If the joint designation is BC-P6, what is the thickness of T2?

In the context of the AWS D1.1 Structural Welding Code, the joint designation BC-P6 refers specifically to a combination of factors from the welding table. The designation "BC" indicates a butt joint with a backing bar and "P6" references a particular procedure that can be used for materials of different thicknesses.

For T2, the second member in the joint designation, the key aspect is that the thickness is often defined by the context of the joint, not constrained by a specific numerical value in this case. Option "C" illustrates this concept correctly by stating "Unlimited," which suggests that T2 can have a thickness that may vary widely without a predefined limit, allowing for greater flexibility in design and welding applications.

Understanding the context helps clarify that in welding codes, certain designations can imply a vast range of acceptable material thicknesses based on other design considerations, which is exactly captured in the correct response.