Materials and Joints in Timber Structures

Higher and larger timber buildings are built today. The building is exposed to static lateral wind loads that cause displacements that might lead to discomfort and non-function of the building. To determine the size and behavior of horizontal displacements, two timber systems has been studied, a light frame system with shear walls and a post and beam system with diagonal bracing. The stabilizing wall segments have been analyzed with a FE model and subjected to static lateral wind load and vertical dead load in serviceability limit state. Both plywood and particle board were used as sheet materials. To reduce flanking transmission Sylomer® is applied in the light frame system. The total lateral displacement varies between 4 to 125 mm in the light frame system and 2 to 5 mm in the post and beam system. Removing the Sylomer® damping material from the light-frame system would decrease the lateral displacements with 1.5-3 times, which needs to be further investigated.

The Korean traditional house,

Hanok

, is a post-and-beam timber structure with a large tiled roof designed to keep the house cool in summer (Fig. 1). The solid wood members are connected together without metallic connectors, using complex wood–wood joints, while the posts are fitted inside cornerstones. Associated with the stabilizing effect of the heavy roof, this provides a basic lateral resistance to the house, which historically proved effective against the few earthquakes which occurred in this area of low seismicity. Nowadays, the Hanok house is still fairly popular among Koreans thanks to its cultural, aesthetic, and eco-friendly aspects.

In comparison to a demolition of existing buildings with severe technical deficits, usually the retrofitting of buildings is more effective in order to prepare them for low energy consumption and new necessities as communication and media connection or HVAC-installation (Heat, Ventilation and Air-Conditioning). Prefabricated retrofit solutions are developed throughout Europe to enable higher levels of industrialization in building envelope modernization and hence additionally improvements in energy efficiency. Five years of experience and a reasonable number of demonstrations done with timber-based element system (TES) façades show tendencies for best-practice building construction.

This paper focuses on the jointing between single façade elements and the connection of those elements to the existing building. Being a crucial construction detail within the TES-façade, the joint area shall meet various requirements and challenges, from load bearing over hygro-thermal to fire safety functionality. The results of in-depth construction detailing lay out the requirements and principles of the TES joint.

The results from bending tests on 107 laminated, green-glued, beams manufactured from Norway spruce side boards are presented. The beams were made by face gluing 21-25 mm thick boards using a commercial one-component moisture curing polyurethane adhesive. In addition to the bending test results, results from shape stability measurements after climatic cycling and bond line strength and durability test results are also presented. The results from the bending tests show that, by applying very simple grading rules, it is possible to obtain beams with high bending strength (with a 5%-percentile characteristic value of 40,1 MPa) and substantial stiffness (mean value of 14360 MPa). Also the shape stability of the beams and the strength and the durability of the interlaminar bonds were found to be satisfactory.

The moment-rotation-behaviour of a post-tensioned beam-column timber joint has been analysed extensively with a series of static bending tests. The timber joint was loaded at the end of the beams in order to apply a moment to the connection. The tests were conducted with various forces in the tendon, from 300 kN up to 700 kN. The bending tests were performed with a controlled load level, so that no failure perpendicular to the grain in the column occurred.

The maximally allowable vertical load to be applied was estimated using a simple spring model. A final bending test was conducted in order to study the failure mode of the post-tensioned timber beam-column joint. The vertical load on the beams was increased until the tendon-elongation got so high that the test had to be stopped for safety reasons.

This paper presents the main results of bending tests on the post-tensioned timber joint. Attention will be given to the structural behaviour and to the influence of the applied post-tensioning force on the connection stiffness. The experimental results will be compared to the results obtained using a simplified analytical calculation model.

In this paper a framework for updating the load and resistance properties of a partly failed timber construction based on different kind of information is presented. That includes information available, such as the resistance of the failed member(s) or the results of destructive and non-destructive measurements. In accordance to the type of information different updating methods are presented. The application of the framework is exemplary illustrated on the case study.

The hardwood species European oak, European chestnut and black locust are hardly used for engineering structures in their naturally grown shape, although such a use is generally possible. From forest thinnings these hardwoods are available as low-cost material to a certain extent and their durability and remarkable strength can fulfil the demands of robust timber structures. However, the irregular shape of naturally grown logs and the subsequent complexity of connections inhibit engineering applications. Thus, there is a gap between the potential of an available low-cost material and the possibility of a value-added application. In order to bridge this gap the present contribution aims first at giving basic ideas to make naturally grown logs more calculable and second at presenting a modular screw connection. Hence, compression tests on naturally grown logs, screw withdrawal tests and tests on screw connections were performed. Based on the experimental results, strength models for the compression capacity and the withdrawal resistance of screws were developed and a high strength and ductile connection type was designed. The findings of the work may widen the possibilities in the engineering design of timber structures made of naturally grown logs.

In consideration of sustainable buildings, closing life cycle loops becomes more and more important. Up to now reuse and recycling is taken rarely into account in building processes. With rising consumption of wood for energetic use recycling of material becomes more important.

Up to now there are various studies in EU market ([1], [2], [3]), which quantify the usage of wood in market shares. Explicit calculations on recycling of wooden material in the building sector have not yet been done. In general the demand for reclaimed wood products in the building sector will rise due to the fact that the preferred option has to be the reuse and the recycling of reclaimed wood. The thermal use of wood is the last option in the cascade of use. On this option the refinement of reclaimed wood for innovative products as well as the broadening and enhancement of the cascades of reuse and recycling is strongly needed for the timber construction industry. Long-term and resource efficient use of wood from premium quality (like laminated wood, plywood, timber frame construction) is necessary to ensure sustainable construction with wood. In the process of planning new wooden construction the dismantling and reuse / recycling of the products has to be considered too.

In this paper outcomes of the woodwisdom-net research project ECO2– wood in carbon efficient construction – as well as calculations on wood consumption of wide-span timber structures and investigated case studies on a very detailed level are brought together to show the state of art and theories to improve resource efficient usage of wood. Aim is a realistic estimation of theoretical scenarios for end of life and their influence on planning processes as well as the influence on life cycle assessment according to EN 15978. In another approach the total demolition of an old wooden house in the Alps was evaluated. It is a typical example for a long-used construction with numerous repair intervals, changes, and additions. This leads to a wide variety of fractions and often to a contamination of wood from preservatives. The fractions of the demolished house mainly consist of small bits and pieces dedicated to different recycling options than wide span structures. The different waste wood fractions in strength, scale, and size will tolerate certain processing options with an emerging range of recycling products.

A better management of its renewable resources supports the material supply of the wood sector to ensure a long-term availability of solid wood products at reasonable prices. This will allow preservation and also gain market shares now and in the future.

Thermo-hygro-mechanical processes can be used to densify and to form wood. This is applied to produce wooden tubes of structural size by shaping of boards, which were densified previously transverse to the grain. Profiles with hollow cross sections, like tubes, have several advantages compared to those with compact cross sections. Due to the larger moments of inertia, a higher load-bearing capacity for bending and buckling with a given amount of material can be reached. Furthermore, the low thickness of the tube walls allows employing smallsized tree sections, which are currently used only energetically or for fibre production. Thus, production of these tubes might increase the added value of the forests. In this contribution, a brief overview over the load-bearing behaviour of circular moulded wooden tubes exposed to axial compression, bending and torsion is given. Moreover, one of the first practical applications of the tubes, the tower of a small-sized wind power plant is presented.

