We're tying two towers together with a section of mostly unsupported pipe (the towers are close together).

In normal analysis when one considers wind in his analysis, it's applied as a static load, and the entire system blows in one direction to its individual calculated displacements, and when the wind goes away, everything returns nicely to its initial state.

Normally this is a decent design point.

However, in the real world, when the wind blows and gusts, everything ends up swaying at its natural frequency. Back and forth, back and forth.

When you have a pipe connecting two different pieces of equipment / structures that are swaying at different natural frequencies, then if you were to take a static snapshot in time, it would look like wind blew one vessel (or structure) in the completely opposite direction from the other.

The analogue would be to perform a time-history seismic analysis with multiple structures to achieve this response. In this case, we could do this statically and assume the worst case opposite deflection.

Questions to the panel:
• Have you performed analysis on a similar piping configuration?
• Have you considered these dynamic effects before?
• Is the proposed methodology sound, or is it over-conservative?

Thanks.

Hi Michael

The Nature of Wind is to flow in one direction only, so didn't get why opposite deflection case needs to be studied.

Please explicate the requirement if my understanding is not correct.

If you note the following video and watch the central power pole very closely, you can see that when the wind dies down, it sways beyond the neutral point. I can't tell if it could be capable of swaying back further if the wind were to die down for a longer period of time, but it's not beyond the scope of engineering to calculate.



I won't hazard too many guesses as to why the two poles are reacting differently, despite being almost practically identical. Possibly the ground's different.

Thanks.

Perhaps the approach used for seismic anchor displacements would apply here.
If you have two swaying structures, apply max displacements from one structure at a time (other structure is not moving) and sum those two independent support motion sets with the wind stresses on the pipe.

Michael,
The following thread gives you the references that you can use for your application if I do understand you correctly.

http://www.eng-tips.com/viewthread.cfm?qid=310179

You can calculate gust wind and cross wind effects on the structures like chimneys (if the towers are with circular pipe sections). This can probably help you to solve the piping problem accordingly.

Ibrahim,

The towers themselves are pressure vessels and are indeed of circular cross section. I've not considered higher modes of vibration as of yet, as I'm just breaking the mold as it were with even low modes of vibration resulting in opposite movements of the towers according to their natural frequencies.

So thanks for that - I'll keep it in the back of my mind and suggest whether or not they wish to consider these effects.

Thanks.

Michael,

I guess I did not explain adequately. I was not implying the second mode of natural frequency, but giving you the the reference only for you to be able to calculate the displacements at the top of vessels for the first mode of vibrations under along (gust) wind and cross wind actions.

Regards.