Christian,

As you are likely aware, the road to LRFD has been a long one, with some recent updates. The road starts back in the 1960s, when ACI introduced "Strength" design as an appendix in 318-64. In 71 this became the primary method and ASD moved to an appendix. Steel was next, in the 1970s with some substantial help from the National Bureau of Standards (now NIST). Movers and shakers in this effort were Bruce Ellingwood, Allin Cornell, and Ted Galambos, but others participated as well. NBS published a recommended strength basis for ANSI A58.1 (the precursor to ASCE 7) in June 1980 as SP577 - "Development of a Probability Based Load Criterion for American National Standard A58." This seminal work is likely the best reference for the underlying basis of LRFD. It states the underlying theory, the formulations of load and resistance factors, and the assumed distribution types and parameters for various loads as well as resistances. The important thing to note is that the "nominal" load values (e.g. 100 psf Live Load in corridors) predated this and were the ones historically used. The load factors established in the load combinations were selected to calibrate the designs achieved by LRFD and ASD, the assumption being that ASD presented an acceptable design in most cases, and the nominal values were ones the profession had grown comfortable with. For the case of structural steel, the calibration was based on producing the same size beam under dead and live loading (of assumed magnitudes) as one would obtain from ASD. Ted MK (Ravi) Ravindra and Ted Galambos published a paper on this in the structural journal in 1978.

Initially the load factor on seismic loads was taken as 1.4, again calibrated against ASD seismic design of the era (which was based on the SEAOC/UBC provisions). In 1998, ASCE 7 adopted a new seismic hazard model based on MCE shaking (similar to what is used now). It was reasoned that the uncertainty in ground motion was so large that it was hard to justify a specific load factor on seismic, so 1.0 was taken. Using the time proven calibration, the values of the MCE and DE shaking parameters were selected to calibrate with Zone 4 designs in the 1994 UBC. In ASCE 7-10, the seismic hazard model was again adjusted so that rather than using 2,500 years as the mean return period for MCE shaking, the MCE return period was adjusted so that with a load factor of 1.0 on seismic, the risk of collapse would not exceed 1% in 50 years, given an assumed structural fragility.

Also in ASCE 7-10, the wind load maps were changed from 50 year MRI to the Risk Category based maps we have today. It was decided to use a load factor of 1.0 on wind, and the MRI for the new maps was selected so as to produce the reliabilities indicated in ASCE 7 Section 1.3. Note these are not really "ultimate" or "strength" loads. They are just the loading that produces the desired target reliability. Due to uncertainty in material strength, engineering abiltiy to compute wind loads and other factors, there is signifciant probability that a structure designed for these wind speeds would not fail.

There present ASCE 7-22 committee expressed no interest in converting the Live loads to a "strength basis". Factors include the "familiarity and comfort" levels cited previously. There has been discussion that the live loads specified in ASCE 7 may need updating for many occupancies given that the present loads were set to typical furnishings and equipment present in typical buildings many years ago. There have been some proposals to undertake the statistical studies necessary to do this, but no funding has yet come forward.

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Ronald Hamburger P.E., F.SEI

Principal

Simpson Gumpertz & Heger

Oakland CA

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Original Message:

Sent: 07-21-2022 04:57 PM

From: Christian Parker

Subject: Ultimate Live Loads at 1.0

The road to LRFD is slow and arduous, but we're almost there with updated maps and 1.0 load factors for wind, snow (as of 7-22), and seismic. I assume ultimate rain loads are around the corner.

Now, dead load is an actual estimation of weight, so the 1.2 and 0.9 factors are important. But live loads are straight out of the code, and would be the easiest to update. Just multiply all of Table 4-1 by 1.6, or revisit those numbers if they're not giving a consistent risk of failure anyway. They're probably not--and if we're embarrased to put the number 160 psf (equivalent to 30 inches of standing water) next to the word "corridors", well, maybe we should be. (In case you can't tell, I'm fishing for a defensive Subcommittee member with these barbs: I know you're out there.)

Load combinations in ASCE 7-22 are:

1.4D

1.2D + 1.6L + (0.5Lr or 0.3S or 0.5R)

1.2D + (1.6Lr or 1.0S or 1.6R) + (L or 0.5W)

1.2D + 1.0(W or Wt) + L + (0.5Lr or 0.3S or 0.5R)

0.9D + 1.0(W or Wt)

With ultimate-level rain and live load, combinations would be far more consistent:

1.4D

1.2D + 1.0L + 0.3(Lr or S or R)

1.2D + 1.0(Lr or S or R) + (0.6L or 0.5W)

1.2D + 1.0(W or Wt) + 0.6L + 0.3(Lr or S or R)

0.9D + 1.0(W or Wt)

Are there plans to update live loads to ultimate in ASCE 7-28? Is there any inherent meaning to the existing floor loads such that the 1.6 factor has significance? Eager to hear any and all thoughts on this topic.

#ASCE7

#ASCE7-22

#LoadCombinations

#LiveLoads

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Christian Parker EIT, P.E., A.M.ASCE

Structural Project Engineer

Washington DC

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