Fixed Connection Strut and Tie Model (AASHTO LRFD 8)

[Bobby46]

I'm working on developing a strut and tie model for the new AASHTO requirements. I have a grade beam between two drilled shafts. The drilled shafts have a fixed connection to the tie beam. It seems like there is a good bit of flexibility in how you choose to model a problem in the strut and tie method. How would you model this fixed connection? The possibilities that I see right now are:

1. The strut and tie model would have a single reaction at each drilled shaft. Model the strut and tie truss with simple supports. Detail rebar to create a fixed connection into the grade beam.
2. The strut and tie model would have a single reaction at each drilled shaft. Model the strut and tie truss with fixed supports to simulate additional forces in struts/ties. Detail rebar to create a fixed head connection into the grade beam.
3. The strut and tie model would have 2 reactions at each drilled shaft. Reactions would be located at centroid of tension and compression stress in drilled shaft cross section. Model the strut and tie truss with simple supports. Detail rebar to create a fixed head connection into the grade beam.

I intend to design the grade beam using strut and tie. I do not think the strut and tie method is particularly well suited for design of the drilled shaft due to quantity/uncertainty of reaction locations. Is there any merit to using a more complex analysis like Option 3 if I don't plan to use strut and tie forces in the drilled shaft?

Does anyone see a better/different approach?

Attached is a sketch showing my general configuration for Option 1/2.

 

[KootK]

At first blush, I like option #1. That said, it depends on what you want the fixity in the drilled shaft for. If it's just a nominal stability thing then, sure, just detail to create fixity. If you're passing serious shear and moment from grade beam to pile, then you might need something fancier like #3 in order to capture the load effects in the grade beam properly. I agree, I wouldn't take the STM model into the piers either.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

You can always go the enveloped design route; perform a strut and tie model for both option 1 and 3. Twice as much work but you ensure that your reinforcement and compression struts are properly sized.

Personally I lean towards option 3. It seems more accurate to the fixity you want to create. But, KootK's definitely much more advanced in his knowledge of S&T than I am; I'd listen to him first.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[Bobby46]

Thanks KootK and TehMightyEngineer.

Fixity is for stability/AASHTO requirement. The geotechnical resistance of the shafts governs their size. Considering their size, loads are relatively small. Some of the larger loads these drilled shafts see are Moment=110kip-ft, Shear=20kip, Axial=110kip on a 36inch (or larger) diameter drilled shaft. Code requirements/minimum reinforcement tend to govern the structural design of the drilled shafts.

I agree that Option 3 is probably most accurate. I am hoping that this level of calculation is overkill. It looks like this would involve multiple iterations of calculations (i.e. design drilled shaft to find compression/tension centroid, run STM model for tie beam, reconsider compression/tension centroid for drilled shaft for additional loads from STM, run STM for tie beam again with revised centroids, redesign drilled shaft).

 

[KootK]

TME is a gentleman but he probably gives me too much credit.

It looks like this would involve multiple iterations of calculations

I would approach this by intentionally underestimating the flexural lever arm in the piers. Then, if you can find a solution that works given that underestimation, you really don't need to go any further or iterate unless you're looking to nickel and dime your pier design (doesn't sound like your intent).

Unless I miss my mark here, this exercise will mostly be about:

1) Getting that somewhat steep, rightmost strut and it's nodes to work. Presumably there's a bunch of gravity load there that wasn't shown in your sketch.

2) Providing enough member depth to support the detailing required to pass your tension load from the column/bent dowels over to the drilled shaft bar extensions.

In my experience, these things sometimes steer you towards not assuming that your support nodes are centered on the piers. And that, of course, introduces some additional complexity.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.