Temporary Structures and LRFD Equations

[CivilEmery]

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?

 

[DTGT2002]

Temporary loadings as well as most falsework are not well defined in most codes, standards, and design books.

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

 

[ishvaaag]

I would say that safety factors in general should be the same than for the final construction. I say this because in the end the work site is not but a place of work for many people, and really statistics show one more dangerous even than the finished place. Hence I don't see why we should intently reduce the safety factors for items or structural systems at the construction sites, and even less for those that are worrying enough as to deserve -even by our present practices- a separate check. But of course this is only one -my- opinion.

 

[SlideRuleEra]

I consider LRFD to be at it's best when used to design new permanent structures, from scratch. For temporary structures, ASD seems to offer advantages:
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.

 

[DTurbojetG]

Scaffolds and workstands have an OSHA requirement for a safety factor of 4. I have seen more films of engineering "disasters" during the construction phase than when completed. Usually, someone rushed and cut corners,
ignoring safety standards.