To evaluate the bond behavior between glulam and GFRP rods, applied according to the nearsurface mounted strengthening technique, an experimental program composed of beam and direct pullout tests was carried. In this experimental program three main variables were analyzed: the GFRP type, the GFRP location into the groove, and the bond length. From the monitoring system it was registered the loaded and free end slips, and the pullout force. Based on these experimental results, and applying an analytical-numerical strategy, the local bond stress-slip relationship was calculated. In this work the tests are described, the obtained results are presented and discussed, and the applicability of the inverse analysis to obtain the local bond law is demonstrated.
With the aim of evaluating the bond behaviour between glulam and carbon fibre reinforced polymer laminates strips, an experimental program using pull-out tests was carried, when the near-surface strengthening technique is applied. Two main variables were studied: the bond length and the type of pull-out test configuration. The instrumentation included the loaded and free-end slips, as well as the pullout force. Based on the obtained experimental results, and applying an analytical-numerical strategy, the local bond stress-slip relationship was determined. In this work the tests are described, the obtained results are presented and analysed, and the applicability of an inverse analysis to obtain the local bond law is demonstrated.
The paper reports on the activities of the RILEM technical committee “Reinforcement of Timber Elements in Existing Structures”. The main objective of the committee is to coordinate the efforts to improve the reinforcement practice of timber structural elements. Recent developments related to structural reinforcements can be grouped into three categories: (i) addition of new structural systems to support the existing structure; (ii) configuration of a composite system; and (iii) incorporation of elements to increase strength and stiffness. The paper specifically deals with research carried out at the Bern University of Applied Sciences Switzerland (BFH), the University of Minho Portugal (UniMinho), and the University of Trento Italy (UNITN). Research at BFH was devoted to improve the structural performance of rounded dovetail joints by means of different reinforcement methods: i) self-tapping screws, ii) adhesive layer, and iii) a combination of selftapping screws and adhesive layer. Research at UNITN targeted the use of “dry” connections for timber-to-timber composites, specifically reversible reinforcement techniques aimed at increasing the load-bearing capacity and the bending stiffness of existing timber floors. At UniMinho, double span continuous glulam slabs were strengthened with fibre-reinforced-polymers. All three examples demonstrate the improved structural performance of timber elements after reinforcing them.
Wood is a natural material, renewable, easily recyclable, and able to store carbon-dioxide, which makes tall timber buildings a solution with potential to answer the main sustainability targets. Cross laminated timber (CLT) has been pointed out as the best wood-based material to make this ambition a real thing. In order to understand why, this paper introduces the material and describe some demonstration buildings recently built. Based on diagnosed weaknesses of CLT buildings, it is presented an initial propose for a new CLT/glulam hybrid construction system, called Urban Timber (UT) system, which aims be able to support taller timber buildings. The main motivation was the development of a wood-based structural solution that provides more spatial flexibility and wider versatility for visual architectural expressions. The system is described and illustrated, considering concerns related with structural behavior, architectural value, structural connections and wood shrinkage.
In past few years, in consequence to the continuous increase of urban densities and seeking for a more sustainable profile for construction, some new proposals for tall timber city housing have emerged. The development of new wood-based materials, like cross laminated timber (CLT), has made possible to believe to build high with timber. Demonstration buildings located in different locations around the world contribute to the development of this new concept of urban housing. With the exception of few recent proposals based on hybrid systems, majority of buildings so far built are fully based in the monolithic construction system offered by CLT panels. Despite all the advantages related with this monolithic system, two main important weaknesses related with architectural freedom have been pointed out: the excessive compartmentalization of inner spaces and the external expression of an extruded box with reduced openings. Inspired on new CLT/steel and CLT/concrete hybrid proposals and their advantages in comparison to the CLT monolithic system, a CLT/glulam hybrid construction system, named UT system (urban timber system), has been developed. CLT remains the main structural material in the UT system but, glulam linear elements are used to reduce the CLT walls both inside and in the building perimeter. Further, based in the bundled tube concept, UT system looks into the possibility of overcome eccentricity problems caused by non-symmetrical location of vertical cores and consequently, offers more design freedom. UT system is described and illustrated, considering concerns related with structural system, tall building specificities, construction sequences, architectural design possibilities, moisture effects, durability, fire resistance, acoustic performance and joints between timber elements.
