View source for Multi-girder composite bridges

__SECTIONLINKTOTOP__
Multi-girder bridges are one of the most common types of medium span composite bridge in the UK. Multi-girder construction is used for single spans and for continuous multiple spans, and it is particularly effective where construction depth is limited. This article provides a description of the features of this type of bridge and introduces some of the structural design considerations.
{{#image_template:image=File:R2_Fig3.png|caption=Multi-girder bridge using variable depth girders<Br>''Westgate bridge, Gloucester''|align=right|wrap=true|width=450}}

==Configuration of a multi-girder bridge==
In multi-girder construction, a number of similarly sized longitudinal [[Steel_construction_products#Fabricated_products|plate girders]] are arranged at uniform spacing across the width of the bridge, as shown in the typical cross section below. The deck slab spans transversely between the longitudinal girders and cantilevers transversely outside the outer girders. The girders are braced together at supports and at some intermediate positions. Composite action between the reinforced concrete deck slab and the longitudinal girders is achieved by means of [[Shear_connection_in_composite_bridge_beams#Headed_stud_shear connectors|shear connectors]] welded on the top flanges of the steel girders.

The arrangement shown in the image below is common where permanent formwork is used and shows four girders of equal depth and with a slab surface that follows the camber of the road. A footway/verge is provided either side of a 2-lane single carriageway and parapets/restraint barriers are mounted on the edge beams.
{{#image_template:image=File:R2_Fig1.png|caption=Cross section of a multi-girder highway bridge|align=left|width=600}}

==Longitudinal girders==
The steel girders are usually fabricated I-section [[Steel_construction_products#Fabricated_products|plate girders]]; for smaller spans, it is possible to use [[Steel_construction_products#Standard_open_sections|rolled section beams]] (Universal Beams) but, for reasons discussed below, [[Steel_construction_products#Standard_open_sections|rolled sections]] are rarely used today. Usually, girders are spaced between about 3.0 and 4.0m apart, and thus, for an ordinary two-lane overbridge, four girders are provided. This suits the deck slab, which has to distribute the vertical loads from the wheels.

===Plate girders===
{{#image_template:image=File:R2_Fig2.png|caption=Typical single span multi-girder bridge<br>''A66 Overbridge, Longnewton''|align=left|wrap=true|width=300}}
The use of [[Steel_construction_products#Fabricated_products|plate girders]] gives scope to vary the girder sections to suit the loads carried at different positions along the bridge. The designer is free to choose the thickness of web and size of flange to suit the internal forces at different positions along the length of the span, though it must be remembered that too many changes may not lead to economy, because of the additional [[Design_for_steel_bridge_construction#Fabricating_the_steelwork|fabrication]] work. Splices are expensive, whether [[Connections in bridges#Bolted connections|bolted]] or [[Connections in bridges#Welded connections|welded]].

Most often, the girders have parallel flanges, that is, they have a constant depth. However, with [[Steel_construction_products#Fabricated_products|plate girders]], the designer can also choose to vary the depth of the girder along its length. For longer spans it is quite common to increase the girder depth over intermediate supports. For spans below about 50m, the choice (constant or varying depth) is often governed by aesthetics. Above 50m, varied depth may offer economy because of the weight savings possible in midspan regions. The variation in depth can be achieved either by straight haunching (tapered girders) or by curving the bottom flange. The shaped web, either for a variable depth or constant depth girder with a vertical camber, is easily achieved by profile cutting during [[Design_for_steel_bridge_construction#Fabricating_the_steelwork|fabrication]]. Generally, webs have a high depth/thickness ratio and this leads to the need for intermediate transverse [[Stiffeners#Types_of_stiffener|web stiffeners]] in regions of high shear (near supports).

Very occasionally, for reasons of appearance, the outermost girders are designed as a J-section; the bottom flange projects only on the inner side of the web. Requests for this detail arise from a dislike of the flange outstand, although there is little visible difference and the distinction is not noticed by most people. Use of such a section introduces [[Design_of_beams_in_composite_bridges#Introduction_to_lateral_torsional_buckling|torsional]] effects (because the shear centre is outside the line of the web) that require very careful consideration during design and construction, with significant penalty on costs an increased hazards during construction. 

