Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Engineer's responsibility -- wind failure

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Thanks Christopher Wright, Ed Schmeckpeper, and Lew Midlam for your comments.
(Incidentally, Lew, I used to live in Naples....)

For the above and anyone else interested in more info:

I do a lot of large fences, 125 feet high in some cases, for driving ranges.
These are cantilevered or guyed poles with cables and one inch polypropylene
or polyester or nylon nettting.  I have done a fair amount of research on
this, including wind tunnel testing the type of netting my clients use.

Since they are non-building structures, ASCE 7-95 and the UBC and the Southern
Building Code must be "massaged" somewhat to apply.  For example, I can use a
"percent solid" factor to modify UBC forces to match those found in the wind
tunnel.  Also, other building effects, such as perpendicular suction forces,
do not occur -- the pressure differential on each side of the netting just
doesn't happen.  (In some cases I design for these anyway since I cannot
convince building departments otherwise....)  Other dynamic effects, such as
Von Karman, are minimized by guys in the plane of the netting (perp. to wind
direction.)  Deflection limits usually do not apply.  This is not rocket
science but there is some thought required.

One competitor in particular has been underbidding my clients and winning many
contracts.  This person is NOT an engineer, and has local engineers overstamp
his designs, such as they are.  I knew that most likely some of the
fundamental assumptions were different.  I was so intrigued by his designs
that I flew to Fort Lauderdale to examine the range and to try to find info on
the engineering.

It is a 110 foot high fence, with poles at 100 feet on center and triple guys
to a single anchor into and out of the range.  This pole spacing is quite
wide, because normally the net will rip at this span.  But apparently they
used more expensive nylon netting, which has greater strength.  (As an aside,
I have seen that stronger netting works to the detriment of the poles, and
have even toyed with the idea of "break-away" netting, much like football
jerseys, but cannot find a tie device with a guaranteed _maximum_ strength.)

I calculated their anchor forces to be in excess of 100 kips.  The anchors
they used typically provide 20k in the sandy soil there.   The poles were
embedded sixteen feet and had three guys -- one at the top and two at the
third points.  This puts all the force into the guy anchor, and very little
into the pole foundation.  For example, designing with the base of the pole
modelled as fixed or pinned makes very little difference in the forces seen by
the cable because of the much higher stiffness of an axial loaded cable versus
a cantilevered pole tip.  This is counterintuitive to non-engineers, and my
client wanted to know why our designs were so much more substantial.  He
couldn't believe that a fixed base would not lessen the guy requirements.  I
spent quite a few nights trying to find something in my calcs, but finally
told my client that I really needed to see the project and the design because
I couldn't make it work.  If anyone has been in this situation, you know it is
very difficult.

I believe the freedom of information act should entitle anyone to see the
plans on a project subject to approval by a tax-payer funded local building
department.  They were not to keen on us looking at the plans though.  The
first guy we sent to examine the plans had his film taken by the local
building department.  Later we tried again, and we did photograph one page
that clearly stated "designed for 75 mph wind loads in accordance with
sections 2309.1(4), 430? & 4403 of 1996 Broward County S.F.B.C." and then were
kicked out.  

The windspeed alone raised a red flag for me, because this is an area subject
to high wind forces and typically has a 110 mph wind design load.  Remembering
that the force increases with the square of the windspeed, this is quite a big
difference.  At that time, which was only a week or two ago, I posted a notice
wondering what the requirements were when one found a design believed to be in
serious error.  I reserved some judgement since these are not "occupied"
during windstorms -- nonetheless they can be dangerous if they fail.

No one responded definitively, I'm not sure one could, but Neil Moore had some
helpful input since he designs a lot of towers (antenna, etc.)

Then literally less than two weeks later, I hear that the ENTIRE range fence
blew down, in winds less than 75 mph.

It's the Golden Bear Driving Range right off 95 (pretty sure it's 95) on the
north side of Fort Lauderdale.  Yes, I AM working with Golden Bear on a
replacement system.  And no, I have no idea what is happening with the
engineer who designed the previous system.

Hear about it in the local news, Lew, or is this of interest only to us crazy
engineers?