Hip rafter span table3/21/2024 It would be interesting to see what restraints were used in your model, and what the horizontal reactions were at the base of the untied jack rafters. Whether they know it or not, the approach by many structural engineers to say the hip rafter takes half the vertical load of each jack rafter means that (if the bottom of the jack rafter is birds-mouthed onto the wall plate) no net axial load exists at supports and so the wall only receives vertical load. Suitably detailed ties perpendicular to the ceiling joists (often found forming double duty as ceiling binders) can address this. As such, the system relies on the wall to resist the thrust of the hipped jack rafters - not ideal. In the hip situation, however, rafters on one side of the hip will (probably) be running perpendicular to ceiling joists so - even assuming there is a set of opposing hip jack rafters on the other end of the roof - there are no tie members to connect and cancel out these opposing thrusts at wall plate level. This is just like a pair of opposing common rafters, where compression is generated in the rafters and the resultant forces at the wall plate resolve into the vertical load (taken by the wall) and opposing horizontal thrusts taken via the ceiling joist (acting as a tie) to cancel each other out. The truss forces you describe depend on the jack rafters working in axial compression which, at wall plate level, will resolve into vertical AND horizontal thrust components. I understand what you're getting at in your post, and agree that according to your analysis the hip beam will have a low load due to the 3D truss type action, however, I think there's more to it… RonnyRaygun: I know this thread is old but.
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