Also, on occasion, relatively small [[Box_girder_bridges|box girders]] are used in multi-girder construction. [[Box_girder_bridges|Box girders]] require special design consideration, because of their high torsional stiffness and high cost of [[Design_for_steel_bridge_construction#Fabrication_the_steelwork|fabrication]]. Advice on construction costs may be obtained from steelwork contractors.

===Rolled section girders===
[[Steel_construction_products#Standard_open_sections|Universal beams]] up to 1016 mm deep are available in the section range covered by BS EN 10365<ref name="BSEN10365">BS EN 10365:2017 Hot rolled steel channels, I and H sections. Dimensions and masses. BSI</ref>. Such [[Steel_construction_products#Standard_open_sections|beams]] would provide sufficient bending resistance for single spans up to about 25m and for continuous spans up to about 30m, although the webs may be rather thin for the high shears associated with longer spans, unless the bridge is lightly loaded – a farm access bridge or a footbridge, for example.

Very little [[Design_for_steel_bridge_construction#Fabrication_the_steelwork|fabrication]] is necessary with universal [[Steel_construction_products#Standard_open_sections|beams]], usually only the fitting of [[Stiffeners|stiffeners]] over support [[Bridge_articulation_and_bearing_specification#Bearings|bearings]] and the attachment of [[Bracing_systems|bracing]]. However, the [[Steel_construction_products#Standard_open_sections|beams]] often need to be curved in elevation to suit either the road profile or the pre-cambering for dead load; this can be carried out by specialist companies using heavy rolling equipment but it does add to cost. Even for smaller spans, universal [[Steel_construction_products#Standard_open_sections|beams]] can often be more economically replaced by similar size [[Steel_construction_products#Fabricated_products|plate girders]]. Steelwork contractors  can advise on the relative economy.

==Bracing==
===Support bracing===
Girders need to be [[Bracing_systems|braced]] together at support positions, for stability and to effect the transfer of horizontal loads (wind and skidding forces) to the [[Bridge_articulation_and_bearing_specification#Bearings|bearings]] that provide transverse restraint (usually one at each support position).

Restraints at supports are provided either by triangulated [[Bracing_systems|bracing systems]] or by horizontal beams, usually [[Steel_construction_products#Standard_open_sections|channel]] sections. The [[Bracing_systems|bracing systems]] at the end supports of non-[[Integral_bridges|integral bridges]] are usually also required to support the end of the deck slab. [[Integral_bridges|Integral bridges]] will require [[Bracing_systems|bracing]] at the end supports for the construction condition.

A typical [[Bracing_systems|bracing]] arrangement at an intermediate support is shown below:
{{#image_template:image=File:R2_Fig4.png|caption=Typical [[Bracing_systems|bracing]] arrangement at an intermediate support (shown for a super-elevated roadway)|align=left|width=600}}

===Intermediate bracing===
In the completed bridge, [[Bracing_systems#Design_of_intermediate_bracing|intermediate bracing]] is usually needed at discrete positions in the spans of multi-span bridges, to stabilise the bottom flanges adjacent to intermediate supports (where they are in compression). During construction, [[Bracing_systems|bracing]] is needed to stabilise both the bottom flanges adjacent to intermediate supports and the top flanges in midspan regions. Where the girders are [[Plan_curvature_in_bridges|curved in plan]], [[Bracing_systems|bracing]] will also be needed to provide ‘radial’ restraint to the bottom flanges.

In most cases, the most effective [[Bracing_systems|bracing system]] is a triangulated frame between adjacent girders. In the completed bridge this provides a very stiff restraint path from the plane of the deck slab through to the bottom flanges. In the construction condition, intermediate bracing between girders, without [[Bracing systems#Plan_bracing|plan bracing]], provides ‘torsional restraint’ – see discussion of the effectiveness of such bracing in Section 7 of [[#Resources|SCI P356]]. As an alternative, ‘channel bracing’ is often used with shallow main girders; the stiff  [[Steel_construction_products#Standard_open_sections|channel]]  has rigid [[Connections_in_bridges#Bolted connections|connections]] to the main girders.