In order to resist lateral loads, modern methods of timber construction are reliant on the in-plane shear strength of the walls orientated parallel to the applied action. In closed panel systems, the shear stresses are transferred to the foundations by the sole plate through the sheathing board, which is usually mechanically jointed to the timber frame. Since closed panels are delivered to site as single units, access to the internal bottom rail is rather restricted and novel efficient solutions to secure the panel to the substrate are required. Sole plate fixing components for open and closed panel systems were tested in isolation and combination in order to validate a simplistic version of the weakest link theory. As a result, findings were embedded into a software database with a direct link to a previously developed sole plate and racking design application. This integrated process facilitates the structural optimization of the sole plate detail.

Due to their efficiency, lightweight, ease of erection and low cost, steel and aluminium thin-walled structures have become very popular in the construction industry over the past few decades. Applications include roof and wall systems (purlins and girts), storage racks, and composite concrete and steel slabs. The effectiveness of these structures lies in the cross-sectional shape of the profiles which enhances their strength by controlling the three fundamental buckling modes: local, distortional, and global. However, despite the attractiveness of these structures, steel and aluminium are greenhouse gas intensive materials and do not produce sustainable structural products. This paper presents an investigation performed at the Griffith School of Engineering, Griffith University, which shows manufacturing these types of profiles in timber is possible. Short composite thinwalled timber Cee-sections (500 mm long) were fabricated by gluing together thin softwood (

Araucaria cunninghamii

) veneers (1 mm thick). Two types of Ceesections were considered, one with a web stiffener to increase the local buckling capacity of the profile and one without. The profiles were tested in compression and the test results are presented and discussed in the paper in terms of structural behaviour and performance. Further research directions are proposed in order to provide efficient and lightweight sustainable structural products to the timber industry.

Complex indeterminate structures with multiple loadpaths are now increasingly common in timber. However, due to connection fit-up and the material variability, one loadpath might be stiffer and attract forces which are perhaps two or three times the level predicted by a simple elastic analysis. To achieve overall ductile behavior is important to ensure that the connections are sufficiently ductile to protect the brittle timber members from damage. This paper examines whether the rules in current design codes are adequate for the design of such structures.

The paper examines the build-up and some aspects of the mechanical behavior of a novel lightweight timber composite called the Keel-web element. The element is a double-skinned composite similar to a multiple box beam element. The flanges consist of finger jointed lumber chords arranged and glued in parallel. The name-giving specific characteristic of the element consists of the multiple S-shaped webs made of plywood or OSB, resembling ship keels, glued in between the flanges. The element, which can be produced in a fully automatized process, as straight or cambered, with lengths of up to 35 m, has recently obtained a German technical building approval. The build-up will first be examined, and afterwards, the engineering design approach for the shear force and compression capacity, partly following Eurocode 5, is shown.

In order to analyse in greater detail the stability and nonlinear bending of the pre-curved webs at end supports, the load deflection behavior of the webs at an increasing support force is studied by 2

nd

and 3

rd

order beam column and plate theory. Additionally, the bending stresses introduced from the manufacturing process of the S-shaped webs which are subject to relaxation, must also be considered. With increasing loads, the out of plane web displacements are restrained by the adjacent webs, leading to a reduction of the free web height. Solving the differential equation for a fixed-end beam, valid results for small deformations can be calculated, while a nonlinear finite element simulation is performed in order to consider large deformations and contact boundaries. It is shown that the stressstate of a 3D shell model with contact simulation of the webs provides a good estimate of the experimental load capacities.

The technology of gluing in wood construction has evolved considerably in recent years. This has been shown especially in timber bridges that Glulam is now the main building material. A further development is the so-called block gluing, which provides a good basis for supporting structures. Numerous bridges, especially in central Europe, appeal by unique design and monolithic and solid construction. These structures base on two main developments: block lamination of glulam and the composite of timber and concrete to one structural system.

Present-day very high strength steel (vhss) grades show high plastic deformation capacity whilst reaching tension strengths of up to 1400 MPa. These properties open new application fields in timber engineering. Replacing mild steel dowels in timber joints with vhss dowels should lead to higher load carrying capacities or to leaner joints (thinner dowels and smaller cross sections) without losing the desired joint failure mode with one or two plastic hinges per shear plane. Extensive test series on double-shear timber joints with slotted-in steel plates have been carried out using 12 mm and 24 mm dowels. The chosen timber species were spruce, beech and azobé. One, three and five dowels in a row were tested and the used dowel steel grades were high strength steel (hss) with a mean tension strength of 590 MPa and vhss with a mean tension strength of 1390 MPa. The test outcomes have shown that joints with vhss dowels reach a higher load carrying capacity than the same joints using hss dowels, but are still able to develop plastic hinges. No correlation between density, load carrying capacity and stiffness within one wood species could be found. The effective number of fasteners showed a trend to be lower for the joints with vhss dowels and at the same time, is dependent on the used wood species as generally, ductile species such as beech show large deformations and subsequently high load carrying capacities if one dowel is used.

The wood engineering community has dedicated a significant amount of effort over the last decades to establish a reliable predictive model for the load-carrying capacity of timber connections under wood failure mechanisms. Test results from various sources (Foschi and Longworth 1975; Johnsson 2003; Quenneville and Mohammad 2000; Stahl et al. 2004; Zarnani and Quenneville 2012a) demonstrate that for multi-fastener connections, failure of wood can be the dominant mode.

This paper presents a study on the implementation of connection ductility in timber structures. Regardless for which purpose ductility in timber structures is needed, it is necessary to avoid a brittle failure of the timber element before the ductile element is in the stage of yielding. An over-strength factor is introduced to consider the required distance of the load-bearing resistance of the beam element from the introduced bending moment initiated by the load-carrying capacity of the fasteners. Hence, a Monte-Carlo simulation was conducted, focusing particularly on the scattering of the material properties to determine a reliability index of a joint loaded in bending. Since the reliability index is based on the application of the ductility, a range of over-strength factors is given for different reliability indices. The Monte-Carlo simulation is based on the mean material properties of the experimental specimens. The experiments are explained and the non-linear behaviour is displayed not only for connections loaded in tension but also for joints loaded in bending.

