Design of anchor plate cast in pile cap.

[BigBar]

Hi, I wonder if anyone can help me with a formula for the following.

I am currently designing an anchor plate for reinforcing bars connecting a pile cap to mini piles. The mini piles will have three no. 63.5mm Dia. high strength reinforcing bars spaced in a triangular arrangement at 104mm c/c terminating within the pile cap.

The three rebar's will be spliced into a single steel anchor plate through three evenly spaced drill holes of 69mm Dia. also at 104m c/c. The bars will be fastened to the plate with a 90m Dia. nut at the bottom and top of the plate on each bar that will be threaded to take the nut. The bars are high grade 555/777 MPa. (1,758 kN Y & 2,217 kN T load per bar), total load per 3 No. bar = 5,274 kN yield & 6,651 kN tensile.

I wish to calculate the required dimensions of anchor plate to accommodate the reinforcing bars in the pile cap (Grade of plate to be determined but not required to be of high a strength as the bars themselves). I would be most grateful if anyone could recommend a formula for calculating my optimal plate dimensions.

Cheers Big Bars

 

[ishvaaag]

I would draw the yield lines joining 2 bolts, and then from the center of the pile through the 2 bolts to the end of the plate in its outer end. Exclude the cntral triangle. Then only the true available lengths (i.e. with deduction of the bolt hole) would be multiplied by the plastic moment capacity per unit length of plate. For the yield line between bolts, use the actual length, and for the inclined ones, use the projected of its actual length on the first.

Equilibrate the moment capacity of the fragment of plate between yield lines with the moment of the reaction forces of the concrete against the plate respect the line joining the center of the bolts to get a required thickness.

 

[Focht3]

Aside from tying the bars together, what is the purpose of the plate? That is very likely to affect or responses -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[BigBar]

Hi again,

The purpose of the plate is to create a cone pull out force in the concrete to prevent the plate and bars from pulling out of the pile cap.

The rebar's cannot be bent as would normally be the case due to the brittleness of the steel as it is a high strength bar. normal reinforcing will run below and above the plate in the pile cap. The plate will need to be strong enough to prevent the bars and nuts punch shearing thought the plate and to transfer the stress to the concrete to create the cone pull out effect.

cheers Big bars.

 

[PEinc]

I don't thimk you will find "the formula" for the arrangement you describe. You will have to invent your own using simplifying assumptions or use finite element analysis.

What is the design load of your minipile? Your yield for the three bars is 5274 kN (1186 kips = 592 tons). This is a very high capacity. This is greater than the yield capacity of a 20 strand tiedown anchor using 15mm (0.6&quot diameter, 1860 MPa (270 ksi) strands. Are you using 63mm diameter threadbars or are you cutting threads into 63 mm bars? If you cut threads, you need to use the reduced bar diameter and area.

What is the design requirement for the bearing plate? Are you designing the plate for the working load of the 3 bars, or the yield capacity of the 3 bars, or some % of the yield?

Based on allowable bearing on the concrete, the total plate area will be much greater than the area of the triple bar pattern. You will need very large, thick plates - probably on the order of 55 cm square by 9 cm thick. This estimate is based on a minipile working load of 0.6 x yield capacity of the 3 bars, 21 MPa concrete, 345 MPa plate steel, and allowable bearing on concrete of 0.3 x f'c x (square root of A2/A1) where (square root of A2/A1) is a maximum of 2.0.

Have you asked the bar supplier/manufacturer to size the bearing plates for you? Frequently, they will provide a bearing plate design which you can review and or submit.

Are you tension testing any minipiles? If so, how and to what load?

 

[ibrov]

I designed cooling tower structure (per UBC'97, Seizmic Zone III) and provided foundation load w/ base plate detail to the customer who will design anchor rods & foundation. I specified standard oversized 1 3/4" hole for 1 1/4" anchor rod and customer asked why 1/2" oversize - their concern is shear transfer. Please advice.

Thanks,

Ibro V.

 

[DRC1]

Standard hole for 1 3/4" is 1 7/16". Otherwise shear capacity of bolts is reduced. See AISC manual. Half inch will induce a lot of slop. Good contractor will set bolts with a steel template - or anchors can be drilled and grouted after pour.

 

[Anchorboy]

In response the initial question above regarding the anchor plate please note:

1. A triple #20 Gr. 80 anchor bar configuration is rare.
Here's what you do: 65 mm th. x 500mm x 500mm brg plate (atleast to span over a 10" diameter borehole - therefore overkill but safe)

2. Use two plates 3 ft spaced vertically to resist both t + c forces, also have a hex nut top and bot each plate

2. Add 10M spiral reinforcing at 8" diam. w/ 2" pitch around the anchor bar inside the pile cap to resist bursting forces!