Intermediate [[Bracing systems|bracing]] that is continuous across more than two main girders will participate in the global action and will distribute loading in any one lane to several main girders. However, such continuity does not provide much benefit to the design of the main girders(because the design case is usually with all lanes loaded) and introduces stress reversals in the [[Bracing_systems|bracing]] and its [[Connections_in_bridges|connections]]; the connection details are potentially prone to [[Fatigue_design_of_bridges|fatigue]]. To avoid this [[Fatigue_design_of_bridges|fatigue]] situation, designers use non-continuous [[Bracing_systems|bracing]], where main girders are connected in pairs, with no [[Bracing_systems|bracing]] between one pair and the next, as shown below.
{{#image_template:image=File:R2_Fig5.png|caption=Typical paired [[Bracing_systems|bracing]] arrangements|align=left|width=600}}

Intermediate [[Bracing systems|bracing]] may also be required if the headroom below the bridge is such that collision loading on the bridge soffit needs to be considered. [[Bracing_systems|Bracing]] at intervals provides restraint to the bottom flange and a load path to the bridge deck. In such cases, the [[Bracing_systems|bracing]] at supports has to be designed to transfer the collision loading down to the restrained [[Bridge_articulation_and_bearing_specification#Bearings|bearings]].

Although continuity of transverse [[Bracing_systems|bracing]] is not needed (and not desirable, for the reason given above), tie/strut members are sometimes provided between the pairs of beams during construction in order either to share wind loads or to [[Bracing systems#Dimensional control|control the spacing]] between the pairs. Such members may need to be removed once the slab has been cast, because of their unwanted structural participation under traffic loading. Removal is a potentially hazardous activity that needs to be considered carefully when planning the [[Design for steel bridge construction|construction]] method. Any [[Design for steel bridge construction|construction]]  [[Bracing_systems|bracing]] that is left in place should be assessed for [[Fatigue_design_of_bridges|fatigue]].
{{#image_template:image=File:R2_Fig6.png|caption=Twin multi-girder bridge structures, showing [[Bracing_systems|bracing]] locations<br>''[[Lagentium Viaduct]], A1(M) Darrington to Dishforth''|align=left|width=600}}

===Plan bracing===
[[Bracing systems#Plan bracing|Plan bracing]] to the top flange is an alternative way to provide a stiff lateral restraint to the top flanges at the bare steel stage. Although such [[Bracing_systems#Plan bracing|bracing]] is very effective in restraining the compression flange in midspan, its presence complicates construction. The two possible locations of [[Bracing systems#Plan bracing|plan bracing]] are above the top flange (connected to cleats on the top flange) and below the top flange, [[Bracing_systems#Plan bracing|bracing]] above the top flange (shown below) adds difficulty to the placing of reinforcement and conflicts with the use of permanent formwork; it is rarely used now.

[[Bracing_systems#Plan bracing|Bracing]] below the top flange, shown below, avoids the clash with reinforcement but needs to avoid any temporary formwork. The [[Bracing_systems#Plan bracing|bracing]]  would need to be removed after casting (because it would attract unwanted forces when the slab is subject to local loading above it) and is thus not favoured by constructors.
<gallery widths=300px heights=220px> 
Image:R2_Fig7.png|[[Bracing_systems#Plan_bracing|Plan bracing]] to top flanges<br>''Humber Road Bridge''
Image:R2_Fig8.png|[[Bracing_systems#Plan_bracing|Plan bracing]] below top flange<br>''A41 Aston Clinton Bypass''
</gallery>
<br>
[[Bracing systems#Plan bracing|Plan bracing]] is occasionally provided to the bottom flanges of narrow bridges when the spans are long (over about 60m) in order to improve the overall torsional stiffness of the bridge (at the completed stage) and thus reduce susceptibility to aerodynamic instability. Such improvement in torsional stiffness would also be beneficial for a bridge with significant [[Plan_curvature_in_bridges|curvature in plan]]. The presence of the [[Bracing_systems#Plan bracing|bracing]] effectively creates a pseudo-box.

