Wood Wall Double Top Plates with Varying Elevations at Corners and Intersections

[gramy13]

I have a standard 4-story wood multi-family project, where the pre-engineered open web floor trusses span between unit demising walls.

Instead of using standard platform framing, the contractor has proposed to raise the elevations of the double top plates in an attempt to eliminate ladder trusses and fire/shearwall blocking between the trusses.

At exterior walls where trusses are parallel, wall double top plates will run to underside of floor sheathing (-0'-3/4"). At exterior walls where trusses are perpendicular, wall double top plates will run to underside of double 2x top chord truss bearing (-0'-3-3/4"). The double top plates at the building corners and jogs will be in different planes, albeit they will still overlap. Should I provide some sort of additional Simpson strapping/clips since the wall top plates will be at different elevations and cannot be finger lapped together?

Additionally at corridor walls, wall double top plates will run to underside of 2x8 corridor joists (-0'-8") spanning between corridor walls. 2x8 joists will bear 4" over top of wall double top plates, and a continuous 2x8 rim board will close off the corridor joists on the unit side. The floor trusses typically span parallel to the corridor walls.

Lastly, at the unit demising walls, the double top plates will run to the underside of double top chord truss bearing (-0'-3-3/4"). The 3" gap between truss top chord bearing ends will be blocked with (2)2x6 for fire blocking. At unit interior bearing walls, the double top plates will run to the underside of the 20" deep trusses (-1'-8-3/4"). Where the demising/interior wall double top plates intersect with the exterior & corridor walls the double top plate elevations will be in different planes. I'm thinking this will require some sort of Simpson strapping/clips since the wall top plates will be at different elevations and cannot be lapped together.

In conventional platform framing, lapping the double top plates at wall corners and intersections seams to be good practice to tie all of the walls together, but quantifying a force for this plate lap connection doesn't seam apparent. If I just provide strapping or clips at the corners and intersections where the double top plates are at different elevations is this acceptable - or is there more to it? Thanks!

 

[KootK]

I vote for doing nothing special at your top plate corners.

I don't actually believe that it is necessary for top plates to lap at corners. You know, other than the general sentiment that it is better to tie things together than not. For diaphragm design, all of your top plate direction changes were always points of hopeless discontinuity.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

I second that KootK. I prefer to have things framed the way your contractor is suggesting. It transfers the loads into and out of the diaphragms better. Also makes it easier to deal with point loads rather than going thru the crappy 2x4 truss ribbon or top chord.

 

[medeek]

I recently ran into a similar situation with a large residence. Discontinuous top chords required the use of some MSTC40 straps to tie different sections of the wall line together.



A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[XR250]

"Discontinuous top chords required the use of some MSTC40 straps to tie different sections of the wall line together."
How are you determining a design tension for the straps?

 

[mike20793]

The only thing I would add is watch out for differential shrinkage. I've seen issues in areas with combination platform and balloon framing for 4 story wood buildings.