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Slab bands

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Some time ago,  there was a discussion of slab bands (wide shallow beams)  and their applicability to non-post-tensioned structures.  The parameters often used for slab band dimensions are h<= 2t and h < = b/3  (b> 3h) , where t is the depth of the slab,  h is the depth of the band and b is the width of the band.

These parameters actually came from a somewhat backwards calculation which involved looking at situations where the UBC requirement that in one-way systems you need bonded reinforcement to support DL + 25% LL would not apply.  

The basis for these parameters are the 318 section (11.5.5.1) that requires minimum shear reinforcement unless  a beam has a total depth not greater than 10 in., 2.5 times thickness of flange, or 0.5 the width of web, whichever is greatest.

To be conservative,  h<2t was used instead of 2.5 times the thickness of the flange,  h<=b/3 was used instead of h<0.5 the width of the web.

Since a 48 in. wide sheet of plywood results in a 45 in. wide band, these parameters worked well with common geometries such as a 8 in. thick slab with a 15 in. deep band.   Finite element analysis of this geometry showed that there was very little increase in the shear attracted by this thickened slab section, hence the assumption that it was still a two-way system.

Presumably, if a thickened slab section in a non-prestressed system had a similar, very small change in shear distribution and met the geometry requirements of 11.5.5.1,  it could be considered a slab band rather than a beam.  It would probably be up to the plan checker to determine what was necessary to verify the assumption with respect to shear though.

Gail Kelley, P.E.