Blodgett's vs. AISC Steel Manual 10

[JD P.E.]

I was wondering if someone could take a look at these simple calcs and offer advice regarding my approach on weld sizing. I use Blodgett's handbook and use the weld line method and then I use AISC fillet weld capacity and get different answers. I suppose that's due to the way I couple the moment and only count on top weld for the tension.

Thoughts?

 

[WARose]

Most likely (just eyeballing it) there are different allowables for the weld (i.e. a newer version of AISC vs. a Blodgett calc from 50 years ago).

 

[driftLimiter]

I am not sure about the AISC part of your calc. Instead of finding the peak stress due to bending using the section modulus you divide the moment to the top and bottom and your ignoring the weld on the side. I would use the Z of the weld to find the required force per unit length then use the AISC equation without the L to determine the strength per unit length. Also be sure to calculate the section modulus Z per unit length or for 'welds treated as a line'

 

[Celt83]

Your weld is significantly undersized. You forgot to account for the direct shear component of your 6,000 lb force. Your Sw calculation also does not appear to be correct.

edit: is the cantilever plate 4" thick or 1/2" thick?

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[JD P.E.]

You are correct this is missing shear.. I was mainly just trying to reconcile the differing weld sizes.

I'll take the recommendations and go about this another way.

 

[JD P.E.]

Its 4x4x.5 tubing

 

[phamENG]

So you're just welding top and bottom? The problem is that you divided your moment by 2" rather than 4". If you fix that, the AISC equation will give you a smaller weld.

Two problems I see otherwise: you're not considering shear and you're acting like you'll have a full 4" of fillet weld top and bottom. You won't. There's a curvature at the corners of those tubes and you lose quite a bit. Especially something as thick as 1/2" wall HSS. You actually can't count on much of anything for that - you'll essentially have a pipe. (Check out the dimensions in the Square HSS table in the steel manual - workable flat.)

You can use Blodgett's analysis equations - once you fix your 2" and 4" mixup, you'll see that it gives you the same demand on the weld. And once you get the demand on the weld, you can use the current AISC equations to size it. Just be mindful of the actual shape and the combined shear stress from bending, tension, direct shear, and compression if the weld is transferring the compression (usually best to assume it is unless you're detailing it otherwise).

 

[JStephen]

Make sure you're not confusing allowable vs ultimate loads, especially when comparing old stuff to new.

 

[JD P.E.]

I might have a conceptual error then.

So let's assume you do have a 4" fillet weld and ignore shear for the sake of this calculation... why would the moment be divided by 4"? I suppose I assume the moment would act in the center of the weld group which would cause a 2" moment arm (fillet's on top/bottom of 4" hss). Or is this the long way of doing this?

 

[phamENG]

Draw it as a free body diagram and do the three summation equations for basic 2-d statics and see what it gives you.

I think I saw in another post you're only a couple years out of school. Do not underestimate the power of a free body diagram. Our work is fundamentally simple if you can break down the more complex stuff into those basic "easy" tasks. Statics is as important in this as it was your first semester of engineering courses. More so when you consider this could keep something from crushing somebody.

 

[SJBombero]

There is no conceptual difference between Blodgett and AISC methods of weld design. Current Steel Manual guidance is an abstraction of the fundamental principles of Blodgett who was a significant contributor to the Steel Construction Manual. The only difference that has occurred over the years is a revision of the allowable stresses. I use Blodgett's "weld as a line" method and then apply current stress limits using LRFD or ASD factors from the Steel Manual.

 

[JD P.E.]

Wow. I did an FBD and that's frankly embarrassing.

I got ahead of myself on that. Thanks for everyone's help...

 

[JLNJ]

Blodgett usually used a lower strength weld, iirc. E60xx vs E70xx.

 

[phamENG]

Wow. I did an FBD and that's frankly embarrassing.

No worries. It's easy to do. There's certainly a temptation to take shortcuts after a while...but it's amazing how many of my own errors I've caught that were nothing more than not doing a proper FBD, not writing out units in my calcs, etc.

 

[ProgrammingPE]

If you use an allowable of 14,847 psi instead of 11,200 psi, then you'll get the same answer for both methods once you correct for the 4" dimension that phamENG mentioned. (Still add in the missing shear component, but this just shows you how the two methods give you the same result.)

Allowable Weld Stress:
fall = 0.6 Fexx / Ω = 0.6 * (70,000 psi) / 2 = 21,000 psi

Allowable Weld Stress w/ Throat Thickness Adjustment:
fall * 0.707 = (21,000 psi) * 0.707 = 14,847 psi

Blodgett:
w = (3,562 lb/in) / (14,847 psi) = 0.24 in

AISC:
(57 in-kip) / 4 in = (0.928 kip/in) D (4 in)
D = 3.84 sixteenths = 0.24 in

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[bhiggins]

Do not put much trust and faith in welders, lot of them are meth addicts. I would definitely weld the full tube together, not just top and bottom. See image below from one of my projects:

 

[phamENG]

Do not put much trust and faith in welders, lot of them are meth addicts.

I get it. You had a welder (or, more likely, a bad contractor's friend who picked up a welding machine at a yard sale) botch a job. But could you paint with a wider brush? Yikes.

 

[bhiggins]

I don't mean to push stereotypes, I've met many welders who are my friends. They openly talk about their drug use which is extraordinarily common in the industry. The point I'm trying to make is to not design welded structures at the bare minimum of what is calculated. There are always other considerations beyond the math and numbers which drive our decisions. It's always better to be proactive, especially when we are hanging truck-sized loads overhead on a 4X4 tube.

 

[dik]

Man, those are some ugly welds... I've not seen anything like that...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[phamENG]

bhiggins - fair enough. Glad I don't work in your area. All the welders I've known are good, hard working people with no apparent drug issues. Now they haven't all been good at their jobs, but that's another issue...