This paper presents the results of long-term experiments performed on three timber-concrete composite (TCC) beams. An innovative fabricated steel plate connection system, which consists of screws and steel plates embedded in concrete slabs, was adopted in the TCC beam specimens. The adopted shear connection can provide dry-type connection for TCC beams. Steel plates were embedded in concrete slabs while the concrete slab was constructed in factories. The timber beam and concrete slab can be assembled together using screws at the construction site. In this experimental programme, the beam specimens were subjected to constant loading for 613 days in indoor uncontrolled environments. The influence of long-term loading levels and the number of shear connections on the long-term performance of TCC beams was investigated and discussed. The mid-span deflection, timber strain, and interface relative slip at the positions of both connections and beam-ends were recorded throughout the long-term tests. It was found the long-term deflection of the TCC beam increased by approximately 60% while the long-term loads were doubled. Under the influence of the variable temperature and humidity, the TCC specimens with 8 shear connections showed slighter fluctuations compared with the TCC beam with 6 shear connections. In the 613-day observation period, the maximum deflection increment recorded was 6.56 mm for the specimen with eight shear connections and 20% loading level. A rheological model consisting of two Kelvin bodies was employed to fit the curves of creep coefficients. The final deflections predicted of all specimens at the end of 50-year service life were 2.1~2.7 times the initial deflections caused by the applied loads. All beam specimens showed relative small increments in mid-span deflection, strain and relative slip over time without any degradations, demonstrating the excellent long-term performance of TCC beams using the innovative steel plate connection system, which is also easily fabricated.
Knowledge on the short and long term deformation behavior of highly loaded components in tall timber buildings is important in view of improving future design possibilities with respect to serviceability, both in the construction and in the operational state. In this paper, we present the results of a monitoring case-study on a tall timber-hybrid building in Switzerland, a 15 storey and 60 m high office building completed in 2019. A fibre-optic measuring system showed an increase of the deformation with increasing load during the construction phase of highly stressed spruce-GLT and beech-LVL columns. However, the highest strain values were not reported in the columns themselves but at the ceiling transitions and in the area near their supports. The measurements on the columns were compared with model calculations for long-term deformation of timber elements in order to differentiate single components of the total deformation caused by load, time, and changes in climate during the construction. Over a monitoring period of a year, good agreement of the modelled deformations could be confirmed, which indicates that such models could be well suited for future usage in serviceability design of tall timber buildings.
The objectives and scope of this study are to conduct long-term experimental test on timber-concrete composite beams, analyse the results to determine the creep coefficient of the composite system and compare the experimental results with the analytical solutions in accordance with Eurocode 5, in which the effective modulus method is used to account the effect of creep. To achieve the aforementioned objectives, a long-term laboratory investigation was started in August 2010 on four 5.8m span TCC beams with four different connector types. The specimens have been under sustained loads of 1.7kPa and subjected to a cyclic humidity conditions whilst the temperature remains quasi constant (22 °C). During the test, the mid-span deflection, moisture content of the timber beams and relative humidity of the air are continuously monitored. The long-term test is still continuing, two TCC beams were unloaded and tested to failure after 550 days, while the other two TCC beams are still being monitored and this report included experimental results up to the first 1400 days only. The long-term investigation on the two timber only composite floor beams commenced on March 2013 and the results are reported for the first 800 days from their commencement.
To estimate the loss of tendon force for a post-tensioned timber connection a series of tests are being conducted at the ETH in Zurich. Several post-tensioned specimens are being observed in different climate conditions. One set of specimens is in a climate chamber, where the relative humidity and temperature are kept constant. The second set of test specimens is positioned in an uncontrolled environment, where temperature and relative humidity change daily. The two environments allow estimating the influence of changes in relative humidity and temperature on the loss rate of tendon force. First results show that the relative humidity influences this rate, making it a key variable to estimate the total loss in post-tensioning force during the lifetime of a building.
Australasian Conference on the Mechanics of Structures and Materials
December 11-14, 2012, Sydney, Australia
Timber-concrete composite (TCC) beams are made up two materials, i.e. wood and concrete, which exhibit different behaviours under long-term loading. The time-dependent behaviour of TCC beam is not only affected by the long-term load but also driven by the variation of the environmental conditions such as temperature and relative humidity. In particular, the maximum deflection under service loads may govern the design requirement for medium to long span TCC beams subjected to heavy environmental conditions. For such structures, application of simplified methods adopted by different codes may lead to significant errors. Hence investigating the long-term behaviour of TCC beams subject to variable environmental condition is of great importance for designers and researchers. In this paper the research undertaken on long-term behaviour of TCC floors is critically reviewed and the recent findings are highlighted. The most important references in the literature were selected to provide more depth into the time-dependent performance of TCC structure.
The excessive use of steel and concrete as energy- and carbon-intensive construction materials have led to a great deal of research on environmentally friendly alternatives to replace conventional construction materials and methods that can reduce the negative environmental impact of the building industry. Application of timber as an environmentally sustainable and light-weight construction material has been highlighted in many studies, but, the widespread use of structural timber has been hindered by significantly different mechanical properties in longitudinal and transverse directions and its variability due to environmental conditions. The recent advancements in manufacturing engineered wood products such as cross-laminated timbers (CLT) with enhanced dimensional stability and similar mechanical properties in both directions have largely addressed the former drawbacks. Accordingly, it has been seen growing interest in mass CLT constructions and/or combining the light-weight CLT panels with steel and reinforced concrete to develop environmentally sustainable structural systems. One such system is steel-timber composite (STC) which comprises prefabricated CLT slabs connected to steel girders using mechanical connectors (e.g. screws and bolts). The adoption of STC floors in practice is however affected by lack of knowledge on the amount of achievable environmental benefit by the trade-off between embodied and operation energy consumption due to the lesser thermal mass of the timber compared to concrete. Furthermore, the long-term behaviour and vibration performance of the steel-CLT composite beams under service loads remains largely unexplored. This study demonstrates the environmental benefits (lower carbon footprint and energy consumption saving) of the STC system in the first step. Then, the hygro-mechanical properties of CLT are measured experimentally as input for numerical simulations. The acceptable long-term performance of the STC connections and beams under sustained service loads are demonstrated by long-term push-out and six-point bending tests in the following part. A simplified numerical model that takes advantage of fibre element is developed and validated against experimental data to predict the long-term creep induced deflections for a service life of 50 years. In the last part of this study, the vibration performance of the STC floors as a governing factor in the design of light-weight low-damping STC systems is studied experimentally and numerically.
Experimental tests of a composite concrete-cross-laminated timber (CLT) floor system were conducted. The floor system was constructed with 5-ply CLT panels (6.75 in. thick) made composite with a 2.25 in. thick reinforced concrete topping slab. Four series of tests were performed using different specimen configurations and laboratory testing methods. Tests included: (1) Comparative one-way bending tests (CB) to evaluate the performance of alternative shear connectors used to join the concrete slab to the CLT panel; (2) Orthotropic stiffness and strength tests (OS) to evaluate the elastic orthotropic stiffness of the deck system and provide strength results for weak-axis bending and negative moment strength; (3) Full-scale system performance tests (FS) of a continuous floor span to establish strength at realistic span lengths and the influence of continuity; and (4) Long-term deformation tests (LT) to investigate creep deflections of the composite concrete-CLT floor system considering positive and negative bending influences.
Results include overall strength, elastic stiffness values, deformation capacity, slip deformations along the concrete-CLT interface, predicted neutral axis locations in the composite concrete-CLT systems, and connection deformations.