Plug shear failure in nailed timber connections

Brittle failures in mechanical timber joints should be avoided, because this often results in low capacity and brittle failure of the structure. Nailed joints experience three ultimate failure modes: embedding, splitting or plug stear failure. To avoid plug shear failure, short and wide joints are preferred, limiting the number of fasteners in line with the load and grain direction. Plug shear failure was examined in short-term experiments on nailed steel-to-timber joints in glulam loaded in tension parallel to the grain with five different joint geometries. The aim of the study was to examine if the fastener group layout can be adjusted to avoid plug shear failure and to gain an insight into the causes of failure initiation. Using spring models, it is shown that the load distribution creates pronounced stresses at the last nail in the joint, which probably initiates the plug shear failure. Test results are compared with prediction models found in the literature. It was found that fasteners placed in groups can be a successful way of reducing the risk of plug shear failure. The failure is probably initiated at the nail farthest from the free end, where tensile stresses perpendicular to grain occur.     

Prefabricated timber walls anchored with glued-in rod connections: racking tests and preliminary design

A new beam and post system for multi-storey timber buildings has been developed in Sweden. The building is braced with timber walls constructed from two Kerto-Q LVL boards glued and screwed onto a glulam frame. The walls are prefabricated off-site and can be connected to the foundation using either glued-in steel rods with metric thread or nail plates. Introductory racking tests of full scale walls anchored with glued-in threaded rods were performed. The paper presents the results of the experiments and discusses the use of the transformed section method to predict racking capacity of the anchored wall. To evaluate the strength of the glued-in rods, a newly proposed model was employed. An analytical study was conducted to investigate the role of the sheathing and the contribution of the axial force on the racking capacity of the walls. The wall panels tested in this experimental programme showed high strength and stiffness under racking load. The anchoring joints with glued-in steel rods with metric thread demonstrated a high load-carrying capacity with, however, large scatter and a brittle failure mode characterized by pull-out from the timber member. The transformed section method was successfully used to predict the racking capacity of timber walls anchored with glued-in steel rods.     

Restraining moisture-related twist in timber structures

Timber changes its shape at varying moisture content levels. In most pieces of timber, this causes distortion, such as twist, spring, bow and cup. Twist is one of the most severe distortion modes and is reversible at varying moisture contents. Restraining a stud from twisting in built-in conditions results in additional forces in the structure. This paper presents a method for measuring the torsional moment while restraining the twist during drying in in-service conditions. The results include the development of torsional moment and the moisture content versus time, as well as the final torsional moment and the corresponding free twist. The magnitude of the torsional moment in studs results in forces affecting the surrounding structure which can then be restrained in an appropriate manner by fasteners. The development of torsional moment versus time demonstrates the importance of controlling the moisture content and the importance of a rapid building process when there is a risk of fast drying. The measured torsional moment was correlated to several measured material properties. However, a good correlation was only found between torsional moment and free twist.     

Air emission from timber drying: high temperature drying and re-drying of CCA treated timber

The nature and mass flows of atmospheric emissions from an industrial kiln, drying radiata pine (Pinus radiata D.Don) timber at 140°C dry bulb and 90°C wet bulb, which was subsequently copper-chrome-arsenic (CCA) treated and redried at 90°C dry bulb and 60°C wet bulb, were assessed by measuring the concentration of chemical components in the kiln atmosphere at regular time intervals and determining air-flows and temperature differentials across the stack. The volatile organic compounds (VOCs) emitted were trapped and analyzed by GC-MS. The two major VOCs found during high temperature pre-drying and CCA re-drying were α-pinene and β-pinene, which made up 87% and 60% of the total discharge (2433 and 145 g/m³ wood), respectively. Most of the VOC fraction was released during the early stages of drying. The polar compounds were trapped in water. Total combined amount of methanol, ethanol, acetic acid and formic acid released over the two runs were 283, 281, 117, and 260 g/m³ wood, respectively. The aldehydes were trapped in 2,4-dinitrophenyl-hydrazine solution and the aldehyde derivatives analysed by HPLC. The total release of formaldehyde and acetaldehyde during the high temperature pre-drying run were 13 and 15 g/m³ wood and the CCA re-drying run were 14 and 4 g/m³ wood, respectively.     

