earthquake Resistant Structure Using Base Isolation Technique”, International Research Journal of Engineering and Technology, Vol. 8, Issue 12, December 2021.

BIOGRAPHY Ma. Jan Nickole F. Estrada is a candidate for the degree Master of Science in Civil Engineering major in Structural Engineering at Tarlac State University in Tarlac City, Tarlac, Philippines.

Specifically for this study, ASCE/SEI 7 consistently provided a lower seismic response compared to UBC 97. This suggests that ASCE/SEI 7 offers a more optimized and potentially less conservative approach to calculating seismic design forces, particularly beneficial when incorporating a base isolation system.

REFERENCES [1]

M. Madhukumar, Helen SM, Vasugi V, “Performance analysis of lead rubber bearing isolation system for low, medium, and high-rise RC buildings”, Research on Engineering Structures and Materials, 2023; 9(1): 263276.

[2]

T. Bermany, S. Osman, M. Yatim, “A state-of-the-art analysis of base isolation systems and future directions for developing a novel multi-directional smart-hybrid isolation system integrated with earthquake early warning system for building structures”, ScienceDirect, Results in Engineering, Vol. 25, March 2025, 104501.

[3]

S. Khan, R. Khan, C. Gupta, “Seismic Performance of RC Structure with and Without Base Isolation System-A Review”, IJTIMES: Research Symposium on “Advancements in Engineering, Science, Management, and Technology”, Vol. 5, Special Issue 04, April 2019.

[4]

J. Weisman, G. Warn, “Stability of Elastomeric and Lead Rubber Bearing Seismic Isolation Bearings”, Journal of Structural Engineering, 138(2):215-223.

[5]

S. Shakeel, S. Khan, S. Saqib, M. Khan, M. Moiz, “Seismic performance and cost comparison of RC moment resisting and dual frames using UBC 97 and IBC 2021”, Scientific Reports, 14:16349, 2024.

[6]

S. Munot, P. Autade, “Design of Lead Rubber Bearing Base Isolator System for High Rise Structure”, International Journal of Creative Research Thoughts, Vol. 9, Issue 7, July 2021.

[7]

F. Amanollah, N. Ostrovskaya, Y. Rutman, “Structural and Parametric Analysis of Lead Rubber Bearings and Effects of their Characteristics on the Response Spectrum Analysis”, ResearchGate: Architecture and Engineering, Vol. 8, 2023.

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