timber construction

1

Timber Engineering – GeneralIntroductionSven Thelandersson

1.1 Timber – our oldest building material 11.2 Modern timber construction 31.3 The timber frame building concept 41.4 Large-scale timber construction 7

1.1 TIMBER – OUR OLDESTBUILDING MATERIAL

Protection and shelter against wind, rain and coldis a very basic need for mankind. Since ancienttimes, wood has been the most important materialused for this purpose. In developed cultures, the artof house construction was already quite advancedseveral thousand years ago. Figure 1.1 shows anarchaeological reconstruction of a so-called longhouse in Central Europe from 3000 BC (Kuklik,2000). The width of this type of house was inthe range of 5.5–7 m and the length varied from20–45 m. The main structural elements were madefrom round timber. This can be seen as an earlyexample of a timber framed house, which in

various forms has been used ever since, especiallyin forested regions.

Some parts of Asia have a very long historyof timber construction. In Japan the oldest tim-ber structure still in existence is from the seventhcentury (Yasumura, 2000). A typical historical tim-ber building is the three storey pagoda, shown inFigure 1.2. This building, which still exists, wasconstructed in 730 AD. It has a double roof in eachstorey supported by a central wooden pole.

Another region with a long tradition of tim-ber construction is Scandinavia, where wood isa resource that has always been easily available.The oldest existing timber building in Scandinaviais the Borgund church in Norway, built in thetwelfth century (see Figure 1.3). The load-bearing

2 Timber Engineering

Figure 1.1 Archaeological reconstruction of longhouse from 3000 BC (Reproduced from Kuklik, 2000by permission of Petr Kuklik, Prague University)

Figure 1.2 Three storey pagoda, Yakusiji Toto,built 730 (Yasemura, 2000). (Reproduced by permissionof M. Yasemura)

structure is a three-dimensional frame with round-wood poles and horizontal timber trusses con-nected by semi-rigid arch-shaped joints.

Figure 1.3 Borgund church, Norway, twelfth century(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 was5–6 m wide, and allowed two lane traffic. 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 fire 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 thestructures are properly designed and maintained.One aspect of long-term durability of timber struc-tures is that they are often designed in such a waythat damaged elements can easily be replaced.

The fact that timber has been used extensivelyas a building material for a very long time does notmean that we have a deep scientific understanding

General Introduction 3

Figure 1.4 Caesar’s bridge across Rhine 55 BC(Jacob, 1726)

Figure 1.5 Chapel bridge in Luzern, Switzerland orig-inally built 1333, restored after fire damage 1993 (Photo:Sven Thelandersson)

of 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 TIMBERCONSTRUCTION

Today, 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-ing applications. Lightweight timber frame systems(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, even

Table 1.1 Production of sawn goods and wood-basedpanels in the world, 1999 (FAO, 2000b)

Product Volume,106 m3

Volume, %

Sawn goods, softwood 323.2 55Sawn goods, hardwood 108.5 18Fibreboard 30.2 5Particle board 75.2 13Veneer sheets 6.4 1Plywood 48.1 8

Total 591.6 100


if 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-pared to that of other building materials. Timberhas a low weight in relation to strength, whichis advantageous for transport, erection and pro-duction. The foundation can be simplified andlow inertia forces make the building less sensitiveto earthquakes. Furthermore, wood has aestheticqualities, which give great possibilities in archi-tectural design.

Building systems based on wood has a greatpotential to be rational and cost-effective. Expe-riences from North America, where timber framebuilding systems have a dominating position in themarket, indicate that it is possible to reduce the costof low-to-medium rise buildings significantly byusing lightweight building systems based on tim-ber and panel products. These systems have theadvantage of simple construction techniques at thebuilding site, and very short construction times canbe achieved.

1.3 THE TIMBER FRAMEBUILDING CONCEPT

Timber frame buildings are built up by a skeletonof timber joists and studs, covered with panelsfastened to the wood elements. Wood-based panels,such as plywood, OSB, fibre-board or chipboard,with structural quality, are commonly used intimber frame buildings. Gypsum panels or similarproducts are also widely used in combination withtimber, mainly to provide fire resistance. A typicaltimber frame house is shown in Figure 1.6.

