Basic Wind Speed - 90 mph or 115 mph

[JohnnyB_]

Hello -

I am trying to calculate wind pressures and I am confused on the basic wind speed as noted in ASCE 7-10 Chpt 26. Per Figure 26.5-1A for Risk category II (residential) structures the design 3-second gust is 115 mph. This is self explanatory to this point.

Where it gets confusing is the local county code states, "Wind Load: The minimum basic wind speed for determining design wind pressure at any site shall not be less than 90 miles per hour, 3 second gusts sustained (115 ultimate)."

Per the county's direction am I to use 90 mph or 115 mph for the basic wind speed, when calculating wind pressure per Chpt 27 of ASCE 7-10?

Thanks in advance.

 

[ChorasDen]

Curious why you are limiting your panel edge nail spacing to 4" on center? Perforated can be analyzed at up to 2" on center panel edge nailing. Might give you what you need for this analysis?

Edit: Detailing requirements may change, such as double studs at panel edges. Also not certain an LTTP would be sufficient, may need to go with a heavier hold down.

 

[phamENG]

Rod - that 90mph is 3-second gust, not fastest mile. Fastest mile hasn't been in the building codes since...7-93? I think it was 1993. Somewhere around then. The 90 to 115 is purely a statistical change from a 50 year MRI to a 700 year MRI for the same wind measurement.

I usually try to limit my shear walls to 3" spacing. 2" spacing requires using 3x or double 2x studs. I don't have a lot of luck with framers following exotic shear wall plans.

OP - is this building in Colorado? If so, you should be using the 2021 SDPWS, right? Building Code of Colorado is based on the 2021 IBC which references the 2021 SDPWS and the 2018 NDS. I don't practice there, that's just what Google told me.

 

[phamENG]

You'd also do well to look into Force Transfer Around Openings. I've found that to be a lifesaver and has stretched what I can do with wood framing well beyond the old segmented and perforated designs.

 

[BridgeSmith]

Rod - that 90mph is 3-second gust, not fastest mile. Fastest mile hasn't been in the building codes since...7-93? I think it was 1993. Somewhere around then. The 90 to 115 is purely a statistical change from a 50 year MRI to a 700 year MRI for the same wind measurement.

Hmm...just an interesting coincidence, then.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[JohnnyB_]

ChorasDen -
Attached is the calc at 2" but which exceeds the 490 plf of shear resistance, so it wouldn't be an applicable consideration for the PSW design. It also requires double 2x's at all sheathing edges, which creates a pain in the ass for the builder and owner.

phamENG -
I am in CO. The codes I have to follow are the 2018 IRC and IBC depending on the zoning. I can't find a 2018 SDPWS and I don't think they published one. Are there major differences from the SDPWS 2018 to 2021?

 

[ChorasDen]

I can't find a 2018 SDPWS and I don't think they published one. Are there major differences from the SDPWS 2018 to 2021?

2018 SDPWS does not exist, SDPWS is on a 6 year code cycle, similar to ASCE 7.

 

[JohnnyB_]

Thanks for the clarification ChorasDen.

I found the APA Force Transfer Around Openings document and will use that to calculate the shear wall capacity.