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Re: Wide Shallow Beams

[Subject Prev][Subject Next][Thread Prev][Thread Next] This may have already been answered by someone,  but if not:

In post-tensioned construction,  wide shallow beams (also known as slab bands)  are essentially a thickening along the column line to allow additional drape.  In both post-tensioned and reinforced concrete construction,  the dimensions of the slab band must be such that the slab is not stiffened so much that two way action is inhibited.

As a rule of thumb,  the depth of the band should not be greater than twice the slab thickness,  i.e. the depth below the slab should not be greater than the slab thickness; in addition the band should be at least three times wider than it is deep (total depth).

Slab depth is typically on the order of span/45  (although this depends somewhat on the loading),  band depth is typically around span/32.

In estimating the width of the band,  consider that the tendons will probably be bundled in groups of five (on the west coast this is sometimes four).  Each bundle of five (four)  will need a foot of width, thus you will need a foot of width for each 135 (108) kips.

Punching shear calculations are based on the band depth.  Note that since this is two-way construction,  requirements for beam stirrups do not apply.

One variation of this construction often used in the DC area is to have post-tensioning in the banded direction but to have a reinforced concrete slab in the other direction.  This effectively eliminates problems with future penetrations hitting tendons - as long as you stay out of the slab band, you are fine.

Gail S. Kelley, P.E.