shear transfer: flexible diaphragm on lg trusses

[HeavyCivil]

In analyzing simple, low rise structures with gable roofs and flexible diaphragms it is not difficult to distribute lateral forces and check shear walls (provided geometry is somewhat simple).

With wood trusses I show blocking in between truss heals and usually use Simpson H2.5A's (they're symetric and are tough to put in wrong) and they have good allowable loads in uplift and in both lateral directions (F1, F2). Its easy to check that lateral forces are transferred to parallel shear walls.

When using masonry walls and LG trusses, I typically see tension ties for the trusses (SST MTSM and similar). The geometry and loading is the same, but to what degree will that tie transfer lateral forces from the perpendicular walls, through the diaphragm to parallel walls? I understand it has little to no stiffeness, but as soon as it starts to deflect, it would go into tension and resist. is it, at that point, too late?

I have been told this is a case of "Engineering Judgement" and that lateral forces will transfer without problem.

 

[vandede427]

Does LG stand for lightgage?

I treat lightgage the same as wood. Don't rely on the clip for diaphragm transfer even if it has a rated value.

For small heel heights, provide full height blocking. There are calcs and material out there for justifying partial height blocking, but it has mixed opinions.

For large heel heights (monosloped trusses), provided blockign for wood, or a blocking truss by lightgage supplier, or diagonal strapping/bracing, or sheathing the wall from truss bearing to roof.

 

[woodman88]

It is the building designer responsibility to provide permanent bracing to maintain the truss in a vertical position. You should follow what vandede427 states for bracing the trusses. Or get the truss engineer to seal the design stating that it can transfer the lateral load this way. You might also talk to Simpson Stronge-Tie about their clips and if rotation of the truss is permited by them for this.

Garth Dreger PE
AZ Phoenix area