A good question.
As background Strength design (LRFD) was developed in the 1960s and 70s as an alternative to ASD which would provide both a more rational basis for design, and also more uniform reliability of structures. When it was developed, the load and resistance factors used in the Strength load combinations were calibrated against the reliability obtained from the ASD provisions of the day, as applied to typical structures. Today the intended reliabilities are memorialized in ASCE 7 Tables 1.3-1, 2, and 3. When adopting new loading maps for environmental hazards, the ASCE 7 committee conducts rigorous reliability studies to confirm that the desired reliabilities will be obtained. For those interested, the basis for Strength design can be found in National Bureau of Standards publication NBS 577. That publication recognized that because the load factors were calibrated against the design of structural elements having typical ratio of Dead Load to Live Load, Snow Load, Wind Load, Earthquake Load, etc.; for structures or (structural elements) having different ratios, the ASD load combinations would not provide the same reliability, sometimes more, sometimes, less.
ASCE 7 adopts a series of ASD Load Combinations that are different from the traditional ASD combinations, in which a 1/3 increase in allowable stress is permitted. The 1/3 stress increase traditionally permitted by the code has been rationalized as acceptable for two different reasons: 1) some transient loads (wind, earthquake) are of short duration, and that some structural materials can be tolerant of these "short term" loads without adverse affect at higher stress levels than is acceptable for long term loading. Examples include wood, with its well-known duration factors. Another are clay soils, where allowable foundation bearing pressures are determined based on considerations of consolidation (a time-dependent phenomenom) and limiting settlement, rather than bearing failure, and which can tolerate short term loads without adverse settlement. A second "excuse" for the 1/3 increase is that it is unlikely that the maximum value of true transient loads will occur at the same time. For example, it is unlikely that maximum live load will occur in combination with maximum wind load. Rather than allowing a 1/3 increase for such load combinations, the ASCE 7 ASD load combinations permit a reduction in the value of transient loads (.75 factor) when two or more transient loads are considered in combination.
Recently the AISC Committee on Specifications rediscovered that for structures with atypical Dead Load to Live Load ratios, ASD and LRFD did not provide the same safety. Further, the AISC Committee found that for structural elements subject to wind and seismic loads, structures designed to the ASCE 7 ASD load combinations, could have demand to capacity ratios as large as 1.3 when evaluated using the LRFD combinations. This was deemed unacceptable. AISC put substantial pressure on the ASCE 7 committee to update its ASD load combinations to reduce this incompatibility between ASD and LRFD safety. After several years of work, the ASCE 7 Committee has approved, for the 2028 edition of ASCE 7, new ASD load combinations that have a format similar to the LRFD combinations and result in much closer compatibility in t. he designs derived from ASD and LRFD. This does not affect the alternative load combinations contained in the IBC and also the Mass. Code. NCSEA recently put forward a successful proposal to ICC to withdraw the alternative ASD load combinations from the IBC. This will appear in the next edition. The Mass Code follow suit, by withdrawing the load combinations.
Now, you may ask, why 2/3 of the LRFD combinations.? If you are familiar with the 2025 AISC 360 specification, you will recall that AISC defines the "allowable strength: of steel members as the LRFD strength divided by 1.5 (the Omega factor). Well, if you use a loading defined by 2/3 of the LRFD load combinations and an allowable strength equal to 1/1.5 times the LRFD strength, you will get identical design solutions from ASD as you do LRFD. This same "trick" does not work with other materials, notably wood, and masonry, but it is close. There was actually a proposal to make this same change to the ASCE 7 combinations, however, because of the material inequality, and instead elected to use a different approach. This will no doubt make its way into the Mass Code (as well as other US codes) with the next IBC adoptions.
For those who are concerned about the loss of the 1/3 allowable stress increase, please keep in mind that the Basic ASD Load Combinations to permit increased allowable stress when this is justified by material behavior. This will typically be the case for foundation bearing pressures, where the allowable pressure is limited by considerations of settlement. Geotechncial enginees will have to include recommendations as to the appropriate increase in allowable pressures for use in design, based on site specific conditions.
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Ronald Hamburger, SE
Consulting Principal
Simpson Gumpertz & Heger
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Original Message:
Sent: 12-02-2025 11:16 AM
From: Chad Morrison
Subject: MA and other State modifications to ASCE 7
For the latest Massachusetts State Building Code, ASD load combinations from ASCE 7 are to be modified.
https://www.mass.gov/doc/10th-edition-chapter-16-structural-design/download
1605.1 Add the following lines before Exceptions
For allowable stress design of structural steel in buildings and other structures, allowable stress design load
combinations from ASCE 7, Section 2.4 that include the effects of wind or earthquake loads shall not be used. Instead
two-thirds (2/3) of strength design load combinations from ASCE 7, Section 2.3 that include the effects of wind or
earthquake loads shall be used.
While this might be a better question for state regulators, I would like to open up this topic to ASCE member input. We know that code is the minimum and should any designer or regulatory agency wish to exceed code, that is certainly permitted. I recall previous versions of IBC permitting an increase of one third for ASD capacities to account for LRFD vs ASD discrepancies.
What is the basis for this adjustment made by Massachusetts? How does this square with IBC and other states? As we increase our reliance on computer models and processing power... do we need more load combinations or less?
ASD remains useful in checking deflections (serviceability checks). LRFD appears to allow for more precise results accounting for types of loads and materials (strength checks). The codes will likely follow ASCE/SEI's lead on adopting any necessary changes. Does ASCE consider accounting for jurisdiction modifications to ASCE 7 when preparing the next edition? Is there any hope for a unified method in the future?
Thank you!
#ASCE7
#ASCE7-22
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Chad Morrison P.E., F.ASCE
Professional Engineer
Greenville RI
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