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O/H Sign failures at baseplate
8

O/H Sign failures at baseplate

O/H Sign failures at baseplate

(OP)
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.

RE: O/H Sign failures at baseplate

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.

RE: O/H Sign failures at baseplate

Gussets between post and base plate?

RE: O/H Sign failures at baseplate

base plate grouted to support load?

RE: O/H Sign failures at baseplate

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.

RE: O/H Sign failures at baseplate

I suggest vibratory stress relieving.

RE: O/H Sign failures at baseplate

(OP)
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?

RE: O/H Sign failures at baseplate

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.

RE: O/H Sign failures at baseplate

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.

RE: O/H Sign failures at baseplate

(OP)
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.

RE: O/H Sign failures at baseplate

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.

RE: O/H Sign failures at baseplate

What are the materials that you are welding
and what type of weld rod?  Why can't you
stress relief the weld?  Do either of the
materials contain any lead?

RE: O/H Sign failures at baseplate

The detail you are talking about is shown in the AASHTO Spec for Structural Supports of Highway Signs, Luminaires and Traffic Signals.  Chapter 11 deals with Fatigue design, and the diagram in Example 7 of that chapter is a fillet welded socket connection as you describe.  I am not an expert in this, but this detail is pretty common in the lighting poles that I have designed, and it shows up in the DOT standard details that I see.  

Part of the reason of having a hole down there is that you can run wires out of the ground and up into the pipe, which may be important for poles where you have small pipes.  That AASHTO spec requires Fatigue design for some components, which limits the stresses on the weld by code.

RE: O/H Sign failures at baseplate

Specifying stress relief may or may not be an easy way out.  It depends on what's required and equipment available.  Call up some of the fabricators you deal with and ask them about resources available for it.

As I understand it, the HAZ is the area outside the weld that is affected metallurgically or mechanically by the weld process- so the actual weld metal itself is not part of the heat affected zone.

Seems like some of the weld specifications I have seen require Charpy impact tests in the HAZ.  I don't know specifically what problem this is trying to avoid, but it very well could be the problem you're describing.

With the normal assumptions made in design, I don't think the distortion from welding the column to the base plate in the manner described would really cause the problem.  If it did, then one other approach would be to pre-heat the base plate before welding it.

RE: O/H Sign failures at baseplate

Dinosaur,
Just out of curiosity, what area of the country is this?  I have had experience with brittle failure at sub-zero (mostly artic) locations.

RE: O/H Sign failures at baseplate

Have holes that allow moisture to escape been placed near the bottom of the column or in the baseplate directly under the post location? May be getting some additional stresses due to freeze/thaw if they are placed in those conditions. We have drilled holes in deformed window frames and HSS frames and witnessed gallons of water coming out.

RE: O/H Sign failures at baseplate

(OP)
The Charpy V-Notch test measures impact toughness.  A notch is cut to specific depth with a radiused bottom of the notch in a specific sample size.  The specimen is broken with a pendulum swing which permits the measurement of energy lost in the fracture.  The test requires the metal dissipate a specific amount of energy.  The perscribed energy varies with temperature.

I am in the eastern US below New York and above Florida, in a place called Jesusland after the 2000 election.

The point of my original post is that I believe the effects of temperature during welding introduce a large stress into the connection that is not accounted for in design.  Heating the baseplate will not eliminate the problem because the same geometric conditions that existed to create the stress are present when the baseplate cools.

Many have theorized that the double fillet is the problem.  Their answer is to not allow the double fillet.  I don't believe this will solve the problem, and may, as an earlier poster said, exacerbate the problem.

A hole at the base is beneficial as it allows access for the wiring and drainage.

If my theory is correct, the tolerances on the hole are crucial to getting a lasting connection.  The circumferential strain is the same as the difference between the original hole diameter divided by the original column diameter.  Since we know the yeild strain is on the order of 0.00125, then the tolerance on the hole in the baseplate is 0.00125 x Col Dia.  For a 12 inch dia column, 1/64 inch - for a 24 inch dia column, 1/32 inch - for a 24 inch dia column 3/64 inch, and so on.  The tolerances will produce a residual circumferential stress about equal to the yeild stress.  Poisons ratio causes a strain of about 1/4 this in the longitudinal direction of the pole.

