View source for Bridges - initial design

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The choice of bridge form is usually made at an early stage and one or more initial configurations with principal dimensions are selected for more detailed evaluation. This article discusses the factors influencing the choice of form and provides guidance on preliminary sizing of the structural members. It focuses on typical composite highway bridges of medium span.
{{#image_template:image=File:A1 Appleyhead.jpg|caption=Typical medium span composite highway bridge, ''[[A1 Peterborough to Blyth|A1, Appleyhead]]|align=right|wrap=true|width=450}}

==Determining factors==
In order to develop a viable concept for a bridge crossing and to choose a preliminary configuration and dimensions, the designer must consider a number of inter-related factors:
*Constraints of the crossing site: e.g. highways, railway lines, rivers, canals or other water courses; deep or steep sided valleys; environmentally sensitive sites requiring minimal disturbance.  Ground conditions can also be a constraint not just for the bridge foundations but the approach embankments as well.
*Service life. The bridge is normally designed for 120 years service life, but items such as a steelwork [[Corrosion_protection|corrosion protection]] system, [[Bridge_articulation_and_bearing_specification#Expansion joints|deck joints]] and [[Bridge_articulation_and_bearing_specification#Bearings|bearings]] will need to be maintained or replaced during the service life of the structure.  Concrete in substructure elements and deck slabs will also require maintenance.
*Costs of material and components. Steelwork is [[Design_for_steel_bridge_construction#Fabricating_the_steelwork|prefabricated]] in factory conditions but the costs of doing so depend on the components used ([[Steel_construction_products#Standard_open_sections|rolled sections]] or plates, for example) the complexity of the structure and the size of the elements. Concrete slabs can be cast in situ or formed partly from precast components. 
*How the bridge can be [[Design_for_steel_bridge_construction#Bridge_erection|assembled and completed]] on site. Is it a [[Design_for_steel_bridge_construction#Erection_on_greenfield_sites|greenfield site]]?; is construction only possible in limited [[Design_for_steel_bridge_construction#Erection during possessions|possessions]]?; what plant can be got to the site? 
*[[Health and safety]] considerations, notably the CDM regulations<ref name="No1"> [http://www.legislation.gov.uk/uksi/2015/51/contents/made Construction (Design and Management) Regulations (CDM) 2015] </ref>
*Appearance, notably the client’s aspirations.
<br>

==Structural form==
{{#image_template:image=File:A34_Wolvercote_Viaduct.JPG|caption=Typical medium span composite highway bridge<br>''[[A34 Wolvercote Viaduct, Oxford]]''|align=left|wrap=true|width=300}}
For medium span highway bridges built in the UK in recent years, by far the most common form has been the deck type steel-and-concrete composite bridge. With bridges of this type the structural steelwork system generally comprises [[Steel_construction_products#Fabricated_products|fabricated 'I' section plate girders]], and these support a concrete deck slab at top flange level. The discussion of considerations at the concept design stage relates principally to that type of bridge and also to other forms of steel bridge construction.

The forms of construction covered below are thus principally [[Multi-girder composite bridges|multi-girder decks]] and [[Ladder deck composite bridges|ladder decks]] although the general guidance will be equally applicable to deck type bridges utilising [[Box_girder_bridges|box girders]] (open and closed top boxes in place of [[Steel_construction_products#Fabricated_products|'I' girders]]) and to [[Half-through_bridges|half-through bridges]].

==Span length==
The designer has to determine the span lengths and number of spans for the bridge from consideration of the alignment, the topography of the site, the physical dimensions of the obstacle (or obstacles, including services that may be too expensive to relocate) to be crossed, the required clearance envelopes, the available locations for the bridge abutments and intermediate piers (if more than one span), the appearance and any particular requirements that the client may impose.  Whilst straight bridges crossing square are the ideal, bridge layouts may need to be [[Skew_bridges|skewed]] and/or [[Plan curvature in bridges|curved]].  For highway bridges refer to CD 127<ref name="No2">[https://www.standardsforhighways.co.uk/tses/attachments/10442706-b592-42c8-85f8-2a0c779a8e37?inline=true CD 127, Cross-sections and headroom, The Design Manual for Roads and Bridges, The Stationary Office]</ref> for cross-sections and headrooms. 

Any pier within 4.5m of the edge of the carriageway has to be designed for impact loads (as given by the National Annex to BS EN 1991-1-7<ref name="No3">NA+A1:2014 to BS EN 1991-1-7:2006+A1:2014. UK National Annex to Eurocode 1. Actions on structures. Accidental actions. BSI</ref>) but abutments do not normally have to be considered. 

