Rafter Tie Required Per IBC & Satisfying Joist End Restraint ?
Rafter Tie Required Per IBC & Satisfying Joist End Restraint ?
(OP)
I have a conventionally framed roof with a collar tie in the upper third. The pitch is 9:12 and the there is no rafter tie or ceiling joists. The ridge is not sized to be structural. The rafter tie needs to be there to prevent thrust which has already pushed these walls out some.
My question is not whether a rafter tie should be there it is whether it is explicitly required by code (IBC)? I was planning to engineer an alternative to keep the ceiling height higher. Perhaps forming a scissor type truss with bracing members from the rafter at the wall end up to the collar tie or incorporating a wide top plate at the wall to give it rigidity from being thrust out from the deflection.
Another question code (IBC) requires Joists to be laterally restrained at ends by blocking or other means. My question is whether anyone could justify joists that overlap and are nailed to eachother over a support provide restraint. I have marine/residential type structure; in looking at the following manual (see link) how are the structures depicted exempt from joist end blocking?
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My question is not whether a rafter tie should be there it is whether it is explicitly required by code (IBC)? I was planning to engineer an alternative to keep the ceiling height higher. Perhaps forming a scissor type truss with bracing members from the rafter at the wall end up to the collar tie or incorporating a wide top plate at the wall to give it rigidity from being thrust out from the deflection.
Another question code (IBC) requires Joists to be laterally restrained at ends by blocking or other means. My question is whether anyone could justify joists that overlap and are nailed to eachother over a support provide restraint. I have marine/residential type structure; in looking at the following manual (see link) how are the structures depicted exempt from joist end blocking?
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RE: Rafter Tie Required Per IBC & Satisfying Joist End Restraint ?
Is this an existing structure? (per your comment that the thrust has "already pushed these walls out some".)
To change a tied rafter system into a sceissor truss system basically would require removal of the roof if that is the case.
A scissor truss could certainly be considered.
Overlapping two joists and nailing them together does not provide for lateral restraint on the the joists in my opinion.
RE: Rafter Tie Required Per IBC & Satisfying Joist End Restraint ?
RE: Rafter Tie Required Per IBC & Satisfying Joist End Restraint ?
RE: Rafter Tie Required Per IBC & Satisfying Joist End Restraint ?
I agree the scissor truss application will likely result in forces that can not easily be managed.
Table R802.5.1 is for rafter spans assuming ceiling joists are installed, see Section 802.3.1
Foot note "a" of the R802.5.1 describes adjustment factors for ceiling joist placement above the wall top plate. If you do not meet the requirements of the table a design professional should be consulted.
It is my opinion, based on your description, wall tie system must be developed, cables or threaded rod for example, or a structural ridge should be installed.
RE: Rafter Tie Required Per IBC & Satisfying Joist End Restraint ?
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RE: Rafter Tie Required Per IBC & Satisfying Joist End Restraint ?
RE: Rafter Tie Required Per IBC & Satisfying Joist End Restraint ?
1. Lower the level of the collar tie;
2. Add doubler or tripler sisters to the existing rafters of the collar tie arrangement;
3. Add the Whaler as mentioned at the top plate level of the wall;
4. Strengthen the roof diaphragm;
5. Add a ridge beam.
Your choice is a function of economics, architecture, and what can feasibly be done structurally.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Rafter Tie Required Per IBC & Satisfying Joist End Restraint ?