element distortion induced by welding heat

[cmrdata]

I have an architecturally exposed steel stair with typical section shown in the attachment. The steel fabricator claimed that the continuous fillet weld I called out between the vertical plate and the horizontal tread plate would generate too much heat to cause the horizontal plate to distort. They therefore suggest using intermittent fillet weld instead, then fill the gap between the welds with something called "plastic filler" to make the appearance the same as the adjacent real welds (because they are exposed to view).

From the strength point of view, intermittent welds would work without any problem, and the only reason I called out continuous welds is to have a uniform and consistent look for this architecturally exposed stair (AESS).

My questions are:

1. The horizontal tread plate is 12" wide and about 6'-6" long overall, the weld between the vertical stiffener plate and this tread plate is 3/16" fillet weld by 5 foot long on each side. In my mind, this is not an excessively heavy and long weld. I doubt that heat distortion would be a problem in this case if appropriate measures are taken. Could it simply be a matter of fabrication procedure in the shop? If so, instead of getting involved in the matter of means and method, what I, as the design engineer, can suggest to prevent such problem?
2. Would it help if they do intermittent welds first, say 6"@12", then come back to weld the gap in between in the 2nd run?
3. I'm not sure about the use of the "plastic filler" as it's something like Automotive Joint and Seam Sealer, such as Bondo. Might it crack or peel off down the road?

Thank you.

 

[Ron]

The long weld is likely to cause distortion. The filler will be a maintenance issue...yes, it will separate and likely peel at some point.

Do intermittent welds, then come back and fill space after complete cooling of the intermittent welds.

 

[oldestguy]

From my welding experience I'd change the spec to 6" stitch welds at 18". Take plenty of time between returns.

 

[cmrdata]

Thank you both, Ron and oldestguy, for your suggestions.
Looks like we are on the same page that doing checkerboard type of welding sequence should help to mitigate the problem.
Just to clarify, oldestguy, when you say ...between "returns", you are saying coming back to weld in between the previous welds, right?

 

[oldestguy]

Yes. I'd cover the whole joint area with the i/3 of the work first pass, then return to the start area, twice.. I see some up-side down welds. Great fun if your suit has any openings!! Of course checking with a welder that does this every day can't hurt either. It is no fun to have your work criticized by others.