Service Wind Speed Drift ASCE 7-16 Load Combination
Service Wind Speed Drift ASCE 7-16 Load Combination
(OP)
In the commentary for appendix C of ASCE 7-16, recommendations are given to check drift under various MRI wind speeds. A load combination is given which has no load factor on wind. It suggests the ultimate design wind speed (MRI 700) is too conservative for drift considerations and one could use MRI 100, 50, 25 or 10 at the engineer's discretion.
In comparing a 0.6xW(MRI 700) asd strength load combination vs. W(MRI 100) for drift...the W(MRI 100) is larger?
Am I am missing something here...
In comparing a 0.6xW(MRI 700) asd strength load combination vs. W(MRI 100) for drift...the W(MRI 100) is larger?
Am I am missing something here...






RE: Service Wind Speed Drift ASCE 7-16 Load Combination
RE: Service Wind Speed Drift ASCE 7-16 Load Combination
The 0.7 is recommended per AISC design guide 3 for a 10-year recurrence interval and the 0.6 is because I design using ASD. Obviously if you design using ultimate loads you'd just apply the 0.7.
RE: Service Wind Speed Drift ASCE 7-16 Load Combination
By using the 0.6 factor to go from MRI 700 to MRI 100 wind, you're implicitly assuming that the difference in magnitude (speed, pressures, etc.) between MRI 100 and MRI 700 storms is the same everywhere in the country. It's not. Same goes for the 0.7 factor in AISC DG 3 and commentary to ASCE 7-05 Appendix C.
That's also why we went to ultimate. The 1.6 factor was intended to convert from like a 50 year return period at service levels to 700 year return at ultimate design, but that factor isn't actually 1.6 for the entire country (or even most of the country). So they just provide maps for the different return periods now and use a 1.0 factor for everything.
RE: Service Wind Speed Drift ASCE 7-16 Load Combination
RE: Service Wind Speed Drift ASCE 7-16 Load Combination
7-10: Cat II 110 mph 1.2D + 1.0W
7-16: Cat II 94 mph 1.2D + 1.0W
That's about a 27% drop in design q.