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Re: steel retro fits

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>I would like to get opinion on how best to approach adding area to a
>steel column without unloading it.
Not with a couple of numbers on the back of an envelope--you need to do a 
careful incremental load analysis. You can probably calculate the column 
response, but I bet it'd take some fairly complex FEA. The added area 
starts out unstressed (except for weld residual stress) with the 
remainder of the structure carrying service loads. You could then 
increment the structure loading and see how the loads get redistributed. 
Then you'd have to figure the limit load of the modified column under the 
redistributed service + incremental load up to plastic collapse. I think 
it's doable but it's complicated. It's something like the problem of 
figuring out the actual stress state as dead load increases during 
construction--maybe techniques and software exist for this problem that 
you could use. 

My guess is that the added area won't pick up much load unless it's 
extremely stiff with respect to the original column and that practical 
modifications won't change the column limit load much. The stress in the 
modified column won't be anything like the assumed stress state for the 
original construction, but if you don't assess the actual stress state is 
you won't know if the column is properly reinforced for the design load. 

You won't really know much about the welds you need until you know how 
much load the added material picks up. You may only need small welds 
because the added material isn't picking up much load. The weld residual 
stress isn't going to affect things much _except_ for heat-treated steel. 
(A modification like this with QT steel isn't very smart based on the 
warning placards on truck chassis structural members.) In carbon steel 
HAZ properties mostly impact fracture and fatigue resistance. Weld 
residual stresses are present in every welded structure, but they shake 
down to elastic response after a couple of load cycles.

Christopher Wright P.E.    |"They couldn't hit an elephant at
chrisw(--nospam--at)skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw


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