timber engineering 1

A Personal View of the Development of Timber Engineering in the UK Dr Geoff Freedman Oct 2008

1

A Personal View of the Development of Timber Engineering in the UK (with examples of some developments by the Author)

Dr Geoff Freedman BSc CEng CEnv FICE FIAgrE

Head of Design - Forestry Civil Engineering

Summary This paper deals with the history and regeneration of timber used as a primary structural material. The UK allowed the old skills to disappear but, today, we are leading the international field in some forms of stress lamination for bridges. There are a small number of Engineers working in the discipline but many more are needed if there is to be a modern revolution in Timber Engineering. Success can bring reduced costs and greater sustainability. 1.1 History of Timber Engineering One hundred and fifty years ago we built with stone and timber, before steel and concrete arrived in quantity. The skills were well honed and wide spread. Apprenticeships were the media for passing on those skills – a training system which evolved and fitted the period. The craftsmen who came from these training programmes took pride in their work and were well enough paid to live a decent life. The industrial revolution changed much but the apprentice system endured. Working with timber moved on from housing and shipbuilding, to factories and machines.

Timber ship frame

Britain’s presence abroad brought home many exotic species of timber- which extended the use of wood. Harder and more durable hardwoods made better machines, vehicles, piers and infrastructure. At this time the future looked bright.


2

Timber Pier

Timber Engineering in this period would probably not have differed much from continental Europe, where skills had been passed down for many centuries more than in the UK. Bridge-building, using timber, on the continent was very advanced at the time where in the UK stone was mostly used. It was more durable for our harsh conditions and our timber was less durable than much of the continental species.

Chapel bridge (around 1300), Lucerne

It was in France in the 17th century that Philip De Lorme invented the stress laminated timber arch/dome which was used often in the USA, after being discovered by a contemporary US President. The timber engineering skills in central Europe were superior to those in the UK and, I have to admit, they still are.


3

1.2 The Twentieth Century Decline The industrial revolution accelerated the use of steel and, in the last century, concrete. These new ‘wonder’ materials were seen as the only way forward.

School Building by Haley Engineering

This was a major error of judgement made by UK building designers. The new materials opened up new possibilities and did some of timber’s work better - but steel and concrete were not always better. Over a very short period of time timber was reduced to a secondary structural material and sent the profession of Timber Engineering to the history books. We were left with a group of semi-skilled joiners who supplied a service to the building industry. A once highly prized craft had been reduced to half trained ‘chippies’ - subservient to steel erectors and reinforced concrete specialists, let alone the brick layers. There were pockets of excellence and specialist units around the country, building railway carriages and looms, but the only highly trained timber workers in quantity were employed in furniture manufacture.

Timber Frame Furniture – thanks to APA


4

The steel and concrete industries formed associations of their users and manufacturers. They ran training programmes for designers and made life easy for them by providing design aids of all kinds. This established these materials so well that, even where timber was the natural choice for say a floor joist, steel or concrete were often used. The timber suppliers were estate owners with no real business acumen. They had their own sawmills or supplied fairly small commercial sawmills. Timber did not have a trade or materials association to compete with the Cement and Concrete Association or British Construction Steel Association. The future looked very grim. 1.3 Reversing the Decline at the end of the Twentieth Century During the past hundred years, plantation timber has become an industry and is now making an impact. This is also true around the world and softwood is now internationally available at a relatively low price. This situation is likely to remain for some time as it is one product that does not require a great deal of capital to produce. You need land and rainfall and the developing world has many places with that combination.

Plantation Forest

Virgin forests are available to provide more volume, but sustainability must be maintained by re-planting and avoiding ground damage. Timber is becoming a very competitive material in the resource-starved world. As China expands its industries it soaks up raw materials particularly steel, coke and cement. This is presenting an opportunity for timber but, do we have the skills and capacity to engineer the product to do the jobs steel and concrete have been doing for the last one hundred years? The general consensus is ‘not at present’ but possibly, given another ten years. While this situation develops, the world’s population is ever expanding – creating pollution and assisting global warming. Environmental damage is accelerating and is caused largely by energy production - the basic necessity of modern man. Steel and


