SJI - Standard Clear Bearing Length vs Span Length at Steel Supports

[StrEng007]

I'm having difficult understanding why SJI defines a joist Span (supported by a steel structure) as the center line of the supporting steel.

They illustrate a "Standard Clear Bearing Length" with the caveat that you only need to maintain 2-1/2" when bearing on steel. Seems like it contradicts itself.

Take the K-Series joists for example. Table 5.4-1 says you need a 4" standard clear bearing length. I interpret that to mean your joist top chord must overlap the edge of the supporting flange by 4" on each side. The same table shows that you're only required to have 2-1/2" of bearing on steel. Note: I realize the significance of this at concrete supports with steel bearing plates.


Here is a scenario:
W16x77 girders with centerline spacing at 25 ft. By illustration, the Span of the beam is 25'-0"

bf/2 of a W16x77 is 5 1/8". So the clear distance between flange edges is 25ft - 10 1/4" = 24'- 1 3/4"
If the joist is required to have 4" of standard clear bearing length on each side, the joists overall length = 24'-1 3/4" + (2)4" = 24'-9 3/4"

But if you're only required to bear 2-1/2" on the steel, can this instead by calculated as = 24'-1 3/4" + (2) 2 1/2" = 24'- 6 3/4"

So which value represents the Span of the beam?

 

[BridgeSmith]

I say if you're fighting for a few inches to make it work, you should just upsize the joist.

Anyway, regardless of the actual length of the joist, one way or another, it's going to be carrying the load of the entire 25' span.

 

[StrEng007]

Not so much that it's a strength design issue. I just want to clearly understand SJI since the information they've presented is unclear.

 

[BridgeSmith]

It's not unclear to me. The design span is center to center of the supporting beams (because that's the tributary length for the loading), and the minimum bearing length is 2.5". They don't know what top flange width of the supporting girders will be.

 

[lexpatrie]

I think you're confusing span with design length. Design length is less than span. Span is center line to center line of the joist supports. The reaction is presumed/designed for 2" inside the edge of the beam. Standard bearing length for K series, 4", minimum is 2 1/2". Those are in the code of standard practice (?) and are "somewhat" debatable, but larger bearing lengths may increase the depth of the seat.

You should really confirm with SJI, as they are the subject matter experts on their standards. Report back, though, so the folks here can access the information as well, if you don't mind.