In recent years, in the field of wood industry, especially in Japan, it is required that planed raw materials switch from import materials to domestic lumber and development of new usage suitable for domestic lumber is strongly demanded. To promote and increase the demand of domestic cedars, the various types of products or building members by use of plywood has been suggested in Japan. We suggested the combined structure which sandwiched a steel material in plywood. And also, to investigate the structural resistant mechanism and performance under compression, the loading test of the sandwiched members has been conducted. Furthermore, we assume that the combined structural system would apply to sheet lightweight section steel structures, and the examination about the stiffening effect with the system is done, too. In addition, it is thought that it is necessary to establish enough clearance between the hole of plywood and bolt because of the accuracy of finishing of materials and precision of constructions. On the other hand, it is desirable that the clearance becomes small as much as possible, because the excessive clearance causes a drop of the structure unity of the combined material and invites the paths for thermal air environmental and humidity. Then, the moderate clearance around the joint would be inevitable, it is necessary to clarify the influence on the stress transfer mechanism and modification quality by the bearing pressure of the circumference of a bolt joint, and the above. In this research, the two types of filling up method for the clearance are studied; the first is a wet construction method by use of filler, and the other is a dry construction method with steel hardware. Herein, to develop and propose the above filling up methods, the structural resistant performance is not only evaluated, the difficulties of production, the costs and the human body effects are estimated. To select the suited filler materials, the construction examination and material testing were carried out. Form this selection, an urethane, an epoxy and a cement were picked up. And the bearing pressure experiment tests with the parameter of materials, the size of clearance, i.e. the thickness of filler, and the procuring period were conducted. The next, the steel hardware is designed with consideration the absorbing the gap resulted from construction and production errors. And the bearing pressure experiment study is conducted. From the experimental result, the relations of rigidity and strength with the parameters of fillers are clarified. And also, from the comparison of the results of filler and the ones of steel hardware, it can be said the steel hardware becomes the one of the most suitable methods.

Self-tapping screws, as simple fasteners with a high load carrying potential if stressed axially, are frequently applied in timber engineering as tensile joints in wide span GLT truss systems or as reinforcements against stresses perpendicular to grain. In fact, force distribution along axially loaded screws has a very important influence on the joint behaviour. Some models based on Volkersen’s theory combined with fundamentals of linear elastic fracture mechanics already exist for glued-in rods or lag screws.

This paper provides a measuring technique estimating the force distribution based on the determined elongation of the threaded part over the inserted length by several strain gauges while the composite “timber-screw” is stressed axially. Therefore, 16 withdrawal “push-pull” tests were carried out in solid timber varying the slenderness

λ

, given as the ratio

l

ef

/

d

, from 5 to 20 and with angles of screw axis to grain direction

α

of 0°, 45° and 90°. The results are used to verify existing models of comparable configurations which are further adapted to self-tapping screws.

Beside the fundamental knowledge of the withdrawal behaviour, also structural analysis of tensile joints with self-tapping screws can be improved considering the location of the stress centre and its impact on eccentricities due to the non-linear distributions, and in regard to recommendations concerning load introduction perpendicular to grain.

Despite its higher strength and stiffness properties as compared to most softwood species, beech wood is today almost entirely used for energetic purposes or non-structural applications. Benefitting from elevated mechanical properties and a reliable high degree of homogeneity, Laminated Veneer Lumber (LVL) made of beech has a great potential for applications in high performance structural elements, for instance in large span truss structures. As the performance of timber truss structures predominantly depends on the efficiency of the connections, an experimental analysis of dowel-type connections in beech LVL was performed. A series of embedment tests was carried out according to EN 383:2007, the parameters being the dowel diameter and the end distance of the connectors. The tests showed a very ductile behaviour of the material, high values of embedment strength and a low scatter in the results (CoV < 5%). These findings were confirmed in subsequent tensile tests on full dowel connections. The tested connections consisted of four dowels and two slotted-in steel plates, the examined parameters were the dowel diameter and the spacing. Provided that an adequate spacing is guaranteed, the full connections also showed a very ductile behaviour. The common problem of premature splitting failure did not occur due to the favourable effect of the cross-layers in the LVL. It was further found, that the adequate spacing has to be determined with regard to shear plug failure. The experimental analysis has confirmed the potential for efficient dowel-type connections with LVL made of beech.

The embedment behaviour of European hardwoods in dowel-type connections is investigated through the analysis of tests performed on beech wood. Experimental results are evaluated using the 5% offset method. They are compared with estimations provided by the Eurocode 5 and the SIA 265 and with similar tests performed on spruce. Influences of the load-to-grain angle

α

and the fastener diameter

d

are investigated. Beech has a stronger hardening behaviour than spruce, and is very influenced by the fastener diameter. The comparison with the standards shows a general overestimation (at least 20%) of the experimental results, especially for loading perpendicular to the grain. The model provided by Eurocode 5 to consider the load-to-grain angle is better than the SIA 265 but is optimized for softwoods only. Thus a formula optimized for both species is proposed.

Models estimating the slip modulus and the load capacity, including temperature dependent effects, of non-metallic timber connections are presented. Previous studies, including the work of Thomson, have shown that ‘Glass Fibre Reinforced Polymer’ (GFRP) rods are suitable connectors for timber and that ‘Densified Veneer Wood’ (DVW) functions effectively as a flitch plate material. Thomson’s model predicting the slip modulus of the connection with GFRP rods and DVW plates is revised in this paper. The revised model is used to predict the slip modulus and the failure load at room temperature and elevated temperatures. The latter is achieved by predicting local temperatures in the connection and taking corresponding reduced material properties into account.

The brittle failure behaviour and the design of double shear connections with mechanical fasteners loaded perpendicular to grain are analysed using experimental and numerical test series and the fracture mechanics methods. The results observed define the important geometry parameters which influence the load carrying capacity. Based on the failure criteria determined for double shear connections, a new design approach are proposed and compared to current international design standards. The correlation between the new design proposal and the experimental test results done or published confirms the methods used and the failure criteria determined.

This paper presents a study to the moment-rotation aspects of two 3-member DVW reinforced timber connections with an inter-connecting steel plate used as middle member. Previous studies showed that reinforcing dowel-type timber connections with ‘densified veneer wood’ (DVW) and using expanded tube fasteners results in connections with superior structural properties compared to all conventional connections. In this connection type, the DVW prevents premature timber splitting. The tube fasteners aid a high initial stiffness, a high ductility and a high reliability. A drawback of the connection, already in a 3-member connection, is the total thickness. By using only two side members and a much thinner, steel middle member, the thickness is strongly reduced. The steel middle member is used as a connecting interface in a flitch plate connection. This generally results in a 50% reduction of the rotational stiffness. However, it is shown by an analytical and numerical study, that the rotational stiffness of two closely spaced, flitch plate DVW connections acting in series remains unchanged if certain conditions are fulfilled. Two full connection tests are performed to confirm the analytical and numerical results. Additionally, the paper presents a comparison to a conventional connection, which confirms the structural quality of the reinforced connection.

In order to benefit from the advantages of fully threaded self-tapping screws as reinforcing elements, it is essential to have detailed knowledge about the strength and stiffness of screws with different shank to grain angles subjected to combined axial and lateral loading. In this paper a design model is proposed for the calculation of the load-carrying capacity and stiffness of screws with different shank to grain angles subjected to loads perpendicular to the grain by accounting for the effective embedment of the screw in the timber. The proposed model is based on commonly used material properties and other established design models and fits well the influence of the angle between shank and grain direction on the joint’s capacity and stiffness. However, detailed knowledge about the specific input parameters in the design approaches is necessary in order to achieve a reliable prediction of the load-carrying capacity and stiffness of the individual screw.

