SCBF - Connection design - To Omega or not to Omega

[DEL2000]

In the 1997 UBC, Special Concentric Braced Frame connections were required to be designed for the lessor of three cases. 1) Tension Strength of brace; 2) Omega times the calculated earthquake force plus all applicable gravity loads; 3) Max force able to be transferred to the system.

Well, now we are in the 2003 IBC, and it references directly the AISC manual (I forget the number, the 1997 version used to be known as the Yellow book, the latest version is blue). In this manual, they give you only items 1 and 3 as listed above; as I read it, they don't allow you to base the connection on Omega times the actual load to the brace.

I have a two story building (architectural art project, really) shaped in an L-configuration with several holes in the floor diaphragm and a mechanical well in the flexible roof diaphragm. Long story short, I have a lot of frames to account for the shape of building and diaphragm, and the braces are not loaded very highly. In my case, designing for the tension capacity of the brace would be twice as big as what the forces in the braces actually are.

1st question: Am I reading the AISC correctly, do you have to do the connection for the tension strength of the brace.

2nd question: Does it seem right that I should have forces so much smaller than the tension strength of the brace? Should I try to downsize my braces to lessen my connection requirements?

Thanks for any help you can provide.

 

[ajh1]

Question 1: Yes, you are reading AISC correctly. Unless you can show reduced loads due to hinges forming elsewhere, etc., then the connection has to be for the tensile capacity of the brace (A BIG BITE!).

Question 2: No, it doesn't seem right, but that is the nature of the beast. The intent is to insure that the member would fail before the connection.
Yes, you should downsize your brace as much as you are comfortable with, since the stress in the brace is irrelevant to the connection design in this case. If your brace is sized for a stress ratio of 0.45 rather than 0.9, it is likely your connection will be at least twice as large.

If you are not in seismic category D, E, or F; consider using R=3 and running this as a "system not specifically detailed for seismic", then you get to design the connections for whatever the loads are.

Good Luck!

 

[DEL2000]

Thanks a lot. That was just what I was looking for.

I'll have to take a look at my braces again to see if I can downsize any of them. I am in seismic category D and the building I am doing requires these special detailing requirements.