The timber frame concept is also very competi-tive for multi-storey, multi-residential buildings upto 5–6 storeys (see Figure 1.7).

Figure 1.6 The anatomy of a typical timber framesmall house

Timber frame systems can be conceived as com-posite wall and floor units built up from timberframing, panel products, insulation, cladding, etc.,with good possibilities to adapt the design to vari-ous requirements. One and the same composite unitin a timber frame system can be utilised for the

• transfer of vertical loads,• stabilization of wind and earthquake loads,

• physical separation,

• fire separation,

• sound insulation, and• thermal insulation.

It is important that the design is made so that all therelevant requirements are met in an optimal way.In the design of walls and floors, different aspectscan be identified as critical. The factors governingthe design of walls are, in order of priority:

• fire resistance,• horizontal stabilisation,• sound insulation, and• vertical loading.

A typical design of a load-bearing, stabilising wallused for separation between flats is shown inFigure 1.8.

For the design of floors, the most importantfactors are, in order of priority:

• impact sound insulation,

• vibration control,


Figure 1.7 Four storey platform frame timber house under construction (Reproduced by permission of HolgerStaffansson, Skanska AB)

Figure 1.8 Horizontal section of double wall separa-tion between flats. Double fire guard gypsum panels areused

• simplicity in production, and

• possibility for the installation of services.

In many countries, customers expect acoustic per-formance of a high standard. It is possible to meetthese requirements with lightweight floor struc-tures, but the solutions often become complicatedand expensive. For this reason, there is a needto develop better floor solutions. One alternativecould be to use floors based on laminated tim-ber decking, where the material cost is higher, butthe floor is still competitive due to a simpler pro-duction process. Composite floors with concreteon top of timber decking, or in combination with

timber joists are to some extent used in CentralEurope.

A very crucial issue for the efficiency of a timberframe system is the solution of wall-floor joints. Inthe design of such joints, a number of aspects mustbe taken into account:

• sound performance,

• load transfer vertically and in shear,

• fire separation,

• shrinkage and settlement of the joint (perpendic-ular to the grain),

• thermal insulation,• air tightness,

• ease of erection,• degree of prefabrication of walls and floors, and• economy.

In the platform frame system commonly used inNorth America, standard solutions are available forwall/floor joints (see Figure 1.9a). Since the verti-cal forces are transferred in compression perpen-dicular to the grain in the whole joint, substantialshrinkage and vertical settlements will occur. Thedeformations created by this can be quite difficultto handle in a multi-storey building. An exampleof a wall-floor joint solution, designed to minimiseshrinkage settlements in the joint, is shown inFigure 1.9b.


Bottom plateRim joistNogging

Top plate

Nogging

Floor support

Floor beams

120145

7045

45

45

220

Figure 1.9 Wall-floor 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 difficultto stabilise the buildings in an economical manner.To ensure good acoustic performance, doublewalls are used between flats (see Figure 1.8), andto prevent flanking transmission, the horizontalfloor diaphragms should be disconnected at thedouble walls. However, continuity in the floordiaphragms is needed for efficient horizontalstabilisation. This conflict must be resolved bythe structural engineer. Recent research has shownthat a better understanding of the force transferin timber frame systems can contribute towardsachieving a more economical and flexible designof the stabilising system in timber frame buildings(e.g. see Andreasson (2000) and Paevere (2001)).

The economy of multi-storey timber houses de-pends very much upon whether a simple bracing

Figure 1.10 Force transfer in multistorey timber framebuilding under lateral loading. (Drawing: SverkerAndreasson. Reproduced by permission of LundUniversity)

system is sufficient, i.e. if the forces can be trans-ferred through the wall boards required for fireprotection, or whether it is necessary to use moreexpensive wood-based panels and extra studs andanchorages.


1.4 LARGE-SCALE TIMBERCONSTRUCTION

The 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 5–7 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-ber trusses produced from structural timber are stillthe most common solution for roof structures insmall residential houses (Figure 1.11). This typeof truss is today almost exclusively designed withpunched metal plate fasteners, creating stiff andstrong truss joints at a low cost. Timber roof trussesare frequently used for spans of up to 12 m, butcan be designed for spans up to 30–40 m.

