ASCE7-10 or IBC 1609.6.3 Which Wind Design Method to Use for Small Scale Buildings and WHY? 4

[TBacon]

I mostly work with small scale construction, rarely encountering buildings over two stories tall. I would like to know which wind design methodology people use and in particular why. Does one method tend to give you more conservative, or more accurate results over the others, again when considering the design for smaller scale buildings? In an effort to understand the different results obtained from the various wind design approaches I ran the numbers on a simple, single story building, with a flat roof with the following criteria and came up with a fairly wide range of design wind pressures for the walls (all calc'd / converted to an ASD force level). My sample building is enclosed, has a simple diaphragm, and so as I understand it the internal pressure coefficient, (GCpi) is equal to zero.

Building dimensions: 80' x 30' x 14' tall oriented with the long dimension facing north/south
Risk Category II
Vult - 110 mph
Exposure Category 'C'

IBC 1609.6.3 P = 15 psf (in both directions)

Chapter 27 Part 1 P = 14.8 psf N/S & P = 12 psf E/W

Chapter 27 Part 2 P = 21 psf N/S & P = 12 psf E/W

Chapter 28 Part 1 P = 13.4 psf N/S & P = 10 psf E/W

Chapter 28 Part 2 P = 14 psf (in both directions)

 

[TehMightyEngineer]

GCpi should not be zero. Per ASCE 7-10 Table 26.11-1 for an enclosed building GCpi = ±0.18 and you must consider both load cases of positive or negative internal pressure applied to all internal surfaces.

As for which method to use; this is the real problem with ASCE 7-10 which has caused much griping in the engineering community. Lots of options and obfuscation for little perceived benefit.

It helps to know which method does what. Here is my very basic summary of each method with the areas I would use them for.

[ul]
[li]IBC 1609.6.3: Conservative, simple, but not as fleshed out intent in atypical situations I feel. I mostly ignore it and use ASCE 7-10 methods due to my greater familiarity, but this section would be a good use for your needs and get both a conservative but not overly conservative value for wind pressure.[/li]

[li]Ch. 27 Part 1: Use this for more complicated high rise structures. This is the "all-heights" method 2 from ASCE 7-05 and covers most structures.[/li]

[li]Ch. 27 Part 2: Use this if you love tables. Excessively conservative for most structures in my opinion. I have never found a need to use this method. This is a new method introduced with ASCE 7-10.[/li]

[li]Ch. 28 Part 1: This is the best method for typical low-rise buildings. Typically results in lowest wind pressures and is probably the most commonly used method by my understanding. It covers most typical building cases, is relatively easy to apply, and generally makes sense to me. This is mostly unchanged from ASCE 7-05 (the low-rise "all-heights" method).[/li]

[li]Ch. 28 Part 2: Use this if you need a quick, conservative answer for a low rise building and the more conservative wind pressures will not result in much additional cost. This is the simplified method 1 from ASCE 7-05. I've used this method a lot for simple single-story structures. Don't use this for anything that's not a simple structure; move to Ch. 28 Part 1 for those.[/li]
[/ul]

There's a number of decent texts on the topic of ASCE 7-10 wind load provisions. A good one I've referenced in the past is

http://www.asce.org/templates/publications-book-detail.aspx?id=6753

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

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[TBacon]

Thanks for your reply. Maybe I misspoke when I said that GCpi = 0. Maybe what I should have said was that for an enclosed building, with a simple diaphragm the internal pressures cancel out for the walls. Is that correct? So for this type of simple diaphragm building that is enclosed, the wind pressure calculation (say from Ch 27 part 2) simplifies to P = q*G*Cp.

Do I understand this right?

 

[TehMightyEngineer]

Ah, yes, for a regular structure the effects of internal pressure will cancel out for MWRFS design.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

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[TBacon]

Great. Thanks for the confirmation. I regret that I find the current wind design to be so complicated and confusing. I truly do want to master wind design per the ASCE, but honestly miss the simplicity and efficiency of the old UBC wind design equation which was maybe not that sophisticated but it was also pretty fool proof. Unofrtunately my experience with engineering design (working for very small firms and working mostly with small scale projects) has been that I've always had a lack of resources available to me for continuing education and have struggled to keep up with the constantly evolving code.

