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.
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.
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.
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.
