Effective flange width of a reinforced concrete T-beam in compression (IS 456 concept) Let l0 be the distance between adjacent points of zero moment, b the rib (web) width, and D the flange thickness. According to IS 456 (conceptual form), the effective flange width beff may be taken as:

Introduction / Context:
In T-beams, compression is carried by a portion of the slab acting monolithically with the web. To reflect load dispersion, codes specify an effective flange width, larger than the web but smaller than the full slab panel width. This prevents overestimation of compression block capacity.

Given Data / Assumptions:

• Reinforced concrete T-beam within a continuous slab system.
• Distance between adjacent zero-moment points = l0.
• Rib (web) width = b; flange thickness = D.
• Standard IS 456 conceptual expression without considering additional span/spacing limits.

Concept / Approach:
IS practice provides a formula of the form b_eff = b + l0/6 + 6D subject to upper limits (e.g., not exceeding the center-to-center spacing of beams, or the clear distance to the adjacent web). The expression given here captures the dispersion from the web into the flange along the span and through the thickness.

Step-by-Step Solution:
1) Identify the three components: web width b, span-based spread l0/6, and thickness-based spread 6D.2) Sum to get b_eff = b + l0/6 + 6D.3) Apply codal caps (not shown in options) when checking against actual slab geometry.4) Choose the option matching the codal form.

Verification / Alternative check:
When D is small and l0 moderate, b_eff modestly exceeds b; checks against spacing limits ensure realism.

Why Other Options Are Wrong:
l0 + 6D omits the web width and is dimensionally excessive; l0/8 + 4D or l0/3 + 3D are not standard IS 456 forms.

Common Pitfalls:
Forgetting the caps based on slab panel width; using entire slab width as effective flange.

Final Answer:
b + l0/6 + 6D