Ductile behavior of timber connections with metal fasteners is essential to achieve a robust structure. Moreover, a ductile behavior of the fastener and the timber prior to failure is necessary to fulfill the boundary conditions for applying the Johansen theory (1949). If these boundary conditions are not fulfilled, the capacity of a connection is overestimated and brittle failure may occur. However, using sufficient spacing, end and edge distances reduces tensile stresses perpendicular to the grain, the main stresses initiating brittle failure. If in addition to that the influence of the relation between fastener diameter and timber volume is considered, brittle failure mechanisms can be avoided.

This paper discusses quasi static tests carried out on dowelled connections with different spacing and loaded end distances chosen in accordance to the above criterion. The results were compared with the capacity calculation for dowelled connections of Eurocode 5, chapter 8.2, the design approach against block shear failure of Eurocode 5, Annex A, and the design proposal for the avoidance of block shear failure proposed by Hanhijärvi and Kevarinmäki (2008). The experimental failure load achieved was in all cases higher than predicted values by all three design approaches.

Screwed connections provide high resistance in strength and stiffness. Arranged to a group the screws interact and influence each other in dependency of their in-between spacings. A test setup was found to investigate (i) the influence of the spacings in-between the screws, and (ii) the anchoring depths on the failure modes and resistances of groups of screws. We conducted tests on axially loaded and under a stress-fiber angle of 90° placed groups of screws in solid timber (ST) and glued laminated timber (GLT) of Norway spruce. Steel fracture, withdrawal failure and also block shear failure mode, till now for self-tapping screws not considered by design codes, were observed. Additionally and based on a simple mechanic load shearing consideration model for the block shear failure mode was developed for the investigated axially loaded groups of screws. Verification with test results confirms congruent but conservative results.

In timber joints with dowel-type fasteners, the yield moment of the fastener is an important parameter. For fasteners of hardened steel, e.g. self-tapping screws, the yield moment cannot be calculated and has to be measured in a bending test as established in EN 409. The yield moment is defined at a plastic bending angle of

α

= 45°/

d

0.7

. Therefore, the determination of the plastic bending angle is important to derive the characteristic yield moment of fasteners. For a proper derivation of the fastener’s yield moment, the onset of plastic deformations must hence be identified in order to be able to determine the correct plastic bending angle. A new method for the determination of the plastic bending angle was developed which is independent of a ductile or brittle behaviour of the fastener. The method is generally applicable to different fastener types with various diameters.

A novel low-damage connection for steel column, timber beam multistorey moment frame buildings is being developed. The connection uses gravity load acting at an eccentricity to the column centre line for moment resistance and self-centring. There is built-in friction damping for energy dissipation. Beams and columns are continuous past the joint, resulting in minimal damage to the floor. 1:20 scale shake table tests have shown expected low-damage, self-centring behaviour. A SAP2000 model with friction isolators for support point gap opening and friction sliding behaviour is presented. The SAP2000 model simulates peak drift well but is stiffer than the test model at low drift levels.

Finger jointing of unseasoned Norway Spruce was studied with respect to tensile strength, adhesive penetration and durability. Finger joints were manufactured with 1) unseasoned wood and one component polyurethane (PUR) adhesive, 2) dried wood and PUR adhesive and 3) dried wood and phenol resorcinol formaldehyde (PRF) adhesive. Two levels of wood density were used. The tensile strength of the finger joints was determined and the deformations within the joint were studied with an optical measurement system (ARAMIS). The penetration of the adhesive was studied with x-ray microtomography. The durability of the joints was determined according to the standard ASTM D 4688. The results show that the tensile strength and the durability of green glued finger joints are on the same level as that of dry glued PUR joints. The penetration of the PUR adhesive is high in the unseasoned wood and cavities within the bonds seem to be smaller than in dry glued PUR joints. The tensile strength of the finger joints is dependent on density, independent on the adhesive system used. The strength of the green glued PUR adhesive bonds in finger joints measured with small scale specimens did not differ from the strength of the dry glued PUR bonds.

Block gluing of glulam beams is an effective production method to obtain massive wooden elements of cross-sections with very large dimensions. Due to the extensive gluing areas, it is difficult to impose clamping pressures similar to those as in standard glulam productions. The paper analyzes the pressure distribution perpendicular and parallel to the clamping bars by means of plain stress continuum analysis and more notably by theory of beams on elastic foundation. Hereby the stiffness perpendicular to the grain of the beams is realized implicitly by the bedding stiffness. Closed form solutions are given to determine pressures and desirable cambers of the clamping bars. The effect of nonlinear pressure distribution was investigated experimentally with a full scale block glued specimen tested in block shear, delamination and bending whereby the test results are compared against glue line thicknesses. The test results showed a good agreement with the analysis for higher glue line thicknesses at the calculated low pressure areas. In the mechanical tests, no correlation between the rather thin glue line thicknesses and strength values was detected.

Emulsion Polymer isocyanate adhesive (EPI) is a two component adhesive system which combines an emulsion component and an isocyanate functional cross-linking component. This combination gives glue line performance with the benefits from both thermoplastic and thermosetting adhesive systems. The glue line is cold curing, has high flexibility, low creep, contains no formaldehyde and gives excellent water resistance in both cold and boiling water.

As the setting and curing behaviour are different from todays primarily used adhesives for laminated beams (PRF, MF, MUF, 1K-PUR), the optimal production parameters have to be verified.

EPI adhesives give very good adhesion and are because of this, well suited for gluing difficult wood species like hardwood, which is a resource with a big potential. EPI adhesives can also be used for gluing of wood to metal. In addition, EPI adhesives open for future production benefits which today are not covered by the European production standards.

This article also describes the properties of EPI adhesives, status for the use of EPI in glued laminated timber in Europe today and future possibilities.

In this research project, the gluability of five soft- and hardwoods (ash, beech, Douglas fir, larch and spruce) in combination with four adhesives (EPI, MUF, PRF and PUR) are investigated. Factors such as extractive content, pH value, wettability, etc., that may have significant influence on the bonding strength and durability of adhesive joints, are analyzed. The results of the research project achieved so far will be reported in this article.

Recent work at the University of Applied Sciences in Wiesbaden / Germany has indicated that wood-steel-connections using adhesive action show a stiff performance, a high ultimate load capacity, a ductile behaviour and at the same time a predicable fatigue performance. This paper shows test results for static as well as dynamic loading conditions. By using adhesive connections in combination with engineered wooden products it is now possible to build a wooden wind tower that provides clean energy.

Structural Health Monitoring (SHM) is a concept, which uses integrated sensors to detect the condition of engineered structures. The sensors can be considered as the nervous system of the structure, which is designed to measure the various loads and damages to the structure during its lifetime. By analyzing the information about loads (wind, snow etc.) and damages (delamination, micro cracks etc.), the general health of the system can be monitored and a prediction of the lifetime becomes possible. While the development of SHM roots in applications of high safety demand (air space and power plant engineering), the decreasing costs of sensors and microelectronic devices supports its utilization in civil engineering.

Timber constructions with glulam members have regularly to be proofed for their performance to avoid structural collapse. For the assessment of glued laminated timber, it is important to know reliable methods and criteria. The requirements given in standard EN 386:2001 are valid for the quality control of the glulam production. The use and application of these two different methods at existing timber structures were investigated. Problems and issues noted during the test series and analyses of the results are discussed.

