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RE: Steel Beam Reinforcing

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Bill,

If your missing it, then I am too.  ;-)

You method is that same as how I was taught.

Scott Maxwell, PE, SE


On Wed, 19 Jul 2000, Bill Allen wrote:

> 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?<<
> ||
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