Wind Load on Temporary Structures

[GoCyclones]

I am looking for a little guidance on reducing the design wind load for a temporary structure with an approximate design life of 1 year. There is Table C6-7 of ASCE, but I am not for sure on how to use it. Do you just multiply the map wind speed from Fig 6-1 by the factor in the table corresponding to an MRI equal to the expected life span.

 

[DWHA]

For all temporary structures we use AASHTO Guide Design specifications for bridge temporary works.

 

[GoCyclones]

Thanks for the response DWHA, but I guess I should have mentioned that my structure is a building.

 

[MiketheEngineer]

I have also struggled with this. But why would a temporary structure "see" any less wind load than a "permanent" structure?? Esp - if it is up for a year??

OF course - once it blows away - it was truly temporary...

I have seen thunderstorms pop up around here that pretty much come very close to design limits - sometimes 2 or 3 times a year!!

 

[GoCyclones]

Thanks for the reply MiketheEngineer. I have considered your argument as well, but you could make the same case for a 100 year event for a permanent structure. Here is my argument for reducing the design loads: A temporary structure designed for some lesser wind load can have the same probability of actually seeing the design wind event as a permanent structure designed for the 50 year MRI wind load. I just am not sure how to arrive at the lesser wind load.

 

[patswfc]

In the UK you cant reduce the wind load for any structure that is subject to at least 6 months of wind.
I see the logic that your trying to use but dont agree with it.

Just because the structure only has a design life of 1 year doesnt mean that it will see less wind in that time. It is likely that it wont see the 1 in 50 year storm during the period that its up, but you never know when that 1 in 50 year storm will occur so i wouldnt try and reduce the wind load.

 

[GoCyclones]

Thanks for the reply patswfc. Isn't it all about probabilities though? Just to play devil's advocate, you could see a 100 year storm in that time as well, so don't you just pick a probability that has been deemed acceptable, which is what the codes have done by selecting the 50 year storm. So couldn't the same probability be used for a short-term building? I am not saying anyone of you are wrong as I have had those same arguments, but someone else used the reduce wind speed based on this probability idea and I am just trying to see if anyone else has done that.

 

[csd72]

A few things to consider:

Firstly, the level of wind load to which we design for is not a set in stone, concrete number. It is based on statistical analysis and an approximation of the likelihood of failure that is acceptable to society. We cannot make building with zero possibility of failure, so we have been forced to pick a number.

Secondly, society does not expect that after a hurricane all temporary structures are going to remain undamaged. It is cheaper to take the risk and accept the consequences in the unlikely event that it is subject to this level of wind.

Thirdly, the shorter something is up, the less likely it is that it will be subject to that load. I would use the reduction factor for short term structures (less than 1 year) that do not have a significant life safety factor.

 

[UcfSE]

Look for some temporary construction loads. In the US we would look in ASCE 37-02. There is a table that relates construction period to construction wind speed, basically a factor less than one which increases as the construction period increases. You might be able to extend that to temporary structures.

 

[frv]

UcfSe..

I don't know about that. A structure under construction is by definition unoccupied. You can get away with lower wind speeds because the danger of loss of life is much lower. If a storm comes during construction, I suspect the workers would not be taking refuge behind an erected, unattached panel.

The structure GoCyclones mentions is Temporary, but occupied (at least that's my interpretation). Consequently, I'd be careful about reducing the design wind speed. The probability of seeing the design wind event in any given year is exactly the same for a structure designed for a life cycle of 50 years as it is for one designed for one year.

 

[UcfSE]

If a storm comes during construction, they will go home and party.

My point was for the OP to look at that information where wind loads are related to a relatively short life and decide if it was appropriate for use in his or her case.

 

[frv]

On this I will agree.. I believe this will fall under "engineering judgment"

I recently attended a seminar where one of the speakers pointed out that if you include load factors into your calculations, the wind event is more along the lines of a 600 year event.. haven't looked at the numbers, but I have no reason to suspect he was lying..

 

[DRC1]

For tempoary steel structrures, ALOWABLE stress were, depending on the desiner occasionally increased by 10% to recognize that a) 50 year design events during a 1 year life were significantly less likelyto occur than in a permenant structure, and b) it may be possible to tolerate some plastifcation of the structure. This was never part of any code, and did require review of the design situation.

 

[SteveGregory]

ASCE 7-02 may have a solution for you. Table 1-1 puts "Certain temporary facilities" into Category I. The wind load importance factor is noted in Table 6-1. For Category I, it is 0.87 or 0.77 if it is in a Hurricance prone region with V>100 mph.

However, Category I is limited to buildings and other structures that represent a low hazard to human life in the event of failure.

 

[csd72]

I thinks situations like this come down to the question:

Could you justify your choice of wind load to a commitee of your peers?

If you cant then you know what will happen if it does fall down.