The use and efficiency of engineered sections in flooring systems

The use and efficiency of engineered sections in flooring systems

1 Literature Review

1.1 The developments in lumber floor traditions ( eleventh century – early twentieth century )

There is a long tradition in Britain of edifice in wood. However, as emphasised by Yeomans ( 1999 ) , lumbers function is frequently overlooked through a combination of most architecturally interesting edifices being of rock building and lumber being a perishable stuff, hence lumber constructions of an early period no longer be.

During the Medieval period lumber building was ‘craft based ‘ as the ocular belongingss were more of import. Buildings at this clip were a mark of wealth and position ; hence, the bulk of lodging existed as individual floor ‘barn wish ‘ edifice i.e. cheap. It was non until the 2nd half of the thirteenth century that first storey floors were accepted into general domestic usage. The carpenters of these early floors preferred to put the joists flat ; they did non hold the apprehension of structural behavior to cognize that greater strength is in increased deepness. This signifier of building was besides adopted for land floor buildings ( Douglas, 1997 ) . Paycocke ‘s House, East Street, Coggeshall, Essex is a 1505 lumber framed house that demonstrates the orientation of joist used at the clip, 1 ( Hewett, 1999 ) shows that the joists used ( in Paycocke ‘s ) were laid horizontally.

Oak was the nucleus structural lumber used in mediaeval Britain. Towards the terminal of the fifteenth century, abroad travel increased the handiness of different stuffs ; with the UK oak supplies consuming, deals took its topographic point.

The Early Period ( 17th and 18th centuries ) saw the development of scientists understanding of structural behavior. This began with Galileo ‘s tenseness testing in 1638 ( Booth, 1999 ) where he observed that the tensile strength was relative to cross-sectional country. Progressing to Coulomb ‘s work in 1773 where the right minute opposition of beams was established. This patterned advance in cognition led to proficient publication and to the alteration in the orientation of joists ; as it was now understood that the joists strength is greatest when ballad upright. This is the orientation adopted by today ‘s floors.

Not merely did the orientation of the joists change during this period but so did the span of the joists. Previously the joists spanned sheer to the external walls onto a cardinal secondary ( summer ) beam that was carried by a cross girder, but during the Early Time period Parissen ( 2008 ) reveals that joists spanned directly onto girders and all in the same way.

Softwoods were normally used at this clip, with Norse Fir replacing British deals ; hence Walker and McGregor ( 2001 ) reasoning that many building techniques found at this clip originated from Europe and Scandinavia i.e. cognition was being shared.

The Modern Period ( 19th and 20th centuries ) saw methods for ciphering the strength of beams being tabulated in the UK by Nicholson and Tredgold ( 1845 ) . This is a important development for lumber floors as today ‘s method of planing floor joist is to mention and utilize TRADA span tabular arraies.

Laminating techniques were besides developed in the nineteenth century as makers were looking towards more economical subdivisions “ but this did non make its full potency until lasting adhesives were introduced during the Second World War ” ( Sunley, 1985 ) . This and the procedure of fabricating lumber in mills to cut down labor costs led to the development of Plywood and Panel Products widely used today.

1.2 The demand for and the procedure of re-engineering lumber joists

Within the recent domestic market there has been a demand for greater flexibleness in footings of internal planning, this has come about because of the current economic clime where households are taking to change their places as apposed to traveling to a different belongings. Therefore, the building industry must look towards solutions that provide a cheaper manner of accommodating layouts in the hereafter. The current manner of making so is to utilize roof infinite or take internal walls. These walls are frequently load bearing as the maximal clear span of the traditional floor joist is limited, which means new steel beams have to be installed to back up the first floor. However, if the first floors comprised of joists capable of greater spans, the divider walls would mostly go non-load bearing, hence changing the internal layout would non impact the structural stableness of the house.

In 1998 the Egan Report ‘Rethinking Construction ‘ highlighted the demand for invention in design and building. The marks within this study included cut downing capital costs by 10 % , cut downing building clip by 10 % , and cut downing the figure of defects on handover by 20 % ( Egan, 1998 ) .

