ASCE Wind 7-16 USD questions 2

[Ron247]

I still use ASD design loads but find myself having to explain USD concepts for wind to someone. Take the following situation. The ASCE 7-16 Risk Cat II wind map shows a 160 mph for his county. Let’s also assume there will be a floor live load of 100 psf for a bar/restaurant. If a USD design is used:
[ul]
[li]Would I tell the Owner his floor is designed for 100 psf or 160 psf (1.6x100)? My answer is 100.[/li]
[li]In wind, I am not referring to what I would write on my drawings, but what would I tell the Owner? Would I tell the Owner his building is designed for 160 mph wind or something much less than 160 mph? Since there is no overload factor applied to wind, I assume the new wind maps have already had the wind speed bumped up to include the overload factor. I also assume the seismic has been bumped up since it also has a 1.0 overload factor. Am I wrong about this?[/li]
[li]If an actual sustained wind speed of 150 mph is recorded, should the Owner expect his building to survive with minor non-structural damage?[/li]
[/ul]
I grew up when only concrete was USD. Now that it is commonplace, I find it harder to explain how it works to a layperson than it is to do the calculations. Usually, it is the other way around. Also, why is Dead only multiplied by 1.2 in the combinations? Overload factors used to have to do with both uncertainty and providing a cushion between the failure load. 20% seems slim to me.

 

[r13]

The basic load specified by ASCE7 is the service load, which is the working load for ASD design. It is then bumped up by load factor to get ultimate load, or factored load. You shall tell your client his building was designed for the basic load, and indicate which design method (ASD, USD, or LRFD/LSD), code (ASCE7, IBC, ACI, or AISC), and version of the code were used .

 

[JLNJ]

Hey R13. Wind loads are not service loads

I bet your client can understand the difference between types of loading. Explain to him about the change in ASCE 7 wind loads and how it’s all related to statistical probabilities.

I don’t think you want to make any guarantees or generalizations about what wind load will cause which parts of a building will fail.

 

[r13]

Understand. This probably explains the difference. Code evolves, but IMO, the basic loads without factor is the service load, which is to be listed on the design documents, then adjust (scale up/down) by the appropriate factor for design.

For the dead load factor, it used to be 1.4 in old USD, and 1.7 for live load compared to 1.6 now. Too many changes for wind, not sure I remember all.

 

[JLNJ]

No, you don’t list the “basic” wind load, you list the Ultimate Wind Speed.

 

[Ron247]

The reason I gave a floor loading of a 100 was to illustrate how we still call it a 100 psf design load even though it gets bumped by the overload factor. So if the wind map already has been bumped up, why would I tell him it is a 160 mph design? The problem for me is incorporating the overload factor into the wind map. The methodology is not consistent.

 

[r13]

I don't think I should argue with an experienced practicing engineer. The image below shows where I got my impression on "basic wind speed", but it does not not indicate I was correct. I hope this ends this argument.

 

[RWW0002]

Don't feel bad R13, the change to ultimate winds speeds is counterintuitive to many of us. I do not want to start in on anASCE-7 is ridiculous or ASD vs LRFD discussion, but I cannot pass up the opportunity to complain a bit about the unintended consequences of codes moving to ultimate wind speeds. Here is my partial list:

-As mentioned above, using ultimate wind speeds further divorces the general public from wind speeds they might hear on the news and safe expected structural performance. It also changes the common practice of giving owners service-level loads and factoring above that for uncertainty and factors of safety.

-For wind-rated components, the change to ultimate wind has caused all kinds of confusion for installers. Will that 90 mph (ASD) rated window work for a 115 mph (Ultimate) building? While it is not that hard to put the ASD wind speed on drawings also, (and is now required in many areas), why make things harder than they have to be?

-Since service-level loading is still used for deflection and serviceability checks and most of the time for design of soil-bearing foundations (along with a good portion of wood, masonry ans steel design), I do not think the change reduced the work for practicing engineers, and in fact made it more cumbersome for many.

-"Net wind uplift" for delegated component designers (prefab wood trusses, bar joists) becomes more complicated than it needs to be. I find myself giving long-winded noted to the joist suppliers just to explain what loads I have provided and what loads they are to design for.

 

[RWW0002]

Now for a more relevant addition to the discussion:

After a tornado came through a town near me recently, there was a news story on how much of the newer construction fared worse than older construction nearby in the storm. Their concerns appeared valid, but instead of focusing on the consequences of taller skinnier construction, whether tornado events should be considered in design, or some shortcomings in residential design or code enforcement, the entire discussion revolved around confusion over design wind speeds.

The story point out that that the municipality needed to update to IBC 2018 in lieu IBC 2012 because "2018 International Building Codes require buildings to be stronger and withstand winds of 115 miles per hour". Not only did they miss that 115 mph in 2018 is equivalent to 90 mph in 2012, but also the fact that that wind speeds for that city were reduced to 105 mph in ASCE7-16 resulting in a reduction in the design wind pressures for the new code.

This is just one more example of the confusion over changing a wind standard. I think this one was worse than changing from fastest mile to 3 second gust. At least that one made a bit of sense when communicating to the public..

 

[Aesur]

This has caused much confusion in the past; technically per code you specify the "Nominal Loads" which per the definition is the loads coming out of the ASCE document, which would mean wind in 7-05 was ASD level, but in 7-10+ it is LRFD. This is the definition on page 1, chapter 1. The issue arises where many engineers don't understand, nor believe this and many firms even publish "learning resources" that say nominal is always ASD. Because of this we specify that wind and seismic loads are ultimate LRFD state, U.N.O. On our plans, we provide LRFD loads for the wind zones and ASD loads for the net uplift zones and specify as such. If we have drag loads on deferred items we specify which state the loads provided in. When verbally telling an owner I would tell them the ultimate wind loading as it's current code. On the plans, per IBC, you are required to specify "Basic Wind Speed, V (this is LRFD for ASCE7-10+) and Allowable Stress Design Wind Speed, Vasd - reference section 1603.1.

 

[dik]

been doing it wrong for years... I stipulate the service design loads... In Canada, our wind and seismic loads are treated as service loads with applicable load factors... when all combined, there are 20 or more different loading conditions... for some of them from an SMath program...

The alpha with the different postscripts are for different load factors... 1.25, 0.9, etc. for DL. and so on...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Rabbit12]

RWW0002, I hear you on the wind-rated components thing. I just recently had an OH door that ended up having like 2" windows because "that's the biggest the manufacturer could make work with the wind load". I asked the question whether they were using ultimate or allowable wind loads but the vendor doesn't understand that stuff. I'm at least three levels away from the actual door designer so it's near impossible to really get any clarification.

On a related note I've dealt with several jurisdictions that obviously don't understand the changes from allowable to ultimate with regards to wind. When you get on a website and see the code listed as 7-10 or 7-16 and then the wind load listed as 90 MPH you know they don't understand. Happens more than you'd think. I do a ton of work in special wind regions so it'd be nice if they had valid information.

 

[r13]

A laundry list of US/ASCE7's changes in wind load.

1982 - 2005

2010

2016

 

[r13]

RWW0002,

Thank for your kind words. Whenever get to the topic about wind, my head start spinning. So miss the good/simple old days, we only need to provide "Wind Load - xx psf; Uplift xx psf" on the project drawing. I won't dispute the necessity in code changes, but....