oh sign failures at baseplate

huangyhg

o/h sign failures at baseplate
if i had a big financial grant i would study this and get a phd or something, but since i don't i'd like to know if anyone has any thoughts on an intermittent failure we are having where steel posts are welded to baseplates and then failing in a brittle manner.
one of these situations exists where you have an overhead sign structure supported by steel posts on each end.  the posts are selected based on the total moment and shear at the base to determine the diameter and the wall thickness of the steel post.  a pattern of anchor rods is selected to resist the same loads.  the posts are welded to the base plates and the base plates are secured to the anchor rods with a nut top and bottom to provide a leveling device during installation.
the trick seems to be in joining the post to the base plate.  to reduce the total volume of weld metal, many fabricators want to cut a circular hole in the base plate to fit the column into.  a fillet weld is then made all around on the top side, and on the underside of the column to the inside of the circular hole in the base plate.  i believe this method of connection has been forbidden in the latest guide specification.  i have seen a fair number of cracks develop in this location in the haz of the circular column.  some have completely failed and appear to be a brittle mode.
i believe additional stresses are being introduced into the column due to the heat introduced into the column at the time of welding.  the heat of welding causes the steel column to expand.  the weld cools and fixes the column in a position above ambient temperature.  after the assembly is completed, it is left with a high tensile residual stress acting circumfrentially at the weld, but because of the weld fixing the tube the stress is permanent.  when the structure is placed in service, the design stresses act normal to the direction of this pre-existing stress condition.  i believe the presence of these two substantial tensile stresses causes the column to fail in fatigue before it would be predicted if the circumferential stress were not present.
if anyone understands this rambling, let me know if you have thought about this problem before and what you think the cause may be.
dinosaur,
i think what you describe, in terms of heating/cooling shrinkage causing residual stresses in the pipe are very possible.
i just returned from an all day conference where one of the speakers, who was a forensic-type engineer, was describing a failure that he investigated some years ago.  the situation was a large billboard structure - with a large diameter pipe support that failed and fell over.
the final analysis revealed that the wedge shaped billboard had a certain resonance with the wind in which there was constant cycling of twist in the poles.  in this case, the weld splice between pipes failed and he found that the original engineer hadn't considered fatigue at all.
in your case, you still have some level of fatigue, but with probably fewer cycles than the situation above.  but in any case, sometimes poles do simply stand there and flex back and forth continually all day.  this increases the cycles/day and thus fatigue enters into consideration.
i could be that the combination of fatigue at the weld (a bad fatigue detail per the code) and the residual shrinkage stresses add to the problem.
gussets between post and base plate?
base plate grouted to support load?
following jae's fatigue suggestion, there are some weld details that set up a triaxial stress state with the residual stresses from weld shrinkage in addition to the stresses due to external loading.  that triaxial stress state prevents the steel material from yielding and forces it to fail in a brittle manner.  i don't know that's what you have in your case but it might be worth looking at.
i suggest vibratory stress relieving.
thanks for the discussion.
foets, fabricators don't often offer to use stiffeners here claiming they are more expensive then what you get back for them.  likewise, the maintenance folks do not want the base plates grouted so they may have a better time inspecting the anchor rods.  i don't think this failure can be attributed to the lack of grout.
jae & ucfse, we have been pushing the fabricators engineer to consider fatigue particularly since we have seen these brittle failures of steel poles.  because of competitive bid, they refuse to do anything that isn't specifically required in the code.  if they were to start, someone else would always win the contract.  however, i am shocked that the failed poles don't spark more inquiry regarding the failure and asking the engineer for more backup documentation into his decision making process.  we have an ongoing inspection program due to this problem which has identified the problems usually resulting in immediate action.  this may be why, so far, we haven't had a fatality (knock on wood).
eddyc, i am not familiar with vibratory stress relieving, but maybe i know it by another name.  can you expand on it including the particular expected benefits?
dinosaur,
stress relieving is used in order to reduce or eliminate residual stresses that result from fabrication processes, such as welding. there are several methods of stress relieving:
a) thermal (put the parts in a controlled oven)
b) vibratory
c) cryogenic
d) shot peening
e) others methods that i don't know about
probably vibratory would be your best bet since i am assuming that your "parts" are of significant size. try to contact: www.stressreliefeng.com. they are involved in vibratory stress relieving of civil engineering structures.
you may also wish to verify that your welded components are not suffering from undetected lamellar tearing, via non-destructive testing.
the idea of putting the shaft partway through the flange and welding inside and out is commonly used for pipe flanges, so it's not a unknown technique.
i'm not sure i follow the problem here exactly.  it sounds like these are publicly bid, so maybe you're with a highway department or something?  in that case, can't you just write the requirement for fatigue design into the specs (or just design it yourself if you know what the problem is?).
you mention this is done to conserve weld metal.  what is the alternative weld detail- a single big fillet on the outside?  in that case, i would expect more problems with fatigue rather than less.  and with the fatique/ stress relief issues, wouldn't you normally expect to see problems in the weld itself rather than the haz?
have you looked at cold-temperature notch toughness?  if you're in the far north, that would be something to check into.
we are moving toward a tighter specification for design and materials, so charpy v-notch toughness and fatigue stress limits are in the works.  however, these provisions address related problems and not the specific effect i believe is going unaddressed.  i am interested in knowing if this heat problem is well founded, as jae seems to agree.
these poles are publicly bid, and are handled as essentially a design-build item in that we specify the requirements and let the contractor submit his design for review and then produce poles.  i could design a pole and base plate but that is not my role.  my organization wants me to do other things and if i were to design the poles, that would likely become my only job.  i wouldn't mind designing a few, but if i start designing them all, i'll never get another promotion.  worse, i'd likely get downsized when the industry got the ear of the right legislator.
so i'm looking for help with the specifications to address what i believe is a problem with the fabrication concept that contributes to failures related to welding, haz and fatigue.
incidently, is the weld in the haz?  or are the haz and weld mutually exclusive?  if so, i should have said some failures are in the haz and others are in the weld.  my bad.
thanks for the help.
residual stresses can be alleviated by pre- and post-weld heat treatment.  depending on the wall thickness of the column and the base plate thickness, the pre-heat and post-heat temperatures and hold times can be developed.  this is not a difficult process and most shops are familiar with it.
as for the fatigue, i agree with jae.  i did a similar investigation about 15 years ago on a power line structure. wind induced vibrations can significantly affect the fatigue life.  keep in mind that all frequencies of vibration felt be a structure are cumulative.