Braced Frame Base Plate Connection Design in SDC D

[ZeroZil]

Hi my fellow structural engineers,

I have not designed something major in California before, but now I am working on a tilt up building with a parking deck roof. As you can tell, the seismic load is a LOT because the mass is so heavy.

I am using Buckling Restrained Braced Frame which is designed by CoreBrace, but I am still responsible to design the column/base plate at the Inverted Cheveron braced frame.

we use RAM SS and RAM connection for the design - my question is should I apply "sesimic amplification load combination" with the 2.5 Omega strength factor to the base plate and anchor design?

The reason I am asking is that with the amplified seismic load combination the reaction at the base is HUGE and it doesn't make any sense to me. It is like no matter how I tried I can't make the base plate design to work.

Anyone who has seismic design experience can shed some light or point me to the right direction will be much appreciated!

 

[driftLimiter]

The relevant requirements are provided in AISC 341 Section F4. Subpart 6c.
"The required strength of brace connections in tension and compression shall be the adjusted brace strength as defined in F4.2a."

For BRBS we don't need to use overstrength because the capacity limited load is well defined.

You will still have a large load at all brace connection points, and you would do well to follow the 341 because any failure of the connection will render the braces useless.

 

[ZeroZil]

my question is what load to use at the brace connection.

Lets say a inverted cheveron brace where diagonal brace coming down to column stiffener base plate.

Lets say there is 200 kips axial force in the brace.

do I just apply an overstrength factor of 2.5 to 200kips and design the weld and anchor bolts for that?

 

[driftLimiter]

I think AISC 341 is pretty clear. ... The required strength of the brace connections ...

 

[Luceid]

If you've got the AISC seismic design manual, they also include some example calculations for BRB connections in section 5.5, specifically around page 5-515 where you can see they design the connection for the capacity of the brace, not overstrength.

Typical braced frames you are using analysis to determine demands and then factoring those demands up by overstrength for the connections to ensure nothing goes wildly wrong once things go inelastic, making sure the connection isn't the problem.

In this case - BRB's have well defined compression and tension capacities, so rather than designing for demands, you design the connection for the BRB capacity. Essentially, BRB can only take x tension and y compression, so the connection must take the forces associated with (at least) x tension and y compression coming from the brace. Designing for capacity rather than overstrength-scaled demand.

 

[Adrdm]

This might help:
Steel Tips - Seismic Behavior and Design of Base Plates in Braced Frames
Link