How do I calculate the strength of a weld joining two crossed rods?

Question

While considering this recent question, I started wondering how to determine the strength of the welds in the steel mesh. As AndyT points out in his comment:

The welds at the connections are only intended to keep the bars at right angles during handling - they are not intended to take any load.

The intent of the manufacturer, of course, doesn't stop someone from using the product to support a load anyway. Just what sort of load can this type of weld support? What is/are the likely failure mode(s)?

This weld has different geometry and loading than fillet welds (which are the only type I'm at all familiar with). I think it's properly called a flare-v groove weld, though most of the examples I can find show that as joining a round bar to a flat bar or plate. The geometry is different enough that I suspect those are not equivalent, from a design standpoint.

This AWS technical manual says:

2.3.3.2 Effective Weld Size (Flare Groove). The effective weld size for flare groove welds when filled flush to the surface of a round bar, a 90° bend in a formed section, or a rectangular tube shall be as shown in Table 2.1, except as permitted by 4.10.5.

Table 2.1 indicates the effective weld size is either 1/2 or 3/8 the radius of the outside surface of the weld. I'm still not sure this applies to the crossed bars geometry, though.

When I was learning about fillet welds, it was always assumed the weld would fail first in shear along its throat. Given that a square mesh has no cross-bracing and there is a single point of contact between the members (rather than a line or plane of contact), can I afford to make that assumption?

Especially in the case where there are only two rods rather than a mesh grid, this weld could be loaded in quite the variety of configurations. Let's consider only the cases of pure torsion, pure tension and pure shear. Assume the weld fails before the members.

Answer 1

The welds on welded wire mesh are for more than just holding the wires together while handling. There are specific weld strength requirements for welded wire. The design of welded wire reinforcing uses the number of cross wires to determine development length. This is the reason why a weld strength is required. You can't have the welds breaking while force is applied. It is because of this mechanical anchorage that welded wire mesh can be considered continuous with only one square overlap.

Weld Specifications

There are two different specifications that might apply depending on the material to be welded.

• Rebar mats would follow AWS D1.4.
• Welded wire should follow ASTM A185 (plain wire) or ASTM A497 (deformed wire)

Welded Rebar - AWS D1.4

AWS D1.4 is the welding specification that covers welding of reinforcing bars.

I'm not sure what the weld is called between two crossing bars. It isn't called out in AWS D1.4, but a detail for the weld is shown in ACI 318 for the welding of cross bars in corbels. The detail from the commentary is shown below.

Also per ACI 318 Section 11.9.6(a), the weld is required to develop the full yield strength of the bar.

Welded Wire Reinforcing

Welded wire gets a complete fusion as shown in this image from the Manual of Standard Practice: Structural Welded Wire Reinforcement by the Wire Reinforcement Institute.

The weld shear strength is specified by either ASTM A185 (plain wire) or ASTM A497 (deformed wire) depending on the type of wire. For all but the thinnest wire, the required weld strength is 35,000 psi (240 MPa). This puts the weld strength at about half of the yield strength of the wire (65,000 psi or 70,000 psi).

Strength in Other Directions

The weld strength requirements as shown above are only for shear. When used in concrete, this is the only direction that force should be applied. It would be difficult to determine how these welds would perform in peeling, torsion, or rotation about the crossing location.