Wind Load 2

[dik]

I'm not familiar with ASCE-7 wind loadings. Does anyone have a formula or description of calculating the q in psf from a Exposure C, 90 mph wind with a 3 second gust?

Dik

 

[phamENG]

90 mph - is this ASCE 7-05? The year makes a difference as the code changes between 7-05 and 7-10. The change is primarily in the MRI of the wind speed used and not the calculation itself, but it's still good to know.

 

[dik]

earlier, ASCE 7-02 and is the calculated load significantly different than the current ASCE 7 if you know?

Thanks

Dik

 

[kissymoose]

Here's what I got for ASCE 7-10, which uses the 300yr, 700yr, 1700yr return periods (varying based on risk category) instead of 50yr from ASCE 7-05 (where the 90mph speed was prevalent). The load combinations were updated in 7-10, accounting for the higher wind speeds.

Dependent on:
• risk category
• mwfrs vs c&c
• topography
• building height

 

[kissymoose]

 

[dik]

and the gust duration is included in the basic wind speed? For a tent structure I could likely consider the K factors as being equal to 1 and use the 0.00256V^2 to get an equivalent pressure. This would then be multiplied by the various exposure coefficients to give the actual pressure on a particular area (roof parts, wall parts, etc.). Return period such as 1:10 years, 1:50 years does not enter into it, except as the basic wind speed?

Dik

 

[r13]

dik,

Try this freeware (so claimed by the site). Link

 

[phamENG]

dik - the load is still calculated in essentially the same way. I was hoping to give you direct equation references, but I don't have a copy of '02.
Everything in ASCE 7 is based on the 3s gust speed and has been since 1995 (used fastest mile before that).

This is for you tent, correct?

From what I recall of your description, K.d=0.85 would be appropriate. It's based on the statistical probability of receiving the design wind at exactly 90degrees to your building in such a way that the worst of the pressure coefficients are valid. You also have to use it in the ASCE 7 load combinations. If you don't, it throws the probabilities off and you're not supposed to take the 15% reduction.

Kzt can be taken as 1 so long as your on relatively flat land.

As long as your mean roof height isn't greater than 33ft and you are in exposure B or C, K.z can be taken either rationally or conservatively as 1.0. If taller or you're in exposure D, you'll need to use >1.

As for the return period, it matters how you are using. If you plug it into the ASCE 7-02 load combinations, then you're fine. You just can't carry it over to a more recent code, as the loads are "calibrated" in the load combinations.

Since you said you're not familiar with ASCE 7 wind loads, I'll also say this. You don't stop at q. After calculating that, you need to apply various pressure coefficients and gust effect factors. The gust effect may be particularly important for your tent if the supporting frame is particularly lightweight and flexible.

 

[dik]

Yup, still the tent... I think I'll use Kd = 1... just to be a little conservative... Kzt = 1... area is flatter than ... height not an issue... we have gust and pressure coef here... coef in ASCE appear to be similar to NBCC. Do the pressures have a typical 'return period'? I've encountered a couple of tutorials on ASCE 7 that appear to be pretty good.

Dik

 

[dik]

I had the spreadsheet in my spreadsheet collection, but have never used it... thanks

Dik

 

[r13]

I had the spreadsheet in my spreadsheet collection, but have never used it...

Typical syndrome of a rich toys collector It would be better if you have the code with you.

 

[dik]

Just a typical packrat... goes with the hundreds or thousands of spreadsheets I written in the last 25 years... I still have my original copy of Visicalc on 5-1/4...

Dik

 

[phamENG]

The 90mph from ASCE 7-02 will be a 50 year return. That was used again in 2005, but in 2010 they dropped the importance factor and introduced individual return periods for each category of structure.

Assuming ASD:

7-02 and 7-05 are both calculated at 50 year MRI, then multiplied by the importance factor (1.0 for typical stuff, 1.15 for important/critical infrastructure). Load combination is 1.0*W.

7-10 calculates typical buildings at 700 year MRI, and critical stuff at 1700 year MRI. There is no importance factor (1.0 for everyone). Load combination is 0.6*W.

So the end result is typically within 5% between the two codes.

That changed with 7-16 as the wind maps have gotten a lot more accurate, so certain regions will start seeing more variation between codes.

 

[dik]

It will be interesting to see how this varies with climate change... glad for the 50 year duration... the calculations I have can adjust for different periods based on 50 year.

Dik