ASCE 7-10 wind loads 1

[keyPitsimplE]

thread176-326107

Does anyone know whether the minimum MWFRS wind loads (16psf walls / 8psf roofs) listed in 27.4.7 are ASD or LRFD? I assume LRFD, but not sure. If so, i would need to multiply by 0.6 in order to compare it to the calculated loads. Funny thing is that most local conditions produce a total load well under this miniumum, which makes the grueling calculations almost pointless. I only noticed this as i was finishing up my new spreadsheet. Awesome.

 

[UcfSE]

ASCE 7-10 wind loads are strength level.

 

[keyPitsimplE]

So, does that then explain why when you muliply 16psf by 0.6, you get the 10 psf minimum load that was in ASCE 7-05? The slow march of structural codes from ASD to LRFD. I must have overlooked a note about that jump in thinking between the 2 codes.

 

[woodman88]

"I must have overlooked a note about that jump in thinking between the 2 codes."

The jump in thinking is IMHO money. The wind load is almost the same between the 7-05 and the 7-10 ASCE. They reduced the wind in the "strength design" and ASD Load Combinations by 1.6. Then increase the minimum and some of the Tables values by the 1.6 factor.

So how do they sell the new code books, if there is very little changes in them? By making it look like they made some big (by a factor of 1.6) changes.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[keyPitsimplE]

I hear you, woodman88, and agree with your frustration. It will be a nice day when we have a code cycle that we dont have to spend so much unbillable time figuring out how to apply.

I am not quite so cynical about the money part. Maybe I'm wrong and have not looked deep enough, but I'm hoping it is an attempt at unifying the codes in the march toward LRFD. Looks like the IRC is staying ASD, keeping its 85 mph wind for the PNW, instead of 110 mph like IBC & ASCE. I'm an old ASD hold-out, since most of my work is residential and light commercial, and I'm a wood guy too.

 

[UcfSE]

Yes, the 16 psf comes from 10 psf times 1.6.

 

[briancpotter]

Complaints about the 3-year update cycle are somewhat misplaced, I think. It's up to local jurisdictions to decide whether they adopt a code or not, and often they decide not to. Of all 50 states, the vast majority are using the 2006 IBC. Many still use the 2003 IBC. Only 1, Maryland, is currently using the 2012 IBC.

In fact, looking at the history of code adoption in some jurisdictions, it appears that in general, code updates are less frequent now then they've ever been.

Rather than greediness, I think ~3 years is just how long it takes for structural research to advance to the point where it can be applied. Even bodies whose publications are completely freely (various FEMA provisions, the army's Unified Facilities Criteria) still update on about a 3-year cycle.

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[woodman88]

briancpotter
I have nothing against structural research and applying it to the codes. Please do some number between the ASCE 7-05 and the 7-10 wind loads. To see for yourself if small change in the wind loads applied to buildings does any good for the public in your opinion. It may apply to buildings over four stories but I see no reason for any change for the four story and lower buildings.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[frv]

Woodman88,

It's not as simple as that. Read the commentary for chapter 26- specifically C26.5.1. The reasoning behind moving to a strength design approach is a reflection of previous issues where there were inconsistencies depending on risk category (read "return period") in hurricane prone regions. In other words, there may be some areas close to the shore in hurricane-prone regions where you would design for higher wind loads for an Occupancy Category IV, but not for a Category I. This can't be accurately reflected by the use of blanket importance factors because return period varies geographically.

And yes- the net result for most of the country is essentially a wash (for 90 mph, Category II, the wind loads are about 2% higher).

 

[woodman88]

frv

If hurricane wind design changes are so important, than they should create special maps/equations/etc. for it. Then they can change the maps/equations/etc. every month if they want or need to.
Why have the rest of the country change if not to get us to help pay for what we do not need?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[frv]

Because I suspect the same people (not naming names) would then complain about the non-uniformity of the code in coastal areas.

I think you greatly overestimate the compensation these folks receive for the thankless work they do. I was fortunate enough to attend a University that is involved in a lot of structural engineering research, and quite a few of my professors were members (even chairmen) of some of these committees (not the ASCE-7 wind loading committee). I can assure you they do this because they are genuinely interested in furthering knowledge and the topics in which they specialize.

I know it's a pain initially, but this is one of the code changes that I think simplified things a bit (I know I'm likely in the minority here). Counter-intuitive as it may seem because it is now spread out over many more chapters, I think the committee did a fairly good job of guiding the engineer through the provisions in a logical way. In the past, ASCE had given out some mock tests to volunteer engineers of all levels with a simple building and they were asked to come up with the loads on the building. The variation in results was sobering. Even experienced engineers would apply the provisions incorrectly. I think this reorganization attempted to address the circuitous nature of the old chapter 6.

 

[keyPitsimplE]

OK guys. I didnt mean to start an argument. You are both right, but who cares. Hopefully ICC and ASCE are getting the message that drastically changing the way we calculate loads, with very little overall net resulting change, is a waste of time and money for everyone. Research must go on, of course. However, would it not make more sense for them to leave something alone if it works, and just submit a NEW way of calculating loads that can be adopted/changed SEPARATELY as needed? Similar to what woodman88 said. God knows we dont need any more ways to skin this cat, and engineers constantly tweaking and complicating things for very little change is incredibly annoying and costly. Yes, it is a complex argument and we must have progress, but lets do it smart and efficiently without punishing the folks who rely on this for a living. I hope you are listening ICC, ASCE and everyone else who falls in this catagory. I'll get of MY stump now.

 

[frv]

keepitsimple,

I completely agree with you about the code complexity issue. I've often argued that we're getting very limited ROI in the codes. It now takes twice as long to design something (for a net savings of 5% in material), as it did not too long ago, and the competition for work is constantly pushing fees down. It seems like the engineers are bearing the brunt of the "cost" while realizing none of the "gains".

