As part of Chapter 27 in ASCE 7-10, Figure 27.4-8 shows the 4 different load cases that should be considered related to the wind loads. Cases 2 and 4 are the two cases that include torsional moment. Many engineers find that when they start considering all of these load cases, with wind blowing in each direction, they end up with a large number of load cases to consider. Moving next to load combinations, and the various iterations, so many combinations are created that it can be difficult and time consuming to solve the problem without using software.
In the September 2014 SE University Session, Designing for ASCE 7-10 Wind Loads – Main Wind Force Resisting System, Don Scott, SE, from PCS Structural Solutions, mentioned that in most cases, if the lateral frames are spaced at least 2/3 of the building width apart, the torsional load cases will not be the controlling load cases for the building. While this doesn’t mean that the load cases don’t have to be checked per the code, they often will not be the controlling load cases.
The question arises, however; are there situations where torsional load cases do not need to be considered? In the Question and Answer portion of the session, there were a few questions about the torsional load cases, asking for the source of the 2/3 of the building width frame spacing. Don directed engineers to ASCE 7-10 Appendix D, Buildings Exempted From Torsional Wind Load Cases. This Appendix lists 5 sets of conditions that a building can meet in order to be exempt from considering Cases 2 & 4. These conditions include:
D1.1 – One and Two Story Buildings (with qualifications)
D1.2 – Buildings Controlled by Seismic Loading (with qualifications)
D1.3 – Buildings Classified as Torsionally Regular Under Wind Load
D1.4 – Buildings with Diaphragms that are Flexible and Designed for Increased Wind Loading
D1.5 – Class 1 and Class 2 Simple Diaphragm Buildings (H = 160’ or less) (with qualifications)
In discussing the origin for the frame spacing of 2/3 the building width, Don pointed to Condition D1.5 Case B, where one of the requirements is that for a square building, “The distance between the two most separated lines of the MWFRS in each principal axis direction shall be at least 66% of the effective building width perpendicular to the axis under consideration.”
In a 4:34 video clip from the Q & A Discussion after the September session, available to SE University Clients through the SEU Resource Center, Don explains the origin of the 2/3 of the building width for frame spacing, and directs engineers to the information in ASCE 7-10 Appendix D.
SE Solutions would like to extend a thank you to Allen Adams, PE, SE, Chief Structural Engineer at Bentley Systems and Don Scott, PE, SE, Vice President/Director of Engineering at PCS Solutions for their contributions on this topic.