Cable Bracing for a Deck

[TonyES]

I'm engineering a deck that is designed independent of the building with cables in the "X" formation designed as tension only cables for the lateral system. The cables are attached to plates that are bolted to wood posts and wood beams. I designed the lateral system with an R of 1.5. Table 12.2-1 seems to forget about wood systems and the closest one is a timber frame of 1.5. The building official says that "timber frames" are a part of section G which is "Cantilevered Column systems" - which is not the case as the wood posts are not embedded into concrete. Most timber frames are not embedded into concrete either. He also won't allow a tension only rod bracing similar to pre engineered metal buildings because there is wood in the lateral system.

I'm stuck and he's requiring us to do testing on this system. My theory is that if the cables were replaced with 2x's than I could use it as a "Timber Frame" with an R of 1.5. Similar to almost all decks built in the world that have either knee braces or "x" bracing with 2x's. Cleary table 12.2-1 is missing many systems with wood, as if the wood guy was gone this day.

I need some kind of proof or documentation that using cables in tension only bracing is acceptable/reasonable with wood. Does anyone have anything?

 

[KootK]

My theory is that if the cables were replaced with 2x's than I could use it as a "Timber Frame" with an R of 1.5.

- It's definitely a gap in the system.

- I don't know of any silver bullet reference that would help.

- Frankly, the argument of yours quoted above is the most convincing that I've heard. If that's not working, this could really be an uphill thing.

I wish that I had more of an answer for your but, for now, you'll have to settle for my commiseration.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Deker]

I also don't know of a specific reference allowing the system. But you might try arguing that the system be classified based on the seismic fuse, which in this case would be your rod bracing. Note that the OCBF section in AISC 341 does not place any compactness limits on beams or columns. If you design the beams, columns, and connections for the expected strength of the rod bracing then you are meeting the intent of the code. So here's what you do:

1. Design your rod bracing using R=3.25 for OCBF. Keep the expected strength of the rod less than 2x the required demand.
2. Design the connections, beams, and columns for the demand amplified by Ω₀=2.
3. Now you have effectively designed your connections, beams, and columns for an R factor of 3.25/2 = 1.625, which probably gives you the same members you originally designed using R=1.5.
4. Resubmit your brand new code compliant design with no changes to the drawings except for the rods which are now half the size you originally submitted!

Of course I'm half joking on #4...plan reviewers typically don't like being shown up. But I'd be willing to bet the reviewer is more likely to accept your first design given that alternative. If that doesn't work, design the entire system to remain elastic using R=1 (or R=2/3 for MCE if the reviewer is really onerous) and throw system classification out the window. Requiring testing for a wood deck lateral system is ridiculous.

 

[haynewp]

I would try the following but none will likely be accepted for a new structure under IBC.

1. Check the IRC if there is anything similar.
2. Find if there is any research on a similar combined system with wood to show him, contact AWC.
3. Propose R=1 and also show that it would meet ASCE 41 if was an existing structure.