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- 1. Composite steel highway bridges Corus Construction & Industrial
- 2. Contents Acknowledgement of author Advantages of steel bridges 1 Design standards 2 Conceptual design 2.1 Spans and component lengths 2.2 Cross sections 2.3 Intermediate supports 2.4 Bracings 2.5 Steel grades 2.6 Further guidance 3 Initial sizes and overall unit weight 3.1 Introduction 3.2 Use of charts 3.2.1 Plate girder flange sizes 3.2.2 Plate girder web sizes 3.2.3 Overall unit weight 3.2.4 Universal beams 3.2.5 List of symbols 4 Worked examples – use of charts 4.1 Continuous plate girder bridge 4.2 Simply supported universal beam bridge 5 References 6 Figures Figure 4 – Simply supported bridges Figure 5 – Continuous bridges – span girders Figure 6 – Continuous bridges – pier girders Figure 7 – Girder spacing factors Figure 8 – Overall unit weights – plate girder bridges Figure 9 – Universal beams – elastic stress analysis Figure 10 – Universal beams – plastic stress analysis 2 Composite steel highway bridges Contents 1. Left: Waterside Bridge Newburgh, Scotland 2. Right: A1(M) Yorkshire, England This guide is an update of a publication originally prepared by A.C.G. Hayward. Corus gratefully acknowledges the work of Mr Hayward and the contribution made by D.C. Iles, The Steel Construction Institute, during this update.
- 3. Composite steel highway bridges 3 Advantages of steel bridges The Author Alan C. G. Hayward FREng CEng FICE FIStructE Alan Hayward was a founding Partner of bridge specialists Cass Hayward & Partners of Chepstow who design and evolve construction methodology for all types of bridges, particularly steel highway, railway, footbridges, movable bridges and Roll-On/Roll-Off linkspans in the UK and overseas. He remains active in the firm as a Consultant. Alan Hayward was continuously involved with the development of bridge codes including BS 5400 and Eurocodes and has been National Technical Contact for the composite bridge code EC4-2. He contributes to the education of engineers by lecturing at Universities on behalf of industry, and has written numerous papers on steel bridge construction. He was a long-standing member of the Steel Bridge Group who disseminate best practice through their published Guidance Notes. Advantages of steel bridges Feature Leading to Advantages Low weight of Fewer piles and smaller sizes of pile caps/foundations. Cheaper foundations. superstructure. Typical 30 – 50% reduction over concrete decks. Composite bridges 6.0 – 8.0kN/m2 typical. Light units for erection. Erection by smaller cranes. Delivery of long pieces. Cheaper site costs. Launch erection with light equipment (skates or rollers). Simple site joints. Bolted joints: easy to form larger pieces from small Flexible site planning. transported components taken to remote sites. Maximum Quality control in good factory conditions avoiding outdoor More reliable product. pre-fabrication in site affected by weather and difficult access. factory. Predictable Commuted painting costs can be calculated. If easy repainting Total life cost known. maintenance costs. is made possible by access and good design then no other maintenance necessary. Low construction Depth/span ratio 1/20 to 1/30 typically. Slender appearance. depth. Lower depth achieved with half-through girders. Reduces costs of earthworks in approaches. Self supporting Falsework eliminated. Falsework costs eliminated. during construction. Slab formwork and falsework also avoided using permanent formwork. Significant if more than 8m above ground. Continuous and Continuity easy with bolted or welded joints. Most expansion Better appearance. integral spans. joints eliminated. Number of bearings reduced. Improved durability. Compliance with BD57. Improved running surface. Adaptable details. Pleasing appearance taking advantage of curves and colour. Aesthetic gain. Re-usable product. Demountable structures and recyclable components which Sustainable product. reduce manufacturing energy input. Composite steel highway bridges
- 4. 4 Composite steel highway bridges The current bridge code BS 5400 (Ref. 1) was conceived in 1967. Its ten parts cover the more common structural media. The 1980 conference in Cardiff introduced the Code relating to steel and made use of research carried out since 1970. Part 3 (Design of Steel Bridges) is compatible with the workmanship standards and tolerances defined in Part 6, drawn up jointly with industry. The Code uses limit state principles. The ultimate limit state (ULS) and serviceability limit state (SLS) must be satisfied. In practice the ULS generally governs, exceptions being the checking at SLS for slip of HSFG bolts and the design of shear connectors. BS 5400 encourages the use of steel for a number of reasons: (i) Plastic stress analysis option offers the use of lighter members and extends the span range of rolled sections. (ii) Design clauses are easier to use than previous Codes. (iii) Workmanship requirements, including tolerances, are rationalised. (iv) Longitudinal web stiffeners to girders are rarely needed. Use of the plastic modulus is permitted for stress analysis of compact sections and where the slenderness is controlled by sufficient restraints, the effects of shrinkage and differential temperature can be neglected. For ‘compact’ sections, the entire load can also be assumed to act on the composite section even if the steelwork is unpropped, provided that SLS checks are made. While most rolled universal beams, columns and channels will be compact, plate girders will often be non-compact and must be stressed elastically. (See also Section 3.2.4.) For structural analysis, elastic methods are utilised using gross sections (i.e. not allowing for shear lag or effective width). 1. M4/M25 Poyle Interchange 1. Design standards