Another possibility to extend the spans for tim-ber structures is to use laminated beams, i.e tim-ber laminations stacked on top of each other andstructurally connected to form members with largecross-sections. Early applications used mechani-cal fasteners, such as bolts, dowels and rods, toconnect the laminations. The potential of the lami-nation technique was, however, not fully exploiteduntil synthetic glues became generally availablein the early twentieth century (see Figure 1.12).Glued laminated timber or glulam became one ofthe first engineered wood products, and is still verycompetitive in modern construction. By bendingthe laminations before gluing, it can be producedin curved shapes. The cross-section depth is in

Figure 1.12 Glulam arch roof for Stockholm centralrailway station, built 1925. Reproduced by permissionof Svenskt Limtra AB

principle unlimited, but for practical reasons max-imum depths are of the order of 2 m. This makesglulam an ideal material to create structures forlarge spans. A variety of structural systems basedon straight and curved glulam members has beendeveloped for roofs with spans of up to 100 m(see Figure 1.13). Today, several other wood-basedproducts are available for large-scale timber struc-tures, such as Laminated Veneer Lumber (LVL)and Parallel Strand Lumber (PSL) (see Chapter 4in Part One). These products are suitable for largerspans in a similar way as straight glulam members.

For straight and tapered beams, spans of 30 mand more can be achieved. Figure 1.14 shows atypical glulam roof with straight tapered beams forhall buildings.

Spans of up to 50 m can be realised by threehinge frames built up by two curved glulam ele-ments, as shown in Figures 1.13 and 1.15. Framescan also be made from straight members, with

Web bracing

Figure 1.11 Typical roof truss for a single family house (Drawing: Jacob Nielsen, Aalborg University, Denmark)


Straight beam

< 30

h ≈ l / 17

Tapered beam 10−30 h ≈ l / 30H ≈ l /16

Pitched cambered beam

10−20 h ≈ l / 30H ≈ l /16

Three-hingedtruss with tie-rod

15−50 h ≈ l / 30

Three-hingedtruss with tie-rod

and trussed beams

20−100 h ≈ l / 40

Three-hinged arch 20−100 h ≈ l / 50

Three-hingedcurved frame

15−50 h ≈ (s1+s2 )/15

Three-hingedframe with finger-

jointed knee

10−35 h ≈ (s1+s2 )/ 13

Three-hingedframe with

trussed knee

10−35 h ≈ (s1+s2 )/ 15

SYSTEM SPAN SECTIONDEPTHSm

DESCRIPTION

h

h

h

h

h

h

h

h

h

S2

S2

S2

S1

S1

S1

H

H

I

I

I

I

I

I

Figure 1.13 Structural systems for glulam (Glulam Manual, 1995. Reproduced by permission of Svenskt Limtra AB)

the moment resisting frame corners manufacturedusing a finger jointing technique. Several types ofmechanical joints are also commonly used for suchframe corners, but they are generally less effectivein resisting moments.

Similarly, plane arches can be constructedby pre-manufactured curved glulam elements forspans of up to 100 m. An example is shown

in Figure 1.16. For spans larger than 60 m, thedifficulty associated with transport of the curvedelements is usually a restricting factor. Glulamarches are normally designed as three-hinged,which gives simpler joints to be arranged at thebuilding site. Also, a statically determinate systemis usually preferred to avoid restraint forces frommoisture movements in the wood.


Figure 1.14 Roof structures with straight glulambeams can be used for spans of up to 30–40 m. Steelwires are used for diagonal wind bracing

Figure 1.15 Three-hinged glulam curved frame.Reproduced by permission of Svenskt Limtra AB

Figure 1.16 Glulam arch structure made from twocurved glulam elements. Reproduced by permission ofSvenskt Limtra AB

For very large spans, glulam trusses may beused. Very efficient truss joints can be madeby slotted-in steel plates combined with dowel

fasteners. Rational methods for manufacturingsuch joints have been developed, making trusssystems of this type competitive. Examplesof trussed glulam structures are shown inFigures 1.17 and 1.18.

