Thrust from Rafters on Wall
Thrust from Rafters on Wall
(OP)
I have been looking at a one story addition to a house that was built without a structural engineer and is now having problems. The addition is conventional wood light framed construction with 9' 2x6 walls supporting 2x10 rafters at a 4:12 pitch over a total building width of 28'. The rafters are collared with 2x6s about 3' above the eave height (12' above the floor) creating a tray ceiling. My analysis shows the rafters failing and this configuration has, of course, created thrust on the exterior walls which have now begun to bow under dead load alone. Normally under these circumstances I would recommend that a strucutural ridge beam be installed, however, the architect does not want to go this route because of accessibility concerns. Instead, what we are looking at doing is reinforcing the rafters. I can get the rafters to work by sistering each. My quesitons is what to do about the horizontal thrust. A finite-element analysis run pin-roller shows the frame with sisters deflecting 1" in the X-direction vs. 2" without sisters. This seems alot better but keep in mind that the present deflection has occurred under dead load only. I'm worried that if all we do is reinforce the rafters the thrust will be just as bad under full loading conditions as it presently is under dead load only. Any thoughts?






RE: Thrust from Rafters on Wall
Suggestion below is just brainstorming without knowing all the factors.....
Would it be possible to shore up the rafters, remove or loosen up the collar ties, install temporary cables with turnbuckles in there place and reduce the horizontal deflection and then refasten the collar ties?
Would this procedure overstress the rafters?
RE: Thrust from Rafters on Wall
RE: Thrust from Rafters on Wall
To describe the situation in a little more detail. At this point in time all drywall is up. The homeowner has ruled out installing any sort of horizontal tie that would be visible below the current tray ceiling line.
RE: Thrust from Rafters on Wall
RE: Thrust from Rafters on Wall
RE: Thrust from Rafters on Wall
RE: Thrust from Rafters on Wall
RE: Thrust from Rafters on Wall
The slip connection alternates with fixed connection, each truss. Each truss has a fixed connx at one end and a slip connx at other end.
RE: Thrust from Rafters on Wall
Jike, you make a good point. The ceiling is in place so I can't see what the connection is right now. However, in my model I'm getting 2k of tension force in the collar. We probably won't be able to do this type of repair because a connection to take this kind of force won't be practical to install on so many rafters.
Low Lax, the thing I don't like about scissor truss clips is that using them takes away restraint from the top of the wall. So when considering wind force on the wall, what holds the wall in place at the top, other than the dead load of the roof?
Thanks for your help guys, I'll think I'll tell the architect to quit being a baby and put in the structural ridge beam.
RE: Thrust from Rafters on Wall
RE: Thrust from Rafters on Wall
RE: Thrust from Rafters on Wall
Or how about doubling the rafters and installing a structural wall plate to span horizontally between shear walls to deal with the thrust?
VB
RE: Thrust from Rafters on Wall
I think your only reasonable solution is the ridg beam. If the architect doesn't like it let him be responsible.
RE: Thrust from Rafters on Wall
With alternating slip connections, doesn't the roof sheathing keep the trusses in line, thereby preventing the movement the slip joints are meant to provide?