Crank Beam End Load Calculation

[GradStructEng]

Hi All,

I need to calculate the forces for a Steel Cranked(Bent) Beam and was wondering if anyone knew of any good books that illustrate this well with example caluclations?

Even better; if anyone knows how I should go about doing this, would be very much appreciated. I've attached the CAD file for the drawing if this helps, and the connection in question is the top joint of the crank beam.

Many Thanks

 

[GradStructEng]

Many many thanks BA. This has helped me alot. I appreciate that you have answered both my questions and were able to stay cool with my lack of knowledge in this area.

Thanks to everyone else who has put their input into this.

Best regards

Aaron

 

[rb1957]

i'd've thought that the web shear would have stayed in the web,
and that the bending loads would have stayed in the caps;
and that the big question is axial load along the cranked beam,
causing bending at the crank.

i'd still draw a full FBD to understand the internal loads.

 

[hokie66]

Just one amendment to the problem which BA has solved...the diagonal stiffener is in tension if the joint is opening, but in compression if the joint is closing.

 

[a2mfk]

I am embarrassed to say that I was a bit confused until I sketched this out myself. And in 13 years of being a structural engineer doing plenty of steel design, I don't think I have ever heard of the term "cranked" beam. The last part I feel the weirdest about. Sounds so red-necky and made up.

Is this what we are all talking about (see sketch)?

I didn't follow all of dhengr's post, over my head I guess, so I may be missing some finer points.

As my fellow ACC'er BA has noted, the plate will be in compression when the moments are reversed. The bottom plates will push against each other forcing the flanges up, and the tension top flanges will pull each other down.

Great thread. Hardly ever have had to design one of these but now I know.

What about sizing the plates? 1/2" PL and call it done? Depending on the moment, angle between the beams, etc., the plate would be sized to resist the resultant perp. forces at the flanges?

 

[BAretired]

I agree that when the moment is reversed, the stiffener plates are in compression. It is usual to use a single stiffener each side of the web as seen in this link:

http://www.pacificcomputing.com/pdf/v14_CrankedBeam41.pdf

It doesn't take a lot of time to design the plates for the forces calculated and that is what I would be inclined to do, but maybe 3/8" or 1/2" would be a reasonable minimum thickness.

BA

 

[ToadJones]

a2mfk-
I was a bit confused myself....the last time this thread came up, I was also confused.
I kept doubting myself as far as the FDB at the joint. Butm I basically had what you have...except I tossed in the shears also.

I have actually design a few cranked beams, but I never put the end-plate-stiffener in. I am not goign to lose sleep over it as they were years back and under the supervision of some good engineers.
The one large one I did design was bolted to the tops of two columns and cantilevered over one side up to a ridge and then back down to another column. The forces at the joint were so low I think I just called for a CJP weld and that was it.
Looking back, that was probably stupid as the end plate will (in my opinion) make fabricating the beam much easier and maybe make it possible to just use fillet welds. so, it is probably cheaper too.

 

[hokie66]

a2mfk,
BA is probably trying to figure out what an "ACC'er" is, and how he came to be one.

 

[hokie66]

If you extend the thoughts in this thread to concrete, the difficulties of making efficient corner joints in reinforced concrete structures becomes apparent, but then we have discussed that a lot.

 

[a2mfk]

Wow, just had a real "duh" moment after looking at that Tekla image. Since these types of connections are usually part of PEMB, and I have never worked for one of those companies, I never have given the end plates in those locations much thought. I had always assumed that was just part of the ease of shipping and erection, to do it all bolted with an end plate. But now I can see it also served the purpose of stiffening those out-of-plane surfaces.

Toad- I'm with you, I know I have designed one or two of them, and I cannot say I considered this. I also can rest easy at night because it was part of a moment frame that was mainly for wind and we used CJP welds all around. I think we may have considered end plates but the forces and architectural restraints would not have allowed that. It was also very much deflection and drift controlled design so I don't think the moment was all that bad.

Hokie- I was crediting you with pointing out the force reversal with a moment reversal. BA gets the credit for the original FBD. And we all get credit for learning something today, and for that, I am going to go get a beer and tip it to you guys.

