The maximum spacing of spiral tie is ____ mm.

Explanation:

As per IS 456:2000, clause 39.4,

Compression Member with Helical Reinforcement

\(Pitch < \begin{Bmatrix} \frac{Core diameter}{6}\\75 mm\end{Bmatrix} \)

In the given question, the core diameter is not given, so we have to take 75 mm only

Important Points

The strength of compression members with helical reinforcement shall be taken as 1.05 times the strength of similar members with lateral ties provided the condition

 \({Volume\; of\; helical\; reinforcement\over Volume\; of\; the\; core}\nless 0.36\left( {\frac{{{A_g}}}{{{A_c}}} - 1} \right)\frac{{{f_{ck}}}}{{{f_y}}}\;\)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.