Base metal capacity calculation at a mitered braced frame connection

[jochav5280]

Good day,

I'm designing a heavily loaded (2000 kip) steel wide-flanged, concentrically braced frame connection where the brace will be mitered and frame directly into the joint, with the flanges CJP welded to the column and beam flange. My question is what calculation should be carried out to check the base metal capacity of both the column and beam flange for both the horizontal and vertical component of the flange force? My intuition is that the flange base metal capacity should be a function of it's thickness, however, there is no clear shear plane through the thickness of the flanges, so I'm unsure as to how to check the base metal.

A colleague of mine insists that the thickness of the beam or column flange does not come into play at all with regard to the strength of the base metal, however, this does not make intuitive sense to me. To illustrate my point, it doesn't make sense that one could weld a 5-inch thick brace flange to a 1/4" beam flange and expect the beam flange to not tear away or fail.

Any light you all could shed on this would be greatly appreciated. This is a very highly loaded connection, one which I cannot find any design example for, but need to ensure it has adequate capacity.

Best regards,

jochav5280

 

[dhengr]

jochav5280:
You’ve really got to provide some reasonably proportioned sketches, plan, elevations, end view, with dimensions, approx. member sizes, loads and their locations, etc. We can’t see it from here. But, 2000k is a fairly sizeable load. Your description so far, leaves us guessing what the detail might look like and that’s what determines how you design it.

 

[jochav5280]

Hi dhengr,

I've attached a detail to this message; the portion I described is regarding the lower right-hand brace connection. Again, what I'm after is what base metal calculation should be carried out to check the column and beam flanges at the locations of the CJP welds on the brace flanges. Since I'm looking for a general answer, I've purposely not given any member sizes or dimensions as the check should be carried out regardless of the member sizes.

Best regards,

jochav5280

 

[dhengr]

jochav5280:
Since you want a general answer... you only have to check the critical elements and you don’t much have to worry about the other parts. If you are using the correct weld process and matching (or better) filler metal and using CJP welds, the welds should be as strong (or stronger) than the original member. And, the original member has been designed for the loads and stresses it will see, so what are you checking? What you guys can do with CAD and FEA, these days, staggers the imagination; and it is also incredibly complex and expensive to fab., all with a flick of the finger. And, with little need to understand how it really works or goes together. You’ve spec’d. the material, that determines Fy, with the exception that some thicker flanges and larger members have some reduction on Fy, don’t they? Or has continuous casting solved that problem? But, the area and the thickness of the flg. do come into play in that they cause large concentrated loadings which don’t have a clean/clear load path, because of their orientation.

Is your question actually, how do size and locate stiff. pls. and what are the governing local stresses in these regions of the connection?

Given the proportions of the various members and the loads those would seem to indicate, and given that the column will be fabed. and shipped with this connection complete, why don’t you extend the W500 brace you are talking about all the way to the column flg.? Then miter the beam into the top flg. of the brace. Then you can take the biggest loads straight through the column, with two sets of sloped stiff. pls. which match/join the center lines of the flgs. on the two sloped members. Thus, you can trade two sets of stiff. pls. for four sets, and make it a little less, more expensive. With the gusset pl. and the size of the brace top flg. (about 125mm thk.?) , the beam connection might not need any stiff. pls., given its orientation to the brace. Maybe another way of saying it, is that all of the forces at that connection have to be resolved in an efficient manner.

I think AISC has several guide books for this kind of joint design. You might want to look into getting their publications on the subject. And, they will have formulas and examples up the waazuu, then you just plug-n-grind. But, the general thinking is that you have to get that concentrated flg. load into the column web, and across the column and into the other sloped member, without drastically exceeding yielding in bearing at the web/flg. juncture, or causing web buckling/crippling. If you are doing this kind of design, you must have seen examples of web and bearing stiffeners in beams and plate girders, at reactions and concentrated loads, and the thought process behind their design and development. This is basically the same thing, with some much more complex geometry and buckling patterns/shapes.

 

[jochav5280]

Thank you dhengr,

I appreciate your response.

What I'm specifically looking for an answer to is what calculation do I perform to verify that the base metal of the beam or column flange can handle the horizontal/vertical components of the imposed brace flange force.

To be more clear, per AISC 360-05, the strength of the welded joint is the lesser of the weld strength or the base metal strength. A nice way to evaluate this problem is to equate the base metal strength to the weld strength and solve for the weld size, which represents the maximum effective weld size based on the base metal capacity. I've attached an example of this taken from my Salmon's "Steel Structures" textbook, (see attachment.)

With that said, there are (3) checks that I need to perform here; check the weld strength, check the brace flange base metal strength and check the beam flange base metal strength. By inspection, the weld strength and brace flange base metal strength are adequate since the CJP weld will fully develop the brace flange, however, the shear/tensile capacity of the beam flange needs to be checked and compared to the horizontal/vertical components of the imposed brace flange force.

I hope this is clear now; I'm not trying to check anything other than that the beam or column flange thickness is enough to support the incoming horizontal/vertical brace flange force.

Best regards,

jochav5280