Self-tapping screws (STS) have been proclaimed as the easiest solution for structural timber connections, in special for cross laminated timber (CLT) constructions. In order to understand deeply the composite model “CLT-STS”, an experimental campaign which comprised 270 withdrawal tests was carried out. Maximum withdrawal load capacity of self-tapping screws inserted in plane side of a three layered CLT panel was evaluated considering three main parameters: moisture levels of CLT (i), number of gaps (ii) and the width of gaps (iii). Regarding (i), connections were tested with CLT at 8%, 12% and 18% of moisture content. Concerning (ii) and (iii), different test configurations with 1, 2 and 3 gaps, with 0 or 4mm, were tested. The influences of moisture content and number of gaps were modeled. Further a correlation between test results and a prediction model developed by Uibel and Blaß (2007) has been proposed.
International Conference on Structural Health Assessment of Timber Structures
Research Status
Complete
Notes
September 9-11, 2015, Wroclaw, Poland
Summary
A timber building made of cross-laminated timber (CLT) panels is a modular system where all panels are pre-cut in factory. On site, the single components are then assembled connecting the panels with mechanical fasteners, mainly angle brackets with nails and/or screws, hold-downs, metal plates and self-tapping screws. CLT wall panels are very rigid in comparison to its connections. Thus, connections play an essential role in maintaining the integrity of the structure providing the necessary strength, stiffness and ductility, and consequently, they need close attention by designers. However, there is still a lack of proper design rules for these connections, in particular under cyclic loads, mainly due to a large variety of connectors and connection systems. In this paper, the different properties of connections for CLT buildings, on both monotonic and cyclic behaviour, are described using recent works from different authors. From the bibliography, it is clear that experimental data, regarding both monotonic and cyclic tests, is required for the assessment of the performance of the CLT structural system attending to the interaction between rigid panels and connections. This work evidences results from experimental campaigns and numerical analysis regarding definition and quantification of the cyclic response of CLT connections. Examples regarding monotonic and cyclic tests aimed to evaluate cyclic behaviour of connections through physical parameters, such as the impairment of strength and the damping ratio, are presented and discussed.
Five full-scale timber floors were tested in order to analyse the in-plane behaviour of these structural systems. The main objective was an assessment of the effectiveness of in-plane strengthening using cross-laminated timber (CLT). To that end, one unstrengthened specimen (original), one specimen strengthened with a second layer of floorboards, two specimens strengthened with three CLT panels, and one specimen strengthened with two CLT panels, were tested. A numerical analysis was then performed in order to analyse the composite behaviour of the timber floors in more detail. Due to its importance as regards composite behaviour, the first phase of the experimental programme was composed of push out tests on specimens representing the shear connection between the timber beams and the CLT pan CLT panels. This paper describes els. This paper describes the tests performed and the numerical modelling applied the tests performed and the numerical modelling applied to evaluate the composite behaviour of the strengthened timber floors. The use of CLT panels is revealed to be an effective way to increase the in-plane stiffness of timber floors, through which the behaviour of the composite structure can be significantly changed, depending on the connection applied, or modified as required.
The Influences of Moisture Content Variation, Number and Width of Gaps on the Withdrawal Resistance of Self Tapping Screws inserted in Cross Laminated Timber
A large experimental campaign comprised of 470 withdrawal tests was carried out, aiming to quantify the withdrawal resistance of self-tapping screws (STS) inserted in the side face of cross laminated timber (CLT) elements. In order to deeply understand the “CLT-STS” composite model, the experimental tests considered two main parameters: (i) simple and cyclic changes on moisture content (MC) and (ii) number and width of gaps. Regarding (i), three individual groups of test specimens were stabilized with 8%, 12% and 18% of moisture content and one group was submitted to a six month RH cycle (between 30% and 90% RH). Concerning (ii), different test configurations with 0 (REF), 1, 2 and 3 gaps, and widths equal to 0mm (GAP0) or 4mm (GAP4), were tested. The influences of MC and number of gaps were modeled by means of least square method. Moreover, a revision of a prediction model developed by Uibel and Blaß (2007) was proposed.
The main findings of the experimental campaign were: the decrease of withdrawal resistance for
specimens tested with MC=18% in most configurations; the unexpected increase of withdrawal resistance as the number of gaps with 0mm increased; and, the surprising increase of withdrawal resistance for REF specimens submitted to the RH cycle.
The characterization of the behaviour of connectors used in Cross-laminated Timber (CLT) structures is an important aspect that needs to be considered in their seismic design. In this paper, the data from shear and axial tests conducted on connectors have been used to define their force-displacement curves under cyclic loads using the SAWS model in OpenSees. The component curves were then incorporated into the corresponding wall models and the results were compared with their experimental counterparts, in order to determine the validity of the finite element model. Thereby, the non-linear behaviour was restricted to the connectors while the walls themselves were composed of linear orthotropic shell elements. The models were found to provide a good estimate of the initial stiffness and maximum load capacity of the wall specimens. The effects of vertical loading and the presence of openings were determined based on analyses run on the calibrated model.
