Timber frame

[phuduhudu]

I am designing a timber frame house (in the UK). It is a particular system designed for minimum cold bridging for Passivhaus performance as shown in the image below using wide flanged wall studs and rafters connected with ply plates at the eaves and ridge. They form portals but are nowhere near stiff enough for frame action. The roof is always fairly steep 35-45 degrees. I would put in a ridge beam but my question is with the stiffness of the steep inclined roof plates I can design the ridge beam to take the load but the roof plates are much deeper and so will surely be much stiffer as diaphragms? The problem is that with this design I can't get a beam in at the tension and compression chords of the diaphragm. All the guidance points to the need to design these diaphragm beams for the forces generated (usually to resist wind loads). Even so if the roof has OSB sheathing it is going to have a stiffness and resist loads whether I design it to or not. In this case the wind loads are not that much less than the roof loads to be resisted so I can't even make the argument that the diaphragm can deal with wind loads but not gravity loads. Any thoughts on what to do in this case?

 

[XR250]

I would just put the ridge in. I reality, you may not need one but proving that and the subsequent detailing would be a lot more expensive than the ridge.

 

[phuduhudu]

That is true but since the wind loads are not much less than the gravity loads I would then still need to justify the roof diaphragm to transfer the wind loads so I may end up needing to do the difficult detailing anyway. However, I know that often with a large enough aspect ratio it is just assumed that the wind loads on a small domestic building will be dealt with by the roof so this has given me pause for thought in terms of what is required of a roof diaphragm.