Braced Frame - connection at baseplate 2

[prsconsultant]

This question relates to braced frame connections at the base plate / foundation level.

For OCBF or SCBF meeting the requirements of IBC2000 / AISC 97 seismic detailing requirements, brace connections are designed per AISC section 13.3 or 14.2 part I, to develop the expected tensile strength of the brace member.

The 1997 Seismic Provisions does not address load transfer to the foundation. The 2002 Seismic Provisions states that "the seismic loads to be transferred to the foundation soil interface shall be as required by the Applicable Model Building Code."

If I design the anchor bolts using the load combinations of IBC 2000 section 1605, will it not become the weakest connection in the entire structure?

What is the philosophy behind forcing the brace connection in the super structure never to fail but no such requirements at the foundation level.

Please let me know if I am interpreting this provision correctly.

Thanks in advance for your help.

 

[Taro]

There's no clear answer to this question in the AISC Seismic Provisions.

Some engineers design the anchor bolts for the capacity of the braces because they don't want this connection to be the weak link.

Some engineers use the "maximum force, indicated by analysis that can be transferred to the brace by the system" to limit the demand. For example if the brace capacity creates a 100 kip tension demand but the footing (that has been properly sized for overturning) only has 20 kips of uplift capacity, then there is no way for 100 kips of tension to get into the anchor bolts.

I usually try to design for the brace capacity, but sometimes invoke the "maximum force" exception if the anchor bolts start getting ridiculous. Note that if the braced frame sits on a concrete basement wall or a pile foundation, then you probably will develop big tension forces and the anchor bolts should be designed accordingly.

 

[dbuzz]

Some of the Steel Tips papers, at

http://www.steeltips.org,

have good information re seismic design of braced frames. You may find a suitable design philosophy amongst them.

 

[prsconsultant]

Thanks to Taro & dbuzz for your reply. I understand the logic behind forcing the brace connections in the superstrucure to develop the expected tensile strength of brace. However, the connection to the foundation is where I have questions.

To be specific I have braced frames with siginificant uplift supported on shallow foundation. We have a mechanism of transferring the uplift forces from the footing to the soil below. But the question is what forces to design the anchor bolts which connects brace/column assembly to the footing? Is this based on just the code level forces or code level forces multiplied by the overstrength factor or the component of the brace force based on its yield capacity? This project falls under seismic design category "D".

The other question is if all anchor bolts (with welded plate at the end) are embedded sufficiently to fully develop, is it ok to use 0.9Fy Ab as the tensile strength of bolt or still base it on the pull out capacity of concrete? Typically the anchor bolts are encased in piers with transverse & longitudinal reinforcement.

Thanks again for your time.

 

[whymrg]

In AISC "Modern Steel COnstruction" June 2002 Steel Interchange (p. 9) there was a question about using of amplified seismic loads for design of anchor rods.

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

Good luck.

 

[whymrg]

Also, AISC Design Guide # 7 (p. 36) suggests to use 1.25*Fy*At for design of development for anchor rods (to insure a ductile failure in case of overload). Transferring of load from anchor rod to rinforcement is discussed as well.

Good luck.