5

concrete both require large quantities of energy to manufacture. Timber requires none- and some uses make it a carbon captor. Timber and timber engineering has had a tough time but it now has the best opportunity for a hundred years. Infrastructure requirements are established and perhaps shaped by what steel and concrete could do. Innovative Timber Engineers are required to invent ways of making their product into a substitute. They need to make big strong structural elements from small weak pieces of fibre by laminating, bracing, reconstituting and with every move, they save a bit of the planet. I would have expected more smart young Engineers to take up this challenge but, unfortunately, the industry is not there yet to offer them a decent career. The challenge is to accelerate the process and make Timber Engineering as important as it was one hundred and fifty years ago. 1.4 Present Day For the last forty years, we have lived with the promise of a developing a timber engineering industry in the UK. We have been saturated by initiatives, forums, centres of excellence and examples of innovation - but no industry. Timber engineering is still not properly off the ground in the UK. There is but one faculty in just one university, in the whole of the UK. It has only been established for five years and is fast drifting into becoming a centre for wood science because the Engineering students are not coming forward.

http://www.cte.napier.ac.uk/

There are a few tiny research facilities around the UK and very little specialist teaching of this form of engineering. At Masters level, there is a fair bit of interest but too many places are taken up by European students who return home with their knowledge and experience - their talent is lost to the UK. There are some large international consultants producing excellent designs but this will not result in a regeneration of timber structures. The future for producing talented Timber Engineers looks grim but some opportunities are appearing. Steel has doubled in price over the last year, partly because of the cost of ore and coke but mostly because China has created a supply and demand shortage. Cement is one of the biggest pollutants in industry because of the heat energy used in its manufacture, rendering it an unpopular material with the modern ‘green’ approach to construction. There is also a ‘China’ factor. Legislation is increasingly demanding sustainability and the reduction of carbon deposits in the atmosphere. Timber can step into the breach to comply with this, but we need the specialist Engineers to innovate the solutions. One way forward in this time of opportunity, and while there is a lack of specially trained Engineers, is to encourage existing designers to design in timber. This could be achieved by adopting the steel and concrete industries’ tactics of the 1960s and 1970s by producing


6

design guides. This will help generate a catching-up process, but sustained and concerted innovation is needed if timber is to do the job of steel and concrete. This will require some real specialists and well funded research centres. Good examples of large timber structures like bridges, towers and large buildings will increase the public confidence in timber – often, unfortunately, regarded by prospective home owners as a material which will rot and result in a poor investment. Only 10% of new houses in England are timber framed but in Scotland it is 60%, where advantage is taken of the speed of construction. This is a potential growth area which, sadly, has taken a severe blow with the recent ‘credit crunch’. On the other hand, the ‘credit crunch’ has caused the price of timber to drop and more bright young people are apparently considering Engineering as a career, instead of investment banking. Over the last twenty five years the Forestry Commission Engineers have been trying to use more timber for structures. During the 1950s and 1960s about one thousand bridges were built in UK forests from steel and concrete - which was inconsistent for an organisation trading on sustainability.

Tramrail Bridge Prestressed Concrete Lorry Bridge

Steel Beam Bridge


7

For that reason, some timber bridges were introduced. The dilemma was that from 1950 to 1980 lorry sizes doubled, so stronger bridge materials seemed necessary and timber was weaker. The challenge would be to engineer timber to produce strong structural sections which could compete with steel and concrete. During this time many composite engineered materials were developed from timber. Before explaining the pros and cons of specialist composites, perhaps some information on structural timber would be useful. 2 Timber as a Construction Material 2.1 Why use Timber We need factory production-line manufacture to utilise the smaller trees which modern plantation forestry produces. We need to develop jointing systems which design out timber’s poor capacity for local bearing and shear. At the same time we must remember to keep structures dry and avoid rot. We must exploit timber’s strengths and composite it with other materials to overcome its weaknesses. Timber’s Strengths

• It is easy to work and fix to. • It has very high strength to weight properties. • It is very adaptable for use as a composite. • The public likes the look and feel of timber. • New design codes permit realistic calculations. • Plantation trees are readily available at low cost.

Timber’s Weaknesses

• Strong joints are expensive. • It rots if it stays wet for a few weeks. • It is not favoured by Structural Engineers.

