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Interior Shear Wall Connections to Trusses

Interior Shear Wall Connections to Trusses

Interior Shear Wall Connections to Trusses

(OP)
I'm a plans examiner looking at a lot of residential designs.  Many have interior shear walls when there are a lot of windows.  The interior shear walls (of course) are either parallel or perpendicular to the trusses. The walls that are parallel may or may not be directly under a truss. The walls which are perpendicular to the trusses will fall under several trusses, but the load will be across the plane of the truss.  I've learned that if the wall is parallel to and directly below the truss, you can sheath the wall up to the roof and design for increased horizontal load. I realize that interior shear walls can have more shear load than end walls.  How can these connections be safely designed?

RE: Interior Shear Wall Connections to Trusses

Sheathing the wall from slab to deck is kind of tough sometimes.  I use Simpson connectors from the truss bottom chord to the top plate.  LTP's work pretty well and so do HGA's.  I have also used blocking between two adjacent truss bottom chords (where the wall was not below a truss) and then clipped the blocking to the wall.  If the walls truely are interior shear walls, the designer should have provided you with the connection from the truss to the wall as that is critical.

RE: Interior Shear Wall Connections to Trusses

(OP)
Thanks scottiesei, those are the types of connections being used.  I guess my real question was do these type of connections adequately transfer the shear loads to the roof diaphram, without overloading the trusses.  Especially for walls connected at a right angle to the truss.  Is the normal truss to truss bracing adequate, or should I ask for extra bracing?  Is a generic prescriptive method OK, or do I need to get a signoff from the truss designer for every instance?

Thanks

RE: Interior Shear Wall Connections to Trusses

Truss loading perpendicular to the plane is the EOR's responsibility, not the truss co.  Extra blocking would depend on the height of the truss at the connection.  I.E: if it is a nominal heel on the wall, then blocking would be less likely required than if it was tall at that point, say in the middle of a common truss.  Some Simpson connectors are tested for the lateral forces and values are given in the book so there is a way to check the forces.  I have seen several trusses checked for drag loads and only in very severe cases was it the controlling design factor.  Usually the DL+LL combo governs.

RE: Interior Shear Wall Connections to Trusses

(OP)
Thanks scottiesei,

I think that gives me what I was looking for.  If you block it right it's OK

RE: Interior Shear Wall Connections to Trusses

Trusses are not designed to withstand perpendicular loading.
For interior shear walls perpendicular to trusses, the truss manufacturer can fabricate shear-blocking panels that install between the trusses and atop the shear wall. This provides a method of transfering the roof diaphragm load, through the panel and into the shear wall. Otherwise the sheetrock ceiling will act as a diaphragm and discontinuity occurs to the roof ply.

A framer can site-build a continuous shear-wall transfer through the attic also.

At exterior walls, and typically a truss heel joint, frieze blocking acts like a short panel. Boundary nailing is needed from the roof-ply into the block, and typically LTP4 transfers the horizontal load to the double top plate.

Interior parallel truss & shear walls that align are the best. Sometimes an additional truss is added to insure alignment. Blocking between trusses with a block-to-top-plate connection will not transfer much load and is not advised.

Multiple shear walls in alignment can rely on the top-plate to act as a collector to transfer the lateral load to the truss.

RE: Interior Shear Wall Connections to Trusses

(OP)
CTruax,
Thanks for your reply.  Your comments are more in line with my initial concerns, and those of a truss design engineer I was able to find.  By his rough calculation the shear force on shear panels built between the trusses (for trusses perpendiculat to the wall) was pretty high.

What I forgot to ask, and seems contradictory, was what happens when there is a porch attached, and the house wall is the shear wall.  This seems to be the same situation.

RE: Interior Shear Wall Connections to Trusses

For porches, either
The roof ply boundary at the truss heel is transfered back to the wall top-plate through a horizontal ceiling diaphragm, or
the shear wall continues up to the roof-ply through truss blocking panels or site built framing, or
an exterior beam supporting the truss heels acts as a collector to drag the total lateral load to another in-alignment shear wall or truss collector.

RE: Interior Shear Wall Connections to Trusses

Another engineer once showed me how to block shear walls to improve strength.  He suggested blocking between the studs, top and bottom, nailing the blocks to the sole plate (bottom) and top plate (top).  I have read excerpts of various earthquake reports suggesting this is useful, but I cannot find good scientific info supporting this practice.  Any help?

lotklear

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