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There is a long tradition in Britain of building in wood. However, as emphasised by Yeomans (1999), timbers role is often overlooked through a combination of most architecturally interesting buildings being of stone construction and timber being a perishable material, hence timber structures of an early period no longer exist.
During the Medieval period timber construction was 'craft based' as the visual properties were more important. Buildings at this time were a sign of wealth and status; hence, the majority of housing existed as single storey 'barn like' building i.e. inexpensive. It was not until the second half of the 13th century that first storey floors were accepted into general domestic use. The carpenters of these early floors preferred to lay the joists flat; they did not have the understanding of structural behaviour to know that greater strength is in increased depth. This form of construction was also adopted for ground floor constructions (Douglas, 1997). Paycocke's House, East Street, Coggeshall, Essex is a 1505 timber framed house that demonstrates the orientation of joist used at the time, 1 (Hewett, 1999) shows that the joists used (in Paycocke's) were laid horizontally.
Oak was the core structural timber used in medieval Britain. Towards the end of the 15th century, overseas travel increased the availability of different materials; with the UK oak supplies depleting, softwoods took its place.
The Early Period (17th and 18th centuries) saw the development of scientists understanding of structural behaviour. This began with Galileo's tension testing in 1638 (Booth, 1999) where he observed that the tensile strength was proportional to cross-sectional area. Progressing to Coulomb's work in 1773 where the correct moment resistance of beams was established. This progression in knowledge led to technical publishing and to the change in the orientation of joists; as it was now understood that the joists strength is greatest when lay upright. This is the orientation adopted by today's floors.
Not only did the orientation of the joists change during this period but so did the span of the joists. Previously the joists spanned perpendicularly to the external walls onto a central secondary(summer) beam that was carried by a cross girder, but during the Early Period Parissen (2008) reveals that joists spanned straight onto girders and all in the same direction.
Softwoods were commonly used at this time, with Scandinavian Fir replacing British softwoods; hence Walker and McGregor (2001) concluding that many construction techniques found at this time originated from Europe and Scandinavia i.e. knowledge was being shared.
The Modern Period (19th and 20th centuries) saw methods for calculating the strength of beams being tabulated in the UK by Nicholson and Tredgold (1845). This is a significant development for timber floors as today's method of designing floor joist is to refer and use TRADA span tables.
Laminating techniques were also developed in the 19th century as manufacturers were looking towards more economical sections "but this did not reach its full potential until durable adhesives were introduced during the Second World War" (Sunley, 1985). This and the process of manufacturing timber in factories to reduce labour costs led to the development of Plywood and Panel Products widely used today.
Within the recent domestic market there has been a demand for greater flexibility in terms of internal planning, this has come about because of the current economic climate where families are choosing to alter their homes as apposed to moving to a different property. Therefore, the construction industry must look towards solutions that provide a cheaper way of adapting layouts in the future. The current mode of doing so is to use roof space or remove internal walls. These walls are often load bearing as the maximum clear span of the traditional floor joist is limited, which means new steel beams have to be installed to support the first floor. However, if the first floors comprised of joists capable of greater spans, the partition walls would largely become non-load bearing, therefore altering the internal layout would not affect the structural stability of the house.
In 1998 the Egan Report 'Rethinking Construction' highlighted the need for innovation in design and construction. The targets within this report included reducing capital costs by 10%, reducing construction time by 10%, and reducing the number of defects on handover by 20% (Egan, 1998).
The importance of Egan's report can be seen by looking at timber's market share at that time. Within the domestic floor market alone, from 1986 to 1997 the suspended timber floor market share dropped by 7%. It lost its market share to precast concrete floor solutions (data sources NHBC and TRADA 1997) as the site labour skill was minimal and the supply was easy; this shows that at the time of the Egan Report traditional timber was not satisfying the construction industry or client requirements i.e. a need to re-engineer to improve "specifying, purchasing, delivery and performance" (Bainbridge & Milner, 1999).
Both TRADA and SCI recognise that to satisfy these targets and increase market share there needs to be an improvement in the construction project process through product development, increasing the use of standardised products, improving productivity and meeting the changing construction and client requirements.
Timber already has made substantial project process improvements with the trussed rafter manufacturing business model. The trussed rafter model was brought over from the US in the 1960s; it introduced machines and jigs to join the timber members together i.e. the use of factory pre-fabrication. Similarly, the UK timber frame industry has industrialised techniques whereas, in comparison to global competitors in North America (major exporters to the UK of structural softwood) who still continue to join members on site (Sunley, 1985). Milner and Bainbridge (1999) cited the timber frame market at 11,000 units per year. Therefore, there is a need for high performance floor solutions as they are the principal component of the timber frame building.