The importance of Egan ‘s study can be seen by looking at timber ‘s market portion at that clip. Within the domestic floor market entirely, from 1986 to 1997 the suspended lumber floor market portion dropped by 7 % . It lost its market portion to precast concrete floor solutions ( informations beginnings NHBC and TRADA 1997 ) as the site labor accomplishment was minimum and the supply was easy ; this shows that at the clip of the Egan Report traditional lumber was non fulfilling the building industry or client demands i.e. a demand to re-engineer to better “ specifying, buying, bringing and public presentation ” ( Bainbridge & A ; Milner, 1999 ) .

Both TRADA and SCI recognise that to fulfill these marks and increase market portion at that place needs to be an betterment in the building undertaking procedure through merchandise development, increasing the usage of standardized merchandises, bettering productiveness and run intoing the changing building and client demands.

Timber already has made significant undertaking procedure betterments with the tied balk fabrication concern theoretical account. The tied balk theoretical account was brought over from the US in the 1960s ; it introduced machines and gigues to fall in the lumber members together i.e. the usage of mill pre-fabrication. Similarly, the UK lumber frame industry has industrialised techniques whereas, in comparing to planetary rivals in North America ( major exporters to the UK of structural deal ) who still continue to fall in members on site ( Sunley, 1985 ) . Milner and Bainbridge ( 1999 ) cited the lumber frame market at 11,000 units per twelvemonth. Therefore, there is a demand for high public presentation floor solutions as they are the chief constituent of the lumber frame edifice.

TRADA ‘s reply to re-engineering is to force frontward factory pre-fabrication. This procedure, harmonizing to Miler and Bainbridge ‘s article in The Structural Engineer ( 1998 ) , has been recognised by the building industry as a manner to better productiveness and meet client demands. Although documents on this topic are mostly from one publishing house, TRADA, it is a procedure that has been proven to work for tied balks and it has restated timber floors as a merchandise. This floor merchandise provides proficient and design support and offers a warrant of public presentation, whereas the current traditional attack to floor design is to utilize span tabular arraies published by TRADA. This traditional method predominately involves on site hard-on, where the stuffs are stored on site ; but these dry constituents can be damaged in moisture conditions conditions as higher wet content reduces strength and the drying out procedure distorts the subdivision ( Abbott, 2008 ) . Hence, a move towards mill production tightens quality control as all handling, choice, checking, boring etc. is done in the mill ( a controlled environment ) by secret agents with the right apprehension and accomplishment.

In add-on to prefabrication, the re-engineering procedure besides satisfies the aims of the Egan Report ( 1998 ) through an improved manner of utilizing the stuff itself. Benefits are added through the usage of structural lumber complexs e.g. timber I-joists with a plyboard web, as they cut down the variableness that natural lumber has ; this increases the mean stuff belongingss and increases the minimal design values. Value is besides added by following alternate building techniques e.g. emphasis tegument panels where the burden sharing belongingss are accounted for ; this minimises over design by optimizing the structural belongingss of the floor ; this in-turn has fiscal benefits.

The ability to incorporate services i.e. pipes and overseas telegrams within the floor nothingness, needs to be incorporated into the re-engineering procedure, non merely to understate floor deepnesss but to cut down the hazard of damaging services incurred through site fiction by the nailing or fuck of panels to joists. By integrating the services as portion of the pre-fabrication, the degree of site operation is reduced but besides the criterions for notching and boring ( BS5268 portion 2 ) can be accurately adhered to, therefore the structural unity is upheld. Through re-engineering the demand for boring and notching, for services, can be wholly eliminated, for illustration the unfastened web joists already have infinites within the joists for the services to go through through.

The first phase resulted in heavier floors that required more material. The heavier floors incur installing jobs, as managing becomes a wellness and safety concern, but significantly the subdivisions are harder to dry out, therefore the full structural strength could be compromised. The 2nd phase, once more, improved the floors stiffness but the adhesive used required the right specification to guarantee the commixture and bring arounding procedure was right. With a stiffer deck the system is heavier but it has an increased serviceableness public presentation due to the composite action achieved. From this 2nd phase the benefits of utilizing a emphasis tegument panel are taken to the concluding phase. Within the concluding phase, larger subdivisions are available ; this widens the market for lumber floors. Using the structural complexs minimises the variableness that you get with natural lumber. However, this to the full prefabricated system has a higher unit cost than the traditional method, but this cost is outweighed by the addition in quality control and structural public presentation. TRADA ( Bainbridge & A ; Milner, 1999 ) conducted research into this phase by patterning a big domestic floor with a span of 4.85m ( this span is about the optimum span of a traditional floor joist ) and compared the public presentation of traditional and prefabricated floor systems. The consequence was that the prefabricated floor system saved 20 % in joist stuff, was capable of crossing farther and could transport higher design loads i.e. more economic. Although prefabricated floors have been proven for the above application and job, traditional floor systems are still predominating, harmonizing to Milner and Bainbridge ( 1998 ) this is due to obstructions caused by regulative demands. However, these systems are being used ; hence, the relevant organic structures have already approved them, so the lone obstruction is the supply concatenation. If the interior decorator looks beyond the conventional TRADA span tabular arraies when designing, the market portion of these merchandises would increase.

