Base Plate to Column Welds

Samwise - is this in seismic design category D? Is this in the Seismic Force Resisting System load path? If so, then you're probably dealing with a capacity design requirement - your weak link is your anchors (which should be designed for a ductile failure mode i.e. steel yielding and NONE of the concrete failure modes) and you don't want a sudden brittle shear failure of your weld before you get yielding in your anchors.

Haven't done one of those in a long time, so I'll leave it to the folks experienced with seismic design to chime in with more detailed answers.

It's a seismic design category B but the description in the design guide just threw me off. I am like the weld doesn't need to be that big to resist shear of uplift forces. But if I have to follow what Design Guide-1 says, its as if the weld needs to be designed to develop the full tensile capacity of anchor rods. What I was not sure about was if this was for the whole tensile capacity of all anchors or just one of the anchors.

1) Why do you have to follow DG-1? Sure, it's a good idea, but it's not a building code. There are other ways to skin the cat (just be careful - standard of care an all that jazz).

2) What you posted doesn't say you have to develop the full strength of the anchor bolts in the fillet weld. Perhaps you could post a reference to where in DG-1 that's stated?

This is the reference. Section 2.4 . I agree Design Guide is just a code, I was just curious as to what the intent was here. I am trying to decrease an all round weld from 5/16" to a 1/4" for a HSS 8x8x and wanted to see if there is something else I need to be checking other than 60kips shear and 20 kip uplift I have.

1) I disagree that it's a code. It's not. It's a design guide.

2) Thank you for posting the reference. I'm happy to help but I spend waaaay too much time on ET as it is...can't go searching through books, too. Reads like a general discussion. For columns with tension loads, it's a good idea to maintain a ductile load path. It's important for seismic, good practice in general. In your case (moment on the base plate, SDC B) I would just design it for the design loads.

Sorry, I meant to say its not a code. Yeah , I am going to ignore this and just design it for the design loads. Thanks for the help ,appreciate it. As you mentioned its likely needed when we need to force a ductile failure in high seismic zones.

Should be able to increase the weld strength by 1.5, since the tension load is 90 degrees to the weld plane.

It sounds like Section 2.4 of DG1 is just recommending to design the base plate welds for the anchor strength since it can typically be met with just small fillet welds. In your case, since you have (8) 1" anchors, the demand is a little bit higher, but it appears that a 1/4" fillet weld still works.

In your initial calculations there are two things you missed. bones206 already mentioned the 1.5 factor you get for tension loads 90 degrees to the weld plane (AISC 360-16, Eq. J2-5). You are also missing a 0.75 factor for the strength of the anchors since Fnt = 0.75*Fu (DG1, Section 3.2.1) (AISC 360-16, Table J3.2).

Bolt Strength, ΦRn = Φ*Fnt*Ab = Φ*(0.75*Fu)*Ab = 0.75*(0.75*58ksi)*(8*0.785in²) = 205 kip
Weld Strength, ΦRn = Φ*Fnw*Awe = 0.75*(0.60*70ksi*1.5)*(0.707*1/4"*30") = 251 kip

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0.75*0.75*58 = 32.6 ksi. So if your weld is sized to develop that bolt stress, it may fail before the anchors are able to yield. And what if the anchor material is stronger than expected? Sizing the weld such that the anchors can yield and stretch requires a conservatively high estimate of the anchor strength as the design load for the weld, IMO.

Designing the anchors themselves would be the more appropriate application of strength reduction factors.

Thank you every for the responses. I missed the 0.75 factor , but I was not completely aware of the 1.5 factor. The reference J2.5 equation doesn't clearly specify the 50% increment for weld strength. Is there another reference which clarifies this.

Check out the (1+0.5sin^1.5(theta)) term in the Fw calculation with theta = 90°