Timber Design Lesson 1 Update PDF

STRUCTURAL TIMBER DESIGN BS 5268 Part 2: 2002 INTRODUCTION Structural timber can be used for; Temporarily works e.g., Trenches, scaffolding, shoring Permanent works e.g., Trusses, joist (Timber beam), floor boards, column(post) Timber can be used in various forms e...

STRUCTURAL TIMBER DESIGN BS 5268 Part 2: 2002 INTRODUCTION Structural timber can be used for; Temporarily works e.g., Trenches, scaffolding, shoring Permanent works e.g., Trusses, joist (Timber beam), floor boards, column(post) Timber can be used in various forms e.g., Solids, built-up, laminated etc. Strength capability of timber is difficult to assess as we have no control over its quality and growth. The strength is defined as the natural resistance of a material to failure. In general situations, failure is deemed to have taken place when the applied stress reaches the elastic limit. Strength tests are carried out to BS373 or to ASTMD143-52 and is determined from small (20mm x 20mm or 50mm x 50mm), oven dried, clear (free from any defects) test pieces. The strength obtained is known as basic strength and must be subjects to a modifying factor to obtain the strength of a specific grade of timber of the same species. Strength tests can also be carried out on selected structural size test pieces of the timber to be used. The sets are carried out to BS5820 or to EN480. The following are the basic methods using factor of safety to achieve safe and workable structures The load factor method – in which the collapsible loads are divide by a factor of safety. The limit state method – the working loads by partial factors of safety and also divides the material ultimate strengths by further partial factor of safety The permissible stress method – the ultimate strength of the materials is multiply by a factor of safety to provide design stresses which are usually within the elastic range. The permissible stress in timber is a product of the grade stress and several modification factors to allow for moisture condition, duration of load, load sharing, slenderness, etc. The permissible stress design philosophy as in BS 5268; part 2, is different from the limit states design philosophy of Eurocode 5 which has 2 basic requirements.  1) The ultimate limit state (safety) is expressed in terms of load-carrying capacity and is achieved by factoring up of load values and factoring down of material strength properties by partial safety factors that reflect the reliability of values that they modify. 2) The serviceability limit state (i.e. deformation and vibration limits) refers to the ability of a structural system and its elements to perform satisfactory in normal use. NB: In permissible stress design philosophy partial safety factors ( i.e. modification factors) are applied only to the material properties i.e. for the calculation of permissible stress and not to the loading. a) Permissible Stress Design or elastic design In this design, the stresses in the structure at working loads are not allowed to exceed a certain proportion of the yield stress of the material. The stress levels are limited within the elastic range. By assuming that the stress-strain relationship over this range is linear, it is possible to calculate the actual stress in the material concerned. Draw backs: - Design process tend to complicate the design process and leads to conservation solutions. - For materials such as concrete the stress-strain relationship may not be linear thus, it is difficult to determine the stress within the elastic range. b) Load Factor Design or plastic design Developed to take account of the behavior of the structure once the yield point of the materials has been reached. This involves calculating the collapse load of the structure. The working load is obtained by dividing the collapse load by a load factor. c) Limit state design This could be seen as a compromise between the permissible and load factor methods. Most modern structural codes of practice are now based on the limit state approach. Principal exceptions are the code of practice for design in timber, BS 5268, and structural steel code, BS 449, both of which are permissible stress codes. The Euro code for timber (EC5), is based on limit state principles. A limit state is simply a state beyond which a structure no longer satisfies its performance requirements. Ultimate limit states are associated with collapse or similar forms of structural failure that may endanger the safety of people, and generally involve the consideration of strength and stability. Serviceability limit states are associated with user discomfort or dissatisfaction or a lack of functionality, and generally involve the consideration of deformation (i.e. the deflections of members or slip in connections). Partial safety factors are used to increase the values of loads and to decrease the material strength values Ways in which the structure may become unfit for use: -excessive bending, shear, compression, deflection and cracking. Each of these mechanisms is a limit state whose effect on