Construction Materials by Adam Nicholson

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Construction Materials: An Introduction for Engineers An ICE Publishing Textbook Dr M Mulheron Department of Civil Engineering, University of Surrey, Guildford, UK Description: ICE Textbooks provide clear, accurate and relevant information on the major principles of civil and structural engineering at a level suitable for undergraduate students worldwide. Divided into easily understandable modules, ICE Textbooks feature worked examples, practice questions and learning point summaries throughout. Construction Materials: An Introduction for Engineers covers each of the key materials used in construction, detailing their properties and application in a civil and structural engineering context. Information is supported by case studies and project examples, developing initial concepts and theory through to practical engineering implications.

Contents: 1. Concrete 2. Steel 3. Composites 4. Glass 5. Aluminium 6. Masonry

Geographical market: Worldwide

Readership: Civil and structural engineering students Civil and structural engineering lecturers and trainers

ICE Manual of Construction Materials: Metals and Alloys | 978 07277 4063 2 | £30.00 | Jan 2010

Related titles: Structural Design: An introduction to the art and science of designing structures | 978 07277 5743 2 | £30.00 | Oct 2012 Coming soon

ICE Manual of Construction Materials: Polymers and Polymer Fibre Composites | 978 07277 4120 2 | £30.00 | Jul 2010

Details: Price: £30.00 ISBN: 978 07277 5741 8 Format: Paperbound Publish Date: October 2012 Publisher: ICE Publishing Imprint: ICE Publishing Page Size: 234 x 156mm Number of Pages: 152pp Illustrations: ~50 b/w figures; 25 tables Subject area: Structural materials

Chapter 1

The Design of Connections in Steel Bridges Learning aims The aims of this chapter are to introduce and explain the inﬂuence which the connection type has on the overall behaviour of steel bridges. The following key points are included. g g g g g

The advantages and disadvantages of both welded and bolted connections. The welding processes. Bolts and high strength friction grip bolts. Code requirements for connection design. Connection design examples.

Introduction The design of the connections forms an important part of the overall design of a bridge structure. Early in the project advice should be sought from steelwork fabricators as to which factors are significant when minimising the cost of the connections. However, cost is not the dominant factor for the bridge designer and careful attention must also be given to the strength and fatigue behaviour of the chosen connections. The bridge designer also has to consider other important factors such as the optimum location of the joint, which connections are to be welded, which are to be bolted and, following on from this, which connections are to be made in the fabricator’s workshop and which connections are to be undertaken on site. When considering the location of joints, it is desirable to position the joints at points of contra-flexure or areas of low stress. Also, consideration must be given to optimising the total number of joints taking into account transportation to site, the weight and corresponding ease of erection of a particular section of the structure. It is generally uneconomic to transport very large sections of steelwork which because of their size, will require a police escort in the UK, and may also demand the ‘one off’ hire of special lifting equipment in order to erect the part. In general, most ‘shop’ joints made in the fabricators workshop are welded. In the workshop, it is easier to obtain the correct ‘fit up’ of the various parts for welding and also to make allowances and, if necessary remedy, any distortions resulting from welding. In bridgework, in situ joints can be welded, bolted or made using a combination of welding and bolting. However, most in situ connections are bolted with extensive use being made of high strength friction grip (HSFG) bolts. 1

The Design of Connections in Steel Bridges

Advantages and disadvantages of bolted and welded in situ connections Due to restrictions imposed by the need to transport to site individual sections of the structure shorter than approximately 25 metres, in situ connections are nearly always required to construct the complete bridge. In order to assist the bridge designer in making the correct choice of connection, Table 1 summarises the advantages and disadvantages of both bolted and welded in situ connections. Table 1. Advantages and disadvantages of bolted and welded in situ connections Bolted joints g g

Advantages Generally cheaper

Connections are almost self-aligning and relatively quick to make g Only semi-skilled labour required g

