increase in allowable stress due to duration

huangyhg

increase in allowable stress due to duration
aub,
we have done similar adjustments for temporary shoring situations as you describe.  what we always do, however, is adjust the applied soil load, not the allowable stresses in the steel or wood.
we normally check with a geotechnical engineer who is familiar with the types of soil that exist behind the shoring and get a somewhat smaller equivalent fluid pressure to use.  the concept is that the build-up of lateral earth pressure would not occur immediately but require a long-term condition.  be careful, however, to make sure that the contractor keeps water runoff from spilling or draining into the material behind your wall.
for many years i have argued strongly against using any overstress for all temporary work.
i have no problem with jae's approach that uses a realistic short term load (similarly for wave, wind, earthquake risk), but...
fatal accidents resulting from failure of temporary work are still all too common (with cofferdams being well represented in the statistics).
good points, austim,
your point is very well taken in that the majority of structural collapses occur during construction.   one visit to any jobsite does a lot for an engineers education in what really gets built.
one point to keep in mind is that a 15 prcent duration factor represents a period of two months during which the wood is fully loaded. if the project lasts, or can be reasonably expected to last, any longer, this factor should not be included.