Thru-beam moment conx/splices
Thru-beam moment conx/splices
(OP)
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
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






RE: Thru-beam moment conx/splices
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.
RE: Thru-beam moment conx/splices
RE: Thru-beam moment conx/splices
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.
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RE: Thru-beam moment conx/splices
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?
RE: Thru-beam moment conx/splices
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.
RE: Thru-beam moment conx/splices
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!
RE: Thru-beam moment conx/splices
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.
RE: Thru-beam moment conx/splices
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.
RE: Thru-beam moment conx/splices
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.
RE: Thru-beam moment conx/splices
Redhead
RE: Thru-beam moment conx/splices
RE: Thru-beam moment conx/splices
With a bolted seat connection, the seat can be installed after the beam is in place obviating the erection problem.
RE: Thru-beam moment conx/splices
RE: Thru-beam moment conx/splices
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.