Strut and Tie Question

[gman8176]

I am currently working on a bridge abutment on caissons. The caisson are 3'-6" diameter with a 3'-0" rock socket. When we had an L-pile analysis performed there was an indication of a spike in shear (600K) about midway up the caisson. When I spoke with Geotech Engineering at the DOH and asked him about the high shear he said it was caused becasue of the soil changed to rock. When we checked the shear reinforcement we could not get anything reasonable to work, we did work backwards and found that if we used a #10 spiral with a 4" pitch we would be ok. I don't think this would be very practical, so I asked the Geotech again if he had some solutions as how to deal witht he shear. He suggested using a Strut and Tie model.

So here is my question/problem. I have never done a strut and tie model, I have found several examples online and think that I could understand once I start working through it. But all of the examples I have found have be rectangular beams and my caisson is round. How do I deal with the difference in the shape. Also, does anyone know of a good example of a caisson designed with a strut and tie model.

I did ask the geotech for an example and he didn't have any handy and he is out of the office this week. I was hoping someone here might be able to help. Thanks.

 

[hokie66]

The geotech is talking out of his hat. There is no reason that a strut and tie model (or truss analogy) will give you "better" results. If the shear is this high, I suggest you look at using battered piles to cope with the lateral loads.

 

[normm]

I will agree with hokie66.

But that apart, the circular section caisson will no doubt have considerable compression which will improve shear resistance. Did you take this compression into account?

 

[vandede427]

imo, strut and tie models weren't inteded for caissons. and yes, he's out of his area talking about it in the first place.

This might seem wacky, but could you have a caisson with varying f'c along the depth?

Also, was your 600 kips factored or unfactored? If it is ultimate, than I got that 1.27 in2 @ 4" o.c. was good for at least phiVn of 600 kips with 4000 psi, but what you've actually got is 2.54 in2 @ 4" o.c. right?

I'm getting a #8 bar on a 5" pitch with 4000 psi.

 

[dcarr82775]

Strut and tie is not unreasonable, just difficult. I have used strut and tie to show beams are acceptable where they exceeded the upper bound shear limit. In a round caisson it would be a bear of a model however.

IMO, if the high shear in the pier is in the portion of rock then it doesn't matter. You can't break the pier because it is in rock. Lpile can give some funny results so you need to take a look and see if they make sense.

 

[gman8176]

I was wondering if a strut and tie model would even be applicable for a caisson. Most examples I was were for some type of beam.

vandede427:

use the 600kips is using the factored loads. I don't think that the DOH would want to vary the f'c along the depths due to increased cost and additional monitoring to make sure that the correct concrete is at the appropriate level. We are using 4500 psi concrete for our caissons.

I have not used spirals all that much is it like a stirrup and you have 2 legs the reason for the 2.54 in2 @ 4"?

Here is how I came up with the 1.27 in2:

Vr = phiVn which inturn becomes Vn = Vr/phi = 600/.9 = 667k

Vn = Vc + Vs + Vp where Vp is 0 we do not have prestressing

Vc = 0.0316(Beta)(sqrt f'c) bv dv = 0.0316*2*sqrt(4.5)*42*26.73 = 150.5k

Vs = Vn-Vc = 667 - 150.5 = 516.5k

Vs = (Av*fy*dv*(COT theta + COT alpha)sin aplha)/s

Solving for Av:

Av=(Vs*s)/(Av*fy*dv*(COT theta + COT alpha)sin aplha) =
Av=(4*516.5)/(60*26.73*(COT 45 + COT 82.41)*sin 82.41)= 1.15 in2

Did I mis-calculate something wrong somewhere?

If I can count the spiral as having 2 legs then I could get by by using a #7 at 4" centers which is not as bad still not the best but would work a little better.