Wind Speeds Used in Outdoor Event ANSI E1.21-2013

[jdgengineer]

I am working on a project that involves some temporary rigging for an outdoor event which will be designed to ANSI E1.21-2013 (previously ESTA E1.21). The document references ASCE 7-10 for the design wind speeds and allows you to use 75% of the design wind speeds. We are providing a certification letter and providing calculations for the trusses and necessary ballast weights.

In our letter, we quote that if 3-second wind speed gusts that exceed our design wind speeds are anticipated, or observed, that the structure needs to be taken down as it is at risk of collapse.

My question involves what these wind speeds should be. In the past, we have used ASCE 7-05 for the design which had the wind speeds calibrated to ASD level. Now that ASCE 7-10 uses strength level wind speeds I'm a little confused as to the wind speeds to quote. We base all of our design using the ASD equations.

Previously, we had used 0.75 * 85 mph (California) = 64 mph. Now we are using 0.75 * 110 mph = 83 mph. However, we obviously reduce this by 0.6 when we do our force calculation and, therefore, arrive at the same wind forces as the ASCE 7-05 approach.

Does anyone have information as to how these design wind speeds relate to actual observed 3 second gusts? I'm tempted to do the calculations for the 83 mph design wind (strength level) but state that the structure needs to be removed at 64 mph wind speeds (ASD level). This would be more in line with what we had quoted on our past designs.

In reality, to some degree this is semantics as no one will be present at an outdoor events with wind speeds this high...

 

[hokie66]

I'm not going to comment on what wind loading should be used, but the philosophy of assuming that "no one will be present" is flawed. High wind events develop very quickly. We had discussions about this on the site a while back, when there were some people killed at a couple of events in the US, one of which was in Indiana. Memory is short....

 

[jdgengineer]

Hokie -- Yes I agree, that statement I made was incorrect, and I agree that the philosophy is incorrect. It's not my intent for this not to be taken seriously, but in the Bay Area peninsula these wind speeds would be highly unusual. So let me retract that statement, and focus more on the original question. Keep in mind that the design remains the same in both instances, it's the information that is conveyed to the event personnel that I'm looking for. In reality, for these structures we typically tell them to take them down at 40mph as it is my understanding that is the OSHA limit for safe wind speeds for working.

 

[hokie66]

OK, but what I am saying is that depending on a substantial structure being disassembled in a wind event is impractical. Maybe we are talking about a different type of "outdoor event".

 

[BAretired]

Summer storms can come up pretty fast. See the link and consider the consequences of using reduced wind speeds. If the event involves a large number of people, I would not reduce wind values below those shown in the code.

http://enterprisesolutions.accuweat...s/AccuWeather_Success_Avoidable_Tragedies.pdf

BA

 

[jdgengineer]

Sorry let me give this some context. It's a few LED videowalls. We tell them to remove the videowalls at 40mph (per direction in ANSI E1.21-2013) as they can be removed in less than 5 minutes. We then say the structure should be removed or the structure abandoned at the design wind speed (75% of ASCE 7-10). I agree it's impractical to remove a large stage like the one present in Indiana. In our case, these are not large structures.

We typically follow ANSI E1.21 which is a document put together by the entertainment industry entitled "Temporary Structures Used for Technical Production of Outdoor Entertainment Events". It is my understanding that this document is the standard for these types of temporary structures and is what the Indiana stage should have been designed to (which it was not). In this document they allow reduced ASCE 7-10 wind speeds (75% of design wind speeds). It's my understanding that using reduced wind speeds the structure would have the same probability of collapse as a structure that is erected for much longer with the full code design forces.

It appears that perhaps, you are in favor of not using this reduction and instead using the same wind speed as the building code for permanent structures. I'm not saying I'm against this, but it's my understanding that this is not typical (at least in my experience).

Regardless, we are diverging from the intent of my post which is to ask does anyone know how the design wind speeds correlate to actual field measured wind speeds. Now that we have changed wind speed from an allowable level to a strength level, how does this correlate to actual measured wind speeds. The design force is essentially the same with some hand waving accomplished by a 0.6 conversion factor to ASD. However, the design wind speeds have changed from 85 mph to 110 mph (in California).

 

[dhengr]

Jdgengineer:
I won’t comment on the wind speed to be used or the design methodology to be used either. But I will second what Hokie and BA have said. These structures are fraught with potential problems, not the least of which are inexperienced erectors, lack of oversight and inspection during this process. These structures can see wind loads from many directions at the same time and are very dynamic. Then you can have crazy things like counter weights moved because they are in a circulation path which the promoter thinks would be nicer, bolts and braces missing, etc. These wind events do happen quickly, often without much warning, although even with warning from the weatherman the promoter is not want to shut down the show if they can help it. Obviously, you are not going to disassemble the structure in short order, when the weather people say there is a storm coming. But, you could design some of the cladding and sail material to be stripped quickly by some means, or just sacrificially rip away with little harm. I thought that the general safety thinking was that at some hazard level the event was supposed to be shut down and the area around the larger stage structure, or whatever, was to be evacuated.

 

[zcp]

jdgengineer,
As part of the Operations Management Plan, wind speed is to be monitored. The wind speed measurements are to be taken at the height of the structure. Actions are to be taken when the measured wind speed reaches the speed specified in the operations management plan.

Check out Figure C6-4 in ASCE 7-05. It will help you convert. Since you only have what is probably an anemometer onsite, we generally lower our 'take X down' wind design speed numbers a relative percent to account for the difference in max and 3 sec. Then publish that number in the Operations Management Plan.

Hope that helps.

ZCP

http://www.phoenix-engineer.com

 

[EngineeringEric]

I believe the loads of both 7-05 and 7-10 are the same when you compare ASD to ASD and STRN to STRN. So if you are comfortable with the old speed then use the same speed. Wind didn't get more powerful in the code revision, the speed-values now include the importance factor.