Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Reinforcing existing steel members

[Subject Prev][Subject Next][Thread Prev][Thread Next]
In the context of the original question,

"This article and several written prior to it, states that the "strength of
columns reinforced under load and under no load are identical."  This
brought me to question some of the practices on my own office where we have
sized reinforcement of steel members by subtracting out the existing
(actual) stresses, resulting in an overly conservative design.  So I thought
I'd write to the audience to see if this philosophy is widely  held.  Also
is this approach true for tension and bending members?"


I think that there may be two different questions getting blended together
in the various responses.  One question is "what is the strength or
allowable stress of the reinforced section after the reinforcement is
added?"

The other question often being asked is "if I reinforce the section, what
will my new remaining capacity or remaining margin for additional stress
be?"

The former may not be related to load history, but the latter is surely
related to the state of stress or demand in the original section at the time
of reinforcement.

For the bending member case, take as an example a simple span bridge girder
where the top flange is not laterally braced by the deck system, but only at
discrete points by the transverse diaphragms.  The geometry results in
stability of the compression flange being  the controlling limit state in
this case.  Say the section modulus is 500 in3, the DL moment is 500 ft-k,
and the DL stress at the top flange is 12 ksi.  To add capacity to the
bridge, the tension flange is reinforced, while the full dead load stress is
maintained on the original section.  The new allowable stress or section
strength is determined by evaluation of the composite section.  This is not
a function of the load history.   Suppose the new allowable stress based
upon stability of the top flange in the now-reinforced section is 17 ksi,
and the new section modulus to the top flange is 600 in3.  But what is the
remaining usable capacity of the now composite section - can this be
determined apart from the knowledge of the state of stress (or demand) at
the time the reinforcement was added?   Or is it (17-12)*600/12 = 250 ft-k,
for this simplistic case?



Mark D.Anderson  PE
Anchorage


----- Original Message -----
From: "Carter, Charlie" <carter(--nospam--at)aisc.org>
To: <seaint(--nospam--at)seaint.org>
Sent: Tuesday, January 28, 2003 8:42 AM
Subject: RE: Reinforcing existing steel members


> >AISC Journal 4th qtr 1990 "Reinforcing Steel members
> >and the Effects of Welding" by R.H.R Tide
> >.... states that the "strength of columns reinforced
> >under load and under no load are identical."  This
> >brought me to question some of the practices on my
> >own office where we have sized reinforcement of steel
> >members by subtracting out the existing (actual) stresses,
> >resulting in an overly conservative design.
>
> The paper is correct. Elastic superposition of stresses is generally not
> necessary because of the redistribution (inelasticity) that will occur
well
> before failure.
>
> This is true whether you use LRFD or ASD. The column curves for both
methods
> are based upon the same data (as they should be -- the column does not
know
> which method was used to design it -- it will behave as it will behave).
>
> This same thing holds for beams and other reinforced members. One caveat:
> deflections and deformations are accumulative. So the existing member
> deflection/deformation is the the starting point for the additional
> deflection/deformation of the reinforced cross section due to the
additional
> loads.
>
> Charlie
>
>
>
>
>
>
>
>
>
>
> ******* ****** ******* ******** ******* ******* ******* ***
> *   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
> *
> *   This email was sent to you via Structural Engineers
> *   Association of Southern California (SEAOSC) server. To
> *   subscribe (no fee) or UnSubscribe, please go to:
> *
> *   http://www.seaint.org/sealist1.asp
> *
> *   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
> *   send to the list is public domain and may be re-posted
> *   without your permission. Make sure you visit our web
> *   site at: http://www.seaint.org
> ******* ****** ****** ****** ******* ****** ****** ********
>
>



******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********