In timber–concrete composite members with notched connections, the notches act as the shear connections between the timber and the concrete part, and have to carry the shear flow necessary for composite action. The shear transfer through the notches generates shear and tensile stresses in both parts of the composite member, which may lead to brittle failure and to an abrupt collapse of the structure. Although simplified design formulas already exist, some structural aspects are still not clear, and a reliable design model is missing. This paper summarizes current design approaches and presents analytical models to understand the shear-carrying mechanism, to estimate the shear stresses acting in the timber and concrete, and to predict failure. The analysis concentrates on three problems: the shearing-off failure of the timber close to the notch, the shear failure of the concrete, and the influence of the shear flow on the gap opening between the timber and concrete. Parts of the model calculations could be compared to experimental observations. The conclusions of this paper contribute to improving current design approaches.
In this study, new design models for cross-laminated timber (CLT) are developed to verify the fire resistance up to 120 minutes. This is done aiming for the popular Effective Cross-Section Method using a so-called zero-strength layer (ZSL) to account for losses in strength and stiffness. This was done using a method earlier presented at WCTE 2010 and discussed with the European industry. To allow for improvements, (a) the current CLT product portfolio was analysed and thermal and mechanical simulations were done accordingly for initially unprotected and unprotected members. Further, (b) new definitions for the ZSL were used to allow for a higher accuracy of the simplified models. As anoutcome, a model with (1) tabulated data between 7.0 and 12.0 mm for the effective ZSL only considering longitudinal layers and (2) a simplified model “twelve and two” is proposed for CLT members in bending.
Cross-laminated timber (CLT) became a popular engineered wood product in recent years for highquality and innovative timber buildings. As for any building product, the fire behaviour of CLT panels requires careful evaluation in the design of such buildings. The adhesive used in the bond lines of CLT plays an important role in the fire design. However, currently, European standards do not provide a test method to assess the adhesive performance in CLT exposed to fire. This paper presents a series of fire tests performed with CLT panels glued with different adhesives. It is shown how the mass loss of the CLT panels in standard fire resistance tests can be used to assess the adhesive performance in CLT exposed to fire.
At the Institute of Structural Engineering at the ETH Zurich numerous of investigations are
conducted to analyse the load bearing capacity of glued laminated timber beams. The investigations are part of the research project ’Influence of varying material properties on the load bearing capacity of glued laminated timber (glulam)’.
The investigations are taking place on 24 glulam beams with well-known material properties.
The glulam beams are fabricated out of 400 timber boards. From those boards the material
properties are investigated non-destructively within a former research project. During the glulam
fabrication it is particularly focused to keep the information of the timber boards; i.e. after the
glulam fabrication the position of each particular timber board within the glulam beam and
thus the position of each particular knot is still known.
The glulam beams are investigated during a 4-point bending test. On the glulam members
the load bearing capacity, the bending stiffness and the density is measured. Furthermore
local strains within the glulam beams are investigated using an optical coordinate-measurement
device. Following the test the failure is investigated in detail. Hereby the type of failure (knot
cluster, finger joint, clear wood) and the amount of failure (number of damaged lamellas) is
documented. Afterwards the failed glulam beams are loaded again to analyse the remaining
bending strength and the corresponding remaining bending stiffness.
The major aim of the experimental analysis is the investigation of the load bearing capacity
of glulam beams with well-known local material properties. The gained results can be used for
an investigation of the influence of local weak zones, such as knot clusters or finger joints, on the
load bearing capacity of glulam. In addition a data basis is produced to develop a new model
(or to evaluate existing models) for the estimation of the load bearing capacity of glulam.
The force-displacement behaviour of structural timber members subjected to axial compression or combined axial compression and bending is distinctively non-linear. This behaviour is caused by the non-linear increase of the deformation due to the increasing eccentricity of the axial load as well as by the non-linear material behaviour of timber when subjected to compression. The present report describes experimental investigations on glued laminated timber members subjected to eccentric compression. The aim of these experimental investigations was to create a data base, which can be used to validate theoretical calculation models and to assess the accurateness of the design approaches given in the design codes for timber structures.
The specimens for the main bunch of experiments were produced using lamellas made of Norway spruce grown in Switzerland. For this purpose, a total of 336 lamellas were available. In the first step, non-destructive tests on the lamellas were performed. These tests aimed at the collection of data in order to characterise the raw material.
In the second step, the lamellas were strength graded. The aim of the grading process was to select two classes of lamellas for the production of the test specimens. The lamellas were selected so that they were suitable to produce glued laminated timber of strength classes GL24h and GL32h. Within the grading process, visual grading criteria as well as machine grading criteria were used.
