The Great Materials Debate Construction Essay

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Introduction.

Timber comes from various species of tree and can be divided into two groups, softwood and hardwood. Softwood comes from conifers, cone bearing trees which have needle like leaves while hardwood comes from deciduous trees which shed their broad leaves at the end of the growing season or during a dry season. Not all hardwoods are hard balsawood is a soft hardwood very light and can take an impression of a finger or thumb, not all softwoods are soft i.e. yew is strong, tough and solid material.

Softwoods are the most commonly used timber materials because they are less costly than hardwoods, are readily obtainable, easy to work with and are less dense than most hardwoods.

There are over 600 species of softwoods throughout the world, around 50 are in commercial use. Most softwood used in the UK comes from Europe within managed forests.

Construction purposes the important characteristics are strength and durability for classification. If stipulating softwood structural timber references should be made and stated, preservative treatments, cross sectional size and type of finish, moisture content and the strength class.

Timber is produced with trees growing in managed forests through rain and sunshine creating photosynthesis, drawing in energy as they absorb Co² from the atmosphere in exchange they release oxygen

As a construction material timber is light, easy to handle and is less of a structural dead weight than that of masonry, with a high strength to weight ratio and very rigid when compared to its strength. Fabrication of its structural units off site reduces skilled labour as joints are simply nailed. The benefit of quality control and efficient manufacturing procedures the high cost of imported timber in UK is offset by effective factory prefabrication of walls, floors, and roof units. Construction Technology

Wood is a natural fibre, consisting of crystalline cellulose hollow cells, fibres made up of mostly carbon, and very important in reducing the atmospheres carbon dioxide, and as long as the timber is preserved helps to reduce Co² once the timber is burnt or allowed to decay it returns to its Co² form and the benefit is lost, even though the disposal of timber is the least of problems compared to all other materials. Materials in Construction An Introduction

Structure

Cells run mainly in two ways longitudinally and radially, mostly longitudinal way so as to provide nutrition throughout the tree, other ways nutrition is carried and stored is radial direction, rays increase the strength of the timber in this direction.

Softwoods are considerably more economical than hardwoods though they are less durable. They are the most widely used type of wood used for general structural purposes, most common used species in UK are Scots pine (redwood) European spruce, Douglas fir and cedar. The cedar contains resins that naturally deter fungi and insects and therefore very durable.

Conversion of timber

This is the process of turning a log into pieces of timber, which can be easily seasoned and appropriate shapes for installation. As soon as the tree is taken down the log should be cut for timber, a series of planks can be cut from

each log and selection and choice of cut is now days done by computer called optimisation, the decision is made on the least waste for the best cuts. The cheapest and quickest cuts is a method called through and through, but is not the best method as some timbers are subject to distortion due to the cut in the run of the grain. An alternative is to use quarter sawn cuts this is more expensive and a lot more waste produced though less distortion occurs. Quarter sawn boards are radial cut from the centre of the tree. It produces the distinctive silver ribbon effect across the whole board. Annual growth rings form an angle greater than 45 degrees.

Image from http://www.woodfx.co.uk/Wood.html

Some timbers have similar moisture movement in the radial and tangential directions, like Douglas fir and are less prone to distortion than timbers with high relative movements like Redwood and Beech.

Quarter sawn timber warps less than flat sawn (through & through) even so the latter has more interesting grain pattern. Pieces that include the pith (centre) are more prone to buckle like bowing, twisting and springing. Twisting is often caused when the grain is not straight or the density of the wood varies due to unequal growth of one side of the tree than the other. Timber having straight and equal grain is less likely to distort. Materials in Construction An Introduction

Preservative Treatments.

Most softwoods contain high proportions of sapwood, and is more susceptible to attack than the inner heartwood which is much denser, preservatives are used to tackle the problems of damp, fungal and wood boring insects, this will be determined by the position of the timber interior, exterior near the damp ground and the geographical location. In parts of southern England there is a supposed risk of attack from the longhorn beetle (origin Asia) The Building Regulations, approved document supporting reg 7 define the particular boroughs where softwood must be treated in any aspect of roof development, BS 8417 clarifies acceptable chemical treatment.

Cross sectional size and type of finish.

Cross sectional size is down to sizes of sawn wood from the mill usually a tree is cut if required into standard sizes of 75mm x 225mmT1 and 50mm x 100mm T1this timber being used for hidden or unseen structural uses, such as roofing or floor structures it has a rough sawn finish and would not be used where its left exposed. When the wood is needed for work that will be on view specifying planed for a smooth finish which takes 1.5mm from the surfaces, so a 50mm x100mm has a new size 47mm x97mm T2, so as not to confuse there are only two tolerance classes T1 rough sawn and T2 planed, to further clarify if a timber is required 50mm sawn and 197mm planed, specification would be 50mm(T1) x 197mm(T2).

Moisture Content. A tree is felled the moisture content is high and is prone to shrinkage distortion and fungal growth if its left long enough, this needs to be dried to be workable either naturally or seasoning in kilns, to reduce the moisture content to between 12% and 20%, this is expressed as the weight of the water in the timber as a percentage of the weight of the dry timber. Moisture content % x Construction Technology 4th Ed

Timber seasoned it needs to remain in a well ventilated stable environment. Timber is prone to absorbing moisture and will expand and contract, deform and twist out of its regular shape in its environment, changes in temperature and humidity affect the timber therefore correct storage is crucial, laid flat on dry surface 200mm above ground under cover with packings every 30cm and in line one on top of the other to keep a flow of air circulating.

