Estimating CMU wall spread with scissor truss

[StrEng007]

What is the best way to estimate the amount of "wall spread" that would be experienced along a masonry wall with a cast-in-place concrete tie beam that runs along the top.

I've seen here the consensus for load bearing walls supporting scissor trusses it to allow the wall to deflect, checking for eccentric loading and all that good stuff. Seems like this is a little easier to accept with wood walls. But with masonry, or concrete walls, I'm not sure exactly how to go about this.

 

[phamENG]

To get a lower bound solution, I'd do two things -

1) Check the tie beam spanning horizontally. Can it deflect as much as the scissor trusses are trying to deflect?
2) Check the wall as fixed at the base. Can it deflect as much as the scissor trusses are trying to deflect? (You could also try a spring constant for the base, but that'll be tough to come up with. A pin doesn't work because the entire wall will "rotate" and not tell you anything.)

If the answer to either of these questions is no, then you need to either develop alternate load paths that allow your walls to remain stable without the roof diaphragm, or design the trusses to be pinned at both ends.

 

[KootK]

I would assume that the scissors don't spread near significant wall returns and spread half their design spread at each bearing elsewhere.

I like this better in block than I do in wood because the wall corners are better held together.

If the design spread is kept to 0.75 in LL / 1.25 in TL per TPI recommendations, I feel that this is a problem not worth worrying about based on a significant history of satisfactory performance. For short walls, consider if the spread would put the out of tolerance if you have code limits on that (h/500 in Canada). One doesn't want a wall visibly out of plumb.

Realistically, most scissors become pin-pin at wall corners and violate their design assumptions.

Obviously, the design spread will not account for the very beneficial help of the sheathed diaphragm in limiting and smithing movement post install.

 

[TRAK.Structural]

I would assume that the scissors don't spread near significant wall returns and spread half their design spread at each bearing elsewhere.

I like this better in block than I do in wood because the wall corners are better held together.

If the design spread is kept to 0.75 in LL / 1.25 in TL per TPI recommendations, I feel that this is a problem not worth worrying about based on a significant history of satisfactory performance. For short walls, consider if the spread would put the out of tolerance if you have code limits on that (h/500 in Canada). One doesn't want a wall visibly out of plumb.

Realistically, most scissors become pin-pin at wall corners and violate their design assumptions.

Obviously, the design spread will not account for the very beneficial help of the sheathed diaphragm in limiting and smithing movement post install.

Hadn't seen those spread limit numbers before. Is this regardless of span/pitch/etc? 3/8" of LL spread (half of the 3/4" total on each side) on wood walls with drywall feels like it would cause cracking.

 

[KootK]

Is this regardless of span/pitch/etc?

Yes, in the sense that the limits will be the same. Obviously, all other things being equal, different configuration will tend to produce different movements in the absence of adherence to the limits.

spread (half of the 3/4" total on each side) on wood walls with drywall feels like it would cause cracking.

The key thing is acknowledging the reality at the corners and transverse interior walls. There, one wall doesn't separate from the other because the top plates are -- or should be -- tied together. At those locations, you get something closer to pin-pin than pin roller.

 

[TRAK.Structural]

The key thing is acknowledging the reality at the corners and transverse interior walls. There, one wall doesn't separate from the other because the top plates are -- or should be -- tied together. At those locations, you get something closer to pin-pin than pin roller.

Yes. I was more so thinking about long runs of walls perp to the trusses. I would think that tilt of these walls due to truss spread would show up in the drywall at the ceiling to wall joint or wall to floor joint.

 

[EZBuilding]

The key thing is acknowledging the reality at the corners and transverse interior walls. There, one wall doesn't separate from the other because the top plates are -- or should be -- tied together. At those locations, you get something closer to pin-pin than pin roller.

phamENG:
1) Check the tie beam spanning horizontally. Can it deflect as much as the scissor trusses are trying to deflect?
2) Check the wall as fixed at the base. Can it deflect as much as the scissor trusses are trying to deflect? (You could also try a spring constant for the base, but that'll be tough to come up with. A pin doesn't work because the entire wall will "rotate" and not tell you anything.)

In a reinforced masonry construction - 3/4" cantilever deflection for your typical wall heights equals a cantilever moment that is generally smaller than your masonry wall out-of-plane flexure due to wind loads for a simply supported condition. With masonry construction typically having centered reinforcement, you tend to find that the cantilever moment is not controlling your wall flexural design. This was a bit of an "AHA" moment for me on my scissor truss project - and possibly why this doesn't manifest itself as a substantial structural issue through out. I do think architect's should detail their drywall framing at joints between ceilings and walls to accommodate the possibility for movement.

Some considerations is due to the foundation to resist that moment, but once again, unlikely to control.

 

[JAE]

I don't have a feel for the numbers - which do vary from project to project - but the issue KootK mentions about orthogonal walls resisting the spread of the scissor truss got me wondering if there might be a significant shear build-up near these wall corners and intersections where the truss tries to spread the wall out but the wall is held back by the intersecting wall. Would shear cracking at the CMU corner possibly occur given a long span scissor truss and heavy snow loads for example?

Might be something to at least look at.

This isn't germane to the original question though.

 

[jerseyshore]

I don't have a feel for the numbers - which do vary from project to project - but the issue KootK mentions about orthogonal walls resisting the spread of the scissor truss got me wondering if there might be a significant shear build-up near these wall corners and intersections where the truss tries to spread the wall out but the wall is held back by the intersecting wall. Would shear cracking at the CMU corner possibly occur given a long span scissor truss and heavy snow loads for example?

Might be something to at least look at.

This isn't germane to the original question though.

I haven't seen it on tighter spaced trusses before, but on bowstring or bigger timber trusses we see CMU walls and piers blown out all the time from the thrust loads. My guess is that if the trusses are 24" o/c you might get some load redistribution happening first, but definitely possible.