huangyhg

shear reinforcement for grade beams of slab on grade
hi, all,
i have designed a slab on grade for a new residential house. i have design the grade beam to be 15" wide x 22" deep w/ 2-#4 @ top & bott & w/ #3 ties @ 32" o.c. in addition, the slab is designed to be 4" thick w/ #4@ 18" o.c. e.w. the contractor came to me indicating that he has been in the construction for almost 20 years and never used shear reinforcement for the grade beams of slab on grade. it confused me. as i always think that putting shear rebars at the grade beam usually is a good practise. i think the shear reinforcement can make the grade beam stronger to resist the unknow shear that might happed to the beam such as temperature, settlement, heave, expansive soil, bad foundation base reparation. i wonder, except to save a little bit construction cost, if there are any other good reasons not using the adequate shear reinforcement.
the second question he imposed on me is that he would like to use wwf to replace #4 rebars @18" o.c. for slab. i knew that using wwf is quite all right. but, what was wrong not using #4? except to save cost.
any comments are appreciated.
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shear values are way less than half allowable, so stirrups not needed. you can't design for unknown loads, else why stop with #4 reinforcing? typical here is #4 on slab as they have them anyways.
i specify shear reinforcement just to help tie the longitudinal reinforcement together (i.e. convenience of installation).
history has shown that residential foundation loads are so low that the cost savings from minimizing steel reinforcing are almost always justified. unless you have special considerations, your contractor's advice sounds reasonable.
here is a link to a typical residential foundation reference (free .pdf dowload)
here is a better reference. "residential structural design guide - 2000 edition". you can download the free document from this page

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