Floor joists over steel

[JStructsteel]

Framing a floor, what is the better detail for joists bearing on a steel beam? I have a 5.25" flange, so I could split the joists right down the middle, and have equal bearing. I see alot of them overlapping at the bearing, is that for stability, just one row of blocking at the beam?

I do have an issue too where I have a cantilever at the other end, so there maybe some uplift at the bearing. I was going to use a tie down, but wondered if lapping and blocking would eliminate that need.

 

[le99]

The best is to bring bearing into the web of the steel beam, so overlapping the joists at the interior beam fits the goal. However, it is better to extend the joist over the edge beam if it is a cantilever beyond the beam.

 

[phamENG]

Lots of reasons. Continuity in tension, especially for ceilings/attic floors to provide rafter ties. Quasi-continuity in bending - even with a few nails it'll provide slightly better performance (especially at lower load levels where these floors usually see the most service - average live load for a house is about 6psf). Blocking is a little easier - you have to put it in just right to effectively brace the joist ends directly if they align, but if the lap then you can stagger them and there's no issue (this is a minor issue, though - while important, just being there is doing more than most houses I see that don't have anything at bearing).

But the "most" important:

Joists framing from opposite sides over a bearing support shall lap not less than 3 inches and shall be nailed together with a minimum three 10d face nails. A wood or metal splice with strength equal to or greater than that provided by the nailed lap is permitted.

 

[ChorasDen]

Personally, my experience is it's tougher on the framer. If your joists are offset, you now have to offset the floor sheathing as well. Therefore, all the panels break overtop this intermediate beam, and have to be installed at a different starting location. This may cause additional issues depending on location of steel beam and if there are other framing changes nearby.

 

[JStructsteel]

Attached is the detail I plan to use. Cantilever is about 2'-0", backspan about 6'-0" This is open below, not a basement. Elevated floor.

 

[KootK]

I see alot of them overlapping at the bearing, is that for stability...

For me, that's the most important reason for the lap other than straight up bearing where that would be an issue. If both joists cross the web of the beam then you can plausibly claim that the beam sees no significant torsion. That said, I often bend to the wishes of contractor who want to avoid the lap detail because it creates the sheathing offset condition that ChorasDen mentioned.

For a cantilevered condition, the lap obviously creates a simple, reliable source of resisting dead load. There are other ways to get that done however.

 

[271828]

Maybe you could try a flush framed connection and make the question go away. These help a lot for vibration and have some other advantages. Vulcraft has info for these. Also:

https://sds2.com/support/how-to/2021/flush-frame-joists

 

[phamENG]

JStructsteel - looks reasonable, though I'd be curious how you're going to finish the detail at the back span, particularly the hold down portion. Unless you can use Simpson's wood to steel screws in the connector, there's a gap in your load path as drawn.

I usually use web packing in a situation like that and tie the joist down with a twist strap.

 

[JStructsteel]

The hold down is fastened to the wood with supplied 1.5" screws. the 2x6 on the beam is fastened with 1/2" bolts every 18" (separate detail)

 

[JStructsteel]

@271828, there is a specific reason for the bottom bearing of the joists. They have to cantilever over another beam to provide overhang. Think of a fort and how the floors cantilever out on all 4 sides.

 

[phamENG]

Got it. I've never liked relying on the plate to resist bending loads in the weak axis like that where it could deflect while the beam stays still. Feels like a good place to get some cracking in finishes. That's probably me being paranoid, though.

As a matter of preference, I like to sync up the bolt spacing in those instances with standard joist spacing in 8" increments. Helps to minimize conflicts. I've seen framers cut off most of the bearing surface of the joist to make room for an anchor bolt in a sill or a bolt in a beam.

 

[XR250]

As others have said, for stability and continuity. I run into situations frequently where the joists butt on top of or into the face of a beam and a joint in the sheathing also lands near that area. You end up with a nice crack down the center of the hardwood floor as the joists shrink.
The framers are used to lapped joists so the offset is no big deal.

 

[HouseBoy]

I very rarely see joist butted together over the top of a steel beam (when the beam is not flush).
I gotta think 99.9% of the time they are lapped so... although the "offset plywood joint" SEEMS like a problem, it must not be too much of one (for the framers anyway).

I like the stability (one joists "braces" the other), availability of continuity (it is possible to nail them together) and added blocking that is provided by lapped joists (often there is a bearing wall on top of the beam).