Manufacture of engineered wood products requires safe adhesive systems. Since their introduction in the mid-1980s, one component polyurethane (1C-PUR) adhesives have proven to provide a reliable alternative to commonly water based aminoplast (UF, MF, MUF) and phenoplast (PR, PRF) adhesive systems. No formaldehyde, no mixing and fast curing at room temperature are some of the reasons, why 1C-PUR adhesives have continuously experienced increasing acceptance in the field of structural timber gluing. Driven by the intention to establish a highest possible level of adhesive performance and security, this successful development was accompanied by extensive scientific and industrial research activities. The present article aims at providing an understandable overview of the most relevant results from those research activities during the past years. Major aspects such as chemical and physical bondline integrity, adhesive behaviour at elevated temperature and fire, and gluing of alternative wood species will be addressed in order to present an insight into 1C-PUR adhesive-technology and an outlook to its future development.

For increased requirements in terms of load-carrying capacity and long spans, a hybrid composite beam made of glulam and high-performance materials was developed at the Bauhaus University Weimar in cooperation with a local SME. The compression zone at the top of the beam is reinforced by an epoxy-based Polymer Concrete (PC). The tension zone at the bottom is strengthened either with Fiber Reinforced Plastics (FRP) - or steel-reinforced lamellas made of Laminated Veneer Lumber (LVL). Further reinforcing elements increase the shear resistance or improve the transverse load-carrying capacity at the bearings. By the use of PC all parts are surface mounted with a stiff connection.

The present paper investigates the mechanical behavior of a novel dowel-type Timber-Concrete Composite system, namely SBB, under monotonic and reversed-cyclic loadings. This system consists of timber beams connected to a concrete slab using a dowel type connection. Because the structural behavior of timber-concrete composite slabs is mainly governed by the shear connection between concrete and timber, an extensive experimental program was carried out in order to assess its behavior under both monotonic (serviceability design) and cyclic loadings (seismic design) and to identify failure modes for each possible configuration. In order to fully characterize the load-slip behavior of the SBB connection under both monotonic and reversed cyclic loading, 24 Push-Out tests (12 under monotonic loading, 12 under reversed cyclic loading) were performed. The experimental program and the results (parameters and phenomenology) are discussed in this paper.

Interest in timber-concrete composite (TCC) floors has increased over the last 30 years. TCC technology relies on timber and concrete members acting compositely together. Both timber and concrete exhibit a brittle behaviour in bending/tension and compression respectively whilst the shear connection is identified as the only contributor of ductile behaviour. Therefore, the strength, stiffness and arrangement of the shear connection play a crucial role in the design parameters of TCC system including deflection and stiffness of floor. Hence, calculation of stiffness is of interest to study the structural performance of TCC floor. Material properties of timber, fastener and concrete influence the overall load-displacement response of shear connection.

There are only few investigations on analytical closed-form equation to predict the stiffness and strength of TCC joints as input values to design a partially composite floor. For example, Eurocode 5 recommends the empirical equations for the slip modulus of dowels and screws which are limited to vertically inserted fasteners only. Eurocode 5 only recommends that the strength and stiffness of unconventional joints should be determined by push-out tests. Previous investigations reported that the inclination of a fastener significantly increase the initial stiffness and ultimate strength of the TCC joints and consequently composite floor.

This paper presents a model for prediction of the stiffness of TCC joint using crossed inclined proprietary screws (SFS Intec). The model assumes the behaviour of inclined screw as a beam on a two-dimensional elastic foundation, and considers the timber as the elastic foundation consisting of orthogonal springs with differing stiffness in the parallel and transverse to the grain directions. The experimental aspect of the research consists of embedding and push-out tests aiming to verify the stiffness model of TCC joints with inclined screws. The model is reasonably accurate in predicting the characteristic stiffness. This research suggests the model to facilitate the design of inclined screw shear connections for TCC construction.

High composite action between members is one of the most important concern to design the wood-concrete hybrid system including the timber bridge (Yeah

et

al

., 2011; Lee

et

al

., 2012). Researches on the timber bridge have been widely conducted in European countries and Northern America. After 1930’s, timber bridges started to be studied and to be constructed. Several experimental approaches which deal with the composite action between wood and concrete member have been performed. Ritter(1990) and Jutila and Salokangas(2010) pointed out that wood-concrete hybrid system can be effectively applied for the timber bridge even the fully-composite action is difficult to be achieved.

The aim of the current research is to develop a new composite timber-concrete structure element as a prefabricated panel. In the Czech Republic, it is possible to see an increased interest in the constructions of timber and wood-based materials in recent years. Increased productivity is evident particularly in realization of family houses, where the number of realizations was doubled during last year in comparison with the situation five years ago. A little bit worse situation is in realization of multi-storey buildings. To some extent, this may be the result of not only the strict fire safety regulations, but also not so good timber properties in terms of serviceability limit states and acoustics. There is a will to spread this kind of construction in our region and one of the possibilities to help solve these constraints is usage of timber-concrete composites primarily for floor structures.

Modern housing and architecture are orientated towards high living quality and low energy consumption. The city of Vienna has put emphasis to introducing wood as a sustainable and ecological material for the building industry in the last decade.

With the purpose to optimize structural performance, usability, energy-efficient and ecological profile of wood-based composite systems in several research projects of the Department of Structural Design and Timber Engineering (ITI) the combination of timber products with other conventional building materials and components was explored [3, 4, 5, 6].

These technologies provide structurally efficient components for low-energy constructions, support rapid-assembly modular construction with increased efficiency using prefabricated dry elements, thereby open opportunities to reduce carbon emissions.

As a representative example for these developments the application of cement bonded wood composites as structural sandwich panels illustrates the extent of the interrelationships involved in the development of complex system solutions to increase resource efficiency.

Historic load bearing structures are often made of timber. The repair of elements, deteriorated by biotical influences or an upgrade for new fields of application with higher loads can require special solutions and methods in rehabilitation. Several studies with new technologies and materials for restoration and strengthening have been carried out at the Bauhaus University Weimar. Reinforcement and repair was realized by high performance materials in direct bond with timber. An epoxy based resin with diverse mineral fillings was used to create a material with high strength and good adhesivity. The properties of this material can be adapted by the fillings in accordance to the application. This can be used for instance as Polymer Concrete (PC) with high stiffness and compression strength. Further materials are Fiber Reinforced Plastics (FRP) with high tension strength for the use as reinforcement or glued-in rods to link structural elements.

Wood-concrete-composite structures in residential and commercial applications are gaining market shares in recent years. The application of the wood-concrete-composite bridges however is very limited. This is mainly because there is a lack of knowledge on the fatigue behaviour of the wood-concrete-composite bridges. The authors believe there is a great potential to design these bridges using wood-concrete-composite cross sections with single span of 30 meters and more.

A sustainable handling of the worldwide available resources requires an innovative and comprehensive ecological concept especially for multi-storied timber buildings. Therefore, profound investigations were done with the aim to develop an efficient composite system, which combines the distinguished properties of wood with common mineral building material anhydrite. This mineral has a high capacity to accumulate thermal energy and moisture for a balanced indoor climate.