TRADA's answer to re-engineering is to push forward factory pre-fabrication. This process, according to Miler and Bainbridge's article in The Structural Engineer (1998), has been recognised by the construction industry as a way to improve productivity and meet client requirements. Although papers on this subject are largely from one publisher, TRADA, it is a process that has been proven to work for trussed rafters and it has restated timber floors as a product. This floor product provides technical and design support and offers a guarantee of performance, whereas the current traditional approach to floor design is to use span tables published by TRADA. This traditional method predominately involves on site erection, where the materials are stored on site; but these dry components can be damaged in wet weather conditions as higher moisture content reduces strength and the drying out process distorts the section (Abbott, 2008). Hence, a move towards factory production tightens quality control as all handling, selection, checking, drilling etc. is done in the factory (a controlled environment) by operatives with the correct understanding and skill.
In addition to prefabrication, the re-engineering process also satisfies the objectives of the Egan Report (1998) through an improved way of using the material itself. Benefits are added through the use of structural timber composites e.g. timber I-joists with a plywood web, as they reduce the variability that natural timber has; this increases the average material properties and increases the minimum design values. Value is also added by adopting alternative construction techniques e.g. stress skin panels where the load sharing properties are accounted for; this minimises over design by optimising the structural properties of the floor; this in-turn has financial benefits.
The ability to integrate services i.e. pipes and cables within the floor void, needs to be incorporated into the re-engineering process, not only to minimise floor depths but to reduce the risk of damaging services incurred through site fabrication by the nailing or screwing of panels to joists. By incorporating the services as part of the pre-fabrication, the level of site operation is reduced but also the standards for notching and drilling (BS5268 part 2) can be accurately adhered to, therefore the structural integrity is upheld. Through re-engineering the need for drilling and notching, for services, can be completely eliminated, for example the open web joists already have spaces within the joists for the services to pass through.
The first stage resulted in heavier floors that required more material. The heavier floors incur installation problems, as handling becomes a health and safety concern, but significantly the sections are harder to dry out, hence the full structural strength could be compromised. The second stage, again, improved the floors stiffness but the adhesive used required the correct specification to ensure the mixing and curing process was correct. With a stiffer deck the system is heavier but it has an increased serviceability performance due to the composite action achieved. From this second stage the benefits of using a stress skin panel are taken to the final stage. Within the final stage, larger sections are available; this widens the market for timber floors. Using the structural composites minimises the variability that you get with natural timber. However, this fully prefabricated system has a higher unit cost than the traditional method, but this cost is outweighed by the increase in quality control and structural performance. TRADA (Bainbridge & Milner, 1999) conducted research into this stage by modelling a large domestic floor with a span of 4.85m (this span is approximately the optimal span of a traditional floor joist) and compared the performance of traditional and prefabricated floor systems. The result was that the prefabricated floor system saved 20% in joist material, was capable of spanning further and could carry higher design loads i.e. more economic. Although prefabricated floors have been proven for the above application and problem, traditional floor systems are still prevailing, according to Milner and Bainbridge (1998) this is due to obstacles caused by regulatory requirements. However, these systems are being used; therefore, the relevant bodies have already approved them, so the only obstacle is the supply chain. If the designer looks beyond the conventional TRADA span tables when designing, the market share of these products would increase.
The souce of the timber product became a large concern in the 1990s as the timber market became aware of illegal logging and poor forest mangement, especially in developing countries (CPET, 2007). As a result of the growing concern the Central Point of Expertise on Timber (CPET) was eatablished in 2002. The CPET certifies providers of sustainable forest and legal timber trade. The CPET main aim is to provide the UK Governement's procurement personnel with support implementing the procurement policy (July 2000); this policy requires all Government departments to buy timber from legal and sustainable. The first assessment of CPET was made in December 2004; the assessment examined five existing citification schemes, of these five only two were found to offer proof of legality and sustainability, the Forestry Stewardship Council (FSC) and the Canadian Standards Association (CSA). Therefore, the full-scale implementation of CPET was delayed by six months to allow further bodies to offer assurance. After this time, two more bodies satisfied the CPET's requirements. Hence, four schemes now have CPET approval, FSC, CSA, the Programme for Endorsement of Forest Certification Schemes (PEFC), and the North American Sustainable Forest Initiative (SFI).