the structure must be individually assessed. The ultimate limit states enables the designer calculate the strength of the structure. Serviceability limit states model the behavior of the structure at working loads. Other limit states are durability and fire resistance. Characteristic and design values When designing, one is never certain about the amount of load that the structure will be subjected and the strength of the material from which the structure is made. The material strength may be less than intended because of: - its variable composition - the variability of manufacturing conditions during construction - other effects such as corrosion The load in a member of a structure may be greater than anticipation because of : - the variability of the occupancy or environmental loading - unforeseen circumstances which may lead to an increase in general level of loading, errors in the analysis, errors during construction e.t.c. The variability in material composition and loading is allowed for by using a characteristic value. Characteristic values are generally 5th percentile values derived directly from a statistical analysis of laboratory test results. The characteristic strength value is determined from test results using statistical principles, and is normally defined as the value below which not more than 5% test results fall. The characteristic load is the value above which only a small percentage (usually 5 %) of the test results of the load recorded over time lies. In both ultimate and serviceability limit states, characteristic values of both the loads and the material properties are modified by specified partial factors of safety that reflect the variability of the values that they modify. These factors increase the loads and decrease the values of the material properties. Factor of safety These are determined using statistics and pre-selected probability of failure. They are used to take care of the following - Variability of construction - Inconsistency of construction material - Possible increase in load - Inaccurate assessment of loads - Unforeseen stress distributions - Variation in dimensional accuracy A factor of unity or less is applied to the resistances of the material, and a factor of unity or greater to the loads. These factors can differ significantly for different materials or even between differing grades of the same material. The factors applied to resistance also account for the degree of scientific confidence in the derivation of the values i.e smaller values used when there isn’t much research on the specific type of 5 failure mode. Factors associated with loads are normally independent on the type of material involved, but can be influenced by the type of construction. The characteristic strength and load are obtained from codes of practice Design code is a document that sets rules for design of new development. It is a tool that can be used in the design and planning process, but goes further and more regulatory than other forms of guidance commonly used. It is a mechanism which operationalize design guidelines or standards which have been established through master plan process. It ensures that aspirations for quality and quantity for development, particularly for large projects, are realized in the final schemes. That is, it has a potential to deliver the consistency of quality. Design stress  Design strength Grade stresses and moduli of elasticity for cypress and pine graded to KS 02 771 are given below (N/mm 2) The strength of timber is a function of several parameters including; Moisture content Density Duration of applied load Size of members Presence of various strength reducing characteristics e.g., slope of grain, knots, fissures and wane. The guidance on the use of timber in building and civil engineering structures in BS 5268; structural use of timber, divided into 7 parts. DESIGN PHILOSOPHY. The design of timber on BS 5268 part 2 is based on permissible stress design; in which stresses are derived on a statistical basis and deformation are also limited. Permissible stresses are calculated by multiplying the “grade stresses” given by the appropriate modification factors; K-factors, to allow for the effects of parameters such as moisture content, load duration, load sharing, section sizes etc. Applied stresses which are derived from the service loads should be less than or equal to the permissible stresses. A summary of the K-factors used for calculations of permissible stresses are given in the table below. K-factor Description. K2 Timber grade stresses and moduli for service class 3. K3 Load duration K4 Bearing stresses K5 Shear at notched end K6 Form factor; bending stresses for non-rectangular sections. K7 Depth factor; bending stress for beams other than 300 mm depth  K8 Load sharing system K9 To modify Emin for deflection in framer beams and lintels. K12 Slenderness in compression members K14 Width factor for tension members DESIGN CONSIDERATIONS a) Loading For design purpose, loading should be in accordance with BS 6399:parts 1,2,3 and CP 3, Chapter