1.3 Timber as a structural stuff

1.3.1 Beginning of structural lumber

The souce of the timber merchandise became a big concern in the 1990s as the lumber market became cognizant of illegal logging and hapless wood mangement, particularly in developing states ( CPET, 2007 ) . As a consequence of the turning concern the Central Point of Expertise on Timber ( CPET ) was eatablished in 2002. The CPET certifies suppliers of sustainable wood and legal lumber trade. The CPET chief purpose is to supply the UK Governement ‘s procurance forces with support implementing the procurance policy ( July 2000 ) ; this policy requires all Government sections to purchase lumber from legal and sustainable. The first appraisal of CPET was made in December 2004 ; the appraisal examined five bing citification strategies, of these five merely two were found to offer cogent evidence of legality and sustainability, the Forestry Stewardship Council ( FSC ) and the Canadian Standards Association ( CSA ) . Therefore, the all-out execution of CPET was delayed by six months to let farther organic structures to offer confidence. After this clip, two more organic structures satisfied the CPET ‘s demands. Hence, four strategies now have CPET blessing, 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 strategy, SFI North America, CSA Canada, and PEFC Switzerland. It makes sense to hold approved strategies across the Earth, in peculiar in countries of big export. However, there is no fit criterion that these strategies work to, for illustration, the CSA trials to Canadian, US, or European criterions. Therefore, the inquiry must be asked, what the differences between these strategies are and what impact they have on the lumber provided.

The UK Government policy is to handle these strategies as equivalents nevertheless “ there have been a figure of elaborate and well-thought-of studies on the comparative virtues of the assorted strategies, carried out by administrations runing from FERN to WWF and the World Bank ” ( Cliffe, 2005 ) . Reports produced by FERN, a non-governmental administration, include ‘Footprints in the Forest ‘ ( Ozinga & A ; Krul, 2004 ) . Within this study it was found that SFI, CSA and PEFC do non hold clear minimal public presentation thresholds for effectual wood direction, but FSC does. Without minimal criterions how is it possible to measure this demand. SFI and CSA besides allow for an single forestry company to custom-make the criterion against which it will be certified ; this has positves and negatives. Positive in the regard that UK woods can be certified against UK stadards. Negative as this proceedure offers no consitency in the enfranchisement procedure as national criterions differ. CSA and FSC make readily available enfranchisement studies to let the consumer to do informed picks. From the study, FERN concludes that FSC provides the best criterion. This is an sentiment which is justifible from the first comparision point associating to minimum stadards. How is one wood differentiated from the other without minimal demands to compare against. Hence the UK Governements statement sing the equivalency of the four strategies, ( Forestry Commission, 2009 ) , should be reviewed.

1.3.2 Strength Rating

Under the term ‘timber ‘ there is many different species. These species are classified as either hardwoods or deals. As defined by Ozelton and Baird ( 2002 ) , hardwoods are timber of a deciduous tree and deals are timber of a cone-bearing tree. It does n’t intend literally difficult or soft. The bulk of lumber used in the UK building industry is softwood, as antecedently mentioned. Approximately 80 % of the imported deal supply comes from ‘Norway, Sweden, Finland, Russia, Poland and the Czech Republic with the balance chiefly from Canade and the USA ‘ ( Ozelton & A ; Baird, 2002 ) . These deals are mostly imported but the usage of place grown deals has late increased. However hardwoods are used for for applications such as farm edifices and Restoration plants.

