3D printed concrete (3DPC) has primarily been used for non-structural applications, with limited exploration into its potential for structural load-bearing applications. This is mainly due to the layered structure of 3DPC and the lack of compatibility of conventional reinforcement strategies to be integrated within the 3D concrete printing process. The post-tensioning of 3DPC structures presents an effective solution to improve the load-carrying capacity and cracking behavior of 3DPC. However, understanding the load transfer behavior and failure modes of 3DPC structures under a particular post-tensioning configuration are crucial to determine the permissible posttensioning load for a structure without premature failure and to understand the distribution of stresses within the concrete structure under post-tensioning loads. The current study aims to investigate the load transfer behavior and failure mode of a 30 mm thick 3DPC formwork designed for one-way ribbed slabs when subjected to endanchorage post-tensioning. For this purpose, a series of mechanical characterization and load transfer experiments were conducted on ribbed 3DPC formwork. The mechanical characterization investigations involved examining the compressive and flexural strength, as well as the elastic modulus of 3DPC, under various loading orientations relative to the print path. In the load transfer experiments, the end anchorage post-tensioning system was simulated by applying the axial load to the specimen via end plates bonded to the specimen. The variable parameters of the load transfer experiments were the boundary conditions, the eccentricity of the axial load from the neutral axis, and the topology of the ribbed 3DPC formwork. A digital image correlation system was used to study the axial and transverse strain evolution during the load transfer experiments. The experimental results showed that, regardless of the eccentricity of the applied load, the ribbed 3DPC formwork specimens exhibited higher axial load capacity when the load was applied near the bottom flange rather than the top flange. This was because of the reduced effective cross-sectional area for compression when the load was positioned near the top flanges, a consequence of the selected formwork topology, where top flanges were discontinuous to allow for pouring of the cast concrete within the formwork.

Reinforced concrete (RC) bridge columns are expected to undergo large inelastic displacements during major earthquake events, which in turn can result in residual displacements that can potentially affect the functionality of the bridge. While post-tensioning has been used to develop self-centering column systems for new bridge construction, the self-centering of existing bridge columns remains a challenge, mainly due to the limitations associated with the application of conventional post-tensioning techniques to existing structures. This study aims to address this important research gap by developing a robust self-centering technique to mitigate the residual displacements of the existing bridge columns. The proposed self-centering technique exploits the unique self prestressing characteristics of iron-based shape memory alloy (Fe-SMA) bars to prestress existing bridge columns. The effectiveness of the proposed technique was evaluated through the large-scale experimental investigation of four columns. The variables of the study included the ratio of steel to Fe-SMA reinforcement, the total longitudinal reinforcement, and the initial prestress. The experimental results showed that the proposed technique could significantly reduce the residual drifts of existing bridge columns. The residual drift of the columns was found to be less than 1% up to a target drift of 4% when the ratio of steel to Fe-SMA reinforcement was <= 0.3. The paper concludes with a discussion of the self-centering mechanism of columns reinforced with prestressed Fe-SMA bars, where it is shown that, unlike post-tensioning tendons which do not contribute much to the energy dissipation of columns, prestressed Fe-SMA bars begin to contribute to the energy dissipation after the initial prestressing is lost at high drifts, resulting in an enhanced seismic resilience of the columns.

The research and development of connecting and strengthening timber structural elements with glued-in rods (GiR) has been ongoing since the 1980s. Despite many successful applications in practice, agreement regarding design criteria has not been reached. This state-of-the-art review summarises results from both research and practical applications regarding connections and reinforcement with GiR. The review considers manufacturing methods, mechanisms and parameters governing the performance and strength of GiR, theoretical approaches to estimate their load-bearing capacity and existing design recommendations.