I don't believe the fabricators are shooting for tolerances as tight as this, and so I believe we are having more strain.  The strain comes from the heat expanding the column during welding.  To get a strain of this magnitude, you only need to raise the temperature 200 degrees F.  Welding causes the local temperature to rise much more.

So there are two questions:  Am I correct in thinking that the tolerance on the diameter of the hole is very important to the state of stress in the pole base? - and - what can we do to help the problem before fabrication?

RE: O/H Sign failures at baseplate

I love this site.  It forces me to learn more.  I found an article on HAZ with respect to SAW (you gotta love acronyms).
http://files.aws.org/wj/supplement/Gunaraj2-02.pdf
Most of you guys may already know this stuff, but I thought I'd add it anyway.  The article states that "Excessive heat input could result in a wide HAZ with low impact strength, particularly in high heat-input submerged arc welds."  Since they are most likely welding the base plate to the column in the shop, they could be using SAW to do it, or some other high-heat welding process.

RE: O/H Sign failures at baseplate

In looking at this some more, sounds like you are viewing as a restrained-joint issue.  That could be.  If so, I would expect the weld problems to be in the weld or in the base plate, but not in the HAZ above the weld.  I did run across this link that discusses restrained-joint welding a bit:
http://www.hatch.ca/Engineering/Structural_%20Assets/newsletters/SAE%20News%20015.pdf

Before trying to tighten up the tolerance on that hole, consider how the hole is cut (IE, tolerance on the torch or other cutting method) and also tolerances on OD and out-of-roundness of the pipe going into the hole.

I'm assuming you or someone has gone through the weld procedures and qualifications and inspections pretty well to make sure that they were actually welded up to snuff in the first place?

And finally, if you have access to a failed pole, it might be worthwhile to hire a specialist to look into the failure.  Assuming a solution and specifying fixes for it could involve a lot of hidden costs down the road, without fixing the problem.

RE: O/H Sign failures at baseplate

(OP)
We have had a couple failures and a few folks have looked at one or two of the failures.  The degree of expertise they bring to the problem is not known to me.  I suspect the review of the weld process did not account for the restrained weld situation.

The out-of-roundness of the pole and the hole cutting methods are a significant contributer to the problem as I believe it is related to the tolerance of the hole diameter explained above.  One of my problems, is that the tolerances I believe are necessary to solve the problem (if I'm correct) are very difficult to acheive.  The pieces would have to be match marked and milled I think.

The problem creates a double curvature bending in the wall of the pole as it transitions from the welded fixed diameter to its unrestrained diameter, again if I'm correct.  This creates bending stresses in the HAZ above the weld in yet another direction.

I expect to receive training to become a CWI in the next calendar year.  At that time, I will look for an opportunity to look into this problem again focusing on the welding specifics.  Right now, I'm just looking at the problem as a structural engineer.

RE: O/H Sign failures at baseplate

Is the hole baseplate chamfered before welding"
Is there a j groove on the pipe.  From what I
have read, you simply have a clearance hole
in the base plate with 1/32 clearance.  Bearings
are sometimes welded to a car body or car bed
and the circular diameter has a split in it
and it may be that it was for the reason that
you are implying. Can you split the pipe for a
short distance before welding?  Are either parts
heated before welding?  Can you weld the base
plates to the columns before assembly?
You might want to post this to the welding forum.
Are there specific specs for this type of welding
that you are using?

RE: O/H Sign failures at baseplate

(OP)
Dimjim,

You are very enthusiastic about designing these supports, but that is not my question.  These supports are designed by the contractor's engineer and reviewed by our structural department.  I am not going to wade into the design.  I left the structural design department because if I stayed, I would be doing these reviews and never get another promotion.  Now I work in another department and I am looking at the specs to see if a better spec can be written to improve the life of these structures.

