Correct Answer - Option 4 : 5%
Explanation
As per IS 456:2000, clause 39.4,
Compression Member with Helical Reinforcement
The strength of compression members with helical reinforcement shall be taken as 1.05 times the strength of similar member with lateral ties provided the condition
\({Volume\; of\; helical\; reinforcement\over Volume\; of\; the\; core}\lt0.36\left( {\frac{{{A_g}}}{{{A_c}}} - 1} \right)\frac{{{f_{ck}}}}{{{f_y}}}\;\)should be satisfied.
where
Ag = gross area of the section
Ac = area of the core of the helically reinforced column measured to the outside diameter of the helix
fck = characteristic compressive strength of the concrete
fy = characteristic strength of the helical reinforcement but not exceeding 415 N/mm2
Hence, the load-carrying capacity of the column with helical reinforcement is increased by 5% due to increased strength as a result of better confinement.