Nowadays for single span bridges, except for the shortest spans, steel composite decks compete with prestressed concrete Y-beam decks and generally will prove more economical at the upper end of the Y-beam span range.  For two or more spans continuous decks are the norm and steel composite decks, because of their superior buildability, should win over Y beam decks.  When optimising span lengths in multi-span bridges, shorter spans (typically 25m to 30m) will be more economical. However where piers are costly (for example when very tall piers or deep piled foundations are needed), or the environmental sensitivity of the site requires minimal disturbance, longer spans with fewer foundations are likely to be more economical.

For bridges with three or more spans, the optimum length of the end span is usually from 0.7 to 0.85 of the adjacent internal span length.  With very short end spans, uplift can occur at the abutments.  Where more than one arrangement of spans could provide a viable crossing, the cost, buildability and appearance of the alternative options are evaluated to obtain the preferred solution. This evaluation will include the costs of the substructure and foundations. 

===Example of options for a bridge over a dual carriageway road===
{{#image_template:image=File:R11_Fig1.PNG|caption=Options to cross a dual carriageway|align=right|wrap=true|width=400}}
Arrangements A, B, C and D shown right are all possible layouts for crossing a dual carriageway. Arrangement A might be considered the best for a greenfield site as it provides the shortest spans and the shortest overall deck length. A high wall type abutment can be more expensive than an additional side span (B) or a longer span (C), particularly with [[Integral bridges|integral construction]]. Where construction depth is at a premium B would be preferable to C. When crossing over an existing motorway or dual carriageway, it is often difficult to construct a pier between the carriageways because the works have to be carried out under traffic management, so arrangement D would be preferred.

===Integral construction===
For highway bridges, [[Integral bridges|integral construction]] must be considered and designers should follow the advice given in PD 6694-1<ref name="No4">PD 6694-1:2011+A1:2020, Recommendations for the design of structures subject to traffic loading to BS EN 1997-1:2004+A1:2013. BSI</ref>. Nowadays the Technical Approval Authorities expect bridge decks up to 60m in length and with [[Skew_bridges|skews]] (at the abutments) not exceeding 30&deg; to be integral with the abutments unless there are valid reasons, such as potential mining subsidence, for them not to be so. Note that 60m is not a limit on maximum length for [[Integral_bridges|integral bridges]], Highways England have accepted [[Integral_bridges|integral bridges]] with lengths greater than 100m.

An integral bridge is necessarily continuous over intermediate supports (there are to be no provisions for movement joints at these locations) but does not need to be integral with the columns or piers beneath.  The girders can simply sit on ordinary [[Bridge_articulation_and_bearing_specification#Bearings|bearings]].  Eliminating the [[Bridge_articulation_and_bearing_specification#Bearings|bearings]] by casting in the main girders or steel crosshead girders does not bring major benefits and does increase complexity in [[Design_for_steel_bridge_construction|construction]]; the tendency to attract moment may also develop potential [[Fatigue_design_of_bridges|fatigue]] problems.

Continuous spans will invariably be more economical in steel weight than simply supported spans of the same length. With non-integral construction provision of an abutment gallery is expensive and can impact on construction programme. Hence [[Integral_bridges|integral bridges]] will almost always be more economical than non-integral, especially when whole life costing is taken into account as user delay costs associated with [[Bridge articulation and bearing specification#Expansion joints|deck joint]] maintenance/replacement are high.

==Steelwork system and deck slab==
The most popular steelwork systems being used today for highway bridge construction are [[Multi-girder_composite_bridges|multi-girder decks]] and [[Ladder_deck_composite_bridges|ladder decks]].  Which system is the more cost effective from the point of view of [[Design_for_steel_bridge_construction#Fabricating_the_steelwork|prefabrication]] and [[Design_for_steel_bridge_construction#Bridge_erection| erection]] for a particular site will depend on the site specific determining factors, so there are no hard and fast rules to aid selection. 

Whichever the system, the structural steelwork will usually be [[Steel_construction_products#Fabricated_products|fabricated plate girders]] of 'I' section. However boxes may be used for appearance reasons (often a look-alike replacement has to be provided for an original design that was a prestressed concrete box), or where the deck is tightly [[Plan_curvature_in_bridges|curved in plan]].

As [[Ladder_deck_composite_bridges|ladder deck]] bridges have only two main girders, the question of structural redundancy might be raised in the choice of [[Ladder_deck_composite_bridges|ladder deck]] configuration – if some accidental event were to damage one girder so severely that it could no longer carry even the dead loads, the bridge would collapse. There is no data on the likelihood of accidental events that could cause such damage, for either [[Ladder_deck_composite_bridges|ladder deck]] or [[Multi-girder_composite_bridges|multi-girder decks]], and it is therefore not possible to make any quantitative assessment of reliability for either type. The girder sections of [[Ladder_deck_composite_bridges|ladder deck]] bridges are generally larger than those of [[Multi-girder_composite_bridges|multi-girder decks]] and they are also restrained at close spacing by the cross girders; designers therefore consider this configuration to be sufficiently robust.