===Crosshead girders===
{{#image_template:image=File:R2_Fig9.png|caption=Crosshead girder in a multi-girder overbridge on the M6 Toll<Br>''(Image courtesy of Mabey Bridge Ltd.)''|align=right|wrap=true|width=400}}
At intermediate supports, it is sometimes desirable to reduce the number of columns and  [[Bridge_articulation_and_bearing_specification#Bearings|bearings]]. Typically, instead of a [[Bridge_articulation_and_bearing_specification#Bearing|bearing]] directly under each girder, one [[Bridge_articulation_and_bearing_specification#Bearing|bearing]] is provided midway between each pair of girders, with a crosshead girder to transfer the reactions. Such an arrangement is particularly common with large [[Skew_bridges|skew]]. An example of a crosshead is shown below. (This illustration also shows a continuity girder between the central girder; such a girder is advantageous for construction, to minimise twist during concreting, but is not normally needed for the permanent condition). 

==Case studies==
<gallery caption=" " widths=180px heights=200px> 
Image:A34_Wolvercote_Viaduct.JPG|[[A34 Wolvercote Viaduct, Oxford]]
Image:A66_Surtees_Bridge_Replacement.jpg|[[A66 Surtees Bridge Replacement]]
Image:Temple_Sowerby_Bypass.jpg|[[River Eden Bridge]]
Image:Lagentium_Viaduct.jpg|[[Lagentium Viaduct]]
Image:Shiremoor_Bypass.jpg|[[Shiremoor Bypass]]
Image:A1_Appleyhead.jpg‎|[[A1 Peterborough to Blyth|A1 Peterborough to Blyth]]
Image:Northside_Bridge.png|[[Northside Bridge, Workington|Northside Bridge, Workington]]
Image:A1_Leeming-Barton-1.jpg|[[Bridges on A1 Leeming to Barton upgrade]]
Image:AWPR_B-T_1.jpg|[[Aberdeen Western Peripheral Route/Balmedie to Tipperty bridges]]
</gallery>

==References==
<references/>

==Resources==
*[[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:SCI_P356.pdf|Iles, D.C. (2010) Composite highway bridge design. (P356 including corrigendum, 2014). SCI]]
*[[Media:SCI_P357.pdf|Iles, D.C. (2010) Composite highway bridge design: Worked examples. (P357 including corrigendum, 2014). SCI]]
*[[Media:SCI_P393.pdf|Iles, D.C. (2012) Design of composite highway bridges curved in plan. (P393). SCI]]
*[[Media:SCI_ED008.pdf|Iles, D.C. (2012) Determining the buckling resistance of steel and composite bridge structures. (ED008). SCI]]
*[[Media:SCI_P406.pdf|Iles, D.C. (2015) Determining design displacements for bridge movement bearings. (P406). SCI]]
*[[Media:BCSA_51-10.pdf|Steel Bridges: A practical approach to design for efficient fabrication and construction, 2010, (Publication no. 51/10), BCSA]], 
*[[Media:BCSA_38-05.pdf|Guide to the Erection of Steel Bridges, 2005, (Publication no. 38/05), BCSA]]
*[[Design_software_and_tools#Bridges_carbon_calculator|Carbon footprint tool for steel composite highway bridges]]
*Preliminary steel bridge design charts:
**Chart finder
**Spreadsheet tool
**User manual
:All three of which can be found on [https://www.steelconstruction.org/resources/technical/bridges-preliminary-design/ the BCSA web site]

==See also==
*[[Ladder deck composite bridges]]
*[[Integral bridges]]
*[[Box girder bridges]]
*[[Bridges - initial design]]
*[[Modelling and analysis of beam bridges]]
*[[Design of beams in composite bridges]]
*[[Shear connection in composite bridge beams]]
*[[Fatigue design of bridges]]
*[[Bracing systems]]
*[[Stiffeners]]
*[[Connections in bridges]]
*[[Bridge articulation and bearing specification]]
*[[Plan curvature in bridges]] 
*[[Skew bridges]]
*[[Design for steel bridge construction]]

==External links==

*[http://www.standardsforhighways.co.uk/ha/standards/dmrb/index.htm Highways England DMRB (Design Manual for Roads and Bridges)]
*[http://www.standardsforhighways.co.uk/ha/standards/mchw/index.htm 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]]