How It Works: What is a Backing Strip for a Welded Joint?

For many applications of welded joints in structures, it is not necessary to design a “full penetration joint”. The designer can use a fillet weld or a partial penetration butt weld to meet many structural requirements.

However, when a full penetration joint, welded from one side, is required, how is the hot weld metal controlled in the initial weld pass located at the root of the joint? In other words, how does a welder ensure that the molten weld metal does not just drop through the root of the joint and thus deposit molten metal all over the floor or, on the opposite pipe wall if it is a pipe/tube that is being welding. 

Bare in mind that with steel the molten metal temperature is in excess of 1400 deg C. There are two ways to approach this:

1. You can attach a backing strip that will hold the weld metal in the joint during welding and which can remain as part of the joint or, can be later removed depending on the design requirements. Such a joint is covered by the “S” classification for a structural welder in the CSA W47.1 certification code for structural steel

2. You can qualify to weld on a structure or pipe that is only accessible from the OD or one side without a backing bar. In Canada this will generally be covered by the pressure welding code ASME IX or the “T” classification for a structural welder in the CSA W47.1 certification code for steel. The level of skill required for this qualification is higher than that necessary in option 1.

The case we will be discussing here is item 1, using a backing strip support. A back strip or backing bar support can be defined as:

A backing strip support is “a metal strip fixed by tack welding on the back side of the plates to be welded or inside the pipes to be welded covering the weld groove”. This is required to achieve proper penetration of the weld metal in the joint. 

The basic backing bar is shown in Figure 1 with further derivations, for corners and T-joints shown in Figure 2.

Figure 1. Illustration of Basic Backing Bar or Strip

Figure 2 Further Examples of Backing Bars-Strips Including Corner and T-Joints

Structural fabrication codes, such as CSA W59 and AWS D1.1, contain many pre-qualified joints that are designed to be welded from one side onto backing bars with various arc welding processes.

In addition to the ease of welding with a backing bar in place, there are many cases of joints designed with these bars where they can be left in the structure after the weld is completed. This is particularly true of situations that are loaded statically or, where the environment is not particularly corrosive. The fabrication code will stipulate that the backing material has to be a similar chemical composition to that of the structure or certainly be drawn from a similar material grouping.

However, if a backing bar/ring is used for the joint and the resulting geometry will be detrimental during service, it should be removed and the internal joint face ground smooth. Some of these situations would be where the structure is subject to corrosion in the cervices formed at the backing bar, subject to vibration, or cyclic loading conditions that can cause fatigue failure.  Ultimately, the choice to leave the backing bar in place or to remove it is made by the designer.

In these cases, when it is impractical, or impossible, to remove the backing bar or ring then other alternatives exist in order to complete the full penetration joint. These would be:

1. Weld with and open root using a T qualification welder or ASME qualified in case of a pipe 

2. Use non fusible backing bars that are made from other materials, typically from copper or ceramic