Exterior tower live load with wind
Exterior tower live load with wind
(OP)
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?
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?






RE: Exterior tower live load with wind
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.
RE: Exterior tower live load with wind
RE: Exterior tower live load with wind
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.
RE: Exterior tower live load with 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.
RE: Exterior tower live load with wind
RE: Exterior tower live load with wind
RE: Exterior tower live load with wind
(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.
RE: Exterior tower live load with wind
RE: Exterior tower live load with wind
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.
RE: Exterior tower live load with wind
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.
RE: Exterior tower live load with wind
I can see the half live and 3/4 wind okay.
RE: Exterior tower live load with wind
RE: Exterior tower live load with wind
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.
RE: Exterior tower live load with wind