Timber construction

Download Timber construction

Post on 06-May-2015

4.445 views

Category:

Education

4 download

Embed Size (px)

TRANSCRIPT

<ul><li>1.1Timber Engineering GeneralIntroductionSven Thelandersson 1.1 Timber our oldest building material1 1.2 Modern timber construction 3 1.3 The timber frame building concept4 1.4 Large-scale timber construction71.1 TIMBER OUR OLDEST various forms has been used ever since, especiallyBUILDING MATERIAL in forested regions. Some parts of Asia have a very long historyProtection and shelter against wind, rain and coldof timber construction. In Japan the oldest tim-is a very basic need for mankind. Since ancient ber structure still in existence is from the seventhtimes, wood has been the most important materialcentury (Yasumura, 2000). A typical historical tim-used for this purpose. In developed cultures, the art ber building is the three storey pagoda, shown inof house construction was already quite advancedFigure 1.2. This building, which still exists, wasseveral thousand years ago. Figure 1.1 shows an constructed in 730 AD. It has a double roof in eacharchaeological reconstruction of a so-called long storey supported by a central wooden pole.house in Central Europe from 3000 BC (Kuklik,Another region with a long tradition of tim-2000). The width of this type of house was in ber construction is Scandinavia, where wood isthe range of 5.57 m and the length varied from a resource that has always been easily available.2045 m. The main structural elements were made The oldest existing timber building in Scandinaviafrom round timber. This can be seen as an early is the Borgund church in Norway, built in theexample of a timber framed house, which intwelfth century (see Figure 1.3). The load-bearing</li></ul><p>2. 2Timber EngineeringFigure 1.1 Archaeological reconstruction of longhouse from 3000 BC (Reproduced from Kuklik, 2000Figure 1.3 Borgund church, Norway, twelfth centuryby permission of Petr Kuklik, Prague University)(Photo: Lund University) Another important application for timber con-struction is bridges. Before the emergence of mod-ern structural materials such as steel and concrete,timber was the dominating structural material inbridge construction. In 55 BC, the emperor JuliusCaesar had a 140 m long temporary timber bridgebuilt across the Rhine (Figure 1.4). The bridge was56 m wide, and allowed two lane trafc. Only10 days were needed to complete the bridge (Tim-ber Bridges, 1996). One of the oldest existing timber bridges is the222 m long Chapel Bridge in Luzern, Switzerland,which was built in 1333 (Stadelmann, 1990). Thisbridge is a well-known tourist attraction. As withmany other bridges in Switzerland, it is covered bya roof, which effectively protects the wood frombiological deterioration. Unfortunately, the bridgewas partly destroyed by a re in 1993, but has beenrebuilt in its original form (see Figure 1.5). Although historic timber structures have disap-peared to a greater degree than, for example, struc-tures made of stone, these examples show thattimber has excellent durability provided that theFigure 1.2 Three storey pagoda, Yakusiji Toto,structures are properly designed and maintained.built 730 (Yasemura, 2000). (Reproduced by permission One aspect of long-term durability of timber struc-of M. Yasemura)tures is that they are often designed in such a waythat damaged elements can easily be replaced.structure is a three-dimensional frame with round- The fact that timber has been used extensivelywood poles and horizontal timber trusses con- as a building material for a very long time does notnected by semi-rigid arch-shaped joints.mean that we have a deep scientic understanding 3. General Introduction3of the behaviour of the material. On the contrary,timber construction has to a large extent been basedon empirical experience and craftsmanship. Woodis therefore often seen as a material with inade-quate control and documentation of its propertiesand behaviour. The purpose with this book is topresent recent advances in research to improveour understanding of the structural performanceof timber.1.2 MODERN TIMBERCONSTRUCTIONToday, the growing stock volume of wood world-wide is estimated to 490 billion m3 (FAO, 2000a).The total world production of timber in 1999 was3275 million m3 (FAO, 2000b). It is estimated thataround 55% of this volume is used as fuel. A sub-stantial part of the raw material is used for pulp andpaper, and 317 million tons of paper was producedworldwide in 1999. The total volume of sawn tim-ber and panel products produced in the same yearwas 592 million m3 , which is 18% of the total rawmaterial production. The relative amounts of dif-ferent wood products are shown in Table 1.1. Timber is used as a major structural materialin a great variety of building and civil engineer-Figure 1.4 Caesars bridge across Rhine 55 BC ing applications. Lightweight timber frame systems(Jacob, 1726) (based on structural timber, engineered wood prod-ucts and panels) may be used for single familyhouses, multi-storey residential buildings and com-mercial buildings. Similar elements are used aswalls and roofs in industrial buildings. Timber isoften used for roof construction in buildings, evenTable 1.1 Production of sawn goods and wood-basedpanels in the world, 1999 (FAO, 2000b)Product Volume,Volume, %106 m3Sawn goods, softwood323.255Sawn goods, hardwood108.518Fibreboard 30.2 5Particle board 75.213Veneer sheets 6.4 1Figure 1.5 Chapel bridge in Luzern, Switzerland orig- Plywood48.1 8inally built 1333, restored after re damage 1993 (Photo: Total 591.6 100Sven Thelandersson) 4. 