Analysis of Concrete Grade Beams spanning over Drilled Piers

[Eng456123]

I am designing concrete grade beams spanning to drilled piers to support 2 floors of data hall and a roof. Our detail has longitudinal reinforcing continuous over the drilled pier with the drilled pier verticals extending into the grade beam with a 180deg hook (see picture). I am looking into whether the assumptions we are making for the design of the grade beams is accurate and complies with the code and analysis practices.

ACI 318-11 has a code provision (8.9.2 and 8.9.3) stating that for continuous construction, the span length shall be from centerline to centerline of supports, and for beams built integrally with supports shall be designed for the moments at the face of the supports. This seemed to disappear in the 318-14 code (governing code for the project). So, the options I believe I have to determine the moments and shears are:

1. Design single span member (fixed-fixed) spanning from face of grade beam to face of grade beam
- I don't like this option because it seems like it ignores the inherent continuous nature of concrete especially when there are adjacent spans of varying lengths (I have some instances with 18' spans adjacent to 24' spans), which would result in the smaller span receiving additional shear and moment from the long span, in addition to end bays where the fixed-pin nature of the end bay grade beam would increase the shear and moment in the 2nd bay.

2. Design continuous member with pins at centerlines of grade beams
- I don't like this option because it feels unconservative and is leading to much higher design moments and shears, which I don't think would be present since vertical load acting above the drilled pier would be directly transferred to the drilled pier.

3. Design continuous member with pins at edges of grade beams
- This option feels like the best of both worlds, but it begs the question of where the moment over the drilled pier goes? I believe it would transfer into the vertical drilled pier bars, but having trouble determine how to quantify that moment, so this one feels a little gray. This option also provides a conservative analysis compared to Option 4 (, which is similar to Option 2 but takes the shear and moment values at the faces of the drilled piers.

Any insight would be appreciated, thanks!

 

[Lomarandil]

I don't have my code in front of me, but I believe the provision you reference is still in -14 and -19, just moved

 

[Huevo]

My personal preference is to avoid 180° hooks on drilled shafts if possible to eliminate construction issues with tremies, beam reinforcing, etc. For that reason, it makes more sense to consider option 2. YMMV

 

[milkshakelake]

If you're using hooked bars, you can go with option 5: continuous member, partially fixed at each support, moment taken at face of support. Though now you have another issue, which is the moment in the drilled pier induced by the grade beams. It doesn't just go into the rebars; the drilled pier needs a P-M calculation now. I'd look into how to design concrete moment frames; I haven't done it for a while so I don't know off the top of my head how to do it. Also, I don't think you can really consider the bottom of drilled pier to be uniformly bearing at this point. Edit: It's definitely doable though. My previous company had a spreadsheet for moments on drilled piers. Sorry I can't be more specific about it.

What I do in practice is option 2. It's much easier because I don't have to consider the vertical concrete and reinforcement below.

 

[BridgeSmith]

I've never done a grade beam for a bridge, but we do cap on columns fairly often, which is similar. Typically, we assume a pin support at the CL of the column, but one time when I had a wide rectangular shaft supporting a cap, I modified the moment over the shaft by applying the axial shaft reaction as a uniformly distributed reaction across the width of the shaft. This reduced the magnitude of the shear at the face of the shaft (linearly) down to zero at the centerline of the shaft. This reduced the moment at the centerline of the support, to a value somewhat less than it would be for a pin support, but still higher than at the face of the shaft.

I considered that it was still a little conservative, since the negative curvature of the cap beam would produce higher pressures at the faces of the shaft than towards the middle of the shaft, so the actual shape of the shear curve would concave, rather than linear, producing a flatter moment curve.