>From what I can see, it looks like Hal is closer to right than some of the
other posts. The legs do seem to be cracked at where the most stress is
going to be, both when up and when being raised and lowered. If the bottom
of the legs are not supported when up, then the plate that is welded to the
leg then takes all the abuse all the time. It does not appear that the
welds are the problem. First step should be to talk with the maker, it they
do not have a recommended solution, it then fall on your shoulders to make
the call. It also appears that the plates have two bolts in each, if one is
not removed in each before moving, that is going to put alot of stress on
the plate and leg (which I have to wonder has not happen at some point).
The common problem of moisture being trapped at the bottom and freezing also
comes to mind.
When and if the legs are welded, and it has beeb galvinized, remember the
concerns with heating and the fumes that will be given off. If a triangular
gusset is added to the top of the plate and welded to the leg, it will
transfer the stress to a much larger area, which is what I would do. This
is not a difficult repair, and if electrical power is close by, it would be
a quick fix.
To me, the small amount of attachment between the plate and leg are really
suspect, and I would have changed that before it was installed the first
time. I would have also more support between the two legs and the third one
(which I would suspect is questionable now). Many fold over towers are
simply a triangular plate that all three legs bolted to, and then the plate
itself pivots. I have made and/or installed many that are that way.
I guess I have a different take - looking at the pictures and having
reviewed these sorts of failures for years in the aerospace industry.
The legs (not the welds) have broken at the point where the side plates
are welded to the legs. Also, the failures show no sign of ductile
deformation - note how sharp the cracks are. Also, the failures have
occurred at the point of maximum applied tensile stress to the leg
members.
This looks like a classic hydrogen embrittlement failure of the legs.
http://www.corrosion-doctors.org/Forms/embrittlement.htm
A few thoughts:
- The third leg will likely fail the same way, at any time.
- This is fixable - the legs need to be realigned and backing stock
welded in place by a professional. All three legs!
- The rest of the tower is suspect, but the failures have occurred at
the point of maximum applied tensile stress - as you move up the tower
the loads get smaller and the chances of a hydrogen embrittlement
failure go down. The rest of the tower may be okay. It should be
THROUGHLY inspected - every leg and tubing weld as a bare minimum.
- It is difficult to detect cracks with the eye and impossible to detect
hydrogen embrittlement without very sophisticated equipment. Applying
some side force with a pry bar may be the best you can do
inspection-wise.
- It is quite possible the manufacturer has seen this before. Get them
to talk to you - if they will.
Good luck -
Hal
N4GG
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|