I’m doing a basement remodel where I’m replacing some existing basement windows on the walkout side. When I pulled the old windows, it was obvious that the old header was sagging (about a 60″ RO with a 2 x 8 box header). There is also a corresponding dip in the floor above, about 5/8″ in 6 ft.
Now, my preference would be to pop a beam under the floor joists, jack it up until the joists are even again, and rebuild the header. Of course this may result in some cracks in the plaster or ceilings in the main floor above. At a minimum, I’m recommending temp walling under the floor joists, and installing a new header tight to the lowest joist (and shimming the rest). At least this would stabilize things and may eliminate the risk of cracks above.
My question, as much for pure curiosity as anything else, is this…
When a wood member deflects, the top fibers are put in compression and the bottom fibers are put in tension. Does it become increasingly difficult to cause further sagging the more a member has been deflected? Or will a piece of wood just continue to defect under the same load until it breaks?
Replies
it wil slow with time, but will continue.
It is not likely to break, unless the load above is extreme and there are wicked knots in the header material.
Odds are 50/50 whether jacking thaat back straight will cause any finish cracking above.
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Does it become increasingly difficult to cause further sagging the more a member has been deflected? Or will a piece of wood just continue to defect under the same load until it breaks?
There is a linear relationship between applied load and deflection in simply loaded beams. That is, if 100 pounds of loading causes a 1/16" deflection, 200 pounds loading will cause an 1/8" deflection, etc. To my knowledge, there is no point of loading at which the beam becomes even more resistant to bending beyond this linear relationship.
With metal beams, there is a point at which the beam becomes permanently deflected and won't "spring back" after the load is removed. I don't know if wood behaves the same way.
With steel, there comes a point of catastrophic collapse with little prior warning, but with wood, you generally observe gradually increasing deflection, checking, cracking, etc as warnings.Wood will also "learn" a degree of deflection - one reason why we install with crown up on floor joists and roof rafters. The cells of the wood slip or creep. They are green and pliable at first, then cure and harden to the learnt location as time goes on.
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"Or will a piece of wood just continue to defect under the same load until it breaks?"
No. Otherwise there wouldn't be a wood building standing over 200 years old.
All beams deflect to some degree, we can just try to keep it minimal or so that we don't notice.
Runnerguy