AD 317: Use of integral crossheads in skew highway bridges
Guidance Note 1.02 in the Steel Bridge Group publication Guidance Notes on best practice in steel bridge construction (P185) includes advice about the use of integral crossheads at the intermediate supports of skew multi-girder deck bridges. However the advice does not discuss the structural behaviour of the deck in service or the possible need for additional bracing for the construction stage. This AD provides supplementary advice from the Steel Bridge Group to cover that deficiency.
Integral crossheads are used when it is necessary or desirable to reduce the number of intermediate columns, the typical arrangement being to position a single column between a pair of longitudinal girders and to provide a crosshead between the girders to transfer the vertical reactions. For skews greater than about 25° the Note advises that the crossheads should be square to the main girders, which would lead to the configuration shown in Figure 1 for a four-girder three-span bridge. As can be seen in the Figure, the crossheads are not continuous across the width of the bridge, they extend only between each pair of girders: there is no continuity, other than the deck slab, between the two inner girders.
At first sight this arrangement might seem to be inadequate to resist transverse bending, with the slab spanning transversely between the columns to carry all the bending. However, for practical configurations, it has been shown that the bending of the main girder contributes to transverse load distribution, with the whole deck behaving somewhat like an orthotropic slab. There is no need for continuity of the crossheads and indeed any continuity would tend to attract fatigue loading − the detailing of the crosshead connections would thus need to consider the stress ranges due to the continuity that would exist. It is of course necessary to model the 3D behaviour of configurations such as that shown in Figure 1, particularly in relation to the transverse bending of the slab adjacent to the crossheads; a simple 2D grillage model is not adequate for that analysis.
However, if no continuity is provided at the construction stage, it becomes quite difficult to control relative deflections during concreting the deck slab unless there are other temporary supports under the girder or the bearing provides rotational restraint – in effect each pair of girders would rotate about a longitudinal axis. For construction purposes, if rotation restraint is not provided by the bearing or by temporary works between the girder and the pier top it may be beneficial to introduce bracing between the inner girders at the crosshead positions (either triangulated bracing or beams with stiff connections to the main girders), as shown in Figure 2 (the intermediate bracing is now aligned with this additional bracing). Such provision also controls the transverse spacing between the inner girders; this provision is especially important where precast plank permanent formwork is used. This extra bracing does not need to be as substantial as the crosshead beams. If the bracing is more flexible than the crossheads, and does not act compositely with the slab, it may be acceptable to leave it in place (thus avoiding the potentially hazardous operation of removal), provided that the fatigue consequences are assessed and found to be acceptable. If the fatigue consequences are not acceptable, the bracing will have to be removed.
Since the need for this continuity bracing depends on the construction operations, the provision of such bracing should be discussed between the principal contractor and the designer at an early stage so that agreement can be reached about what should be provided and whether it must be removed after construction. Early involvement of the steelwork contractor will also assist in ensuring that any additional steelwork can be incorporated efficiently with minimum disruption to the fabrication process. The cost and responsibility for the additional steelwork should also be established at an early stage.
Where the skew is small (and where the supports are square to the main girders), the provision of continuity (with the webs of both crossheads and the continuity bracing in a common plane) is less onerous in fatigue. There is however still the option for either the permanent works designer or the contractor to provide this bracing. In the latter case, the bracing would probably be designed to be left in place, rather than removed after construction.
Contact: David Iles
Tel: 01344 636525