DIAPHRAGM ACTION

[ick2]

in an existing house the roof joists have been raised 2 feet above the top of the brick wall, and frame into the rafters. the roof joists support an uninhabitable attic floor. There are two wood plates at the top of the wall which i do not think that are connected to the wall. the rafters sit on these plates with a bird's mouth cut. How can i provide diaphragm action at the top of the wall for wind loads?

 

[ick2]

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cross section showing wall, attic floor and roof

 

[JAE]

Isn't it just sort of a bent shearwall? Just create a proper load path using sheathing and nailing to track the horizontal loads from the attic floor - to the diagonal diaphragm - to the vertical diaphragm.

 

[ckar]

this is an existing house. the rafters just sit on top of the wall while the attic floor is two feet above. my concern is the point of connection between rafters and wall. the wall is two wythe brick and i do not think that the top plate is connected to the wall and now the headroom is too low to try to connect with bolts. i am not clear to what diagonal diaphragm you are referring to.

 

[ckar]

may be you mean the portion of the rafter under the attic floor. this is ok for in plane loads but what about in and out of plane?

 

[csd72]

For lateral loads(across page) if the rafters are continuous from the bend point to the top level of the roof, then you use these as propped cantilevers to support the top of the wall and then provide a diaphragm at attic level to transer this to the end walls.

For longitudinal loads(into page) if you provide a diaphram in the sloped portion of the wall that is continuous between the end walls then this will give a push/pull at the end walls (and therefore the lateral component can be taken out by the end walls) you are then left with a lateral load along the top of the vertical wall that can be taken out as if by a normal shear wall.

 

[JAE]

Yes..that's what I meant - agree 100% with csd72.