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O/H Sign failures at baseplate 8

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Dinosaur

Dinosaur

Structural
Mar 14, 2002
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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.
 
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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?
 
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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
 
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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.
 
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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.

BigInch[worm]-born in the trenches.
 
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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.
 
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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.




BigInch[worm]-born in the trenches.
 
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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.




BigInch[worm]-born in the trenches.
 
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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?
 
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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.
 
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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.

BigInch[worm]-born in the trenches.
 
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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
 
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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!
 
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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.
 
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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
 
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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
 
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