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Re: A structural engineering journal

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At 04:14  19/11/00 -0500, you wrote:
Paul Feather wrote:

>>Roger Turk wrote:

>
> Concrete:
>
The flat plates were that were tested were designed using the equivalent
frame method, a method developed in the late 1940's to simplify hand
calculations of two-way flat plates and slabs, with the simplification being
that *all* of the applied forces are carried by an equivalent frame in one
direction *and* all of the applied forces are *also* carried by an equivalent
frame in the other direction.  This is a very conservative method that lends
itself to hand calculations of two-way flat plate and slabs.

Roger,

In what way is the equivalent frame method "very conservative". A finite element analysis results in the same total moments in a panel in each direction as an equivalent frame analysis for a rectangular grid of columns. This also shows that the total load needs to be carried in both directions. The difference from a FE analysis is that it gives an elastic distribution of those moments across the width of the panel and will also show the distribution of moments for irregularly placed columns. The total design moments in a panel, however, are the same from the 2 approaches for the rectangular grid of columns.

Using a banded tendon system design approach completely ignores this distribution of the moments throughout a flat plate and an FE analysis is therefore gross overkill for the analysis and could also lead to misleading results as the FE analysis is distributing moments and shears to walls, columns and beams based on a two-way elastic distribution of moments in the slab while the failure mode of the slab is as a one-way slab as you have mentioned in your email. An FE analysis cannot easily account for the inelastic design approach being used and should be used with caution.



Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd.
5 Cameron Street Beenleigh Qld 4207 Australia
Ph +61 7 3807 8022              Fax +61 7 3807 8422
email:  gil(--nospam--at)raptsoftware.com
webpage:        http://www.raptsoftware.com/