First edition published in Great Britain by Crosby Lockwood Staples 1976, reprinted by Granada Publishing Ltd 1981, 1982. Practical and special considerations 221 12.1 Deflection limits 221 12.2 Buckling 222 12.3 Deflection due to dead load on unpunctured beams only 223 12.4 Deflection due to wind uplift on roofs or wind on walls 223 12.5 Degrees of deflection due to assembly sequence 224 12.6 Examples of cases requiring special treatment at.
Preface
Truss-Joist MacMillan, Boisse Cascade, James Jones & Sons Ltd, Fillcrete Masonite and Finnforest Corporation for their contributions to chapters 8 and 10. Janet Brown, Andrew Hughes and Richard Adams at Arch Timber Protection, Knottingley, for their contribution on conservation in chapter 24.
The Materials Used in Timber Engineering
INTRODUCTION
In addition to expanding the range of existing materials, new materials that can generally be described as 'engineered wood products' have become available, ie. For the purposes of the allowable stress design code (BS 5268), these characteristic values are further reduced by including factors of safety to arrive at stress rate values or strength values for use in design.
TIMBER .1 General
- Strength grading of timber .1 General
- Commercial grades .1 General
- Sizes and processing of timber .1 General
- Moisture content and movement of timber .1 Measurement of moisture content
- Specifying timber
Before the introduction of the European grading standards, BS 4978 also covered machine grading of timber for use in the UK. Fortunately, most of the wood in the J&P, SLF, LF and Stud grades (see section 1.2.2.5) is available kiln-dried to a moisture content of 19%.
PLYWOOD .1 General
- Available sizes and quality of Canadian plywood
- Available sizes and quality of Finnish construction plywood
- Available sizes and quality of American construction plywood
- Available sizes and quality of Swedish construction plywood
Grade C–D' unpolished, with grade C face veneers of Group 1 species, posterior grade D veneers of Group 1 species, and inner grade D veneers of groups 1 or 2. Substrate C-D closed' , with Class C or Class C closed face veneers of Group 1 species, Class D posterior veneers of Group 1 species and Class D internal veneers and Group 1 or 2 species.
PARTICLEBOARD, ORIENTED STRAND BOARD, CEMENT-BONDED PARTICLEBOARD AND WOOD FIBREBOARDS
- General
- Environmental conditions
- Structural usage
- Particleboard
- Oriented strand board
- Cement-bonded particleboard
- Wood fibreboards
Tiles without moisture resistance characteristics will suffer irreparable swelling and loss of strength when wet – a situation that can be described as "wet Weetabix syndrome". Fiberboard type: state of use + purpose of application + load duration + load category. the last two are optional codes) so, for example, a heavy-duty hard plate for use in wet conditions for all types of loading is HB.HLA2.
ENGINEERED WOOD PRODUCTS .1 General
- Laminated Veneer Lumber
- Parallel Strand Lumber
- Laminated Strand Lumber
There is further classification and marking:. for use: general without symbol. load capacity for all load durations Load capacity for current or short duration load S. Load capacity is further classified as 1 for normal load and 2 for heavy loads. Regarding the properties of the material, it is currently necessary to rely on the certificate of consent.
MECHANICAL FASTENERS .1 General
- Nails
- Improved nails
- Staples
- Screws
- Bolts and dowels
- Toothed plate connector units
- Split ring connector units
- Shear plate connector units
- Punched metal plate fasteners
- Other fasteners, gussets and hangers
It may be necessary to 'tap out' the nut or 'run down' the thread before countersinking. For the joint slip properties of a particular plate, the manufacturers of the plate should be consulted.
ADHESIVES USED IN TIMBER ENGINEERING .1 General
- Weatherproof and boil proof glues
- Boil-resistant or moisture-resistant glues
- Interior glues
- Epoxy resins
- Gluing
The thickness of the plate must not be less than 0.9 mm and not more than 2.5 mm. The strength capacities of the various nail configurations are determined by test and published in the plate manufacturer's Certificate of Agreement.
Stress Levels for Solid Timber
- INTRODUCTION
- DERIVATION OF BASIC STRESS AND CHARACTERISTIC STRENGTH VALUES
- Background
- The UK methods up to 1984
- The methods in BS 5268-2: 1984
- The methods in BS 5268-2: 1996
- MODULUS OF ELASTICITY AND SHEAR MODULUS
- GRADE STRESS .1 General
- North American timbers
- Grade stresses for compression perpendicular to the grain
- Grade shear stress
- LOAD SHARING .1 General
- MOISTURE CONTENT
Grade stress is defined as 'the stress which can be permanently sustained with safety by material of a specific section size and of a particular strength class, or species and (strength) grade'. Two values are given for compression perpendicular to the grain in the strength class table (Table 7) of BS 5268-2.