Anhydrite layered walls, made of stacks of boards or solid crosswise nailed timber, are developed and investigated for modern residential multi storey buildings. The mineral layers are in direct connection with the timber. The serviceability under vertical and horizontal load was examined. Numerical structural models were developed and verified with the results of measurement by Close-Range Photogrammetry (CRP) experimental tests.

A novel type of timber-concrete composite floor, consisting of longitudinal glulam beams with a fibre reinforced concrete (FRC) slab on the top is proposed. In order to check some relevant mechanical properties of such a floor, full-scale laboratory tests along with numerical analyses were carried out. The shear connector system used in the investigation consisted of self-tapping screws driven at an angle of 45° to the grain direction of the glulam beams. The manufacture of the structure occurred according to the following steps: (a) the screws were inserted on the top of the glulam beams; (b) the beams were rotated 180° about the longitudinal axis and placed in a concrete formwork; (c) the FRC was cast into the formwork; (d) after curing of the FRC, the composite floor was again rotated 180° about the longitudinal axis into its right position, i.e. with the FRC slab on the top side. Long term tests and quasi-static bending tests were performed. It was found that the proposed connection system showed a very high degree of composite action both during the long-term testing and at load levels close to the failure load. Furthermore, the assembly of the prefabricated timber-concrete composite system revealed to be very fast and easy.

Wooden structures have become widespread in several regions of the world, including earthquake-prone areas. An estimation of their dissipative capacity is of fundamental importance for an accurate design for seismic actions. To reproduce the structural behaviour under earthquake excitation, a numerical model was developed and validated on experimental results. The cyclic behaviour of connections for cross-laminated (X-lam) buildings, light-frame construction and moment-resisting frames were schematised by a piecewise linear hysteretic relationship taking into account strength/stiffness degradation, pinching and friction between elements. This relationship was assigned to a non-linear elasto-plastic spring implemented in Abaqus software package. The spring has to be calibrated on experimental tests performed on single connections, which are the only components dissipating energy in the structure. All the other timber components (Xlam panels, timber beams, sheathing panels, etc.) are regarded as linear elastic. In this work, several examples of cyclic analyses of wooden structures are presented, including X-lam buildings, light-frame construction, and moment-resisting frames. Numerical predictions are compared with experimental results demonstrating the effectiveness of the model.

Cross-laminated Timber (CLT) has been extensively used in Europe and is now gaining momentum in North America; both Canada and more recently the U.S. Construction projects have shown that CLT can effectively be used as an alternative construction material in mid-rise structures and has significant potential in commercial and industrial buildings. In the United States, the CLT system is not currently recognized in seismic design codes and therefore a seismic design can only be performed through alternative methods specified in the codes. The FEMA P695 report published in 2009 presents a methodology to determine seismic performance factors namely the response modification factor, overstrength factor, and deflection amplification factor for a proposed seismic resisting system. The methodology consists of a number of steps to characterize system behavior and evaluate its performance under seismic loading. The additional benefit of the methodology is that it considers variability in ground motions and uncertainties in tests, design, and modeling. This paper presents an overview of the P695 methodology and more specifically the approach adopted to apply the methodology to Cross Laminated Timber (cross lam) systems in the United States. The type of tests and testing configurations conducted as part of this study and development of the CLT archetypes are discussed. Nonlinear models used to simulate CLT behavior at the connection, wall, and system levels are presented and the procedure to determine collapse margin ratio is explained.

Cross-laminated timber (CLT) as a structural system is not introduced in European or North American building codes, and there is limited information available on the seismic factors for design of such structures. The objective of the study was to derive suitable ductility-based force modification factors (R

d

-factors) for seismic design of CLT buildings in the National Building Code of Canada (NBCC). For that purpose, the well-known six-storey NEESWood Capstone wood-frame building was redesigned as a CLT structure. Non-linear analytical models of the building designed with different R

d

-factors were developed using the SAPWood program. CLT walls were modeled using the output from mechanics models developed in Matlab that were verified against CLT wall tests conducted at FPInnovations. Each of the 48 building models was subjected to a series of 22 bi-axial input earthquake motions suggested in the FEMA P-695 pro cedure. Results showed that an R

d

-factor of 2.0 is appropriate for the building studied.

The use of glued in rods as a method for connecting timber has received much attention over the past decade with an increasing number of examples of their use in industry. The objective of this study is to understand the failure mechanism of a moment resisting portal frame connection in timber using glued in steel rods. A series of pull out tests are used to determine axial strength of glued in rods with varying anchorage length and bond line thickness. The results are compared with the current design equations and the discrepancies are highlighted. Further pull out tests using rods glued in at angles are used to assess the influence of lateral loading on the axial pull out strength. Small amounts of lateral loading combined with axial loading are seen to significantly reduce the pull out strength of glued in rods and splitting failures are seen at relatively low levels of load. The findings from the pull out tests are used to design a portal frame connection and the moment capacity is estimated using linear elastic analysis. Results from the destructive testing of the portal frame connection are presented and parallels are drawn between the observed behaviour in the angled pull out tests. The linear elastic analysis is found to accurately predict the failure load of the tested connection. The moment rotation (

M

-

ϕ

) relationship of the portal frame connection is extracted from the measured test data.

The paper describes tests carried out on structural glued laminated timber (glulam) beams and finger-jointed boards made out of thermally modified hardwood (beech,

fagus sylvatica

) in the following named as TMTB. The finger joints were bonded with a two-component PRF adhesive and the lamellas were edge-bonded using a two-component MUF adhesive. The finger jointed lamellas were tested in tension, flatwise- and edgewise bending. While automatically produced finger joints mostly showed unsatisfactory strengths, it was possible with manually produced finger joints to achieve higher strength values. Fifty glulam TMTB beams were produced to evaluate their load carrying behaviour. The beams were tested in 4-point bending and the integrity of the glue lines was verified by means of delamination tests and shear tests. Usually it is expected that combining lamellas of a certain strength class to a glulam beam will enhance certain characteristic mechanical properties of the final product compared to the properties of single boards. The results of the tests could not confirm this behaviour for the TMTB glulam beams even if the bond lines proved to be of a satisfactory quality. Hence, a structural use of TMTB glulam seems to be restricted to a limited range of applications.

In Australia, plantation forests have increased in area by around 50% in the last 10 years. While this expansion has seen a modest 8% increase for softwoods, hardwood plantations have dramatically increased by over 150%. Hardwood plantations grown for high quality sawn timber are slow to mature, with a crop rotation time potentially reaching 35 years. With this long lead-time, each year the risk from fire, pests and adverse weather events dramatically increases, while not translating into substantially higher financial returns to the grower. To justify continued expansion of Australia’s current hardwood plantation estate, it is becoming necessary to develop higher value end-uses for both pulpwood and smaller ‘sawlog’ resources. The use of the low commercial value stems currently culled during thinning appears to be a necessary option to improve the industry profitability and win new markets. This paper provides background information on Australian forests and plantations and gives an overview of potential uses of Australian hardwood plantation thinning logs, as their mechanical properties. More specifically, this paper reports on the development of structural Veneer Based Composite (VBC) products from hardwood plantation thinning logs, taking advantage of a recent technology developed to optimise the processing of this resource. The process used to manufacture a range of hollow-form veneer laminated structural products is presented and the mechanical characteristics of these products are investigated in the companion paper. The market applications and future opportunities for the proposed products are also discussed, as potential benefits to the timber industry.

