Top and bottom slab cut distances
'Top slab cut distance' (t1) and 'Bottom slab cut distance' (t2) are user-defined dimensions which specify the depths of the top and bottom slab portions of a box girder, as shown in Figure 1:
Figure 1 - Top and bottom slab cut distance
Computation
Computation depends on these slab cut distances as follows:
- Equivalent slab thickness is evaluated, assuming a rectangular shape, based on slab area and width (t_slabarea = A_slab/b_slab).
- Dimensions dictate the sectional polygon, used to compute slab tributary area for each girder web. The tributary region includes all material above the top slab cut and half-way between girders. Exterior girders include the entire outside cantilever. Top and bottom slab-cut dimensions may be different at interior and exterior girder locations.
Please refer to the 'Bridge Superstructure Design Manual' for additional details.
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Top slab cut distance and bottom slab cut distance
The "Top slab cut distance" and "Bottom slab cut distance" are user-defined distances that specify what depth of the box girder will be considered as top or bottom slab for the purpose of design. The "Top slab cut distance" is measured downwards from the top of deck and the "Bottom slab cut distance" is measured upwards from the bottom of the box girder - please see the figure below:
Use in design
The slab cut distances are used in design as follows (refer to the bridge superstructure design manual for additional details):
- The equivalent slab thickness used for design is evaluated based on the slab area and slab width, assuming a rectangular shape, as t_slabarea = A_slab/b_slab.
- The slab tributary area for each girder (web) is determined from the sectional polygon. For the top slab height-wise it includes everything above the top cut line. Width-wise it includes half distance between girders, for exterior girder the entire outside cantilever is included.
The values of ThSlabTop and ThSlabBot can be different for interior and exterior girders, because the exterior girder may include the entire cantilever slab (top slab) or no slab on the outside face of the web for bottom slab.
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