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Lateral Analysis - Attic Trusses

Lateral Analysis - Attic Trusses

Lateral Analysis - Attic Trusses

(OP)
I am designing a simple shop (30x50) that will incorporate a second floor into the attic trusses.  At 30'x50', this building requires engineering, but I am unsure how to treat the roof diaphragm shear transfer from the roof ply to plate level.  Floor ply applied to the bottom chords of the trusses obviously does not extend to the plates due to web members of the trusses.  I have been told that the load path from the second floor ply travels through the vertical members of the trusses (floor ply to bottom chord to vertical member) to the roof plywood and back down to plate level.  This doesn't seem very realistic.

Any thoughts?

RE: Lateral Analysis - Attic Trusses

IF YOU HAVE A RIGID CEILING APPLIED TO THE UNDERSIDE OF THE BOTTOM CHORD AND YOUR ATTIC ROOM KNEEWALLS ARE SHEETED, THIS SHOULD TAKE CARE OF LOAD TRANSFER TO THE PLATES. THEN I WOULD CHECK ONE OF THE SIMPSON HURRICANE TIES FOR THE MECHANICAL CONX AT THE PLATE. NOTE THAT IT YOUR ATTIC TRUSS HAS A RELATIVELY TALL HEEL YOU MAY WANT TO HOLD BACK THE TRUSS FLUSH WITH THE TOP PLATE SO YOU CAN EXTEND YOUR WALL SHEATHING UP THE END OF THE TRUSS.

RE: Lateral Analysis - Attic Trusses

(OP)
Is it typical to design the underside of the BC with sheetrock? Or, is it typical to sheet with a more rigid material such as 3/8" cdx.  I have seen some standard details required by the local building department that show 3/8" cdx for non-engineered systems.  Also, the UBC has an allowable shear value table for vertical diaphragms (sheetrock), but I don't see a table for horizontal diaphragms?

RE: Lateral Analysis - Attic Trusses

WITHOUT LOOKING INTO IT FURTHER, I WOULD SPEND THE EXTRA FEW HUNDRED DOLLARS IN 7/16 OSB FOR THE UNDERSIDE OF THE B.C AND SHEETROCK OVER IT IF DESIRED.

RE: Lateral Analysis - Attic Trusses

Between the floor plywood, ceiling drywall and roof sheathing - you should get all the shear transfer you need.  However - blocking between the trusses at the sill plate is warranted.  This will help transfer any shear to the walls.

Good Luck!

RE: Lateral Analysis - Attic Trusses

Roof sheathing should really be enough.  Any ceiling and flooring will help more, but they both need to span the length of the building.  I assume 30 foot span for the trusses, leaving 50' open?  I would be more concerned with having enough shear walls, since these things typically have big openings on the ends, where you need shear walls the most.

RE: Lateral Analysis - Attic Trusses

(OP)
Thanks to all for the information! It has been a great help!

RE: Lateral Analysis - Attic Trusses

Could you build a tall heel into the truss and incorporate typical shear panel's between the trusses at the heel on both ends?

If so this would help to raise the ceiling in the room.

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