The poles are not perfectly round.  The holes are not perfectly round.  I would say that the combination of "out of roundness" results in a gap typically about one quarter inch, but sometimes more.  As you can see from my earlier post, this results in a strain that is over 4x the strain necessary to yeild the pole.  Is this a problem from the point of view of a restrained weld condition?  If so, how can we spec the connection to eliminate the problem?

Currently, I can't advance this with the structural department because they don't see this as a problem.  I don't think they understand the restrained weld problem and how temperature causes this strain.

RE: O/H Sign failures at baseplate

According to Article 5.15.3 of the AASHTO Spec for Structural Supports of Highway Signs, Luminaires and Traffic Signals, the alternate connection is full-penetration groove weld.  Using a solid base plate, a small hole in the base plate can be made for wiring/drainage.

What is the relative thickness between the pole and the base plate?  I seem to remember that welding thin members to thick ones is results in greater residual stresses.  Something to do with the thicker member acting as a heat sink, and the thin one getting extra hot.

Extensive experimental/finite element studies of tube to base plate connections have been made at Lehigh.  These studies were mainly concerned with bolt stresses induced by the flexibility of the base plate.  But flexure of the base plate would impact the weld as well.   Cutting out the hole for the entire post would sgnificantly alter the flexibility of the base plate.  

RE: O/H Sign failures at baseplate

Most base plates and columns/poles are full penetration (small hole for wires and water) or slip fit (stronger-double fillet) but both are usualy rose bud heated to make a better weld. But to change the material to a brittle condition I would suspect over spec'ng the material/rod or the fab shop had some high strenght stuff laying around. Or bad location designing ( one size don't always fit all).
just wandering.
rent

RE: O/H Sign failures at baseplate

Dinosaur,

So far, I haven't seen an overwhelming response indicating that this is a universal problem, which makes me think you may be having a local issue with fabrication.  As far as specs go, I suggest contacting AASHTO, AISC, and AWS to see if they can shed some light on this issue.  Maybe this is a bigger problem than we know.

Something else occurred to me.  If they're welding the pipe at the same location above and below the base plate, does this increase the size of the HAZ for both the plate and pipe?  Maybe they could weld a ring sleave to the base plate first and then insert the pipe support into the ring sleave and weld at the top of the sleave.  That way, you get some separation between the two welds.  Just brain storming here.

RE: O/H Sign failures at baseplate

We were having trouble with some weld materials
cracking due to too much sulphur in the parent
materials.  Controlled the sulphur in our specs
and the problems went away.

RE: O/H Sign failures at baseplate

(OP)
One reason I am posting the question here is because I think I need structural engineers to contemplate the problem with me.  Welding guys I ask don't understand the relationship between the heat and the strain, but these guys are not engineers.  The welding guys are looking at the C.E. for the steel and the weld rod, the need for preheat to eliminate HAZ concerns and this sort of stuff.  All of this is good for improving the quality but does not zero in on my concern.

Structural engineers understand poisons ratio, and the significance of the circumferential strain being 6x the yeild strain, and the potential stress caused by double curvature bending through the wall thickness as it transitions from an artificially large radius back to the "at rest" radius, and that if all these problems could be cause by 200 deg F then it is very likely to be a problem.

I have been brain storming this off and on for about two years.  Every solution I think of sounds too expensive.  Checking into sulfer content had not occured to me except that it is addressed somewhat in the C.E. formula.

There is an engineer that works for a fabricator that I believe has all the right skills.  I plan to chat him up next time I am in that shop.

Anyway, I appreciate the help.

RE: O/H Sign failures at baseplate

2
Forensic building engineers have found lots of brittle failures after major earthquakes.  Many are caused by the steel being restrained in the orthogonal directions.  I.E. it could not yield by necking down.  I did not see a reply to the above question about the relative thicknesses of the pole and the plate but if the failure is right at the top of the top fillet weld then I believe restraint may be a contributing factor along with the residual welding stress and possibly some bending due to the anchor bolts not being in line wth the pipe wall.  Exactly where is the failure?  On those that have only cracked do the cracks show up near the bolt locations?  Ken

RE: O/H Sign failures at baseplate

(OP)
The failures are usually in the weld or the piece of the pole extending from the weld, maybe up to 1 inch away from the weld.

