---
title: "Factors affecting strength and permissible stresses"
subject: "Design of Timber and Masonry Structures"
chapter: 1
key: "semester_5/design_of_timber_and_masonry_structures:factors-affecting-timber-strength"
source: https://www.bctengineer.com/q/semester_5/design_of_timber_and_masonry_structures/factors-affecting-timber-strength
site: BCT Engineering
---

# Factors affecting strength and permissible stresses

Design of Timber and Masonry Structures · Chapter 1: Structural Timbers

- Asked in: year not given, 6 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/design_of_timber_and_masonry_structures/factors-affecting-timber-strength
- Whole chapter: https://www.bctengineer.com/md/semester_5/design_of_timber_and_masonry_structures/chapter-1

## Question

(a) Discuss the main factors that affect the strength of structural timber. (b) How are the permissible stresses of a timber obtained from the strength of clear specimens? Explain the grading of timber.

## Answer

#### (a) Factors affecting strength
1. **Moisture content**: below the fibre saturation point (about 25–30 percent) strength and stiffness increase as the wood dries. Above it, strength is nearly constant. Timber is tested and designed at a standard moisture content (about 12 percent).
2. **Density / specific gravity**: denser wood has more cell wall material and so is stronger and stiffer.
3. **Defects**: knots, slope of grain, shakes, checks, splits, wane, decay and insect attack reduce the effective section and cause stress concentration. Slope of grain and knots mainly reduce tensile and bending strength.
4. **Direction of loading**: strength parallel to grain is many times that perpendicular to grain; compression, tension and shear all depend on the angle to the grain (Hankinson's formula).
5. **Duration of load**: the longer the load acts, the lower the strength. Codes allow higher stresses for short-term loads (for example wind or earthquake).
6. **Temperature, seasoning and treatment**: high temperature and poor seasoning reduce strength; some preservatives and fire-retardants slightly reduce it.
7. **Size and position of the member**: larger sections contain more defects and have lower strength per unit area.

#### (b) Permissible stresses
Small clear specimens are tested to failure and the mean and standard deviation of strength are found. The basic stress is taken near the 5 percent exclusion value and then divided by a factor of safety:

$$
\sigma_{\text{permissible}} = \frac{\text{Basic (clear) strength}}{\text{Factor of safety}} \times \text{strength ratio (grade)} \times \text{modification factors}
$$

The factor of safety allows for variability, reduction due to long-term loading and local defects.

#### Grading
Timber is visually graded according to the size and number of defects permitted (knots, slope of grain, shakes).

| Grade | Approx. strength ratio |
|---|---|
| Select | 1.00 |
| Grade I | about 0.80 |
| Grade II | about 0.63 |

The permissible stresses of the select grade are listed in the code for each strength group; stresses for lower grades are obtained by multiplying by the grade ratio. Modification factors are then applied for moisture, load duration and size of the member.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does moisture content affect strength below the fibre saturation point (about 25-30 percent)?
   A. Strength falls as the wood dries
   B. Strength is unaffected by moisture
   C. Strength is constant below it and rises above it
   D. Strength and stiffness increase as the wood dries
   Answer: D. Below the fibre saturation point, drying increases strength and stiffness; above it strength is nearly constant, so tests use a standard moisture content (about 12 percent). (Key point: Moisture content)

2. Why do codes allow higher permissible stresses for wind or earthquake loads?
   A. They act for a short time, and strength is lower for longer-acting loads
   B. They act in a direction across the grain
   C. Wind and earthquake load make timber denser
   D. They are always smaller than dead loads
   Answer: A. The longer a load acts, the lower the strength, so short-term loads such as wind or earthquake can be given higher stresses. (Key point: Duration of load)

3. How is the basic stress obtained from tests on small clear specimens?
   A. It is the highest strength found in the tests
   B. It is taken near the 5 percent exclusion value of the strength distribution
   C. It is the mean strength with no reduction
   D. It is the mean strength divided by the grade ratio
   Answer: B. Mean and standard deviation of strength are found and the basic stress is taken near the 5 percent exclusion value, then divided by a factor of safety. (Key point: Basic stress and factor of safety)

4. How are permissible stresses for Grade I and Grade II timber obtained?
   A. By testing each piece to failure
   B. By adding modification factors to the Grade II value
   C. By multiplying the select grade stresses by the grade strength ratio (about 0.80 and 0.63)
   D. By dividing the select grade stresses by the grade ratio
   Answer: C. The code lists select grade stresses; lower grades are found by multiplying by the strength ratio, about 0.80 for Grade I and 0.63 for Grade II. (Key point: Grading and strength ratio)

## Related in Design of Timber and Masonry Structures

- [Classification and characteristics of structural timber](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Atimber-classification-characteristics) — Design of Timber and Masonry Structures, chapter 1
- [Permissible compressive stress of masonry as per IS 1905](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Amasonry-permissible-stress-factors-code) — Design of Timber and Masonry Structures, chapter 5
- [Design of a simply supported timber beam](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Adesign-timber-floor-beam) — Design of Timber and Masonry Structures, chapter 3
- [Flitched timber–steel beam capacity](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Aflitched-beam-moment-capacity) — Design of Timber and Masonry Structures, chapter 3
- [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
- [Safe axial load on a timber column](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Aaxial-capacity-timber-column) — Design of Timber and Masonry Structures, chapter 3

## Related in other subjects

- [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
- [Timber-steel plate composite member: permissible load](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Atimber-steel-composite-permissible-load) — Strength of Materials, chapter 3
- [Permissible stresses as per IS 800](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Apermissible-stresses-is800) — Design of Steel and Timber Structure, chapter 2
- [Reduction factors in permissible stress of masonry](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Areduction-factors-permissible-stress) — Concrete Technology and Masonry Structure, chapter 8, asked 2×