However, this is one area I think the committee did the right thing. They didn't really reduce (or increase) the complexity of the provisions, they simply reorganized one confusing chapter into a more logical arrangement of several separate chapters. In my opinion, the change to strength-level wind loads and corresponding change in load factors is really a trivial matter.

 

[keyPitsimplE]

I agree with you, frv. Now that things are more organized and at least starting to consolidate into a segregated LRFD mindset, hopefully we see them settle down with all the calc method changes from here on out. Somehow I doubt that will be the case, but lets hope.

I also hope they always keep ASD on the side for its simplicity and hand-applicability. Even tho most of us DO have a computer at hand at all times, we SHOULD NOT have to rely on it in total. We are engineers. Lets be a little smarter than so many other economic sectors that will bite the dust when the first electromagnetic pulse, from man or nature, knocks out the grid for an extended time.

 

[AELLC]

The latest ASCE wind code reqmt's may be fine for a PEMB structure, so that it can be designed to within 0.1% of allowable wind capacity and save the Mfr some dollars on steel.

But to apply all this to a custom home design is impossible. If the plan looks like something as far removed from a simple rectangle, it is impossible to figure out end zone reqm'ts and apply wind at 45 degree to orthogonal directions.

At least back in UBC days there was Method 1 that gave a fairly sophisticated analysis that most engineers could understand, and come up with consistent results...unlike the current situation.

I think it would make more sense if there were 3 different analysis methods - for PEMB and commercial low-rise, medium and high-rise, and low-rise residential.

 

[frv]

AELLC,

Again, the newest ASCE wind load provisions didn't really change from a procedure standpoint. The wind loads are calculated essentially the same way as they had been the last 10 years. They are now simply organized in a more logical way, with some "big picture" guidance at the beginning of each of the chapters. Although there is a new method, it is really not new- it is simply a tabular application of one of the other methods, for buildings that meet certain constraints. I used it recently for a particular project that didn't fit our Excel wind-load spreadsheet constraints and found it to be pretty simple to navigate.

The change to strength-level winds is really trivial. I've had to submit a few calcs based on ASCE 7-10 wind load provisions, and it quickly becomes apparent that the "difference" (in computation time and resulting loads) is essentially zero. The change to strength-level wind loads simply allowed for more consistent reliability, depending on occupancy category.

I know the knee-jerk reaction from our community is to initially oppose any change, and it's probably the right reaction in most cases. It's a pain to have to learn new codes when it is not really necessary to change the existing ones. The new ones tend to be 50% more complex, 3% more efficient and .05% safer. But for this one change, I think you ought to go through the new chapters with some patience (and your favorite adult beverage) and familiarize yourself with it. I think you will see that it is not only not bad, but it sort of makes sense.

 

[woodman88]

frv

Here is my "knee-jerk reaction" I already had the procedures "organized in a more logical way" done for the wind conditions I typically do. I can not imagine any engineer who would not have written the procedure out, for the next time it was needed.

What I do not like is that they decreased the factors applied to wind in the Load Combination by 1/1.6=.6. Then up the wind forces and values for the calculations by 1.6 to offset the decrease. So that "the "difference" (in computation time and resulting loads) is essentially zero."

Why??? As I see it it is merely so they can say "look the wind speed was 90 mph and now it is 115 mph" don't you feel safer! When 115^2/90^2 = 1.63 for an increase of only 1.63/1.6 = 1.02.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[woodman88]

To clarify my concern, the problem is that after all the "news" reports on the damage done by hurricanes. The ASCE is merely saying to the public "hey we increase the wind speed from 90 to 115, an 115/90 = 1.28 increase. When the force to the structure is increased only 1.02.

This is a disservice to the public.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[briancpotter]

woodman88-

I'm not sure where you get the impression that ASCE is implying, to the public or anyone else, that wind design is now more stringent. It's certainly nothing I've encountered designing in my years of doing hurricane design.

And in fact, one of the changes in 7-10 wind is a more accurate hurricane model which actually DECREASES design hurricane loads. From the commentary:

"The new hurricane hazard model yields hurricane wind speeds that are lower than those given in ASCE 7-05, even though the
overall rate of intense storms (as defined by central pressure) produced by the new model is increased"

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[frv]

Hmm. I'm not sure I follow. The process to get to the final result (i.e., calculation of q, determination of Kz, kzt, G, Cp, GCpi, etc.) is essentially unchanged, other than the number in is higher and the factor is lower. I don't see how this would change your calculations other than perhaps having to update the reference section in your templates.

But big-picture, I think you have to remember that you are not the only practicing engineer. There are a lot of new guys coming in and they will certainly benefit from a more logical arrangement of wind load provisions. Frankly, a lot of practicing engineers with many years of experience also apply the code incorrectly (

http://ascelibrary.org/doi/abs/10.1061/41130(369)157).

If a reorganization allows more engineers to get more accurate results, I think it's a good thing.

As to your second point, I haven't heard that ASCE is doing any such thing. I'm absolutely certain they'd be called out on that immediately. If I heard such a claim, I'd call whatever news outlet gave that information and correct them. I know engineers well enough to know that I'm not the only one that would not allow such a claim to go unchallenged. And, once again, the purpose of the different wind speed maps was an attempt at uniformity that previously lacked in the codes, and that was not possible to achieve using a single importance factor based on occupancy.

Look, I'm not saying that the change is completely without challenges. My argument is that I think some engineers are making it out to be this enormous reset where everything we knew goes out the window. My argument is that we should not overreact to it. That it seems to make sense and that the pain (at least compared to some other changes in other codes) is relatively minor.