It is not regarded as a primary structural material by the public but the cost, availability, sustainability and ease of use means we must learn more about how to use it to our best advantage. 2.2 Species used for Structural Timber Timber is the only renewable structural resource available to man. It is carbon neutral because at the end of its useful life the CO2 which it puts into the atmosphere is equal to the quantity that it removed while growing. There has been resistance from modern Engineers to the use of timber, partly because of their general lack of understanding of species, properties of strength and durability, and of the construction details necessary for sound construction. Timber frame housing has flourished over recent years but some poor workmanship and detailing has led to a number of premature failures. This has generated new resistance to the use of timber for permanent structures. However, the use of timber for major high profile structures like bridges could help renew confidence. There are three groups of trees – softwood, temperate hardwood and tropical hardwood.


8

Within these groups there are a number of species in each with different properties. The softwoods generally come from temperate zones and the species which are readily available in the UK are Pine, Spruce, Larch and Douglas Fir. Within these species there are a number of particular tree types. The type which has been planted in the UK over the last eighty years, due to its toleration of the low winter temperatures in Scotland, together with its ability to thrive in the acidic soil resulting from deforestation, is Sitka Spruce. This is very good for paper and pulp but weak as a structural timber, partly because of the rate of growth. The average rate of growth in the UK is approximately forty years to maturity. Scots Pine is very useful, reasonably abundant and very good at taking up preservative treatment. Larch is more durable in its natural state and takes up less preservative treatment. Douglas Fir is a fine structural timber with good resistance to decay. Common temperate hardwoods, or deciduous trees, in this part of the world are Oak, Beech, Birch and Lime. These have very variable durability and their ‘hard’ description is not always deserved. In fact, some ‘softwoods’ in the world are harder than hardwoods. Oak is particularly useful in the UK as it is plentiful, durable and very hard, though it is very expensive. Beech and Birch are generally used for furniture or other indoor products. The tropical hardwoods are dense, strong and durable. They are very effective as structural materials and their chemistry gives them resistance against predators, in some cases even marine borers. Greenheart is a famous example and originates from Guyana in South America. In colonial times, this was harvested and used in many UK Victorian maritime piers. The Author built a new pier three years ago using Greenheart which, although it had already given one hundred years’ service in Helensburgh pier, was still in perfect condition. These hardwoods however are difficult to work, produce toxic splinters and are difficult to obtain from truly sustainable sources. Their natural habitat tends to be in politically sensitive areas, so their use is best discouraged in preference of plantation softwood. Plantation softwood is available from most countries in the world so the cost should remain competitive for some time to come. International competition should ensure control of the price, in contrast to that of oil and gas which are only available in a few locations and where the price is controlled politically. This is one very good reason to develop the use of timber for structural applications, even if it is not the strongest, most durable material. 2.3 Properties of Structural Timbers Timber is not a homogenous material like steel, which has the same properties in all directions. Timber is orthotropic, which means it has different properties in different directions. This is because it is formed from a group of parallel vertical tubes which convey water to the extremities, where sunlight converts it to food through photosynthesis. These tubes form what we call the ‘grain’ and vary in diameter, depending on growth rate. Along the grain they provide substantial stress resistance but at right angles to the tubes the timber is easily compressed, especially in softwood species.


9

Breakdown of components of a tree from molecular structure to whole tree

A perfect piece of timber is very strong. Scots Pine can achieve 46N/mm2 [3] in bending stress before breaking, but natural defects like knots, slope of grain, microfibril angle, compression wood etc. mean that a safe stress would only be one tenth of the ultimate. Fresh timber is made up of about 50% water by volume and considerably more by weight. Most of this water is lost over time after the tree is cut down. Usually it is kiln dried nowadays, to accelerate that process until it has reached the in-service moisture content (MC) which is 16% - 18% for protected external timbers. Even after optimum MC is reached there are seasonable shrinkages, dependent on humidity. Softwood shrinkage is less than temperate hardwoods, which is very satisfactory for Stress Laminated Timber (SLT) bridges. 2.4 Durability of Timber This is a description given to the resistance to fungal or insect attack. In this part of the world insect borers are only a problem near seawater and fungus only grows if there is sufficient moisture. Some timbers have natural toxins which resist organic attack while others are very vulnerable. Timber can be impregnated with chemicals to help the resistance to decay. Hardwoods are generally more durable but this is very variable e.g., whilst home grown oak is reasonably resistant, tropical Greenheart is almost totally resistant. Softwoods are all susceptible to decay though Larch, while resistant, still benefits from chemical treatment. Scots Pine requires treatment and absorbs it well, whereas Douglas Fir, although moderately resistant, will still be treated.


