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Observation Platform

Observation Platform

Observation Platform

(OP)
I'm designing an observation platform that will be supported on three steel pipe columns and be about 30 feet up. In plan, the columns form a triangle with 4 foot sides. I've derived the wind loads but I've bumped these up to be about 10% of the dead load in my prelim analysis because I want the tower to feel stiff.

My questions are:

a) what's the best way to have a greater confidence that the structure will 'feel' sturdy to the people up there? I can look at vertical vibration, but my bigger concern is people moving in a way to get the tower rocking.

b) Is there a frequency associated with people on a platform that my structure should be designed around?

Thanks.

RE: Observation Platform

Can you calculate a natural frequency of the tower?
The period can give you a feel if people moving laterally can get it to oscillating.

The higher the frequency the better of course. Not sure what would be an adequate threshold though.

RE: Observation Platform

(OP)
JAE, Thanks for the feedback.

I (my software) can calculate frequency of the structure:

mode #1: 3.1 Hz
mode #2: 6.0 Hz

I find that I can increase the frequency (reduce the period) by changing the size of the diagonal members I've modeled. Now I just need to decide whether I want to do this.

RE: Observation Platform

You can also add some mass to the structure and increase the stiffness by grouting the pipe columns.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

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