The thickness of the composite concrete deck slab on deck type bridges typically will be 250mm.  An in-situ slab of this thickness, cast either on timber formwork or permanent formwork, will span around 3.5m, hence girder spacing (main girders in [[Multi-girder_composite_bridges|multi-girder decks]], cross girders in [[Ladder_deck_composite_bridges|ladder deck]] systems) will typically be at 3.5m centres, or a little more as the slab spans between the flange outstands. Generally contractors like to use permanent formwork (Omnia planks are the market leader) rather than conventional timber formwork. Use of proprietary parapet cantilever formwork systems is now widespread for the deck slab cantilevers. Cantilever length is usually not more than half the girder spacing on [[Multi-girder_composite_bridges|multi-girder decks]], typically 1.5m. The designer needs to consider how the steelwork system and deck slab are arranged geometrically to accommodate crossfall and superelevation, and whether to split dual carriageway underbridge decks down the middle. CD 377<ref name="No7">[https://www.standardsforhighways.co.uk/tses/attachments/1fe48581-82ba-4b6f-95a1-ee93309bd1b5?inline=true CD 377, Requirements for road restraint systems, The Design Manual for Roads and Bridges, The Stationary Office]</ref> has specific requirements for designing a divided structure with a longitudinal gap between the two bridge decks.

When determining the form of the substructure, particularly the type of pier, it should be borne in mind that it is preferable to put the [[Bridge_articulation_and_bearing_specification#Bearings|bearings]] directly under the main girder. The tops of piers and columns should provide sufficient space for jacks to be inserted for [[Bridge_articulation_and_bearing_specification#Bearings|bearing]] replacement, and additional jacking [[Stiffeners|stiffeners]] should be provided in corresponding positions adjacent to the main [[Stiffeners#Bearing_stiffeners|bearing stiffeners]]. [[Ladder_deck_composite_bridges|Ladder deck]] steelwork systems and [[Multi-girder_composite_bridges|multi-girder]] systems with [[Ladder_deck_composite_bridges#Integral_crossheads_at_internal_support|integral crossheads]] reduce the number of intermediate support columns and [[Bridge_articulation_and_bearing_specification#Bearings|bearings]]. However [[Ladder_deck_composite_bridges#Integral_crossheads_at_internal_support|integral crossheads]] are expensive to [[Design_for_steel_bridge_construction#Fabricating_the_steelwork|fabricate]] and complicate [[Design_for_steel_bridge_construction#Bridge_erection|erection]], so their use should be avoided unless there are particular constraints due to limited space or appearance. When used considerable thought needs to be given as to how to detail the steelwork to accommodate the crossfall and the longitudinal gradient of the carriageway.

==Girder spacing and layout==
{{#image_template:image=File:R11_Fig3.png|caption=Sirhowy River Bridge – Form chosen at tender stage to avoid falsework at height over the river.<Br>''(Image courtesy of Rob Watkins)''|align=right|wrap=true|width=250}}

Girder spacing is controlled by the spanning ability of the concrete deck slab. For a 250mm deck slab this leads to a maximum span for the slab of about 4m.  This spacing is consistent with the maximum span of permanent formwork, typically about 3.8m. Deck slab cantilevers at the edge of the deck will control the position of the outer main girders. A cantilever of 1.5m is common and can work economically with a 250mm deck slab. 
With these primary dimensional constraints identified, the main steel grid can be defined to suit the particular geometry and dimensions of the bridge.

For a [[Multi-girder composite bridges|multi-girder]] bridge, an even number of main girders should be selected, such that the limits on cantilever length and girder spacing are observed,  Longer cantilevers] can be achieved with care and may be preferable for aesthetics: the edge cantilever should ideally be similar to the depth of the outer girder.

For [[Ladder_deck_composite_bridges|ladder decks]], the spacing of the main girder is dictated by the chosen length of cantilever, with a maximum spacing of about 18m. Cross girders are generally spaced at between 3.5m and 3.8m but the spacing will need to be adjusted at the ends and intermediate supports of [[Skew_bridges|skew bridges]]. [[Ladder_deck_composite_bridges#Cantilever_girders|Steel cantilevers]] can be added to support longer cantilevers on [[Ladder_deck_composite_bridges|ladder decks]] if required and can also be used to avoid cantilever falsework. They do, however, have cost implications.
{{#image_template:image=File:R11_Fig2.png|caption=[[Ladder_deck_composite_bridges#Cantilever_girders|Steel cantilevers]] used to support precast concrete permanent formwork for a [[Ladder deck composite bridges|ladder deck]]<br>''(Image courtesy of Costain)''|align=left|width=450}}

==Initial size estimates for main girders==
Previous experience will often be the first guide to selection of flange and web sizes for the main girders and cross girders. The selection can be refined using simple ‘rules of thumb’ or by the use of [[Bridges - initial design#Design charts and software|design charts and software]].

