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steel retro fits (also, Multiple Yield Stresses)

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Reinforcing a column under load is a little tricky, but manageable.  There are several points to consider.
 
1.  What's this column carrying?  Usually at least some of the load is roof LL or snow, and it's usually not there.  Floor LL is trickier, but it's usually not at its design level.  Or maybe some of it can be removed temporarily.
 
2.  What was it originally designed for?  There are a lot of columns out there whose design load is half of their allowable load.  And that's before you consider incidental base fixity, or hit up the safety factor for a short term loan.
 
3.  How long is it?  If it's slender, a temporary brace will double its capacity.
 
4.  If after all this you're still worried, perhaps it's feasible to put up a temporary jacking post next to it.
 
 
Enough about load.  On to construction.
 
5.  In welding cover plates to columns, you don't need to make a continuous weld unless you've got severe fatigue concerns or you need to make it weathertight.  I strongly recommend a stitch weld, because the welding cost is a large percentage of the cost of a retrofit like this.
 
6.  AISC spec section E4 has some rules for stitch welding in such a situation, the most pertinent of which is to limit the kl/r of the plate to 75 percent of that of the gross column by limiting the stitch weld spacing.
 
7.  If you still want to do a continuous weld in 4 bites, you'll need 100 percent inspection and a big stick to make it happen.  Every time you leave the area to take a leak they'll lay 4 feet of continuous weld.
 
 
The analysis of a reinforced column has generated a lot of discussion.  The best methodology I've run across (Ted Galambos recommended it to me) is in an article entitled "Reinforcing Loaded Steel Compression Members" by jack Brown in the AISC EJ, 4th quarter 1988.  It is a little involved.  To Zachary Goswick's question of a week ago: access this article.
 
I haven't answered your original question, about the heat-affected zone and all.  We've gotten away with ignoring it.
 
Finally, after you've got your nice new column, where does it go?  Can the footing and underlying earth take it?  That's usually where the real problem is.
 
HTH
 
Mike Hemstad
TKDA
St. Paul, Minnesota
 
Robert M. Hanson wrote:
List,
I would like to get opinion on how best to approach adding area to a
steel column without unloading it. My thinking was to use small welds
and stitch maybe 3=94 @ 12=94 and go up the column. Then go with another =
3=94
@ 12=94 adjacent to original and fill in with the 3rd and 4th passes of =
3=94
welds. As far as evaluating how safe the column is during welding I am
wondering if I can justify the column by neglecting the heat effected
zone (how big is that) and checking the column. It=92s a fully braced
frame in both directions.=20

I will use strain compatibility to look at existing area vs. new area
and how much each will carry. My thinking is I will not be able to add
much strain so new plate will be at low allowable stress while column is
high. Any insight on the weld effects using this approach would be
appreciated.=20

Client intends to increase column loads after retro fit.

Robert M. Hanson,S.E.
Kappa Engineers
Carson, CA.
(310) 233-3800