Welded joints Advantages Good aesthetics forming a visually unobtrusive connection g Minimises girder weight with no deductions required for holes in the tension flange g Full strength connection can be achieved by using full penetration butt welds g g

Allows adjustment to vertical and longitudinal alignment if normal clearance holes are used g Easy to inspect g Normally does not govern bridge fatigue life g Not weather sensitive during â&#x20AC;&#x2DC;bolting upâ&#x20AC;&#x2122; g

Disadvantages May be visually unacceptable g Deduction for holes in tension flange may lead to the use of a larger section g Can be prone to corrosion if not properly protected g

Disadvantages g Expensive unless in large numbers g Weather sensitive, must be protected from moisture g Pre-heat of steelwork is usually necessary requiring a tent/shelter to prevent heat loss and protection against inclement weather g Expensive to inspect for defects g If defects are present it is expensive and time consuming to repair with subsequent risk of delay to project g May govern the fatigue life of the structure g Skilled operatives required, working to an approved specification g Temporary cleats needed to assist in preweld alignment. Cleats normally have to be carefully removed and parent material restored to original condition to prevent fatigue problems

The Design of Connections in Steel Bridges

Welded connections The welding process The process of welding requires the formation of a molten metal weld pool which is used to fuse together the faces of the parts to be joined. Arc welding is the predominant method used for welding structural steelwork and with this process a weld pool is formed by passing a high current through an electrode which when placed close to the earthed work piece allows a plasma arc to form. The high current passing through the electrode and subsequently through the arc into the earthed work piece causes the tip of the electrode to melt and allows the transfer of molten metal from the electrode to the work piece, to form the weld bead. Several arc welding techniques are available for use by the fabricator and the major methods are briefly outlined below. Manual metal arc process (MMA) This process is the oldest of all of the arc processes and is widely used by fabricators for attaching stiffeners, etc. The process is a manual operation with the quality of the finished weld depending to a large extent on the skill of the welder. The electrode consists of a steel wire coated with a flux formed from cellulose, silicates, iron oxide, etc. The primary purpose of the flux is to shield the weld pool to prevent the molten metal oxidising and in addition taking into solution nitrogen and other unwanted gasses. Some of these gasses, if present, are released back into the air as the weld cools and in the process cause porosity in the weld metal. The flux on the electrode also helps to stabilise the plasma arc and can be used to control the deposition rate of the molten metal. It is most important that the electrodes chosen are of the correct grade of steel, at least equivalent to that of the parent parts to be joined, and that they are completely dry and corrosion free before use. Eurocode 3 (2005) specifies that the filler metal in the electrodes must have mechanical properties, namely yield strength, ultimate tensile strength, elongation at failure and a minimum Charpy V – notch energy value, equal or better than the values specified for the steel grade being welded. The electrodes used in the manual metal arc process are of a relatively short length and hence for long continuous welds there will be numerous stop – start positions, each the potential source of an imperfection. Submerged arc welding (SAW) Unlike the manual metal arc process the submerged arc method is a fully automatic method, employing a travelling gantry or robotic unit. The electrode is a continuous bare steel wire which is unwound from a storage drum as required, and fed, via the welding nozzle, into a bed of flux which has been deposited on the surfaces to be welded. The flux, which completely encapsulates the plasma arc, can also be used to enhance the composition of the weld metal. The submerged arc process can only be successfully used in the ‘down hand’ position and is generally the preferred method for making long welds, such as the web to flange welds, required for plate girder fabrication. Gas shielded processes These processes, like the submerged arc process, use a continuous bare wire electrode. However, the weld pool is protected from the atmosphere not by a flux, but by a gas 3

The Design of Connections in Steel Bridges

which is also fed via the welding nozzle, around the plasma arc and weld pool. The shielding gas used may be inert argon or non-reactive carbon dioxide and the processes are termed metal inert gas (MIG) and metal active gas (MAG) respectively. These processes are very versatile and can be used in both the ‘down hand’ and ‘overhead’ positions but must be sheltered from draughts which tend to disturb the gas ﬂow.

