Observation Platform

[kipfoot]

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.

 

[JAE]

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.

 

[JAE]

Possibly charts like the attached might help.

 

[JAE]

Or this one....

 

[kipfoot]

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.

 

[msquared48]

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