existing overstresses
existing overstresses
(OP)
When doing an evaluation of an existing building, what is the maximum stress for members where you can say the member is still safe, I have heard that 5% to 10% overstresses are still ok. Is there any code reference in the United States regarding this issue?
Thanks
Thanks






RE: existing overstresses
RE: existing overstresses
New construction -- 5% overstress allowed
Verifying existing construction -- 10% overstress allowed
Justifying my own mistakes after something is already built -- 15% overstress allowed
Remember that loads are in general conservative, that structures are in general redundant, and that our methods of analyzing structures are generally oversimplifications of the actual behavior of structures.
If you are uncomfortable with allowing overstress, look for ways to justify the existing structure (e.g., are there other load paths you are not accounting for?).
DaveAtkins
RE: existing overstresses
1. Do you check it using code required loads at the time it was constructed or current code loads? The same questions could be asked regarding material stresses.
2. Do you make an assement of the actual loads the building is seeing and check it with those? If so, would this control over the code requirements? In otherwords, if the ACTUAL loads are less than code specified minimums, would you "pass" an existing building even if it didn't meet code loads? Would it then be appropriate to post load limits?
This all assumes the structural members are in good condition. If there is deterioration, then I would assume that some discount to orginal strength would have to be made.
RE: existing overstresses
All the calculation procedures to verify the adequacy of existing elements shall be based on current code.
Material strengths shall be based on the older stresses that were allowed at the time of construction. If it is not available, test some samples to get an estimate or use whatever stresses that are reasonable.
Especially in wood, the current allowable stresses are so low (due to the depletion of older forests and the available lumber today are from younger forests and are not as dense). When dealing with older wood structures, framing maybe overstressed even without adding new loads if checked with current allowable values.