raise roof on existing tilt up in southern california

[efsinc]

the owner would like the bottom of the new roof structure to be at 25' for most of the structure. the top of existing tilt up wall is 25'-0".

if i use 30" deep truss with bottom chord bearing on top of wall secured for seismic wall anchorage, can i transfer horizontal diaphragm shear at top of truss via short shear wall (say 30" high +/-)down to existing tilt up?

is there a way to use the existing chord steel in the tiltup wall to justify my chord stresses although the new diaphragm is 30" +/- above top of tilt up wall?

i can have the diaphragm slope down to wall at the two perpendicular walls.

 

[msquared48]

You will have a real can of worms to justify the chord loads as the wind and seismic forces are likely to increase.

Plus, unless it is designed properly, you will have a knuckle joint at the top of the existing tilt-up wall, with the trusses, rather than the roof diaphragm, having to provide lateral stasbility to the walls.

The load to the walls will also be an issue, depending on the frequency of the new trusses with respect to the existing framing. This also could alter the required steel in the walls.

You might think of using steel columns inside the tilt-up walls to support the new roof and tying the new structure toi the existing walls. They will see less load that wan and it may work.

Mike McCann
MMC Engineering