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New Bigger Wind Loads?
3

New Bigger Wind Loads?

New Bigger Wind Loads?

(OP)
I got this Email today. I don't understand if the wind loads are going up by 1.3 or not. Or are they increasing anyhow? I can't believe they need to get bigger. And are the PEMB and roofing manufacturer's wanting to increase loads? That seems unusual.

Email Excerpt:

"Request to Oppose Assembly Motion "As Submitted"

ASCE 7-2016 has been proposed for adoption into the 2018 IBC, and Code Change Proposal S105-16 would allow for the continued use of ASCE 7-2010 (with a modification factor of 1.3) for roof pressure calculations while the remainder of the wind pressure calculations would be based on ASCE 7-2016. This would require the use of two ASCE 7 standards which would lead to potential confusion and would place an undue burden on both the practicing structural engineer and the code officials

We urge you to participate in the ICC voting process that is currently open and to vote to uphold the ICC Structural Committee's action on proposal S105-16 by disapproving the Assembly Motion.

Vote Oppose on S105-16 Assembly Motion as Submitted. ICC members can vote using cdpAccess from May 12-26; go to www.cdpaccess.com, log-in, go to Group B 2016 Online Assembly Vote, and search for code change S105-16.

Please assist us in opposing the attempt to retain the ASCE 7-10 Roof Pressure Coefficients modified by a factor of 1.3 within the 2018 IBC. All ICC members can vote during an Assembly Motion!
Background
At the code hearings in Louisville, KY, a few weeks ago, the ICC Structural Committee voted to disapprove Code Change Proposal S105-16, which would have allowed the ASCE 7-2010 roof pressure coefficients to be utilized for the calculation of wind pressures on the roof. The new roof pressure coefficients have been determined utilizing the results of wind tunnel studies in comparison with full scale studies on instrumented buildings. This data and our understanding of these wind pressure coefficients have been known for up to 10 years now and just reviewed and presented to the ASCE 7 Wind Load Subcommittee during this last cycle. This information was presented, deliberated and voted on in an open, consensus process involving both the Wind Load Subcommittee and the ASCE 7 Main Committee.

Unfortunately many individuals associated with the pre-engineered metal building and roof industries brought this code change proposal forward in the ICC 2018 hearings. Many of these same organizations funded a peer review of the proposed roof pressure coefficients during the ASCE 7 process, and the peer reviewer found the coefficients to be accurate.

During testimony at the hearings in Louisville, the proponent of this proposal delivered inaccurate testimony indicating that the factor of 1.3 was supported by research by Dr. Timothy Reinhold. Dr. Reinhold is currently out of the country, but has noted the following when contacted concerning this testimony by the proponent:

I completely support the pressure coefficients in ASCE 7-16. In order to properly obtain design wind pressures for low slope roofs, it is critical that both the new ASCE 7-16 pressure coefficients and new definition of zones are used. For the particular building tested in our laboratory, the last two figures really tell the story.

When you look at pressure coefficients from all wind directions, nearly all of the clips would be exposed to pressures that exceeded ASCE 7-10 pressures (nearly 200 out of about 225 clips). When the ASCE 7-16 pressure coefficients and zone definitions are used, the number of clips that would experience pressures greater than those prescribed by the code would drop to about 50 out of about 225 clips - next to last figure. The load transfer mechanism help a bit as shown in the last figure where roughly 25 out of the about 225 clips experienced pressures in excess of what would have been computed using ASCE 7-16 pressure coefficients and zones.

It is important to note that the results from our laboratory are for a single building shape. The coefficients and zones developed for ASCE 7-16 represent data from a much broader range of building sizes, making those results much more robust. Nevertheless, the results from our full-scale study definitely show that ASCE 7-16 coefficients and zones move the bar in the right direction and are not overly conservative since we still had a number of clips that experienced pressures/loads in excess of what would be computed using ASCE 7-16 coefficients and zones."

RE: New Bigger Wind Loads?

