--- title: "Simple timber beam and flitched beam" subject: "Design of Steel and Timber Structure" chapter: 12 key: "semester_6/design_of_steel_and_timber_structure:simple-flitched-beam-sketch" source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/simple-flitched-beam-sketch site: BCT Engineering --- # Simple timber beam and flitched beam Design of Steel and Timber Structure · Chapter 12: Design of Flexure Member - Asked in: 2069 Bhadra, 4 marks - Video: [Flitch Beam - Design example of flitch beam](https://www.youtube.com/watch?v=XrVdFNEjJU8) — FGJ Engineering - 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/simple-flitched-beam-sketch - Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-12 ## Question Explain simple timber beam and flitched beam with neat sketches. ## Short answer A simple timber beam is a single rectangular timber piece on supports, designed for bending, shear, bearing and deflection. A flitched beam comprises steel plates sandwiched between timber members and bolted; an equivalent section with modular ratio m=Es/Et is used, and steel plates increase strength and stiffness without adding depth. ## Answer #### Simple timber beam A **simple beam** is a single rectangular piece of sawn timber, simply supported on two supports, that carries the load by bending. ```text load v v v v v v +------------------+ | solid timber | b x d +------------------+ ^ ^ support support ``` It is used for short spans of up to 4 to 5 m. Section modulus $Z = bd^2/6$, and the design checks bending, shear, bearing and deflection (IS 883:1994 cl. 7.5). #### Flitched beam A **flitched beam** is a built-up beam in which one or more steel plates (flitch plates) are sandwiched between timber members and bolted through so that all parts act as one. The steel plates add strength and stiffness without increasing the depth. ```text timber steel timber steel timber +-----+-+-----+-+-----+ bolts through all | | | | | | parts at intervals | | | | | | +-----+-+-----+-+-----+ ``` - Timber and steel have the same strain at the same level, so with the modular ratio $m = E_s/E_t$ each steel plate is replaced by timber of width $m\,t_s$ (equivalent section). The load is shared in proportion to the stiffness $EI$ of each part. - Bending stresses: $f_t = \dfrac{M y}{I_{eq}}$ for timber and $f_s = m\,\dfrac{M y}{I_{eq}}$ for steel, with $I_{eq} = I_t + m\,I_s$; each must not exceed its permissible stress. - The bolts (or screws), placed at a regular pitch, hold the parts together and take the horizontal shear. - It is used where a deeper timber beam is not available, or for long spans and heavy loads. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a simple timber beam? A. A beam built up of timber and a steel plate B. A single rectangular piece of sawn timber on two supports, carrying load by bending C. A beam fixed at both ends D. A timber beam with only a nominal section Answer: B. A simple beam is a single solid sawn timber member, simply supported, with Z = bd^2/6. (Key point: Simple beam definition) 2. What is a flitched beam? A. Two timbers glued side by side with no other material B. A tapered timber beam C. Timber members sandwiching a steel (or timber) plate and bolted to act together D. A beam supported at three points Answer: C. The plate is flitched between two timber members and bolted through, increasing strength and stiffness without more depth. (Key point: Flitched beam construction) 3. In flitched beam analysis, how is the steel plate treated? A. It is ignored B. It carries all the moment C. It is replaced by timber of width m x t_s, with m = E_s/E_t D. It is replaced by timber of width t_s/m Answer: C. Equal strain at the same level means steel is converted to equivalent timber using the modular ratio m = E_s/E_t. (Key point: Equivalent section) 4. Which statement is correct for a flitched beam? A. Steel stress = m x My/I_eq, and bolts resist horizontal shear B. Steel stress = My/(m I_eq), and bolts only carry vertical load C. Steel stress equals timber stress, and bolts are decorative D. Steel stress = My/I_eq, and bolts carry bending Answer: A. Steel stress is m times the timber stress at the same level, and the bolts hold parts together and take horizontal shear. 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