- 5. Composite steel highway bridges 5 Redistribution of moments arising from the formation of plastic hinges is not permitted, but redistribution due to cracking of concrete over intermediate supports may be assumed using Part 5. Combined bending and shear is dealt with using interaction formulae. This is sometimes critical at intermediate supports. The Code contains no specific limits on slenderness of members or proportion of plate panels. Longitudinal web stiffeners are usually only necessary for very deep girders or those with curved soffits. For rolled sections the full shear yield stress can generally be used without the need for intermediate stiffeners. Bearing stiffeners are virtually mandatory at supports, together with lateral bracing or a system of bracing to maintain verticality. Fatigue is checked to Part 10, although for highway bridges this rarely demands a reduction in working stresses provided good detailing practice is used. For example: (i) Do not locate welded attachments close to or on flange edges (class 'G'). (ii) Re-entrant corners should be radiused. (iii) Use HSFG bolts for permanent bolted connections. (iv) Restrict doubler flange ends to areas of low stress (class 'G'). (v) Avoid single sided partial penetration butt welded joints which are subject to tensile stress. (vi) Avoid welded cruciform joints, which are subject to significant tensile stresses. An example is when using integral crossheads (see Figs. 1B & 1F) where fillet welds should be used in preference to full penetration butt welds. If butt welds are necessary, the use of steel with through-thickness quality (Z-grades to BS EN 10164 – Ref 14) may be considered in view of the strains which will be caused during welding.
- 6. 1. This page: A69 Haltwhistle Viaduct (Photo courtesy of Cleveland Bridge (UK) Ltd.) Northumberland, England 2. Right: Festival Park Flyover Stoke, England 3. Far right: Simon De Montford Bridge Evesham, England
- 7. Composite steel highway bridges 7 Conceptual design 2.1 Spans and component lengths Spans are usually fixed by site restrictions and clearances. Where freedom exists, budget costing – including foundations – is desirable to determine the economic span. A range of 25m to 50m is likely. Where deep piled foundations are needed, cost will encourage the use of longer spans, thus keeping foundations to a minimum. Multiple spans Multiple spans of approximately 24m suit universal beams, this being the longest readily available length and because continuous spans are convenient and economic. Site splices may be bolted with HSFG bolts or welded near points of contraflexure. The length of end spans should ideally be about 0.8 of the penultimate span. Continuous spans The optimum for using plate or box girders for continuous spans is about 45m, because 27m long ‘span girders’ can be spliced with ‘pier girders’ of a single plate 18m long. For longer spans, more shop or site splices are needed. Component lengths for shop fabrication should be the maximum possible consistent with delivery and site restrictions to reduce the amount of on-site assembly. The maximum length for road delivery without restrictions is normally 27.4m although longer lengths can readily be transported by arrangement. A minimum number of shop butt welds should be used consistent with plate sizes available. The decision whether to introduce thickness changes within a fabricated length should take account of the cost of butt welds compared with the potential for material saving (Ref. Documents in Section 2.6). Curved bridges Curved bridges in plan may be formed using straight fabricated girders, with direction changes introduced at each site splice. However, steel girders can be curved in plan which simplifies the cantilever formwork and permits the use of standard systems. An example is the A69 Haltwhistle Viaduct (radius 540m) Skew and plan tapered bridges may also be built in steel. Ideally, plan layout should be as simple as possible (Ref. Documents in Section 2.6). Integral bridges The Highways Agency requires consideration of integral bridge forms for spans up to 60m with the objective of improved durability by elimination of bridge deck movement joints (Ref. 4 & 5). Girders may then be required to develop a degree of continuity with substructures at end supports such that axial forces and reverse moment effects need to be considered in the design of the composite deck. Design principles remain the same but girder sizes and bracing provision may be influenced. Further guidance is available from the Steel Construction Institute (Ref. 8, 9, 10 & 10a). 2.2 Cross sections Deck type construction Deck type construction is common and is suitable for highway bridges as shown in Fig. 1. A span-to-girder depth ratio of 20 is economic although 30 or more can be achieved. A half-through bridge (‘U’ frame) can be appropriate in cases of severely limited depth, such as where approach lengths are restricted. Footbridges and rail under-bridges are common examples. 2. Conceptual design
- 8. 8 Composite steel highway bridges Conceptual design Where permanent formwork is envisaged, the slab should be made sufficiently thick to accommodate the details taking account of reinforcement cover and practical tolerances (Ref. 7). When using composite part depth planks such as Omnia then a minimum thickness of 250mm may be needed. Universal beams and plate girders Universal beams may be appropriate for bridges up to 25m span and above when continuous, or when use can be made of the plastic modulus. For spans above 22m, plate girders, especially if continuous, can be economic because lighter sections can be inserted in mid-span regions. Costs per tonne of painted and erected universal beams were traditionally lower but, more recently, automated fabrication and less expensive plate material has allowed economic supply of plate girders for the shorter spans. A girder spacing of 3.0m to 3.5m is usual with a deck slab of about 250mm thick (see Figs. 1A and 1B). Edge cantilevers should not exceed half the beam spacing and to simplify falsework should, where possible be less than 1.5m. Shorter cantilevers are usually necessary with a locally thickened slab where very high containment parapets are specified, e.g. over rail tracks. An even number of girders achieves better optimisation of material (ordering) and allows bracing in pairs. For wide girder spacings, the slab may be haunched, but use of standardised