10

Treatment of timber is a very emotive environmental subject because the best treatments, designed to kill carbon based insects and fungus, are also toxic to humans. The most superior is Creosote, followed by Copper Chromium Arsenic (CCA) and then Copper Chromium Phosphate (CCP). Organic preservatives can leach out so have a poor performance, externally. Creosote and CCA were legislated out of use in the UK in June 2004 but derogations were introduced for a number of professional uses, like railway sleepers and bridge decks. However, in practice, these treatments have not been available for bridges due to the low demand, so specifications have to accept the available weaker treatments. With modern pressure/vacuum treatment processes, the chemical only penetrates a few millimetres so all cuts and holes have to be made before treatment. It is always good practice to protect timber from prolonged wetting therefore, to ensure this, good detailing is essential for timber bridges especially since treatment yards willing to use the more toxic treatments for commercial structures are difficult to find. There are some new treatments coming to the market for softwoods which may well become cost effective in the near future if timber finds favour as a structural material and the markets grow. The most likely to succeed, in my opinion, are acetylation and some similar form of extreme heat treatment. Acetylation is an expensive treatment using acetic acid and heat and is now available in the UK marketed under the trade name ‘Accoya’. The advantage of this treatment is that the chemical is all the way through the cross section and when on site, if timber is cut it is still fully protected at the cut. Timber treated this way is available only in smaller sections and large orders are necessary for economy. Timber window manufacture may use this process. Another interesting development is microwaving or extreme heating where the chemical composition of the timber is amorphised into an inert compound. The heating actually causes the cellulose and lignin in the timber to combine chemically. Again this treatment is homogeneous. 2.5 Strength of Structural Timber The strength of a piece of timber depends on its species, growth rate and defects. The growth rate determines the number of growth rings per width of a piece of timber and the closer they are, the stronger the sample. Defects are - clusters of knots, slope of grain and ‘wane’, which is the loss of edge pieces caused by milling too near the extremities of the round timber. An assessment of all of these factors gives a piece of timber a specific grade which is related to its ultimate strength, that being a proportion of its yield strength or breaking strength. That strength is preceded by a ‘C’ for coniferous if it is softwood and ‘D’ for deciduous if it is hardwood. The latest Code of Practice for structural timber design in the UK/Europe is ‘Eurocode 5’, [51] which is a Limit State Code and uses these Ultimate strength Grades. The old UK Code, ‘BS 5268’ [50] is still in use and is based on permissible stresses, which are generally about one third of the ultimate. The two common Grades of timber are ‘C16’ and ‘C24’, which relate to visual grading categories ‘General Structural’ and ‘Special Structural’. Home grown Oak will be graded as ‘D30’ or ‘D40’.


11

3.0 Using Timber for Structures 3.1 Designing out Defects in Timber One of the most important goals of timber engineering is to avoid the defects and utilise the good straight grained timber or minimise the effects of the defects. Today high quality whole wood has the knots simply removed and the good timber is then jointed using a modern technique called finger-jointing.

Finger Joint

This is a glued joint between two pieces of timber whose ends have been cut to fine ‘V’ joints which are pushed together in a tight fit. Sheet materials are formed from thin laminates glued to make plywood or chipped and glued to make Orientated Strand Board (OSB). A mixture of the above processes is used to make ‘Engineered’ timber which is dealt with below. A structural process to produce the same result with whole timber planks is laminated timber where planks are dried and planed carefully to a precise cross section and then stuck together under pressure to form timber beams.


12

Glue Laminated Portal Frames – UK Glue Lam Association All of the above processes require factories and large capital investments which leads to the reason for stress laminated timber. This is where planks are mechanically stressed laterally to form strong timber plates. This form of structure has been used extensively by the Author for bridges and will be explained in detail later. 3.2 Engineered Timber There are basically two main types of timber composite in production for the manufacture of timber beams for flooring and roofs. Laminated Veneer Lumber (LVL) and Parallel Strand Lumber (PSL).

