What is the average span of a beam bridge

In multi-girder decks , bracing between pairs of girders is often provided using a system of torsional bracing.

This can comprise either cross bracing made up from angles , or channel sections. Channels will often need to be to mm deep. Larger sections and sections in weather resistant steel can be economically fabricated from plate. Bracings are often provided at about 5 m to 1 m centres, with a minimum of 3 or 4 bracings along the length of the girder span.

Cross bracing arrangement M6 Toll, Bridge Plan bracing of the top flanges is a very efficient way of controlling lateral torsional buckling: especially useful for simply supported single spans.

In the example illustrated below, the plan bracing was detailed to be clear of the deck construction and was designed to be removed after construction, to avoid future maintenance. During the concreting stage, the cross girders of ladder decks provide torsional restraint to the main girder flanges. 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 fabricate.

Knee bracing at supports prior to erection M6 Toll Bridge For wide 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 channel bracing at midspan. Some basic rules for provision of stiffeners at preliminary design stage are given below.

Stiffeners are required along the main girders for the following:. For ladder decks , the provision for the third requirement above usually deals adequately with the provisions for the first, so that once the bracings and cross girder arrangements are resolved, the resulting web stiffening is normally adequate. Thin webs on multi-girder decks could need additional stiffeners in between bracing positions. However, UK fabricators will normally advise that it more cost effective to make the web thicker than to add further web stiffeners , if the provisions the third requirement do not provide sufficient control to satisfy the first.

UK practice is to use transverse vertical web stiffeners. Continental Europe often uses more extensive stiffening including longitudinal horizontal stiffeners. Normally, 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.

Transverse web stiffeners should be provided at the position of each cross girder or bracing. Transverse web stiffeners at supports are usually referred to as bearing stiffeners. Additional stiffeners should be provided at positions for jacking for bearing replacement. Intermediate web stiffeners can commonly be proportioned as flat plates mm by 25mm or mm by 20mm thick: generally be proportioned in the ratio width to thickness.

This is convenient for the size of the top flange and for detailing bolted splice connections. Where the stiffener is part of a U frame providing support to the bottom flange against buckling, then they may need to be larger. Bearing stiffeners are usually thicker than intermediate web stiffeners as they need to resist additional lateral forces to be transmitted to supports.

Bearing stiffeners will normally be between 30 and 50 mm thick. 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 and cantilevers. Plywood formwork and traditional falsework system for edge cantilever construction Porth Bypass, Rheola Bridge.

Occasionally, a precast concrete cantilever system may be preferred to avoid falsework below the bridge deck level. 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. 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 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:. Bearings are typically used to make the connection between bridge and supports, to accommodate rotations and movements arising from these effects, unless an integral support is provided. 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 and the substructure.

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 erection of the steelwork and for the concreting of the deck slab.

The principal options for bridge erection are:. The most common method of erecting bridge girders is direct 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.

There are physical limitations on the length of girder pieces that can be fabricated and transported to site. Under normal circumstances in the UK the maximum length for 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 transported by road.

Using card and glue make the three beams above. Test the beams individually for structural strength and resistance to loads stress in compression and tension. We can achieve this by placing the beams between either desks or chairs.

If weights are hung or placed on top of the beams we should eventually be able to witness the shape of the beams altering. It will indicate the points of weakness. They may fold or bend depending upon how much weight is hung from them. Beam Bridges - Design Technology Beam bridges A beam or "girder" bridge is the simplest kind of bridge.

Beam bridges A beam or "girder" bridge is the simplest kind of bridge. To create very tall beams, bridge designers add supporting latticework , or a truss , to the bridge's beam. This support truss adds rigidity to the existing beam, greatly increasing its ability to dissipate the compression and tension. Once the beam begins to compress, the force spreads through the truss. Yet even with a truss, a beam bridge is only good for a limited distance.

To reach across a greater length, you have to build a bigger truss until you eventually reach the point at which the truss can't support the bridge's own weight. Brace yourself for some serious stats on truss bridges on the next page.

Sign up for our Newsletter! Mobile Newsletter banner close. The beams can be I-Beams, box girders, or trusses. They can also be braced or half-through. They are not limited to a single span either. Concrete elements can be used to improve the strength of the bridge.

Some, such as the Feiyunjiang Bridge in China, have viaducts that are supported spans. Multiple material options are available to use for beam bridges.

Planks, stone slabs, steel, concrete, and virtually any other material can be used to create this type of bridge. Each material option has specific placement pros and cons to consider, so not every material will work for every specific bridge.

It would not be cost-effective to build a small steel bridge to cross a creek, nor would it to be to build a large, wooden bridge for highway traffic. They can be placed almost anywhere. Some beam bridges perform better than others because of their placement, but all perform adequately in virtually any environment.

From walking trails to interstates, this bridge design is proven to be effective. There are span limitations to beam bridges. A single span for a beam bridge is somewhat limited. With current technologies, a single span can handle about meters of length for modern weight requirements.

Anything longer than that will require additional spans, which can be linked together as needed to form a longer bridge. Chaining multiple beams together adds time to the project, however, which reduces some of the advantages this bridge design tends to offer.

Beam bridges can be susceptible to sagging.