permanent formwork is unlikely to be possible and construction depth is increased (see Fig. 1C). Where spans exceed 40m, twin plate girders with a central stringer have been used on some single carriageway decks up to about 13m wide (see Fig. 1D). Twin girders and cross beams (often referred to as ladder decks) have proved economic for a wide range of spans (Ref. 10b). They can be used for single carriageway decks (see Fig. 1E) and for wider decks supporting more lanes. Box girders Where spans exceed 100m box girders are likely to be more economic than plate girders with which flange sizes would be excessive. Other reasons for using box girders include aesthetics (where justifiable), aerodynamic stability, severe plan curvature, the need for single column supports or very limited depth. Other than in the cases noted, box girders – being heavier than plate girders – are more expensive because although less flange material may be demanded due to inherent torsional properties, this is usually more than offset by the amount of internal stiffening and extra costs for workmanship. Fabrication costs are higher because the assembly/welding processes take longer and more shop space is needed. However, erection work is often reduced because box girders require little or no external bracing. Multiple box girders have in the past proved to be economic for spans of around 50m in particular situations. Using narrow cross sections eliminates the need for longitudinal stiffeners (see Fig. 1F). An example of which is the M25/M4 Poyle Interchange. For box girders, consideration of the safety of personnel in confined spaces is essential during fabrication, erection and for maintenance. Detailing must recognise the need to avoid internal welding as far as possible and to allow sufficient ventilation and openings for access and recovery in emergency situations. Open-topped trapezoidal and rectangular shaped box girders have been used efficiently, but provisions are needed to preserve stability during erection, for example the Forrest Way Bridge, Warrington. Plate girder flanges Plate girder flanges should be as wide as possible but consistent with outstand limitations in BS 5400 (i.e. 12t in compression if fully stressed and up to the 20t robustness
- 9. Composite steel highway bridges 9 Conceptual design 1. Far Left: Nene Bridge Peterborough, England 2. Left: Forrest Way Bridge Warrington, England 3. Right: M20 Road Bridge Folkstone, England limit), to give the best achievable stability during erection and to reduce the number of bracings. For practical reasons a desirable minimum width is about 400mm to accommodate detailing for certain types of permanent formwork, especially precast concrete. A maximum flange thickness of 63mm is recommended to avoid heavy welds, minimise pre-heating requirements and also limit the reduction in design yield strength. Limiting the thickness also has benefits in terms of notch toughness specification. 2.3 Intermediate supports Piers can take the form of reinforced concrete, leaf, column or portal. Steel columns are also used. For example, tubular steel columns (concrete filled composite), were used in the M5 Almondsbury Interchange and deserve consideration. Leaf piers or multiple columns supporting every girder are convenient but where fewer columns are demanded for aesthetic reasons, integral steel crossheads provide a solution. The popularity of these crossheads has recently increased following earlier examples on M25 bridges including Brook Street Viaduct, Mar Dyke Viaduct and South Mimms Interchange Bridges (see Figs. 1B and 1F). They were extensively used for the Second Severn crossing approach roads and for the new Thelwall Viaduct. It should, however, be recognised that the introduction of these additional members is only likely to be economic where the use of fewer supports is essential. Costs can increase especially if column spacing is not arranged to allow balanced erection and temporary trestles become necessary. Care is also needed detailing cruciform welded joints at the crosshead/main girder connection (Ref. Section 1 (vi)). 2.4 Bracings For most universal beam or plate girder bridges, lateral bracings are needed for erection stability and during deck concreting. Intermediate bracings require to be spaced at about 20 x top flange width and need to be adequate to prevent lateral torsional buckling. Bracing is necessary at supports if only to prevent overturning during erection. At abutments this can be a channel trimmer composite with the slab and supporting its free end. Over piers a channel section can be used between each pair of girders of up to about 1.2m deep. For deeper girders triangulated angle bracings are usual (see Fig. 1B). Intermediate lateral bracings are usually necessary in hogging regions with a maximum spacing of about 12 x bottom flange width. If the bridge is curved they should be close to the site splices where curvature induces torsion. Bracings may be of a triangulated form or of single channel sections between each pair of girders of up to 1.2m deep (see Fig. 1A). Alternatively, bracings can take the form of inverted 'U' frames, but for spans exceeding around 35m it may be necessary to interconnect all the girders by bracings during erection so that transverse flexure from wind is adequately shared. Although plan bracing systems are uneconomic and should be avoided, they may be required for spans exceeding 55m for temporary stability, especially if launch erection is used (Ref. Documents in Section 2.6). Use may be made of bracings in distributing live loads between girders. This may offer reduced flange sizes under HB loading but the uniformity of current loading to BD37 across the carriageway (HB + 2 lanes HA + 0.6 HA other lanes) tends to discourage this. An optimum design is likely to include bracings only between pairs of girders, such discontinuous bracings attracting minimal effects under deck loading except in cases of heavy skew or curvature where a different system may be appropriate. Bracings should be included in the global analysis to check for possible overload or fatigue effects.