In almost all cases, steel grade S355 (to BS EN 10025<ref name="No8">BS EN 10025: 2019 Hot rolled products of structural steels (in 6 Parts). BSI</ref>) will offer the most economic solutions and bridge steelwork contractors have extensive experience of processes to [[Fabrication|fabricate]] steelwork of that grade. 
{{#image_template:image=File:R11_Fig4.png|caption=Steel [[Steel_construction_products#Fabricated_products|plate girder]]  terminology|align=left|width=500}}

===Simple rules for proportioning girders===
The main girders can be proportioned according to the following rules of thumb, which are based upon a typical highway bridge of two or more spans carrying a single carriageway over a dual 2 or dual 3 lane highway. Large bridges will need to have their sizes adjusted upwards in a proportional manner, e.g. wide ladder decks will require proportionally wider and thicker flanges. 

{|class="wikitable" width=900 style="text-align: left"
|+''' Recommended girder proportions'''
|-
!style="width: 18%"|'''Element'''!!style="width: 40%"|'''Proportioning'''!!'''Comments'''
|-
|Girder Depth
|Span/20 to Span/30
|For wide ladder decks and simply supported spans, ratios should be kept to 20.<br>
A higher ratio will likely result in a heavier steel weight.<br>
Flange and web proportions below suit these main girder dimensions.<br>
On wide but short span ladder deck bridges, the main girder depth can be governed by the depth of the cross girders.
|-
|Top flange width
|Minimum width 350 mm
|Minimum width to facilitate shear studs, splices and precast formwork.<br>
Minimum width can be applied to most single carriageway bridges with spans up to about 30 m.<br>
Variable width top flanges are not recommended.<br>
Larger spans and wide ladder decks may need wider top flanges.
|-
|Top flange thickness
|In sagging zones: 21 mm for 350 mm minimum flange width.<br>
In hogging zones thickness increases significantly.<br>
Generous depth girders could expect to see 40 to 50 mm of top flange at hogging zones. 
|This is based upon an 8:1 outstand ratio for EN1993-1-1<ref name="BSEN1993-1-1">BS EN 1993-1-1:2005+A1:2014, Eurocode 3: Design of steel structures. General rules and rules for buildings, BSI</ref> table 5.2 for grade 355 steel for a class 2 section. <br>
Preliminary analysis best to ascertain top flange thickness in hogging zones.
|-
|Bottom flange width
|Typically approx. half the girder depth.
| 
|-
|Bottom flange thickness
|Mid span sagging or hogging at supports. <br>
Ladder deck: 55 to 60 mm typical. <br>
Multi-girder deck: 40 to 50 mm maximum. <br>
Can reduce to near minimum thickness at points of contraflexure.
|This is the most difficult dimension to estimate without analysis. <br>
Flange thickness should be varied through the length of the girder to optimise design. <br>
Refer to product information for guidance on available lengths of plate in order to choose flange thickness change points. 17m lengths of plate of a single thickness are a good starting point. <br>
Minimum flange thickness away from high stress areas can be based upon EN 1993-1-1<ref name="BSEN1993-1-1"></ref> outstand ratio of 8.1 for class 2 sections in compression. <br>
Be careful to choose correct grade of steel selection for thick plates.
|-
|Webs
|Minimum web = 15 mm in mid span zones, increasing to 20 mm or 25 mm at supports. <br>
Multi girder decks will probably work at 20 mm webs at supports. Ladder decks likely to need 25 mm webs.
|This is a practical minimum for robustness during construction. <br>
Webs should be assumed to be working near their maximum allowable stresses for efficiency: web stiffeners will enable.
|-
|Web to flange welds
|Smaller girders: 6 mm at midspan. Up to 8 or 10 mm at supports. <br>
Larger spans, ladder decks: 8mm at midspan; up to 10 mm at supports.
|Weld sizes are expressed as either leg length or throat length; one run of weld laid down each side of the web. <br>
6 mm (leg length) is a practical minimum weld size. <br>
Fabricators will sometimes propose partial penetration fillet welds as an alternative which provide the same throat.
|-
|Shear Studs
|Use 19mm diameter by 150 mm high studs. <br>
Typically rows of 3 at 300 mm centres at mid span increasing to rows of 3 or 4 at 150 mm centres at supports.
|The spacing needs to be coordinated with the detailing of the transverse reinforcement.
|}
<br>
Plate thicknesses may be chosen to the nearest millimetre (or perhaps rounded up to the nearest 5mm at this stage, leaving a more precise selection to be made at the detailed design stage). Plate widths would normally be refined to the nearest 50mm.

