Exterior tower live load with wind

[JAE]

For an exterior framed observation tower (park-like setting - open to the public) I am using 100 psf for the live load. I'm satisfied with the use of this.

For wind - this is in an outdoor environment and I've calculated the loads based on 90 mph Exposure C based on its location.

In combining D+L+W, ASCE 7-05 allows the use of the following load combination (ASD):

D + .75L + .75W

My observation of this is that they have statistically observed that the chance of a full live load and a full wind load occurring simultaneously is low - thus the 75% reductions are allowed.

This makes sense for a buildng where people WOULD occupy the building during a windstorm. However, this tower is outdoors and it seems that the chance of a full live load with full wind would be EVEN LOWER. Would you ever get even close to 100% LL with a heavy wind blowing? I don't think so. But it appears that I'm locked into the 75% reduction.

Does it make sense to lower that combination to this:

D + 0.5L + 0.75W
D + .75L + 0.5W

There's no code justification for it - just common sense?

 

[JedClampett]

I ran into almost the exact same situation recently. The building is in Florida and I neglected to check .75L + D + .75W case. The wind is 140 mph and I just couldn't justify a case when someone would be up on a roof during a hurricane wind (there's no equipment up there). But technically, I have an overstress (actually a reduction in the factor of safety) for this case. Anyway, I can live with it and I think I can justify it if challenged.
There used to be a 1/3 overstress allowed for cases where wind or earthquake was combined with live load. That seems to be out of style. I wish high wind speed regions would allow the 1/3 overstress for this case. It seems to make good sense for this ridiculous case.

 

[slickdeals]

Unfortunately, there is no justification for it. It's like forcing you to design a topping slab with a 1.5 safety factor so that it does not fly away during a hurricane.

 

[JoshPlumSE]

There is a section in ASCE 2.3.6 (and the commentary) which provide criteria for which you can create your own LRFD type load combinations. That may not directly relate to you because it is LRFD type load combinations. But, the criteria and methodology may help.

Also, the ASCE commentary to the ASD load combinations cites a paper by Turkstra and Madsen (published in 1980) which formed the basis for the 0.75 load reduction. If you can get hold of that paper, it may go into more detail.

 

[enginerding]

If you look at the combinations you have suggested, even they seem quite high under some conditions. 0.5 W corresponds to a 64 mph wind (because pressure varies with the square of velocity). 0.75 W is a 78 mph wind.

I would think about it like this: would people seek shelter on/under this tower in a storm? This type of wind is to be expected with a severe thunderstorm. If people caught outside would run to this structure to seek shelter from the storm, I would not feel comfortable reducing the load combinations.

On the other hand, if the people (or other "stuff") on the tower would blow off of it in a wind this high (unlikely because there are probably rails and guards), then it would seem silly to include this load simultaneously with the wind.

But the code does not support any reduction besides the 0.75 factor on each. This would have to approved by the building official.

 

[DWHA]

Just remember that live load is anything not permanently attached to the structure. Objects may be placed up there, and would exist during a high wind event. So, I feel that you should have some live load accounted for during a high wind event.

 

[hawkaz]

You can use your own engineering judgment (as long as the jurisdiction agrees to it)- but if something went wrong, I wouldn't want to defend why I varied from code in court

 

[ishvaaag]

Persistent or transitory situations limit states spanish code CTE (4.3):
(just cases that have wind and live load)

(1.35 ó 0.8)*DL + (1.5 ó 0)*LL + (0.60 ó 0)*WL
(1.35 ó 0.8)*DL + (1.5 ó 0)*WL + (0.7 ó 0)*LL

Assume you might normalize to DL to 1 by division of all by 1.35 (something difficult to necessarily sustain given the different nature of the loads), the hypotheres then become for the nonzero cases

1*DL + 1.11*LL + 0.44*WL
1*DL + 1.11*WL + 0.52*LL

looks as if the opinion is more or less keep one at full value, then halve the other, or better, both somewhat over that.

 

[ishvaaag]

A case more of code fiction.

 

[azcats]

Could it be considered a Special Purpose Roof and have the loading then be under the Lr combinations? See ASCE 4.9.2.

However, you have to slide over to the alternative basic load combinations in the IBC (not in ASCE 7) to find combinations where Lr is not included in conjunction with W.

 

[JLNJ]

If you have a tall tower and a low "evacuation" occupancy you might never be able to get close to 100 psf live load on the entire stair height. That's something like two people on EVERY tread of every rise on a 44" wide stair. This loading is unreasonable, but there is no code provision I know of to reduce it.

I am currently working on an exit tower of some 100' feet out of a industrial process building. The proces is manned by just a few people. Maybe we could get 100 people during a construction or process maintenance event. Using 100 psf LL on the entire tower stair height seems unreasonable.

I feel comfortable applying some engineering judgement to reduce this load if I get in a tight spot.

 

[fancypants]

I certainly do not know the specifics of this tower. However, I want to play devils advocate a bit and remind everyone of the stage that just collapsed in a quick approaching storm.


I can see the half live and 3/4 wind okay.

 

[JAE]

I went 60% live and 75% wind. I feel comfortable with that combination as the 0.6D + W, the D+L control most of the member designs anyway.

 

[asixth]

I think so. I work to a different code to you but the wind combinations I would be using are:

1.2DL+0.4LL+W
0.9DL+W

These are ultimate limit state combos as well. Only the brave would be prepared to stand on an observation deck in 60mph winds.

 

[WillisV]

JAE - I do not see how that would stand up in court if something did happen. Not saying I disagree with your reasoning, but it does not meet code.