- 10. 10 Composite steel highway bridges Conceptual design 1. Left: Humber Road Bridge Immingham, England 2. Right: Thelwall Viaduct M6, Warrington, England DECK WIDTH 230 TO 250 mm 2.5 TO 3.5 1.0 TO 1.75 TYPICAL 300 TO 350 mm 1.0 TO 3.3 4.0 TO 5.5 W 230 TO 250 mm D D D 1B Multiple P.G. (N=4) 1C Twin P.G. Haunch Slab (N=2) 1A Multiple U.B. (N=4) AT MID-SPAN AT PIER Figures 1A – 1F Typical deck type cross-sections
- 11. Composite steel highway bridges 11 Conceptual design 230 TO 320 mm 6.0 TO 7.0 3.0 TO 3.5 c/c 0.9 TO 1.2 2.5 TO 3.5 AT MID-SPAN AT PIER 1.0 TO 3.3 >7.0 230 TO 250 mm 230 TO 250 mm D D D 1D Twin P.G. & Stringer (N=2) 1E Twin P.G. & Cross Girders (N=2) 1F Multiple Box (N=6)
- 12. 12 Composite steel highway bridges 2.5 Steel grades BS EN 10025-2: 2004 Grade S355 steels (Ref. 12) are usual for bridges as they offer a lower cost-to-strength ratio than Grade S275. BS 5400 requires all steel parts to achieve a specified notch toughness, depending upon design minimum temperature, stress level and construction features (e.g. welding details). Subgrades J2 and K2 will be most common. Composite bridge decks are specifically categorised in the composite version of BS 5400: Part 2 (implemented by BD37), to allow a range of effective bridge temperatures to be determined from isotherms of minimum and maximum shade air temperature for a particular site location. Limiting thicknesses for steel parts are prescribed in BS 5400: Part 3, as implemented by BD13 (Ref. 3), as appropriate to these effective bridge temperatures, and the other factors mentioned above. Weathering steel To eliminate the need for painting, weathering steels to BS EN 10025-5: 2004 (Ref. 13) should be considered. Although it can be shown that the commuted costs of repainting are less than 1% of the initial bridge cost, weathering steel bridges can be more economical on a 1. Above: Findhorn Viaduct Inverness, Scotland 2. Left: Westgate Bridge Gloucester, England 3. Right: Slochd Beag Bridge Inverness, Scotland
- 13. Composite steel highway bridges 13 first cost basis and are particularly useful in eliminating maintenance where access is difficult – over a railway, for example. Weathering steel is not suitable at or near the coast, (i.e. within about 2km from the sea) due to the chloride laden environment or in areas of severe pollution. The Highways Agency requires sacrificial thickness to be added to all exposed surfaces for possible long term corrosion (1.5mm per face in a severe marine or industrial environment, 1mm in mild environments and 0.5mm inside box girders) and detailed guidance is given in design standard BD 7 (Ref. 6) and Corus Publication ‘Weathering steel bridges’ (Ref. 11). 2.6 Further guidance Particularly relevant information for initial (and detailed) design is included within two publications: • BCSA Publication No. 34/02 ‘Steel Bridges’ Alan Hayward, Neil Sadler and Derek Tordoff, 2002. • SCI-P-185, Steel Bridge Group: Guidance notes on Best Practice in Steel Bridge Construction.
