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This article explains the meaning and the use of the top and bottom slab cut distances specified on the "Define Bridge Section Data" form.
Top slab cut distance and bottom cut distances
The Top Slab Cut Distance (t1) and bottom slab cut distance Bottom Slab Cut Distance (t2) are user-defined dimensions which specify the depths of the top and bottom slab portions of a box girder used for designdistances 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 distance
Computation
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distances
Use in bridge superstructure design
The slab cut distances are used in the bridge superstructure design as follows:
- Equivalent 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 (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.