Challenge 9

1. The Dead Load Calculations for Pier and Pier Cap are as mentioned below:

Pier:

Diameter of Circular Pier= 2m

Area of Cross-section of Pier= pi * D^2/4 = 3.14 sq.m

Length of Pier = 30m

Volume of Pier = Area x Length = 3.14 x 30 = 94.2 cum

Dead Load of Pier = Volume x Density = 94.2 x 25 = 2355 kN

Dead Load of Pier Cap = Volume x Density = 35 x 25 =875 kN

2. The pier are provided with a sacrificial element called pier protection in order to avoid the damages to pier from a vehicle collision. This is similar to a crash barrier. The pier proetection is supported by Pedestal or Pile cap. There is a gap between the pier and pier protection which is more than the predicted deformation of pier proetection under collision load. This gap is generally ensure during pier protection casting by using soft material like thermacol. The pier protection is designed for vehicle collison loads as per IRC 6.

3. Axle Loads of a metro rolling stock are the loads that are considered for design of metro structure. These loads are placed at certain intervals as per the Metro Train Configuration. These loads are 12 T to 14 T for light metro loading, 15 T to 16 T for medium metro loading and 17 T to 18 T for heavy metro loadings. A typical metro car is generally defined with 4 Axles with spacing between axle and spacing between 2 cars defined. Based on the spacing between 2 cars maximum 6 successive cars shall be considered for design of metro structure.

4. Pile caps are the structural elements which transfer the load from pier to piles. These are heavy and robust elements which transfer the moments in pier as axial forces (Push & Pull Effect) in the piles.

Pile caps have following 3 significant functions:

a. It acts as a structural element to transfer the vertical force horizontal force and moments from pier to piles

b. It also accomodate the construction tolerances in the piles which occur due to tilt and shift of piles during construction.

c. It also acts as a supporting for pier shuttering and pier construction activities

Additionally, piles caps also provide stability to the pile group system even after scouring of bed material surrounding top piles. It also reduces the chances of differential settlements.

5. Advantages of Monopiles:

a. They are fast in construction as we need to construct only one pile at one pier location

b. They are generally used in 25m to 30m depth and in sea conditions where time of construction in challenging weather conditions is very less as compared to group piles

c. The monopile substructre are asthetically more pleasing due to continuity in structre and absense of pile cap

Disadvantages of Monopiles:

a. They are generally not efficient structural element

b. They require more material as compared to pile group system

c. It is difficulty to accomodate the construction errors due to tilt and shift in monopiles due to absense of pile caps

d. They require high precision equipments and highly skilled manpower for construction to minimise tilt and shift

6. Following are the assumptions made in pile and pile cap design of metro viaduct:

a. All the horizontal forces from the pier are assumed to be resisted by piles only. The passive soil resistance form pile cap is ignored 

b. The pile cap is assumed as a rigid structure for calculation of vertical forces in the pile due to moments from pier.

c. The bending of pile caps is assumed to be predominant in longitudinal and transverse direction and the design checks in these directions are carried out.

d. Piles are assumed to be fixed at top for bending moment calculations in piles

e. The vertical load from pier is assumed to be equally distributed in all the piles

f. The horizontal force for pier are assumed to be equally distributed in all the piles of the group

7. Pile cap analysis based on bending theory is as per following procedure:

a. The pile cap is assumed as rigid element

b. The vertical loads from pier are equally distributed in all the piles

c. The moment in the pier are transfered to piles of the group as axial forces. This is calculated using rivet theory which is based on the distance of the supporting elements(piles) from thier c.g. i.e. c.g. of piles group

d. The final forces in all the piles ( direct axial due to vertical loads + axial due to bending moments) are calculated.

e. These forces in the piles are then considered as external loads acting on pile cap which is supported by pier

f. The bending moment at critical sections at the face of the pier in longitudinal and transverse direction are calculated.