===Design charts and software===
An early estimation of the sizes of steel sections in a medium-span composite highway bridge can be made by using [[#Resources|Preliminary steel bridge design charts]]. 

The design charts cover both [[Ladder deck composite bridges|ladder deck]] and [[Multi-girder composite bridges|multi-girder]] forms of construction and account for the differences between inner and outer girders in [[Multi-girder_composite_bridges|multi-girder bridges]]. They also cover both elastic and plastic design of sections. The charts are fully consistent with Eurocode loading, as implemented by the National Annex to BS EN 1991-2<ref name="No9">NA to BS EN 1991-2:2003, UK National Annex to Eurocode 1. Actions on structures. Traffic loads on bridges. BSI</ref>, and design resistances given in accordance with the relevant Parts of Eurocode 3 and Eurocode 4.  

A user manual is also provided that outlines the assumptions behind the design charts and explains how to use them. The design charts may either be used manually, or alternatively an accompanying spreadsheet tool may be used that automates the process and interpolates between charts.

==Cross girder proportions==
Cross girders can be proportioned using the following rules of thumb.

{|class="wikitable" width=900 style="text-align: left"
|+''' Recommended proportions for cross girders '''
|-
!style="width: 18%"|'''Element'''!!style="width: 40%"|'''Proportioning'''!!'''Comments'''
|-
|Girder Depth
|Span/12 to Span/20
|Flange and web proportions below suit these main girder dimensions.
|-
|Top flange width
|Minimum width 300 mm can apply to typical single carriageway bridges.<br>
Dual carriageway bridges will need wider top flanges.
|Minimum width to facilitate shear studs, splices and precast formwork.<br>
Variable width top flanges are not recommended.
|-
|Top flange thickness
|Typically minimum of 18 mm
|The outstand ratio of 8.1 from EN 1993-1-1<ref name="BSEN1993-1-1"></ref> table 5.2 for grade S355 steel and class 2 cross sections.<br>
Flange thickness usually constant throughout the length of the cross girder.
|-
|Bottom flange width
|Typically approx. half the girder depth or less
| 
|-
|Bottom flange thickness
|Mid span sagging.
|This is the most difficult dimension to estimate without analysis.<br>
Flange thickness usually constant throughout the length of the cross girder.
|-
|Webs 
|Typically 15 to 20 mm through the length of the cross girder.
|Refine to the nearest mm through detailed design.<br>
Thickness usually constant throughout the length of the cross girder.
|-
|Web to flange welds
|Typically: 6 mm at midspan. Up to 8 or 10 mm at supports
|Weld sizes are expressed as either leg length or throat length; one run of weld laid down each side of the web.<br>
6 mm (leg length) is a practical minimum weld size.<br>
Steelwork contractors will sometimes propose partial penetration fillet welds as an alternative which provide the same throat.
|-
|Shear Studs
|Use 19mm diameter by 150 mm high studs.<br>
For 19 mm diameter, typically rows of 2 at 300 mm centres at mid span increasing to 150 mm centres at supports.
|
|}
<br>
{{#image_template:image=File:R11_Fig8.png|caption=Cross girders, seen during erection<Br>''Porth Bypass, Rheola Bridge''|align=left|wrap=true|width=400}}
One important aspect of structural behaviour for cross girders is the bending moment at the ends where they connect to the main girder.  Although a substantial [[Connections_in_bridges|connection]] may be needed to transfer shear force, the moment generated at the ends of the cross girders is generated only by the torsional stiffness of the main girder.  Since these are typically 'I' beams, the torsional stiffness and hence cross girder end moments are small in most cases.

==Bracing==
Basic rules for provision of [[Bracing systems|bracing]] at preliminary design stage are given below.

===General===
Main girders will require [[Bracing systems|bracing]] to stabilise the girders against lateral torsional buckling both during the construction concreting phase and to stabilise the bottom flange during service near supports.

At the supports the steelwork must transfer any horizontal actions on the bridge deck to the supports.  Such loads are significant and consequently require more substantial bracing against lateral sway.