- 14. 14 Composite steel highway bridges Initial sizes and overall unit weight 3. Initial sizes and overall unit weight 3.1 Introduction Charts are given to provide initial estimates of flange area (Af) web thickness (tw) and overall unit weight of steelwork (kg/m2 ) for typical composite bridge cross sections as shown in Fig. 1. Continuous or simply supported span plate girders and simply supported universal beams are included. The charts were derived from approximate BS 5400 designs using simplifying assumptions for loads, transverse distribution and to achieve correlation with modern bridges. The charts take account of the latest highway loading requirements in BD37. It is emphasised that the sizes obtained do not represent final designs, which must always be executed to take account of all factors, such as bridge configuration and loading. Adjustments will need to be made to take account of the likely effects of end continuity if integral construction is intended. The charts are based on the following assumptions: (i) Deck slab 250mm average thickness (6.25kN/m2 ). (ii) Superimposed dead loads equivalent to 100mm of surfacing (2.40 kN/m2 ). (iii) Permanent formwork weight 0.50 kN/m2 of slab soffit area. (iv) Steel grade S355. (v) Span to depth ratios L/D of 20 & 30. (vi) Plate girder webs have vertical stiffeners at approx. 2.0m centres where such stiffening is required. (vii) Elastic stress analysis is used for plate girders. If however the plastic modulus is used for compact cross sections, then economies may be possible. (viii) Steelwork is unpropped and therefore not acting compositely under its own weight and that of the concrete slab. The steel is however composite for all superimposed loads after the concrete has cured. (ix) Sufficient transverse bracings are included such that bending stresses are not significantly reduced due to buckling criteria. (x) Top flanges in sagging regions are dictated by the maximum stress during concreting allowing for formwork and live load – to BS 5975 (Ref. 15). Continuous bridge mid-span regions are concreted in turn followed by portions over the piers. (xi) Live loading HA (assuming 3.5m wide lanes), or alternatively 45 units of HB loading with co-existent HA loading (BD37). (xii) Continuous spans are approximately equal. 3.2 Use of charts 3.2.1 Plate girder flange sizes Flange areas (Af in m2 ) are read against the span L. (a) For simply supported bridges – (refer Fig. 4) (b) For continuous bridges – Size of span girder (refer Fig. 5) Size of pier girder (refer Fig. 6) Figures 4, 5 and 6 are applicable to an average girder spacing ‘s’ of 3.5m. Fig. 7 gives a girder spacing factor Kaf which is multiplied by the flange areas, obtained above, to give values appropriate to the actual average girder spacing. i.e. Top Flange Aft = Aft (Figs. 4, 5 or 6) x Kaf (Fig. 7) i.e. Bottom Flange Afb = Afb (Figs. 4, 5 or 6) x Kaf ( Fig. 7) Two different span-to-depth ratios, L/D = 20 and L/D = 30, are included for either HB or alternatively HA loading. Values for intermediate L/D ratios can be read by interpolation.
- 15. Composite steel highway bridges 15 Initial sizes and overall unit weight The charts also show actual flange sizes using 400mm x 15mm to 1000mm x 75mm. Flange area of pier girders of continuous unequal spans can be estimated by taking the greater of the two adjacent spans. End spans of continuous bridges may be estimated using L = 1.25 x actual span. 3.2.2 Plate girder web sizes Web thicknesses are similarly obtained using Figs. 4, 5 and 6 applicable to 's' = 3.5m. Adjustment for the actual average girder spacing 's' is obtainable from Fig. 7 using girder spacing factor ktw. i.e. Web thickness tw = tw (Figs. 4, 5 or 6) x ktw (Fig. 7). The thickness obtained may be regarded as reasonably typical. However, designers may prefer to opt for thicker webs to reduce the number of web stiffeners. 3.2.3 Overall unit weight Overall unit weight (kg/m2 of gross deck area) for plate girders is read against the span L from Fig. 8 for simply supported or continuous bridges with L/D ratios of 20 or 30, under HB or alternatively HA loading and applicable to ‘s’ = 3.5m. Adjustment for average girder spacing 's' other than 3.5m is obtainable from Fig. 7 using girder spacing factor kw. i.e. Unit weight kg/m2 = kg/m2 (Fig. 8) x kw (Fig. 7). The unit weight provides an approximate first estimate of steelwork tonnage allowing for all stiffeners and bracings. For continuous bridges with variable depth, Fig. 8 may be used to provide a rough guide, assuming a span-to- depth (L/D) ratio for each span based upon the average girder depth (D) within that span. For box girder bridges a rough estimate may be obtained assuming that N = 2 x number of box girders in the cross section (see Fig. 1F where N = 2 x 3 = 6). For continuous bridges the end spans should be assumed as 1.25 x actual span, following which the mean span for use in Fig. 8 may be determined as follows: Mean span L = L1 4 + L2 4 ...Ln 4 n where n = number of spans. 