Questions and Answers
The depth factor for bending stress for beams other than 300 mm depth is K______.
7
The load sharing system is referred to as K______.
8
To modify Emin for deflection in framer beams and lintels, the code is K______.
9
Slenderness in compression members is addressed by K______.
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The width factor for tension members is indicated as K______.
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Service class 1 refers to timber used internally in a continuously heated building with an average moisture content of ______%.
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In service class 3 conditions, the average moisture content is likely to be attained at ______%.
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The duration of load affects timber strength and therefore the permissible ______.
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The permissible stress design philosophy is based on ______ design principles.
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The ultimate limit state refers to safety and is expressed in terms of ______-carrying capacity.
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Partial safety factors are applied only to the material properties in the ______ design philosophy.
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In load factor design, the working load is calculated by dividing the collapse load by a ______ factor.
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Serviceability limit state refers to deformation and ______ limits.
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Limit state design is a compromise between the permissible and load factor ______ methods.
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The stress levels in permissible stress design are limited within the ______ range.
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Most modern structural codes of practice are now based on the ______ state approach.
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Characteristic values are generally the ______ percentile values derived from a statistical analysis.
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The characteristic strength value is determined from test results using ______ principles.
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In both ultimate and serviceability limit states, characteristic values of both the loads and material properties are modified by specified ______ factors of safety.
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Factors increase the ______ and decrease the values of material properties.
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A factor of unity or ______ is applied to the resistances of the material.
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Factors applied to resistance account for the degree of scientific ______ in the derivation of the values.
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Factors associated with loads are normally ______ on the type of material involved.
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Design code is a document that sets ______ for design of new development.
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The Euro code for timber (EC5) is based on ______ state principles.
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Ultimate limit states are associated with ______ or similar forms of structural failure.
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Serviceability limit states are associated with user discomfort or dissatisfaction or a lack of ______.
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Each limit state must be individually assessed for its effect on the ______.
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The ultimate limit states enable the designer to calculate the ______ of the structure.
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The material strength may be less than intended due to its variable ______.
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The load in a member of a structure may be greater than anticipated because of the variability of ______ loading.
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Errors in the analysis can lead to unforeseen circumstances which may result in an increase in the general level of ______.
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K5 = 1.0 for a > ______
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For a notch on the downside, K5 = ______ / h
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K6 = 1.18 for ______ sections.
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K6 = 1.41 for solid ______ section loaded diagonally.
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The grade bending stresses in BS 5268 apply to beams with a depth of h = ______ mm.
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The load sharing modification factor K8 has a value of ______.
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In load sharing systems, the mean modulus of elasticity is denoted as ______.
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The grade compression stresses must be modified by K12 when the slenderness ratio is more than ______.
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Study Notes
Design Philosophies in Structural Engineering
- Permissible Stress Design (PSD) limits stresses to a proportion of yield stress, focusing on maintaining elastic behavior under working loads.
- Partial safety factors in PSD are applied only to material properties, not to loads, complicating the design process and leading to conservative solutions.
- Load Factor Design (LFD) accounts for post-yield behavior, calculating collapse load by dividing it by a load factor for working load assessment.
- Limit State Design (LSD) is a modern approach combining PSD and LFD, addressing both ultimate and serviceability limit states to ensure safety and functionality.
Limit States
- Ultimate Limit States (ULS) prevent structural failure or collapse endangering safety, involving strength and stability considerations.
- Serviceability Limit States (SLS) address user comfort and functionality, focusing on deformation and vibrations in normal use.
- Structures must remain feasible under bending, shear, compression, deflection, and cracking, each requiring individual assessment for limit states.
Characteristic and Design Values
- Characteristic Values represent the inherent variability in material properties and loads, determined statistically: 5th percentile values for material strength and loads.
- Factors of Safety enhance reliability against construction variability, responsibility for material inconsistency, and potential unforeseen loads, typically incorporating statistical analysis for accuracy.
Design Codes and Modifications
- National codes like BS 5268 inform permissible limits and modification factors (K factors) affecting properties according to conditions such as timber moisture content and load duration.
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Service Classes classify timber exposure:
- Service Class 1: Internal and heated, avg. moisture 12%.
- Service Class 2: Covered buildings, avg. moisture 15% heated, 18% unheated.
- Service Class 3: Exposed timber, avg. moisture 20%.
Modification Factors for Timber
- Depth Factor (K7) adjusts bending stress values for timber sections over 300 mm deep, ensuring sustained performance.
- Load Sharing Systems (K8) apply to interconnected structural elements like joists, increasing grade stresses by 1.1 when adequate lateral load distribution is present.
- Form Factor (K6) applies to non-rectangular timber shapes, modifying grade bending stress values based on cross-section.
Additional Design Considerations
- Moisture content at installation should reflect expected service conditions to mitigate movement and creep in timber and wood-based panels.
- The Duration of Loading (K3) impacts timber strength; adjustments are necessary for increased load durations to accommodate changes in permissible stresses.
- Slenderness Ratio of compression members must be evaluated; alterations are required for ratios exceeding 5 using modification factor K12 to maintain structural integrity.
Summary of Stresses and Loading Factors
- Bending and Tension Parallel to Grain: Maximum permissible stresses are adjusted in accordance with service class conditions.
- The mean modulus of elasticity is employed for calculating deflections in load-sharing systems, emphasizing the need for accurate assessment during structural design.
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Description
This quiz explores the differences between the permissible stress design philosophy in BS 5268 Part 2 and the limit states design philosophy of Eurocode 5. It covers the ultimate and serviceability limit states along with safety factors and material strength. Test your understanding of these two key engineering design philosophies!