The paper presents a study about the glass transition of commercially available epoxy resins used for structural strengthening of concrete members for instance by means of Carbon Fiber Reinforced Polymer (CFRP) strips. Prior to an experimental investigation with a dynamic mechanical analysis (DMA), an overview on differences between definitions for the glass transition temperature Tg is given. Several testing recommendations are listed in this respect. Subsequently, DMA tests on three commercially available products are presented. A first focus is put on the different evaluation methods for one specific test result. It is visible that considerable differences in the finally adapted glass transition temperature might arise if one or the other procedure is followed. Additional parameters, such as curing procedure, specimen age, temperature history, and ultimate temperature during heating are considered, too. In all the above mentioned cases, differences in the glass transition can be found. Higher specimen age, higher ultimate temperature during testing, accelerated curing, as well as a lower heating rate implicate higher glass transition temperatures, showing that the glass transition temperature is not a fixed material characteristic. In a final step, the relevance for Tg for civil engineering applications is described. The various design code provisions for defining the service temperature in structures related to Tg are presented. The overall aim of the investigation is to show that structural engineers and end users have to be aware of the different influential parameters on the final results regarding the glass transition temperature, which also highlights the need of a potential deeper product investigation in case technical data sheets lack detailed information.

This paper describes theoretical and experimental work undertaken to obtain a structural model for assessment of historical Latin-American vertically laminated planked timber arches, as built by the Spanish, mainly in the seventeenth and eighteenth centuries. Many such arches still stand, and represent an important historical heritage. Following initial historical and construction studies, a structural analysis is being undertaken; the first stage is described here. Two wire frame analysis models are developed and assessed: a finite-element model using elasto-plastic constitutive equations (implemented in SAP2000), and another model using limit analysis. An experimental programme is also conducted. The structure presents elasto-plastic behaviour with an important plastic range, which is a relevant and favourable property for a structure. The numerical elasto-plastic model shows good agreement with experimental results for the elastic range and deviations around 60% for the plastic range. The plastic model shows a deviation of around 40%, which is an important outcome at this stage. The models show the key problem of the structure: the transference of the bending moment along planks is poor, restricting its overall load-bearing capacity. Additional tests and more complex material models (including failure in tension perpendicular to the grain), using two-dimensional and three-dimensional finite elements, are now under development to recalibrate the models presented.

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.

This paper summarizes the test recommendations for selected semi-destructive testing techniques as developed by members of the RILEM Technical Committee AST 215 “In-situ assessment of structural timber”. The recommendations cover resistance drilling, core drilling, glue line test, tension micro-specimens, screw withdrawal, and several hardness tests. The paper includes a matrix of common non-destructive testing to assess structural timber. The discussion of each technique is intended to provide users with sufficient information to understand the theoretical basis, typical equipment set up, and basic capabilities and limitations.

Abstract Glued laminated timber (glulam) is known in timber constructions since more than 100 years. Glulam members can delaminate due to aging and excessive changes of temperature and humidity. This results in significantly reduced load bearing capability of the affected structural members. This contribution focuses on the ultrasonic point-contact inspection of gluing plane delamination as a nondestructive method. Ultrasonic measurements on a section of a 90-year-old roofing glulam member are presented. The results are compared with manual detection and evaluation of delamination with a feeler gauge, with X-ray computed tomography analyses, and with numerical simulations. Appropriate data evaluation of the mechanized ultrasonic results allows the determination of material separation that are deeper than 20 mm in the signature of the surface wave and large-scale delamination (>80% of the complete bonding width) in the back-wall echo. Numerical simulations based on the finite-difference time-domain method shed light into the details of the wave propagation and support the experimental findings.

The work presented in this paper is part of an ongoing Swiss federal research project that deals with the assessment and strengthening of glulam members. In the course of the tests some of these methods are not only used to repair delaminated glulam beams subjected to tension perpendicular to grain and shear but also to reinforce weak parts of the members, e.g. at the supports and the loading points. Two different strengthening methods, one based of selftapping screws and one on base of CFRP-meshes are compared to each other regarding their potential for strengthening glulam members that show delaminations. With the help of short-span 3-point bending tests of missglued glulam beams, the shear strength and stiffness of the original and reinforced beams were evaluated. It was shown that the applied strengthening techniques helped to restore the required shear strength and to increase the shear stiffness of delaminated beams significantly but without reaching the level of the original beams.