3.2.4 Universal beams An indication of beam size for simply supported spans may be obtained from Figs. 9 and 10 for elastic or plastic stress analysis respectively. BS 5400 permits the use of either option, provided that the cross section is ‘compact’; this condition being satisfied for all sections shown in Fig. 10. Sufficient ductility is also required. It is apparent that plastic stress analysis can achieve significant economy in extending the span range of universal beams. In practice, a serviceability stress check (SLS) must be made including the effects of shear lag. There is advantage also in using the plastic design option for continuous spans but some universal beams may need to be classed as 'non-compact', requiring elastic analysis in hogging regions because the web depth between the (elastic) neutral axis and its compressive edge may exceed 28tw, depending upon the amount of longitudinal slab reinforcement. An overall unit weight for universal beam bridges may be estimated at the conceptual stage by adding an allowance of approximately 8% to the weight of the main beams to allow for any bracings and stiffeners etc. Figs. 9 and 10 refer to mass per metre of universal beams. 4 1. Left: Milton Bridge Lesmahagow, Scotland 2. Right: Fossdyke Bridge (Photo courtesy of Cleveland Bridge (UK) Ltd.) Lincoln, England
- 16. 16 Composite steel highway bridges Initial sizes and overall unit weight Reference Universal beam size Actual depth (mm) figures 9 & 10 Serial Mass per size (mm) metre (kg/m) 388 914 x 419 388 921.0 343 343 911.8 289 914 x 305 289 926.6 253 253 918.4 224 224 910.4 201 201 903.0 226 838 x 292 226 850.9 194 194 840.7 176 176 834.9 197 762 x 267 197 769.8 173 173 762.2 147 147 754.0 170 686 x 254 170 692.9 152 152 687.5 140 140 683.5 125 125 677.9 238 610 x 305 238 635.8 179 179 620.2 149 149 612.4 140 610 x 229 140 617.2 125 125 612.2 113 113 607.6 101 101 602.6 Table 1 (with reference to sizes in Figs. 9 and 10) Table 1 above defines the referencing system for the serial sizes in Figs. 9 and 10, which is based on the mass per metre of universal beams. Larger sizes are available (e.g. 1016), but are unlikely to be economic compared to fabricated plate girders. 3.2.5 List of symbols Af Flange area (m2 ) Afb Bottom flange area (m2 ) Aft Top flange area (m2 ) D Girder or beam overall depth excluding slab or finishes (m) HA Standard highway loading defined in BD37 HB Abnormal highway loading defined in BD37, 45 units assumed Kaf Girder spacing factor for flange area Ktw Girder spacing factor for web thickness Kw Girder spacing factor for unit weight L Span centre to centre of bearings (taken as 1.25 x span for end span of continuous bridges) kg/m2 Unit weight of steelwork in bridge expressed as: total steelwork weight (kg) W x overall bridge length s Average girder spacing defined as W/N (m) tw Web thickness (mm) W Overall deck width including parapets (m) n Number of spans N Number of girders (refer to Section 3.2.3 for box girders) Notes (i) Where relevant, symbols correspond with BS 5400 Part 3. (ii) Units where relevant are shown in parentheses.
- 17. Composite steel highway bridges 17 Worked examples – use of charts 4.1 Continuous plate girder bridge A composite highway bridge has 3 continuous spans – A, B and C of 24, 40 and 32m. Overall deck width is 12m and it carries 45 units of HB loading (as shown in figure 2). There are 4 plate girders in the cross section of 1.75m depth. Estimate the main girder sizes and the total weight of structural steel. Average girder spacing 's' = W/N =12m/4 No. = 3.0m Figure 2 Worked example Flange and web sizes Girder spacing factors: for 'S' = 3.0m From Fig. 7: Kaf = 0.87, Kaf = 0.85*, Ktw = 0.95 (*top flange span girders only). Span girder Pier girderPier girder Span girderSpan girder W = 12m 24m 40m 32m Span A Span B Span C D = 1.75m 1. Left: Trent Viaduct Newark, England 2. Right: A69 Haltwhistle Viaduct (Photo courtesy of Cleveland Bridge (UK) Ltd.) Northumberland, England 4. Worked examples – use of charts