The first lumber codification CP112:1952 specified that structural lumber should be obtained by the pattern of deriving a commercial class so utilizing ocular review, delegate an approporate structural class. At present lumber is strength graded either by ocular or machine methods. Strength scaling, or stress rating as used in British Standards, is ‘the procedure by which indivual pieces of sawn structural lumber are sorted into classs to which strength values are assigned for each species ‘ ( British Research Establishment, 1996 ) and was devleoped as a manner to get the better of the variableness of lumber belongingss.

Ocular rating visually assesses each piece against allowable defect bounds given in the criterions listed below. The ocular scaling regulations define the size, type and figure of characteristic that cut down overall strength e.g. knots, ebb, deformation and incline of grain. The grader assesses the subdivision and so casts to advise the provider of its class, either General Structural ( GS ) or Particular Structural ( SS ) .

For the ocular strength classs, flexing strength is influenced chiefly by the presence of knots as they cut down the first minute of are of the subdivision. Therefore, it is of import to cognize the are occupied by the knots, known as the knot country ratio ( KAR ) . BS 4978 defines the KAR as ‘the ratio of the amount of the projected cross-sectional countries of the knots to the cross-sectional country of the piece ‘ . The codification besides states that knots less than 5mm can be disregarded and there should be no differentiation between knot holes, dead knots, and unrecorded knots. A knot near the border of the subdivision has a greater consequence on the bending strength than a knot at the Centre because, a knot at the border weakens the borders intending they could shear off under lading go forthing a smaller cross-section and hence a lower first minute of country. Therefore, BS 4978 introduced the construct of a border and a border status. The border is an outer one-fourth of the cross-section and the border status is known as the border knot country ratio ( MKAR ) . MKAR is ‘the ratio of the amount of the projected cross-sectional countries of all the knots or parts of knots in a border to the cross-sectional country of that border ‘ ( Ozelton & A ; Baird, 2002 ) . A entire knot country ratio ( TKAR ) is besides used in the scaling procedure and is defined as the ratio of the projected cross-sectional country of all knots to the cross-sectional country of the piece.

For a SS class, the MKAR must non transcend A? and the TKAR must non transcend a…“ , if the MKAR exceeds A? so the piece can still be graded every bit SS every bit long as the TKAR does non transcend 1/5. For a GS class, both the TKAR and MKAR must non transcend A? , if the MKAR exceeds A? so the TKAR must non transcend a…“ ( Ozelton & A ; Baird, 2002 ) .

Upon the appraisal of the knots, if no more than 3mm is removed from a 100mm subdivision and 5mm from a larger subdivision, so BS 4978 provinces that the initial class remains unchanged. If the subdivision is surfaced or sawn past these limitations so the subdivision must be re-graded. If, nevertheless, the length of the piece is cut the class is non reduced and may be increased if the defect is removed.

Machine rating steps parametric quantities such as stiffness, which is straight related to strength. The mahcine classs each single piece and once more stomp the subdivision. A farther ocular review is so carried out to measure strength cut downing features non routinely sensed by the machine. ADVANCES.

The British Regulations require structural lumber to be both ranked and marked by sanctioned organic structures certified by the UK Timber Trade Federation. Stresss assigned within the British Standards ( BS 5286-2:2002 ) apply merely to timber that is graded to the following

For visually graded deals:

* BS 4978:1996 Specification for ocular strength scaling of deal.

A· ECE Recommended Standard for strength scaling of cone-bearing sawn lumber. 1982.

A· National Grading Rules for Dimension Lumber ( NGLA ) , Canada, 1994.

A· National Grading Rules for Softwood Dimension Lumber ( NGRDL ) , USA, 1975.

For machine graded deals:

A· BS EN 519:1995 Structural lumber. Rating. Requirements for machine strength graded lumber and rating machines.

A· North American Export Standard for machine stress-rated timber, 1987 ( NAMSR ) .

The rating systems above trade with the size of the timber piece in different ways. TRADA ( 2006 ) note that BS 4978 rating system assigns grade emphasiss independent of the size of the timber piece, hence alteration factors ( thousand factors ) are included in the design phase. EXPAND. The North American systems are baased on the map the lumber will function ; this relates to size. Therefore no alteration factors are required as size has been accounted for. Hence in the British Standards the North American lumber of the same class and species but of different sizes can look in different strength categories. Therefore, it becomes hard for the interior decorator and provider to bring forth alternate designs, if the preferable option is unavailable or excessively expensive, as there are so many species and grade combinations. If the above scaling criterions where more parallel so the figure of combinaions would diminish and the design procedure would go more efficient with fluctuating lumber monetary values.

