Rock Socket for Confinement

[Lion06]

I'd like to get some opinions on development length and confinement.

The development length for rebar (at least in my office) is not often thought about too hard. We typically take the basic equation (with the (cb+Ktr)/db factor = 1) and call it a day.

I've come across a situation where I want to use the cb+Ktr factor, but it's a bit of deviation from the typical case and I'd like to get some opinions.

cb is the clear distance (center of bar to edge of concrete or half of the nearest bar spacing). That's straightforward enough. Ktr is a a measure of confinement which helps with a splitting failure for development of bars. Ktr is a function of the transverse reinforcement, which is a measure of the confinement. As far as I can tell from ACI and from my advanced concrete class, it is the confinement that matters and the transverse steel is just the means to achieve that confinement.

Let's say you have confinement by other means......... like say the rock socket of a caisson. Can you get any more confined than dumping concrete into a hole drilled 15' into bedrock? My thought was to say that even though there is no transverse reinforcement in the rock socket that the concrete could not possibly be more confined by any level of reinforcement than it is by being surrounded by bedrock. With that thought in mind, I don't think it inappropriate to use a (cb+Ktr)/db factor equal to 2.5 to reduce the developement length of the rods into the rock socket for uplift. What is everyone else's opinion on this?

This makes a huge difference in the required development length, and consequently, the required rock socket length (on the order of 15' of rock socket per caisson). This would mean a huge savings for the job, but I don't want to do anything that is unsafe or deviates substantially from the norm.

 

[rowingengineer]

StructuralEIT,
I can see what you’re thinking, but I am a little confused, normally the bond between the concrete and the rock would size your anchor, not the development length of the bar.

However I do remember confinement of pier due to soil/rock has been studied in seismic areas to see what happens at plastic hinges. They found he confining provide by the soil improved the plastic strains. Here is a link to a similar article, don’t know if it helps, but your theory sounds ok, however is 2.5 a good number?

http://www.quakewrap.com/frp papers...ctility - The Good, the Bad, and the Ugly.pdf

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[Lion06]

rowing-

Right, the bond from concrete to rock is sizing the socket, but, we are taking anchor rods straight from base connection into the rock socket for uplift. Therefore, we need to ensure that the rock socket is, at a minimum, as deep as we need to develop the rod from the top of the rock socket down into the rock socket. Since the load doesn't start shedding out of the concrete until it hits the rock socket, we can't count on the caisson length above teh socket for development.

 

[Ron]

SEIT....I disagree that the load doesn't start shedding from the concrete until it hits the rock socket. It starts shedding as soon as shear stress in the soil above the rock is mobilized. We don't have the luxury of rock in my area, so our uplift design is entirely contingent upon the soil-pile interface. We are in a high wind area, so uplift is almost always a consideration.

 

[Lion06]

Ron-

Fair enough. I agree that the load will start to shed from the concrete into the soil, but we have no information on this load transfer. The rock sockets are sized to take all of the uplift. As a result, we have no information on how much of this load would be taken into the soil as compared to the rock. Additionally, I think the rock is so much stiffer than the soil (axially, considering both as separate spring stiffnesses) that the load will naturally migrate to the rock (again, not all of it, but most).

That would be an interesting look that I think I'll do. I'm going to set up a parametric study with a set length in soil and a set rock socket length (sized to take all of the uplift). I'll study how the loads distribute based on spring stiffness ratios of rock to soil of 2,3,4,5,10). I'll do this by hand, so it might take several hours next weekend. My advanced structural mechanics notes are in the office and I can honestly say that I don't remember the differential equations and solutions for continuously supported axial members.

Thanks for the idea, Ron! This will be very interesting and enlightening.

 

[Ron]

SEIT...well, actually there's quite a bit of information on soil-pile interaction. If you have a soil profile to work with, you can get a lot of info.

Assuming since you are designing a deep foundation system, you have a geotechnical report. If not.....not good!

 

[Lion06]

We have a geotech on board. We gave him foundation loads, and he gives us max expected displacement at top of caisson, required rock socket lengths, require caisson diameters (this is really a chart that we pick off the diameter based on caisson length and axial load), and shear and moment diagrams for the lateral caissons so that we can design the reinforcement for the caissons.

We've been in contact with the geotech almost daily as this is a very fast track project, but won't have the report in hand for a couple of weeks yet.

What are you thoughts on the confinement provided by the rock socket and the use of 2.5 for the (cb+Ktr)/db factor for development?