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Re: Precomposite Stresses in Reinforced Steel Sections

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Matt, John,

This discussion is reminiscent of similar discussions which took place in course lectures which I took several years ago. At that time the subject under discussion was shored vs. unshored composite construction and the examples were concrete deck upon main steel beams. Since our professor was a world class bridge engineer it is reasonable for me to suggest that you may find technical references on this subject in some of the bridge engineering literature (or codes).

Your approach, Matt, is certainly correct (at least it's not incorrect); but it may be overkill for some applications. For structures where the dead load (hence the residual stresses) is relatively high and where fatigue is a consideration (such as highway bridges, for example) your approach is probably mandatory. For structures where stress redistribution (read that as local yielding over a small part of the cross section) can be tolerated for the very infrequent design load application (such as, I think, most buildings) the approach used by your colleague is probably quite adequate.

       I hope these thoughts are useful to you.

Regards,

H. Daryl Richardson

----- Original Message ----- From: "John Riley" <jpriley485(--nospam--at)yahoo.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Monday, June 27, 2005 9:14 AM
Subject: Re: Precomposite Stresses in Reinforced Steel Sections


I would have taken your approach; your collegue's
method seems accurate to me if the member is jacked to
horizontal before reinforcing.

John Riley

--- Matt & Priscilla Muhlenkamp
<muhlenkamp(--nospam--at)mindspring.com> wrote:

When strengthening a steel section (an open-web
joist, beam, or girder in my
most recent case), I always did the following (ASD):
1. assume a more or less permanent load (mainly the
DL taken by the member),
and compute the stresses and deflection due to this
load on the original
section.
2. compute the additional stresses and deflection
based on the reinforced
section (due mainly to LL and any additional DL -
like permanent mechanical
equipment)
3. add the two together (at appropriate points on
the section) to determine
the total stress, and compare with allowable.  If
the section is
overstressed, then require some load removal prior
to reinforcing (e.g.
jacking to remove some DL).

Recently, an engineer in my office told me he only
checks the 1st condition.
In step 2, he puts the entire DL + LL on the new
reinforced (i.e. increased)
section, and doesn't consider the locked in stresses
due to the permanent
load on the original section.

While I know AISC ASD allows this for composite
beams, it seems this is
based on certain approximations and simplifications.
 I just think that it
cannot be correct in an elastic approach to the
problem, especially when
considering things like coverplating, etc.   (I know
using a plastic section
approach would make a big difference, but I only
talking elastic in this
case)

Any thoughts?

Thanks,

Matt





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