On second thought, maybe Hokie and BA were just teaching and not learning....

 

[ToadJones]

I am unilaterally taking credit for the FBD.
I know BA gave me the confidence to do it, but I want full credit, Okay?

ToadJones (Structural)
29 Mar 11 22:12
BA-
I guess I get a similar result.
If I draw a FDB at the joint with the forces in the compression and tension flanges at the joint along with the shears in the beams at he joint I wind up with what amounts to axial load in the stiffener plate. Make sense?

BAretired (Structural)
29 Mar 11 22:26
Yup!

 

[ToadJones]

speaking of beer, IPA time?

 

[a2mfk]

Gotta do the work, son! Did you break into BA's account to post that sketch?

I love IPAs, but for some reason a little Irish Whisky on the rocks with a splash of water is what I selected for tonight's nightcap.

Been struggling with this project that has an L-shaped diaphragm with extra architectural funkiness added in to just give me a headache. Drag struts? Drag struts? We sittin here talkin 'bout drag struts?

 

[ToadJones]

A2mfk- sounds like you need a little practice...we talkin bout practice, its practice man, we talkin bout practice?

 

[dhengr]

Hokie’s terminology for the joints is a good way to describe this. If the moments on the jnt. tend to increase the angle btwn. the two members (the jnt. is opening) and the web forces or stiffener forces will be in tension. That’s the top jnt. in BA’s sketch from his post 29MAR11, 18:56. And, that’s a2mfk’s sketch also, which was exactly what I was trying to describe, except I would use only one stiffener on each side, right at the miter line. If the moments on the jnt. tend to decrease the angle btwn. the two members (the jnt. is closing) and the web forces or stiffener forces will be in compression. They might cause web buckling which must be accounted for. That’s the lower jnt. in BA’s sketch. The flg. forces, basically (flg. area)(the avg. flg. stress) = T, in BA’s sketch and are concentrated at that sharp transition and act akin to a point load, F, and cause a heck of a tensile stress concentration or causing web crippling just as a heavy, narrow point load on a top flg. of a beam must have stiffeners under it to prevent web crippling.

My comment about a curve as opposed to the sharp transition had to do with the fact that I saw this problem often on curved members with a radius to the inner flange “r” and a radius to the outer flange of “r + member depth”. And, the normal weld btwn. the flg. and the web which carries VQ/I shear flow, now also has a radial stress component perpendicular to the flg., and the weld must take both of these, or stiffeners must be used. This radial force component either pushes the flgs. into the web (BA’s lower jnt.) or pulls the flgs. away from the web (BA’s upper jnt.). This same phenomenon happens at the haunch in a laminated wood, three hinged arch, and shows itself as higher horiz. shear stress and a radial component perpendicular to the lams, and that radial stress must be limited or it’ll pull the lams apart, in tension perpendicular to the grain.

 

[BAretired]

What is an ACC'er and how did I become one? Is there a cure?

Toad...you get full credit for the FBD (and the FDB).

dhengr, it really was not flagrant self promotion...simply a standard way of referring to...oh, to heck with it, I think I'll join Toad in an IAP...API...AIP...whatever.

BA

 

[ToadJones]

Free Diagram of the Body.

 

[hokie66]

BA,
a2mfk said it about you, but meant it for me. ACC is the Atlantic Coast Conference. His school, Florida State, and mine, Virginia Tech, are members of that conference and friendly (sometimes) rivals in sporting activities.

 

[ToadJones]

Now I'm feeling good and I'm looking for extra credit.
Anyone have any questions for me?

 

[dhengr]

Hell, on 29MAR11, 23:53, Toad was going to retire out of frustration, he said, “cant even emoticon right.
I'm retiring.”
Now he wants the full patent rights on FBD’s and FDB’s too, and he thinks since BA said “Yup!,” he must have invented them and has the right to them, we owe it to him. These whiz kids are gettin out of control.

BA.... I wouldn’t have complained nearly as much if you had labeled those nodes D & H.

 

[ToadJones]

BA-
The part that they left out is that the best conference is the Big Ten, Go Buckeyes!