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Re: Steel Beam Reinforcing

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A steel beam that is overstressed by 40% will require some creative and
"heavy duty" reinforcing, unless you shore/jack the beam to relieve
existing stresses.

First, your options...some possiblilities for reinforcing steel beams:

1) Weld plate to bottom of bottom flange (plate can either be wider than
the bottom flange which requires a standard weld or the plate can be
selected so as the flange is wider which would require overhead welding) 
2) Weld WT to bottom of bottom flange
3) Weld plates to bottom of bottom flange and bottom (or top if
accessible) of top flange
4) Weld angles to top of bottom flange & web and to bottom of top flange &
web
5) Weld channel to bottom of bottom flange.
6) Combinations of the above.

The different options are affected by access, clearence requirements (i.e.
welding a WT to the bottom of a beam may interfer with head room), amount
of overstress, type/position of weld desired, and other factors.
Unfortunately, sketches showing the above information would be more
useful.  I have seen or used most of the above options.

One other item impacting the selection can be the stiffness of member
selected to reinforce the beam.  A plate welded flat to the bottom of the
beam is much easier to install when matching the deflected shape of the
beam than say a WT. 

In general you want to add you material as close to your flanges as
possible (recall the basic of flexural stresses...fb=My/I...the further
from the "centroid" the material is the greater the resistance to flexural
stresses).  The above option all accomplish this basic trait.  Another
tip is to TRY (will not always work) to add the reinforcing symmetrically.
If the material is not symmetric about the major access, the neutral axis
will shift for the composite section.

As others mentioned, when designing the reinforcing, the combined section
must be designed for "residual" stresses in the beam due to non-removable
(dead) loads based upon the ORIGINAL cross section.  The additional
stresses due to removable (maybe some dead load, live load and all new
load) loads will be based upon the NEW cross section.  Remember to check
the stresses in the original beam (there will be stresses in the original
beam section due to both non-removable and removable loads).  The
exception to this is IF the decision is made to jack the beam while the
reinfocing is installed.  This will effectively "remove" all or most
(depending on the level of jacking selected) existing stresses.

The amount/size of the weld for attaching the reinforcment to the beam is
determined by shear flow (v=VQ/I).

Hope this helps,

Scott Maxwell, PE, SE

On Wed, 19 Jul 2000, 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?
> 
> TIA
> Randy Diviney
> Structural Engineering Dept.
> Hayes Large Architects
> Altoona, Pa
> (814)946-0451
> 
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