ASCE 7-10 Wind Loads 2

[SteelPE]

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.

 

[JedClampett]

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.

 

[Archie264]

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?

 

[msquared48]

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

 

[SteelPE]

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?

 

[UcfSE]

How does this affect P-Delta?

 

[Archie264]

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.

 

[SteelPE]

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.

 

[sforesman]

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.

 

[msquared48]

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.

Mike McCann
MMC Engineering

 

[XR250]

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.

 

[Archie264]

>>>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?

 

[SAIL3]

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..

 

[msquared48]

SAIL3:

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

Mike McCann
MMC Engineering

 

[UcfSE]

SteelPE, understood. FWIW, the V[sub]asd[/sub] 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.

 

[SteelPE]

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.

 

[TDIengineer]

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!!

 

[bookowski]

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.

 

[Archie264]

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.

 

[wannabeSE]

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 V[sub]ult[/sub] and V[sub]asd[/sub]. That way there is no confusion on the construction documents during the transition period.
3. For those that have trouble calculating V[sub]asd[/sub] = V[sub]ult[/sub] 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. [p[sub]drift[/sub] = p[sub]design[/sub] x V[sub]drift[/sub][sup]2[/sup] / V[sub]design[/sub][sup]2[/sup]].
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.