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Re: Wide Shallow Beams

[Subject Prev][Subject Next][Thread Prev][Thread Next] Ken,

There is no code rule because there are no fixed figures for this. The dimensions are dependant on the requirements for ultimate strength, crack control and deflections in the 2 directions. If you can satisfy all of these (with a calculation method properly allowing for cracking, shrinkage and creep for the deflection calculations) with a particular set of dimensions then your shape is acceptable.

The relative depth of band to depth of slab cannot be regulated as it is dependant on the relative span lengths in the 2 directions and is not limited to a 2 to 1 ratio. Either direction could control the depth of the band depending on the span lengths. The transverse span length will normally control the width of the band.

At 01:13 PM 18/03/02 -0800, you wrote:

 Beside the rule of thumb, where I can find the code reference for the rule of thum?

  GSKWY(--nospam--at) wrote:
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.

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Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd. (ABN 99003351504)
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