The pole has a comparatively thin wall thickness, probably up to 3/8 to 1/2 inch, but may get thinner than 1/4 inch.

The base plates are usually from one to two inches thick.

The anchor rods are a minimum of four-one inch diameter anchor rods of A36 steel.  We have had anchor rods fractured in the top third and we have had anchor rods work loose in the concrete.

I don't recall the specific grade of steels.  I believe it varies from one manufacturer to another.  It depends on where they go to get their supply of steel.  Our requirements are for a minimum yeild of 36 ksi, an 18% elongation in the test specimen, a C.E. below 0.45, and soon a Charpy V-notch test.

RE: O/H Sign failures at baseplate

Dino,

Without reviewing any of the calc's, my best guess is that the designer is ignoring the local bending due to the anchor rods not aligning with the shell of the pipe.  In building design the base plate is continuous and you check bending in the plate due to the distance from the flange of the column to the center of the bolt.  In your case the plate is not continuous therefore the bending has to be taken by the wall of the coulmn or the anchor rods.  Without grout there will be stress reversals.  You seem to have had failures at both locations.

Can you convince your structural engineers to check for this in the shop drawing review?  Can you specify that the loads from the pipe be transfered with stiffeners without counting on the circumferential weld?  It may cost a little more but if they are all bidding the same spec no one should complain.  Good luck.

RE: O/H Sign failures at baseplate

(OP)
MWPC,

The forces and moments causing load in the anchor rods are a result of the forces and moments used to design the poles.  It seems the forces and moments in the pole have to have been checked.

In a building column, you have an axial force and moment in the column, and the base plate is checked against the effects of the eccentric anchor rod to the column flange tip usually.

Unless you are alluding to a likely unequal load distribution in the pole due to the flexibility of the baseplate transfering four point loads into the pole, which I believe would have to be checked using a fairly rudimentary FE model, I don't get the disconnect between the design of the pole and the design of the anchor rods being non-conforming.

Stiffeners have proven to be hard points that increase the likelyhood of introducing fatigue problems.

Maybe I'll have to ask our Structural department to perform a few representative FE model analases.  Unfortunately, due to personalities and budget issues, that will be a long time obtaining.

It is my suspiscion that the failures in the anchor rods are largely due to poor construction practices.

RE: O/H Sign failures at baseplate

Dino,

All I am saying is that there is an eccentricity between the wall of the column and the centerline of the bolt.  If the base plate is not grouted to provide some fixity, this local bending must be taken in the bolt and/or the the wall of the column.  I am not an expert at sign base design.  Maybe designing for this local bending is SOP and is in the calcs.  I think it would be worth a quick check.  I have seen much worse overlooked by designers.

RE: O/H Sign failures at baseplate

(OP)
When I was designing O/H supports, I accounted for the bending in the anchor rods due to the ungrouted condition since I knew the agency would not permit grouting.  I don't think many were as rigorous.  I have also seen much more significant mechanisms overlooked by designers.

I am currently fixated on the cracks at the pole base.

I wish I had the resources to prepare a quick FE model to look at some of these other effects.

RE: O/H Sign failures at baseplate

2
Perhaps you are seeing a local effect from an abrupt change in stiffness of the resisting material.  In theory, when you weld (2) pices of steel together, they become one.  Following this theory, the cross section of the element at the base plate would be orders of magnitude stiffer than the pipe directly above the base plate.  The abrupt change in stiffness equates to an abrupt change in stress gradient.  Tapered stiffeners tend to smooth out the stress gradient to the base plate.  The doctorial thesis would be to examine the finite element analysis on lengths of stiffener required to transition the stresses within an acceptable range.  Good luck.

RE: O/H Sign failures at baseplate

In addition to your concerns about residual shrinkage stress, the double fillet weld would seem to load the upper weld more than the lower.  Under a static load condition, it would be safe to assume that, at ultimate, the two welds share the load.  But under a stress reversal, I expect the upper weld is going through larger stress cycles.

If fatigue is the concern, than I would think the groove weld allowed by AASHTO Article 5.15.3 would be a better option.