Grade emphasiss in BS 5268-2:2002 are based on the lumber being service category 1 or 2 therefore alteration factors are given by which the emphasiss and moduli should be multiplied to give emphasis values appropriate for service category 3. Differentiation between service categories is explain in Section 2.3.5.

The emphasiss besides merely use to long term burden, such as dead burden. Again a alteration factor ( thousand factor ) needs to be applied to the emphasis if the applied burden is deemed to be average term, short term or really short term i.e. snow burden would be really short term in this state, but for North America the snow burden would be deemed medium or long. Hence, finding the class emphasiss for long term loads allows the class emphasiss, obtained by legion systems, to be more widley applicable.


1.3.3 Strength categories

‘A strength category is the grouping of lumbers that possess similar strength features irrespective of species. ‘ ( Ozelton & A ; Baird, 2002 ) . Stipulating a strength category ensures the interior decorator knows that the lumber selected with be an economically feasible solution.

Ocular scaling produces two chief strength categories, General Structural ( GS ) , and Particular Structural ( SS ) . As the strength cut downing features varies between species, the strength belongingss of one SS class of one species may be different to thee SS class of another species. This becomes an issue for interior decorator and providers seeking alternate solutions for the ground mentioned in Section 2.3.2. Hence, in 1984, BS 5268 introduced a strength category system ; nevertheless, 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 demands for structural lumber into ocular class and species is assigned to a category on the footing of its mechanical belongingss ‘ ( British Research Establishment, 1996 ) . There are eightteen defined strength categories ; these are 12 deals 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 deals being C16, C24 and C27. The Numberss represent the characteristic bending strength ( TRADA, 2006 ) ; these are the values used for design to Eurocode 5, which uses a bound province attack. BS 5286-2:2002 replaced the 1996 edition to run parallel with Eurocode 5. Harmonizing to TRADA ( 2003 ) BS 5286-2:2002 is based on a allowable emphasis attack, although it does now mention to European criterions for stuff belongingss that have been prepared for the bound province attack. This alteration was a necessary measure towards the full integrating of Eurocodes in 2010.

1.3.4 Section sizes, tolerances, and procedures

The current ordinance for which all deal lumber dimensions have to run into is the European CEN Standard BS EN 336, the sizes are referred to as mark sizes. This must be followed for both BS 5268 and Eurocode 5 designs.

Target size is defined as the size of the subdivision at 20 % wet content that is required by the interior decorator, and is the size that is used in computations. However, divergences and tolerances are non included. Work size is the size of the subdivision at 20 % wet content before any machining is carried out to accomplish he aim size. The allowable tolerances for mark sizes are split into two classs:

Tolerance category 1 ( T1 ) : Dimensions & lt ; 100mm +3, -1 millimeter

Dimensions & gt ; 100mm +4, -2 millimeter

Tolerance category 2 ( T2 ) : Dimensions & lt ; 100mm +1, -1 millimeter

Dimensions & gt ; 100mm +1.5, -1.5 millimeter

T1 is normally used for sawn subdivisions and T2 is more normally used with surfaced, planed, or processed dimensions. It is common pattern for UK interior decorators to presume that 3mm is removed when processing or surfacing subdivisions up to 150mm initial dimension and 5mm for initial dimensions greater than 150mm ( Ozelton & A ; Baird, 2002 ) . If interior decorators are presuming a tolerance degree so they are non using the current criterions. They could plan solutions that are more economical if tolerance categories were used and taken note of, alternatively of general premises. To follow BS EN 336 the interior decorator should, for illustration, province 47mm ( T2 ) x100mm ( T2 ) to clear up their demand, so the demand to presume a tolerance would n’t be necessary.

The national extension to BS EN 336 tabulates the most readily available length and cross-sectional sizes with expected tolerances, therefore the interior decorator should utilize this tabular array to avoid supply troubles and cost punishments. These subdivisions and tolerances assume moisture content of 20 % . For other wet contents, the codification states that the alteration in dimension from the base value should be taken as 0.25 % alteration in the face dimension for a 1 % alteration in wet content. For wet contents over 30 % it should be assumed that the dimensions remain the same. Lengths of lumber are available in 300mm increases up to a common bound of 3.9m. Greater lengths, inherently, incur costs.