The proposed revision to the IBC is on ICC's website http://media.iccsafe.org/codes/2015-2017/GroupB/CA... (see page S311, pdf page 313). The proposal is made by a representative for the MBMA

Quote (MBMA Rep)

Reason: ASCE 7-16, that is under public review at the time of this code change submittal, incorporates the latest information and knowledge with respect to wind loads. There was considerable ASCE 7 committee debate on revisions that drastically affected the roof component and cladding loads for gable and hip roofs with a mean roof height less than 60 feet. The ASCE 7-16 Commentary explains that "the negative roof (GCp) values given in these figures are significantly greater (in magnitude) than those given in previous versions (2010 and earlier)...". The GCp pressure coefficients have been traditionally derived from wind tunnel tests. The most recent wind tunnel testing utilized a finer grid of pressure taps on the models that revealed that the GCp values were significantly higher in some cases. However, the GCp pressure coefficients are one of six factors that are combined in the equation to calculate the component and cladding wind loads. The other five factors include wind speed (V), directionality factor (Kd), internal pressure (GCpi), topographic factor (Kzt), and velocity pressure exposure coefficient (Kz). The ASCE 7 debate did not focus on whether the roof component and cladding GCp values were in fact greater based on the latest wind tunnel tests, but on the resulting wind loads. It did not seem realistic that we were underestimating design wind loads on roof components and cladding by a factor of more than 2 in some cases. For example fasteners in the field of a gable roof with a roof slope between 7 and 20 degrees would see the negative GCp value increase from -0.9 to -2.0. If, in factwould have been more widespread performance issues raised over the years associated with a design wind load deficiency rather than the most common failure issues that are sporadic and most often associated with poor quality of construction.
The question raised was – are there conservatisms in the other factors and systematic biases that have been offsetting the GCp pressure coefficients? The resounding answer to that question from the ASCE 7 Committee was yes, absolutely. But, the debate was a philosophical one that divided the committee into two camps. Some said we know there are probably other conservatisms and the wind loads aren't as high as they will be with the proposed GCp revisions, but we will start with GCp and address the other factors in subsequent revisions. Others said that piecemeal approach would increase construction costs too severely, given the lack of performance issues. It was not a resounding decision – a swing of one vote would have changed the outcome.
It should be pointed out that the wind speed maps (V) are also revised in ASCE 7-16, and that there were some reductions in the interior portions of the United States (non-hurricane regions) that would partially offset the total wind load increase due to the increase in GCp. However this reduction in wind speed is not manifested in the hurricane regions, where the largest wind loads would occur.
This proposal would cap the wind load on roof components and cladding to 30% higher than those that we are currently designing for. This is still a substantial increase, but it is felt that would be a prudent compromise while the other conservatisms are studied in the next cycle of ASCE 7. Several industry groups, including MBMA and AISI have committed funding to begin these studies. It is felt this would be the reasonable approach and this would also even out the severe swings in the wind loads from one cycle to the next that would ensue.


RE: New Bigger Wind Loads?

I was one of the negative voters on ASCE7-16 winds during the recent public review cycle. As noted in the attachment regarding the rationale behind S105, wind coefficients on gable buildings under 7 degree roof slope are going up in some cases by more than double. Note this will apply to ALL buildings of this nature, not just PEMB, as it is in the components and cladding section of the code. As noted in the above attachment, the concern is that there are other factors involved in addition to the coefficients that may have had counterbalancing impacts, i.e., low coefficients coupled with over-conservative other aspects that the committee recognizes still need to be evaluated. S105 is intended as a stopgap, not a permanent part of the code, to allow full study of all aspects of wind, not just coefficients that have been pulled from small scale models in a wind tunnel. Given that at least for steel, safety factors are around 1.67, having the coefficient jump by over 100% would suggest that previous designs were being designed with a safety factor less than 1.00. If that were true, we should have expected lots of roof failures in wind storms and in a broad scope, that has not been happening.
The other issue here is consistency. The wind coefficients and wind zones for gables less than 7 degrees are completely at odds with all other roofs, including monoslope buildings less than 7 degrees and gables in the range of 7 to 27 degrees. Some variations could be expected but none of those other types are being changed. You basically have ASCE7-10 coefficients for all of the other conditions except this one case that had some later testing done.

RE: New Bigger Wind Loads?

3
What happens if you need to renovate a building during this code cycle? Are you supposed to do that with this code provision in mind knowing that the building won't have been designed to it and the number is likely going to be reduced again later?

I'm very much of the opinion that if you have failures or safety risks you should throw in a brute force solution to make sure things are safe while you systemically fix it. However if it looks like your model is wrong but, historically, is safe you should really take the time to step back and figure out a systemic fix even if it takes time rather than going off half cocked.

Also, this kind of points out the fact that we're doing incredibly detailed analysis on stuff using models and inputs that aren't necessarily all that accurate. If we honestly think our model could be out by this much why are we separating buildings into so many different pressure areas and things. Being precise is useless and a waste of time if the basis isn't accurate enough to justify the amount of precision being used.

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