===Multi girder decks===
In [[Multi-girder composite bridges|multi-girder decks]], [[Bracing systems|bracing]] between pairs of girders is often provided using a system of [[Bracing_systems#Torsional_bracing|torsional bracing]].  This can comprise either cross bracing made up from [[Steel_construction_products#Standard_open_sections|angles]], or [[Steel_construction_products#Standard_open_sections|channel]] sections. [[Steel_construction_products#Standard_open_sections|Channels]] will often need to be 300 to 430mm deep.  Larger sections and sections in [[Weathering_steel|weather resistant steel]] can be economically [[Design_for_steel_bridge_construction#Fabricating_the_steelwork|fabricated]] from plate.

[[Bracing_systems|Bracings]] are often provided at about 5 m to 1 m centres, with a minimum of 3 or 4 [[Bracing_systems|bracings]] along the length of the girder span.
<gallery widths=300px heights=250px> 
Image:R11_Fig9.png|Cross [[Bracing_systems|bracing]] arrangement<Br>''M6 Toll, Bridge 523'' 
Image:R11_Fig10.png|[[Bracing_systems#Torsional_bracing|Torsional bracing]] arrangement using [[Steel_construction_products#Standard_open_sections|channel]] sections<Br>''Sirhowy Enterprise Way Pioneer Viaduct''
</gallery>
<br>
[[Bracing_systems#Plan_bracing|Plan bracing]] of the top flanges is a very efficient way of controlling lateral torsional buckling: especially useful for simply supported single spans.  It is, however, not always the steelwork contractor’s preferred method of [[Bracing_systems|bracing]] as there can be issues with accessibility for final [[Paint_coatings|painting]] and clashes with deck formwork if [[Connections_in_bridges|connections]] are not carefully detailed. In the example illustrated below, the [[Bracing_systems#Plan_bracing|plan bracing]] was detailed to be clear of the deck construction and was designed to be removed after construction, to avoid future maintenance.
{{#image_template:image=File:R11_Fig11.png|caption=Plan bracing to top flanges<Br>''A41 Aston Clinton Bypass''|align=left|width=400}}

===Ladder decks===
During the concreting stage, the cross girders of [[Ladder_deck_composite_bridges|ladder decks]] provide torsional restraint to the main girder flanges.

At the in-service stage, restraint is provided to the bottom flanges near supports (where they are in compression) either by U-frame action (deck slab plus web [[Bridges_-_initial_design#Stiffeners|stiffeners]]) or by adding “knee braces” from the cross girders down to the bottom of the web.  The choice will depend upon the relative depths of the main and cross girders, but it should be noted that knee bracing is relatively expensive to [[Design_for_steel_bridge_construction#Fabricating_the_steelwork|fabricate]].
<gallery widths=250px heights=190px> 
Image:R11_Fig12.png|Cross girders providing torsional bracing<Br>''M6 Toll Bridge 443''
Image:R11_Fig13.png|Knee [[Bracing_systems|bracing]] at supports (prior to erection)<Br>''M6 Toll Bridge 334'' 
Image:R11_Fig14.png|Narrow ladder deck torsional bracing<Br>''M6 Toll Bridge 450''
</gallery>
<br>
For wide [[Ladder deck composite bridges|ladder decks]] with long cross girders, it will be more economical brace the cross girders than to increase the flange thickness. This can be achieved by pairing the cross girders with [[Steel_construction_products#Standard_open_sections|channel]] bracing at midspan.

==Stiffeners==
Some basic rules for provision of [[Stiffeners|stiffeners]] at preliminary design stage are given below.

===Provision===
{{#image_template:image=File:R11_Fig15.png|caption=Transverse web stiffeners<Br>''Porth Bypass, Rheola Bridge''|align=right|wrap=true|width=400}}
[[Stiffeners]] are required along the main girders for the following:
*To limit the dimension of the web panel to control web buckling
*At support positions
*At positions of cross girders or bracings to form [[Connections_in_bridges|connections]].
<br>
For [[Ladder_deck_composite_bridges|ladder decks]], the provision for the third requirement above usually deals adequately with the provisions for the first, so that once the [[Bracing systems|bracings]] and cross girder arrangements are resolved, the resulting web stiffening is normally adequate.

Thin webs on [[Multi-girder_composite_bridges|multi-girder decks]] could need additional [[Stiffeners|stiffeners]] in between [[Bracing systems|bracing]] positions. However, UK fabricators will normally advise that it more cost effective to make the web thicker than to add further web [[Stiffeners|stiffeners]], if the provisions the third requirement do not provide sufficient control to satisfy the first.

UK practice is to use [[Stiffeners#Intermediate_transverse_web_stiffeners|transverse (vertical) web stiffeners]].  Continental Europe often uses more extensive stiffening including [[Stiffeners#Longitidunal_stiffeners|longitudinal (horizontal) stiffeners]].  Normally, [[Stiffeners#Longitidunal_stiffeners|longitudinal (horizontal) stiffeners]] would not be considered economical in everyday highway bridge construction.  (They may become appropriate for long span bridges, such as cable stayed bridges, to help control compression buckling of the web.)