4Timber Engineeringif the rest of the structure is made from concreteor steel. Large-scale timber systems (based on glu-lam, LVL and other engineered wood products)may be used for industrial and commercial build-ings with long spans, as well as for bridges, park-ing decks, etc. Worldwide, the potential to increasethe utilisation of timber for these applications inthe future is large. There are many general advantages in using tim-ber for building purposes. It is an environmentallyfriendly, easily recyclable material. The energyconsumption during production is very low com-Figure 1.6 The anatomy of a typical timber framepared to that of other building materials. Timber small househas a low weight in relation to strength, whichis advantageous for transport, erection and pro- Timber frame systems can be conceived as com-duction. The foundation can be simplied andposite wall and oor units built up from timberlow inertia forces make the building less sensitive framing, panel products, insulation, cladding, etc.,to earthquakes. Furthermore, wood has aesthetic with good possibilities to adapt the design to vari-qualities, which give great possibilities in archi- ous requirements. One and the same composite unittectural design.in a timber frame system can be utilised for the Building systems based on wood has a greatpotential to be rational and cost-effective. Expe- transfer of vertical loads,riences from North America, where timber frame stabilization of wind and earthquake loads,building systems have a dominating position in the physical separation,market, indicate that it is possible to reduce the costof low-to-medium rise buildings signicantly by re separation,using lightweight building systems based on tim- sound insulation, andber and panel products. These systems have the thermal insulation.advantage of simple construction techniques at theIt is important that the design is made so that all thebuilding site, and very short construction times canrelevant requirements are met in an optimal way.be achieved.In the design of walls and oors, different aspectscan be identied as critical. The factors governing1.3 THE TIMBER FRAMEthe design of walls are, in order of priority:BUILDING CONCEPT re resistance,Timber frame buildings are built up by a skeleton horizontal stabilisation,of timber joists and studs, covered with panels sound insulation, andfastened to the wood elements. Wood-based panels, vertical loading.such as plywood, OSB, bre-board or chipboard,with structural quality, are commonly used in A typical design of a load-bearing, stabilising walltimber frame buildings. Gypsum panels or similarused for separation between ats is shown inproducts are also widely used in combination with Figure 1.8.timber, mainly to provide re resistance. A typical For the design of oors, the most importanttimber frame house is shown in Figure 1.6.factors are, in order of priority: The timber frame concept is also very competi-tive for multi-storey, multi-residential buildings up impact sound insulation,to 56 storeys (see Figure 1.7). vibration control, 5. General Introduction 5Figure 1.7 Four storey platform frame timber house under construction (Reproduced by permission of HolgerStaffansson, Skanska AB)timber joists are to some extent used in CentralEurope. A very crucial issue for the efciency of a timberframe system is the solution of wall-oor joints. Inthe design of such joints, a number of aspects mustbe taken into account: sound performance, load transfer vertically and in shear, re separation, shrinkage and settlement of the joint (perpendic-Figure 1.8 Horizontal section of double wall separa-ular to the grain),tion between ats. Double re guard gypsum panels are thermal insulation,used air tightness, ease of erection, simplicity in production, and degree of prefabrication of walls and oors, and possibility for the installation of services. economy.In many countries, customers expect acoustic per- In the platform frame system commonly used informance of a high standard. It is possible to meet North America, standard solutions are available forthese requirements with lightweight oor struc- wall/oor joints (see Figure 1.9a). Since the verti-tures, but the solutions often become complicated cal forces are transferred in compression perpen-and expensive. For this reason, there is a need dicular to the grain in the whole joint, substantialto develop better oor solutions. One alternative shrinkage and vertical settlements will occur. Thecould be to use oors based on laminated tim- deformations created by this can be quite difcultber decking, where the material cost is higher, but to handle in a multi-storey building. An examplethe oor is still competitive due to a simpler pro- of a wall-oor joint solution, designed to minimiseduction process. Composite oors with concreteshrinkage settlements in the joint, is shown inon top of timber decking, or in combination withFigure 1.9b. 6. 