When equipment anchorage on the roof in Miami area is designed using ASD method, wind loads in the ASD load combinations, are the loads calculated based on ASD wind speed or ultimate wind speed?
It appears calculation using ASD wind speed will result in wind pressures that are much less than that based on earlier code editions since 0.6 is used twice (one is in converting ultimate wind speed to ASD wind speed, the 2nd is in load combinations).
Interpretation #8496
Published January 19, 2021
Original Request
- Code Version
- 2010
- Code
- BUILDING
- Code Description
- Chapter
- 16
- Section
- 1605.3.1
- Topic
- Basic load combinations
Question
Answer Published 01/19/2021 at 6:57 AM
The loads are calculated using the ultimate wind speed (V(ult)). This results in the nominal, unfactored loads.
Section 1609.1.1 "determination of wind Loads, state: "The wind speeds in Figures 1609.3(1), 1609.3(2) and 1609.3(3) are ultimate design wind speeds, Vult, and shall be converted in accordance with Section 1609.3.1 to nominal design wind speeds, Vasd, when the provisions of the standards referenced in Exceptions 4 and 5 are used."
The 0.6 factor is used only once, when the nominal load is factored to provide the ASD values.
Section 1609.1.1 "determination of wind Loads, state: "The wind speeds in Figures 1609.3(1), 1609.3(2) and 1609.3(3) are ultimate design wind speeds, Vult, and shall be converted in accordance with Section 1609.3.1 to nominal design wind speeds, Vasd, when the provisions of the standards referenced in Exceptions 4 and 5 are used."
The 0.6 factor is used only once, when the nominal load is factored to provide the ASD values.
Commentary
For future editions, ASCE 7-16 uses the term ‘nominal’ for the unfactored loads used in the loading combinations, either for Strength Design (Limit States Design or Ultimate Strength Design are other terminologies for this) or for Allowable Stress Design.