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.

 

[XR250]

@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!

 

[dcarr82775]

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

 

[SteelPE]

dcarr,

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

 

[dcarr82775]

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.

 

[SteelPE]

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?

 

[XR250]

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

 

[msquared48]

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

Mike McCann
MMC Engineering

 

[SteelPE]

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.

 

[msquared48]

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

Mike McCann
MMC Engineering

 

[dcarr82775]

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.

 

[SAIL3]

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

 

[DST148]

@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.

 

[bookowski]

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.

 

[JedClampett]

It's not only software providers who like these changes. $299 members, $349 non-members.

 

[dcarr82775]

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.

 

[SteelPE]

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.

 

[dcarr82775]

Misunderstood, apologies.

 

[XR250]

@JedClampett

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

 

[steellion]

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.

 

[Hokie93]

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).