Seismic Detailing - "Inadequate Embedment into Beam-Column Joint"

[steellion]

I am using ASCE 41-06, "Seismic Rehabilitation of Existing Buildings" as a guide to upgrade an existing RC concrete building for progressive collapse resistance. Table 6-11 relates m-factors to beam properties, one of the limits is "Beams Controlled by Inadequate Embedment into Beam-Column Joint".

I have determined that existing beams are insufficient and I am designing new beams with proper detailing that will dowel into existing beams. I am counting on new beams to take ALL of the load in a life safety scenario. The question is, with the new beams being placed inside the existing beams around the perimeter, they barely pass through the existing columns in some locations and not at all in others. What, if anything, can I do to get "Adequate Embedment"? What does "adequate" even mean quantitatively?

 

[hokie66]

Add columns to match the width of the new beams?

 

[Teguci]

It means the rebar will yield away from the column-beam joint and not at the rebar development.

I'm not so sure you want to do what you are doing? What seismic zone are you in?

 

[steellion]

Hokie, I don't think that is an option for this project. The columns were generally acceptable in analysis, so it would be hard to justify losing all of that extra useable SF for that purpose. I'm just trying to determine if getting adequate embedment is possible. It may be possible to detail it in some way to create an acceptable joint at the interface.

Teguci, it's Seismic Design Category A actually. The reason for the upgrade is for AT/FP, ASCE-41 is used to determine ways to increase redundancy of the building.

 

[Teguci]

If you are still brainstorming consider:
- Adding depth to the existing beams with additional reinforcement on bottom and coring through existing columns for development
- Wrapping beams with FRP for additional shear capacity

The interior beams will add considerable eccentricity at the columns. For redundancy, the new beams will have to carry the column load plus the torsion caused by the eccentricity. Since there is already a problem getting the new beams tied to the columns, I imagine the problem will be compounded.

 

[steellion]

-Adding depth to the beams would be a great option, but unfortunately not possible because the perimeter beams are spandrel beams already at max depth to not cover the window opening.

-FRP is definitely a possibility, especially if wrapping the joints would provide the joints sufficient rigidity to get away from "inadequate development" (m=2) to beams controlled by flexure (m=4-16 for Prog. Collapse). Perhaps FRP-jacketing plus a series of post-installed seismic-rated anchors at the interface between the column and the new beam will be sufficient to "lock" them together?

 

[hokie66]

Seismic reinforcing of existing buildings usually involves some changes in the architecture. I wouldn't confine myself to working within the existing member profiles, as that won't get the job done. You can usually look at a building which has been upgraded and identify the seismic upgrade elements.