In North America subdivision sizes and mark sizes are deemed as tolerance category 2, but 10 % of pieces of NLGA graded lumber are permitted to hold tolerance degrees in conformity with subdivision 747 of the NLGA. These tolerances are in inches and in add-on to drying decreases ( Ozelton & A ; Baird, 2002 ) . This may confound the interior decorator or provider if T1 or T2 has been specified with dimensions in millimeter.

Discrepancies in size descriptions is deepened as the subdivisions can be referred to as sawn, regularized, planed all unit of ammunition, or surfaced four sides. A log delivered to a factory is sawn to big baulks or flitches for cargo. The buyer either re-saws the subdivision or straight uses the subdivision. Sawn lumber is normally used when dimensional tolerances are non of import e.g. for roof balks, 3mm will non interfere with the visual aspect of roof tiles. When dimensional demands are set e.g. floor joists must supply a consistent top and bottom surface to have floor and ceiling coatings, so the two parallel surfaces are planned, this is regulating and coded as S2S. For an open member where all four faces have dimensional demands, all four faces are planned. This is called planed all unit of ammunition or surfaced four sides and coded as S4S.

1.3.5 Moisture content and motion

Moisture content is defined as ‘the sum of H2O in the lumber expressed as a per centum of its oven-dry mass ‘ ( British Research Establishment, 1996 ) . The wet is found in the cell pits, when dried to a wet content of 30 % Hodgkinson ( 1980 ) states that the free H2O has evaporated and the lumber reaches the fibre impregnation point. With more drying the wet leaves the cell walls, the lumber psychiatrists, and the strength and stiffness additions due to the hardened cell walls. This continues until equilibrium is reached with the environing ambiance.

For that ground, wet content is an indispensable natural parametric quantity to find as it straight affects strength, size, susceptibleness to assail by Fungi, and lastingness of the lumber ( a lower wet content means a higher strength ) . Both BS 5268-2:2002 and Eurocode 5 classify lumber as service category 1, 2 or 3. Service category 1 has a wet content that relates to a temperature of 20A°C and a comparative humidness of the environing air that does non transcend 65 % for 4 hebdomads each twelvemonth. Service category 2 is the same as service category 1 except the comparative humidness should non transcend 85 % . Service category 3 is classified as holding a higher wet content than service category 2.

Moisture content is of import to see during the scaling procedure. BS 4978, BS 5756 and BS EN 14081-1 province that lumber is either dry-graded or wet-graded. Dry-graded is deemed to be service categories 1 and 2 with an mean wet content of 20 % ( 19 % for North American lumber ) . Wet-graded lumber has a wet content transcending 20 % and is classified as service category 3. Wet-graded lumber is non required to be stamped, hence, unmarked lumber should be assumed to be service category 3 ( TRADA, 2006 ) . With wet content ordering the strength so all lumber should be stamped with a service category to guarantee the design of the lumber is the most economical solution.

Another short autumn of the scaling system is that lumber subdivisions over 100mm in thickness are inherently more hard to dry ( TRADA, 2006 ) , hence, they are graded and installed moisture. Hence, in design, wet emphasiss must be considered and shrinkage must be understood. BS 5268-2:2002 adopts alteration factors that should be applied to emphasiss obtained for service categories 1 and 2 so service category 3 members can be more economically designed. The deductions of these alteration factors have been discussed in Section 2.3.2.

Moisture content is determined by weighing the lumber, oven drying and so re-weighing, hence a weight of H2O can be determined. This is a destructive trial laid down by BS 373 with the consequence being an mean value for the sample tested. A less timely method is to utilize a wet metre. The wet metre measures the electrical opposition of the lumber utilizing two investigations, one at either terminal. Hence, with a higher H2O content the electrical opposition will be low. The job with this method is that single species of lumber have their ain degree of opposition ; hence, the metre needs to be calibrated. However, even within a individual species the opposition inherently varies. In add-on, as stated by Ozelton and Baird ( 2002 ) , the deepness to which the investigation is inserted at the terminal besides affects opposition readings as the wet content within the surface bed ( 20mm ) differs to the wet content in the nucleus. Therefore, the truth of this method is dictated by the ability of the maker to graduate the metre. Even so, as one species can change to another and even within the same species, the metre can non be used as the exclusive method, but it is deemed a satisfactory manner of look intoing the lumber on site ( British Research Establishment, 1996 ) and gives an instant reading to within A±2 % of the true wet content ( Hodgkinson, 1980 ) .