Loading
- TYPES OF LOADING
- LOAD DURATION
- CONCENTRATED LOADINGS
- DEAD LOADING
- IMPOSED LOADINGS FOR FLOORS
- IMPOSED LOADINGS FOR ROOFS
- SNOW LOADING
- ROOF LOADINGS ON SMALL BUILDINGS
- WIND LOADING
- UNBALANCED LOADING
- COMBINATIONS OF LOADING
- SPECIAL LOADINGS
- Sliding doors
- Water tanks
- Roofs under high towers or masts
- Accidental loadings and disproportionate collapse
Not surprisingly, the magnitude of the concentrated load is often equivalent to the effect of 2.4 m of uniformly distributed load. This potential problem should be discussed in the design of such roofs and possibly in relation to the location of the building itself.
The Design of Beams: General Notes
RELATED CHAPTERS
DESIGN CONSIDERATIONS
EFFECTIVE DESIGN SPAN
LOAD-SHARING SYSTEMS .1 Lateral distribution of load
- Concentrated load. Load-sharing system
Where members are glued together to form a vertically laminated part with the strong axis of the members in the vertical bending plane, the bending, tension and shear stress parallel to grain stress can be increased by factor K27 while Eminand compression parallel to grain can be increased with factor K28 (see section 7.3.2). In a load sharing system, the lateral distribution of the load will reduce the effect on any member, especially if the deck is quite thick and can bear the effect.
LOAD–DURATION FACTOR
Provided this will not result in a significant difference in deflection between beams A and B, it is probably better to assume that beams B carry the total weight of the interface and part of the floor load, excluding any imposed load under the partition wall. For example, due to the duration of the load factors, it is possible that the design case of dead load is merely more critical than the case of dead plus imposed roof load.
LATERAL STABILITY
- Maximum depth-to-breadth ratios (solid and laminated members)
- Maximum second moment of area ratios (built-up beams)
Although this does not conform to a pedantic interpretation of BS 5268-2, it provides more restraint and more than meets the intent of the clause. Such an approach is conservative, it makes no recognition of the stabilizing influence of the web and tension flange or the location of the load in relation to the depth of the beam.
MOISTURE CONTENT
Use class 1: dry exposure conditions where the average moisture content will not exceed 12% (indoor use in buildings with constant heating). Use Class 2: Dry exposure conditions where the average moisture content will not exceed 20% for any extended period of time.
BENDING STRESSES
Wood that is fully exposed to outdoor use is defined as service class 3, which is characterized by conditions of wet exposure, where the moisture content of solid wood exceeds 20% for long periods. If the beams are placed at a moisture content of 22% or less in a situation where drying is free to proceed and only a fraction of the dead load is applied, there is very little likelihood of serious problems due to creep deflection.
DEPTH AND FORM FACTORS .1 Depth factor for flexural members
- Form factor for flexural members
The scale stresses in BS 5268-2 for bending members apply to solid and laminated timber of a rectangular cross-section (including square sections with the load parallel to one of the principal axes). For solid circular sections and solid square sections loaded in the direction of a diagonal, the grade stresses should be multiplied by the factor K6, as shown in Fig.
BEARING
- Bearing across and along the grain
- Plywood. Bearing on the face
- Plywood. Bearing on edge
The degree stresses in BS 5268-2 for compression (or bearing) parallel to the grain are not reduced to account for subsidence. BS 5268-2 does not give guidance on this point, but it would be sensible to calculate the reduced end grain area that would remain if tapering up to the grading limit occurred.
SHEAR .1 Solid sections
- Ply web beams. Panel shear stress
- Glulam
EfN=the E-value of the flanges taking into account the number of pieces N acting together in the whole beam, not just one flange. SXf = the first moment of area of the flange elements above (or below) the x–x axis.
THE EFFECT OF NOTCHES AND HOLES
- Shear stress concentrationsKh haah
- Shear capacity of a notch in the bottom edge
- Shear capacity of a notch in the top edge
The shear projection, bh, beyond the inner edge of the bearing line affects the shear capacity of the reduced section. Provided that a is at least 50% of the full section, the shear capacity of the full-depth section is realized.
