--- title: "What is plate girder? Preliminary design" subject: "Design of Steel and Timber Structure" chapter: 7 key: "semester_6/design_of_steel_and_timber_structure:plate-girder-preliminary-design" source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/plate-girder-preliminary-design site: BCT Engineering --- # What is plate girder? Preliminary design Design of Steel and Timber Structure · Chapter 7: Flexure Members - Asked in: 2076 Bhadra, 8 marks - Video: [5.3 Preliminary sizing of plate girder](https://www.youtube.com/watch?v=fXihGupDd4I) — Eng-aim - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/plate-girder-preliminary-design - Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-7 ## Question What is the plate girder? Explain about preliminary design of plate girder. ## Short answer A plate girder is a built-up beam of web and flange plates used for large spans/loads. Preliminary design: estimate depth d ≈ L/10 to L/12, web thickness t_w = d/k (k≈100–200), flange area A_f ≈ M/(f_y d/γ_m0) – A_w/6, flange width b_f ≈ d/3 to d/5, check b/t_f limits, then verify moment, shear, and deflection. ## Answer A **plate girder** is a built-up beam of steel plates (a web plate and two flange plates, joined by welds or bolts) used for spans or loads beyond the range of rolled sections, usually for spans above about 15 m, such as in bridges and industrial buildings. The web is deep and thin and is stiffened against buckling. #### Preliminary design (sizing) steps 1. **Design forces.** Find the factored maximum moment $M$ at mid-span and the maximum shear $V$ at the supports (from the loads, including an assumed self-weight). 2. **Depth of girder.** For economy $d \approx \dfrac{L}{10}$ to $\dfrac{L}{12}$; or, for minimum weight, $d = \left(\dfrac{M\,k}{f}\right)^{1/3}$, with $f = f_y/\gamma_{m0}$ and $k = d/t_w$ (about 100 to 200). 3. **Web thickness.** $t_w = d/k$. Check the serviceability limits, $d/t_w \le 200\varepsilon$ (unstiffened or widely stiffened) up to about $270\varepsilon$ (closely stiffened), and the flange-buckling limit $d/t_w \le 345\varepsilon_f^2$ (IS 800:2007 cl. 8.6.1). Minimum thickness 8 mm (6 mm sheltered). 4. **Flange area.** $A_f = \dfrac{M\gamma_{m0}}{f_y\,d} - \dfrac{A_w}{6}$ (web contribution included approximately), where $A_w = d\,t_w$. 5. **Flange size.** Width $b_f \approx d/3$ to $d/5$ (also $\ge L/40$ for lateral stability); thickness $t_f = A_f/b_f$, with the outstand $b/t_f \le 13.6\varepsilon$ (semi-compact, welded). Round to available plate sizes. 6. **Trial check.** Find $I_{zz} = \dfrac{t_wd^3}{12} + 2\,b_ft_f\left(\dfrac{d+t_f}{2}\right)^2$ and check the moment capacity $M_d = \beta_bZ_pf_y/\gamma_{m0}$, which for a semi-compact section ($\beta_b = Z_e/Z_p$) becomes $M_d = Z_ef_y/\gamma_{m0}$; the shear capacity $V_d = d\,t_wf_y/(\sqrt3\gamma_{m0})$, or the shear buckling resistance of cl. 8.4.2 when $d/t_w > 67\varepsilon$; and the deflection. 7. **After preliminary sizing,** the detailed design proceeds with the stiffeners (bearing and intermediate), the flange-to-web welds ($q = VA\bar y/I$), curtailment of the flange plates, and splices. #### Remark The thinner the web, the lighter the girder, but the more stiffeners it needs; the usual choice balances the web weight against the cost of stiffeners and fabrication. The flanges carry the bending as a couple $F = M/d$, and the web carries the shear. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a plate girder? A. A rolled I-section with holes in the web B. A built-up beam of web and flange plates used for long spans or heavy loads, with a deep thin stiffened web C. A timber beam strengthened with steel plates D. A truss with plate gusset joints Answer: B. A plate girder is built up from plates joined by welds or bolts, used beyond rolled-section capacity (often spans above about 15 m). (Key point: Plate girder definition) 2. What depth is usually chosen for an economical plate girder of span L? A. L/20 to L/25 B. L/5 to L/8 C. L/10 to L/12 D. L/30 to L/40 Answer: C. For economy the depth is about L/10 to L/12, or from minimum weight d = (Mk/f)^(1/3). (Key point: Economical depth) 3. How is the required flange area estimated? A. Af = M gamma_m0/(fy d) - Aw/6 B. Af = V/(fy d) C. Af = d tw/6 only D. Af = M/(fy tw) Answer: A. The flanges carry the bending as a couple F = M/d, so Af = M gamma_m0/(fy d), reduced by about Aw/6 for the web's contribution. (Key point: Flange area) 4. What is the effect of making the web thinner? A. Lighter girder but more stiffeners are needed B. Heavier girder but fewer stiffeners C. Higher shear capacity without stiffeners D. No effect on the number of stiffeners Answer: A. A thinner web saves weight but buckles more easily, so more stiffeners and fabrication are needed; the design balances the two. 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