Total Lateral Building Drift

[acwooten]

I have a concentrically braced frame, steel, stair tower, with approximate plan dimensions of 24'x9' and an approximate height of 50'. This is an industrial application, where the tower is being used to access a bridge between two tanks for general maintenance. What is an appropriate lateral drift for the total structure (i.e. not story drift or inter-story drift as I wouldn't think those are really applicable).

H/200? 300? 400? 500? 600? Is there really a limit at all since there will be no cladding on this?

Thanks!

 

[jayrod12]

I would try to stick to H/400 for user comfort.

 

[acwooten]

Do you have a code reference for that? Or is that more of a general rule?

I've seen code stipulations of H/100 to H/600 for story and inter-story and H/400 to H/600 for cladding and materials. But haven't been able to find anything for human perception really.

 

[jayrod12]

I tend not to design new structures for less than that. think about what H/400 means at 50ft, 1.5" of sway. Trust me when I say if you're on those stairs and they're going back and forth in a high wind event 1.5" you notice.

 

[UKJim]

No code reference, strictly judgment/experience: h/500. If the cost to provide h/500 is significant (I'm guessing it is not), only then sharpen your pencil. Making a quantitative estimate of the velocities a person on the top level would sense from wind gusts would be a pain, as would a non-linear analysis to be confident of the stability of an uber-flexible braced frame.

 

[vmirat]

Human tolerance of sway is subjective. If you've ever been high up on a scissor lift, you know what it's like. To a construction worker with experience working such lifts, it's not a problem. I was once on a single-man lift up 20 feet that was quite wobbly. I couldn't wait to get down!

The cost/benefit has to be looked at. If it's a major concern of the customer, then I would use H/600. The wind speed you use is also a factor. An exterior tower of that type won't be used during a code-maximum speed of 120 mph. ASCE 7 commentary discusses this.

I would approach this from a different way. Set the max drift at 3/8" at a speed of 50 mph, then figure the structure from that. Nobody should be up there if the wind speed is greater than that. I would also research OSHA to see if they have any restrictions on working on heights at high winds.