Timber stud supported 60' Scissor Truss
Timber stud supported 60' Scissor Truss
(OP)
What tips can you offer for dealing with the horizontal thrust and deflection of a pre-engineered timber scissor truss supported by tall load bearing timber studs typical of church or atrium construction?





RE: Timber stud supported 60' Scissor Truss
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Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm
RE: Timber stud supported 60' Scissor Truss
On a tall wall (or long length) building you will want zero thrust perpendicular to the top plate to eliminate bowing. You can see this effect when you look at old barns which have a sag of their ridge line near the building center due to creep/slip of the plate/collar-tie mechanism used.
Framing crews still 8d toe-nail heel joints to top plates when trusses are rolled into place, and then later add a mechanical anchor such as Simpson H1 or H2.5 per recent wind/uplift code changes.
To minimize this effect when using scissor trusses:
-Detail horizontally slotted connector(s) at the truss heel(s) such as Simpson TC24 or TC26.
-Make sure the framing crew is aware of where to place the truss nailing in the slots and not to toe-nail.
-Inspect for correct installation (and non-bowed plates) AFTER the dead load roofing materials have been applied. Remember that the application of sheetrock, insulation, and mechanical will further increase deflections.
-Specify an aesthetic molding/trim at the wall/ceiling corner which is connected only to the wall and allows for the truss movement.
The truss calculatons will provide LL & LL+DL deflections. If the anticipated horizontal deflections exceed the connector(s) slot widths allowed (one or both ends specified), then request a different (stiffer) truss design such as increasing the top chord slope, or lowering the bottom chord slope. Reducing truss spacing will not be as cost effective.
Attic ventilation, temperature changes, and snow loads cause all trusses to deflect (breathe); but the scissor, cathedral, and vaulted shapes amplify this condition.
RE: Timber stud supported 60' Scissor Truss
RE: Timber stud supported 60' Scissor Truss
This made me think - if every truss bearing is detailed for "slip", and every scissors truss thrusts outward, say 1", due to the effect of its vertical deflection with gravity loading, won't the top of stud wall tilt outward as a result? This is assuming that there is some value of friction between the bottom of truss and top plate.
RE: Timber stud supported 60' Scissor Truss
Slip outward 1"
and/or
Thrust outward with the frictional force
You can design a
-single top plate
-double (no splice allowed) top plate
-triple or more top plate
-steel reinforced top plate
to withstand the horizontal load equal to the friction-adjusted multiple-load-case reactions of the truss bottom chord onto the Simpson TC steel surface area, hopefully you don't find the need to require additional intermediate tension tie-walls (or cables) in the building design.
The point is; get the DeadLoad slip to occur during construction, and don't ignore the Hz deflection numbers on truss calcs.
If you're running scared at this point; switch to 3-hinged glu-lam arches or steel frames and focus your attention to the design of the tension tie in the floor slab.
The saving grace here is that truss software will flag large horizontal deflections and truss component engineers usually limit their calcs to less than 1.5" before a new re-designed shape is requested. The truss though, will perform better with pin-pin reactions making top plate design of more concern.
One more thing;
Strongbacking the trusses together near centerline helps to eliminate differential settlements between trusses.
RE: Timber stud supported 60' Scissor Truss
Still trying to understand this - Simpson TC at each end of every truss, or at alternate ends? (with "fixed" tie at other alternate ends?)
RE: Timber stud supported 60' Scissor Truss