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Blodgett's vs. AISC Steel Manual
8

Blodgett's vs. AISC Steel Manual

Blodgett's vs. AISC Steel Manual

(OP)
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?

RE: Blodgett's vs. AISC Steel Manual

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).

RE: Blodgett's vs. AISC Steel Manual

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'

RE: Blodgett's vs. AISC Steel Manual

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?

I'm making a thing: www.thestructuraltoolbox.com
(It's no Kootware and it will probably break but it's alive!)

RE: Blodgett's vs. AISC Steel Manual

(OP)
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.

RE: Blodgett's vs. AISC Steel Manual

(OP)
Its 4x4x.5 tubing

RE: Blodgett's vs. AISC Steel Manual

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).

RE: Blodgett's vs. AISC Steel Manual

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

RE: Blodgett's vs. AISC Steel Manual

(OP)
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?

RE: Blodgett's vs. AISC Steel Manual

3
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.

RE: Blodgett's vs. AISC Steel Manual

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.

RE: Blodgett's vs. AISC Steel Manual

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

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

RE: Blodgett's vs. AISC Steel Manual

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

RE: Blodgett's vs. AISC Steel Manual

Quote (JD P.E.)

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.

RE: Blodgett's vs. AISC Steel Manual

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

Structural Engineering Software: www.structuralcentral.com
Structural Engineering Videos: www.youtube.com/structuralcentral

RE: Blodgett's vs. AISC Steel Manual

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:



RE: Blodgett's vs. AISC Steel Manual

Quote (bhiggins)

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.

RE: Blodgett's vs. AISC Steel Manual

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.

RE: Blodgett's vs. AISC Steel Manual

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

RE: Blodgett's vs. AISC Steel Manual

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...

RE: Blodgett's vs. AISC Steel Manual

2
bhiggins - your comment about welders is way off the mark. Whoever "welded" that joint in your picture was not a welder, that was someone with a welding machine claiming to be a welder. The solution here isn't to increase your weld size until you get enough shit weld to be acceptable, the solution here is to reject that horrible work and not allow it to continue until a qualified welder is available to do the work. I've seen plenty of bad welds, similar to the picture you posted, but I've also worked with lots of welders that produce world class products. Drug use is not limited to any particular job-type or class of people - your comment says more about you than any welder.

RE: Blodgett's vs. AISC Steel Manual

Star for CANPRO, from someone who works on a daily base with welders.
I agree, the problem with the picture above is the supervision, not the so-called welder. Whoever let that guy perform structural welds without assessing his capabilities upfront, should be fired. The "welder" simply shouldn't have been hired for this job.

RE: Blodgett's vs. AISC Steel Manual

I never said ALL welders are meth addicts, only a certain percentage which is a 100% true statement. Ya'll need to hang out more with the people who build the structures, I was one of them. They are great people with their own share of problems like everyone in this world. Drug use and alcoholism is pretty rampant in the construction industry, moreso than the general population. It's a real problem which must be accounted for with realistic expectations. I have nothing against welders, however you must be prepared for when the bad ones show up, which happens to everyone at some point. On this particular project all of the welds were rejected and the entire structure had to be re-welded. I would never advocate for more shitty weld, just don't design the bare minimum because you never know what defects are lurking and are invisible. Don't blindly trust your calcs.

When you load fillet welds transversely like the OP's sketch you lose ductility, which makes the connection even more critical. I would personally modify the detail to have some more redundancy or ductility.

RE: Blodgett's vs. AISC Steel Manual

3
As much as I want this site to be mostly about engineering, these claims need to be addressed and not permitted to stand on their own.

"...a certain percentage which is 100% true".

Boy howdy, that is some serious obfuscation/double speak. This is what happens when hasty generalizations are called out and someone isn't willing to own up to it.

"Ya'll need to hang out more with the people who build the structures, I was one of them."

Translation: his anecdotes are bigger than everyone else's combined, so here is an ad hominem to get everyone to go away.

"It's a real problem which must be accounted for with realistic expectations."

"Of course drug and alcohol addiction are high amongst construction workers (there is mental health data that supports it), and it would be nice to reduce it. However, you were making a weird claim about the likeness of a welder to be a meth addict. Oddly specific that encourages people to think "likely a meth addict" when they come across a welder rather than just assess the situation at hand and give people you don't know a charitable starting point. Back in the office... when designing a welded joint, there are reasonable ways to evaluate the risk of the joint that can certainly include the difficulty of field conditions, redundancy/plastic redistribution, % sustained loading, ductility, etc. Oversizing a joint to some degree from theoretical vectors/magnitudes is a normal practice when determined with care. Randomly increasing weld sizes and lengths while dreaming the imagery of a stereotypical movie/tv show meth addict is not one of them.

"I have nothing against welders, however you must be prepared for when the bad ones show up, which happens to everyone at some point."

First, if you have nothing against welders, you would own up to the generalization and implication of your words. Second, when I am called out to the field to address poor welding, there is literally ZERO value in maintaining the presumption of the welder, who I don't know, quite possibly being a meth addict. And, "must be prepared", is an absurd retort to no one in this thread downplaying the need to be prepared to fix improper weld joints.



-Mac

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