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Questions and Answers
The depth factor for bending stress for beams other than 300 mm depth is K______.
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______.
The load sharing system is referred to as K______.
8
To modify Emin for deflection in framer beams and lintels, the code is K______.
To modify Emin for deflection in framer beams and lintels, the code is K______.
9
Slenderness in compression members is addressed by K______.
Slenderness in compression members is addressed by K______.
The width factor for tension members is indicated as K______.
The width factor for tension members is indicated as K______.
Service class 1 refers to timber used internally in a continuously heated building with an average moisture content of ______%.
Service class 1 refers to timber used internally in a continuously heated building with an average moisture content of ______%.
In service class 3 conditions, the average moisture content is likely to be attained at ______%.
In service class 3 conditions, the average moisture content is likely to be attained at ______%.
The duration of load affects timber strength and therefore the permissible ______.
The duration of load affects timber strength and therefore the permissible ______.
The permissible stress design philosophy is based on ______ design principles.
The permissible stress design philosophy is based on ______ design principles.
The ultimate limit state refers to safety and is expressed in terms of ______-carrying capacity.
The ultimate limit state refers to safety and is expressed in terms of ______-carrying capacity.
Partial safety factors are applied only to the material properties in the ______ design philosophy.
Partial safety factors are applied only to the material properties in the ______ design philosophy.
In load factor design, the working load is calculated by dividing the collapse load by a ______ factor.
In load factor design, the working load is calculated by dividing the collapse load by a ______ factor.
Serviceability limit state refers to deformation and ______ limits.
Serviceability limit state refers to deformation and ______ limits.
Limit state design is a compromise between the permissible and load factor ______ methods.
Limit state design is a compromise between the permissible and load factor ______ methods.
The stress levels in permissible stress design are limited within the ______ range.
The stress levels in permissible stress design are limited within the ______ range.
Most modern structural codes of practice are now based on the ______ state approach.
Most modern structural codes of practice are now based on the ______ state approach.
Characteristic values are generally the ______ percentile values derived from a statistical analysis.
Characteristic values are generally the ______ percentile values derived from a statistical analysis.
The characteristic strength value is determined from test results using ______ principles.
The characteristic strength value is determined from test results using ______ principles.
In both ultimate and serviceability limit states, characteristic values of both the loads and material properties are modified by specified ______ factors of safety.
In both ultimate and serviceability limit states, characteristic values of both the loads and material properties are modified by specified ______ factors of safety.
Factors increase the ______ and decrease the values of material properties.
Factors increase the ______ and decrease the values of material properties.
A factor of unity or ______ is applied to the resistances of the material.
A factor of unity or ______ is applied to the resistances of the material.
Factors applied to resistance account for the degree of scientific ______ in the derivation of the values.
Factors applied to resistance account for the degree of scientific ______ in the derivation of the values.
Factors associated with loads are normally ______ on the type of material involved.
Factors associated with loads are normally ______ on the type of material involved.
Design code is a document that sets ______ for design of new development.
Design code is a document that sets ______ for design of new development.
The Euro code for timber (EC5) is based on ______ state principles.
The Euro code for timber (EC5) is based on ______ state principles.
Ultimate limit states are associated with ______ or similar forms of structural failure.
Ultimate limit states are associated with ______ or similar forms of structural failure.
Serviceability limit states are associated with user discomfort or dissatisfaction or a lack of ______.
Serviceability limit states are associated with user discomfort or dissatisfaction or a lack of ______.
Each limit state must be individually assessed for its effect on the ______.
Each limit state must be individually assessed for its effect on the ______.
The ultimate limit states enable the designer to calculate the ______ of the structure.
The ultimate limit states enable the designer to calculate the ______ of the structure.
The material strength may be less than intended due to its variable ______.
The material strength may be less than intended due to its variable ______.
The load in a member of a structure may be greater than anticipated because of the variability of ______ loading.
The load in a member of a structure may be greater than anticipated because of the variability of ______ loading.
Errors in the analysis can lead to unforeseen circumstances which may result in an increase in the general level of ______.
Errors in the analysis can lead to unforeseen circumstances which may result in an increase in the general level of ______.
K5 = 1.0 for a > ______
K5 = 1.0 for a > ______
For a notch on the downside, K5 = ______ / h
For a notch on the downside, K5 = ______ / h
K6 = 1.18 for ______ sections.
K6 = 1.18 for ______ sections.
K6 = 1.41 for solid ______ section loaded diagonally.
K6 = 1.41 for solid ______ section loaded diagonally.
The grade bending stresses in BS 5268 apply to beams with a depth of h = ______ mm.
The grade bending stresses in BS 5268 apply to beams with a depth of h = ______ mm.
The load sharing modification factor K8 has a value of ______.
The load sharing modification factor K8 has a value of ______.
In load sharing systems, the mean modulus of elasticity is denoted as ______.
In load sharing systems, the mean modulus of elasticity is denoted as ______.
The grade compression stresses must be modified by K12 when the slenderness ratio is more than ______.
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.
- 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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