In the third step, the graded lamellas were used to produce glued laminated timber members. Five tests series were produced. Each of the test series consisted of ten specimens. Three series were made of glued laminated timber GL24h and two series were made of glued laminated timber GL32h. The length of the timber members was varied between the different test series. The lengths were L = 1’400 mm, L = 2’300 mm and L = 3’200 mm respectively. During the production, the setup of the test specimens was recorded. Hence, the position and the orientation of every lamella within the test specimen were documented. Additionally, some non-destructive tests were performed using the test specimens.
In the last step, the glued laminated timber members were subjected to buckling tests. The test specimens were loaded with an eccentric compression force up to failure. During the tests, different measurements were carried out in order to document the experimental investigations as accurate as possible. Amongst others, the applied loads as well as horizontal and vertical deformations were recorded. For a subsample of 20 test specimens, additional local deformation measurements were performed using an optical measurement device.
The performance of timber in fire is often assessed by measuring the temperature at different positions in the specimen. As timber is a low conductive material, it can be difficult to measure the correct temperature.Therefore, this paper shows how to correctly measure the temperature in timber members and how to describe temperature measurements of fire tests and experiments non-ambiguously.Typical temperature measurement setups used in tests and experiments were experimentally assessed under ISO/EN fire exposure and a constant incident radiant heat flux. By comparing the charring depth and the thermocouple readings(charring temperature 300°C) it was found that only the wire thermocouples inlaid parallel to the isotherms deliver correct temperature readings. For other temperature measurement setups, the underestimation was between 5 and 20 minutes.Due to the numerous factors influencing the measurement error, no correction factor could be defined.
The paper examines the behaviour of structural timber members subjected to axial compression or combined axial compression and bending. Based on experimental and numerical investigations, the accuracy of the existing approach in Eurocode 5 for the design of timber members subjected to axial compression or combined axial compression and bending is assessed and modifications are suggested. By means of extensive experimental investigations, a data base was created for the validation of calculation models and for the assessment of design concepts. In order to assess the behaviour of timber members subjected to axial compression or combined axial compression and bending, strain-based calculation models were developed.
The investigations indicate that the existing approach of Eurocode 5 based on 2nd order analysis can lead to an overestimation of the load-bearing capacity. Hence, a modified design approach was developed which agrees with the results of the Monte Carlo simulations very well and thus ensures a safe and economical design of timber members subjected to compression or combined compression and bending.
The mechanical behaviour of timber-to-timber connections with internal panels of densified veneer wood (DVW) and fibre-reinforced polymer (FRP) dowels was experimentally assessed and a design method, based on EN 1995-1-1, was developed. Embedment tests on DVW plates and bending/shear tests on FRP dowels were performed to characterise these components, followed by full-scale tests of connections assembled with these materials. The results show that these connections exhibit a mechanical behaviour compatible with structural applications, regarding both load-carrying capacity and ductility. The proposed design model is based on EN 1995-1-1’s expressions for connections with dowel-type fasteners and gives good predictions of the experimental load-carrying capacities.
The results of an experimental programme on the structural behaviour, fire behaviour, and fire resistance of CLT rib panels are presented. The floor system consists of cross-laminated timber (CLT) plates rigidly bonded to glued-laminated timber ribs by means of screw-press gluing.
The experimental programme comprised ultimate-load tests at normal temperature as reference tests and full-scale fire resistance tests on four cross-sections. In addition to the reference tests, shear tests of the glue line between CLT plate and glued-laminated timber rib were performed for analysis of the cross-sections’ composite action.
The results of the reference tests show good agreement with results based on the simplified method according to EN 1995-1-1 [1] and its final draft of CLT design [2]. The importance of the glue line’s quality was confirmed. The fire resistance tests show results on the safe side compared to predictions of the fire behaviour according to EN 1995-1-2 [3] and its actual draft [4]. However, the fire resistance was underestimated due to conservative assumptions about the composite cross-section’s structural behaviour.
The experimental programme addressed the fire behaviour and fire resistance of CLT rib panels currently not covered in standards. The project’s overall aim is the development of design rules in fire for EN 1995-1-2.
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.
In existing wood strength prediction models for parallel to grain failure in timber connections using dowel-type fasteners, different methods consider the minimum, maximum or the summation of the tensile and shear capacities of the failed wood block planes. This results in disagreements between the experimental values and the predictions. It is postulated that these methods are not appropriate since the stiffness in the wood blocks adjacent to the tensile and shear planes differs and this leads to uneven load distribution amongst the resisting planes (Johnsson 2004; Zarnani and Quenneville 2012a).
The present study focuses on the nailed connections. A closed-form analytical method to determine the load-carrying capacity of wood under parallel-to-grain loading in small dowel-type connections in timber products is thus proposed. The proposed stiffness-based model has already been verified in brittle and mixed failure modes of timber rivet connections (Zarnani and Quenneville 2013b).