[[Stiffeners#Intermediate_transverse_web_stiffeners|Transverse web stiffeners]] should be provided at the position of each cross girder or [[Bracing systems|bracing]].

Transverse web stiffeners at supports are usually referred to as [[Stiffeners#Bearing stiffeners|bearing stiffeners]].  Additional stiffeners should be provided at positions for jacking for [[Bridge articulation and bearing specification#Bearings|bearing]] replacement.
{{#image_template:image=File:R11_Fig16.png|caption=Transverse web stiffeners<Br>''M6 Toll Bridge 450, [[Ladder deck composite bridges|ladder deck]] with knee bracing''|align=left|width=700}}

===Proportions===
[[Stiffeners#Intermediate_stiffeners|Intermediate web stiffeners]] can commonly be proportioned as flat plates 250mm by 25mm or 200mm by 20mm thick: generally be proportioned in the ratio 10:1 width to thickness. This is convenient for the size of the top flange and for detailing [[Connections_in_bridges#Bolted_connections|bolted splice connections]]. 

Where the [[Stiffeners|stiffener]] is part of a U frame providing support to the bottom flange against buckling, then they may need to be larger.

[[Stiffeners#Bearing_stiffeners|Bearing stiffeners]] are usually thicker than [[Stiffeners#Intermediate_stiffeners|intermediate web stiffeners]] as they need to resist additional lateral forces to be transmitted to supports.  [[Stiffeners#Bearing_stiffeners|Bearing stiffeners]] will normally be between 30 and 50 mm thick.

==Deck slab==
The most common way of constructing the deck slab is to use precast concrete permanent participating formwork. Other formwork includes glass fibre reinforced plastic (GRP) and traditional timber plywood, the latter being commonly used for irregular areas such as corners of [[Skew_bridges|skew bridges]] and cantilevers.
<gallery widths=380px heights=300px> 
Image:R11_Fig17.png|Precast concrete permanent formwork<Br>''M6 Toll, Bridge 295'' 
Image:R11_Fig18.png|Plywood formwork and traditional falsework system for edge cantilever construction<Br>''Porth Bypass, Rheola Bridge''
</gallery>
<br>
Occasionally, a precast concrete cantilever system may be preferred to avoid falsework below the bridge deck level.
<gallery caption="Precast concrete edge beam and cantilevers" perrow=2 widths=380px heights=300px> 
Image:R11_Fig19.png|''A650 Bingley relief Road, Cottingley Viaduct'' 
Image:R11_Fig20.png|''A650 Bingley relief Road, Cottingley Viaduct''
</gallery>
{{#image_template:image=File:R11_Fig21.png|caption=Waterproofing and bridge deck detailing around an expansion joint<Br>''M6 Toll Bridge 501''|align=right|wrap=true|width=400}}
<br>
The typical features of a bridge deck of this type will be:
*250 mm overall thickness, (including precast concrete permanent participating formwork).
*Grade C40/50 concrete (to BS EN 206<ref name="No10">BS EN 206:2013+A2:2021 Concrete. Specification, performance, production and conformity.
BSI</ref>).
*Reinforcement grade 500B (to BS EN 10080<ref name="No11">BS EN 10080:2005 Steel for the reinforcement of concrete. Weldable reinforcing steel. General. BSI</ref> and BS 4449<ref name="No12">BS 4449:2005+A3:2016 Steel for the reinforcement of concrete. Weldable reinforcing steel. Bar, coil and decoiled product. Specification. BSI</ref>), typically 250kg/m&sup3;.
<br>
There are many aspects of the deck slab that affect the durability of the bridge: (concrete grade, cover, details to shed water away from critical areas).  Durability and buildability should be considered at the preliminary stage - see CIRIA Report 155<ref name="No13">Ray, S.S; Barr, J.; Clark, L. (1996) Bridge detailing guide. (Report R155). CIRIA</ref> and Report C543<ref name="No14">Souby, M. (2001) Bridges – design for improved durability. (Report C543). CIRIA</ref>. The combination of concrete grade and cover is important and careful reinforcement detailing is required to ensure that the correct covers are maintained. The top of the bridge deck has a waterproofing membrane applied.

==Articulation==
Articulation is the way in which the bridge is arranged to deal with movements that occur as a result of actions on the bridge arising from:
*Temperature
*Wind
*Traffic loading (vehicles, trains, people)
*Self weight
<br>
[[Bridge_articulation_and_bearing_specification#Bearings|Bearings]] are typically used to make the connection between bridge and supports, to accommodate rotations and movements arising from these effects, unless an [[Integral bridges|integral]] support is provided.