PSL, LVL & Whole Timber


13

The first in made as a continuous sheet of very thick plywood with plies of about 5mm. The sheet is the thickness of the beam – perhaps 200mm – which is sawn to any length necessary. These beams will not shrink or twist or creek like whole timber. PSL is made from crushed timber which is re-glued into beams with all strands in one direction which makes it very strong and because it has no knots it has no defects to reduce its strength. These beams also show dimensional stability and can be any convenient length. These timber products are imported to the UK and are therefore expensive and only used for specialist construction but may well become more economic with time. 3.3 Engineered Structural Components The most important items in this category are Structural Insulated Panels (SIPS), New Age Flitch Beams and timber ‘I’ beams. Structural Insulted Panels are made up of two Orientated Strand Boards (OSB) sandwiching polystyrene which is glued to the boards. These panels are commonly 100mm thick and 2400x1200mm in elevation.

SIPS Wall Panels

They are used as wall and roof panels for housing and commercial buildings. Although use is reasonably common in the USA, Europe and Japan various panels configurations have been load tested at Building Research Establishment (BRE) and Napier University in recent years to assimilate results for UK Codes of Practice. This form of construction, using wood based products, promises significant cost advantages where factory production reduces site operations. The New Age Flitch Beam was developed as commercial project carried out by TRADA (Timber Research & Development Association), backed by the Forestry Commission and


14

some industry partners. A series of tests were carried out on combinations of timber joists acting compositely with steel plates sandwiched between timbers. The New Age title is

New Age Flitch Beam with shot fired dowels

based on the fixing method which is a pattern of shot fired nails acting as dowels through the matrix. This allows quick, low cost assembly of beams in situ or in a factory. Further research is ongoing at Napier University as part of a research programme. The results have shown a structural advantage against the cost of using a bigger timber beam or a steel beam. The focus is in using low grade Sitka Spruce grown in the UK. The probable uses will be in timber frame housing and in bridges. Where there is to be no fixings to the bottom of the beams e.g. a ceiling, there is scope for shot firing a strip of metal to the bottom of a timber joist. The resulting composite action is likely to produce a more significant structural gain than a plate between beams as in the New Age Flitch. ‘I’ beam production has been common in a number of locations in the UK for ten years now. It is a development of an idea used by the Author in the construction of school roof beams in the 1960s.

Timber ‘I’ Beam


15

They are manufactured sections using a timber board as a web and whole timbers as flanges. Originally the webs were ply but now are made from OSB. The whole wood flanges are generally grooved and the web glued in place. These components have been very successful because, again they come in ordered lengths and are dimensionally stable. One manufacturer markets them as ‘Silent Floor’ because, unlike timber joists, they do not warp and squeak. These beams were the subject of a successful PhD study with insulation between a double web which gives extra rigidity and makes a very strong lightweight beam which has been taken up commercially.

Insulated ‘I’ Beam

3.4 Conclusion The above ‘Engineered’ products involve costly production and most involve glues and highly stressed joints which will suffer in an external location. Durability is one of the most important considerations for bridges and other external structures. Timber is still a relatively low cost material so using more of it to build a structure with in built lateral restraint can be cost effective. These considerations have lead to a new form of timber engineering known as Mechanical Stress Lamination.

4.0 Stress Lamination of Timber for Bridges Stress lamination is different from the known glued lamination which requires strict quality control and results in high quality expensive products. A vertically stress laminated element is formed by drilling holes in the wide face of sawn timbers at centres of between 300mm and 900mm. These timbers are laid side by side on their narrow edge, stressing bars are passed through the holes and then stressed to between 100kN and 200kN tension.


16

Typical cross section of SLT deck

The friction between the faces, induced by mechanical stressing, permits transfer of load from one laminate to the next. This converts the whole into a solid load-bearing orthotropic deck with the ability to transfer load laterally and longitudinally. The natural variability and defects found in timber are responsible for the reduced allowable stresses in whole timber.

Dismantling a Stress Laminated Bridge

However, when vertically laminated the defects and natural variability are dispersed and the efficiency of the system is greatly enhanced. This form of construction utilises a low cost sustainable material with the minimum input of quality control and production energy. The resultant life cycle carbon footprint is very low compared with other bridge construction materials.

20m span test bridge at Glentress during construction

t =50mm

100

1000mm


17

4.1 Recent Research A recent research study by the Author investigated the performance characteristics of stress laminated timber arches in detail. The significant findings include the ability of the structures to sustain large deformations and the redistribution of resulting stresses and also the effectiveness of the arching action which contributed to their stiffness and strength characteristics. The arch shape allowed the timbers to take forces in compression parallel to the grain through the friction between the laminates and end bearing. Unlike a masonry arch, which would fail as a result of a very small shape change, the stress laminated timber arch can sustain large deformations, is very good in bending and recovers on load removal. All load tests to failure over a three-year period resulted in ductile (slow plastic) failures which would give plenty of warning in a real structure.

