Truss Deflection

[2jdevine]

A brief overview:
I'm working on a project in which the client wants to move a existing column. The column supports two 100' trusses. I do not want to try and reinforce the existing trusses, so I have designed a new transfer truss to provide support at the column to be removed.
The new transfer truss will deflect, putting additional stress back into the existing trusses. To nullify/minimize this I would like to jack up the column to the calculated deflection prior to attachment, such that a net zero stress is sent back into the existing trusses.

My question is should I include any live load when calculating my deflection to cancel out?

 

[CCox]

Can you see if the two trusses can handle the deflection? Find the reaction of the two trusses on the column. Model transfer truss and apply reaction from the two trusses. Determine deflection at point of interest on transfer truss. Go back to original trusses and apply a spring to account for this settlement.

Should be pretty straightforward.

Cody

 

[KootK]

This is an interior support for continuous multi-span trusses, right? I'd be hesitant to balance more than the reliable dead load unless you're going to evaluate the existing truss members for potential load reversal. Additional member bracing might be required. For the most part, I'd hang your hat on making that new truss as stiff as possible.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[2jdevine]

Thanks for the quick replies.
Cody, that's is pretty much the exact process I'm using.
KootK, I share your hesitation about using live load in the jacking up deflection calc since its "not really there". However, I'm also trying to keep any induced stress (from the deflection) in the existing trusses below 5 percent, that way I can justify not having to touch/reinforce them.

I am thinking I may jack up the column to eliminate the dead load deflection plus a small amount of the live load deflections such that at no time during construction or after will the induced stress get above 5 percent.

 

[WillisV]

Sandwich the existing column between two new trusses and jack the ends of the trusses up until the existing column begins to lift up - then cut the remainder below out. The existing trusses see no additional deflection that way and there is no guessing about camber amounts etc.

 

[CCox]

So you've determined that the trusses are overstressed with the induced deflection? What is the deflection of the transfer truss?