Setting long reinforcing cage for drilled shaft 2

[PEinc]

Can anyone guide me to a source of information or make recommendations on the handling of long, heavy, double, reinforcing cages for drilled shafts? The cages are about 130 feet long and weigh about 700 pounds per foot. The cages have double circular rings of vertical #18 bars with #7 spirals with 3 inch pitch. The inner cage is 40 inch diameter. The outer cage is 60 inch diameter. In addition, the cages will have instrumentation equipment attached inside. The drilled shafts are 72 inch diameter with 74 inch outside diameter steel casings. The drilled shafts are being installed out in the river using barge mounted equipment.

 

[Focht3]

Wow! Now that's a real rebar cage! The cage length is a bit longer than "typical", but No. 18 bars for the vertical bars, with two cages. Wow. Where is this to be installed?

I'd call and talk to Bill Eisenhower at Ensoft to see if he is aware of anyone that has done this "recently." Or maybe the guys at Fugro's office in Houston; also STS in Chicago - try to get Clyde Baker. He's semi-retired, but I think you can find him through STS.

I've done a 40 foot pier 96 inches in diameter with a single cage of No. 18's and 2.25 inch anchor bolts. It was heavy but short. It still took two cranes and multipoint pickup. The cage used hoops, not spirals, and had lots of wire ties. I'd be worried about the vertical bars shifting "up/down" in the cage because of the spiral. This can happen with hoops, too, but it seems to occur less frequently.

You do have one thing going for you: the double cage. It should provide extra flexural strength during pickup by reducing the likelihood the cage will "egg", collapse and damage the bars. The cross bracing is pretty damn important.

Have you thought about a structural analysis of the pickup?

 

[PEinc]

The project is a bridge in NJ. Another engineer is "picking my brain" for ideas. I'm sure someone must have done something similar. I have an idea or two but would hate to "reinvent the wheel" if I don't have to. It's a fairly well known project around here so I don't think the drilled shaft/caisson contractors will be too eager to help the GC who apparently intends to self-perform the work.

 

[krd]

PEinc - what's the problem? The cage should be prefabricated with adequate internal bracing and picked and set into the shaft with cranes of sufficient capacity. Long rebar cages often require a 2 crane pick, more because they're too flimsy to be stood up vertical by a single crane than because they're too heavy. The internal bracing should be made out of rebar welded into cross frames and tied inside the cage at some appropriate spacing.

 

[Focht3]

The cross bracing could be tricky. Because of the double cage, maintaining a clear center space is probably important. The tremie pipe or pump hose can't get snagged in this one! And the concrete will be placed under slurry or water, so visibility - in case of any problems - will be difficult or impossible. And the ID of the inner cage is too small to send a diver down to remove the bracing after the steel is placed.

It looks to me that the bracing will need to be placed between the two cages...

A wild idea: why not use a 36 inch pipe as an "internal mandrel" to help the cage hold its' shape? No bracing inside the inner cage this way. But it will add to the overall pickup weight...

 

[PEinc]

"What's the problem?" you say. Focht3 understands the problem very well. The cage is very long, very heavy, very tight, and is held together with tie wire. Some cages also are full of O-Cell instrumentation and pressure lines.

I agree that multiple cranes and pick points will be needed. I was hoping someone could recommend suitable tilt-up rigging equipment and a supplier.

 

[BigHarvey]

I think it would be wiser to have the reinforcement cage built in two sections and splice them over the pile. This implies very strict construction procedures because the bars will have to fit perfectly during the splicing. Given the size of the bars they won't move if they are not at the right place : gauges ( or combs as we call them in France ) should be used to construct the cages.
You can't rely on ties for handling cages horizontal an vertical reinforcement will have to be welded together at least on a few rows for the bottom and upper parts of each section and the splicing should also be welded.
Hanging bars and positioning bars will have also to be designed and welded carefully. Bracing will be necesary to go from horizontal to vertical position and cut with a torch when the cage is lowered in the pile. You will need at least 350 to 400mm free space to lower the tremie pipe.
It seems very unwise to let this operation carried out by a GC alone if he has not got its own experience of this kind of operation which is not common for a specialized sub-contractor.
Good luck

 

[PEinc]

BigHarvey, I agree with you about the GC's experience and specialized sub-contractors. However, nobody put me in charge or asked my opinion! The GC did hire an experienced caisson superintendent last year, for what that's worth.