In today’s trend of sustainable development, there is a renewed interest to use bamboo for modern building and bridge structures. However, traditional use of raw bamboo culms is not the only and nor the most effective application. The authors developed a laminated bamboo or glubam for general structural applications. This paper describes the manufacturing process of glubam, investigates and analyzes its energy consumption and carbon dioxide emission, and provides main mechanical properties through material testing. Analysis results and comparison with other comparable construction materials show the eco-friendly performance of glubam. The mechanical properties of glubam are promising for general use in construction. Research on connections using steel bolts shows the good connectivity of glubam components similar to timber structures. The paper also summarizes the authors’ extensive experimental tests on various glubam components, such as full-scale girders under static and fatigue loads, wall panels under monotonic and cyclic loads, full-scale room models under simulated earthquake load and fire, etc. Several recent practical design and construction of residential and industrial buildings are also briefly introduced.

Compared to most softwoods, the hardwood species beech (Fagus sylvatica) exhibits higher strength and stiffness properties, but – for structural use – also a number of drawbacks. The drawbacks, however, can be overcome partly by processing beech wood to laminated veneer lumber (LVL). In order to utilise beech LVL not only for plate-like structures, but also for beams or columns with deliberate cross-sectional dimensions, the beech LVL can be further processed to glued laminated beams (glulam) made of LVL laminations. The paper reports on experimental investigations of innovative high end structural beech glulam and demonstrates the system effect on the load capacity in compression loading parallel to the grain.

The acetylation of wood gives enhanced resistance against fungal decay and improved dimensional stability, benefits which make Accoya

®

wood particularly suitable for use in external applications. With increasing interest in using acetylated wood in structural applications, Accsys Technologies are continually undertaking studies which add to the database of structural properties of Accoya

®

wood. This paper describes one such study investigating the structural performance of Accoya

®

wood under service class 3 conditions and in particular the relative structural performance of Accoya

®

wood between service classes 1 and 3 and how Accoya compares with solid timber in this regard.

Australian utility pole network is aging and reaching its end of life, with 70% of the 5 million poles currently in-service nationally installed within the 20 years following the end of World War II. The estimated investment required for the replacement or remedial maintenance of the aging 3.5 millions poles is as high as 1.75 billion dollars. Additionally, an estimated 21,700 high-durability new poles are required each year, representing further investment of 13.5 million dollars per year. Yet, agreements which progressively phase out logging of native forests around Australia have been signed, giving the industry about 25 years to make the transition from Crown native forests to plantations and private forests. As utility poles were traditionally cut from native forest hardwood species, finding solutions to source new poles currently presents a challenge. This paper presents tests on Veneer Based Composite hardwood hollow utility poles manufactured from Gympie messmate (

Eucalyptus cloeziana

) plantation thinning. Small diameter poles of nominal 115 mm internal diameter and 15 mm wall-thickness were manufactured in two half-poles butt jointed together, using 9 veneers per halfpole. The poles were tested in bending and shear, and experimental test results are presented. The mechanical performance of the hollow poles is discussed and compared to hardwood poles cut from mature trees and of similar size. Future research and different options for improving the current concept are proposed in order to provide a more reliable and cost effective technical solution to the current shortage of utility poles.

Glued laminated timber consists today predominantly of softwoods as they are currently the main source of structural timber in the northern hemisphere. Due to several reasons, hardwoods will increasingly gain a more important role as a sustainable material resource which will of course therefore also impact glulam production. In Europe, the wood species white oak (Quercus Robur, Quercus Petraea) is the most important species other than beech, and is being increasingly used in structural applications. Recently, the first national and European technical approvals were issued for oak glulam. The design strength values were based on extensive testing campaigns, with emphasis on the differences inherent between growth regions of the raw material itself. The paper reports first on basic strength properties and requirements for oak laminations and beams as specified in the technical approvals. Further, essential test results which form the basis for the characteristic strength and stiffness parameters are given, showing a clear effect of size on the bending strength. In order to assess the beam characteristics based on basic lamination and finger joint properties, the applicability of the new European strength equations for softwood glulam is discussed. In the case where the important ratio of finger joint vs. lamination strength is at least one, as is typical for softwoods, the model applies for the considered hardwood databases as well. However, when the lamination strength reaches very high values with means in the range of 80 MPa to 120 MPa, finger joint strength vs. lamination strength drops significantly below one. It is then shown that the bending capacity and size effect can be well predicted for specific high strength hardwood glulams by a serial model, effectively including the bending stress gradient between adjacent laminations.

A

Phyllostachys edulis

(Moso Bamboo)sample’s density, heat capacity and thermal effusivity were obtained by a series of experiments. The porosity, thermal conductivity and thermal diffusivity were calculated. Based on these experimental values, this study discusses the

Phyllostachys edulis

sample’s microstructure characteristics and the causes of the variation of thermal properties along the radial direction.

A method to determine strength reduction factors for tapered beamsmade of cross laminated timber (CLT) is presented. The method is based on EC5 equations for the calculation of strength reduction factors for tapered glulam beams. For CLT-beams, however, the required strength properties, i.e. the shear strength and the tensile or compressive strength perpendicular to the longitudinal direction, are determined considering the beam layup and the different failure modes both affecting the characteristics of CLT-beams. The analytical approach was substantiated by the good agreement that was found between the strength reduction factors obtained from the analytical approach and the results of tests performed with double tapered CLT-beams.

The paper investigates the influence of modelling different types of connections in multi-storey cross-laminated timber buildings. The importance of modelling the connection flexibility in the prediction of the natural vibration periods and the base shear force of a crosslam building is demonstrated using linear-dynamic analysis. The building’s global ductility and peak ground acceleration were compared using non-linear static and dynamic analyses, demonstrating that the former may lead to non-conservative result. The effect of gravity loads does not seem to be crucial in the analyses, whereas including friction in the model leads to a lower seismic response of the structure. This beneficial influence may however be reduced by the vertical component of the seismic acceleration, not considered in this study.

In this paper, the behaviour of cross-lam (CLT) wall systems under cyclic loads is examined. Experimental investigations of single walls and adjacent wall panels (coupled walls) in terms of cyclic behaviour under lateral loading carried out ìn Italy at IVALSA Trees and Timber Institute and in Canada at FPInnovations are presented. Different classifications of the global behaviour of CLT wall systems are introduced. Typical failure mechanisms are discussed and provisions for a proper CLT wall seismic design are given. The influences of different types of global behaviour on mechanical properties and energy dissipation of the CLT wall systems are critically discussed. The outcomes of this experimental study provides better understanding of the seismic behaviour and energy dissipation capacities of CLT wall systems.

