Seal welds in water storage tanks 5

[coolin]

I am working on a steel water storage tank. We are using steel W beams to support a steel plate roof. The steel tank fabricator does not want to continuously weld the flanges to the plate because of possible thermal differences between the top and underside of the plate roof that could cause the welds to fail. The civil engineer wants continuous seal welds to prevent bacteria from growing in the spaces between the top flange and roof plate.

Any opinions?

 

[JedClampett]

Although the bacterial growth reason is a little wacky, I would request that it be done for corrosion reasons. There's no way to coat the top flange once the plates are in place and it's a prime area for corrosion.
We're currently rehabbing some tanks with severe corrosion issues and requiring the rehab contractor lift the roof plates and recoat the top flange and plate. Then the joint is caulked. As you might expect, this is a major pain you know where.
Expect the tank supplier to fight you to the death on this.

 

[civilperson]

Weld the beams to the roof. Steel is poor insulator and good conductor of heat. The temperature differential is therefore small, (<10 degrees). This differential gives about 1.63 ksi of stress or 0.61 kips/inch along weld assuming 3/8" thick plate.

 

[RockEngineer]

With the continuous welding you will have more problems with warping thand from temperature differential during use. If heat controls are used during the weling process like skipping areas and then coming back and filling them in rather than one continuous weld, the distortion can be held to a minimum. I agree you want the continuous welds for corrosion control.

 

[SteveWag]

Corrosion is the problem. In cases like this what I have done is taken a W-Shape 2-inches larger than needed and cut the web in a zig-zag to produce two T-Shapes having the height of the needed shape. That is you get two identical pieces. The zig-zag pattern creates a series of VVVVVV’s, in the web of the T-Shape, flat on top and bottom for about two inches and spaced about the thickness of the desired W-Shape height. This piece is positioned against the underside of the tank roof and then welded, leaving the tank roof as the upper flange. The tops of the VVVVVV pattern only require a two inch weld on both sides. This way there is no areas that cannot be blasted and painted, and one W-Shape returns to life as two beams. Saves welding and steel.

Steve Wagner

 

[JStephen]

Normally, if seal welding is done, it is done for corrosion and repaint reasons. You do need to make provision for weld distortion. Just going in and welding rafters in a tank that's already built is not necessarily a good idea.

The tank standards leave seal welding as an option. If it's not been specified, it would be an extra to a contract- it's not just a detailing issue.