--- title: "Horizontal load resisting system in buildings" subject: "Design of Steel and Timber Structure" chapter: 1 key: "semester_6/design_of_steel_and_timber_structure:horizontal-load-resisting-system" source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/horizontal-load-resisting-system site: BCT Engineering --- # Horizontal load resisting system in buildings Design of Steel and Timber Structure · Chapter 1: Steel Structures and their Analysis and Design - Asked in: 2068 Bhadra (old course), 6 marks - Video: [How does a steel bracing works structurally?](https://www.youtube.com/watch?v=9v_P13l2tlY) — TeleTraining - 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/horizontal-load-resisting-system - Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-1 ## Question Describe the horizontal load resisting system in steel structure building. ## Short answer Horizontal loads are resisted by braced frames (diagonal braces), moment frames (rigid joints), shear walls, or dual systems. Roof and floor diaphragms distribute force to vertical elements. Design follows IS 800 and NBC 105, considering drift and architectural needs. ## Answer Horizontal loads on steel buildings (wind and earthquake) are carried to the foundation by a lateral load resisting system. Floors and roofs act as diaphragms that distribute the lateral force to vertical resisting frames. #### Main systems 1. **Braced frames (concentric / eccentric):** diagonal, X, K or V bracings between columns carry the lateral load by axial force in members. Stiff and economical for low and medium rise buildings. 2. **Moment-resisting (rigid) frames:** beam-to-column joints are rigid and resist the lateral load by bending of beams and columns. Gives open bays but larger sections and costly joints. 3. **Shear walls / core:** RC or steel plate shear walls in lift and stair cores resist storey shear. 4. **Dual systems:** moment frame combined with bracing or shear wall; share the load by relative stiffness. 5. **Tube systems:** closely spaced perimeter columns and deep spandrels form a stiff tube for tall buildings. 6. **Roof/floor bracing and diaphragms:** horizontal bracing in plan and rigid slabs transmit forces to the vertical frames. ```text Braced frame Moment frame |\ /| |==|==| | \/ | | | | | /\ | |==|==| |/ \| | | | ``` Choice depends on height, drift limit, architectural openness and cost. IS 800 and IS 1893 / NBC 105 control the load and detailing. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the function of floors and roofs in resisting horizontal loads? A. They act as diaphragms distributing lateral force to the vertical resisting frames B. They resist wind by bending of columns C. They are ignored D. They carry lateral load directly to the foundation Answer: A. Floor and roof diaphragms transmit lateral forces to vertical frames, which carry them to the foundation. (Key point: Diaphragm role) 2. How does a braced frame resist lateral load? A. By bending of beams and columns B. By axial force in the diagonal bracing members C. By rigid slab action only D. By tube action of perimeter columns Answer: B. Diagonal, X, K or V bracings carry lateral load as axial forces, making stiff economical frames for low and medium rise. (Key point: Braced frame action) 3. What is a feature of a moment-resisting frame? A. Pin joints and diagonal bracing B. Open bays but larger sections and costly rigid joints C. Cheapest system for all heights D. Lateral load carried by axial force only Answer: B. Rigid beam-column joints resist lateral load by bending, giving open bays at the cost of larger sections and joints. (Key point: Moment frame) 4. What is a dual system? A. Two layers of floor slab B. Bracing without a frame C. A moment frame combined with bracing or shear wall, sharing load by relative stiffness D. Perimeter tube without a core Answer: C. Dual systems combine a moment frame with bracing or a shear wall, and the load is shared in proportion to stiffness. 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