RE: O/H Sign failures at baseplate

happened across this on another post.

Fatigue and Fracture Control

As a crystalline material, steel is very susceptible to brittle fracture due to fatigue.  There are other mechanisms that lead to brittle fracture as well.  The SCM discussion on pg 2-33 is a must read section for engineers designing in steel.

Wind and Seismic Design

Both wind and seismic events have very high strain rates which can lead to brittle behavior.  In addition, current practices in seismic design typically do not design structures to resist forces elastically.  There is an inherent dependence on material ductility in the design philosophies.  Special detailing is required to ensure ductile behavior of steel structures in these events.  There has been significant advancements in this area in recent years.  These detailing requirements, again, are topics for a more advanced course, however, you should take the time to read the SCM discussion on pg 2-35

RE: O/H Sign failures at baseplate

(OP)
jmiec,

Yes, I agree the upper weld is receiving more load but only a working stress methodology will reveal it.  Unfortunately, most engineers are only learning ultimate strength/plastic design concepts so they don't perceive this.  My thinking on this is that if the upper weld has to yeild to permit the lower weld to share the load, then under fatigue considerations the upper weld is undergoing too much strain.

The problem with a groove weld is how do you do it?  You can't groove weld it from both sides, so you have to do it from one side only.  This requires the backing bar to remain in place which is a very poor fatigue performer.  And then when it is over, how do you inspect the weld to insure its soundness?  I doubt you can radiograph it and ultrasound will probably pick up the installed crack at the backup bar.

RE: O/H Sign failures at baseplate

According to Table 11-2 of AASHTO's Luminaire Spec, the groove weld (Ex. 5) with the backer gets a fatigue category E`, same (to my surprise) as the fillet welded connection (Ex. 7.)

The groove welded connection can be upgraded to category E by attaching the backer ring to the plate with a full penetration groove weld.

Even though the stress category is similar to the fillet welded connection, the stress range in the groove weld will be much less since there's a lot more weld there, especially when compared to the upper weld of the double fillet detail.

RE: O/H Sign failures at baseplate

(OP)
Back to the original question, do the stresses induced on the column due to the restrained weld cause any concern among the rest of you fellows?

RE: O/H Sign failures at baseplate

I don't think that should be a concern, if post weld treatment is used where the material thickness would so require.

It sounds like a connection detail problem to me. The only weld I would use in this situation is a full penetration bevel weld from column to plate without cutting a hole in the plate.  If you don't have time to put that on a drawing, outsource the design.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: O/H Sign failures at baseplate

(OP)
Big Inch,

You wouldn't cut a hole in the base plate?  How would you run the wiring to the lights?  In a conduit along the outside of the pole?

What kind of post weld treatment would aleviate these stresses?

As for out-sourcing, the whole thing is out-sourced already.  Did you read the thread?

RE: O/H Sign failures at baseplate

Little holes are fine, a cable, a weep hole, no problem, but not one big enough for the whole pipe to fit.  Its a base plate, not a pipe flange.

Weld, then heat if necessary, or at least slow the weld cooldown period.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: O/H Sign failures at baseplate

In this case I don't think the restraint and residual stresses are a problem with most moderate strength steels.  I think it is local bending in the column wall  and anchor bolts due to the eccentricity and/or lack of grout.  Ken

RE: O/H Sign failures at baseplate

Dino
I would be more concerned about the thickness
variations in the pipe.  I would like to know
more about the welding procedure for the pipe
to the base plate that is 2 inches thick
and lack of control for the roundness of the
pipe and hole in the base plate.  Not what you
want to hear however.

