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Steel Interchange


Window Washing Davits
Do you have any info on the design of davits for window washing equipment? I understand there is a required factor of safety of 4. How does this work when using the LRFD load combinations?

Answer
In my experience, most erection and window washing support evaluations historically have be made using unfactored load assumptions, and then a higher safety factor is applied than defined for the AISC Specification limit states. Since there is a consistent relationship between Ω (the safety factor used in ASD) and  (the resistance factor used in LRFD), it is possible to use LRFD if you wish with some engineering judgment and numerical manipulation.

Remember that the two load approaches are equilibrated when the live load to dead load ratio is 3.0, resulting in the relationship wherein Ω = 1.5/. If your case is such that the live load is much more than 3 times the dead load, you may need to rederive this relationship. See the Commentary to Section B3.4 in the AISC Specification.

 Kurt Gustafson, S.E., P.E.

Posted on August 1, 2010


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