In reinforcement detailing, the straight extension beyond the end of a standard hook (i.e., the length of the straight portion past the bend) shall be at least how many times the bar diameter?

Introduction / Context:
Hooks and bends provide mechanical anchorage for tension bars. However, a hook is not effective unless it is followed by a minimum straight length that develops the steel and ensures proper force transfer to the concrete.

Given Data / Assumptions:

• Standard hook at the end of a tension bar.
• Ordinary R.C.C. detailing for anchorage.

Concept / Approach:
Detailing rules prescribe a minimum straight length beyond the bend (tail). A widely practiced minimum is 4d (four times bar diameter), which, along with the bend, provides adequate development against pullout and splitting forces.

Step-by-Step Solution:
Recognize the requirement for a minimum tail beyond the hook.Adopt the commonly specified value of 4d for standard hooks.Select the correct multiple from the options.

Verification / Alternative check:
Bar bending schedules and detailing handbooks list the tail beyond 90°/135° hooks as not less than 4d (subject to code-specific refinements for seismic detailing).

Why Other Options Are Wrong:

• 2d or 3d: Often insufficient to mobilize full anchorage capacity.
• 5d or 7d: Conservative and may be used in special cases, but exceed the standard minimum requirement referenced here.

Common Pitfalls:
Confusing development length with hook tail length; ignoring bar coating effects (e.g., epoxy) that may require longer development.

Final Answer:
four times the diameter