(a) Laboratory tests (2.1m to 15m spans) (b) Field tests (20m span) Arch structures during testing

The study showed that it is possible to make a good prediction of structural performance using a step-wise elastic linear analysis based on large-deformation concept. Semi-empirical models were also developed to assist Engineers with the analysis-design process, taking into account many possible boundary conditions.

Because this type of structure proved successful with span to depth ratios of 1 to 100 and the material is essentially all timber, these structures were extremely material and energy efficient. This has formed the basis for further development of different types of structure using the stress lamination technique.


18

4.2 Markets and Uses for SLT Structures which are Driving Research The stress lamination technique was first developed in Canada and has been successfully used for thirty years in the USA where many hundreds of flat deck bridges have been built. The arch form was first developed in the UK in 2000 and has since lead to about forty commercial bridges being built.

These bridges are mostly arches for pedestrians but there are some flat decks for 44 tonne lorries. In 2009 the first long span arch and flat deck combination will be built for 44 tonne lorries. Commissions have also been received to design a 40m span railway arch - flat deck combination as well as a 50m span triple arch horse bridge in the Yorkshire Dales.

Arran Golf Club Salcey Walkway – Northampton

Examples of a stress laminated arch structures built in the UK

Stress laminated arches have been used for roofs, though there is still much development required, and there is great interest in the use of the technique for industrial floors. There are also plans to build tall towers using stress laminated legs and bracing panels stressed between legs to provide the lateral restraint. Also, there is a new market in Forestry for temporary bridges for forwarders and harvesters which are large machines with total weights up to 40 tonnes. Because they have a very high ratio of strength to weight, the forest machine can carry its own bridge without the need for a crane.

However, the biggest market will be for footbridges. In the UK there is a need for at least two thousand replacement bridges in rural areas. Development of these kinds of bridges, made in factories and light enough to be transported to site complete and lifted into place, will be in demand. The largest span transportable bridge was completed on an estate at Coignafearn. This is the first bridge of this type to take a hydro electric feed pipe and the timber species chosen specifically so that the timber could be used untreated for environmental reasons.


19

Hydro Bridge Coignafearn – factory produced 4.3 Construction Modules These bridges were first built in situ and involved large scaffolding which was expensive.

Carribber Bridge showing the use of Scaffolding for Construction

To develop a more economic construction method the idea of building full span modules which could be lifted by an excavator looked to be worth trying. A project was set up to explore this.

The aim of this project was to determine the viability, dimensional limits and the type of fixings that would be required to produce self-supporting modules to take construction loads. The modules would have to span the full distance from abutment to abutment and be of a weight that could be easily lifted. The aim was to produce modules weighing less than one tonne so that an excavator (often available on site for other purposes) can lift easily. This means that there will be no need to hire a crane, so keeping cost down.

The width of a module was selected to be a multiple of the number of stressing holes in each full-section of the lamina. For example, for a system with laminates of four holes per section, the width of a module would be a multiple (n) of four times the thickness of a laminate i.e. n × 200mm for 50mm laminates. It was therefore decided to start with 400mm wide modules which would keep even the longest spans under a tonne in weight.


20

The laminates were temporarily fixed by nails or screws to one another to form the modules. The modules would then be lifted into position and laid side by side, while being temporarily supported laterally, and stressed together to form the permanent bridge deck.

4.4 Modules The module fixings are temporary fixings and as such they need to be low cost provided that the required stiffness and strength are achieved.

Assembly of a 12m span test bridge module in the laboratory.

The first trial was to make a roof for a Royal Highland Show exhibit. The laminates were nailed using 3mm diameter × 90mm long ridged nails fired from a gas powered gun. These modules were 4m span and they held together but it soon became clear that greater rigidity would be required for larger modules and some method was required to align the holes.