The FSC is a UK based scheme, SFI North America, CSA Canada, and PEFC Switzerland. It makes sense to have approved schemes across the globe, in particular in areas of large export. However, there is no set standard that these schemes work to, for example, the CSA tests to Canadian, US, or European standards. Therefore, the question must be asked, what the differences between these schemes are and what impact they have on the timber provided.
The UK Government policy is to treat these schemes as equivalents however "there have been a number of detailed and respected reports on the relative merits of the various schemes, carried out by organisations ranging from FERN to WWF and the World Bank" (Cliffe, 2005). Reports produced by FERN, a non-governmental organisation, include 'Footprints in the Forest' (Ozinga & Krul, 2004). Within this report it was found that SFI, CSA and PEFC do not have clear minimum performance thresholds for effective forest management, but FSC does. Without minimum standards how is it possible to assess this requirement. SFI and CSA also allow for an individual forestry company to customise the standard against which it will be certified; this has positves and negatives. Positive in the respect that UK forests can be certified against UK stadards. Negative as this proceedure offers no consitency in the certification process as national standards differ. CSA and FSC make readily available certification reports to allow the consumer to make informed choices. From the report, FERN concludes that FSC provides the best standard. This is an opinion which is justifible from the first comparision point relating to minimum stadards. How is one forest differentiated from the other without minimum requirements to compare against. Hence the UK Governements statement regarding the equivalency of the four schemes, (Forestry Commission, 2009), should be reviewed.
1.3.2 Strength Grading
Under the term 'timber' there is many different species. These species are classified as either hardwoods or softwoods. As defined by Ozelton and Baird (2002), hardwoods are timber of a deciduous tree and softwoods are timber of a coniferous tree. It doesn't mean literally hard or soft. The majority of timber used in the UK construction industry is softwood, as previously mentioned. Approximately 80% of the imported softwood supply comes from 'Norway, Sweden, Finland, Russia, Poland and the Czech Republic with the balance mainly from Canade and the USA' (Ozelton & Baird, 2002). These softwoods are largely imported but the use of home grown softwoods has recently increased. However hardwoods are used for for applications such as farm buildings and restoration works.
The first timber code CP112:1952 specified that structural timber should be obtained by the practice of gaining a commercial grade then using visual inspection, assign an approporate structural grade. At present timber is strength graded either by visual or machine methods. Strength grading, or stress grading as used in British Standards, is 'the process by which indivual pieces of sawn structural timber are sorted into grades to which strength values are assigned for each species' (British Research Establishment, 1996) and was devleoped as a way to overcome the variability of timber properties.
Visual grading visually assesses each piece against permissible defect limits given in the standards listed below. The visual grading rules define the size, type and number of characteristic that reduce overall strength e.g. knots, wane, distortion and slope of grain. The grader assesses the section and then stamps to notify the supplier of its grade, either General Structural (GS) or Special Structural (SS).
For the visual strength grades, bending strength is influenced primarily by the presence of knots as they reduce the first moment of are of the section. Therefore, it is important to know the are occupied by the knots, known as the knot area ratio (KAR). BS 4978 defines the KAR as 'the ratio of the sum of the projected cross-sectional areas of the knots to the cross-sectional area of the piece'. The code also states that knots less than 5mm can be disregarded and there should be no distinction between knot holes, dead knots, and live knots. A knot near the edge of the section has a greater effect on the bending strength than a knot at the centre because, a knot at the edge weakens the edges meaning they could shear off under loading leaving a smaller cross-section and hence a lower first moment of area. Therefore, BS 4978 introduced the concept of a margin and a margin condition. The margin is an outer quarter of the cross-section and the margin condition is known as the margin knot area ratio (MKAR). MKAR is 'the ratio of the sum of the projected cross-sectional areas of all the knots or portions of knots in a margin to the cross-sectional area of that margin' (Ozelton & Baird, 2002). A total knot area ratio (TKAR) is also used in the grading process and is defined as the ratio of the projected cross-sectional area of all knots to the cross-sectional area of the piece.
For a SS grade, the MKAR must not exceed ½ and the TKAR must not exceed â…“, if the MKAR exceeds ½ then the piece can still be graded as SS as long as the TKAR does not exceed 1/5. For a GS grade, both the TKAR and MKAR must not exceed ½, if the MKAR exceeds ½ then the TKAR must not exceed â…“ (Ozelton & Baird, 2002).