[[Bridge_articulation_and_bearing_specification#Articulation|Articulation]] needs to be considered at the preliminary design stage in order to determine where restraint forces will be provided, thus influencing design of support [[Bracing systems|bracing systems]] and the substructure.

==Method of construction==
The designer of a steel bridge has to determine the method of construction as this must be taken into account in the design of the steelwork.  It is also incumbent upon the designer to indicate in the contract documents (usually on the drawings) the construction sequence assumed in the design, both for the [[Design_for_steel_bridge_construction#Bridge_erection|erection]] of the steelwork and for the concreting of the deck slab. 
The principal options for [[Design_for_steel_bridge_construction#Bridge_erection|bridge erection]] are:
*[[Design_for_steel_bridge_construction#Erection_using_cranes|Erection by crane]]
*[[Design_for_steel_bridge_construction#Launching|Launching]]
*Sliding
*Rolling
*Lifting large preassembled sections.
<br>
The most common method of erecting bridge girders is direct [[Design_for_steel_bridge_construction#Erection_using_cranes|erection by mobile crane]] lifting the girders (termed erection pieces) from the ground onto the bridge substructure. Typically, girders for single span bridges will be placed either singly or in braced pairs spanning full length between the end supports. For multiple spans the girders are erected (again either singly or in braced pairs) in a span and cantilever sequence involving erection pieces that cantilever over the piers to the point of contraflexure in the next span as shown below.
{{#image_template:image=File:R10_Fig6.png|caption=Sequential erection by [[Design_for_steel_bridge_construction#Erection_using_cranes|erection by crane]]|align=left|width=600}}
There are physical limitations on the length of girder pieces that can be fabricated and [[Design_for_steel_bridge_construction#Transportation|transported to site]]. Under normal circumstances in the UK the maximum length for [[Design_for_steel_bridge_construction#Transportation|transport by road]] is 30m without a movement order but steelwork contractors are quite familiar with this procedure and girders up to 50m long have been [[Design_for_steel_bridge_construction#Transportation|transported by road]].

When determining the most appropriate structural arrangement and associated [[Design_for_steel_bridge_construction#Bridge_erection|erection]] method, the designer will be carry out hazard assessments and determine mitigation measures as required under the CDM regulations<ref name="No1"></ref>. Guidance on the designer’s responsibilities and common hazards in bridge construction is given in CIRIA publication C604<ref name="No15">CDM Regulations–work sector guidance for designers. (Report C604 Second Edition) 2004. CIRIA</ref>. Guidance is also available in [[#Resources|BCSA 38/05]].

==References==
<references responsive="0"/>

==Resources==
*[[Media:SCI_P356.pdf|Iles, D.C. (2010) Composite highway bridge design. (P356 including corrigendum, 2014). SCI]]
*[[Media:SCI_P185.pdf|Hendy, C.R.; Iles, D.C. (2015) Steel Bridge Group: Guidance Notes on best practice in steel bridge construction (6th Issue). (P185). SCI]]
**[[Media:GN_1-05.pdf|Guidance Note 1.05, Structural arrangement]] 
**[[Media:GN_2-01.pdf|Guidance Note 2.01, Main girder make-up]] 
*[[Media:BCSA_38-05.pdf|BCSA Guide to the erection of steel bridges (38/05). BCSA]]
*[[Media:BCSA_51-10.pdf|Steel Bridges: A practical approach to design for efficient fabrication and construction. (51/10). BCSA]]
**Chapter 1: Design conception 
*Preliminary steel bridge design charts:
**Chart finder
**Spreadsheet tool
**User manual
:All three of which can be found on [https://bcsa.org.uk/resources/fabrication-technical-design/bridges-preliminary-design/ the BCSA web site]

==See also==
*[[Multi-girder composite bridges]]
*[[Ladder deck composite bridges]]
*[[Integral bridges]]
*[[Material selection and product specification]]
*[[Weathering steel]]
*[[Bracing systems]]
*[[Stiffeners]]
*[[Connections in bridges]]
*[[Bridge articulation and bearing specification]]
*[[Plan curvature in bridges]]
*[[Skew bridges]]
*[[Design for steel bridge construction]]

==External links==

*[https://www.standardsforhighways.co.uk/dmrb/ Highways England DMRB (Design Manual for Roads and Bridges)]
*[https://www.standardsforhighways.co.uk/mchw Highways England MCDHW (Manual of Contract Documents for Highway Works)]
*[http://www.steel-sci.com/the-steel-bridge-group.html The Steel Bridge Group (SBG)]

[[Category:Bridges]]
[[Category:Index of main articles]]