Personell Hoist Wind Design

[anchorengineer]

I'm designing the tiebacks for a Personell Hoist on the east coast. The building is 126ft to top of roof. The top of the hoist is 145ft. I calculated the wind force using ASCE 7-02, section 6.5.13 (Open buildings and other structures). My question is: How conservative should I be? I'm getting a wind force of 29 psf +/-. I started to design for both cars to be at the top of building, one above top floor and one at the floor below it creating the worst case surface area. I used the 50 yr wind speed which induces a huge load for the tie back. Is this too conservative? I decided to multiply the wind load by .75 because of low probability of both cars being at the top of the building during this event but it is still very high. Do I assume they lower the car during the 50 yr wind? If I do then what speed should I use? I'm starting with an existing drawing that notes an "in service" wind speed of 35 mph and a "Out service" wind speed of 115 mph. I placed a note on the drawings that the cars need to be lowered to the ground when the wind velocity is greater than the "in service" speed. 35 mph creates a very small load and not conservative enough.
Thanks for you help!

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[JedClampett]

Where are the cars normally stored? If it's on the ground, I'd say you were OK. But if they're stored hanging off the hoist at roof level, I'd say you need to design for full wind. My reasoning is that you might get a windstorm during a weekend or at night. Even if it's predicted, what's the chances of someone sending a crew back to work, in bad weather, to lower the car? And if they geet there and the wind is already howling, how will they lower the car? By the time soneone can react, the wind might be 50 mph and the car is swaying severely all the way to the ground.
I'd say you were on the right track with the huge load on the tieback.

 

[MiketheEngineer]

I go with Jed -

This is exactly what caused a window washer's platform to fall off a Chicago skyscraper a few years back - on I think a Sunday morning. Unfortunately, it "flew" all the way to an intersection crushing a car and killing 2 or 3 women.

 

[anchorengineer]

Thanks guys. I just realized that I was using the same Cf factor for the car at the top of the hoist (solid rectangle)as the hoist tower (truss structure). See ASCE 7-02, Wind section, table 6-22 for tower and 6-19 for elevator car. I was getting Cf = 2.1 for the tower and I get Cf = 1.4 for the car/rooftop equipment. I believe this is a more logical assumption. The loads are still large but seem more "reasonable"... whatever that means. I just don't want to be too conservative when something has been done for many years before I entered the game. I still get a wind force of 45 psf for the car vs. 65 psf for the tower.

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