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VictorAK (Structural) (OP)
20 Jul 10 21:33
Does anyone know where I can find the design of a cranked steel beam along with some examples?
Thank you.
paddingtongreen (Structural)
20 Jul 10 21:57
If there are no fixities at the supports, it is simple statics to find moments and shears. Stabilities are determined by connecting members (I assume there will be members connecting at the top and bottom of the crank).

Timing has a lot to do with the outcome of a rain dance.

Helpful Member!  csd72 (Structural)
21 Jul 10 6:44

the bending moment does not change but the deflection will increase.

Be careful of overall stability at the crank as well as the strees transfer at the actual crank.
ToadJones (Structural)
21 Jul 10 8:39
Are we referring to a beam that cut/ mitered and spliced to make a ridge?  
VictorAK (Structural) (OP)
21 Jul 10 8:56
Yes - the beam is 31'-6" long with a 135 degree bend 8' from one end and a factored point load of 5,800 lbs. located 5' from the opposite end of the crank.
slickdeals (Structural)
21 Jul 10 8:57
Can you explain why the deflection would increase? Won't a cranked beam peaking upward have less deflections? Would it not behave as an truss of some sorts?

We are Virginia Tech

ToadJones (Structural)
21 Jul 10 9:12
I have actually done several of these within the last year or two. I guess I never gave them any special design consideration.
My intuition was that the deflections would be lower but I checked them using FEA.

I'd be interested in what ConnectEngr has to say about the splice location.  
ToadJones (Structural)
21 Jul 10 9:38
I guess another thing to watch is the amount of compression or tension you get in the member.  
nutte (Structural)
21 Jul 10 10:06
Blodgett discusses this type of connection, including local reinforcing required at the crank.
ToadJones (Structural)
21 Jul 10 10:07
You know where in Blodgett? (I'm being lazy)
slickdeals (Structural)
21 Jul 10 10:13
Blodgett is a vast resource, it always suprises me how much good information is contained in a single book (and each one is so practical)

We are Virginia Tech

nutte (Structural)
21 Jul 10 10:20
Starting on page 5.11-7 is the example I was thinking of, with the forces at the crank.  This is actually for a rigid frame knee, but the concept would apply to the cranked beam.
ToadJones (Structural)
21 Jul 10 10:26
Thanks nutte
Slick- Other than Moby Dick, Blodgett's Design of Welded Structures is the greatest book of all time.  
csd72 (Structural)
21 Jul 10 11:35
You would only get significant axial load in this if there was restraint at the walls preventing spread.

If it is like most beams of this type then it is effectively on roller supports and therefore cannot have axial loads. You would therefore have the same bending moment as this is based on the distance between supports whereas deflection is more dictated by the actual length. You can verify this using first principles and force vectors.

As per the detail at the ridge, there has been several different controvertial discussions on this. My personal opinion if that you need a gusset plate between the flanges to equalise the out of plane force vectors in the flanges. I have been through this several times and will not be convinced otherwise.

If this is of the roller type then you also need to limit the horizontal spread of the rafter under vertical loads.
ToadJones (Structural)
21 Jul 10 11:42
can you describe further what you mean about "a gusset plate between the flanges"?
csd72 (Structural)
21 Jul 10 12:02
A stiffener plate/ web plate/fin plate whic runs between the two flanges at the point where they change direction (i.e. the ridge)
paddingtongreen (Structural)
21 Jul 10 12:10
Toad, I think csd means to use stiffeners as continuations of the cut flanges. I don't like that detail, I think it is over-restrained. I prefer to weld a piece of tee under the joint to deepen it. I agree with him that deflections are greater if the ends are not restrained horizontally.

I started off thinking that a "cranked beam" was actually a cranked beam, horizontal to vertical to horizontal again, a very different animal.

I don't have Kleinlogel anymore, but I found that I still have a copy of Crosby Lockwood's "The Steel Designer's Manual", this case is in there.

Timing has a lot to do with the outcome of a rain dance.

csd72 (Structural)
21 Jul 10 12:30

No I mean a vertical stiffener at the point where you weld the two sections together. The problem with the two solutions you have suggested is that they do not restrain the top flange against buckling at the tips where it all of a sudden changes direction.
ToadJones (Structural)
21 Jul 10 13:08
Well, I guess I was wondering why the stiffener would be necessary always.
I have done a few beams like this that were lightly loaded and due to the availability of materials the beams were way over capacity. In this case I did not use a stiffener where csd suggests, but I do like the idea.  
csd72 (Structural)
21 Jul 10 13:27

Yes I think that why this is not a common failure point is because most cranked beams are dictated by deflection and therefore over capacity for strength.

I always look at these things like a truss with the flanges/stiffeners able to take compression or tension and the webs only able to take diagonal tension. If you sketch that apex force diagram you will see what my issue is with it.
ToadJones (Structural)
21 Jul 10 13:51
I sketched it out very quickly and what seems to result is:
Compression in the top flanges due to moment result in a  vertically upward shear force
Tension in the bottom flanges due to moment result in a vertically downward shear force.
These two would combine to put the web of the beam at the join in tension, right?

The shear forces on the beam would want to put the same point in compression, correct?  
hokie66 (Structural)
21 Jul 10 19:52
Follow the forces in the flanges.  The change in direction causes forces which have to be resisted.  A plate across the joint as csd72 described will do this.  The force in the splice plate changes across the joint from tension to compression as the web absorbs the force.
paddingtongreen (Structural)
21 Jul 10 21:25
The problem with that plate is the welding. It is placed over the butt weld in the web and needs welds on both sides of the plate to the webs and in the oblique angles to the flanges.

That is why I would rather put a tee under the bottom flanges to reduce the flange forces.

Timing has a lot to do with the outcome of a rain dance.

hokie66 (Structural)
22 Jul 10 1:22
That's not the way I detail it.  The plate is butt welded to the beam on both sides.  It's an end plate for both members.

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