Welded connection design There are two types of structural weld in common use namely butt welds and the ﬁllet welds. A butt weld is normally made within the cross-section of the abutting plates, whereas a ﬁllet weld is a weld of approximately triangular cross-section applied to the surface of the plates to be joined. When plates greater than about 5 mm thick are to be butt welded together, the plate edges will have to be prepared before welding in order to obtain a full penetration weld. Figure 1 shows typical butt welds together

Figure 1. Typical butt welds showing the required edge preparation (Needham, 1983) 1:5 taper 60°

Top flange butt 60°

1:5 taper

60° Webb butt 60°

1:5 taper Bottom flange butt

The Design of Connections in Steel Bridges

Figure 2. Effective throat thickness of fillet welds (BS 5400 Part 3, 1982) t1

g is the effective throat of the weld If either t1 or t2 is greater than 4 mm, g has to be at least 3 mm

g 90° P

P is the penetration of the weld g is the throat of the weld

with the required edge preparation. A full strength penetration butt weld in structural steel, made correctly with the appropriate electrodes, is considered to be a strong as the parent steel plates, and consequently no strength calculations are necessary, unless fatigue is a consideration. However, fillet welds do not require any edge preparation for the parent plates and consequently are usually cheaper to undertake than butt welds. Fillet welds are usually specified by requiring a minimum leg length for the weld. These welds are considered to carry all of the loads applied to them, through shear which acts on the effective throat of the fillet weld. Figure 2, taken from Figures 53 and 54 of BS 5400 Part 3 (1982), shows the method for calculating the effective throat area which is described in Clause 14.6.3.9 of BS 5400 Part 3 (1982) as the height of a triangle that can be inscribed within the weld cross-section and measured perpendicular to its outer side. The UK Code of Practice BS 5400 Part 3 suggests two alternative procedures for assessing the stress in a fillet weld. The first procedure requires that the vector addition of all of the shear stresses acting on the weld should not exceed the shear stress capacity of the weld tD given by: D ¼

kð y þ 455Þ pﬃﬃ m f 3 2 3

Clause 14.6.3.11.1, BS 5400 Part 3 (1982)

The Design of Connections in Steel Bridges

Where:

sy k

gm gf3

is the nominal yield stress of the weaker part joined. ¼ 0.9 for side ﬁllets, or ¼ 1.4 for end ﬁllets in end connections, and ¼ 1.0 for all other welds. is the material partial safety factor given in Table 2 of BS 5400 Part 3 (1982) for the ultimate limit state as equal to 1.10. is a loading partial safety factor taken to be equal to 1.1 for the ultimate limit state as given in Clause 4.3.3 of BS 5400 Part 3 (1982).

The second method of assessment states that the following stress condition should be satisfied: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kð þ 455Þ y Clause 14.6.3.11.2, BS 5400 Part 3 (1982) 2 þ 3ð 12 þ 22 Þ 4 2 m f 3 Where:

s t1 t2

is the stress normal to a section through the throat of the weld. is the shear stress acting at right angles to the length of the weld on a section through the throat. is the shear stress acting along the length of the weld on a section through the throat.

The terms sy, k, gm and gf3 are defined as given above. By permitting the parameter k to be equal 1.4, both methods allow for the fact that end fillet welds have proved to be stronger than side fillets. The latter procedure, where it is required to calculate the stresses acting on the weld throat, is more time consuming but less conservative than the former method.

High strength friction grip bolted connections It is generally a requirement that for bridgework all bolted structural connections are made using high strength friction grip (HSFG) bolts. These bolts, which are tightened up to achieve a speciﬁed shank tension, act by clamping together the plates to be joined, so that under normal loading conditions, the applied forces are transferred through the connection by friction acting at the plate interfaces. High strength friction grip bolts are manufactured to the requirements given in BS 4395 (1969) which covers three different grades of bolts namely: Part 1 Part 2 Part 3

General grade – with a strength grade for the bolts of about 8.8 together with grade 10 nuts. Higher grade – with a strength grade 10.9 for the bolts together with grade 12 nuts. Higher grade – with a waisted shank – with a strength grade 10.9 for the bolts together with grade 12 nuts.

