---
title: "Flange width concept in T and L beams"
subject: "Design of RCC Structure"
chapter: 4
key: "semester_7/design_of_rcc_structure:flange-width-concept-in-t-and"
source: https://www.bctengineer.com/q/semester_7/design_of_rcc_structure/flange-width-concept-in-t-and
site: BCT Engineering
---

# Flange width concept in T and L beams

Design of RCC Structure · Chapter 4: Design of beams: Behavior in Flexure

- Asked in: 2082 Bhadra, 2+2 marks
- Video: [Lec 15 | RCC | FLANGE SEC,PART2 | CONCEPT OF EFFECTIVE WIDTH OF ISOLATED T AND L BEAMS](https://www.youtube.com/watch?v=FcE7ARdFi0k) — CONCEPT DECODER
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/design_of_rcc_structure/flange-width-concept-in-t-and
- Whole chapter: https://www.bctengineer.com/md/semester_7/design_of_rcc_structure/chapter-4

## Question

What is the concept behind the typical flange width in T and L beam? Explain with necessary sketches.

## Short answer

Shear lag causes non-uniform compressive stress across the slab, so design uses a reduced effective flange width over which uniform stress is assumed. For T-beam, bf = l0/6 + bw + 6Df; for L-beam, bf = l0/12 + bw + 3Df, limited by actual width and beam spacing.

## Answer

#### Concept

In a monolithic slab-beam system, the slab on either side of the beam web acts with the web in resisting bending, forming a T-beam (slab on both sides) or L-beam (slab on one side, e.g. edge beam). The compressive stress in the slab is not uniform across its width; it is highest at the web and falls away from it, because the shear that transfers compression from the web to the flange is carried by shear deformation of the slab (**shear lag**).

```text
 compressive stress across flange      T-beam section
   ___                                 +------------------+  Df
  /   \___                             +----+      +------+
 /        \___                              |  bw  |
 |  web     slab edge                      |      |
```

For simple design, the actual variable stress is replaced by a uniform stress over a smaller **effective flange width** $b_f$ which gives the same total compression.

#### Effective flange width (IS 456 cl. 23.1.2)

- T-beam: $b_f=\dfrac{l_0}{6}+b_w+6D_f$
- L-beam: $b_f=\dfrac{l_0}{12}+b_w+3D_f$
- Isolated T-beam: $b_f=\dfrac{l_0}{\dfrac{l_0}{b}+4}+b_w$
- $b_f$ must not exceed the actual width of the flange, or the centre-to-centre spacing of beams.

Here $l_0$ is the distance between points of zero moment (equal to the effective span for a simply supported beam), $b_w$ the web width, $D_f$ the flange thickness and $b$ the actual flange width.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why does compressive stress vary across the slab flange?
   A. Shear lag: the slab transfers compression from the web by shear deformation
   B. Slab has no stress
   C. Steel is in the slab
   D. Temperature
   Answer: A. Stress is highest at the web and falls away from it. (Key point: Shear lag)

2. What does the effective flange width represent?
   A. Actual slab width
   B. A reduced width carrying uniform stress that gives the same total compression
   C. The web width
   D. The span
   Answer: B. It simplifies design by replacing variable stress with uniform stress. (Key point: Effective width idea)

3. Effective flange width of an L-beam is:
   A. l0/6 + bw + 6Df
   B. l0/12 + bw + 3Df
   C. l0/4 + bw
   D. l0/(l0/b + 4) + bw
   Answer: B. L-beams have a flange on one side only so the contributions are halved. (Key point: L-beam formula)

## Related in Design of RCC Structure

- [Ultimate moment of T-beam (2250 mm flange)](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aultimate-moment-of-t-beam-2250) — Design of RCC Structure, chapter 4
- [Importance of ductile detailing, sketches](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aimportance-of-ductile-detailing-sketches) — Design of RCC Structure, chapter 12
- [Ductile detailing of beams and columns](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aductile-detailing-of-beams-and-columns) — Design of RCC Structure, chapter 12, asked 2×
- [Slab rigidly fixed with beams, 6.5x4 m](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aslab-rigidly-fixed-with-beams-6) — Design of RCC Structure, chapter 9
- [Ductility requirements of RC beam with sketch](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aductility-requirements-of-rc-beam-with) — Design of RCC Structure, chapter 12, asked 4×
- [Limiting moment of T-beam](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Alimiting-moment-of-t-beam) — Design of RCC Structure, chapter 4

## Related in other subjects

- [Simply supported I-beam: stress at flange-web junction](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Ai-beam-flange-web-junction-stress) — Strength of Materials, chapter 7
- [Symmetrical I-beam: moment, flange and web share](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Asymmetrical-i-beam-max-moment-flange) — Strength of Materials, chapter 7
- [Beam with flange plates, 30 kN/m + 16 kN](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Abeam-additional-plates-30knm-16kn) — Design of Steel and Timber Structure, chapter 7
- [Bracket plate on column flange, 5 M20 bolts](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Abracket-bolted-5-m20) — Design of Steel and Timber Structure, chapter 4