Anchorage of masonry wall to wood diaphragm

[ronstruc]

Does anyone have a good solution for providing out-of-plane anchorage of the top of masonry walls to a wood trussed roof? It can easily be done when the masonry walls extend past the diaphragm (parapet) with a Simpson strap embedded into the CMU. The condition I am referring to is where the top of the masonry wall stops just below the truss bearing elevation and a wood sill plate is attached to the top of the masonry wall with anchor bolts and then the trusses bear on the wood sill plate. I haven't been able to find a Simpson strap that will work well in that condition. It would be preferable to have a strap attach to the side of the truss top chord (assuming the roof is pitched). In the past I have used a peice of steel angle that rests on top of the sill plate, parallel with the truss, and the vertical leg buts against the side of a truss. The angle is bolted down into the top of the masonry and then bolted into the side of the truss and through a Simpson sheet metal strap that is nailed to the side of the truss bottom chord.
I would like to hear if anyone has an better solution.
Thank you

 

[ronstruc]

I forgot to mention that the situation would be where the trusses run perpendicular to the wall.

 

[egc]

By "out-of-plane" do you mean lateral forces at the truss support (parallel to the truss)? If so, then strap type anchors loaded in their weak axis direction supply minimum resistance (to me replace minimum with zero). Depending on the magnitude of the lateral load, Simpson and USP have various connectors that may work better for you.

 

[ronstruc]

Out-of-Plane meaning lateral forces (usually seismic) against the wall (parallel with the truss); the diaphragm to provide support to the wall. The load is around 300 plf, so a Simpson strap is capable and that is what I have used in the past. What kind of strap and how have you attached it to the wall? That is where I have used the angle to be a transfer device from the truss to the wall. The strap I have used is a flat strap like a MSTC28. I would like to use a simpler solution without the angle and have the strap attached to the side of the top chord.

 

[lsmfse]

How tall is the wall? Can you provide a wide footing and cantilever the wall? Maybe you could use 2-HGA10, one at the inner face of wall & one at the outer face so you won't have cross grain tension. Maybe a custom made HS24 that would fit over the nailer. Simpson does have a lot of anchors for Masonry, but rarely if at all have I seen truss bearing directly on the block as shown on pages 129-133.

 

[UcfSE]

I would consider trying the H10, an A-clip (A23 for instance), or using META's or HETA's and drilling an access hole through the bearing plate. H2.5A's may also work, all depending on the magnitude of your uplift and in- and out-of-plane forces.

 

[ronstruc]

Thank you for the ideas. The wall is 18' high, but regardless the code (IBC 2003 1620.3.1) requires this anchorage to meet a minimum force requirement based on the formula 16-62. The code also says the sill plate can't be used in cross-rain bending. The problem with a HGA10, an A-clip, and others is the cross-grain bending problem. I am a little worried about a META or HETA used in this application through an access hole. There would be 1-1/2" (assuming a 2x plate)eccentricity between the lateral load and point of resistance (masonry). I think the slender strap would want to twist or buckle with cyclic action that could occur in any direction. I have asked Simspon reps. this question and have never received a good solution. Simpson connectors work well without a sill plate but I need a sill plate to transfer the in plane forces out of the roof diaphragm and into the masonry wall through truss blocking. Again I appreciate the ideas. If you or anyone else has any more ideas I would appreciate it.

 

[Lutfi]

I encounter this situation a lot. On my truss bearing detail I show a strap/hurricane anchor and request that the strap meets the following criteria:

1. Resist up lift force as determined by truss supplier.
2. In plane load to transfer the roof diaphragm from decking to walls.
3. Out of plane wind load. This is important because I design my CMU wall as being pinned-pinned condition. Of course the tie beam will support the top of the wall along with the trusses.

For items 2 and 3, I compute these forces based on number of trusses and their spacing. I found lots of resistance from local contractors when they saw my criteria. They were used to providing straps for uplift only. However, this is not the case. Simpson and southeaster Metals in the southeast do provide straps that have the three capacities.


Regards,
Lutfi

 

[MWPC]

ronstruc,

IMHO, I believe the "no cross grain bending" is to eliminate a ledger bolted to the side of the wall where the diaphragm would pull on the top of the ledger and the bolts would restrain it at mid-point causing "cross grain" bending. The plate on top of the wall would be loaded in bending parallel to the grain.

I use metal clips like your angles just making sure that they are long enough or installed in pairs such that regardless of the direction of the force, they are not causing tension perpendicular to the grain at the bolt. The edge distance requirement for the bolts may take care of that as well.