IRC 502.6.1 purpose

[smb4050]

Does anyone know the history - reasoning - purpose behind IRC section R502.6.1 - Opposite floor joist over interior supports lapped 3"? In this case there are 2x8 floor joist spanning 8' to 9' butt together over a double LVL beam and NO wall above it. A home inspector is saying telling a contractor it doesn't meet code. Granted it doesn't, but I don't see where its critical. There is adequate end bearing, and 3" isn't long enough of a lap to consider it a continuous member. LTB won't be a factor because continuous solid blocking is installed. I plan to show have a 2x4 scabbed across the butt joint, but really don't see the need for it. I've ran across this same issue a few years ago when an insurance company denied a claim after a wash machine ran over, and the floor sheathing buckled claiming it wasn't built to code. Appears to be an in the IRC because it's always been there, and nobody has a motivation to remove it. Any thoughts someone is willing to share will be appreciated. Thanks,

 

[jayrod12]

My bet (and I'm not familiar with the IRC) would be twofold. Firstly to ensure that even if a sheathing panel joint happens right at the overlap you are still getting fasteners into each piece. Secondly, when the floor joists deflect it causes the ends to rise up slightly moving the top of the member inwards towards the span. if you have a butt joint instead of an overlap you would have this gap in the floor that was opening and closing constantly as the members deflect and rebound.

That's just my $0.02

 

[msquared48]

Two reasons I can think of:

1. To provide enough end distance so the individual joists can be toe nailed to the wall top plate, with the intention of minimize the tendency of the joist to split out at the end, and

2. To provide enough distance to maintain joist bearing in a seismic situation should strapping between the joists not be used, and a minimum of shear attachment to the wall top plate. This should not be a problem in a low seismic area.

Mike McCann, PE, SE (WA)

 

[KootK]

It's just good practice to ensure sufficient bearing. Given construction tolerances on the joist lengths and the wall placement, 1.75" bearing can turn into 1.00" bearing in no time. If things are snug tight and accurate out on sight, I can't think of any great engineering reason to enforce the 3" lap provision. IRC is prescriptive, right? The designer isn't necessarily obligated to check bearing stress?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[DBronson]

smb4050:

The commentary to that section of the code states the the overlap is " to provide a concentric application of load the supporting beams or girders...". Of course, that assumes the reactions are symmetrical. KootK is correct that the IRC is prescriptive. If you supplied calc's showing sufficient bearing exists I would think the problem would go away.

Regards,

DB

 

[woodman88]

It is per the IRC code if there is a calculated design for it by the building designer.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[smb4050]

A short note to say thanks and I appreciate everyone who took time to reply. I hadn't thought of an unsymmetrical joist bearing causing torsion in the beam. A legitimate reason that can be checked. Thanks again, smb4050

 

[KootK]

The rotation of the joist ends tends to move the bearing points out towards the edges of the beam matter what. True shear centre load application is usually an illusion.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[madmantrapper]

It used to be a note or something in the IRC that said something to the effect that if the joist butted you needed to place a strap across the top. I'll see if I can find it.

 

[jfmann]

Questions about whether some condition or other "meets code" arise often during inspections and alteration work. New Jersey (my home area) does not use IRC provisions for existing buildings, instead relying on the home-grown "Rehab Code", which allows an existing building to remain in place without alterations unless a condition is clearly an imminent safety problem. This basic concept should apply to most areas, except for seismic upgrades..........otherwise huge disruptions would be mandated for buildings that are in good condition and have served successfully for decades.

As for lapping floor joists......basic reason (as noted in other posts) is adequate bearing. Better resistance to overturning is another consideration, since all beams must have adequate lateral bracing at supports.

Based on my understanding of the role of home inspectors......which almost all even include in standard verbiage with their reports that tends to greatly exceed any written descriptions of observations.......is that they are not supposed to checking code violations anyway. They should be checking general condition to determine if there might be damage or a grossly unsafe condition such as a girder completely cut through by some exuberant HVAC installer.

John F Mann, PE

http://www.structural101.com

 

[medeek]

The comments made by msquared48 and madmantrapper make the most sense to me. In a seismic event everything is going to shake and shuffle a bit. With the very minimum bearing length chances are the joist will drop right off the beam without some additional strapping or other restraint.

Another thought I had is that the solid blocking that is often typical over beams or bearing points is harder to achieve without the overlap. If the joist butt up and no overlap is present there is a good possibility that the solid blocking between joists will block one set of the joists and not the other set given the tolerances of most framers. Requiring the overlap ensures that this blocking is working positively for both joist sets.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com