2x4 Floor Truss 1

[XR250]

Just saw an apartment complex being built with 2x4 floor trusses instead of 4x2. - i.e the trusses are 1 1/2" wide with the top and bottom chords having the the 3 1/2" axis oriented vertically. Have never seen a floor truss like this. What is the benefit and/or downsides?

 

[Aesur]

I can't think of any benefits, but if it's engineered for that I don't necessarily see a structural concern.

My first thoughts on downsides:
1. May not be economical due to the orientation of the members as technically there is a difference in stiffness/moment of inertia for same depth members with the centroid of the 2x4's being further apart when flat versus vertical, this could affect floor stiffness/vibrations, but could be eliminated with properly engineered trusses.
2. More difficult to run ducting, electrical and plumbing through the truss, depending on depth maybe not a concern, but normally I see very shallow trusses on apartment complexes.
3. Truss bearing area is less, thereby maybe needed TBE's or sim to achieve bearing, depends on forces, but a 2x vertical has much less are of bearing than a flat member.

 

[Flotsam7018]

I don't think there's any mechanical advantage, if anything less due to the smaller distance between the chord areas. Any benefit is likely in the fabrication and shipping.

 

[XR250]

Maybe the webs could be farther apart due to the increases chord stiffness?

 

[Zoobie777]

For us it’s a manufacturing thing. Our machine can only do max 24” depth. Most of the time that’s enough. Anything bigger goes to our regular table. Last year I did a floor with ~4’ depth. There was no reason from a structural viewpoint. Just what the architect had in his head.

 

[Craig_H]

I had a supplier propose this once. Their explanation was that their tooling couldn't accommodate lumber on its edge on their press table. I'll admit I didn't quite understand why that would be the case, but just went with another supplier who could fabricate the trusses as-normal.

 

[RontheRedneck]

I'd echo what Zoobie777 said. I've never seen a floor truss machine that would accommodate a 4X2 truss more than 24" deep.

I have quoted deeper 2X4 floor trusses before. But no one has ever followed through and bought them.

 

[JAE]

Gang nail connection plates would have more "meat" to fasten to with vertical oriented chords and diagonals....that's the only benefit I see.

 

[Zidan Yang]

The following reasons are typically the cause:

• If the truss height exceeds 24 inches, it cannot be manufactured using floor truss equipment.​
• Balcony trusses that require a step or 1/4'' slope are usually made as 2x4 trusses.​
• Girder trusses require more plies, and 2x4s are used to better connect plies to plies and support higher loads.​
• For floor trusses with too long of a span, 2x4s provide a larger plate contact area, which effectively reduces the plate size.​

 

[XR250]

Balcony trusses that require a step or 1/4'' slope are usually made as 2x4 trusses.

Ahh, makes sense. They do cantilever out for a balcony.

 

[lexpatrie]

The "benefit" of being a weaker diaphragm?