 

[krd]

PEinc, where I work we never rely on tie wire to hold shaft rebar cages together, we always use welded interior bracing designed to provide plenty of clearance for the concrete pump line and the bracing is never removed. CSL tubes are tied to the outer mat. If there is other instrumentation it must be kept clear of the shaft center. A double mat of rebar is out of the ordinary but 3 foot clear in the center is not bad. It certainly doesn't preclude properly engineered bracing. The bracing X'es can be made offcenter if need be to get the max clearance down the center of the shaft(caisson).

As far as cranes go, the maximum pick may be the rebar cage or it could be the weight of a vibro hammer and temporary casing being pulled during the shaft pour. I don't know what you have. In any case the shaft service crane is always sized to do the job at hand. A 240 ton barged mounted crawler will pick 120,000 lb at 40 ft radius with 160 ft boom and 2 degree list. A 240 ton barge mounted ringer will pick 120,000 lb at 190 ft radius with 260 ft boom and 2 degree list. Use the proper tool for the job.

I'll not get into rigging since anyone qualified to perform this work will know how to do their own rigging.

 

[AZdinak]

You are indeed speaking of a large cage. Based on our recent experiences with some concentric cages up to 84" in diameter and 115 feet long, I have two comments. 1) More than 5 pick points and a good crane operator(s) are a must. 2) using an internal "mandrel" to help the pick is not a popular option. We identified that as one option and the contractor has refused to comply. Diagonal stiffners work well in addition to spiral stiffners on the outside of the cage. Be careful, we have watched a few hit the ground when some of the spiral steel slipped which was the tie-off point.

Make sure inspectors are watching the cages as they go in. If there is any rupture of the tiewire, have them redo it. If any of the CSL tubes rupture, have them reinstall them. After a few of these problems, the contractor becomes much more willing to take better care of the cage when suspending it.

Also, multi-section cages can work well, but you have to make sure you have the alignment issues undercontrol. Typically, we have used a short lower section (at that time the one loaded with 2 levels of O-cells), so that the splice is really not within an area of "high moment or shear" (I expect that 100 feet down, most of this has been disipated). Let's not use my last comment as a "theoretical discussion maker."

Be ready for some flexure and quantify what is acceptable in the specs. A well written spec makes it much easier to deal with the problems when they arise.

Zdinak

 

[Focht3]

AZdinak:

You said, Let's not use my last comment as a "theoretical discussion maker." Aw, you're no fun!



Actually, I agree with your comment - that's about 14 to 15 diameters below the top of the pier, so it should be well below any areas of "high moment or shear." The only exception might be if the pier is socketed into rock at about that depth.

I'm not surprised your contractor balked at using an "internal mandrel." I made that suggestion with some tepidation; I'm still waiting to see a contractor use one.

 

[caissondriller]

In that area Cook Drilling Corp installed 4 130' cages in 6' diameter holes, loaded with csl and o cell equipment. I believe they made a custom jig out of smaller pipe, and had two cranes involved

 

[PEinc]

Would that be Cook Drilling from Trevose, PA?

 

[caissondriller]

Yes. I spoke to the forman from a project completed in phil Pa. The cages were spliced. The only problem in picking the cage were the csl tubes. The first pick the cage bent enough to bend the csl tubes within the cage. The picks were adjusted and no other problems encountered.

 

[PEinc]

When you say "spliced' were the picks made in lengths less than 130 feet and then spliced to 130 feet or were the cages spliced to 130 feet and then the entire 130 feet length picked?

 

[caissondriller]

The first cage set was one piece 130+-', due to O-cell set up. All others were spliced, I don't know lengths but I believe to be close to your scope.

 

[Zambo]

If an internal mandrel was used how would concreting proceed as the tremie is raised. I feel with the quantity of concrete involved this would cause difficulties.

Zambo

 

[Focht3]

The (internal pipe) mandrel is only necessary to pick up the cage. It can be removed once the cage is vertical, although this is usually impractical. It is typically removed after the steel is placed into the excavation, and before the concrete is placed. Although, come to think f it, the mandrel could double as a tremie pipe...

 

[PEinc]

It seems to me that you guys could be wasting your time talking about an inner mandrel as a lifting/stiffening member. With 130 LF double cages weighing 700 PLF, it would be a monsterous pipe even with a multi-point pick. With a double rebar cage, rebar supports, O-Cell, and instrumentation, there's probably no room for a mandrel that would be strong enough to support the cages.