For bridgework the majority of HSFG bolts used are General grade. 6

The Design of Connections in Steel Bridges

Figure 3. A tension control bolt

Bolt tension The reliable control of bolt tension is essential for ensuring predictable performance in HSFG bolted joints. The Code of Practice BS 4604 (1970) which is published in two parts corresponding to Parts 1 and 2 of BS 4395 (1969), outlines the procedures acceptable for tightening these bolts. The Code of Practice gives two main procedures for tightening namely, the ‘Part Turn Method’, used for General grade bolts, and the ‘Torque Control Method’, used for higher grade bolts. In the ‘Part Turn Method’, the nut is rotated a speciﬁc number of turns from the snug position which tensions the bolt. The ‘Torque Control Method’ requires the use of a manually operated torque wrench or power driven wrench to achieve the required bolt tension. There are other methods of obtaining the required bolt tension the most frequently used method employing load indicator washers. This method relies on the plastic compression of nibs, which are incorporated into the washer, down to a pre-determined gap, giving a reliable and relatively easily inspected bolt assembly. Another propriety system available for use is the ‘Tension Controlled Bolt’ (TCB). These HSFG bolts have a special spline section at the end of the bolt which shears off at a pre-determined torque. Figure 3 shows a typical tension control bolt, tightened using a special shear wrench which simultaneously tightens the nut whilst holding on to the spline end. Figure 4 also shows a typical splice connection in a plate girder made using tension control bolts. HSFG bolt connection design High strength friction grip bolted connections designed to comply with BS 5400 Part 3 (1982) are not permitted to slip at the serviceability limit state. At the ultimate limit state, the connection is allowed to slip and the strength of the joint is governed by the bearing or shear capacity of the bolts, whichever is the lower. Clause 14.5.4.2 of BS 5400 Part 3 (1982) states that the design capacity of a HSFG bolt at the serviceability limit state is the friction capacity PD given by: PD ¼ kh

F v N m f 3

Where:

gm is the material partial safety factor given in Table 2(b) of BS 5400 Part 3 (1982) for the serviceability limit state as equal to 1.2. 7

The Design of Connections in Steel Bridges

Figure 4. Typical splice connection in a plate girder using tension control bolts

m N kh

gf3

is the prestress load equal to the proof load in the absence of external applied tensions. is the slip factor given in Clause 14.5.4.4 of BS 5400 Part 3 (1982). is the number of friction interfaces equal to 1.0 for a single lap joint and equal to 2 for a double lap joint. ¼ 1.0 where the holes in all the plies are of normal size as speciﬁed in BS 4604 (1970). is a loading partial safety factor taken to be equal to 1.0 for the serviceability limit state as given in Clause 4.3.3 of BS 5400 Part 3 (1982).

At the ultimate limit state Clause 14.5.4.1.1, of BS 5400 Part 3 (1982) states that the design capacity is the greater of: The friction capacity using: PD ¼ kh

F v N m f 3

Where gm ¼ 1.3 Table 2(a) BS 5400 Part 3 (1982). 8

The Design of Connections in Steel Bridges

Figure 5. Plate girder bolted splice 300

325

300 50 75

50 75 75 100

325 75 50 50

60 50 15 mm thick web plate

1500

12 @ 100

M22 HSFG bolts general grade

2 No. 200 by 12 web cover plates

250 by 20 cover plates

50 60

600 by 20 cover plate

or the lesser of the shear capacity or the bearing capacity given by Clause 14.5.3.4 and Clause 14.5.3.6 of BS 5400 Part 3 (1982) respectively. Clause 14.5.3.4 states that t the average shear stress given by t ¼ V/nAeq must comply with the following condition: ¼

