Pile foundation. Types of piles. Classifications of piles by material. Functions of pile cap Load calculation(both Dead load and live load). Impact factor for live load. Moment calculation. Shear force calculation. Design of pile (for piles acting individually and group). Reinforcement details of a pile. Efficiency of pile group. Design of pile cap. Reinforcement details of pile cap. Conclusion Reference
1. Pile foundation.
Pile foundations are the part of a structure used to carry and transfer the load of the structure to the bearing ground, located at certain depth below ground surface.
The main components of the foundation are the pile cap and t he piles.
2. Types of Piles Types of piles based on the mechanism of load transfer. There are two types of piles. They are,
3. Classification of piles by material Timber piles, Concrete piles, Steel piles.
In this project, cast in- situ concrete piles are used.
4.Functions of piles
To transmit a foundation load to a firm ground
To resist vertical, lateral and uplift load
Piles can also be used in normal ground conditions to resist horizontal loads. Piles are a convenient method of foundation for works over water, such as jetties or bridge piers.
5. Load calculation (Dead load) Load from Box girder Total area of box girder = 7.4 m 2 Self weight of the box girder Self weight of the box girder is 18,500 kN. Load from wearing course
Load from wearing course is 1237.5 kN Load from pier Average solid rectangular area = 640.000m3
Combined semi-circular area (cut water) = 251.327 m3 Total volume = 891.327 m 3 Self weight of pier is 22283.175 kN
Total dead load = Self weight of box girder + load due to weathering coarse + Self weight of pier. Total dead load is 42021 kN.
Load calculation (Live Load – Tracked vehicle) Max Live load = (70 x 9.81) = 686.7 KN
6. Impact factor for live load From graph, curve for reinforced concrete bridges denotes 8.8 percent for spans more than 45m. Impact factor of live load for bridge = 60.43 kN
Total load on pile cap Total load on pile cap
7. Moment calculation Max. B.M. Occurs when the load is at mid span. B.M. at mid span = 1510.75 kNm. Moment on pier due to water current in lateral direction
Projected Area of pier = 70.55 m2 k = 35, co-efficient of pier with semi circular cut water. F = kAV2 = 96.89 kN Mean velocity at distance = 5.67 m
Moment = Force x Distance Moment on pier due to water current = 550.00 kNm.
8. Shear force due to Impact factor of live load Maximum shear force is occurs in the supports of the girder. Max. Shear force at Supports = 30.215 KN.
of a pile
Cohesive factor, c = 50 kN/m2 Adopted length of pile = 30m. Diameter of each pile is (h p) 1.5m. Spacing between two piles S = 4.5m
To find the number of piles: Qug = n c Л d L
108402.75 = n (50 x Л x 1.5 x 23) n = 15 For piles acting individually Qug = n x Qup Qug = 15 x (c x Ap)
Where, c = 50
A = ЛDL = 141.37 m2 Qug = 106028.75 kN
Check for group action For group action of piles, Qug = 4 BpcLpc + Apc c Nc For clay, Nc = 9 Apc = Lpc x Bpc = 287.5 m2
Qug = 198375 kN Ultimate load bearing capacity of group of piles, Qug = 198375 kN Hence the design of pile foundation for the bridge is safe.
10. Reinforcement details of a pile Diameter of pile = 1500mm Pitch
Clear cover to longitudinal bars = 75mm (min. cover = 40mm) Area of longitudinal steel, Asc = 10799.22 mm2 Diameter of core, Dk = 1318 mm Area of core, Ak = 1364334 mm2
Dia. of column corresponding to centre of helical bars, d h dh = Dk – Dia. of helical rod dh = 1302 mm Gross area of pile, Ag= 1767145.868 mm 2
Area of Concrete, Ac = 1756346.648 mm2 Ultimate load carrying capacity for the pile, P u Pu = 1.05(0.4 f ck Ac + 0.67 f y Asc) = 17906.2 kN Load for each pile = 7166.261 kN
7166.261 kN < 17906.2 kN Therefore a pile can carry the ultimate load from above structures. Consider one pitch length of the column P = 50 mm
Length of helix per pitch length = √ (Л x dh) 2 + P2 = 4090.659 mm Volume of helix per pitch length = 822475.837 mm3 Volume of core per pitch length = Ak x P = 68216700 mm3
Ratio of volume of helical steel to volume of core = Volume of helix per pitch length / Volume of core per pitch length = 0.0121
This value should be ≥ 0.36 [(Ag / Ak) – 1](f ck/f y) ≥ 0.005 0.0121 ≥ 0.005 Hence the provision of helical reinforcement is satisfactory.
Spacing of longitudinal bars Diameter of core, Dk = 1318 mm
Л x Dk = 4140.619 Spacing of longitudinal 32 mm dia. rods = 197.17 Therefore provide 195 mm spacing between each longitudinal bars.
11. Efficiency of pile group By using Converse Labarre formulae, we can find the efficiency of pile group
12. Design of pile cap Choosing depth of the pile cap: hpc = 1/3 (8hp + 600) mm for hp ≥550 mm hpc = 4200 mm Therefore depth or thickness of the pile cap is 4.2m Self weight of pile cap = 30187.500 kN Total load including pile cap = 72268.5 kN Factored load = 108402.75 kN
13. Reinforcement details of pile cap To find area of steel by Truss action, H = PL/4d = 29036.45 kN. Area of steel required, As = (PL) / 4(0.87 f y) d
As = 80422 mm2 Minimum reinforcement, A st = 0.12 % x b x d = 5040 mm2 Provide 32 mm diameter bars Spacing = ast / Ast x 1000 = 159 mm Ast (Provided) =196237mm2 Ast (Provided) > Ast (Required) Due to the larger depth of the pile cap, provide 2 legged stirrups. Hence the provided reinforcement for pile cap is safe.
CONCLUSION This project has given opportunity to understand the basic principles of pile foundation design. In the project pile and pile cap have been designed with reinforcement details During the course of the project we have gained knowledge about designing of pile cap and loading criteria. The pile foundation is designed as per IS 2911- 1979, Part 1 – concrete piles, section 2 – Bored cast in situ piles. We have learnt the application of AUTOCAD during the course of this project.
References Books referred Bridge Engineering by S. Ponnusamy. Soil mechanics and Foundations by B.C. Punmia. Limit State Design of Reinforced Concrete by P.C.Varghese. Design of reinforced concrete structures by S. Ramamrutham. Design of Pile Foundation by Satyendra mittal.
Indian Standards referred IS 456:2000 - code of practice for reinforced concrete. IS 2911- 1979, Part 1 – concrete piles / section 2 – Bored cast-in-situ piles. Indian Road Congress 6 -2000. Tamil Nadu Highways Manual, Standard Specifications for Bridges.