Glulam or other engineered wood products mayalso be used efficiently for spatial frames and domestructures. Special detailing solutions are usually

Figure 1.17 Glulam arch truss with a free span of86 m for a sports facility in Lillehammer, Norway(Photo: Sven Thelandersson)

Figure 1.18 Roof structure for Gardemoen air termi-nal, Oslo, Norway. The main girder is a slightly curvedglulam truss, covered by plywood for the sake of appear-ance. Reproduced by permission of Svenskt Limtra AB


Figure 1.19 Interior from the Tacoma Dome, Wash-ington, USA with a 162 m span (Photo: Sven Thelander-sson)

Figure 1.20 Glulam roof for a swimming hall inBad Durrheim, Germany, built 1987 (Reproduced bypermission of Arge Holz, Dusseldorf)

developed for the three-dimensional joints in suchsystems. Figure 1.19 shows one of the largesttimber structures in the world, with a diameterof 162 m.

Many good architects around the world preferto work with wood as a major structural material.There are numerous examples worldwide of largespan timber buildings with excellent architectureand innovative design. An example is shown inFigure 1.20.

Another important application for large-scaletimber construction is bridges. For small spans,straight beams of solid wood, glulam or otherengineered wood products can be used as theprimary load-bearing elements. Trusses, arches orframed structures can be used as primary structuresfor larger spans. A very common solution for thebridge deck is to use laterally prestressed timberplates of the type shown in Figure 1.21. As analternative to lateral prestress, the planks can benailed to each other, although this gives lowertransverse stiffness of the deck.

In modern bridge construction, timber isgrowing in popularity for foot and bicycle bridgesas well as road bridges with moderate spans,especially in the USA, Central Europe andScandinavia. One reason for this is environmentalawareness and the trend towards the use ofecologically sound materials in construction. Newefficient jointing techniques developed in recentyears are also very important for competitivenessin timber bridge construction. An excellentexample of a modern timber bridge for road trafficis shown in Figure 1.22.

Figure 1.21 Bridge with laminated timber decking (Reproduced by permission of Svenskt Tra)


Figure 1.22 Europabrucke, Murau, Austria, built1993. Glulam structure with concrete bridge deck(Reproduced by permission of Holger Gross)

A key factor for timber bridge design is dura-bility. Preservative chemical treatment is not anattractive alternative considering environmentalpolicies of today. However, by careful design anddetailing, the wood material in a timber bridge canbe kept more or less constantly dry, so that bio-logical decay is avoided and long lifetimes canbe achieved with or without very limited use ofpreservative treatment.

Laminated decks of the type shown inFigure 3.16 have also become popular for floors

in house construction. With such floor structures,combined with concrete or other materials, goodsolutions for sound insulation and fire can beachieved at reasonable costs. Massive timberconstructions are sometimes also used for thewhole structural building system, including wallunits.

REFERENCES

Andreasson S. (2000) Three-Dimensional Interaction inStabilisation of Multi-Storey Timber Frame BuildingSystems. Report TVBK-1017, Division of StructuralEngineering, Lund University, Sweden.

FAO (2000a) The Global Forest Resource Assess-ment 2000, Summary report.

FAO (2000b) Yearbook of Forest Products.Glulam Manual (1995) Svenskt Limtra, Stockholm (in

Swedish).Jacob L. (1776) Brucken und Bruckenbau (Theatrum

Pontificale, oder Schauplatz der Bruckenbaues). In ThSchafer GmbH Hannover, 1982.

Kuklik P. (2000) Development of timber framed housesin Central Europe. Proc. of COST E5 Workshop,Timber Frame Building Systems, Ed. A. Cecotti,S. Thelandersson, Venice, Italy.

Paevere P. (2001) Full-Scale Testing, Modelling andAnalysis of Light-Framed Structures Under Earth-quake Loading. PhD thesis, Civil and EnvironmentalEngineering, University of Melbourne.

Stadelmann W. (1990) Holzbrucken der Schweiz – einInventar. Verlag Bundner Monatsblatt, Chur, Switzer-land.

Timber Bridges (1996) Trainformation AB, Stockholm,Sweden (in Swedish).

Yasemura M. (2000) Seismic performance of timberstructures in Japan. Proc. of COST E5 Workshop,Timber Frame Building Systems, Ed. A. Cecotti,S. Thelandersson, Venice, Italy.

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