Evaluation of glue line shear strength of laminated timber structures using block and core type specimens

Monitoring and evaluation of glued laminated timber structures in service is essential to warrant its integrity, where non-destructive techniques play an important role. This paper describes the results obtained on glued laminated timber beams of maritime pine, with and without preservative treatment, and of spruce, exposed to natural outdoor environment as well as to artificial weathering. The assessment of glue lines’ shear strength was investigated through the extraction and testing of cores parallel and perpendicular to the glued lines and block shear specimens with 50 mm?×?50 mm and 20 mm?×?20 mm shear area. The results highlight the different performances of the various types of specimens tested, showing that the measured shear strength depends on the specimen size and that the size effect depends on the material strength giving a decreasing modification factor with the increase of strength in opposition to the constant factor suggested by standard EN 14080. They also show that shear testing of cores drilled perpendicular to the glued joints may be a promising tool in the assessment of glulam structures on site.     

Shear strength of glued laminated timber made from European beech timber

This paper describes an investigation into the shear strength of glued laminated timber (GLT) made from European beech. Special consideration was paid to the possible strength influence of a frequently occurring discolouration of the timber, termed red heartwood, which is inherent to the species. The beech lamellae were visually graded according to German hardwood grading standard DIN?4074-5. Grade LS13, conforming to European hardwood strength class D40, was split into two sub-grades LS13? and LS13+. Additionally, modulus of elasticity (MOE) was determined by ultrasound pulse, longitudinal vibration and static tension tests. Sub-grade LS13+ showed a mean density and MOE of 690 kg/m³ and 14,800 N/mm², respectively. The GLT shear strength was evaluated by means of four-point bending tests on structural sized I-shaped beams with a depth of 0.6?m and a span to depth ratio of 5:1. The slightly inhomogeneous build-up of the cross-section conformed to glulam strength class GL42c. Two beam samples were investigated, each with seven specimens, where one grouping had no red heartwood and the other with a high red heartwood in the web laminations. Additionally block shear tests on bond line strength were performed with standardized small specimens according to EN?392. Neither the beam shear capacity tests nor the bond line block shear tests revealed an influence of the red heartwood discolouration on strength. The fifth-percentile value of shear strength of all beams was 3.5?N/mm². The results of the block shear tests indicate that the present requirements on minimum block shear strength are set too low in the European standard EN 386 with regard to beech GLT.     

Feasibility of glued laminated timber beams with tropical hardwoods

A feasibility study of glulam was carried out in French Guiana using local wood species. The aim was to determine gluing parameters affording satisfactory behaviour to manufactured glulam in a tropical climate. Three abundant wood species, with special properties, were selected for the study and resorcinol–phenol–formaldehyde resin was used for bonding. Three industrial parameters were considered: adhesive spread rate, closed assembly time and gluing pressure. Delamination and shearing tests were carried out in accordance with European Standards. The tests revealed the influence of wood properties and manufacturing parameters on joint resistance. In fact, the results showed that specific gravity and the shrinkage coefficient greatly influenced the gluing step. Indeed, wood with a medium specific gravity needed more adhesive and more pressure than wood with a high specific gravity. In addition, planing and lamella thickness were found to affect glue joint resistance.     

Deflections in sawn timber beams with stochastic properties

A stochastic model is proposed to study the behavior of structural sawn beams of Argentinean Eucalyptus grandis with the aim of improving the predictability of the elastic deformations. The enhancement of the mid-span deflection calculation is based on a probabilistic model of the Modulus of Elasticity (MOE) and the representation of its lengthwise variability through a random field. The standard model that uses a MOE variable assumed random from piece to piece but deterministic (constant) within each piece is obtained as a particular case. In order to obtain a statistical representation of the MOE, the Principle of Maximum Entropy (PME) is employed. Experimental data obtained from bending tests are employed to find the parameters of the derived Probability Density Function (PDF). The PDF of the mid-span deformations is numerically obtained through the Stochastic Finite Element Method (SFEM) and Monte Carlo Simulations (MCS). Numerical results are validated with experimental values. Deflections of structural sized beams under usual loads are obtained. Finally, the stochastic model is used to compare with the serviceability requirements established in the Argentinean design code. It is shown that the structural performance of timber beams is found through a more realistic material approach.     

指接集成材力学性能的探讨

对结构用集成材定义、特点以及结构可靠性原因加以阐述;探讨了国内外关于指接集成材层板、成品和指接接头的力学性能研究情况;根据我国指接集成材力学性能的研究现状,提出自己的几点思考。     

木材复式弯曲成形方法分析

在分析以前木材复式弯曲成形方法的基础上,提出了木材数控复式弯曲方法,解决了小曲率和无固定旋转中心弯曲构件的弯曲成形问题,并分析了复式弯曲方式对木材密度等方面的影响,在利用钢带弯曲的基础上完善了木材弯曲工艺,为家具和门窗上应用的复杂曲线形状弯曲构件提供新的加工方法,对于木材加工企业新的木质弯曲件产品开发利用提供了新的思路和解决方案。