- 18. 18 Composite steel highway bridges Worked examples – use of charts Span A: 24m This is an end span so take L = 1.25 x 24m = 30m Therefore L/D = 30m/1.75m = 17, so assume L/D = 20 Top flange Aft = Aft (from Fig. 5) x Kaf = 0.006 x 0.85 = 0.0051m2 400 x 15 top flange Bottom flange Afb = Afb(from Fig. 5) x Kaf = 0.014 x 0.87 = 0.012m2 500 x 25 bottom flange Web tw = tw (from Fig. 5) x Ktw = 10 x 0.95 = 9.5mm Use 10mm web Span B: 40m Span girder L/D = 40m/1.75m = 22.9 Top flange Aft = Aft (from Fig. 5) x Kaf = 0.009 x 0.85 = 0.0077m2 400 x 20 top flange Bottom flange Afb = Afb (from Fig. 5) x Kaf = 0.020 x 0.87 = 0.017m2 500 x 35 bottom flange Web tw = tw (from Fig. 5) x Ktw = 10 x 0.95 = 9.5mm Use 10mm web Span C: 32m This is an end span so take L = 1.25 x 32m = 40m therefore sizes as 40m span. Pier girders Take L as the greater of the two adjacent spans, i.e. assume L = 40m at both supports, hence, L/D = 40m/1.75m = 22.9 Top flange Aft = Aft (from Fig. 6) x Kaf = 0.017 x 0.87 = 0.015m2 400 x 40 top flange Bottom flange Afb = Afb(from Fig. 6) x Kaf = 0.033 x 0.87 = 0.029m2 500 x 60 bottom flange Web tw = tw (from Fig. 6) x Ktw = 16.8 x 0.95 = 16mm Therefore use 18mm web Steel tonnage Girder spacing = 3.0m for end span A: L = 1.25 x 24m = 30m for centre span B: L = 40m for end span C: L = 1.25 x 32m = 40m Therefore mean span 4 L1 4 + L2 4 ...Ln 4 n 4 304 + 404 + 404 = 37.5m 3 L/D = 37.5m/1.75m = 21 = kg/m2 (from Fig. 8) x Kw (from Fig. 7) = 145kg/m2 x 1.04 = 151kg/m2 Hence, steel weight = 151 kg/m2 /1000 x (24m + 40m + 32m) x 12m wide = 174 tonnes
- 19. Composite steel highway bridges 19 Worked examples – use of charts 4.2 Simply supported universal beam bridge A composite bridge has a simply supported span of 24m. (as shown in figure 3). Overall deck width is 9.6m and it carries HA loading only. Estimate the beam size and total weight of structural steel assuming there are 4 beams in the cross section. Figure 3 Worked example (a) For an elastic stress analysis refer to Fig. 9 For 4 beams 'S' = 9.6m/4No. = 2.4m. Use 388 i.e. 914 x 419 x 388kg/m Universal Beam Total weight approx. (388kg/m/1000) x 4No. x 24m x 1.08 (the 1.08 factor allows for 8% bracing + stiffener allowance) = 40.2 tonnes (i.e. 174kg/m ) (b) For a plastic stress analysis refer to Fig. 10 For 'S' = 2.4m. Use 289 i.e. 914 x 305 x 289kg/m universal beam Total weight approx. (289kg/m /1000) x 4No. x 24m x 1.08 = 30 tonnes (i.e. 130kg/m ) Thus, plastic stress analysis offers a significant reduction in beam size but SLS checks must be made. W = 9.6m 24m 2 2 1. Left: A9 Bridge Pitlochry, Scotland 2. Right: A1(M) Yorkshire, England
- 20. 20 Composite steel highway bridges References 5. References 1. BS5400, Steel, Concrete and Composite Bridges. British Standards Institution. Design Manual for Roads and Bridges (DMRB): 2. DMRB 1.3 BD37 Loads for Highway Bridges. 3. DMRB 1.3 BD13 Codes of Practice for Design of Steel Bridges. 4. DMRB 1.3 BD & BA 57 Design for Durability. 5. DMRB 1.3 BA 42 Design of Integral Bridges. 6. DMRB 2.3 BD7 Weathering Steel for Highway Structures. 7. DMRB 2.3 BA36 The Use of Permanent Formwork. Steel Construction Institute Publications 8. P163: Integral Steel Bridges – Design Guidance. 9. P180: Integral Steel Bridges – Design of a Single Span Bridge. 10. P250: Integral Steel Bridges – Design of a Multi Span Bridge. 10a. P340: Technical Report on Integral Steel Bridges. 10b. P339: Design Guide for Ladder Deck Bridges. 11. Corus Publication – Weathering steel bridges. Material Standards (EN) 12. BS EN 10025-2 – Non-alloy structural steels. 13. BS EN 10025-5 – Structural Steels with improved atmospheric corrosion resistance. 14. BS EN 10164 – Steel products with improved deformation properties perpendicular to the surface of the product. Other Standards (BS) 15. BS 5975 Code of Practice for Falsework. BS 5400 Title DMRB MCDHW Part Document* Document** 1 General Statement BD15 – 2 Specification for Loads BD37 – 3 Code of Practice for Design of Steel Bridges BD13 – 4 Code of Practice for Design of Concrete Bridges BD 24 – 5 Code of Practice for Design of Composite Bridges BD16 – 6 Specification for Materials & Workmanship, Steel – Volume 1 Series 1800 7 Specification for Materials & Workmanship, Concrete, Volume 1 Series 1700 Reinforcement & Prestressing Tendons – 8 Recommendations for Materials & Workmanship, Concrete, Volume 2 Series NG1700 Reinforcement & Prestressing Tendons – 9 Bridge Bearings BD20 – 10 Code of Practice for Fatigue BD9 – * Design Manual for Roads and Bridges published by the Stationery Office for the Overseeing Organisations. ** Manual of Contract Document for Highway Work published by the Stationery Office for the Overseeing Organisations.
