--- title: "Design steps of flanged beam sections" subject: "Design of RCC Structure" chapter: 4 key: "semester_7/design_of_rcc_structure:design-steps-of-flanged-beam-sections" source: https://www.bctengineer.com/q/semester_7/design_of_rcc_structure/design-steps-of-flanged-beam-sections site: BCT Engineering --- # Design steps of flanged beam sections Design of RCC Structure · Chapter 4: Design of beams: Behavior in Flexure - Asked in: 2081 Baisakh, 4 marks; 2078 Bhadra, 5 marks (asked 2 times) - Video: [Analysis of Flanged Beam (T- Beam) | Design of RCC Strctures | Mumbai University | IOE , TU , PU](https://www.youtube.com/watch?v=9FtHa8RZHZM) — Parash Joshi - Civil Construction and Tutor - 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/design-steps-of-flanged-beam-sections - Whole chapter: https://www.bctengineer.com/md/semester_7/design_of_rcc_structure/chapter-4 ## Question Describe/explain the design steps of flanged beam sections. ## Short answer Fix section dimensions and effective flange width (T-beam: b_f = l0/6 + b_w + 6Df, limited to spacing). Compute factored moment M_u and flange moment capacity M_u,f assuming x_u = Df. If M_u ≤ M_u,f design as rectangular beam of width b_f. Otherwise NA lies in web; solve for x_u considering Df/d ratio, ensure x_u ≤ x_u,max, then find tension steel from equilibrium. Check shear and deflection. ## Answer T- and L-beams occur when a slab is cast monolithically with the beam, so the slab acts as the flange. Design steps (IS 456 cl. 23.1.2 and Annex G-2) are as follows. 1. **Fix dimensions:** assume web width $b_w$ (usually 230-300 mm) and depth $D\approx L/12$ to $L/10$; slab thickness $D_f$ is known from slab design; $d=D-$ cover. 2. **Effective flange width $b_f$:** - T-beam: $b_f=\dfrac{l_0}{6}+b_w+6D_f$ - L-beam: $b_f=\dfrac{l_0}{12}+b_w+3D_f$ - limited to the actual centre-to-centre spacing of beams. $l_0$ is the distance between points of zero moment (span for simply supported). 3. **Loads:** self weight of web and slab, finishes, live load; factor by 1.5. 4. **Moment:** $M_u=w_uL^2/8$ (or from analysis). 5. **Locate neutral axis:** find the moment capacity with $x_u=D_f$: $$ M_{u,f}=0.36f_{ck}b_fD_f(d-0.42D_f) $$ - If $M_u\le M_{u,f}$, NA lies in the flange: design as a rectangular beam of width $b_f$. - If $M_u>M_{u,f}$, NA lies in the web: go to step 6. 6. **NA in the web:** - If $D_f/d\le0.2$: $M_u=0.36f_{ck}b_wx_u(d-0.416x_u)+0.45f_{ck}(b_f-b_w)D_f(d-D_f/2)$. - If $D_f/d>0.2$: replace $D_f$ by $y_f=0.15x_u+0.65D_f$. - Solve for $x_u$ and check $x_u\le x_{u,max}$ (if not, increase section or add compression steel). 7. **Tension steel:** from $0.87f_yA_{st}=0.36f_{ck}b_wx_u+0.45f_{ck}(b_f-b_w)D_f$ (or $y_f$). 8. **Checks:** minimum steel $0.85b_wd/f_y$, maximum 4% of $b_wD$, shear, deflection ($L/d$ with modification factor and $b_w/b_f$ factor), cracking and detailing. Flange transverse steel is provided in the slab. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the effective flange width of a T-beam? A. l0/6 + bw + 6Df B. l0/12 + bw + 3Df C. l0/4 + bw D. bw + 12Df Answer: A. L-beams use l0/12 + bw + 3Df, while T-beams use l0/6 + bw + 6Df. (Key point: Effective flange width) 2. How is the position of the NA decided? A. Compare Mu with Mu,f = 0.36 fck bf Df (d - 0.42 Df); if Mu <= Mu,f the NA is in the flange B. Compare Mu with 0.138 fck bw d^2 only C. Always assume it is in the web D. Compare d with Df Answer: A. If the moment is less than the flange capacity at xu = Df, the NA is in the flange and the beam is designed as a rectangle of width bf. (Key point: Locating NA) 3. When NA is in the web and Df/d > 0.2, what is done? A. Ignore the flange B. Replace Df by yf = 0.15 xu + 0.65 Df C. Use Df = 0.2d always D. Double bw Answer: B. The equivalent flange thickness yf accounts for the non-uniform stress in a deep flange. (Key point: Thick flange) 4. What limits the effective flange width further? A. Actual centre-to-centre spacing of beams B. The depth of the web C. The steel percentage D. Nothing Answer: A. bf cannot exceed the actual flange width or beam spacing. (Key point: Flange width limit) ## Related in Design of RCC Structure - [Design shear reinforcement of flanged beam](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Adesign-shear-reinforcement-of-flanged-beam) — Design of RCC Structure, chapter 5 - [Types of rectangular beam sections](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Atypes-of-rectangular-beam-sections) — Design of RCC Structure, chapter 4 - [Design steps of doubly reinforced section](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Adesign-steps-of-doubly-reinforced-section) — Design of RCC Structure, chapter 4 - [Design steps for long column, unbraced frame](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Adesign-steps-for-long-column-unbraced) — Design of RCC Structure, chapter 10 - [Steps of rectangular footing design](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Asteps-of-rectangular-footing-design) — Design of RCC Structure, chapter 11 - [Beam design, 250x475 section](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Abeam-design-250x475-section) — Design of RCC Structure, chapter 4, asked 2× ## Related in other subjects - [Bell's guide bund plan, sections, design steps](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Abells-bund-plan-design-steps) — Irrigation and Drainage Engineering, chapter 6 - [Steps of grounding mat design](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Agrounding-mat-design-steps) — Power Plant Design, chapter 4, asked 5× - [Design steps and criteria of distribution system](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Adistribution-design-steps) — Water Supply Engineering, chapter 7, asked 3× - [Design steps of superelevation](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asuper-design-steps) — Transportation Engineering I, chapter 3, asked 3×