RE: O/H Sign failures at baseplate

Sorry, but I can't help think the wall thickness to base plate thickness is totally incompatible for welding.  At least in pipeline work, we have maximum thickness differentials between materials that can be joined together, which are usually not more than 3/16" difference in wall thicknesses of the joined pipes.  Joining pipe with wt difference > 3/16 requires a transition piece inbetween.  Anything over a 3/4 to 1" thickness requires a preheat and controlled cooling.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: O/H Sign failures at baseplate

(OP)
Biginch,

Hypothetically, let's say the column was 18 inch O.D. with a 3/8 inch wall thickness socket welded into a 1.5 inch thick baseplate using 1/4 inch all around fillet welds inside the socket and on top.  The overall size of the baseplate is 20 x 20.  The baseplate is secured to a concrete foundation of 3000 psi concrete using (4) 1-1/4" dia F1554 anchor rods with a minimum yeild of 36 ksi.  The structural steel is all A36 passing a typical Charpy V-Notch requirement.  The pole and anchor rods are galvanized in accordance with current ASTMs.  The baseplate is 1-1/2 inch clear above the concrete foundation with no grout.  There are no stiffeners.

What would be your concerns?

csd72,

The reaearch I have been reading also states stiffeners are not he answer to the problem for the same reasons I believe these Japanese folks conclude; the stiffeners create hard points that magnify fatigue problems.

For those petroleum guys out there, tell me about your process for joining a flange to a pipe and the design concerns including limits on thickness of the flange to the pipe wall?  How are pipe flanges designed?

Thanks.

RE: O/H Sign failures at baseplate

I'm really not a welding expert, but I'll tell you what I know and give you some of the notes I have on the subject.

I'd worry about the heat sink effect on the little fillet welds by the base plate and any restraint added by weld heating when fitted in the hole.  There doesn't seem to be any way the plate and pipe can expand and contract uniformly.  I just can't believe there are not excessive stresses remaining.  Since cooling time to about 100ºC is critical to the crack susceptibility, that just might be where the problem is originating.  the 1/4" weld must be cooling very fast.

Pipeline welding requires specific written procedures which are based upon four factors,
(1) the hydrogen level of the welding process
(2) the pipe wall thickness
(3) the expected cooling rate of the weld
(4) the chemical composition
Each procedure is qualified and the welders are tested and qualified to make the welds on the line pipe.

Pipe flanges are forged steel of varying grades from low carbon 30ksi yield all the way up to 60 ksi, so they're not at all similar to an end welded base plate, since they are welded to the pipe on the welding neck.  Fittings arn't as easy to weld as pipe due to the thickness. Thickness increases the restraint on the weld and acts as a greater heat sink, resulting in faster cooling and greater hardening of the weld. You should be a bit careful when welding fittings, particularly if high hydrogen electrodes are used. It is common practice to use a higher preheat temperature when welding a fitting than when welding linepipe, and some companies require the use of low hydrogen electrodes for large diameter fittings. An alternative approach for butt welded fittings is to shop weld short lengths of line pipe onto the ends of the fittings using a low hydrogen welding procedure before the fittings are delivered to the site.

Pipe can be from low carbon steel to the latest fine grain, low sulphur, high strength and high toughness steel with yields to 80 ksi.  

Differences in wall thickness are limited to 3/16", otherwise a transition pipe with an intermediate wall thickness is inserted into the assembly.

Weld-Neck Flanges to or pipe to pipe welds are full penetration welds made on joints beveled to 30º.  The weld is done with tacks for maintaining lineup, a root weld, filler weld and a final cap weld.  Typically the weld is performed using manual shield metal arc, gas metal arc, but wire is being used in many automatic welding jobs now.  

MMA and TIG processes are widely used for pipe welding, and MIG process is used when components are assembled in workshops.  Lately FCAW-G  Gas shielded flux-cored wire and gas metal arc wire is making an appearance for x80 pipe.

Pre/post heat treatments are always (I think) used when the material thicknesses are 3/4" or above.  Induction Heating blankets can do this very nicely.

I mentioned I didn't like the idea of the heat sink effect on a 1/4" fillet weld.  The HAZ undergoes a rapid thermal cycle during welding. The area closest to the weld sees a high peak temperature, resulting in coarsening of the grain structure in the steel. The rapid cooling after welding hardens the structure. This area, the grain-coarsened HAZ, is therefore normally the hardest and least tough part of the weld. The hard microstructure in the HAZ of the weld can be susceptible to delayed hydrogen cracking after welding, particularly if consumables that deposit weld metal with a high hydrogen content are used for speeding up the welding process.

