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Re: Light Pole Failure

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Steven ---

I was involved in a forensic investigation in 1999 resulting from one
such failure - a freeway message sign in Southern California.  Apparently
failed in a moderate wind after only 8 months in service.  Somewhat
larger, and with 20 inch steel pipe mast, and with the "socket" base
plate detail as you described.  CalTrans inspected widely, and took 17
similar structures out of service due to fatigue cracking just above the
weld-affected zone on the back side.  In our case, a smaller fatigue
crack was also found also on the road side, indicating some severe
dynamic conditions.  The crack leading to total failure had existed for
months (fully oxidized), with the final load contributing only the last
few cycles to failure.

Three factors lead to the failure, and to the similar damage to other
signs.  First, the thick base plate with a large hole - without
reinforcing gussets - leads to large local bending stresses in the
thinner pipe just where you described the damage.  This will occur
regardless of the degree of grout support, and can be readily seen in a
fine-mesh finite element analyis.  Even a hand analysis showed that this
local stress was close to yield with dead loads alone.  A solid (small
hole only) base plate will reduce, but not eliminate this local stress.

Second, most signs are loaded dynamically by wind due to the
vortex-shedding phenomena.  In my case, the vortex shedding frequency
closely matched the natural first mode for the sign. Coupled with the
local stress problem, this load also peaked out at close to yield.

Third, the cantilevered freeway signs are subject to a wind pulse from
passing trucks.  This was measured in the field using strain gages on
another location, and was a significant peak stress.  The relatively
undamped sign structure would see at least two significant such strain
cycles per each truck passing.  Excursion at the end of the sign was
nearly 12 inches.

Fatigue analysis for the message sign concluded that either wind or the
trucks were sufficient to induce a short useful life.  Unfortunately, the
designer had extrapolated the (static) design from lighter non-message
signs, and had not incorporated 20 years research into this newer
application.  Further, the lack of gussets, while saving a few dollars,
made the difference between a 30+ year calculated life and 8-12 months
total failure.

Do a literature search - much research over the last 20 years.  More
recently, many types of wind dampers have been used, some very
successfully. I would recommend to always use gussets for the
pole-to-base connection. Our analysis showed that without them, there is
no reasonable design.  You may have to go beyond AASHTO design standards
to ensure a long-lasting pole structure.  Our paper was published in the
December 1999 ASCE Conference on Natural Hazards in Washington DC.
Better grouting techniques are not the answer, and are truly a waste of
time.  All the grout can do is hide the bolts and provide some corrosion
protection.  Design for the bolts to carry full loads.

Russ Nester
Simi Valley, CA

*************************************************************************
***

On Thu, 7 Feb 2002 16:34:35 -0800 Steve Hiner <shiner(--nospam--at)folsom.ca.us>
writes:
> Two light pole failures in the last three years ... doesn't sound
> like many,
> but the failure mechanism is of some concern.
>
> Central California - 75 mph design wind speed
> Some strong winds occurred several months back (40-50 mph gusts)
> (Poles did not fail/drop during those storms)
>
> 30' tall poles with light standard (about 3 years old)
> 5" square steel tubes, 1/8" thickness (material spec - not sure)
> 12" +/- x 1" thick square base plate w/ 4 anchor bolts
> The base plates have 5" square holes such that the tube slides
> within the
> base plate thickness
> Fillet welds - all around (3/16" or 1/4"? +/-) at top of base plate
> to tube
> AND at bottom edge of tube to 5" square slotted edge of base plate.
>
> Failure has occurred in the steel tube just above the fillet weld
> between
> the top of base plate and the tube section.
>
> Would be interested to hear some opinions or from those who may have
> experience with similar failures.
>
> You can also contact me personally at shiner(--nospam--at)folsom.ca.us
>
> Regards,
> Steven T. Hiner, SE
>
>
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