Upon the assessment of the knots, if no more than 3mm is removed from a 100mm section and 5mm from a larger section, then BS 4978 states that the initial grade remains unchanged. If the section is surfaced or sawn past these restrictions then the section must be re-graded. If, however, the length of the piece is cut the grade is not reduced and may be increased if the defect is removed.
Machine grading measures parameters such as stiffness, which is directly related to strength. The mahcine grades each individual piece and again stamps the section. A further visual inspection is then carried out to assess strength reducing characteristics not routinely sensed by the machine. ADVANCES.
The British Regulations require structural timber to be both graded and marked by approved bodies certified by the UK Timber Trade Federation. Stresses assigned within the British Standards (BS 5286-2:2002) apply only to timber that is graded to the following
For visually graded softwoods:
* BS 4978:1996 Specification for visual strength grading of softwood.
· ECE Recommended Standard for strength grading of coniferous sawn timber. 1982.
· National Grading Rules for Dimension Lumber (NGLA), Canada, 1994.
· National Grading Rules for Softwood Dimension Lumber (NGRDL), USA, 1975.
For machine graded softwoods:
· BS EN 519:1995 Structural timber. Grading. Requirements for machine strength graded timber and grading machines.
· North American Export Standard for machine stress-rated lumber, 1987 (NAMSR).
The grading systems above deal with the size of the timber piece in different ways. TRADA (2006) note that BS 4978 grading system assigns grade stresses independent of the size of the timber piece, therefore modification factors (k factors) are included in the design stage. EXPAND. The North American systems are baased on the function the timber will serve; this relates to size. Therefore no modification factors are required as size has been accounted for. Hence in the British Standards the North American timber of the same grade and species but of different sizes can appear in different strength classes. Therefore, it becomes difficult for the designer and supplier to produce alternative designs, if the preferred option is unavailable or too expensive, as there are so many species and grade combinations. If the above grading standards where more parallel then the number of combinaions would decrease and the design process would become more efficient with fluctuating timber prices.
Grade stresses in BS 5268-2:2002 are based on the timber being service class 1 or 2 therefore modification factors are given by which the stresses and moduli should be multiplied to give stress values appropriate for service class 3. Distinction between service classes is explain in Section 2.3.5.
The stresses also only apply to long term loading, such as dead load. Again a modification factor (k factor) needs to be applied to the stress if the applied loading is deemed to be medium term, short term or very short term i.e. snow load would be very short term in this country, but for North America the snow load would be deemed medium or long. Hence, determining the grade stresses for long term loads allows the grade stresses, obtained by numerous systems, to be more widley applicable.
'A strength class is the grouping of timbers that possess similar strength characteristics irrespective of species.' (Ozelton & Baird, 2002). Specifying a strength class ensures the designer knows that the timber selected with be an economically viable solution.
Visual grading produces two principal strength classes, General Structural (GS), and Special Structural (SS). As the strength reducing characteristics varies between species, the strength properties of one SS grade of one species may be different to thee SS grade of another species. This becomes an issue for designer and suppliers seeking alternative solutions for the reason mentioned in Section 2.3.2. Hence, in 1984, BS 5268 introduced a strength class system; however, this has now been replaced by the European system BS EN 339:1995.
The current British Standard BS 5286-2:2002 has adopted the European strength system BS EN 338:1995. The system 'divides the strength requirements for structural timber into visual grade and species is assigned to a class on the basis of its mechanical properties' (British Research Establishment, 1996). There are eightteen defined strength classes; these are twelve softwoods prefixed with a C and six hardwoods prefixed with a D. C14, C16, C18, C20, C22, C24, C27, C30, C35, C40, C45, C50, D30, D35, D40, D50, D60 and D70. With the most readily available softwoods being C16, C24 and C27. The numbers represent the characteristic bending strength (TRADA, 2006); these are the values used for design to Eurocode 5, which uses a limit state approach. BS 5286-2:2002 replaced the 1996 edition to run parallel with Eurocode 5. According to TRADA (2003) BS 5286-2:2002 is based on a permissible stress approach, although it does now refer to European standards for material properties that have been prepared for the limit state approach. This revision was a necessary step towards the full integration of Eurocodes in 2010.
The current regulation for which all softwood timber dimensions have to meet is the European CEN Standard BS EN 336, the sizes are referred to as target sizes. This must be followed for both BS 5268 and Eurocode 5 designs.
