Half-through bridges are useful in the IStructE exam as they minimise construction depth and are therefore ideal for situations where there is limited headroom clearance, a common constraint.
If you are using a U-frame bridge then you need to demonstrate that you understand it will have a significant reduction in bending capacity due to buckling.
Some people just use engineering judgement to "guess" a buckling reduction factor (say 0.7 depending on span), but it's possible to calculate it reasonably quickly using this method.
The method essentially treats the compression chord as a column with a series of spring restraints. The spring value is calculated based on the stiffness of the vertical posts, the stiffness of the crossbeam, and the stiffness of the joint between the post and crossbeam. Generally I have found that if the crossbeam proportions are sensible then the stiffness of the crossbeam has a small effect (order of magnitude lower) compared to the stiffness of the post and joint, and could be ignored for speed of calculation.
For half-through trusses, the U-frame post will commonly be a hollow section, so section properties (e.g. second moment of area I) can be taken from the relevant pages of the Blue Book (note the paper version is SCI P363).
For plate girders, the U-frame 'post' will be a virtual 'T' section comprising a stiffener and an effective section of the girder web either side. The cheat sheet includes pre-calculated values for the second moment of area (I) of various T sections based on top flange width and web thickness, which assumes:
- The stiffener (the web of the T section) will be as long as possible, i.e. (flange width - web thickness)/2
- The "flange" width / the length of girder web participating will be equal to 15Ɛt
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