Wind Pressure Minimum requirements 1

[S3Engineers]

Hello,
Both ASCE 7-10 (28.4.4) and IBC 2012 (1609.6.3) list minimum wind pressures at 16 psf for walls. ASCE 7-10 also lists 8 psf minimum for vertical projections of roofs.

The minimum wind load comes into play on single story structures particularly in exposure B. There has been much written about LFRD vs ASD wind design. IBC has a conversion factor (1609.3.1). It seems obvious to me that 16 psf is an LFRD minimum wind pressure. 16/1.6 = 10 psf should be the minimum ASD wind pressure similar to ASCE 7-05.

The issue I am having is that neither code specifically lists this minimum value as an LFRD value and many Governing Agency building officials are now requiring the use 16 psf minimum for ASD lateral design of one story wood buildings. What literature is available that addresses this item - particularly ASCE or IBC sourced?

Thanks,
Rich Warren
S3 Engineers

 

[CURVEB]

I believe that the 16 psf minimum is an ultimate pressure, which should be multiplied by 0.6 or 0.45 to convert to ASD levels (depending on which load combination controls), or 1.0 for LRFD levels. It's my understanding that this minimum is essentially synonymous with the previously used 10 psf pressure from the 05 code.

If the building department is requiring a minimum 16 psf ASD pressure, it could be that they do not understand how the code was revised. You could argue that the ASD load combinations include a 0.6 factor for wind forces.

 

[XR250]

I call it 10 psf when I use ASD (I never use LRFD except for concrete beams))

 

[S3Engineers]

The plans examiners understand the difference between ASD and LFRD. They are "hanging their hat" on the fact that the codes do not specifically list the minimum wind pressures as LFRD in the sections cited above. They would like to have an IBC or ASCE sourced document that clarifies the minimum value.

 

[steellion]

Don't mix and match codes. The IBC 2012/ASCE 7-10 that has 16 PSF minimum also uses (0.6 x Wind) in all ASD load combinations. Therefore, by strict code compliance, the minimum ASD wind load has actually decreased slightly (16 PSF x 0.6 = 9.6 PSF). Give them the code references to minimum wind pressure AND load combinations from the same code and that should resolve the issue.

 

[structSU10]

It is dealt with by the load combinations, within sections 2.3 and 2.4 of ASCE 7-10 - wind has a 1.0 factor for LRFD and a 0.6 factor for ASD. There is some discussion of the change in the commentary for the general provisions chapter, stating that wind was changed to an ultimate load to match how we deal with seismic, and try to create a more uniform recurrence interval.

 

[DaveAtkins]

Call the plan examiner, thank him for prompting you to look further into the issue, and then tell him ASD wind loads are now LOWER than previous codes (9.6 psf versus 10 psf). Then laugh maniacally--HA HA HA HA HA HA (this last step is optional)

DaveAtkins

 

[manstrom]

Forget LRFD vs. ASD. Use the 0.6W combination factor in the ASD load combos or the 1.0W factor for LRFD. It's right there.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[S3Engineers]

Guys,
I have met the the plans examiner on this item. I have attempted to explain the use of load factors. He understands them very well. He is just saying that the code specifies a minimum value of 16 psf and that he needs something in writing from IBC or ACSE... I have had this issue with more than one plans examiner and they usually get it after a brief discussion. However, I have one examiner that refuses to budge...

Guess my next step is to request a code interpretation from IBC. However, I am not a member. I will call them to ask if it is possible to get one as a non-member.

 

[RFreund]

I would agree the code minimum is 16psf, but when you design the member the load combination is 0.6*WL, which is 9.6psf so you are actually getting a slightly lower minimum design pressure.

Maybe I don't understand the argument.

EIT

http://www.HowToEngineer.com

 

[S3Engineers]

We use 16 psf minimum in member design and apply load combinations. However, when we run lateral analysis we use Excel spreadsheets that were written based on ASD procedures. Perhaps I can convert our spreadsheet to LRFD and then apply the load combination to the resultant forces. This would be built in to the sheet and the plans examiner would only see "16 psf". That's got to be the answer...

Thanks,

Rich

 

[RFreund]

I see what you mean. To me that is a good catch by the examiner but once he has pointed out to you and you've explained things it should end there. Odd that he is refusing the calcs.

EIT

http://www.HowToEngineer.com

 

[TehMightyEngineer]

Switch your design over to LRFD, problem solved.

Maine Professional and Structural Engineer.

http://www.fepc.us

 

[gte447f]

So, the plans examiner is reviewing your calculations, not just your construction drawings where the design criteria are listed?... that is the problem. I'm glad I don't have to suffer that injustice in the jurisdictions where I practice.

As suggested above, it seems like the best thing to do is change your calc spreadsheet to deal with the conversion from LRFD to ASD inside the black box so that the plans examiner can only see what he is able to understand and is none the wiser to the things he doesn't understand.

 

[mrsutton]

The min is 9.6(typically rounded up 10)PSF ASD and 16PSF ULT.

 

[sekwahrovert]

All pressures listed in ASCE 7-05 and ASCE 7-10, including those in the tables for simplified methods and the minimum pressures you mentioned, are 1.0W. You should modify these pressures according to applicable load combinations. You are correct that 16 psf is ultimate because the LRFD load factor on wind is 1.0 for ASCE 7-10 load combinations. If you want the ASD pressure, it's 0.6*16 = 9.6 psf.