Target size is defined as the size of the section at 20% moisture content that is required by the designer, and is the size that is used in calculations. However, deviations and tolerances are not included. Work size is the size of the section at 20% moisture content before any machining is carried out to achieve he target size. The allowable tolerances for target sizes are split into two categories:
Tolerance class 1 (T1): Dimensions < 100mm +3,-1 mm
Dimensions > 100mm +4,-2 mm
Tolerance class 2 (T2): Dimensions < 100mm +1,-1 mm
Dimensions > 100mm +1.5,-1.5 mm
T1 is commonly used for sawn sections and T2 is more commonly used with surfaced, planed, or processed dimensions. It is common practice for UK designers to assume that 3mm is removed when processing or surfacing sections up to 150mm initial dimension and 5mm for initial dimensions greater than 150mm (Ozelton & Baird, 2002). If designers are assuming a tolerance level then they are not utilising the current standards. They could design solutions that are more economical if tolerance classes were used and taken note of, instead of general assumptions. To follow BS EN 336 the designer should, for example, state 47mm(T2)x100mm(T2) to clarify their requirement, then the need to assume a tolerance wouldn't be necessary.
The national annex to BS EN 336 tabulates the most readily available length and cross-sectional sizes with expected tolerances, therefore the designer should use this table to avoid supply difficulties and cost penalties. These sections and tolerances assume moisture content of 20%. For other moisture contents, the code states that the change in dimension from the base value should be taken as 0.25% change in the face dimension for a 1% change in moisture content. For moisture contents over 30% it should be assumed that the dimensions remain the same. Lengths of timber are available in 300mm increments up to a common limit of 3.9m. Greater lengths, inherently, incur costs.
In North America section sizes and target sizes are deemed as tolerance class 2, but 10% of pieces of NLGA graded timber are permitted to have tolerance levels in accordance with section 747 of the NLGA. These tolerances are in inches and in addition to drying reductions (Ozelton & Baird, 2002). This may confuse the designer or supplier if T1 or T2 has been specified with dimensions in mm.
Discrepancies in size descriptions is deepened as the sections can be referred to as sawn, regularized, planed all round, or surfaced four sides. A log delivered to a mill is sawn to large balks or flitches for shipment. The purchaser either re-saws the section or directly uses the section. Sawn timber is commonly used when dimensional tolerances are not important e.g. for roof rafters, 3mm will not interfere with the appearance of roof tiles. When dimensional requirements are set e.g. floor joists must provide a consistent top and bottom surface to receive floor and ceiling finishes, then the two parallel surfaces are planned, this is regularising and coded as S2S. For an exposed member where all four faces have dimensional requirements, all four faces are planned. This is called planed all round or surfaced four sides and coded as S4S.
Moisture content is defined as 'the amount of water in the timber expressed as a percentage of its oven-dry mass' (British Research Establishment, 1996). The moisture is found in the cell cavities, when dried to a moisture content of 30% Hodgkinson (1980) states that the free water has evaporated and the timber reaches the fibre saturation point. With more drying the moisture leaves the cell walls, the timber shrinks, and the strength and stiffness increases due to the hardened cell walls. This continues until equilibrium is reached with the surrounding atmosphere.
For that reason, moisture content is an essential natural parameter to determine as it directly affects strength, size, susceptibility to attack by fungi, and durability of the timber (a lower moisture content means a higher strength). Both BS 5268-2:2002 and Eurocode 5 classify timber as service class 1, 2 or 3. Service class 1 has a moisture content that relates to a temperature of 20°C and a relative humidity of the surrounding air that does not exceed 65% for 4 weeks each year. Service class 2 is the same as service class 1 except the relative humidity should not exceed 85%. Service class 3 is classified as having a higher moisture content than service class 2.
Moisture content is important to consider during the grading process. BS 4978, BS 5756 and BS EN 14081-1 state that timber is either dry-graded or wet-graded. Dry-graded is deemed to be service classes 1 and 2 with an average moisture content of 20% (19% for North American timber). Wet-graded timber has a moisture content exceeding 20% and is classified as service class 3. Wet-graded timber is not required to be stamped, therefore, unmarked timber should be assumed to be service class 3 (TRADA, 2006). With moisture content dictating the strength then all timber should be stamped with a service class to ensure the design of the timber is the most economical solution.