SHEAR IN BEAMS SUPPORTED BY FASTENINGS AND IN ECCENTRIC JOINTS
When the top edge is trimmed (Fig. 4.15), the above formulas apply with ah measured on the inside of the bearing and bh measured horizontally from the actual depth point. In addition, the connectors and supporting members must of course be able to withstand the force F, and the entire shear must be checked on the beam.
GLUE-LINE STRESSES
In the special case of the flange-to-web connection of a ply web beam and the connection of plywood (or other board) to the outer beam of a glued tension skin panel (Fig. 4.24), clause 4.7 of BS 5268- 2 requires the allowable shear stress at the glue line to be multiplied by 0.5(K37). The maximum stress depends on the geometry of the beam section, the combination of shear force and moment and the load on the beam.
DEFLECTION .1 Introduction
- Deflection limits and appropriate E values
- Bending deflection and shear deflection
- Bending deflection
- Shear deflection
- Magnitude of shear deflection of solid rectangular sections
- Creep and dynamic effects
If more than one type of load occurs on a span, Fe is the sum of the individual FKm values. If more than one type of load occurs on a span, F0 is the sum of the individual FKv values.
BENDING AND SHEAR DEFLECTION COEFFICIENTS
Beams of Solid Timber
- INTRODUCTION
- GENERAL DESIGN
- PRINCIPAL BEAMS OF SOLID TIMBER
- Example of checking a previously selected floor trimmer beam (principal member)
- Example where the section, species/grade are to be determined
- LOAD-SHARING SYSTEMS OF SOLID TIMBER
- Example of checking a previously selected load-sharing floor joist system Check the suitability of 38 ¥ 235 C16 joisting spaced at 0.6 m centres over a clear
- Example where the section, species/grade are to be determined
- GEOMETRICAL PROPERTIES OF SOLID TIMBER SECTIONS IN SERVICE CLASSES 1 AND 2
- PRINCIPAL MEMBERS BENDING ABOUT BOTH THE x–x AND y–y AXES When the direction of load does not coincide with one of the principal axes of a
By inspection it can be seen that the 1.8 kN concentrated applied load is less critical than the uniformly applied load condition, i.e. Similarly, the equation for total deflection can be simplified to avoid the need to determine cross-sectional properties about the y-axis.
Multiple Section Beams
- INTRODUCTION
- MODIFICATION FACTORS
- CONNECTION OF MEMBERS
- STANDARD TABLES
- DESIGN EXAMPLE
The fastening arrangement between the members will depend on the relative loading on each side of the beam and must be such as to carry the corresponding change in loads in each component of the member. Determine a suitable 2-member section to carry a uniform dead load (including dead weight) of 3.95 kN/m at an effective span of 3.0 m.
Glulam Beams
INTRODUCTION
It is important to check the moisture content of the wood at the time of manufacture. There is also an element of flattening the laminates required to remove minor cupping distortion.
TIMBER STRESS GRADES FOR GLULAM
Either of these strength values may be used, but the modification factors in both cases are based on the C16 grade listed in Table 21 of the standard. For vertically laminated elements, the modification factors are only related to the number of laminates, regardless of the level of strength.
STRENGTH VALUES FOR HORIZONTALLY OR VERTICALLY LAMINATED BEAMS
- Horizontally laminated beams
- Vertically laminated beams
- Glulam beams with bending about both axes
This reduction does not apply to compression perpendicular to grain, shear, or modulus of elasticity. Stripping is not permitted with glulam, therefore the graded strength value for compression perpendicular to the grain is either the higher value given for the strength class or the species/quality value multiplied by 1.33.
APPEARANCE GRADES FOR GLULAM MEMBERS
The deflection of the radius d = and will be at an angle q to the direction of the load on the x axis, where tanq = dy/dx. Fixed. Not less than 50% of the sawn or planed surface to remove the protruding lamination.
JOINTS IN LAMINATIONS .1 Joints in horizontal laminations
- Joints in vertical laminations
Consider a horizontal laminated beam 540 mm deep of 12 laminates and calculate the allowable extreme fiber stress in bending for the outer laminate under medium load. When designing with C24 or C16 laminates and unable to provide an end joint efficiency of 70% or 55% efficiency respectively, clause 3.4 of BS 5268-2 requires the allowable stresses to be reduced accordingly.
CHOICE OF GLUE FOR GLULAM
With an element consisting of vertical laminates, the stress grades that apply are those for solid wood (eg C24, C16, etc.). Efficiency ratings for a range of commercial finger joint profiles are given in Annex C of BS 5268-2 and in Table 7.3.
