AD317: Use of integral crossheads in Skew Highway Bridges.

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Advisory Desk AD 317 Use of integral crossheads in skew highway bridges Guidance Note 1.02 in the Steel inner girders. effect each pair of girders would provision of such bracing should be Bridge Group publication Guidance At first sight this arrangement rotate about a longitudinal axis. For discussed between the principal Notes on best practice in steel might seem to be inadequate to construction purposes, if rotation contractor and the designer at an bridge construction (P185) includes resist transverse bending, with restraint is not provided by the early stage so that agreement can advice about the use of integral the slab spanning transversely bearing or by temporary works be reached about what should crossheads at the intermediate between the columns to carry all between the girder and the pier top be provided and whether it must supports of skew multi-girder deck the bending. However, for practical it may be beneficial to introduce be removed after construction. bridges. However the advice does configurations, it has been shown bracing between the inner girders Early involvement of the steelwork not discuss the structural behaviour that the bending of the main at the crosshead positions (either contractor will also assist in of the deck in service or the girder contributes to transverse triangulated bracing or beams ensuring that any additional possible need for additional bracing load distribution, with the whole with stiff connections to the main steelwork can be incorporated for the construction stage. This AD deck behaving somewhat like an girders), as shown in Figure 2 efficiently with minimum disruption provides supplementary advice from orthotropic slab. There is no need (the intermediate bracing is now to the fabrication process. The the Steel Bridge Group to cover that for continuity of the crossheads aligned with this additional bracing). cost and responsibility for the deficiency. and indeed any continuity would Such provision also controls the additional steelwork should also be Integral crossheads are used tend to attract fatigue loading − transverse spacing between the established at an early stage. when it is necessary or desirable to the detailing of the crosshead inner girders; this provision is Where the skew is small (and reduce the number of intermediate connections would thus need to especially important where precast where the supports are square to columns, the typical arrangement consider the stress ranges due to plank permanent formwork is the main girders), the provision of being to position a single column the continuity that would exist. It used. This extra bracing does not continuity (with the webs of both between a pair of longitudinal is of course necessary to model need to be as substantial as the crossheads and the continuity girders and to provide a crosshead the 3D behaviour of configurations crosshead beams. If the bracing is bracing in a common plane) is between the girders to transfer such as that shown in Figure more flexible than the crossheads, less onerous in fatigue. There is the vertical reactions. For skews 1, particularly in relation to the and does not act compositely with however still the option for either greater than about 25° the Note transverse bending of the slab the slab, it may be acceptable to the permanent works designer advises that the crossheads adjacent to the crossheads; a leave it in place (thus avoiding the or the contractor to provide this should be square to the main simple 2D grillage model is not potentially hazardous operation of bracing. In the latter case, the girders, which would lead to the adequate for that analysis. removal), provided that the fatigue bracing would probably be designed configuration shown in Figure 1 for However, if no continuity is consequences are assessed and to be left in place, rather than a four-girder three-span bridge. provided at the construction found to be acceptable. If the removed after construction. As can be seen in the Figure, the stage, it becomes quite difficult to fatigue consequences are not crossheads are not continuous control relative deflections during acceptable, the bracing will have to Contact: David Iles across the width of the bridge, they concreting the deck slab unless be removed. Tel: 01344 636525 extend only between each pair of there are other temporary supports Since the need for this Email: advisory@steel-sci.com girders: there is no continuity, other under the girder or the bearing continuity bracing depends on than the deck slab, between the two provides rotational restraint - in the construction operations, the Figure 1. Arrangement of main girders and crosshead girders for a skew multi-girder bridge Figure 2 Provision of cross beams for the construction stage 36 NSC January 2008


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