The motion of lumber does non happen until drying reaches the fibre impregnation point, explained above. At wet contents below this point the dimensions of the subdivision alteration ; these motions are linearly related to the alteration in wet content for wet contents between 8 % and 30 % ( fibre impregnation point ) . This motion has a more important consequence on timber-framed edifices as the perpendicular members that are installed as service category 1 or 2 could dry down to a wet content of 12 % in a het edifice ( Ozelton & A ; Baird, 2002 ) .

As the motion is dimensional and natural, the orientation of the cross-section is an of import parametric quantity. The motion ‘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 procedure of change overing logs into beams means that the motion is a combination of digressive and radial effects ; hence, an mean value is taken.

An illustration of the affect of this motion can be found in timber-framed platform building where most traverse grain motions result from the lumber joists. A typical 200mm service class1 joist that prohibitionists in a het edifice to 12 % wet content could travel 4mm. In comparing, a typical 2.4m high he-man back uping the floor that would travel 0.8mm in its length under the same conditions. The motion of 4mm may look little, but in footings of warp and connexion inside informations, it could do breaks.

1.3.6 Distortion

Following on from Section 2.3.5, a alteration in wet content will ensue in dimension alterations due to differing features both tangentially and radially to the growing rings. If the sum of the digressive alteration additions and wet content decreases, the cross-section will ‘cup ‘ . This deformation is of importance when pasting timber together as the gum lines will travel off from one another. To get the better of this, the orientation of the members should be such that upon cupping they nest together. However, this solution has restrictions, the maximal wet content difference between any piece at the clip of pasting and in its equilibrium province, can non transcend 5 % , with a maximal difference of 3 % between next pieces.

The way of the grain within a timber subdivision does non co-occur with the common geometrical axes. Therefore, the effects of wet alteration consequence in asymmetric motion. This gives rise to bow, spring and turn of the subdivision, which are worst locally at knots.

Distortion has been found to be caused by coiling grain with lumber cut from the Centre of a log being prone to more distortion ( Johansson, 2002 ) . At present lumber is sorted chiefly on the footing of incline of grain, but as the location the lumber is taken from is besides of import methods of automatic optical maser measuring have been under probe by Nystrom ( 2002 ) with triasl being carried out in Sweden ( Holland & A ; Reynolds, 2005 ) .

Bow and spring is caused by an inbalance in longitudinal shrinking. This shrinking is a consequence of an inbalance between juvenile wood and compaction wood from one side to another. Fast turning lumber has a high proportion of juvenile lumber. Whereas compaction wood is a reaction that affects softwwods that have grown on inclines, and the wood below heavy subdivisions ( Desch & A ; Dinwoodie, 1996 ) . Scaning is presently being used to find the sum of juvenile and compaction woode in each face, but, this is simply a manner of screening the lumber. It would be more good to implement a drying and managing procedure that would restrict the deformation as apposed to simply placing it.

1.3.7 Thermal Conductivity

Timber has a low thermic conduction value, hence for floor joists it is non necessary to see thermic bridging as the floor joists are portion of a thermally insulated system. Thermal bridging is where heat is allowed to flux through a system that contains stuffs that are hapless dielectrics. A typical value for lumber is taken as 0.14W/m K ( Ozelton & A ; Baird, 2002 ) or 0.12W/m K ( Eurocode 5 2004 ) . This value may besides be adopted for plyboard. The usage of fiberboards in floor systems can do a important part to the insularity of the edifice as they have lower values of thermic conduction.

The thermic conduction of lumber additions with temperature as the degree of wet lessenings, this rule will be discussed in subdivision XXX where lumbers behaviour in fire is considered.

1.3.8 Thermal Motion

Under regular clime conditions, the motion of lumber is little and normally neglected, except for joists of longer spans where the motion is more noticeable and may hold a greater consequence.

