This article explains the meaning and the use of the top and bottom slab cut distances specified on the "Define Bridge Section Data" form.

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Top slab cut distance and bottom cut distances

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 bridge superstructure 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, as shown in Figure 1:

Figure 1: Top and bottom slab cut distances

Use in bridge superstructure design

The slab cut distances are used in the bridge superstructure design as follows:

• For each girder (web), the tributary slab area is determined from a section polygon that is constructed as follows:
  • For top slab in the vertical direction, all concrete area above the Top Slab Cut Line is included.
  • For bottom slab in the vertical direction, all concrete area below the Bottom Slab Cut Line is included.
  • For interior girders in the horizontal direction, all concrete area between half distance to adjacent girders is included.
  • For exterior girders, the entire outside cantilever is included.
• Once the tributary slab area, A _slab , is determined, the equivalent slab thickness is evaluated, assuming a rectangular shape, based on slab area and width as t _slabarea = A _slab / b _slab

Please refer to the Bridge Superstructure Design Manual for additional details.