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

RE: Steel Beam Reinforcing

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Roger-

Either I don't understand what you are saying or I don't agree.

First of all, if the existing beam is neither "unloaded" or shored, then
whatever stresses are in the flanges of the beam before a plate is welded to
the bottom flange will still be there after the plate is welded to the
bottom flange and, neglecting the loads imposed by the weight of the
reinforcing plate, there will be no stresses in the reinforcing plate until
superimposed loads are applied.

Secondly, yes, the neutral axis will move down based on the composite
section and due to superimposed loads, but the moment of interia will
increase as well. For example, if you weld a 3/4"x6" plate to the bottom of
a W12x40, the section modulus with respect to the top flange will increase
from 51.9in^3 to 56.61 in^3. So, if the existing beam was loaded up to it's
capacity and the beam was shored to relieve this load and a 3/4"x6" plate
was welded to the bottom flange, the moment capacity would increase by about
9%.

More realistically, the existing beam is not stressed to capacity but is not
capable of supporting new, additonal load. So, one would analyze the
stresses in the top flange based on the beam section itself, weld
reinforcement to the bottom flange and add the stresses due to the
superimposed load based on the composite section properties and compare the
total to the allowables. Only if the existing beam were stressed nearly to
it's capacity would I mess with shoring. For one thing, what would you shore
it to? Another can of worms, IMO.

Unless I'm missing something (hint: it's happened before).

Regards,

Bill Allen, S.E. (CA #2607)
ALLEN DESIGNS
Laguna Niguel, CA
http://www.AllenDesigns.com


||-----Original Message-----
||From: Roger Turk [mailto:73527.1356(--nospam--at)compuserve.com]
||Sent: Wednesday, July 19, 2000 7:11 AM
||To: seaint(--nospam--at)seaint.org
||Subject: Steel Beam Reinforcing
||
||
||Randy,
||
||Your post left a few things unclear, but I hope that my
||"assumptions" will
||address them.
||
||I am assuming that the existing beams will *not* be unloaded
||during the
||remodeling process, therefore stresses due to loading will
||exist in the
||original beam before any beam reinforcing is added.
||Therefore, consideration
||of the existing (when modified) stress condition has to be taken into
||consideration.
||
||I have never had very good luck in adding a cover plate or
||channel to one
||flange of a beam to take additional loading if the existing
||beam, unmodified,
||would be overloaded.  What I have observed is that although
||the section
||modulus is increased for one flange, the neutral axis is
||lowered, and the
||section modulus for the other flange is decreased.  Therefore, if the
||built-up section is designed based on the tension stress in the added
||reinforcing, the compressive stress in the top flange is
||going to increase
||due to the increase of its distance from the N.A.  If the top
||flange is
||overstressed under the new loading without modification, it will be
||overstressed a greater amount if reinforcing is added only to
||the bottom
||flange.
||
||If it is possible, angles, bars, or plates welded to the
||underside of the the
||top flange could help offset the change in the N.A.
||
||Hope this helps.
||
||A. Roger Turk, P.E.(Structural)
||Tucson, Arizona
||
||Randy Diviney wrote:
||
||>>I have a few beams in an existing building that will be 40%
||overstressed
||when new loading conditions are applied. I planned on using a channel
||stitch welded to either side of the web to make up the difference in
||required section modulus. The beams have adequate shear
||strength. I noticed
||in our office standards book, we have a chart designed to
||reinforce beams
||with rebar, welded to the top flanges.
||
||My questions are:
||
||1. Am I on the right track using channels?
||
||2. Any better suggestions?
||
||3. Any Internet sites or literature that deals with this?
||
||4. Should I design the channel stitch welding for only the
||moment I expect
||channels to carry
||
||5. Will rebar really work?<<
||
||******* ****** ******* ******** ******* ******* ******* ***
||*   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) to the list, send email to
||*   admin(--nospam--at)seaint.org and in the body of the message type
||*   "join seaint" (no quotes). To Unsubscribe, send email
||*   to admin(--nospam--at)seaint.org and in the body of the message
||*   type "leave seaint" (no quotes). For questions, send
||*   email 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) to the list, send email to 
*   admin(--nospam--at)seaint.org and in the body of the message type 
*   "join seaint" (no quotes). To Unsubscribe, send email 
*   to admin(--nospam--at)seaint.org and in the body of the message 
*   type "leave seaint" (no quotes). For questions, send 
*   email 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 
******* ****** ****** ****** ******* ****** ****** ********