Area Light Pole Wind Zone

[EngrSTR]

We are designing a natural gas compressor station using Risk Category of IV per ASCE 7-10, wind speeds for the area is 120 mph. Per ASCE 7-10 wind speeds are 120 mph, but per AASHTO 2013 it is 90 mph. Which wind speeds should I use ?

 

[JedClampett]

Unless you're designing a highway component, you should use the building code and by reference, ASCE 7-10 (if that's referenced). But they're basically the same. When you look at the load combinations, ASCE 7-10 multiplies wind loads by .6 to get to working stress level. But AASHTO is already at working stress level. I'm not sure AASHTO has an importance factor, which is where the Risk Category enters in.

 

[EngrSTR]

I agree ASCE 7-10 should be used, .6 is only for buildings, under 2.4 basic combinations exceptions for other structures 1.0 applies for other structures below 1000 ft^2 surface area.
Im trying to pick light pole from vendor catalogs and they are based on AASHTO wind speeds and they suggest per that.

 

[azcats]

I'm not sure I agree with your interpretation of exception #2 in 2.4.1. Where does it say to use 1.0 instead of 0.6 in combo #7?

If I had to up-rate a wind speed in an AASHTO map to pluck things from a table, I'd use the square of 90 x 1.15 (Importance Factor from old ASCE7) and take the square root of that [((90^2) x 1.15)^.5] for an equivalent speed of 96.5 and probably round up to 100.

 

[EngrSTR]

ASCE 7-10 2.4.1 Exceptions 2 - For nonbuilding structures...., it shall be permitted to replace W with 0.9W in combination 7 for design of the foundation.
if the projected area is less that 1000 ft^2, we must use W and not 0.6W. isn't it ?


Agree with your AASHTO take, but since its inside a facility could we apply AASHTO winds for ligh poles in the site.

 

[azcats]

I disagree. IMO, the .6 never goes away. The exception is saying that for large areas, the .6 can be additionally multiplied by .9 for a slight reduction in wind load. And that reduction only applies to foundation design. Seems like a very narrow scope exception especially as it applies to only one load combination. If they really wished to alter wind loads for those non-building structures, they would alter them in the coefficients in the tables and not just in one specific instance of a load combination that is typically only used for foundation design.

The ASSHTO wind speed doesn't really address the risk category...and the vendors could possibly even use an importance factor of .87 bringing the fixtures down to category 1. Probably fine for light poles...but I don't know what the facility is expecting. If they're expecting category 4 design for the entire facility, I'd use the higher wind speed. And now I question my initial wind speed equivalency as I don't know what importance factor is used in the catalogs.

 

[BridgeSmith]

I'm not familiar with the ASCE spec, but I am with the AASHTO spec. The AASHTO wind speed is the basic wind speed, which is multiplied by a gust factor of 1.3 to get the design wind pressure for a pole design. Is the 120mph in the ASCE the gust speed?

 

[EngrSTR]

Yes, 120mph is ASCE 3sec gust speed

 

[BridgeSmith]

Thank you EngrSTR; that explains the discrepancy then. 90mph * 1.3 = 117mph

 

[Hokie93]

The difference is not the 1.3 gust effect factor. The 120 MPH wind speed from ASCE 7-10 is a strength-level or ultimate wind speed based on a 1700-year mean recurrence interval (MRI) for a Risk Category IV structure. To convert that to a nominal or service-level wind speed, you would multiply by the square root of 0.6, which results in a wind speed equal to 93 MPH.

 

[BridgeSmith]

Sounds like a alot of theoretical mumbo-jumbo by "egghead" spec writers so they can all feel smart about arriving at the same answer in different ways. At the end of the day, I'll bet when it gets to the bottom line of design wind pressure, there's no significant difference between the 2.

 

[EngrSTR]

Agree with you azcats. Thank you everyone.
That's right HotRod10