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ASCE 7-10 Wind Loads
2

ASCE 7-10 Wind Loads

ASCE 7-10 Wind Loads

(OP)
What was the purpose of switching the wind provisions from ASCE7-05 from allowable to strength methods in ASCE 7-10? IBC requires you to calculate wind loads in accordance with ASCE 7-10 but then requires you to calculate a Vasd?????? and Vasd is the determining factor when you get into the Structural Observations for Wind Requirements of chapter 17. Not to mention that now all of my allowable wind drift and deflection limits (H/400 under allowable loads ect.) are all different because of the increase in the wind speed.

Very frustrating.

RE: ASCE 7-10 Wind Loads

I think they're trying to "seismicize" the wind sections of the code to make it seem as difficult as the seismic.
Anyway, I've attached a slick website (for all of you not familiar with it) that gives site specific wind speeds. You can even show that you're at the right site if you magnify the little map on the pdf.

RE: ASCE 7-10 Wind Loads

Now, now...remember our brethren in the engineering software industry. Where would they be if building codes were designed in a manner to be understood by humans and left unchanged?

RE: ASCE 7-10 Wind Loads

And the national structural exam is using the provisions of ASCE7-10 in the test now....

This ain't gonna go away anytime soon folks!

Mike McCann
MMC Engineering

RE: ASCE 7-10 Wind Loads

(OP)
Yes, but the software doesn't set the limits on building drift.... as far as I know, the code doesn't really set these limits either. This is something that needs to be taken out by the engineer and evaluated against the intended use of the building. Now you get a bunch of amplified forces being driven into the building.... and what about p-delta, how does that figure into the equation?

RE: ASCE 7-10 Wind Loads

How does this affect P-Delta?

RE: ASCE 7-10 Wind Loads

SteelPE,

My real point, and perhaps yours as well, is that I have trouble keeping up with all the modifications, refinements, complications, and multiple disparate requirements from various sources. Further, it seems to me that said complexities have long since passed the point of diminishing returns and are now well into the realm of being a hindrance, rather than an aid, to good design.

And believe me, I abbreviated and tempered my thoughts on this topic in both of my posts. To call it a source of frustration would be an understatement.

RE: ASCE 7-10 Wind Loads

(OP)
UcfSE, I suppose it doesn't really have an effect on Pdelta. I had a thought at the time that I wrote the post that with a larger wind applied load the more drift the frame will have the larger the pdelta.... however, now that I think about it, I usually have my programs (and I assume others do as well) calculate pdelta for each specific load combination.... and if the load combination has 0.6W you are really not having and increase in pdelta.

However, I do like to look at my building drift under the pure wind load case.... which is now amplified. I also find it funny that IBC has you calculate Vasd and uses this figure for a few different bits in the code.

RE: ASCE 7-10 Wind Loads

msquared48,

Not yet, the 2014 NCEES Exams are still based on the 2009 IBC and subsequently ACSE 7-05, ACI 318-08, AISC 360-05, NDS 05, etc. I checked, just in case I didn't pass this October, I'd get another two shots at my current codes.

RE: ASCE 7-10 Wind Loads

sforesman:

Well, my son will be taking the structural test within the next year or so, and he is tearing that code apart right now, I assume for a logical reason. peace

Mike McCann
MMC Engineering

RE: ASCE 7-10 Wind Loads

They also seem to have made the layout of the wind provisions chapters much more complicated. Looks like they were trying to simplify it somehow but failed in an epic manner. We were all used to the confusing nature of the 7-05 and prior layout, why did they need to change that aspect too?
FWIW, there is an old timer engineer around here who still just uses 15 psf for the MWFRS, 20 psf for C&C and .05w for seismic.

RE: ASCE 7-10 Wind Loads

>>>FWIW, there is an old timer engineer around here who still just uses 15 psf for the MWFRS, 20 psf for C&C and .05w for seismic.<<<

Excel,

I can assure you that there are a lot of engineers who do that, or a variation thereof. But your example might be a tad complicated; I've heard of 20 psf for wind and .1w for seismic.

Isn't it interesting that first year engineering students have the concept of significant digits battered into their heads only to have the codes impose absurdly "refined" design parameters that completely fly in the face of that?

RE: ASCE 7-10 Wind Loads

it has now become an "immaginary" wind speed...nothing to do with reality...an old-timer once said that it is not so much engineering as it was immagineering...and that day has arrived now for sure..

RE: ASCE 7-10 Wind Loads

SAIL3:

Hull speeds are "im'm'aginary" too, right? I always exceeded mine.

Mike McCann
MMC Engineering

RE: ASCE 7-10 Wind Loads

SteelPE, understood. FWIW, the Vasd is there to give product manufacturers something to justify the use of their product until their own testing requirements catch up to the use of strength winds instead of service winds. Its just so we don't jump the design wind speed from 110 mph to 140 mph and immediately disqualify all of the products in use, that should be acceptable, or have to use hurricane hardware and such in Kansas.

