Foundation Design for Fixed Base Steel Moment Frame

[smwa]

I'm currently designing a school building with Special Steel Moment Frames. The columns of MF run through the depth of a 30" deep mat slab and sits on a bearing pad below the mat slab. I used fixed condition at the base in the MF analysis to limit the drift. My question is, do I need to to design the foundation using the over strength load factor?

ASCE-07 Section 12.2.5.2 requires using the over strength load factor for cantilever columns foundation. Does the same logic apply to moment frame columns?

Thanks in advance.

 

[sandman21]

Is it a DSA project? If so the answer is yes. It can be found in chapter 18 if I remember correctly. I will pull the code section tomorrow.

 

[InDepth]

You may want to also figure out if the base of your column will yield....

 

[sandman21]

Yep I was wrong, it is CBC 1614A.1.10

 

[paddingtongreen]

In my book, there is no such thing as a "fixed base". It might be very stiff, but...

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[CTW]

I'm curious why the columns run through the 30" thick mat. Is the concrete cast against the column?

 

[smwa]

Sandman,
Thanks for the reference to CBC. I didn't catch that section. It's pretty clear what I need to consider for the foundation design.

CTW,
The columns run through the mat foundation to establish fixity of the MF base and also to allow steel erection before the mat slab is complete. The columns are erected on bearing pads (spread footing) below the mat slab. We specify that the concrete fill on the floor decks above can only be poured after the mat slab is poured and gains adequate strength. So the bearing pads are design for steel erection loads only.

Michael,
I'm curious as to your comment "there is no such thing as fixed base". I agree that nothing is infinitely rigid, but for analysis purpose, do you not agree with the fixed base moment frame analysis? I can't imagine there is much rotation within the 30" mat slab compare to the rotation of the steel column above the the foundation.

Thank you all for your comments and input.

 

[sandman21]

There are requirements for column base plates in AISC 341 Section 8.5 page 6.1-28, for axial, shear and flexure. As long as you apply overstrength you will comply with the sections.

Make sure you provide rebar welded to the column at the top and bottom of the mat slab to resolve the moment, we also provide HSA’s on the column for uplift and as overlap for thje moment.

We have not had an issue with our detail going through DSA, best of luck.

 

[ToadJones]

smwa-
I'd say that's about as close to fixed as you'll ever get in practice.
The debate of base fixity has been hashed out on this forum many many times.

 

[smwa]

Sandman,
We are using Nelson headed studs and stiffener(bearing) plates welded to the column to resist the column axial load, checked with ACI 318 Appendix D. In addition, we check for punching shear for combined axial load and fixed base moment.
As for your recommendation for welded rebars, we are using Lenton welded couplers T&B at the edge of the mat slab only. In the middle of the mat, it seems like we should be able to resolve the moment using concrete bearing against the column flanges.
In addition, is it your interpretation that non-DSA projects, can design the foundation without the over strength factor? My project is a DSA project , so I plan on following CBC 1614A.
Thanks a bunch.

 

[sandman21]

For non-DSA projects, you are not required to use overstrength for foundation connections unless required by a ref. standard. Such as the requirements in Section 8.5 of the AISC 341. If you had a shear wall building (CMU or Concrete) jamb and shear reinforcement at the base would not need over strength for non-DSA but would be required for DSA.

 

[paddingtongreen]

smwa, are you placing the 30" slab against the column to transmit the moment in horizontal bearing? Do you not expect the top edges of the concrete to spall? I'm sure they will, maximum bearing pressure (due to the moment) will be there at the top edge.

Are you using stiffeners to stop the flanges bending inward at maximum horizontal bearing?

If you include the stiffness of the concrete into the analysis you will find the moment is far lees than fixed. Before we had the fancy tools, I did one or two by moment distribution to find out for myself. I used to use 50% of fixed for the structure and 100% for the foundation design in case it was higher.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[rscassar]

I use 90% fixity for deflection considerations. I assume the specifications clearly state that it is the builders responsibility to ensure that the steelwork is stable at all times during construction.

Otherwise I think it is a good detail, definitely more work involved then typical pinned baseplate connections but the structure must remain serviceable for all loading scenarios.