O/H Sign failures at baseplate 8

[Dinosaur]

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.

 

[BTIGUY]

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 -

http://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

 

[BTIGUY]

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 -

http://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