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Re: A structural engineering journal

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I am agreeing that it is a myth that needs to be debunked. Too many designers are accepting it as fact and do not understand/realize how wrong their designs will be if they extend them outside the lightly loaded structures with low levels of tension in the concrete at service they are have previously used for this type of design.

Designers are pushing towards partially prestressed solutions but are still relying on this "design logic" and this will cause problems with the resulting structures. I have heard of designers ignoring the 6root F'c limit for this type of design approach because they feel they are being disadvantaged by having to conform to it.

They (and the PTI who push this approach) do not realize that the 6 root limit was put into ACI318 to protect designers from the approximations inherent in this design approach: the redistributions of moments/shears/strains that are required to make it work in practice. If service stresses are allowed to increase to levels at which cracking will occur which will result in these redistributions beginning under service loads and not under higher failure loads as is supposed to happen. The resulting increase in deflections which will result from this will be disastrous as will the unrestrained cracking. Fortunately, they may not fall down, just deflect and crack a lot.

What was a design approximation introduced to make design easier has developed into a definition of how prestressed slabs actually function in practice throughout the loading range and that is a myth which needs to be debunked before it gets even further out of hand.


At 01:52  17/11/00 -0800, you wrote:
Gil,  Are you agreeing with Roger?  Or am I misplacing the heavy sarcasm?

Paul Feather
----- Original Message -----
From: "Gil Brock" <gil(--nospam--at)raptsoftware.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, November 17, 2000 12:54 PM
Subject: RE: A structural engineering journal


>
> >Expand on Raucher's(sp?) (Texas Transportation Institute) research on
banded
> >tendons in post-tensioned concrete and debunk the myth that distribution
of
> >tendons is not relevant in P/T slab construction.
>
> Hear hear! Anyone who believes this one would believe in the Tooth Fairy,
> Santa Claus and honest politicians who are acting for the good of the
> masses (had to get that one in in the current climate in US).
>
>
>
>
>
> 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/
>
>
>





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/