Temporary Structures and LRFD Equations
Temporary Structures and LRFD Equations
(OP)
I am an EIT and am wondering whether the LRFD Equations which add "safety factors" to dead, live, wind, and other typical loads are generally used in determining the critical loadings for temporary structures (Bridge Falsework, Pile Cap Forms, temporary retaining walls, and the like). I have yet to find anything requiring the use of LRFD Equations(such as 1.2D + 1.6L + 0.8W) in the specs of the jobs that I am looking at nor in the referred to design manuals.
Is it general practice to simply determine unfactored loads and at the end see if the stresses produced by these loads are lower than so-called "allowable stresses" or that the stress are only a portion of the Ultimate Stresses (application of a safety factor)? This would be what I understand to be Allowable Stress Design, which I was under the impression is being gradually replaced by the LRFD Strength Design approach. Is the LRFD only for permanent structure design?
Is it general practice to simply determine unfactored loads and at the end see if the stresses produced by these loads are lower than so-called "allowable stresses" or that the stress are only a portion of the Ultimate Stresses (application of a safety factor)? This would be what I understand to be Allowable Stress Design, which I was under the impression is being gradually replaced by the LRFD Strength Design approach. Is the LRFD only for permanent structure design?





RE: Temporary Structures and LRFD Equations
However, many code committees are moving to address temporary loading conditions.
A good start in my mind is ASCE Standard SEI/ASCE 37-02 - Design Loads on Structures During Construction.
This documnet lists load combinations and load factors appropriate for falsework for construction, though more geared towards construction of buildings not as much bridges, though structural formwork for buildings is not that far removed from formwork for bridges.
As far as referring to load factors as "safety" factors, the load factors are based on statistical data amassed over the years and give a weight to the probability of the varience of a load case. A safety factor is not the same animal.
Good luck, and keep posting.
Daniel Toon
RE: Temporary Structures and LRFD Equations
RE: Temporary Structures and LRFD Equations
1. Often the materials in temporary structures are reused many time. Say a beam used in falsework; during it's life holes have probably been cut into it, maybe items welded to it and removed. If this beam had an allowable bending stress of 24 KSI when new, the engineer may choose to reduce the allowable value - to maybe 20 KSI or so. This would be done to take into account the beams actual conditon.
2. Temporary structures may be constructed using materials in ways that were never intended. For example, several years ago we successfully used PZ-27 steel sheet pile as beams in a temporary tressel for a crawler crane.
You probably could make the necessary caluclations for situations like these with LRFD, but with ASD it seems to be more of a "direct" approach.
RE: Temporary Structures and LRFD Equations
ignoring safety standards.