Maybe I should have given you this link first, before making you read all that above, but I think you may find this guy very useful.  There are a lot of hints here, how to prevent weld failure, weld procedures, weld fatigue, metalurgy, and even has a weld preheat calculator.  Maybe he's worth the phone call to England.  Sure he is, there's VOIP these days.

http://www.gowelding.com/

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: O/H Sign failures at baseplate

I'm really not a welding expert, but here goes...some of the thoughts and notes I have.

I'd worry about the heat sink effect on the little fillet welds by the base plate and any restraint added by weld heating when fitted in the hole.  There doesn't seem to be any way the plate and pipe can expand and contract uniformly.  I just can't believe there are not excessive stresses remaining.  Since cooling time to about 100ºC is critical to the crack susceptibility, that just might be where the problem is originating.  the 1/4" weld must be cooling very fast.

Pipeline welding requires specific written procedures which are based upon four factors,
(1) the hydrogen level of the welding process
(2) the pipe wall thickness
(3) the expected cooling rate of the weld
(4) the chemical composition
Each procedure is qualified and the welders are tested and qualified to make the welds on the line pipe.

Pipe flanges are forged steel of varying grades from low carbon 30ksi yield all the way up to 60 ksi, so they're not at all similar to an end welded base plate, since they are welded to the pipe on the welding neck.  Fittings arn't as easy to weld as pipe due to the thickness. Thickness increases the restraint on the weld and acts as a greater heat sink, resulting in faster cooling and greater hardening of the weld. You should be a bit careful when welding fittings, particularly if high hydrogen electrodes are used. It is common practice to use a higher preheat temperature when welding a fitting than when welding linepipe, and some companies require the use of low hydrogen electrodes for large diameter fittings. An alternative approach for butt welded fittings is to shop weld short lengths of line pipe onto the ends of the fittings using a low hydrogen welding procedure before the fittings are delivered to the site.

Pipe can be from low carbon steel to the latest fine grain, low sulphur, high strength and high toughness steel with yields to 80 ksi.  

Differences in wall thickness are limited to 3/16", otherwise a transition pipe with an intermediate wall thickness is inserted into the assembly.

Weld-Neck Flanges to or pipe to pipe welds are full penetration welds made on joints beveled to 30º.  The weld is done with tacks for maintaining lineup, a root weld, filler weld and a final cap weld.  Typically the weld is performed using manual shield metal arc, gas metal arc, but wire is being used in many automatic welding jobs now.  

MMA and TIG processes are widely used for pipe welding, and MIG process is used when components are assembled in workshops.  Lately FCAW-G  Gas shielded flux-cored wire and gas metal arc wire is making an appearance for x80 pipe.

Pre/post heat treatments are always (I think) used when the material thicknesses are 3/4" or above.  Induction Heating blankets can do this very nicely.

I mentioned I didn't like the idea of the heat sink effect on a 1/4" fillet weld.  The HAZ undergoes a rapid thermal cycle during welding. The area closest to the weld sees a high peak temperature, resulting in coarsening of the grain structure in the steel. The rapid cooling after welding hardens the structure. This area, the grain-coarsened HAZ, is therefore normally the hardest and least tough part of the weld. The hard microstructure in the HAZ of the weld can be susceptible to delayed hydrogen cracking after welding, particularly if consumables that deposit weld metal with a high hydrogen content are used for speeding up the welding process.

Maybe I should have given you this link first, before making you read all that above. There are a lot of hints here, how to prevent weld failure, weld procedures, weld fatigue, metalurgy, and even has a weld preheat calculator.  

http://www.gowelding.com/

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: O/H Sign failures at baseplate

Do you require low hydrogen electrodes?  Could hydrogen embrittlement contribute to these failures?  Pipeline flanges are usually butt welded to the flanges using a "weld neck" flange.  This allows a full thickness complete fusion butt weld.  The flange neck then tapers to the bolting portion of flange, providing a good transition.  Are these assemblies galvanized after fabrication, and if so could that contribute to the issue by contaminating the weld?

