jar546
Forum Coordinator
Since 2000, the IBC has contained three separate groups of load combinations, including the following: (1) Strength Load Combinations (1605.2), (2) Basic Allowable Stress Load Combinations (1605.3.1), and (3) Alternative Allowable Stress Load Combinations (1605.3.2). Two of these, the Strength Load Combinations and Basic Allowable Stress Load Combinations, are transcribed directly from an earlier edition of the ASCE 7 Standard. The third set of combinations is a legacy from the codes that predate the IBC.
This proposal is intended to remove minor discrepancies in requirements between the IBC and ASCE 7 Standard versions of the Strength and Basic Allowable Stress Load Combinations by eliminating the duplication of this material from the IBC. Further, it is intended that the removal of the duplicative Strength and Basic Allowable Stress Load Combinations from the IBC will reduce the likelihood of design errors that many engineers have been making when applying the Basic Allowable Stress Design Load Combinations.
The Alternative Allowable Stress Design Load Combinations permit the use of a 1/3 increase in allowable stresses when evaluating Load Combinations containing short-term transient loads including wind and seismic. The Basic Allowable Stress Combinations do not permit this but instead apply a factor of 0.75 to the transient loads including live, snow, wind, and seismic, when more than one of these loadings is considered simultaneously.
The ASCE 7 Load Combinations further permit increases in allowable stresses only when the material, such as wood, has increased available strength under short-term loading, as opposed to long-term loading. These further increases are not intended to be used for the design of masonry, concrete, or steel structures when using the Basic Allowable Stress Design Load Combinations because the strength of these materials does not have significant duration dependence. Unfortunately, and despite specific commentary within the IBC to discourage this, many engineers routinely apply the 1/3 increase to all allowable stresses when designing using the Basic Allowable Stress Design Load Combinations. This creates a potentially dangerous situation in which the safety margins of structures designed in this manner are substantially reduced.
By removing the transcription of the ASCE 7 Load Combinations from the IBC, in addition to avoiding duplication of nearly identical material, we expect to reduce the likelihood that design engineers will misapply the 1/3 increase factor applicable to the Alternate Allowable Stress Design Load Combinations. With the approval of this proposal, the IBC will point to ASCE 7 for the Strength and Basic Allowable Stress Design Load Combinations, where there is no mention of the 1/3 increase factor. The Alternate Allowable Stress Design Load Combinations will remain in the IBC with the permissible 1/3 increase.
It is important to note that this proposal does not result in any substantive technical change as all Load Combinations presently used by engineers will remain available to them including the exceptions for flat roof snow loads in combinations with seismic loads. The requirement that engineers reference ASCE 7 to determine the Strength and Basic Allowable Stress Design Load Combinations is not burdensome to engineers as they already must reference ASCE 7 to compute the values of the various loadings required by the load combinations for design.
This proposal is intended to remove minor discrepancies in requirements between the IBC and ASCE 7 Standard versions of the Strength and Basic Allowable Stress Load Combinations by eliminating the duplication of this material from the IBC. Further, it is intended that the removal of the duplicative Strength and Basic Allowable Stress Load Combinations from the IBC will reduce the likelihood of design errors that many engineers have been making when applying the Basic Allowable Stress Design Load Combinations.
The Alternative Allowable Stress Design Load Combinations permit the use of a 1/3 increase in allowable stresses when evaluating Load Combinations containing short-term transient loads including wind and seismic. The Basic Allowable Stress Combinations do not permit this but instead apply a factor of 0.75 to the transient loads including live, snow, wind, and seismic, when more than one of these loadings is considered simultaneously.
The ASCE 7 Load Combinations further permit increases in allowable stresses only when the material, such as wood, has increased available strength under short-term loading, as opposed to long-term loading. These further increases are not intended to be used for the design of masonry, concrete, or steel structures when using the Basic Allowable Stress Design Load Combinations because the strength of these materials does not have significant duration dependence. Unfortunately, and despite specific commentary within the IBC to discourage this, many engineers routinely apply the 1/3 increase to all allowable stresses when designing using the Basic Allowable Stress Design Load Combinations. This creates a potentially dangerous situation in which the safety margins of structures designed in this manner are substantially reduced.
By removing the transcription of the ASCE 7 Load Combinations from the IBC, in addition to avoiding duplication of nearly identical material, we expect to reduce the likelihood that design engineers will misapply the 1/3 increase factor applicable to the Alternate Allowable Stress Design Load Combinations. With the approval of this proposal, the IBC will point to ASCE 7 for the Strength and Basic Allowable Stress Design Load Combinations, where there is no mention of the 1/3 increase factor. The Alternate Allowable Stress Design Load Combinations will remain in the IBC with the permissible 1/3 increase.
It is important to note that this proposal does not result in any substantive technical change as all Load Combinations presently used by engineers will remain available to them including the exceptions for flat roof snow loads in combinations with seismic loads. The requirement that engineers reference ASCE 7 to determine the Strength and Basic Allowable Stress Design Load Combinations is not burdensome to engineers as they already must reference ASCE 7 to compute the values of the various loadings required by the load combinations for design.