The present paper describes collapses and failures of three large-span roof structures in Switzerland: In February 2009 the steel roof of a three years old gym in eastern Switzerland collapsed. Based on visual findings and on a detailed investigation it could be found that the cause of the collapse was a deficient detailing in each of the seven 26 m long, simply supported main steel plate girders. The collapse was triggered by increasing snow load although at the day of collapse the load was 25% lower than the characteristic value according to the Swiss design code. In November 2003 the roof of a timber multi-purpose hall partly collapsed after a period of rain. The investigations showed that the most relevant reason for the collapse was the incorrect execution of welds at the joints of supporting shoes in conjunction with the marginal design of that detail. From other factors that contributed to the collapse an insufficient drainage system of the roof could be identified as having played an important role. In 2011 a 180 x 1120 mm2 glued-laminated timber beam with a span of 18 m being part of the secondary structural system supporting the flat roof of a DIY superstore near Zurich failed in bending. The failure had been triggered to a considerable extent due to overloading of parts of the roof by a gravel layer compared to other parts of the roof being of higher depth and specific weight. From all three incidents it could be concluded that a closer orientation of the design to available design codes and a strict quality control during design, execution and use of the building would have reduced the probability of collapse / failure of the roof structures considerably.

In European Beech (Fagus sylvatica L.) large growth stresses lead to severe log end splitting that devaluate beech timber. Our study aimed at detecting relationships between growth stress and some morphology parameters in trees.Growth stress indicators were recorded for 440 mature trees in nine stands from five European countries, together with morphology parameters.Most trees displayed an uneven distribution of growth stress around the trunk. Moreover, growth stress intensity varied largely between individual trees. Geometry of the trunk was a poor predictor of growth stress intensity. Crown asymmetry resulted in a larger stress dissymmetry within trees. Trunk inclination was not correlated to max tension stress, contrary to what is usually found in younger trees. In the case of small inclination, growth stress was close to expected from biomechanics of restoring verticality. Trees exhibiting a larger inclination probably evolved a different mechanical solution: a rather large crown, lower tree slenderness and a sufficient asymmetry in growth stress as to prevent a higher inclination due to growth.A large slenderness is the best accurate predictor of a large growth stress, although variations in the ratio height/diameter at breast height explained only 10 % of the variability of growth stress. A large crown surface was the best predictor of a low level of growth stress. A large spacing between trees seems a good solution to lower the risk of growth stress in mature beech.

The use of thermally modified timber for structural purposes is of increasing interest. In order to guarantee sufficient reliability in terms of load bearing capacity and fitness for use the strength and stiffness properties of this modified wood have to be assessed. Industrially produced, thermally modified structural timber members made of beech (Fagus sylvatica L.) were subject of the tests presented in this paper. Bending, tension parallel and perpendicular to grain and compression parallel and perpendicular to grain properties were determined. The derived mechanical properties were benchmarked to the European EN 338 strength class system for structural timber. It turned out that the used strong thermal treatment of the raw material resulted in a significant reduction of most of the strength properties. However, stiffness properties were not affected. In particular the strength properties perpendicular to grain suffered a lot due to the thermal modification whereas compression strength parallel to grain was unchanged. The main drawbacks found along the experiments were a pronounced brittle behaviour of the specimens and big variations in strength. For the determination of strength values it is proposed not to use correlations as provided in European standards but to test and state these properties discretely. On the basis of these results a general use of strongly thermally modified beech as structural timber cannot be recommended. However, for selected purposes, like e.g., for structural façade elements or for columns, the use of this material might be an option.