RE: O/H Sign failures at baseplate

(OP)
Our specifications require low hydrogen electrodes and the steel grades and carbon equivalence are established so these should not be a concern.  The cooling rates are probably not considered in the design as well as any connection between the tube wall thickness and the weld size and the base plate thickness.  A research study recently presented declared that thicker base plates performed better than thin plates.  Material I have indicates that galvanizing after fabrication does not contribute to any fillet weld problems if the structure is properly detailed.  This would include seal welds around all joints and supports welding the socket as I had indicated earlier.  Using a backup bar that can not be removed can lead to problems during the HDG process.

I wish there were a welding robot that would be able to do the full pen weld economically.

RE: O/H Sign failures at baseplate

As you know, I'd obviously tend to favor a preheat, a full penetration weld on a thicker pipe transition piece to the plate, controlled cooling, then drilling a hole as big as you like for the wires.

Try checking into the offshore fabricator welds.  They use some thick plates and tubes.

Takes a lot of joints to feed a robot.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: O/H Sign failures at baseplate

(OP)
Let's look at the problem from another point of view.  Can the poles be secured without welding to a base plate?

RE: O/H Sign failures at baseplate

This kind of failure occurs in South Africa as well. We use gussets to strengthen the base but on the inside of the mast we weld a "spreader" plate at the height of the gussets. This apparently takes care of the high stress points

RE: O/H Sign failures at baseplate

I have noticed that the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals (with interims through 2003;  a new interim is supposed to be coming out soon) shows no welded pole-to-base-plate details that are in a higher fatigue category than E.

It would certainly be nice if they could come up with something better than that, but in the meantime I guess you just have to design for fatigue according to Chapter 11 in that manual.

If you crunched the numbers according to that manual, would the failed poles have passed this code?

Please bring it to everyone's attention (including AASHTO) if the answer is YES!

Thanks very much!

RE: O/H Sign failures at baseplate

(OP)
As far as crunching the numbers, I'll have to wait until I am provided the information.  My suspiscion is that these would not pass the new fatigue limits in the code.  My jurisdiction has interpreted many fatigue checks out of the requirements because they believe the cost for the structures would increase too much.  My own thinking is that there is something besides fatigue happening here, but I don't have the resources to study it myself.

RE: O/H Sign failures at baseplate

(OP)
HgTx,

That is the research I was refering to earlier that indicates stiffeners don't help and that the base plate thickness appears to have some effect on the problem.  Thanks though.  Dino

RE: O/H Sign failures at baseplate

Those base plates appeared to be bolted to a relatively solid base.  Imagine how much effect the thickness would have in Dinosaur's case where they are floated on anchor bolts and nuts.  Ken

RE: O/H Sign failures at baseplate

Dinosaur

Can't help you with design or material selection, but I recommend that you use sub-harmonic energy "during welding" which will make the weld metal up to 400% more ductile while keeping the same strength.  This makes the weld metal more "forgiving" in service and lasts longer.  Impact (charpy) is also higher - up to 75%.  Check out this website for more info on a process called Meta-Lax Weld Conditioning - www.meta-lax.com.

There may be some people (without firsthand experience) that will disagree with the possibility of improving ductility, but I assure you it is possible and done everyday for the specific purpose of improving the fatigue life of the weldment.  Check out page 2 of the Photo Gallery for numerous examples.  Better yet, apply the technology and you'll see for yourself.  FYI, those sky high HD towers are built using this sub-harmonic technology.

BTIGuy

RE: O/H Sign failures at baseplate

Dinosaur

Can't help you with design or material selection, but I recommend that you use sub-harmonic energy "during welding" which will make the weld up to 400% more ductile while keeping the same strength.  This makes the weld metal more "forgiving" in service and lasts longer.  Impact (charpy) is also higher - up to 75%.  Check out this website for more info on a process called Meta-Lax Weld Conditioning - www.meta-lax.com.

There may be some people (without firsthand experience) that will disagree with the possibility of improving ductility, but I assure you it is possible and done everyday for the specific purpose of improving fatigue life of the weldment.  Check out page 2 of the Photo Gallery for numerous examples.  Better yet, apply the technology and you'll see for yourself.

BTIGuy

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