Connecting Element Rupture Strength at Welds

[RMC123]

Hello,

This is my first post, so bear with me. I am looking to design a Single-Plate Shear Connection using AISC Steel Construction Manual, 14th Edition.

Ideally, I want to use Table 10-10a, following the conventional configuration, to quickly/easily pick a connection that will work based on their tabulated geometry/parameters. I think I understand all the limit states of Section J that tie in to using this table. However, to my understanding, one of the additional checks required to supplement the use of this Table comes from Part 9, specifically Equations 9-2 and 9-3, under the "Connecting Element Rupture Strength at Welds" portion.

In my case, I want to use a single-plate connection on a Square HSS column member using welds between the column and the plate. If using a 1/4" weld on both sides of the plate, then Equation 9-3 must be checked.
Equation 9-3 reads t_min = 6.19 * D / F_u. With D=4 (for the 1/4" weld), and F_u=58ksi for the square HSS, it gives you t_min=0.427". This t_min is independent of the shape size. In this situation, I want to use an HSS4x4x__ shape. With the calculated t_min, that drives me to an HSS4x4x1/2 which has a design wall thickness t_design=0.465".

To me it just seems impractical to have every HSS column with a weld connection to have at least 1/2" nominal wall thickness. I don't imagine it is practical to use a weld on only one side of the plate which would drive you to Equation 9-2 instead of 9-3, giving a smaller t_min. Am I interpreting this design check correctly? The AISC Design Examples (v13 and v14.1) both consider this check. The older engineers I have talked to (I am an EIT in my first four months as a structural engineer) are not aware of this design check. Is this something that is new to the 13th/14th Editions of AISC? Has anybody else run into this issue of practical concerns?

On a related note, to use Table 10-10a, does anybody know if they are assuming that the weld is applied on both sides of the plate, or a single side? I cannot find it explicitly defined/stated anywhere.

Thanks in advance!

 

[Hokie93]

You may be able to use a lesser wall thickness by using the size of the weld you need for strength, not the minimum weld size to develop the shear plate (which is the size given in Table 10-10a). In other words, say you only need a 3/16" fillet weld on both sides of the plate to transfer the shear force. I believe it is permissible to plug D=3 into Equation (9-3). Put another way, you just need to make sure the strength of the weld to the supporting element is taken as the lowest value of the weld metal, shear yielding of the base metal (AISC 360-10 Section J4.2a), and shear rupture of the base metal (AISC 360-10 Section J4.2b).

The weld to the supporting element must be provided on both sides of the shear plate per the first paragraph at the top of page 10-102 in the 14th edition AISC Steel Construction Manual.

You also need to check punching shear of the HSS wall per Chapter K, Table K1.2, Equation (K1-3).

 

[RMC123]

Thanks for the reply, Hokie!

So really the only way to reduce the HSS wall thickness is to reduce the weld size? Is a 3/16" fillet weld on both sides common on a single-plate connection detail? It seems small.

When looking at the AISC Design Example K-6 (v14.1), they used Equation K1-3 to preclude a shear yielding failure of the HSS wall, but you are saying that Equation K1-3 checks punching shear? Also, doesn't Equation 9-2 check shear rupture of the HSS, so why do you also have to use section J4.2b?
When you talk about taking the lowest value to check the weld strength, am I correct that you would use Table 8-4 to check the weld metal? When you refer to checking shear yielding and rupture of the base metal, you are referring to the base metal being the plate, or the HSS?

 

[Hokie93]

In thinking about this connection a bit more, you should be using Equation (9-2), not (9-3). The "connecting element" in your case is the HSS wall and you only have a shear plate on one side of the wall. Thus, you should be using Equation (9-2). If you had shear plates aligned on both sides of a wide-flange beam web, for example, using Equation (9-3) would be correct. This should resolve your issue and result in an HSS wall thickness considerably thinner than 1/2".

Equation (K1-3) checks punching shear which is the same as the shear yielding failure in the AISC example problem.

Equation (9-2) and (9-3) both check shear rupture by equating the weld metal strength to the shear rupture strength of the base metal. I was suggesting to use either Equation (9-2) or Section J4.2b, not both. For me, it is easier to think about the behavior of the situation by looking at Section J4.2.

You can use Table 8-4 or, since the weld is resisting pure shear, you could just divide the load by the total weld length and compare the result to the weld shear strength based on Equation (J2-3) and Table J2.5. Someone in your office should have a table of design (LRFD) or allowable (ASD) weld shear strengths based on Equation (J2-3). I have my tables taped to the inside cover of my AISC manual. You can find tables in many structural steel textbooks. For example, see Table 5.14.2 (LRFD) and Table 5.15.1 (ASD) in the fifth edition Salmon & Johnson textbook.