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This message was also sent by h.d.richardson(--nospam--at)shaw.ca to: rconnor(--nospam--at)purdue.edu

Fellow engineers,

        Following is a copy of my correspondence with Dr. Robert Connor, of 
 Perdue University on the subject of wire rope for strakes which he has 
kindly allowed me to post.  His report is impressive and may be helpful for 
 those of you who are interested in this subject.

        Thanks again for your earlier postings on my behalf, particularly 
Ray Minor who reference Dr. Connor's work.

Regards,

H. Daryl Richardson

----- Original Message -----
From: Robert Connor
To: h.d.richardson(--nospam--at)shaw.ca
Sent: Tuesday, February 05, 2013 8:40 PM
Subject: Re: Strakes For Small Diameter Stacks

Hi Daryl
We did not really perform any complex calcs to size the strakes, but we 
found 1 inch diameter rope worked very well for the geometry of our poles. 
We did some back of envelope calcs to estimate the size and we found about 
1 inch should work.  There are some general guidelines in the literature 
that you should be able to find on sizing strakes.   They need to be spaced 
 properly, which we discuss in the report.  Frankly, we sort of estimated 
what would work and did not spend a lot of time calculating spacing either, 
 just looked at references and did our best estimates.  Again, I am sure 
you can find guidance on how to lay them out that is more rigorous.

The poles were about 150 feet tall and tapered multi sided.  For our type 
of poles, the strakes really "killed" vortex shedding.  The best is for you 
 to download our final report from NCHRP 
(http://www.trb.org/Design/Blurbs/167099.aspx).  You can download the 
entire report and all appendices for free.  See sections 2.3.4  and 3.1.4.1 
 of the report for the details on the strakes.

The issue is durability of the rope in the environment.  I imagine if you 
had something that would last a long time, they should work.  Of course, 
they need to be secured properly as well.  We did leave strakes on a pole 
in Iowa which as far as we know have been in place for about 2 years now. 
I have not heard of any problems from Iowa DOT to date.

As for natural frequencies, we have a lot of data in the report on what we 
measured.  Our analytical predictions were in reasonable agreement for 
first 3 modes or so.  Damping is less then what the old AASHTO specs 
suggest.  As for support effects on frequencies, I can't really speak to 
that.

Hope this helps
Robert Connor

On 2/5/2013 6:19 PM, h.d.richardson(--nospam--at)shaw.ca wrote:

  Dear Dr. Connor,

          I am a structural engineer practicing in Calgary, Alberta, 
Canada.  I have recently come up with the idea of using wire rope for 
strakes instead of fabricated flat plate as an inexpensive solution for 
small diameter flare stacks for the petrochemical industry.

          I posted this problem on the engineering bulletin board at

    SEAINT-SEAOSC(--nospam--at)mail-list.com.

  and received a number of helpful replies including some from Ray Minor, 
who indicated that you had done work on tall lighting poles wrapped with 
fibre rope.  I would be interested in any information you might provide on 
the subject.

          Two topics of particular interest would be:
  1.) Would rope (wire or fibre) be suitable for use as strakes?
  2.) What experience have you had regarding natural frequencies being 
lower that calculated due to flexibility of the foundation?

          Thank you for any help you may provide.

  Regards,

  H. Daryl Richardson, P.Eng.

  P.S.  I have included information for the engineering bulletin board 
below if you are interested.  I have found it to be helpful, informative, 
and entertaining for the last 15 years.

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