Welded Moment Conn Backing Bar

[CTSeng]

Involved in a project in a non-seismic region with field welded (CJP flanges) moment connections. Erector is using backing bars and wants to leave them in-place. I see a lot of information saying this is not permitted for seismic or fatigue conditions but is it acceptable for a non-seismic moment frame system.

Thanks,

 

[nutte]

Yes.

 

[Lion06]

In seismic areas when the backing bars are removed, does the weld need to be backgouged and welded from the other side?

 

[hokie66]

Not to answer your question, SEIT, but if the backing bars have to be removed for backgouging, what is the point of using backing bars? What would be the ruling with inaccessible welds, e.g. the inside of closed shapes?

 

[TXStructural]

StrEIT: in general, only if the weld is called out that way. More welding can actually cause more risk of fatigue due to residual stresses. Removing the backer and grinding is an effort to reduce stress risers resulting from discontinuities in and around the weld.

This is also why rough, flame cut access holes are prohibited in fatigue applications. Each notch is a focal point for local stress. The risk of fatigue, as a mechanism for failure, increases as the stress in the metal increases, beginning at around half of yield.

 

[Ron]

As long as the backing bar is tacked within the weld and the tacks will be consumed by subsequent welding, fatigue is usually not an issue. If a highly critical location, the backing bar can be removed, the weld can be backgouged and welded; however, as hokie66 implied, you usually only use a backing bar when the weld is inaccessible from both sides.

Keep in mind that welding without backing requires a higher level of qualification than welding with a backing.

Most often, rejectable weld anomalies occur at the root of a CJP weld. These are most commonly lack of penetration and poor fusion. While other anomalies may occur (inclusions, porosity,etc.), these are common.

 

[nutte]

Chapter 3 of AISC 358-05, Prequalified Connections ..., addresses several areas where backing bars may be used. It looks like in all cases where the backer is removed, they do require that the root pass be backgouged and a reinforcing fillet added.

The seismic specification, section 11.2a, says the same thing for the cases where the backing bar is removed.