Since wood products for structural elements, such as cross-laminated timber (CLT), have gained importance in the building sector, the need for appropriate and reliable design codes has become essential. For this reason, this work focuses on global failure mechanisms and the corresponding evolution of different crack modes in CLT plates, depending on geometric and/or material related influence quantities. Therefore, plate-bending experiments on 3- and 5-layered CLT-plates were carried out. In addition to standard evaluation methods, each specimen were cut into cubes (10/10/10 cm) to get information about the failure modes inside the plates. Areas and location of dominant shear failure, tensile failure, delamination, and mixed failuremodes could be clearly defined and connected to geometry and loading situation. Based on this evaluation well known but not yet in detail described effects, such as the ductile structural behavior of CLT plates, can be explained.

The paper deals with the possibility of using crosslam timber panels for strengthening existing buildings against seismic forces. Three types of buildings are considered - unreinforced masonry (URM), reinforced concrete frames and reinforced concrete frames with masonry infill. Results from an extensive testing series are presented, namely quasi-static cyclic test on URM and shaking table tests of reinforced concrete frames with and without infill.

Five full-scale timber floors were tested in order to analyze the in-plane behaviour of these structural systems. The main objective was to assess the effectiveness of the in-plane strengthening using cross laminated timber (CLT). For that, one unstrengthened specimen (original), one specimen strengthened with a second wood board, two specimens strengthened with 3 CLT panels and one specimen strengthened with 2 CLT panels were tested. Moreover, because of its importance in the composite behaviour, the first phase of the experimental program was composed by push-out tests on specimens representing the shear connection between the timber beams and the CLT panels. This paper describes the tests performed and the numerical modelling aimed to evaluate the composite behaviour of the strengthened timber floors.

The load carrying capacity of dowel type joint in Cross Laminated Timber (CLT) was derived based on the Johansen’s yield model. The steel plate inserted drift pin joint with CLT (5 layered; thickness of laminae were same in one CLT, all of laminae were orthogonally arranged) was chosen as the specimen. Stiffness and nonlinear load - deformation relationships were calculated by numerical analysis using Rigid Body Spring Model (RBSM). Estimation showed the good agreement with the tensile test results on the joints.

Strength properties in timber disperse remarkable. A regressive course of strength with increasing volume is observed. This phenomenon is known as size effect, dominated by the stochastic part, the dispersion in strengths locally. Although boundary conditions of this theory are violated, traditionally, size effects have been modelled by means of Weibull’s brittle failure theory. We address stochastic length effects on the tensile strength parallel to grain of timber members with and without finger joints by means of a probabilistic approach. For jointed members regulations of minimum requirements on finger joint tensile strength are discussed. We assume lognormal distributed strengths and use a second-order hierarchical model together with equicorrelation to account for within and between members’ strength variations. As outcome, the effect of length on the mean and 5 %-quantile of tensile strength is quantified. Simplified models and parameters for the design of timber structures are provided. Minimum requirements on finger joint tensile strength, relevant for modelling of timber products as well as factory production control, are defined.

An important aspect in machine strength grading of timber is the prediction of strength. This property is mainly based on grain direction and its deviations from a main direction, which can cause a dramatic loss of strength. Therefore, an essential demand of the wood industry is to evaluate the direction of wood fibers in a fast and non-invasive way to identify specimens with low strength values. For three decades now polarized microwave radiation has been investigated to identify the main direction of wood fibers in timber in a noncontact and non-destructive way. This paper deals with the use of microwave radiation to identify specimens with severe slope of grain to be able to reject them in the grading process.

The modulus of elasticity of structural timber (MOE) may be determined by 2 methods according to the European standard EN 408 [1]. The ratio between the so-called local and global MOE that is found from tests according to these two methods, cannot be explained by the ratio between the MOE and shear modulus G that is assumed in the strength class tables of EN 338 [2]. The relationship between MOE

local

and MOE

global

from EN 384 [3] is not consistent with the shear values given in [2]. In this study the shear modulus for samples of the tropical hardwood species massaranduba and of softwood species spruce was determined. The shear modulus G was found to be not related to the MOE and was shown to be constant at around 550 N/mm

2

for massaranduba and 190 N/mm

2

for spruce. With these values, the ratios between MOE-local and MOE-global that were found in the test series could be explained. The found values for the shear moduli differ from previous research. The study concludes that it is unclear which parameters determine the magnitude of the shear modulus of a single piece of timber and that this needs to be investigated.

Australia and New Zealand are debating how they should develop a new specification for structural glulam to replace AS/NZS 1328.1:1998. The new document will include i) performance requirements and ii) methods for computing characteristic strength and stiffness properties from lamination data. With respect to item ii) ASTM D3737 and prEN 14080 have been considered as potential models but, it is argued, both are indirect and involve arbitrary assumptions and a series of computational steps that are difficult to justify on a rational basis. Two drafting principles will guide the format of the new standard: it will contain minimal manufacturing prescriptions and the link between the lamination test data and the bending strength of the glulam will involve as little simulation and calibration as possible. Although the testing of both laminations and glulam members has shown that Australian glulam meets the stated characteristic strength and stiffness values listed in AS/1720.1, it is difficult to justify this claim on the basis of lamination test data. The paper addresses those issues and recommends taking an alternative approach to both ASTM D3737 and prEN14080 methodologies in developing a new glulam standard. It is a work in progress at the time of writing.

Recent research has shown that glulam laminations of Norway spruce side boards possess excellent structural properties. This investigation concerns the possibility of improving the performance of such laminations through elimination of weak board sections by means of finger jointing. Sections to be removed were identified using profiles of edgewise bending stiffness determined on the basis of scanned fibre angle fields on board surfaces. The difference in average tension strength and average tension stiffness, respectively, between a group of finger jointed boards and a reference group of non-jointed boards was evaluated. Joints were inserted in the first group with an average distance of 2.4 m. It was found that the finger jointing gave a considerable increase of strength (36 %), whereas the stiffness improvement was not as evident. Based upon the results, it can be assumed that application of finger jointed side board laminations will result in glulam beams with very high strength.

Full-scale slab strips were tested in order to analyze the flexural response of GFRP-glulam slab systems under monotonic loading. The type of strengthening technique (externally bonded reinforcement - EBR and near-surface mounted - NSM) and the increase target in terms of ultimate load capacity (20% and 40%) were the main studied parameters. GFRP sheets were utilized in the EBR technique, while GFRP rods were applied in NSM technique. In this work the tests are described in detail, and the obtained results are presented and discussed.

This paper presents a numerical study on the torsional moment in lateral torsional stability analysis of glued laminated timber girders. A numerical model is developed solving the bending-torsion problem. This numerical model according to second order theory also takes single supports at the upper chord into account,which are typical for wide span girders as roof beams with horizontal bracings in order to reduce the relative slenderness of the girders. The numerical studies on straight parallel-chorded glued laminated beams show, that the torsional stresses on buckling girders are in the range of 4-12% of the allowable shear stresses in cross-section at the end supports considering two different curvatures of imperfection. In practice the torsional moment due to stability analysis on the lateral torsional buckling should be derived in a simple and secure way to be able to verify it against the relatively low shear strength of timber without using a calculation according to second order theory.