Another short fall of the grading system is that timber sections over 100mm in thickness are inherently more difficult to dry (TRADA, 2006), therefore, they are graded and installed wet. Hence, in design, wet stresses must be considered and shrinkage must be understood. BS 5268-2:2002 adopts modification factors that should be applied to stresses obtained for service classes 1 and 2 so service class 3 members can be more economically designed. The implications of these modification factors have been discussed in Section 2.3.2.
Moisture content is determined by weighing the timber, oven drying and then re-weighing, hence a weight of water can be determined. This is a destructive test laid down by BS 373 with the result being an average value for the sample tested. A less timely method is to use a moisture meter. The moisture meter measures the electrical resistance of the timber using two probes, one at either end. Hence, with a higher water content the electrical resistance will be low. The problem with this method is that individual species of timber have their own level of resistance; therefore, the meter needs to be calibrated. However, even within a single species the resistance inherently varies. In addition, as stated by Ozelton and Baird (2002), the depth to which the probe is inserted at the end also affects resistance readings as the moisture content within the surface layer (20mm) differs to the moisture content in the core. Therefore, the accuracy of this method is dictated by the ability of the manufacturer to calibrate the meter. Even so, as one species can vary to another and even within the same species, the meter cannot be used as the sole method, but it is deemed a satisfactory way of checking the timber on site (British Research Establishment, 1996) and gives an instant reading to within ±2% of the true moisture content (Hodgkinson, 1980).
The movement of timber does not occur until drying reaches the fibre saturation point, explained above. At moisture contents below this point the dimensions of the section change; these movements are linearly related to the change in moisture content for moisture contents between 8% and 30% (fibre saturation point). This movement has a more significant effect on timber-framed buildings as the vertical members that are installed as service class 1 or 2 could dry down to a moisture content of 12% in a heated building (Ozelton & Baird, 2002).
As the movement is dimensional and natural, the orientation of the cross-section is an important parameter. The movement 'is about 40 to 50 times greater across the grain than along it and may be between 1.5 and 2 times greater tangentially than radially' (Hodgkinson, 1980). The process of converting logs into beams means that the movement is a combination of tangential and radial effects; therefore, an average value is taken.
An example of the affect of this movement can be found in timber-framed platform construction where most cross grain movements result from the timber joists. A typical 200mm service class1 joist that dries in a heated building to 12% moisture content could move 4mm. In comparison, a typical 2.4m high stud supporting the floor that would move 0.8mm in its length under the same conditions. The movement of 4mm may seem small, but in terms of deflection and connection details, it could cause disruptions.
Following on from Section 2.3.5, a change in moisture content will result in dimension changes due to differing characteristics both tangentially and radially to the growth rings. If the amount of the tangential change increases and moisture content decreases, the cross-section will 'cup'. This distortion is of importance when gluing timber together as the glue lines will move away from one another. To overcome this, the orientation of the members should be such that upon cupping they nest together. However, this solution has limitations, the maximum moisture content difference between any piece at the time of gluing and in its equilibrium state, cannot exceed 5%, with a maximum difference of 3% between adjacent pieces.
The direction of the grain within a timber section does not coincide with the common geometrical axes. Therefore, the effects of moisture change result in asymmetric movement. This gives rise to bow, spring and twist of the section, which are worst locally at knots.
Twisting has been found to be caused by spiral grain with timber cut from the centre of a log being prone to more twisting (Johansson, 2002). At present timber is sorted primarily on the basis of slope of grain, but as the location the timber is taken from is also important methods of automatic laser measurement have been under investigation by Nystrom (2002) with triasl being carried out in Sweden (Holland & Reynolds, 2005).
Bow and spring is caused by an inbalance in longitudinal shrinkage. This shrinkage is a result of an inbalance between juvenile wood and compression wood from one side to another. Fast growing timber has a high proportion of juvenile timber. Whereas compression wood is a reaction that affects softwwods that have grown on slopes, and the wood below heavy branches (Desch & Dinwoodie, 1996). Scanning is currently being used to determine the amount of juvenile and compression woode in each face, but, this is merely a way of sorting the timber. It would be more beneficial to implement a drying and handling process that would limit the distortion as apposed to merely identifying it.