It's certainly not your responsibility to "educate me" in this regard, but is there a simple way to explain (or a good resource you could point me to) that explains the whole concept of internal pressure? Like when you are referring to the IBC 1609.6.3 (conservative and simple approach likely sufficient for many buildings I would work on) and you are looking at table 1609.6.2 for Walls of an Enclosed building, I am using the + Cnet factor for Windward Walls and subtracting the - Cnet factor for Leeward walls which gives you a combined Cnet value of 0.94 [0.43- (-0.51) = 0.94]. This combined Cnet value for an enclosed building of 0.94 is the same for +Internal Pressure as it is for -Internal Pressure (0.73- (-0.21) = 0.94). So I guess don't understand how / when you use these +/- internal pressure coefficients to get any different value of Cnet other than 0.94.

Again, certainly not your responsibility to explain this to me, but if you are interested in educating an eager student I would welcome your reply!

 

[TehMightyEngineer]

I regret that I find the current wind design to be so complicated and confusing.

I truly do want to master wind design per the ASCE, but honestly miss the simplicity and efficiency of the old UBC wind design equation which was maybe not that sophisticated but it was also pretty fool proof.

I've always had a lack of resources available to me for continuing education and have struggled to keep up with the constantly evolving code.

You're not alone on any of these. This is a huge complaint by many, myself included.

is there a simple way to explain the whole concept of internal pressure?

Think of a structure as a aircraft wing. As the air is forced to move around the structure it causes a low pressure inside the structure (essentially it's sucking air out of the structure as the structure is not airtight). This is represented as a negative internal pressure, this increases the forces on the windward wall (it's being pushed on from the exterior wind pressure while simultaneously being pulled on by the negative interior pressure).

However, if we open up a bunch of windows and doors on the windward wall then the building turns into a parachute and not a wing. You're blowing it up like a balloon. This causes a positive internal pressure.

For the MWFRS this often wont matter much as the end result is it all cancels out (assuming both the windward and leeward walls provide similar areas for the pressures to act on). However, the roof structure external pressure will likely be controlled by the suction external pressure; adding a large positive internal pressure onto it adds to the net pressure and may overload the roof hold-downs. This is why people board up windows for hurricanes, they're trying to prevent large, windward openings forming in the building envelope, generating a large positive pressure. This is also why partially enclosed structures have higher internal pressures in ASCE 7.

Simplified designs like the IBC essentially combine internal and external pressure into a unified "net" pressure via the tables, so one way or another you're always considering it. Where differentiating between internal and external pressures matter is when your building is not symmetrical, then the internal pressures do not cancel out nicely.

I could go on and on but realistically you'll want to read up a bit on this on your own, don't forget the ASCE 7-10 commentary and having an example or two that you can follow will likely make everything much clearer. I'd personally recommend buying a book or two on your own dime if your work can't/won't provide them. Loads are the most basic part of structural engineering and the most important design step in my mind. While a pain, it's worth learning how to calculate them correctly for modern codes and to make the effort to keep up with things.

All said, ASCE 7-70 does have one advantage. You don't need to know all the methods, just learn the most applicable one for your needs and design everything with that one method. If you find the IBC method or simplified method of ch 28 part 2 the easiest and most applicable then there is nothing incorrect about that.

P.S. Don't forget components and cladding wind loads and the minimum wind load cases.
P.P.S. The minimum wind load cases for MWFRS design are just that, minimum load cases. Not minimum wind pressures. However, for components and cladding the minimum pressures are just that, minimum pressures.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

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[TBacon]

Thank you so very much. Your explanation of wind as far as internal pressure (positive and negative) and how this relates to windward and leeward pressures really helps a lot! I also appreciate your calling attention to the minimum wind load cases (which I had not ever looked at) and the minimum wind pressures (which I was aware of).

Given the information that you've given me I'm going to revisit my sample building again and rework through the different methods. I like your suggestion to find a method that works for me (and for most of the buildings that I work with) and then stick with that one method.

So much yet to learn...thanks again for taking the time to provide me with the information you did and sharing your knowledge. It is greatly appreciated!!

 

[TehMightyEngineer]

One last thing to note; at the start of each wind part in ASCE 7-10 there is a checklist of steps to take to utilize that method. It's quite handy and I wish more texts did this.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

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[TBacon]

Yes. I am familiar with the step-by-step checklist. And agree, it is indeed handy. Cheers!