Top vs. Bottom Chord Bearing for Flat Wood Truss 1

[StrEng007]

When specifying flat wood trusses for roof and floor systems, are there any clear advantages/disadvantages to using top chord vs. bottom chord bearing?
Roof trusses would consists of 2x4 or 2x6 members in their strong axis. Floor trusses would be the typical System 42 (4x2, flat) open web truss.

Also, for situations where these trusses sit inside a CMU shell (example, roof trusses at a parapet wall), does it make more sense to use a ledger with a pre-engineered hanger? Or specify top chord bearing at the truss ends?

 

[jerseyshore]

These are the types of truss details that no matter which one I put on the plans they always want to do the opposite. I'll show a ledger one job, but they'll want to make it easier and sit the trusses on top of the block. Next job I'll update my detail and they'll go back to the ledger.

 

[Eng16080]

Ha ha! jerseyshore speaks the truth. Whatever detail you prefer, just do the opposite since the builder will hate it!

From my experience, here are some advantages/disadvantages:
[ol 1]
[li]TC can often be better for shear transfer from a wall above to a wall below. With BC, a large rim board or blocking is often necessary.[/li]
[li]With TC, walls will be framed taller (more material). Also, in most cases, there will need to be fire blocking installed between wall studs at the ceiling level (truss btm chord level).[/li]
[li]BC can support a higher shear force at supports, whereas TC will be limited due to the top chord depth (usually 2 or 3 plies).[/li]
[li]For TC, the connection to supporting framing (walls/beams) is usually simpler.[/li]
[li]I'm not certain, but I assume BC trusses are slightly less expensive as a general rule.[/li]
[/ol]

Also, for situations where these trusses sit inside a CMU shell (example, roof trusses at a parapet wall), does it make more sense to use a ledger with a pre-engineered hanger? Or specify top chord bearing at the truss ends?

I'm not sure which is necessarily better in your case. For a TC truss, the ledger may need to be wider than for a BC truss with a hanger. The overall install is probably simpler for TC.

 

[KootK]

This one all day long:

BC can support a higher shear force at supports, whereas TC will be limited due to the top chord depth (usually 2 or 3 plies).

The top chord seat connections that make some sense in steel(great shear capacity) make much less sense wood. They're fragile and I'd have them outlawed if I had my druthers.

 

[XR250]

The top chord seat connections that make some sense in steel(great shear capacity) make much less sense wood. They're fragile and I'd have them outlawed if I had my druthers.

Never thought about that. Good to know.

 

[jayrod12]

I'm also on board with bottom chord bearing. Mostly because I've had to specify repairs for enough rotten ones and fought through ways of maintaining the top chord bearing detail. But that's never pretty. Bottom chord bearing details are far easier to reconstitute when required.

That being said, I love using standard dimensional lumber rim boards. So although the truss supplier would prefer a 3" deep top chord bearing detail, I often specify either 5 1/2" or 7 1/4" so that I can easily use a 2x6 or 2x8 as a rimboard. I've found those depths of seats often allow for larger plates at the top node, and therefore more capacity as well.

 

[RontheRedneck]

A couple of thoughts from my perspective -

I tend to like BC bearing, as it allows more variation in the lengths of the trusses. With TC bearing, the walls and trusses have to be pretty well right. That doesn't always happen in reality.

TC bearing trusses have more pieces, more plates, and more labor. So they're slightly more expensive to build.

I often get plans that call for the beams to be flush. They're not thinking about getting mechanicals through the floor system later, so I strongly discourage it.

I was once on a jobsite with TC bearing flat trusses that spanned maybe 40'. The framer thought he could just set them in place and let them hang there with no bracing.

Don't know if any of this is helpful, but it' what I deal with from my end of the process.

 

[StrEng007]

Very helpful information. Thanks to everyone.

Looks like bottom chord takes the lead on this one.