RE: ASCE 7-10 Wind Loads

(OP)
Is requiring hurricane hardware in Kansas all that bad? I would think they would want something a little more substantial in Tornado Alley than hurricane hardware.

Again, I find it frustrating...... and I haven't even expressed my frustration with the fact that I will need to update my software soon. Kind up puts some pressure on the small shops like myself who don't have the resources larger companies do.

Excel,

I find it interesting what you see with older engineers. I find many large engineering shops will have people go out for the AISC and give lectures on how to go about engineering. Then I get a set of drawings in from them to do some connection design and I find that they are not even following their own guidelines. I seem to remember on lecturer saying that he only does modal analysis... only to find out a few weeks later that they design most of their systems using the equivalent lateral force procedure using R=3. Then there was the large engineering shop that doesn't even remotely follow the guidelines of IBC Chapter 17.

RE: ASCE 7-10 Wind Loads

I agree with the general consensus on this thread, the code has gone too far in determining exact wind pressures that it has become a hindrance instead of an aid. For example corner wall pressures of zone 4 versus zone 5. Would you really change your stud spacing at a dimension "a" from the edge of the building? Seems very impractical to me. I always just use zone 5 for all walls and not worry about it. This is just an example I have selected.

Just like tax codes, the wind loads need to be simplified. It shouldn't take twice as long just to determine your load magnitude versus your static analysis of the loading applied!!

RE: ASCE 7-10 Wind Loads

The NYC building code has a 'Simplified Method 2' which covers a large majority of the buildings in the city and allows 20psf up to 100ft and 25psf from 100ft to 300ft.

RE: ASCE 7-10 Wind Loads

Good to hear it bookowski and there's quite a lesson there. If that's good enough for the skyscrapers in NYC located as they are in the canyons of building with their swirling winds, it ought to be adequate for the design of a convenience store or some such in most other places.

RE: ASCE 7-10 Wind Loads

1. See ASCE 7-10 commentary section C26.5.1 for the "official" reason for changing the basic wind speed.
2 I like that the IBC has added the subscripts for Vult and Vasd. That way there is no confusion on the construction documents during the transition period.
3. For those that have trouble calculating Vasd = Vult sqrt(0.6), the IBC has table 1609.3.1.
4. For drift, I prefer to scale the design loads with the wind speeds in the ASCE 7 commentary to appendix C. [pdrift = pdesign x Vdrift2 / Vdesign2].
5. For deflection, IBC table 1604.3 changed footnote f from 0.7 times the C&C wind load to 0.42 times the C&C wind load for the deflection limits.
6. The IBC requires the design wind pressure on CDs for the components and cladding not designed by a "registered design professional". The design load, "W", in the 2012 IBC is at ultimate not ASD. Until the components, cladding, and equipment manufactures get up to speed with the changes, it may be a good idea to provide both ASD and ultimate design pressures. However, I think the manufacture may figure it out quickly once the start seeing huge design pressures.

RE: ASCE 7-10 Wind Loads

@TDIEngineer:

I actually do change the spacing or gage of the studs at corners if the building is big enough to make it economical.
The metal stud subs I do engineering for always underbid these things so I have to find them some money where I can without making things too complicated in the process.
But I agree, it is a pain - though not as much as dealing with parapets!

RE: ASCE 7-10 Wind Loads

If it makes you feel any better I believe they are changing Snow loads to ultimate for the next ASCE7.

RE: ASCE 7-10 Wind Loads

(OP)
dcarr,

Is that a joke.... because I don't get jokes.

RE: ASCE 7-10 Wind Loads

Sadly no. It really is in the works. I believe some are trying to get it done for the next ASCE7, and if not that one, than the one after.

RE: ASCE 7-10 Wind Loads

(OP)
Great, now when we have a bad winter and I get a call from a client saying they think their roof is going to fall in and I go out onsite to measure the snow I will have no idea how it compares to the design load.

What is the reasoning behind the switch?

RE: ASCE 7-10 Wind Loads

NC has not even adopted 7-10 yet. Hopefully, I will be retired when the next one comes around and gets adopted.

RE: ASCE 7-10 Wind Loads

Job security. (No one else understands what you are doing scenario)

Mike McCann
MMC Engineering

RE: ASCE 7-10 Wind Loads

(OP)
If they keep this up, I'm not even going to know what I am doing. For the first time today I finally have a building where the wind load is larger than the seismic load. Thanks ASCE 7-10.

RE: ASCE 7-10 Wind Loads

This is the perfect example of the "KISS" theory of analysis and design being torn to pieces and totally disregarded.

Mike McCann
MMC Engineering

RE: ASCE 7-10 Wind Loads

I get the rational of specifying limit state loads where possible since the capacity side is that way for many materials. I wouldn't be surprised if the standard live loads are all changed to be 1.6* what they are now. The only loads it gets a bit too funky on are the dead loads.

