Rafter Tie Required Per IBC & Satisfying Joist End Restraint ?

[jgeng]

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?

http://newstore.southernpine.com/cgi-bin/newsopine/product?;26;

 

[JAE]

A rafter tie is not explicitly required by the IBC. Keeping the structure from falling down is required, however.

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.

 

[jgeng]

Thanks for your response JAE. Yes this is an existing structure, why do you say I would need to take the roof off? I was thinking adding bottom chord members to the existing rafters and colar ties to form a scissor type truss. I am trying to minimize modifications to the original structure and ensure it meets minimum code requirements.

 

[MiketheEngineer]

You can look at a scissors truss - but I am afraid the forces you will see will make it very difficult to fabricate with nails and/or bolts.

 

[cdhse]

A topic of many a conversation!!

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.

 

[BAretired]

The answer is there are probably several ways to remedy the problem. Provide a sketch of what you are proposing. In doing so, you may answer some of your own questions.

BA

 

[tolchijb]

I have put in 2x10 whalers at the wall's top plate before to resist the outward thrust, but you have to have some perpendicular walls somewhere for the force in the whalers to get down to the foundation.

 

[msquared48]

There are five basic ways to limit lateral spreading here:

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

 

[JAE]

jgeng - I agree with what MiketheEngineer said. Getting nails/screws/bolts to work and create a competent scissor truss in place, with existing framing, would be very difficult and the cost would extend past what would be required to simply take off the roof and lift in new scissor trusses.