Grade beam seismic hoops - overkill?

[WAstruc10]

For SDC of D, looking at ACI 318-14 section 18.13.3.4: "Grade beams and beams that are part of a mat foundation subjected to flexure from columns that are part of the seismic-force-resisting system shall be in accordance with 18.6."

My situation requires large grade beams NOT spanning between pile caps, rather they are direct bearing on soil (compression-only spring model) and sized to prevent overturning. Lateral system above grade has BRBF's, which cause large downward or upward forces on the grade beams but do not have "flexure" in the column. The commentary for this code section says "Grade beams resisting flexural stresses from column moments should have reinforcement details similar to the beams of the frame above the foundation." It makes no reference to those details for braced-frame type upward or downward loading.

First thought is that these should be designed to those 18.6 SMF provisions, because why is the flexure in the GB any different if caused by upward/downward loading vs. column moments. But for a 6' wide x 4' deep grade beam that would require 6-leg stirrups at 6" on-center. Which feels pretty crazy for a grade beam.

Any references to why this SMF requirement should or should not apply for this situation? Thanks!

 

[KootK]

...because why is the flexure in the GB any different if caused by upward/downward loading vs. column moments.

I don't have a reference but I would say that it's different because:

1) With column moments it is anticipated that the beam will dissipate the seismic energy by undergoing cyclic, plastic hinge development at the joint.

2) With the brace overturning forces, it is anticipated that the braces will dissipate the seismic energy and that the beam will either be capacity designed or designed with overstrenght to largely preclude the development of plastic hinging within the beam.

 

[HTURKAK]

ACI 318-14 section 18.13.3.3 is for grade beams which are part of the SFR frame and clause 18.6 is for Beams of SMF..
It is not reasonable to design the grade beam ( which is a part of continuous footing ) with assuming SMF beam.

You are expected to design the ftg at least for the axial strength of the steel columns to make sure that , plastic hinge will not develop at found.

Sometimes useful knowledge is free of charge... The following is excerpt from NIST GCR 15-917-34 which is one of the priceless documents..

((Column bases need to be designed for the maximum axial compressive and axial tensile loads to which the column will be subjected, including the effect of BRBs attached directly to column bases. Per AISC 341 §F4.3, these loads are determined assuming that the forces in all BRBs correspond to their adjusted brace strengths in tension and compression to ensure a complete load path from the top of the BRBF to the foundation. Included in the definition of column bases are the column-to-base welds, the base plate, and the anchor rods. Although not explicitly required by codes, the concrete foundation
receiving the anchor rods and providing bearing support to the base plate should also have a design strength greater
than the required axial strength of the column base. If the foundation were to be designed to a lower strength, although allowed by code, it would imply that the foundation would have to undergo inelastic deformations, which would not align with the AISC 341 intent that inelastic deformations occur primarily in the BRBs. If the best practice approach of designing the foundations for the strength of the column bases is adopted, reinforcing bars in those foundations need not comply with ductile detailing requirements, because inelastic deformations are not anticipated.))

I will suggest you to provide more info. ( some descriptive sketches of found. and SFRS plan ) to get better responds..