Kneewalls, ties and thrust

[Don_P]

I came across this pic of a barn being dismantled. It had an inadequately tied roof that was delivering thrust to the wall posts and you can see the results. The steel looks like a retrofit added after the problem began to show itself.

A kneewall design needs to be thought about with this in mind, the floor does provide a tie but not a direct tie across the rafter feet. The vertical leg of the wall has to be strong enough not only in vertical compression but also in bending to resist the thrust. As the kneewall gets taller or the roof more heavily loaded this becomes more critical.

[poppy]

I wonder if maybe there was a lack of collar ties as well?

[Don_P]

I don't know and am not sure if it would have done much if it was high enough to give headroom. This gets into semantics somewhat so bear with me.
A collar tie in the "new" sense is used to prevent uplift. It ties the tops of the rafters together to prevent the rafters from being able to be seperated from the ridge. Imagine a strong wind getting inside and trying to inflate the roof apart. It's really a gusset.
A collar tie in the old sense was a compression member, a wind beam or a collar beam, meant to transfer compression forces between rafters.
A collar tie as most of us use the word, in tension, is a rafter tie. The floor provided one, but was obviously too low. If another was provided it was either too high or not well enough connected to do the job needed.

As the tie is raised two things happen. First, the leverage against the joint increases geometrically as the tie is lifted. Second, the tie puts an increased bending load on the rafter, which is already under a bending load from the weight of the roof and whatever is on it.

Obviously the floor and kneewalls are helping as a tie so the situation is not typically that dire but this picture did help reinforce my thinking that there is a tipping point when making the kneewalls taller, away from the lower tie, and raising the tension "collar tie" to give headroom.

I posted a drawing the other day when someone was talking about using tall kneewalls. It was of a method timberframers used to redirect the load down to the lower tie, a jambe de force, or "leg of strength"

[rwanders]

   DonP, how does the force equation or picture change when you replace a ridge board design with an appropriately sized and supported ridge beam?  Does that mitigate some of the kneewall stresses?

[devildog]

Just wondering...... Is there less outward force on a steeper pitch(12/12), than say alower(4/12). I dont know physics but makes since
Darrell

[Don_P]

rwanders, a ridgebeam or a wall under the ridge eliminates the horizontal thrust load on the kneewall. The rafters are supported on the upper end, rather than pushing against one another and down to their feet they are hanging from a beam. Works very well when you can do it.

devildog, yes there is less horizontal thrust from a steeper pitch than from a shallower one. An example, if you need to force the last pair of green deck boards down, prop them against each other and stand on the pair, if they form a teepee you'll never get them to go down. If they are nearly flat your weight is multiplied as a horizontal load.

These three pics show the same load, 200, hanging from the peak of three different rafter pitches. I drew the bottom one just to show what happens as it approaches flat (my deck boards being forced in, I can generate screw shearing force).  Notice the difference in the tension in the tie and in the compressive load in the rafter in each example.  The ratio of the lengths of the sides is also the ratio of the forces.




In a kneewalled roof the tie has been moved down below the rafter feet, below a pair of hinges at each plate/rafter joint, and is relying on the stiffness of the wall, to transmit the horizontal force to the floor joists.

Look at the heeljoint connection requirements at the end of the rafter span tables in the codebook. As the pitch decreases they require more and more nails in the rafter to ceiling joist connection, this is why. That connection load and requirement is at the loft floor in these designs.

[rwanders]

Thanks Don P, Excellent clear explanation and illustration of these design principles.