Circular tie details for drilled piers 5

[khkw]

I have two concerns about the circular ties (#6) for my drilled piers (for river embankment stabilization, under mostly large bending and shear, minimum axial, 36"dia, 24#11 with #6 ties @ 3"-4" spa)

1. The lap length - is 48 db sufficient (36" as lap length)?
2. the lap details - should the lap steel be overlaping with the circular tie or turning into the interior of the cross section(seems to be preferred by FHWA)? If the lap is 48db (36") there is barely enough room for the lap. What about the constructibility?

Thanks for the help!

 

[mudflaps]

Use spiral ties.

Ols CA SE

 

[JedClampett]

Is there a reason you need #6 ties? Usually #3 or #4 are used.
Also, if the member is taking mostly bending and shear, it's classified as a tension controlled member and the requirements for column ties don't apply. However, the requirements for shear stirrups take over.

 

[khkw]

Shear (500 kips factored)is too high for spirals - size of spirals are limited to #5 unless you go with special heavy equipment. I can not size up the shaft either. It seems the only option is #6 ties.

 

[JedClampett]

If the direction of the shear can be predicted, can you use smaller ties and add #6 or #7's spanning across the interior of the piers (parallel to the load direction)? They'll be tough to install, but there should be room to get the mud in.

 

[JAE]

Also increase f'c?

 

[khkw]

fc'is 4000psi. Anything above that will have special requirement by ACI318.

These shafts are one of the many sections of shafts along the river emankment. The clients wanted use same diameter shafts (36") and similar details....same strength of concrete...therefore to cut their cost. They claimed that #6 ties have been used in another section. Although #6 circular ties may be out of the ordinary but will it be a solution? I am mostly interested in the details of it......

Thanks!

khkw

 

[JAE]

I'd use #4 circular ties but then add rectangular #5 or #6 per JedClampett's suggestion above if the load is of a known direction. You could create a rectangular rebar cage within the circular cage to fit together - and perhaps do this only in the top portions of the pier.

Getting the concrete down the shaft might be an issue (as Jed suggested above). One thought would be to fill the pier up to a point and then drop in an internal rectangular cage, tie it off to the cirular cage and then continue with the concrete placement.

But tying it all together at the start is preferred.

 

[hokie66]

I would stick with the circular #6 ties if you can't get #6 made into a spiral. But as for the laps, that is not the way to go. You should be able to bend the ends of the ties 135 degrees around alternate bars; that is, one vertical bar space between the bends. Just check with a reinforcing supplier in your area about bending these ties, but I have seen it done where I am.

 

[khkw]

Thanks for all your responses! JedClampett and JAE both have very good points. But I think introducing #6 or #7's spanning across the interior of the piers might jack up the cost more than they wanted. Hokie66's idea is refreshing but how do you achieve the splice length (36") with 135 degree between bends. Also by doing this it will create honey cones in the space between the splice and the ties - an issue for concrete to get thru......

 

[hokie66]

Perhaps my post was confusing. The ties are made continuous by 135 degree hooks at each end rather than by splice length. These hooks are not around the same vertical bar, but rather around alternate bars, which allows space for the vertical bars and avoids the type of congestion that may cause honeycombing. Also, the location of the tie terminations should be varied around the perimeter of the pier.

 

[khkw]

Thanks! It was my confusion. I had that splice length on my mind!
What is the standard hook length for this 135 degree bend between two legs of the tie? My vertical bar spacing is 3.5"......

 

[Teguci]

With your high moment and shear and minimal axial load, it might be worth looking at a PT system. Of course this would be a novel approach but the principals should be the same. With a high eccentric compressive load on the pier, the shear capacity can be increased and you can reduce some of your vertical steel.

Otherwise, I would defer to CRSI and use the 48 bar diameter lap length. I suspect some of the (24)-#11 get cut off about 15 feet down along with most of your shear reinforcement?

 

[hokie66]

khkw,

It takes about 9" or 10" for a standard hook of a #6 bar. Again, check with a reinforcing supplier. With a 3.5" bar spacing, you may want to make the hooks overlap 4 spaces or so.

 

[khkw]

Thanks!

Hokie66,
Out of the 9" to 10" hook length half is bent and half is straight in the section. It may not be as bad as it sounds. I will check details of ties with the reinforcing supply.

Teguci,

As far as the location of the max moment and shear -according to the geotech analysis, since the shale layer is down 30' in depth and the layers above is not good stuff, the max shear happens at the sliding surface (30' in depth). And max moment occurs at the same location. Geotech engr used slope stability analysis and L-Pile to compute the loading (M and V) in the shaft - lots of assumptions and uncertainties incolved. I don't plan on cutting the vertical bars. As for the ties I will beefup the middle section with 3" spacing and cut down to 6" tie spacing at top and bottom 10' sections (total shaft length 45'. This might be an overkill but it covers the uncertainties.

 

[JedClampett]

Even though the hooks should fully develop the bar, I would ask that the installer stagger the location of the lap or hook, such that they're not always in the same spot.

 

[dik]

If the shear is high enough, can you weld up a 'rebar truss' to be placed in the concrete caisson?

Dik

 

[miecz]

With tie spacings of 3 to 4 inches, things will be tight at the splice locations. Could you use a second inner cage with a diameter 10" smaller than the first?

 

[khkw]

On the plans I have given the options of splicing or hooks for the ties. I will be checking with manufactures and figure out what is the most constructible and go from there. I agree that #4 or #5 spirals and inner layer or lateral spannings will be the next thing to do if manufactures do not aggree with the ties and the crowded rebars bars around the perimeter.

Thanks so much for all the great input!