Proper R value for wood knee brace

[nickky]

Hi

Considering the cost of the moment frames and cantilever columns, sometimes when the subject structure is not so big I try to utilize wood knee brace for lateral loads. For example right now I have a wood deck at second floor level added to an existing building. The deck is only 8.5'x16' and let me assure you that no homeowner will pay the cost of a cantilever column and grade-beam and footing and....

Questions:

1- Do you think utilizing the wood knee braces are proper solution for small tributary areas?

2- If the answer is yes, then what would be a proper R-value per new IBC-06?

Thanks

 

[COEngineeer]

If it is a deck, it is not part of MWFRS. R is just to determine how much goes to each system. If that is all you got, does it matter what R is?

Never, but never question engineer's judgement

 

[nickky]

Does anybody has any other opinion?

Thank you

 

[civilperson]

Solve for the stress in the knee brace with horizontal loads. Keep stresses less than recommended for that species of wood. Any connection restraint, (semi-moment connection), can be the unused safety blanket. I sometimes use the adjacent house for bracing purposes by steel straps under the decking tied to the house joists/sill plates. Stairs can also brace the deck against horizontal loading.

 

[tolchijb]

ASCE 7-05 lists Timber Frames in table 12.2-1 having an R=1.5. This seems to be the closest I can find.

 

[nickky]

Thank you Tolchijb,

I saw that but the problem is that it does not define clearly what is a timber frame. I mean what kind of connections, or general requirements. Probably ASCE does not want to recognize the knee brace system as an accepted lateral resiting system.

 

[nickky]

Hi All

What I meant by "R"-value is "Response modification factor" in earthquake design. This value kind of represents the level of ductility of the lateral resisting system.

ASCE 7-05 table 12.2-1 gives a complete list of different systems with their R, omega and Cd factors. The problem is that the table does not give any value for knee braces which we use to laterally support a car port or a deck.

My question is that if anybody has any suggestion for a proper R value for wood knee brace.

Thanks for the responses

 

[civilperson]

R of 1 is conservative since it is in the denominator for calculating shear from earthquake. Larger values would have to prove energy dissipation during a seismic event using wood as structural element. The 1.5 cited by tolchijb is a good estimate and compares poorly with steel systems.

 

[Whiffle]

I agree with civilperson. Taking an R of 1 will be conservative since there are no guidlines in ASCE 7-05 for wood knee braces. ASCE 7 section 12.2.1 says other systems [not included in 12.2-1] are permitted if analytical and test data are submitted. Sounds like a lot of work to get a 1/2 higher R. Plus if it's just a deck, the tributary dead load will be relatively light, so taking all of the weight in knee braces will probably work.

 

[nickky]

Thank you Guys,

The more I think, the more I become convinced that R=1 is the answer because of it firm technical justification. I think it is a safe and defendable decision.

Therefore, Thank you for the valuable answers.

nickky

 

[youngstructural]

You came to the right decision in my opinion; As you said yourself, the R factor is a reflection of how much ductility your system will show... Ductility in timber is a matter of flexure, and given that timber braces behave in a push-pull manner, have no ductility. Yes, you could argue that there is some in the push instance, but I do not think this would be measurable.

Since your knee braces will take a great deal of load compared with the other components of your frame, it is very unlikely that any overall flexural mechanism will occur.

R=1.0 for sure.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...