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Re: Light Pole Failure

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FWIW, the commentary in the fatigue design section of the
AASHTO's highway sign and luminaire 2001 standard confirms that
square sections are much more susceptible to fatigue problems
than round sections, and it states that square sections have a
history of fatigue failures.  It also urges caution using square
sections even if a fatigue analysis is performed...

Dave Evans, P.E.
TNH, Inc.


To:             	seaint(--nospam--at)seaint.org
Date sent:      	Fri, 8 Feb 2002 01:02:50 -0800
Subject:        	Re: Light Pole Failure
From:           	r nester <rnester(--nospam--at)juno.com>
Send reply to:  	<seaint(--nospam--at)seaint.org>
Organization:   	http://www.seaint.org

> 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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