Assn 14 - Rafter Tie Investigation

Gang, here is assn 14 - it is the investigation of the effect of the location of the tie in a `rafter' construction roof. Rafters are 2 x 6 DFL No. 1 spaced 24 in. o.c. and span 12 ft on the plan. Roof slope is 5/12. Each pair of rafters is connected by a tie ... however, the location of this tie will be allowed to vary, per our investigation. Ties are 2 x 4 DFL No.1. Yeah, they are potentially long ...

Scenarios:

1) tie connects rafters at their ends (ceiling tie)

2) tie connects rafters 1/3rd of the way up from the bottom.

3) tie connects rafters 1/3rd of the way down from the top (kind of a collar-tie scenario)

Roof load: 40 psf (snow)

Dead load: 15 psf

Roof load values are `as projected onto a horizontal plane' and may be applied to the rafters. And they may be assumed to include self weight.

The purpose of the tie is to prevent `pushout' of the ends of the rafters.

Find: for each rafter scenario:

a) maximum bending moment in rafter (lb-in., and indicate positive or negative ... positive being flexural tension on bottom),

b) pushout of right end relative to left, in in., and

c) axial force in tie, lb (indicate T or C).

Model one end as a pin and the other as a roller ... though in real life, perhaps both are `springs' as attached to the tops of walls.

For E use 1,700,000 psi x 1.03 ... (I'll explain the 1.03 in recitation).

Discuss ... especially how each scenario is effective at preventing pushout, or not.

Do in GROUPS.

Due before you leave (or I leave) for Thanksgiving Break.

Doc