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Re: Wide Shallow Beams

[Subject Prev][Subject Next][Thread Prev][Thread Next] In a message dated 3/18/2002 4:18:01 PM Eastern Standard Time,

kenmail234(--nospam--at) writes:

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

There is no code reference for sizing wide shallow beams for either post-tensioned or conventially reinforced construction.  In general, the codes tend to lag far behind the industry when it comes to post-tensioning.  Much of what is considered common practice is not covered in the codes.  What does get into the codes is often written in such a way that noone understands it anyways so they just keep on doing what they were doing.  

The codes,  ACI 318 in particular,  are often written by people who, although they may have a great deal of education and even experience in a given area,  are typically not in a position where they ever have to apply what they have written.



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.