framed roof with a knee wall

[bylar]

In a framed roof with a knee wall how can one determine the load from the roof through the wall to the floor.

If the point at which the knee wall touches the rafter Is taken as a support then basically half the roof load transfers but in reality only a part of the load is going to transfer based on the deflection of the rafters. This of course is assuming the rafters are designed to carry the load without the knee wall.

I normally design the rafters to carry the load in order to get the insulation required in our area. I call for 2x12’s. I then specify a gap between the knee wall and the rafters greater then the design calculated deflection. Then I only take the weight of the wall down to the floor framing.

In the rare occasion that I use smaller rafters and use the knee wall to support the rafters I take the reaction of the rafters at the knee wall and the weigh of the knee wall as a load on the floor joists.

My question is, “is there a way to determine the load based on the deflection of the rafter at the knee wall”?

 

[AlmostPE]

I am not sure what you mean by knee wall. Do you mean interior wall? Just design the rafters to span the whole way. If you want the interior wall to take some load, then you need to declare it on your drawing that the interior wall is load bearing wall and you need to make sure you transfer all the load down to foundation. I think thats why it is important to follow the deflection limit on rafters and joists so non load bearing wall will not deflect too much since theoratically they do bear loads.

Not almost anymore!

 

[CJJS]

If you want 0 vertical deflection at the knee wall you just need to use the stiffness relationship to figure out what the force will be. Remember: Force = Stiffness * Deflection. Figure out what your stiffness is and you've got it.
To accomplish this in reality, you need to make sure your knee wall and it's supporting structure are sufficiently stiff enough to prevent vertical deflection.

 

[hokie66]

You have to have consistent deflections between the rafters and the joists. Apply a unit load on both the rafter and joist at the location of the knee wall, compare deflections, and adjust the load carried by each accordingly.

 

[msquared48]

"Knee walls" were commonly used in older residences with 2X4@24 rafters spanning ungodly distances, but they still worked.

What I saw regularly in the attic spaces where they were used was 2X4 rafters with no ridge beam spanning continuously over a Knee wall at each side to an outside wall. The rafters were periodically nailed directly off to the floor joists to serve as periodic collar ties. Thus no ridge beam was needed. The purpose of the Knee Wall was to support the rafters.

If you could model via computer this arrangement, or whatever arrangement you have, you will find the load to the Knee Wall.

And as hokie66 says, the rafters, Knee Wall and floor joists will all deflect the same and together if the Knee Wall is a bearing wall.

Mike McCann
McCann Engineering