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 Dead Load vs Live Load Cosideration

  • ASCE Standards
  • Dead Loads
  • Live Loads
Justin Fields's profile image
Justin Fields posted 12-29-2022 05:15 PM
I was hoping to connect with someone when considering the Dead Load vs Live Load cases and get more clarity on items not expressly stated in the two designations. I often come across having a moving load that is almost always present, and I'm trying to get clarity on the code intent of whether this is to be considered dead load (though its moving, it is mostly constant) or live load (which per code is everything else that is not DL, WL, EQ, etc).  When considering the reductions to be had with the load combinations (namely 0.75 LLr and 0.75 LL) I think this is overly liberal in the approach in our case.  However, I cannot find justification in the code language that states it as such.  
James Williams's profile image
James Williams posted 12-30-2022 04:41 AM
In the early pages of ASCE 7 you find text that says "ASCE does not intend, nor should anyone interpret, ASCE's standards to replace the sound judgment of a competent professional, having knowledge and experience in the appropriate field(s) of practice, ... ."

In my professional opinion, documenting your justification is part of the process when wisely deviating from a standard, guide or code. Some of the greatest challenges are the undocumented deviations that, upon first glance, appear incorrect and resources are then spent coming to the same conclusion. A few minutes to document can save hours of manhours and minimize delays. 

I look forward to reading the feedback on this discussion and 1) whether selection of dead vs live loading is motion and/or time dependent; and 2) can it be a DL locally (structural component) but a LL globally (other structural elements of an structural assembly periodically loaded).
Christian Parker's profile image
Christian Parker posted 01-04-2023 02:37 PM
Hello Justin,

I always consider a moving load to be live, but to James's point I would be hesitant to take advantage of a reduction for a live load that will be present concurrently with a full design floor or roof load.  For the 0.75 reduction, are you referring to ASD load combination 4 in ASCE 7, and do you find that it governs over 1.0D + 1.0L?

If so, commentary section C2.4 (ASCE 7-16) may be helpful:

"Most loads, other than dead loads, vary significantly with time.  When these variable loads are combined with dead loads, their combined effect should be sufficient to reduce the risk of unsatisfactory performance to an acceptably low level.  However, when more than one variable load is considered, it is extremely unlikely that they will all attain thier maximum value at the same time (Turkstra and Madsen 1980).  Accordingly, some reduction in the total of the combined load effects is appropriate.  This reduction is accomplished through the 0.75 load combination factor.  The 0.75 factor applies only to the variable loads because the dead loads ... do not vary in time."

I hope this helps.

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