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Light Framing Wood - Room Addition Lateral question

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I have a client adding approximately 295 square feet onto a home that is 3711 sf. The plan is controlled by seismic. The contractor representing the owner wants to remove approximately 38-feet of a 70’-0” long wall. I am having the truss manufacturer design a 38’-0” long Grider/Drag truss to pick up the ends of the existing trusses (minus the truss tails) at the bottom chord of the girder truss with a Simpson THA Adjustable Truss Hangers. This will allow a smooth transition of the existing 10’-0” ceiling from the existing to the new without having to drop a beam below the ceiling level.  The truss manufacturer is a high-end company out here who does very good work.


The existing roof is pitched 5:12 and slopes down to the addition. The new trusses (including the girder/drag truss slopes perpendicular at 5:12 with a gable end in the front of the addition. The roof tile will be removed and a series of diminishing height trusses will be set on the existing sheathing with Simpson VTC-2 Valley Truss Clips that are found in their New C-2007 Catalog (page 149). The tile will be relocated to the higher roof.


Parallel to the main home, I will need to resolve the shear in the wall removed (20-feet of shear wall at 460 plf max. or 9200 pounds of shear) into an area closer to the front of the home where there is somewhat more than 20-feet of wall that I can retrofit a new proprietary shear wall (two Hardy Panels) or a new plywood wall if I can resolve the 3x plate requirement and retrofit something in or decrease the spacing of anchors. The new Girder/Drag truss will sit flush to the outside face of the existing wall and bear on the new construction. I thought I could get a Simpson DSC Drag Strut Connector as long as the load developed by the time the force reaches the portion of the roof being replaced by the drag / girder. At 10,000 pounds (Existing + New) of shear into 70-feet of wall, the diaphragm boundary shear is only 143 plf. The total connection at each end of the truss should not exceed 4,235 pounds of shear. If it does, I’ll pick up the additional drag load in blocking below and between the lower truss blocking. This is doable.


There is sufficient solid wall for almost 30 feet I the front of the building to remove stucco, sheath with plywood. Worst case, I’ll remove 80-inches of wall or (2) 48” panels to install and HF1048 Hardy Frame (or two to meet the load). Uplift is not a big problem when length is considered with a 15-foot tributary tile roof load . I can pick this up with an epoxy anchor.


Here is the question – in the direction normal to the home, the majority of the new roof is a California Framed roof plus the 295 s.f. added to the structure. I originally designed it to act independent, but after some consideration it seems that all I am adding is 7.9% to the overall roof area (seismic controls over wind). If I design the structure as connected but braced to handle the shear calculated by the area of the addition I will have a stiffness problem between the new and the existing with the new being the stiffer of the two as I would need to add Hardy Panels inasmuch as there are large openings in the 8’-0” wide walls for cart entry and at the back side, for window view of the mountains.


Here is what I would like to get advice for:

1.       I have location of all drag trusses that will connect to the new girder truss and at the opposite end of the home are connected to three heavy shear walls.

2.       I know the design capacity of the drag trusses and I can calculate the capacity of the plywood shear walls based on the shear wall schedule from the original engineers drawings. The home was built to the 97 UBC.

3.       I suspect that there is a reserve capacity in the drag trusses and the shear walls on the plans. I would like to omit the shear in the addition and have the calculated shear at the addition drag into the existing roof without changing the stiffness by introducing a proprietary shear wall.

4.       At the worst case, the truss company can check the existing drag trusses for the additional capacity that the new addition will provide.

5.       I would also add blocking to the sub-diaphragm in order to transfer the drag from the new to the existing by blocking the bottom chord as far as necessary into the existing addition.


My gut tells me that if I design the addition as independent but make a positive connection, it will have a stiffness greater than the original residence shear walls by nature of the Proprietary panels I would need to use. This can cause the stiffer elements to take the initial jolt, fail after a number of cycles and then yield the new load into the existing load into the existing shear walls of below the drag trusses. I don’t like this idea.


My same gut is telling me to try and drag the addition into the existing diaphragm and to the aligned drag trusses since most of the nailing is based on capacity of shear rather than actual shear (this is unknown or assumed as the capacity the drag trusses were ordered designed for). If the reserve capacity exists then I can eliminate the shear normal to the main home and have both structures move together sharing common stiffness in the existing shear walls.


Finally, the total increase in square footage amounts to a conservative 7.9% and the design weight is insignificant because the only additional weight added to the roof is the new decreasing 5:12 pitched California Framed Trusses. From experience, the capacity of the manufactured trusses usually exceeds the weight of the engineers calculated design loads due to the separate 10-psf added to the bottom chord of the truss and not deducted from the top chord.


If this is as it appears, do any of you see a flaw in my design assumptions and performance intuition? I think it is a much greater risk to add stiffer shear elements in the addition that will be within 2-feet of a drag truss (essentially putting the shear in the same line).


I’d appreciate any comments you may have. Current code is still the 97 UBC here in Southern California.