Cantilevered Floor Truss Backspan 1

[Zoobie777]

I'm designing a floor truss system that has a short cantilever (1'6") that is perpendicular to the trusses. Generally I follow the 150% rule of thumb which would give a 4'6" back span, which will end up being 5'4" to keep the 16"oc truss spacing nice and neat. The question I have is, can you reduce the required backspan when using a truss instead of a joist? Its going to be hung from a girder truss. The cantilever is supporting a doorway/entry and a gable truss with 2' OH.

It got me thinking because of how jack trusses are often designed. Very normal to have a 2'+ overhang or cantilever with only a 2' span back to the girder truss.

EDIT: Should also mention truss depth is also 1'6".

 

[KootK]

Like most rules of thumb, the back span thing can be circumvented if it's engineered properly. To that end, some things to consider:

1) Do the overhanging bits have robust uplift anchorage to the supporting girder?

2) Does the supporting girder have robust uplift anchorage to its supports?

3) When the girder bows upwards due to the load on the cantilever tips, will that be a service ability problem, either out at the cantilever tips (downwards) or over the hogging girder? At the cantilever tips, this will not be a uniform deflection along the length of the exterior wall.

4) Does this condition put the bottom chord of the girder into compression such that it would require unconventional bracing?

.... probably a bunch more stuff that I've not thought of.

 

[jerseyshore]

As usual Koot hit the major thinking points.

We recently had a building where truss designer flipped some of the framing we showed to create short 2ft cants with 2ft backspans.

Guess what happened after the roof was framed...major uplift problem. Felt like a speed-bump as the girder truss was bowing upwards with the hangers also bent.

I hate short backspan to cant ratio members without careful consideration of the forces. I almost always run them with 0 dead load on the backspan because in reality (like our issue above) during construction there is almost no DL on the backspan and that's when issues arise.

 

[Zoobie777]

Like most rules of thumb, the back span thing can be circumvented if it's engineered properly. To that end, some things to consider:

1) Do the overhanging bits have robust uplift anchorage to the supporting girder?

2) Does the supporting girder have robust uplift anchorage to its supports?

3) When the girder bows upwards due to the load on the cantilever tips, will that be a service ability problem, either out at the cantilever tips (downwards) or over the hogging girder? At the cantilever tips, this will not be a uniform deflection along the length of the exterior wall.

4) Does this condition put the bottom chord of the girder into compression such that it would require unconventional bracing?

.... probably a bunch more stuff that I've not thought of.

Thanks. Good food for thought.

 

[Zoobie777]

As usual Koot hit the major thinking points.

We recently had a building where truss designer flipped some of the framing we showed to create short 2ft cants with 2ft backspans.

Guess what happened after the roof was framed...major uplift problem. Felt like a speed-bump as the girder truss was bowing upwards with the hangers also bent.

I hate short backspan to cant ratio members without careful consideration of the forces. I almost always run them with 0 dead load on the backspan because in reality (like our issue above) during construction there is almost no DL on the backspan and that's when issues arise.

Fortunately here I an designing the floor and the roof. I do share your concerns though. I have put the girder as far back as I can get (against the stair well). Once I actually got into the design I noticed that they have a post at the corner of the cant that supporting the porch roof beam. I can design for it but I think I will just push back and say pour a concrete pile under the corner. They are also hanging the porch floor off of the cantilever (additional 4 ft of trib). The more I look at this the more stupid it seems. It would literally be 9 lin. fit of footing.

 

[RontheRedneck]

I typically try to go somewhere around doubling the backspan. I figure the wall on the cantilever will always be there. Live load on the backspan will not.

I try to be mindful of plywood widths when laying out the floor. (Keep in mind that subfloor is not a full 4' wide - More like 47 3/8")

Sometimes I've used 2 of the Simpson THA422 hangers on the cantilevered truss - One right side up, and the other upside down.

 

[Zoobie777]

I typically try to go somewhere around doubling the backspan. I figure the wall on the cantilever will always be there. Live load on the backspan will not.

I try to be mindful of plywood widths when laying out the floor. (Keep in mind that subfloor is not a full 4' wide - More like 47 3/8")

Sometimes I've use 2 of the Simpson THJA422 hangers on the cantilevered truss - One right side up, and the other upside down.

Thanks! I was literally just looking through the Simpson catalogue for this hanger (I forgot the model#). BTW, its actually THA422. The THJAs are hip/jack hangers.

 

[RontheRedneck]

You're correct on the hanger type. Sorry about the type.