Base Plate Design in Seismic Design Category D 2

[aquacata]

1. AISC Seismic Provision 2002 Section 8.5 doesn't specify the use of amplified seismic loads for base connection design. However, at the end of the commentary C8.5, it says,

" In the case of Moment Frames, if the building system performance intends column yielding at the base plate, the connection between the column and the base plate should be designed for the Amplified Seismic Load."

My building is in Seismic Design Category D, the frames are intermediate steel moment frame, and the lateral columns are fixed based. Does that mean that I have to design the frame to let the column yield at the base plate?

2. If I used the Amplified Seismic Loads for base plate design, how about the anchor rod embedded in concrete footing? We are required to use ACI 7-02 Appendix D to check the anchorage. I don't think the concrete will work for the huge break out force resulting from the amplified seismic loads. We can put a lot reinforcing bars around the anchorage, but first of all, is it necessary to use the amplified seismic loads to check this anchorage?

Thanks a lot!

 

[Taro]

If you analyzed the column bases as fixed, then you need to design and detail them to achieve the fixity. The amplified forces would need to be used for every component including anchor rods, etc. As you noted, this can be difficult to achieve, especially with a shallow foundation system. But that is the cost that you pay in order to save money in the superstructure design by fixing the bases.

 

[prsconsultant]

Below is an excerpt from AISC, which answers your question.

Shall I use the load combinations (shown below) with amplified earthquake load (Eqs. 4-1 and 4-2 of the 1997 AISC Seismic Provisions for Structural Steel Buildings) for the design of column base plate and anchor rods?
1.2D + 0.5L + 0.2S + Omega0QE
0.9D - Omega0QE

The answer to the question depends on the expected system performance of the structure. In general the System Overstrength Factor, Omega0, is prescribed to assure sufficient design strength to allow selected members to yield. When a Building Code requires design of a connection in accordance with Special Load Combinations that include the System Overstrength Factor, the intent is to assure that the connection is strong enough and stiff enough to allow yielding of the member.

In the case of moment frames, if the building system performance intends column yielding at the base plate, the connection between the column and the base plate should be designed for the System Overstrength Factor.

In the case of braced frames, if the building system performance intends brace yielding at the base plate, the connection between the brace and the base plate should be designed for the System Overstrength Factor.

In all cases, the design of the anchor rods should consider ductile behavior, without using the System Overstrength Factor. In other words, the system performance of all frames will be enhanced by designing anchor rods to yield before they “pull out” of the concrete. Designing anchor rods to yield requires sufficient concrete embedment to preclude concrete “shear cone” failure. The top half of the anchor rod should also have a bond breaker to increase the length of rod that will strain.