Timber has a low thermal conductivity value, therefore for floor joists it is not necessary to consider thermal bridging as the floor joists are part of a thermally insulated system. Thermal bridging is where heat is allowed to flow through a system that contains materials that are poor insulators. A typical value for timber is taken as 0.14W/m K (Ozelton & Baird, 2002) or 0.12W/m K (Eurocode 5 2004). This value may also be adopted for plywood. The use of fibreboards in floor systems can make a significant contribution to the insulation of the building as they have lower values of thermal conductivity.
The thermal conductivity of timber increases with temperature as the level of moisture decreases, this principle will be discussed in section XXX where timbers behaviour in fire is considered.
Under regular climate conditions, the movement of timber is small and usually neglected, except for joists of longer spans where the movement is more noticeable and may have a greater effect.
In fire, damage is the result of the thermal expansion of the material. This leads to disruptions at connections and supports before failure of the member itself. When subjected to fire, the core of the timber joist effectively remains at room temperature due to the low thermal conductivity of the outer charred layers. Combined with the low coefficient of linear thermal expansion timber suffers little trouble due to thermal movement. Timber has a coefficient of linear thermal expansion about a third of that of steel and concrete (4x10-6/°C), therefore its behaviour in fire is more favourable.
Timber's behaviour in fire will be discussed further in Section XXX.
BS 6566 defines plywood as 'a wood based panel product consisting of an assembly of plies bonded together, some or all of which are wood'. It is the most variable and oldest panel product, examples have been found in Egyptian tombs (Dinwoodie & Enjily, 2004). Plywood is not the first choice for use in domestic suspended floors but is in the commercial sector due to its impact resistance and high stiffness. Because of the high stiffness, plywood is more commonly being used within high structural performance systems such as stress skin panels. More recently, the use of plywood has extended to engineered sections such as I-joists and box beams.
At present plywood is made from softwoods mainly imported from Scandinavia, USA and Canada (Ozelton & Baird, 2002). Imports from other countries may not comply with European Regulations, so the source of the softwood must be checked before use in construction.
There are four principle stages in plywood manufacturing. Step 1involes selected logs being rotated and sliced into veneers then cut into panel-sized widths; this is known as peel and cut. Step 2, known as dry and sort, is where the veneers are died to a moisture content of between 4% and 8%. At this stage, strength-reducing characteristics such as knots can be removed. Within step 3 the veneers are coated with adhesive and assembled so the direction of the grain of alternate layers are perpendicular to one another; this is known as glue and assemble. The final step, step 4, is the press and finish stage. The glued and assembled products are fed into a hot press so the glue is cured. After the pressing, the sheets are cut to size. (Manufacturing steps as laid out by BRE (1994)). Adhesives will be covered within Section XXX
The plywood either contains an unequal or equal number of plies. The unequal system means that the piles are balanced around a single central veneer. Whereas the equal system has plies balanced around two central plies that have the same orientation. The number of plies the plywood has dictates the performance as the thickness and stiffness increases. These multi-layered plywoods are known as veneer plywoods.
Structural applications of plywood, in Europe, must satisfy the Construction Products Directive by complying with the European Harmonsied Standard BS EN 13986. Currently the design of structural plywood can take two approaches, either testing or calculation.
Testing ensures the most economical use of the plywood but is only regularly used when large numbers of identical products are being produced from the same design, as the procedure does not allow for test modification. The current standard to which floors, if tested, must comply with is BS EN 1195. The test results can be converted into design loads that can be used for either permissible stress of limit state design, depending on whether the design will be designed using BS 5268 or Eurocode 5.
Design by calculation, in comparison, is more conservative but less costly. For calculations carried out using permissible stresses the working stresses are given in BS 5268, this is current UK practice. Calculations carried out using limit state design use characteristic stresses found in Eurocode 5. The limit state approach is soon to be adopted in the UK; hence, there is a transitional period underway. This transitional period is highly confusing as there are now twice as many regulations with which designers could comply with, which leaves the question as to which ones to adopt. UK designers are reluctant to use European standards until they have to, especially those who have been in the industry for many years and base most design on experience. A good example of this would be that previously for a certain set of design parameters, experience demonstrated design compliance and a set of 'deemed-to-satisfy' tables were produced in BS 8103-3. These tables provided a panel thickness for a given joist spacing and load. The tables were produced using a different impact test method to that in new European codes. Hence the tables 'do not comply with the European Harmonised Standard and EN 12871 and by 1 April 2004 they must be withdrawn from use and they must not be used' (Dinwoodie & Enjily, 2004). These tables had proved to work satisfactory before to it asks the question of whether the permissible stress method is comparable to the limit state method.