- 21. Composite steel highway bridges 21 Figures 20 75 70 65 60 55 50 45 75 70 65 60 55 50 45 40 35 S=3.5m 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 202530354045505560 10 11 12 13 14 15 tw(mm) Span(m) 1000 x 800 x 650 x 600 x 500 x 400 x Flangesize(mm) 75 75 75 75 70 65 60 55 50 45 40 35 30 25 70 65 60 55 50 45 40 35 30 25 70 65 60 55 50 45 40 35 30 25 20 70 65 60 55 50 45 40 35 30 25 20 15 L/D 30 tw Afb HB Afb HA Afb Aft30 HB 20 Afb HA Aft 20 tw 30 Af (m2 ) 30 20 HA/HB HA/HB 6.Figures Figure4:Simplysupportedbridges–flange(atmid-span)andweb(atsupport)
- 22. 22 Composite steel highway bridges Figures Figure5:Continuousbridges–flangeandwebsizesofspangirders 7075 75 75 0.0465 60 55 50 45 40 35 30 25 70 65 60 55 50 45 40 35 30 25 70 65 60 55 50 45 40 35 30 25 20 70 65 60 55 50 45 40 35 30 25 20 15 0.03 0.02 0.01 0 S=3.5m Af (m2 ) 650 x 600 x 500 x 400 x 02530354045505560 10 11 12 13 14 15 tw(mm) Span(m) L/D 30 Flangesize(mm) HB Afb HA HB Afb HA Aft Aft tw tw 20 20 20 30 30 30 20 Afb Afb HA/HB HA/HB
- 23. Composite steel highway bridges 23 Figures 60 75 75 75 75 75 55 50 45 70 65 60 55 50 45 40 35 70 65 60 55 50 45 40 35 30 25 70 65 60 55 50 45 40 35 30 25 70 65 60 55 50 45 40 35 30 25 20 70 65 60 55 50 45 40 35 30 25 20 15 S=3.5 Af (m2 ) 0.05 20 10 Flangesize(mm) 1000 x 0.04 0.03 0.02 0.01 0 2530354045505560 12 14 11 13 15 16 17 18 19 20 21 800 x 650 x 600 x 500 x 400 x Span(m) L/D 30 Afb Afb tw tw 20 20 20 30 30 Aft Aft tw(mm) HA/HB HA/HB HA/HB HA/HB Figure6:Continuousbridges–flangeandwebsizesofpiergirders
- 24. 24 Composite steel highway bridges Figures Figure7:Girderspacingfactors 2.0 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 12345678 Girderspacing-S(m) Kaf,Ktw,Kw Kw Ktw Kaf TopFlangemid-spanonly L=40 L=60 Ktw Kaf Kaf Girders&slab Haunch slabStringerCrossgirders
- 25. Composite steel highway bridges 25 Figures 80 20 ContinuousSimplysupported Kg/m2 L/D 30 Span(m) 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 2530354045505560 HB HA Universalbeam s 20 HB HA 3020 HB HB HA HA 30 20 20 Figure8:Overallunitweights–plategirderbridges(S=3.5)
- 26. 26 Composite steel highway bridges Figures HA HB Beamspacing-S(m) 2.2 12 Span(m) 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 13141516171819202122232425 388 343 289 253 224 201 194/238 197 176 173 179 147 388 343 289 253 224 226 201 194/238 176 173 Figure9:Universalbeams–elasticstressanalysis
- 27. Composite steel highway bridges 27 Figures HA HB 2.2 12 Beamspacing-S(m) Span(m) 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 1314151617181920212223242526272829 388 343 289 253 224 226 201 194/238 179/147 170 173 176 197 152 140/149(686) 140(610) 125 113 101 388 343 289 253 224 226 201 194 197/238 176 173 170/179 147 152 140(686) 149 140(610) 125 Figure10:Universalbeams–plasticstressanalysis
- 28. Care has been taken to ensure that this information is accurate, but Corus Group plc, including its subsidiaries, does not accept responsibility or liability for errors or information which is found to be misleading. Copyright 2005 Corus Designed and produced by Orchard Corporate Ltd. www.corusgroup.com Corus Construction & Industrial Technical Sales & Marketing PO Box 1 Brigg Road Scunthorpe North Lincolnshire DN16 1BP T +44 (0) 1724 405060 F +44 (0) 1724 404224 E tsm@corusgroup.com www.corusconstruction.com English language version CC&I:CD:3000:UK:04/2005/r

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