q V pﬃﬃ 4 nAeq m f 3 2

Where V is the load on the bolt and Aeq is the cross-sectional area of the unthreaded shank, provided the shear plane or planes pass through the unthreaded part, or the tensile stress area of the bolt if any shear plane passes through the threaded part. The parameter n is the number of shear planes resisting the applied shear; sq is the yield stress of the bolts and the partial safety factors gm and gf3 are both equal to 1.1 as given in Table 2(a) and Clause 4.3.3. BS 5400 Part 3 (1982) respectively. The bearing capacity is given by Clause 14.5.3.6 BS 5400 Part 3 (1982) which states that sb, the bearing pressure between a fastener and each of the parts, given by sb ¼ V/Aeb 9

The Design of Connections in Steel Bridges

must comply with the following condition: k1 k2 k3 k4 y V 4 Aeb m f 3 Where Aeb is the product of the shank diameter of the bolt and the thickness of each connected part loaded in the same direction, irrespective of the location of the thread. k1 ¼ 1.0 for HSFG bolts. k2 varies with edge distance and is equal to 2.5 when the edge distance, measured from the centre of the hole, is greater than three times the hole diameter. k3 depends on whether the part being checked is enclosed on both faces when k3 ¼ 1.2 or k3 ¼ 0.95 in all other cases. k4 depends on bolt tension and is equal to 1.5 when fasteners are HSFG bolts acting in friction or equal to 1.0 for all other cases. Further details concerning this parameter are given by Needham (1984). sy is the yield stress of the bolt or plate, whichever is the lesser. gm ¼ 1.05 Table 2(a) BS 5400 Part 3 (1982). gf3 ¼ 1.1 Clause 4.3.3. BS 5400 Part 3 (1982).

Summary box For ultimate limit state design a high strength friction grip bolted connection is allowed to slip and the design is governed by the lowest value of either the shear or bearing capacity. For serviceability limit state design a high strength friction grip bolted connection is not allowed to slip and the design is governed by the friction capacity.

Other considerations Lack of fit In a HSFG bolted connection, good ‘fit up’ of the parent and cover plates is essential if a load bearing connection is to be achieved. If this is not the case, and the joint faces are not flat and/or not properly aligned, then some or possibly all of the pre-stress in the bolts will be used to bring the plates into contact with little or no bolt tension available to induce the friction resistance in the faying surfaces. CIRIA has published a Report on the lack of fit in steel structures which offers valuable advice on how to circumvent such problems (CIRIA, 1981). Relaxation of bolt tensions Research has shown that the resistance to slip along the faying surfaces is a result of shearing of the contact interfaces of microscopic protrusions on the surface of the plates. 10

The Design of Connections in Steel Bridges

This resistance can be enhanced with surface contamination and roughness. In a HSFG bolted joint, these surface effects, together with a small reduction in joint plate thickness caused by in-plane plate tensile stresses due to the applied load, can cause a loss of bolt pre-tension. The loss of bolt tension due to plate thinning has been shown to increase rapidly when the in-plane tensile stress becomes high enough, which when combined with the normal bolt pressure, causes local yielding of the plate material (Cullimore and Eckhart, 1974). The loss of bolt tension from this cause will therefore increase with the ratio of bolt pre-stress to the yield stress of the parent plates, and with other parameters remaining constant, the slip factor will decrease as the bolt tension increases. Slip factor Clause 14.5.4.4 of BS 5400 Part 3 (1982) gives values of the slip factor m which can be used when assessing the friction capacity of a HSFG bolted connection. Generally, for UK bridge construction, the faying surfaces are grit blasted and then masked until the erection of the steelwork, allowing the use of a slip factor m equal to 0.5. As mentioned previously, the slip load of a connection will be improved by increasing the friction coefﬁcient of the faying surfaces. If this is achieved by increasing the roughness of the faying surfaces this will generally result in offsetting the accompanying loss of bolt tension. Where difﬁculties arise in assessing a suitable value for m, the characteristic value of the slip factor should be determined in accordance with the procedure given in BS 4604 (1970).