In fire, harm is the consequence of the thermic enlargement of the stuff. This leads to breaks at connexions and supports before failure of the member itself. When subjected to fire, the nucleus of the timber joist efficaciously remains at room temperature due to the low thermic conduction of the outer charred beds. Combined with the low coefficient of additive thermic enlargement lumber suffers small problem due to thermic motion. Timber has a coefficient of additive thermic enlargement about a 3rd of that of steel and concrete ( 4×10-6/A°C ) , therefore its behavior in fire is more favorable.

Timber ‘s behavior in fire will be discussed further in Section XXX.

1.3.9 Plywood

BS 6566 defines plywood as ‘a wood based panel merchandise consisting of an assembly of plies bonded together, some or all of which are wood ‘ . It is the most variable and oldest panel merchandise, illustrations have been found in Egyptian grave ( Dinwoodie & A ; Enjily, 2004 ) . Plywood is non the first pick for usage in domestic suspended floors but is in the commercial sector due to its impact opposition and high stiffness. Because of the high stiffness, plyboard is more normally being used within high structural public presentation systems such as emphasis tegument panels. More late, the usage of plyboard has extended to engineered subdivisions such as I-joists and box beams.

At present plyboard is made from deals chiefly imported from Scandinavia, USA and Canada ( Ozelton & A ; Baird, 2002 ) . Imports from other states may non follow with European Regulations, so the beginning of the deal must be checked before usage in building.

There are four rule phases in plyboard fabrication. Step 1involes selected logs being rotated and sliced into veneers so cut into panel-sized breadths ; this is known as Peel and cut. Step 2, known as dry and kind, is where the veneers are died to a wet content of between 4 % and 8 % . At this phase, strength-reducing features such as knots can be removed. Within measure 3 the veneers are coated with adhesive and assembled so the way of the grain of surrogate beds are perpendicular to one another ; this is known as gum and assemble. The concluding measure, measure 4, is the imperativeness and finish phase. The glued and assembled merchandises are fed into a hot imperativeness so the gum is cured. After the pressure, the sheets are cut to size. ( Manufacturing stairss as laid out by BRE ( 1994 ) ) . Adhesive materials will be covered within Section XXX

The plyboard either contains an unequal or equal figure of plies. The unequal system means that the hemorrhoids are balanced around a individual cardinal veneer. Whereas the equal system has plies balanced around two cardinal plies that have the same orientation. The figure of plies the plyboard has dictates the public presentation as the thickness and stiffness additions. These multi-layered plyboards are known as veneer plyboards.

Structural applications of plyboard, in Europe, must fulfill the Construction Products Directive by following with the European Harmonsied Standard BS EN 13986. Presently the design of structural plyboard can take two attacks, either proving or computation.

Testing ensures the most economical usage of the plyboard but is merely on a regular basis used when big Numberss of indistinguishable merchandises are being produced from the same design, as the process does non let for trial alteration. The current criterion to which floors, if tested, must follow with is BS EN 1195. The trial consequences can be converted into design loads that can be used for either allowable emphasis of bound province design, depending on whether the design will be designed utilizing BS 5268 or Eurocode 5.

Design by computation, in comparing, is more conservative but less dearly-won. For computations carried out utilizing allowable emphasiss the working emphasiss are given in BS 5268, this is current UK pattern. Calculations carried out utilizing bound province design usage characteristic emphasiss found in Eurocode 5. The bound province attack is shortly to be adopted in the UK ; hence, there is a transitional period underway. This transitional period is extremely confusing as there are now twice as many ordinances with which interior decorators could follow with, which leaves the inquiry as to which 1s to follow. UK interior decorators are loath to utilize European criterions until they have to, particularly those who have been in the industry for many old ages and base most design on experience. A good illustration of this would be that antecedently for a certain set of design parametric quantities, experience demonstrated design conformity and a set of ‘deemed-to-satisfy ‘ tabular arraies were produced in BS 8103-3. These tabular arraies provided a panel thickness for a given joist spacing and burden. The tabular arraies were produced utilizing a different impact trial method to that in new European codifications. Hence the tabular arraies ‘do non follow with the European Harmonised Standard and EN 12871 and by 1 April 2004 they must be withdrawn from usage and they must non be used ‘ ( Dinwoodie & A ; Enjily, 2004 ) . These tabular arraies had proved to work satisfactory before to it asks the inquiry of whether the allowable emphasis method is comparable to the bound province method.