Service Wind Speed Drift ASCE 7-16 Load Combination 1

[engjg]

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...

 

[MotorCity]

Unless your client has asked for tighter drift control, I would design for the required drift based on the code governed wind speed (and whatever MRI is associated with that windspeed). The required drift could be based on the serviceability requirements of the building fascade, client's operational requirements, aesthetics, etc

 

[bearjew]

I'm not sure about ASCE 7-16, but in 7-10 I usually design for drift by using (0.7)(0.6)=0.42 for my wind load factor.

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.

 

[MrHershey]

What you've stumbled upon is one of the primary reasons they went to the different maps in ASCE 7-10.

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.

 

[Hokie93]

For a Risk Category II structure (700-year MRI), the effective mean recurrence interval (MRI) after multiplying by 0.6 is 50-years. So, yes, you should expect a greater wind pressure associated with a 100-year MRI wind speed. That, however, begs the question as to why you are using a 100-year MRI wind speed to check building drift. Is your structure very sensitive to drift? For non drift-sensitive structures, I believe it is common to check drift against a 10-year or 25-year MRI wind speed.

 

[ElliottJames]

Speaking of 7-16 wind speeds, I'm somewhat amazed at the drop in basic design speeds on the west coast. Using Fresno, Calif. as an example:

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.