I don't like it and I think it is a huge complicating waste of time and money, but I can see where those folks are coming from. I also think it is a bit of earthquake envy.

RE: ASCE 7-10 Wind Loads

next time I get stopped for a speeding ticket I am going to try this new and improved speed logic out..

RE: ASCE 7-10 Wind Loads

@bookowski> NYC had those simplified wind pressures of 20 psf / 25 psf including 33% increase in stresses. But those simplified
wind pressures were based on NYC 1968 Building Code which was used until 2008.
The new NYCBC 2008 is based on IBC 2003 / ASCE 7 -05. Those simplifications were thrown to the winds.

RE: ASCE 7-10 Wind Loads

DST: NYCBC 2008 still has them - they are referred to as Simplified Method II, there are some restrictions but not many. They are used all the time.

RE: ASCE 7-10 Wind Loads

While they made it look a bit different, the wind loads are essentially the same between 05 and 10. The differences are in rounding of wind speeds and ultimate v service. So I don't understand STEELPE's comment that the wind loads are now bigger.

RE: ASCE 7-10 Wind Loads

(OP)
Where did I say the loads were much bigger? I recently had a structure where the wind was larger than the seismic which is about the first time this has ever happened to me. Of course, the jurisdiction switched from ASCE 7-05 to ASCE7-10..... they also reduced Ss by 25% which helped (can't figure that one out).

So, if the numbers at the same........ why switch?????????? Oh, and I know all the numbers end up being the same in the end.

RE: ASCE 7-10 Wind Loads

Misunderstood, apologies.

RE: ASCE 7-10 Wind Loads

@JedClampett

...and the academics or scientific community who justify their existence by promoting these changes.

RE: ASCE 7-10 Wind Loads

When I first heard that they were making sweeping changes to the wind code and expanding it from one chapter to six, I was frustrated as many of you are. But honestly, the changes make a lot of sense. I find the wind chapters to be much better organized that in previous editions. It is useful to be able to make an apples-to-apples comparison between wind strength loads and seismic strength loads without the multiplier. And the equations and charts for the regular all-heights method we are used to are essentially unchanged.

I would love for them to simplify the codes for basic structures, but we all know that's not going to happen. While the wind load chapters have grown in size, I find ASCE 7-10 actually easier to use. I know I'm in the minority though.

RE: ASCE 7-10 Wind Loads

I have only used ASCE 7-10 for two projects so far but I have no issues with the revised layout and the use of strength-level basic wind speeds. I think it is an improvement to provide separate chapters for main wind-force resisting systems, components and cladding, and other structures. The basic provisions are essentially unchanged from ASCE 7-05 (Chapter 27 (MWFRS - Directional Procedure) is Method 2 from ASCE 7-05 and Chapter 28 (MWFRS - Envelope Procedure) is Method 1 from ASCE 7-05), so if you are familiar with ASCE 7-05, the learning curve for ASCE 7-10 should be relatively flat. Chapters 27 and 28 do include alternative, simplified procedures that I have not yet tried. For those who design structures in hurricane-prone regions, you will find a nominal 20% to 30% reduction in design wind pressures with ASCE 7-10 compared to ASCE 7-05 due to better data and improved hurricane simulation modeling utilized in the development of the provisions. The design wind pressures in non-hurricane regions appear to be the same or slightly less compared to ASCE 7-05. I also like the 10-year, 25-year, and 50-year MRI wind speed maps that are provided in the Commentary to Appendix C as they are handy for drift computations. I do regret that we will no longer be able to utilize a single basic wind speed for a particular area, as the basic wind speed is now linked to a particular Risk Category (so for a given area, there are three basic wind speeds associated, respectively, with Risk Category I, II, and III/IV).

RE: ASCE 7-10 Wind Loads

It's kind of a book-keeping problem. We design multi-structure water or wastewater treatment plants. On the structural general notes, we used to put the design wind speed. We also have to put the design wind speed in the specifications for the electrical and mechanical equipment suppliers to use. Some of the buildings have different Design Categories (some are storage only, some have hazardous chemicals in them, etc.), so we might have Category II, III or IV on the same project. Now instead of one working level wind load and assigning Importance Factors, technically we need to give different wind speeds for each structure. Add to that the fact that the suppliers need to adjust the wind loads by the .6 load factor is just something else to add to the confusion.
I actually don't mind the whole concept and having site unique site specific wind speeds is pretty awesome. And it's not any harder to use than ASCE 7-05. I just have a problem with the fact that just because we have bigger computers that we need to add several layers of complications to the code that was OK. I'm carrying wind pressures to three figures for pity's sake. Does the wind know that?

RE: ASCE 7-10 Wind Loads

>>>I'm carrying wind pressures to three figures for pity's sake. Does the wind know that? <<<

It must. It seems to know what category the building is.

RE: ASCE 7-10 Wind Loads

@Bookowski> Thanks for the info about simplified wind pressures still being used in NYC.

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