Plate girder connection design example The connection design is for a 1500 mm by 600 mm plate girder, with a 15 mm thick web and 40 mm thick ﬂange, fabricated from grade S275 steel. The plate girder has to support the following forces: Ultimate Limit State: Bending Moment Axial Tension Shear

4800 kNm 115 kN 650 kN

Serviceability Limit State: Bending Moment Axial Tension Shear

3570 kNm 85 kN 495 kN

Plate girder properties Second moment of area about the neutral axis: " # ! 15 14203 600 403 2 ¼ 2:916 1010 mm4 I¼ þ2 þ 600 40 730 12 12 Area ¼ (2 600 40) þ (1420 15) ¼ 69 300 mm2. 11

The Design of Connections in Steel Bridges

Web splice Try a single line of 13 bolts at 100 mm centres, 50 mm from the centre of the joint as shown in the ﬁgure. Ultimate limit state: Calculation of the bending stress in the web plate of the girder at the level of the top and bottom web bolts due to the bending moment: ! My 4800 106 600 ¼ ¼ 98:8 N=mm2 ¼ b ¼ I 2:916 1010 Horizontal shear force on bolt due to the bending moment: ¼ sb (bolt pitch) (web thickness) ¼ 98.8 100 15 10 3 ¼ 148.2 kN Average stress on the girder due to axial load: ! Axial tension force 115 103 a ¼ ¼ ¼ 1:63 N=mm2 Area 693 102 REFERENCES

BSI (1969) BS4395: Specification for high strength friction grip bolts and associated nuts and washers for structural engineering. British Standards Instituition. BSI (1970) BS4604: Specification for the use of high strength friction grip bolts in structural steelwork. British Standards Institution. BSI (1980) BS5400, Part 10: Steel, concrete and composite bridges, Code of practice for fatigue, British Standards Institution. BSI (2000) BS5400, Part 3: Steel, concrete and composite bridges, Code of practice for the design of steel bridges, British Standards Institution. Cullimore MSG and Eckhart JB (1974) The distribution of Clamping Pressure on Friction Grip Bolted Joints, The Structural Engineer, Vol. 52, No. 5. Eurocode 3, Design of steel structures, ENV1993-1-1: Part 1.1 (1992) General rules and rules for buildings, CEN. European recommendations for the design of longitudinally stiffened webs and stiffened compression flanges (1990) Publication 60, ECCS. Hicks JG (1987) Welded Joint Design. BSP Professional Books, London. Malik AS (ed.) (2002) Joints in Steel Construction: Simple Connections, The Steel Construction Institute and The British Constructional Steelwork Association Ltd. Dowling PJ, Harding JE and Bjorhovde R (eds) (1992) Constructional Steel Design An International Guide. Elsevier Applied Science Publishers, London. Needham FH (1983) Site Connections to BS 5400 Part 3, The Structural Engineer, Vol. 61A, No. 3. Needham FH (1984) Discussion Site Connections to BS 5400 Part3, The Structural Engineer, Vol. 62A, No. 3, pp. 93–100. 12

The Design of Connections in Steel Bridges

Owens GW and Cheal BD (1989) Structural Steelwork Connections. Butterworths, London. Pratt JL (1989) Introduction to the Welding of Structural Steelwork. Ascot, Berks, UK, The Steel Construction Institute. Galambos TV (ed.) (1998) Guide to stability design criteria for metal structures, 5th edn. John Wiley. FURTHER READING

Blodgett OS (1966) Design of welded structures. Cleveland, James F Lincoln Arc Welding Foundation. Bjorhovde R, et al. (1987) Connections in steel structures. Elsevier Applied Science, London. Galambos TV (ed.) (1998) Guide to stability design criteria for metal structures, 5th edn. John Wiley.