The bowstring arch bridge concept has been realized with thermoplastic Carbon Fiber Reinforced Polymer (CFRP) tendons for six longitudinal arranged bowstrings and glulam (glued laminated timber) for the finally arch shaped bridge deck. This deck is stress-laminated as a flat plate. It is constructed by placing sawn lumber laminations on edge and stressing the laminations laterally together on the wide face with thin high-strength thermoplastic CFRP tapes. It is causing the deck to act as a large orthotropic wooden plate. The bowstrings are composed of non-laminated pin loaded thermoplastic CFRP strap elements. These elements enable the individual layers to move relative to each other which allow an equalization of forces in the layers as the strap is tensioned. The approach allows excellent use of the strength of CFRP and great flexibility in terms of the geometry of the strap elements as tendons. After prefabrication of the flat glulam plate and the CFRP straps the bow has been drawn. The bridge deck, which has the arch function, was axially loaded with the CFRP straps and elastically bent with a deflection of 1/75 of the span. The described very sustainable bowstring arch concept allows an extremely slender and therefore very elegant bridge design. The first bridge of this kind with a span of 12 m was built end of 2006.

The bowstring arch bridge concept has been realized with thermoplastic CFRP tendons for six longitudinal arranged bowstrings and glulam for the finally arch shaped bridge deck. This deck is stress-laminated as a flat plate. It is constructed by placing sawn lumber laminations on edge and stressing the laminations laterally together on the wide face with thin high-strength thermoplastic CFRP tapes. It is causing the deck to act as a large orthotropic wooden plate. The bowstrings are composed of non-laminated pin loaded thermoplastic CFRP strap elements. These elements enable the individual layers to move relative to each other which allow an equalization of forces in the layers as the strap is tensioned. The approach allows excellent use of the strength of CFRP and great flexibility in terms of the geometry of the strap elements as tendons. After prefabrication of the flat glulam plate and the CFRP straps the bow has been drawn. The bridge deck, which has the arch function, was axially loaded with the CFRP straps and elastically bent with a deflection of 1/75 of the span. The described bowstring arch concept allows an extremely slender and therefore very elegant bridge design. The first bridge of this kind with a span of 12 m was built end of 2006.

Beech is one of the most important wood species in Europe. Beech wood quality is related to the occurrence of two important defaults often observed on logs : red heartwood and cracks. This last aspect has been investigated in an European project in which the ability to crack is related to wood properties, growth stress and tree shape. The aim of the study was to point out some external parameters that could be related to stress level, indicating at it turns the ability to crack. Observations have been realised in eight stands taken out 5 European countries where 50 trees were selected. The results indicate that, unfortunately, there are few relationships between « tree shape » and « Growth Stress Indicator (GSI) » parameters. Nevertheless, some tendencies can be pointed out : the direction of leaning is a good way to find the zone of maximum GSI, maximum GSI is very highly correlated both to mean GSI, and difference between maximum and minimum GSI, diameter at breast height is highly correlated to crown surface and tree slenderness. Correlation between cracks and tree morphology indicators (diameter, crown surface, slenderness) is mostly rather weak. Mots clefs : growth stress, cracks, tree shape, tree morphology

In this paper the evaluation of strength and stiffness properties for thermally modified beech (fagus sylvatica) structural timber (TMTB) is described. On base of test results the possibilities and limits for relevant grading and factory production control of TMTB are shown and discussed. Within the EC-FP6 funded and still ongoing project HOLIWOOD it is intended to use TMTB for load bearing members. The relevant static calculations shall be based on the requirements of the appropriate European standards, e.g. Eurocode 5 and EN 338. The knowledge of characteristic strength and stiffness values is indispensable for the static design and therefore the determination of these values is a key task within the project. The respective tests were executed with TMTB "Buche forte" produced by Mitteramskogler GmbH in Austria. Results of preliminary tests showed, that the standard procedure - determination of bending strength, MOE and density followed by a calculation of the remaining properties using given conversion equations - cannot be used for TMTB. In consequence all properties have to be determined by tests. The tests confirmed the known behaviour of TMTB: a significant reduction of most strength properties and a more or less unchanged stiffness compared to untreated beech timber. These premises also influence possible set-ups for grading and factory production control procedures. Therefore several parameters for machine grading were investigated and the current status is presented. The paper shows the difficulties in introducing TMTB - which has to be regarded as a completely new material, rather than just a slight modification of a known wood species - into the European strength class system (EN 338).