--- title: "Design methods and basis for timber structures" subject: "Design of Steel and Timber Structure" chapter: 9 key: "semester_6/design_of_steel_and_timber_structure:timber-design-methods-basis" source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/timber-design-methods-basis site: BCT Engineering --- # Design methods and basis for timber structures Design of Steel and Timber Structure · Chapter 9: Timber Structures and Design Methods - Asked in: 2069 Bhadra, 4 marks - Video: [Team B Design of Steel and Timber Members.wmv](https://www.youtube.com/watch?v=B_rxFV1Q5qM) — SlimeyGrimey - 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/timber-design-methods-basis - Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-9 ## Question Describe design methods and design basis for timber structure. ## Short answer Design methods: working stress method (IS 883:1994, used in Nepal/India), limit state method (Eurocode 5), and prototype testing. Design basis: permissible stresses from Table 1 modified by grade (Select ×1.16, Grade II ×0.84), location, duration (K2=1.33 for wind), slope-of-grain, form factors; loads from IS 875; beam checks for bending, shear, bearing, deflection; column slenderness limits; tension on net section. ## Answer #### Design methods for timber structures 1. **Working stress (permissible stress) method** (IS 883:1994, used in Nepal and India): the stresses produced by service loads must not exceed the safe permissible stresses of the species and grade. It is simple and is the method used in this course. 2. **Limit state method:** partial safety factors on loads and material strength, with checks for strength and serviceability, as used in Eurocode 5 and other modern codes; it is more rational, but IS 883 does not use it. 3. **Load factor / ultimate strength approach** and **tests on prototypes** (IS 883 allows design by calculation or by prototype testing). #### Design basis (IS 883:1994) - **Permissible stresses** from Table 1 for the species (Grade I values), modified as follows: Select grade $\times1.16$, Grade II $\times0.84$; low-durability species used outside $\times0.8$; slope of grain ($K_1$), duration of load ($K_2$, e.g. 1.33 for wind), form factors ($K_3$ to $K_6$), bearing length ($K_7$). - **Loads** from IS 875 (dead, imposed, wind, snow); wind and seismic loads are not combined. - **Beams:** check bending, horizontal shear, bearing and deflection (limits span/360 with brittle finishes and span/240 otherwise, with the dead load doubled for initially dry timber); minimum width 50 mm or span/50; depth not more than 3 times the width unless laterally stiffened. - **Columns:** classified as short, intermediate or long by the slenderness $S/d$; $S/d \le 50$ for solid columns; combined bending and axial compression is checked by $f_{ac}/f_c + f_{ab}/f_b \le 1$. - **Tension members:** net section; notches may not remove more than one quarter of the section. - **Moisture content, grading and preservation** as per IS 287, IS 3629 and IS 401. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which design method does IS 883 use? A. Limit state method B. Ultimate load method only C. Plastic design D. Working stress (permissible stress) method Answer: D. IS 883:1994 uses working stresses; limit state is used in codes such as Eurocode 5. (Key point: Method of IS 883) 2. How do grade factors modify Table 1 stresses (given for Grade I)? A. Select grade x 1.16 and Grade II x 0.84 B. Select x 0.84 and Grade II x 1.16 C. Both grades x 1.0 D. Select x 1.33 and Grade II x 0.75 Answer: A. Better select timber is allowed a higher stress, poorer Grade II a lower one. (Key point: Grade modification) 3. How are wind and seismic loads treated? A. They are always added B. They are not combined C. Wind acts with 50% of seismic D. Only seismic is considered Answer: B. IS 883 design takes loads from IS 875, and wind and earthquake are not applied together. (Key point: Loads) 4. What deflection limits apply to timber beams? A. Span/150 and span/180 B. Span/500 for all C. Span/360 with brittle finishes and span/240 otherwise D. Span/240 with brittle finishes and span/360 otherwise Answer: C. Stiffer limit for beams supporting brittle finishes; dead load is also doubled for initially dry timber. (Key point: Beam deflection) ## Related in Design of Steel and Timber Structure - [Design process and basis of design](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Adesign-process-basis) — Design of Steel and Timber Structure, chapter 1, asked 2× - [Steel versus timber structures](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Asteel-vs-timber-advantages) — Design of Steel and Timber Structure, chapter 1 - [Design philosophies for steel structures](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Adesign-philosophies-steel) — Design of Steel and Timber Structure, chapter 1 - [Structural timber and factors affecting strength](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Astructural-timber-factors-strength) — Design of Steel and Timber Structure, chapter 9 - [Types of timber and its defects](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Atimber-types-defects) — Design of Steel and Timber Structure, chapter 9 - [Bolted joints in timber](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Atimber-bolted-joints) — Design of Steel and Timber Structure, chapter 10 ## Related in other subjects - [Internal structure of earth and basis of study](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Ainternal-structure-of-earth-and) — Engineering Geology I, chapter 2 - [Project financing; structures; capital structure steps](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Aproject-finance-define-structures-steps) — Project Engineering, chapter 6 - [Design of a nailed timber lap joint](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Anailed-lap-joint-design) — Design of Timber and Masonry Structures, chapter 2 - [Design of a square timber column](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Adesign-axially-loaded-timber-column) — Design of Timber and Masonry Structures, chapter 3