The importance of seasoning timber is that it stops moisture loss and this always takes place at or near the surface of the timber meaning shrinkage occurs normally in these places. If shrinkage does happen surface layers tend to contract resulting in surface rupture or they may split. Timber also has natural variations such as grain pattern and defects known as knots which are the weakest point in the timber.

Strength Classes Grading

"Tensile strength of perfect timber taken in relation to its self weight compares favourably with that of steel, values of over 100 N/mm² being in common with a material which has a density equal to about 7% that of steel, compressive strength is much lower, due to the fibres to buckle on compression."

Timber grading can be visual each piece examined by eye for a number of defects which would limit strength, categorised into 1 of 3 grades Special structural (SS), General structural (GS) and reject. The SS grade having strength of 50 to 60 % of perfect or clear timber of that species, and GS grade having 30 to 50%, timber should be graded by a qualified grader and marked with a grading of recognised authority TRADA is the main body in UK.

Now days grading can done by a more efficient computerised grading machine. Single pieces are gauged against allowable defects, marks, knots (KAR), fissures, limits and scored accordingly see BS4978 for defects. Example of grading mark below including from left bottom line what company has graded, British Standard 519, Strength graded, Certification authority, FSC supplier Chain of Custody sustainable source of timber. Timber can be Graded in the UK or the origin country.

BS 5189 (European standards for visual and machine grading) control a lot of the timber graded in Europe and the neighbouring countries, but uncertainty can happen if timber is graded elsewhere with the lack of an international standard. Construction Technology 4th Edition

Image from www.bsw.co.uk/processes/strength/

It is not enough to rely on stress grade SS as different species have different stress capacities if one species is in short supply and designer may have difficulties finding replacement with similar structural properties. The introduction of strength classes in 1991 BS 5268 eased the problem, grouping different species and grades together so designer no need specify individual species. Ten classes were defined 1 to 5 being softwoods, and classes 6 to 9 being hardwoods, SC3 and SC4 equivalent to European redwood & whitewood in GS and SS grades. Timber design tables in building regs are based on strength classes SC3 and SC4 which are a simple method of design for domestic floors and roofs. To blend BS 5268 with the EC5 (European code for the structural use of timber) new strength classes have been introduced with prefixes of C for softwoods and D for hardwoods, the numbers correspond to bending strength of the timber in each class. (TR standing for trussed rafter) European redwood and whitewood in the GS and SS grades would match to C16 and C24 respectively.

Materials in Construction An Introduction

Insects and beetles can attack timber like the common furniture beetle and the deathwatch beetle. Woodworm is probably the most common cause of insect attack on timber in buildings. The beetle will lay its eggs in any susceptible timber and once hatched the larvae burrow into the timber. Once the beetle has hatched it will leave the timber creating a hole approximately 1 to 2mm.

Dry Rot is a fungus and a living organism with a fruiting body that can attack softwood and hardwood and damages timber with restricted ventilation or where it has not been seasoned. It can also be caused by high humidity with a moisture content between 30- 40% but can also attack dry timber. Dry rot can spread through a building if the above conditions are present by penetrating brickwork and masonry and behind plaster.

Wet rot is not a major problem compared to that of dry rot as the problem is caused by the timber decaying naturally. It decays when the moisture levels are about 20 - 40%. Most of the time wet rot is caused by damp such as a wall close to the timber or rain water collecting on the wood. In most cases the problem will be due to damaged external paint work covering the timber as this will allow the timber to absorb any excessive moisture. But if any damage is caused it will only be limited to the affected timber.

Some species of trees like cedar are more effective at resisting biological hazards such as fungi and termites than other species. Designers when working with timber can match the natural durability of a particular species with hazard classification and can specify an appropriate species for a specific application

Finally many structures will be dismantled or demolished and it is a fact of life that waste is formed. However timber can be reused, recycled or recovered.

Environmental Considerations

At the start of this subject timber was a explanation of the ecological benefits low energy growing trees, helping reduce carbon levels in the environment.

In the construction industry at certain times decisions in its choice of timber, some hardwoods being shipped from countries which have mass deforestation projects which might have in relation to Co² progressed further and inherited Land instability, Soil erosion, climatic changes on a local or wider scale, interruption of wildlife balance.

Hardwood forests in UK have vanished long ago and in large parts of America and Africa. Present supplies and sources are often more distant and in less developed part of the world where financial and political demands are often short term rather than long term focus. The continued misuse of natural resources on this time scale may have severe long term penalty and efforts are being made to ensure that timber supplies are obtained from sustainable sources.

Reduce - Reclaim - Reprocess - Recover.

There are opportunities in prefabrication for waste to be reduced.

Dismantling yards where sources of timber stripped from sites are stored.

Possibilities of reprocessing timber or even broken down and used as an insulator.

Timber could be pelletised and used to heat the program.

Materials in Construction An Introduction

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