6Timber Engineering7045 Floor beams Bottom plate45 Rim joist Nogging 220Nogging Top plateFloor support 45145120Figure 1.9 Wall-oor joints. (a) Platform frame design, (b) design to minimise shrinkage In concrete and masonry buildings, staircaseand elevator shafts as well as cross walls canbe used to stabilise the building. In timberframe construction, timber frame shear walls areused for lateral stabilisation against wind andearthquakes (see Figure 1.10). For multi-storeytimber framed buildings, the issue of stabilisationis not trivial. A trend towards narrow houses toprovide good daylight, as well as requirements ofhigh acoustic performance, makes it more difcultto stabilise the buildings in an economical manner.To ensure good acoustic performance, doublewalls are used between ats (see Figure 1.8), andto prevent anking transmission, the horizontaloor diaphragms should be disconnected at thedouble walls. However, continuity in the oor Figure 1.10 Force transfer in multistorey timber framediaphragms is needed for efcient horizontal building under lateral loading. (Drawing: Sverkerstabilisation. This conict must be resolved byAndreasson. Reproduced by permission of Lundthe structural engineer. Recent research has shown University)that a better understanding of the force transferin timber frame systems can contribute towardsachieving a more economical and exible design system is sufcient, i.e. if the forces can be trans-of the stabilising system in timber frame buildingsferred through the wall boards required for re(e.g. see Andreasson (2000) and Paevere (2001)). protection, or whether it is necessary to use more The economy of multi-storey timber houses de- expensive wood-based panels and extra studs andpends very much upon whether a simple bracinganchorages. 7. General Introduction 71.4 LARGE-SCALE TIMBERCONSTRUCTIONThe maximum dimension of solid timber sawnfrom logs is of the order of 300 mm or evenless, depending on species and growth region. Thismeans that the maximum possible span of struc-tural timber beams in practice is limited to 57 m.Before the appearance of engineered wood prod-ucts such as glulam, timber trusses were thereforecommonly used to achieve larger spans, which isoften needed in roof and bridge construction. Tim-Figure 1.12 Glulam arch roof for Stockholm centralber trusses produced from structural timber are still railway station, built 1925. Reproduced by permissionthe most common solution for roof structures in of Svenskt Limtr AB asmall residential houses (Figure 1.11). This typeof truss is today almost exclusively designed withprinciple unlimited, but for practical reasons max-punched metal plate fasteners, creating stiff and imum depths are of the order of 2 m. This makesstrong truss joints at a low cost. Timber roof trussesglulam an ideal material to create structures forare frequently used for spans of up to 12 m, butlarge spans. A variety of structural systems basedcan be designed for spans up to 3040 m.on straight and curved glulam members has been Another possibility to extend the spans for tim- developed for roofs with spans of up to 100 mber structures is to use laminated beams, i.e tim-(see Figure 1.13). Today, several other wood-basedber laminations stacked on top of each other andproducts are available for large-scale timber struc-structurally connected to form members with large tures, such as Laminated Veneer Lumber (LVL)cross-sections. Early applications used mechani-and Parallel Strand Lumber (PSL) (see Chapter 4cal fasteners, such as bolts, dowels and rods, to in Part One). These products are suitable for largerconnect the laminations. The potential of the lami- spans in a similar way as straight glulam members.nation technique was, however, not fully exploited For straight and tapered beams, spans of 30 muntil synthetic glues became generally availableand more can be achieved. Figure 1.14 shows ain the early twentieth century (see Figure 1.12). typical glulam roof with straight tapered beams forGlued laminated timber or glulam became one ofhall buildings.the rst engineered wood products, and is still very Spans of up to 50 m can be realised by threecompetitive in modern construction. By bendinghinge frames built up by two curved glulam ele-the laminations before gluing, it can be produced ments, as shown in Figures 1.13 and 1.15. Framesin curved shapes. The cross-section depth is in can also be made from straight members, withWeb bracingFigure 1.11 Typical roof truss for a single family house (Drawing: Jacob Nielsen, Aalborg University, Denmark) 8. 8 Timber Eng...</p>