Live Load Determination 2

[akastud]

All,

I serve on an ASTM committee and have been tasked with rewriting the loads and strength portion of our particular standard. Historically our standard has used Factors of Safety for determination of acceptance criteria (essentially borrowed from another standard). These FOS do not translate well to building codes, plan reviewers or even interpretation between similar engineers. It leaves interpretation very vague and not well applied. I have proposed that we convert to an LRFD/ASD approach where we can return to ASCE 7 for guidance without having to rewrite every piece and component. I understand the principals involved in both determining LRFD and ASD "code level forces", but I wonder how specifically to convert Live Load forces experienced in our industry into "code Level Forces" such that similar levels of safety are applied.

In general our live load forces are very unpredictable. Should we try and estimate some return level (say 50 year event) and make that our L level load for inclusion into LRFD or ASD load combinations? I know that is how they do it for wind or seismic, but I have never seen this discussion on Live Loads. For example is a 50 PSF office load a maximum design event or a 50 year return event? I believe once I know how the code level design foices are considered and determined, I can then make appropriate adjustments to our Live Load forces to consider them appropriately within current schemes.

Thanks for your help!

 

[RobertHale]

From what I remember, live loads utilize some survey information and then use a 2 dimensional spatial probability distribution as well as a probability function for the magnitude. This was one part of the loading class that exceeded my abilities in probability and statistics. There is a little information about the determination of occupancy live loads in the commentary of ASCE 7, as well as a number of references that will be far more specific on the mathematics and base data. Since you are an official committee you might consider retaining a true expert on the subject. Bruce Ellingwood is very knowledgeable and the last I checked he consults on this type of work. If he can't help you or point you toward someone that can I would be very surprised.

 

[JAE]

I would second the use of Bruce Ellingwood material (along with Galambos and others).

RobertHale is right that the use of load factors with strength reduction factors, along with establishing an overall ASD safety factor, all depend on the statistical variability of the loads as well as your ability to zero in on the actual resistance (phi factor) as well.

I used much of Ellingwood's work during my graduate days trying to establish a phi factor for use in composite metal deck slabs.
A lot of Ellingwood's work that I found was in the ASCE Structural Journal.



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