It was therefore decided to use plastic tubes through the holes to align them and use the same nailing specification to make the modules for testing. This technique worked well but the very dense Douglas fir was resistant to the gas powered nail gun. Shot firing the nails using cartridges would have driven them to the full depth. This was considered too expensive and would render the module approach non viable. However it was judged that future modules would use less dense timber so the nails which did not drive fully into the laminates were hammered home. Nails were driven in approximately 50mm from each side of every hole which amounted to eight nails per laminate. 4.5 Module types and sizes The experimental work was designed to examine a range of different sizes of modules and spans to determine the viability and dimensional limits of modules that could be made. A recent PhD study on stress laminated arch bridges concentrated mostly on span- to-rise ratios of 12:1 and 6:1 so it was decided to maintain these ratios so that test results could be compared to previous experimental work and provide some valuable cross-referencing.


21

As design had shown that a 100mm deck depth is strong enough to withstand pedestrian loading over a span of 6m, 150mm over a span of 12m and 200mm over a span of 18m, it was decided to construct and test two replicate modules of each of these three spans with two different rises. The rises of these arches were 0.5m and 1m for the 6m span, 1m and 2m for the 12m span and 1.5m and 3m for the 18m span.

4.6 Structural load tests Modules were load tested for construction personnel loads by applying a line load at the weakest quarter point. They were tested for handling by lifting at mid span using the laboratory crane and surging the biggest arch up and down.

A 12m span nailed bridge module during lifting and under quarter-point load test.

5.0 Factory Production Tests had shown that modular construction was limited to 12m and it was calculated that a whole bridge of that span could be lifted by a small crane if access was good. Because timber is so much less dense than steel or concrete new opportunities were opening up so a number of demonstration projects were set up to try various factory production units. The object of this exercise was to calculate the cost savings against building on site.

As an example, the building of Arran Golf Course bridge, shown below, demonstrated that factory production of a 12.6m bridge, built traditionally using stressing bars, would lead to total installed cost of £20k for 30m2 of bridge, which is very competitive. The handrails were added on site.


22

Arran Golf Course bridge during installation – the completed bridge is shown above

6. Further Developments The limiting strength of the gas gun driven nails led to considering using screws. In a parallel research, the efficiency of using a combination of glued and nailed/screwed lamination for flat decks and arches and their effects on enhancing structural performance were examined. The findings of this research and possible applications are encouraging and are likely to lead to the use of such techniques for production of long span arch modules (see Coignafearn above). The results of this study are currently being processed for publication in due course.

A possible application of the use of glued and screw method is for the standard steel beam and timber plank road bridges built by Forestry Civil Engineering which have been suffering premature failure of the decks while new health and safety laws are making them difficult to build. To explore this, factory-produced glued and screwed vertically laminated timber slab modules supported on steel beams were tested for wheel patch loads and line loads.

Load testing of glued and screw laminated flat deck.

The modules were glued and screwed with two different types of screw and at two different centres. All screws were 130mm long thus passing through three modules. All


23

screws had a 50mm unthreaded shank so that as they were tightened they pulled the last module into the two adjacent ones. Coach screws, requiring pre-drilling, were used on half the deck while the other half was pulled together using narrow gauge screws without pre-drilling. The two different spacings for each screw type were 300mm and 600mm centres.

The load tests consisted of 200kN on a 300mm square patch, at different locations, to simulate a wheel on four different locations. Finally a line load was applied at mid-span and increased until failure occurred in bending. The success of the tests led to the production of a number of small footbridges using these modules with glue and narrow gauge screws at 450mm centres. These modules have no holes for stressing bars, which makes the preservative treatment easier. The surface is sealed with resin or bitumen and non-slip aggregate is added.

Glued and Screwed Footbridge Lorry Bridge Deck Modules

7.0 Conclusion Timber Engineering is working with the plantation timber coming on stream in great quantities, and at low cost, to take back some quality construction from steel and concrete. These structures are not only competitive, they are sustainable and lightweight. Factory production will bring down costs and improve the quality of production. In time, cost may become low enough for short term, replaceable, structures to become the most attractive strategy. Short enough economic life span may supersede the need for ‘polluting’ preservative treatment and high cost maintenance. The time may come when a minor road bridge can be replaced every ten years by closing the road for only one day. Timber houses could easily be recycled every fifty years economically using specialist timber composites. Steel and concrete will always have their place but in future, timber can be a credible substitute for reasonably large structures - while saving money and reducing pollution.

timber engineering 1

Documents

timber pier timber engineering

timber engineering skills

history of timber engineering

regeneration of timber

exotic species of timber

short period of time

haley engineering

uk stone