Thru-beam moment conx/splices 1

[dougantholz]

I have a connection that I have used a couple of times, but I'm not sure that the analysis method that I am using is correct. Here is the connection: Two steel beams, perpendicular to one another. The purlin is simple span in one bay and cantilevers thru one girder to a couple of feet beyond. The tops of steel for both beams is the same so the purlin is spliced with a top plate welded to the purlin on either side of the girder flange. Since the beams are of different depths, no bottom plate can be welded without an extranous connection to reach around the girder. The shear is taken out with a simple shear connection in web of the purlins.
My question is:In the analyis, I usually design the top plate for the plastic capacity of the beam (Mu = phi *Zx*Fy)/depth to size the top plate. I take out the compression in the shear connection. Is there an example of this type of connection anywhere, especially by AISC or ASCE? I looked lots of places, but I haven't found one. This seems to be a fairly common connection, but I can't find an example anywhere.
I appreciate any guidence.
Doug

 

[ishvaaag]

Design in Structural Steel
John E. Lothers
Prentice Hall

has a problem simlar to yours in bolts, but is not developed (prob 521 in my spanish edition).

Main concerns on what you describe would be in passing the compression, varying with the detailing of the shear connection. Alignment since forced by the coverplate shouldn't be a problem, I think.

 

[LPPE]

I can't help you with any literature, but I have come across that same situation many times and have done the same thing, with the exception of a bolted top plate (easier erection in the field).

 

[JAE]

I can't see how a shear tab plate takes out your compression directly. We usually add a seat plate at the bottom flanges, welded to the web of the supporting (and deeper) beam. The seat plate is usually detailed wider than the beam flanges so a top weld can be applied along the bottom flanges to the seat plate.

The seat plate helps by providing a construction seat for ease of erection. Once the shear tab plate is bolted to the beam, your top cover plate and your bottom flange seat plates can be welded. The seat plate can then take the compressive force that comes in through the bottom flange and passes it through the supporting beam web to the cantilever section.
______________
------ ===== -------------
|
|
|
_______ | _______________
----|----
|
=====

 

[dougantholz]

Thanks for the input. We like to put in a plate at the bottom as well, shop welded to the girder, but we were told recently that erectors don't like this connection because they have to swing the beam in. They didn't have any suggestions beyond a backer bar on the girder web, then directly welding the purlin flange to the girder web. Problem with this is you can't get the entire flange engaged without a rat hole...but that's the best I can come up with.
Does anyone out there have a better way of doing a cantilever...sometimes it gets rediculuos to have a W18x35 cantiliever thru to catch a slab edge 3 ft beyond the girder?

 

[JAE]

If the beams are fairly regular, you could simply build some angle frames, spaced say at 3 to 6 feet on center. A triangular-like bracket that reaches horizontal from your edge beam top flange and diagonally up from your edge beam bottom flange. A continuous edge angle can then be added to frame the edge of the floor.

With this, you get torsional forces on the edge beam which can be resisted (or taken out of the beam) with other diagonal brackets extending from the bottom flange of the edge beam, upward toward any intersecting or parallel beams.

 

[LPPE]

JAE - You dont think a double angle cxn at the stub beam and on the inside strut would transfer the compression? Design the bolts for the added horizontal shear.
Granted, the double angle cxn isnt located at the bottom flange, so the compressive force is greater. Usually the stub beams are rather small and the forces end up not being to large. If the bolts are adequate to transfer the shear and you have enough bearing, I don't see much of a problem. Also, it would be tough to buckle the double angle between the bolt and the web of the girder. Let me know what you think, in case I have to modify my design!

 

[Taro]

I have used the bolted web connection to resist the cantilever beam moment only when the loads are very small. It's a perfectly valid design when you can make it pencil out. For larger forces, I transfer the forces out at the bottom flange.

Another consideration in high seismic zones is that cantilever components may need to be designed for a net upward load to account for vertical acceleration effects. This should be considered if applicable.

 

[JAE]

I agree that the bolts could probably be used to take the compression (as you both state - for smaller loads). I guess I'm just a purist and when I design the cantilever as a beam, and the wf on the inside span as a beam, I'm assuming a full section transferring the moment through the supporting beam. When you count on the bolts, I think the following would apply:

1. The rigidity of the cantilever would be lower than you assume with a full beam. Thus, the cantilever deflection would be higher.
2. The bolts, using N or X type, would slip, causing a slight rotation and perhaps some cracking along the length of your supporting beam.
3. For higher loads, taking the compressive force through the bolts would place a strange stress gradiant through the cantilever web - not sure how to check for web local buckling there and this would make me ask myself...what is the definition of "smaller" loads for this design assumption.

That's all I can think of at the moment. None of the above, I believe, would kill the concept of bolts taking compression.

 

[LPPE]

Indeed, what is the definition of "smaller" loads. I would guess judgement would come in to play.
I also agree that the bolts would slip (hole is 1/16" larger than bolt), therefore causing a greater deflection. However, with non-slip critical bolts, I think the "extra" deflection would occur during construction. Once the wall or floor or roof finishes are applied, I dont think the additional initial deflection will hurt them.

 

[redhead]

Anybody tried referring to pages 4-128 and 4-129 of the AISC Manual of Steel Construction, 9th edition.

Redhead

 

[LPPE]

Yes, the bolted and welded show that "seat" connection that erectors like to avoid.

 

[redhead]

Erectors aren't structural engineers. In order to have a moment connection a force couple must be created to resist the applied moment. Without some form of connection at the bottom flange, this couple cannot be created.

With a bolted seat connection, the seat can be installed after the beam is in place obviating the erection problem.

 

[LPPE]

Carpenters aren't structural engineers either, but I try to base all my designs on constructability.

 

[Taro]

redhead,
Bolt groups certainly can resist moments. Since you seem to be fond of the ASD manual, check pages 4-57 through 4-69. It's simply a question of magnitude. If the applied moment is greater than the bolted web connection can resist, then you must revise the design. I wouldn't advocate using this type of connection for a primary structural member, but it might work out for a minor stub-out.

Doug,
Another thing to consider is that AISC requires connections to be designed for a minimum reaction of 6 kips for ASD or 10 kips for LRFD (see spec section J.1). This might control the design for small stub-outs.