Challenge 10

Challenge-10

Ans of question-1

Codal provisions for pile cap of metro viaducts in detail

i)Find out Neutral axis depth (Xu)-

Total tension=0.87*fy*Ast

Total compression= 0.36*fck*xu*b

Where,

B=width of section, d=effective depth of section, Ast= area of steel reinforcement, xu=depth of N.A

C=total compression, T= total tension.

For equilibrium of force, Total tension= Total compression

0.87*fy*Ast=0.36*fck*xu*b

Xu/d=0.87*fy*Ast/0.36*fck*b*d

1. ii) Limiting depth of N.A-

Grade of steel	Fy(N/mm^2)	Xumax/d
Fe 250	250	0.53
Fe 415	415	0.48
Fe 500	500	0.46

iii) Moment of resistance-

Mu=C* lever arm=T*lever arm

For compressive zone, C= 0.36*fck*xu*b

Mu= 0.36*fck*xu*b(d-0.42xu)

For tensile zone, T= 0.87*fy*Ast

Mu= 0.87*fy*Ast(d-0.42xu)

1. iv) Percentage of steel-

The percentage of tensile reinforcement can be found by equating the tensile and compressive forces.

T=C

0.87*fy*Ast=0.36*fck*xu*b

Ast/bd=0.36*fck*xu/0.87*fy*d

Pt=0.36*fck*xu/0.87*fy*d

Ans of question-2

Design philosophies involved in the design of pile cap

Bending theory-

Pile cap can be designed by bending theory for shear and flexture effects if distances between piles is large. i,e grater than 2 times the diameter of the pile.

Bending considered when pile group have more than 4 piles are normally designed with bending theory and shear reinforcement need to be provided and more reinforcement is required.

Shear of pile cap is often required to be changed during site execution due to utilities in fringment.

This happens mostly in metro projects as they are constructed in densely populated cities with many underground utilities like telephone lines water and sewar pipelines.

Strut and tie theory-

When there is a possibility of direct flow of loads from the base of the pier to pile cap with in a 45 envelop then we used strut and tie theory normally upto 4 pile groups are designed with strut and tie theory and shear reinforcement are not to be required in that case.