PRESERVATIVE TREATMENT
If a waterborne process is used, it is likely that some amount of surface deterioration will occur and either time must be allowed for air drying or the cost of oven drying must be factored in (which can also lead to further surface deterioration (cracking)). If the treatment is to be carried out on finished elements, it will probably be necessary to delay the treatment until the adhesive has fully cured (which may be seven days or more) and compatibility must be checked.
STANDARD SIZES
If individual laminates are preserved by an organic solvent process after machining, this will not affect the moisture content or cause surface deterioration. There is some doubt as to whether water repellants or other additives can be included in the preservative.
TABLES OF PROPERTIES AND CAPACITIES OF STANDARD SIZES IN C24 GRADE
- Introduction
- Horizontally laminated beams
- Vertically laminated beams
The capacities of sections are given in Tables 7.4 to 7.8 for C24 horizontally laminated single grade beams determined from the following considerations. The capacities of sections are given in Tables 7.9 to 7.13 for C24 vertically laminated beams determined from the following considerations.
TYPICAL DESIGNS
- Typical design of C24 grade laminated beam loaded about major axis
- Typical design of C24 grade laminated beam loaded about both the major x–x and minor y–y axes
The shear stress in a rectangular section will be at its maximum along the y–y axis for loading about the x–x axis and will similarly be at a maximum along the x–x axis for loading about the y–y -as. Therefore, it is only at the intersection of the x–x axis and y–yas that the combined shear stress reaches its maximum value.
THE CALCULATION OF DEFLECTION AND BENDING STRESS OF GLULAM BEAMS WITH TAPERED PROFILES
- Introduction
- Formulae for calculating bending deflection of glulam beams with tapered profiles
- Example of calculating bending deflection of a tapered glulam beam
- Formulae for calculating shear deflection of glulam beams with tapered profiles The formulae for calculating the shear deflection of glulam beams with tapered
- Example of calculating shear deflection of a tapered glulam beam
- Formulae for calculating position and value of maximum bending stress of glulam beams with tapered profiles
Calculate the flexural deflection at mid-span (a) considering the elemental strips and using the strain energy method and (b) using the coefficients from Fig. Calculate the mid-span shear deflection for the beam profile and load condition shown in the figure.
Thin Web Beams
INTRODUCTION
PRIMARY DESIGN CONSIDERATIONS
- Bending deflection
- Shear deflection
- Bending
- Panel shear
- Web–flange interface shear
- Rolling shear stress
The shear deflection will be a significant proportion of the total deflection (often 20% or more), and must be taken into account when determining deflection of thin web beams. If the surface grain of the plywood runs perpendicular to the general grain direction of the wood in the flange, the roll-off occurs at this interface.
DESIGN EXAMPLES
- Design of full depth web section
- Design of rebated web section
- Introduction
- Stresses in ply web splices
The allowable stress is the rolling shear stress of the plywood, modified by the load duration factor K3. The maximum resultant stress acting on the adhesive area of the weld plate is the vector sum of tv and sm.
WEB STIFFENERS .1 Introduction
- Canadian design method for Douglas fir ply webs
- A method for Finnish birch-faced plywood
Currently there are no recommendations for the distance between stiffeners in Finnish birch plywood (or all-birch plywood) comparable to the method given in section 8.5.2 for Douglas fir, and the following suggestions have been given in previous editions of this manual on the more general recommendations which Hanson set out in his book Timber Engineers' Handbook (Wiley, New York, 1948). Applying this formula to the range of Finnish plywoods listed in Tables 40–46 of BS 5268-2 gives a maximum slenderness ratio hW/t @ 28 for all grades and thicknesses.
HOLES OR SLOTS IN PLY WEB BEAMS
The above formulas are based on the assumption that the moment MA produces a bending stress given by the first part of each formula, while the shear is divided between the upper and lower parts of the beam in proportion to the sectional stiffness of the upper part. and the lower parts of the beam. For slender web beams, it is usually convenient to form a symmetrical crack about the mid-depth of the section, and then reinforce the perimeter of the crack.
PROPRIETARY SECTIONS .1 Range of sections and properties
X = distance from section AA to BB ZA = section modulus of the entire section at AA Zu = section modulus of the section at BB above the gap ZL = section modulus at BB below the gap (EI)u= stiffness of the section section at BB above the gap (EI)L = section stiffness at BB below the slot. In the case of simple span beams, the gap is often provided at mid-span where there is no shear under conditions of symmetrical uniform loading, and the gap may be of a clear depth hw between the flanges, provided that the remaining section of the flange is adequate to resist the moment MA.