V: Part 2. b) Service classes Due to the effects of moisture content on mechanical properties of timber, the permissible property values should be those corresponding to one of the 3 described and summarized as i. Service class 1; refers to the timber used internally in a continuously heated building. The average moisture content likely to be attained in service conditions is 12% ii. Service class 2; refers to the timber used in a covered building. The average moisture content likely to be attained is 15% heated and 18% unheated. iii. Service class 3; refers to the timber used externally and fully exposed. The average moisture content is likely to be attained is 20%. For the service class 3 condition they should be multiplied by the modification factor K2 as shown below. Property Value of K2 Bending parallel to grain. 0.8 Tension parallel to grain. 0.8 Compression parallel to grain 0.6 Compression perpendicular to grain 0.6 Shear parallel to grain 0.9 Mean and minimum modulus of elasticity 0.8 c) Moisture content As moisture content affects the structural properties of timber significantly BS 5268 part 2, recommends that in order to reduce movement and creep under the load the moisture content of timber and wood-based panels when installed should be close to that likely to be attained in service.  d) Duration of loading, K3 Duration of load affects timber strength and therefore the permissible stresses. Because timber and wood-based material can sustain a much greater load for a short period (a few minutes) than for a longer period (several years) the grade stresses and joint loads may increased for other conditions of loading by modification factors given below, K3, by which all grade stresses (Excluding moduli of elasticity and shear moduli) should be multiplied for various durations of loading. Duration of loading Value of K3 Long-term (Dead + permanent imposed) 1.00 Medium-term (dead+ temporary imposed + snow) 1.25 Short term (Dead + Imposed+ wind) 1.50 Very short term (Dead + imposed + wind) 1.75 e) Length and position of bearing BS 5268: Part 2, Recommends that the grade stresses for compression perpendicular to the grain apply to bearings of any length at the ends of a member and bearings 150mm or more in length at any position. For bearing less than 150mm long located 75mm or more from the end of a member, the grade stress should be multiplied by the modification factor K4. Modification factor K4, for bearing stress. Length of bearing (mm) K4 10 1.74 15 1.67 25 1.53 40 1.33 50 1.20 75 1.14 100 1.10 150 or more 1.00 f) Notched end, K5 Notches at the end of flexural member generally result to high shear concentration which may cause structural failure and therefore must be taken into account during design. In a notched member, the grade shear parallel to the grain are multiplied by a modification factor K5 as shown. i. For a notch on top edge. ℎ(ℎ𝑒−𝑎)+𝑎ℎ𝑒 𝐾5 = for a≤he ℎ𝑒2 K5=1.0 for a>he ii. For a notch on the downside. ℎ𝑒 K5 = ℎ NB; BS 5268 notes that the effective depth, he should not be less than 0.5h g) Form factor, k6 Grade bending stress value given in the code apply to solid timber members of rectangular cross-section. For shapes other than rectangular the grade bending stress value should be multiplied by the modification factor K6 where K6= 1.18 for circular sections. K6= 1.41 for solid square section loaded diagonally.  h) Depth factor, k7 The grade bending stresses in BS 5268, is only applicable to the timber section having a depth of h=300 mm. For other depths of beams, the grade bending stresses are multiplied by a modification factor, K7, defined as i) Load sharing systems, K8 The grade stresses given in the code apply for individual members e.g. isolated beams, columns In load sharing system such as rafters, joists, trusses or wall studs spaced at maximum of 610mm centre to centre, and which has adequate provision for the lateral distribution of loads by means of purlins, binders, boardings, battens etc., the appropriate grade stresses can be multiplied by the load sharing modification factor K8, which has a value of 1.1. BS 5268 part 2, recommends that the mean modulus of elasticity should be used to calculate deflections and displacements induced by static loading conditions. Therefore, in load sharing system; Modification factor K8=1.1 Modulus of elasticity E=Emean j) K9 used to modify the minimum modulus of elasticity for trimmer joists and lintels. k) Compression members, K12 The grade compression stresses parallel to the grain given in the code are used to design struts, these values apply to compression members with slenderness ratio less than 5, when the slenderness ratio is more than 5, the grade stresses must be modified by K12. K12 takes into account the tendency of a member failing by buckling and also allows for imperfection such as out of straight and accidental load eccentricity K12 is based on the minimum modulus of elasticity, irrespective of whether the member acts alone or forms a part of load sharing systems. K13 for an effective length of spaced columns.