Experimental and numerical assessment of new precast concrete connections under bending loads
Introduction
The collapses of the prefabricated structures in the last decades, mainly relate to weakness in their connections. On the other hand, the use of bulky and complicated elements to compensate for the low resistance of the connections against bending moments along with the mandatory allocation of prefabricated connections to limited zones (e.g. beam to column crossing point), may lead to uneconomic structures. Nontheless, studies which calculate the optimum locations for the connections, indicate that the majority of these connections must be assigned to the length of the columns and beams [1], demanding new types of precast connections for optimal designs.
Normally, the use of steel elements such as reinforcing bars (rebars) is essential for the junctions of precast concrete units, while injection of concrete mortar next to their insertion, leads to non-detachable structures with more construction time, and need of scaffolding. Whereas, dry jointing systems have higher capability of disassembling and swapping the damaged structural parts. The innovative connections, generally, ease proposing new construction approaches, especially by means of the new analyzing tools e.g. optimization algorithms [1]. As, different companies practically benefit from steel made connections in developing novel precast construction approaches [2].
The designers are looking for novel connections in precast structures [3], with particular consideration to material properties, ease of assembly, type of usages and loads, most of which are steel-based connections, due to its high strength [4]. Nevertheless, steel structures are more readily affected by ambient loads such as temperature change that could reduce the integrity of the structure. In this regard, the advanced system of ‘hybrid connections’ could work well with the independent post-tensioned steel and typical rebars embedded in concrete. It has the benefits of pre-stressed steel and prefabrication with good performance in frames with bending moments developed due to the seismic loads [5]. Its advanced performance could incorporate other joint details as well, even when the joint involves energy dissipative devices.
Furthermore, a wide range of efforts for developing ’dry-wooden-connections’ and adapting the traditional efficient wooden joints to building structures have been made [6], 1. As a consequence, analysing method and performance of these carpentry joints under different types of the loads were discussed, [7], [8].
The inspiration of carpentry joints along with the advancement of and utilizing Ultra-High Performance Fibre-Reinforced Concrete (UHPFRC), enabled the research projects to produce and evaluate new precise concrete made connections [9]. such as ‘dove-tail finger’ joints were tested under various bending moments and in-plane shear loads, concluding an efficient performance with more than of the flexural capacity of the integrated (beam) element [10]. Despite the infrequent usage of dry joints, they can be practically used in design of bridges [11] and assemblage of semi-printed-precast elements [12].
A good number of experimental and numerical studies on Scarf joints are done to assess their mechanical behaviour. The assessments indicate that the numerical modelling is capable of designing and analysing the joints [13] for instance, Abaqus was used by many researchers to evaluate the stresses in these joints [14] and to clarify its various features. It has been found that the Scarf joint retains of the beam’s solidity and of its integrated bearing capacity [15]. Another wooden joint detail that can be used for braces of new precast systems is Tenon joint[7].
The proper use of non-adherent post-tensioning techniques in these connections can significantly improve the performance of the joints [19]. Evidently, more information could be obtained and monitored from the finite element simulation of the joints than the experimental works on them. In this regard, some researchers comment that the calculations of the joints are more complex than the simplified method of the Displacement Based Design approach [20]. Along with the numerical and analytical works, a number of full-size experiments [21], as well as -size scaled experimental studies on connections are reported [22], which could provide valuable information on the ‘true’ behavior of such connections.
In many studies, the connections are classified by their mechanical characteristics [23] and their types as a crucial component of prefabricated systems [24]. The behavior of a connection is basically evaluated by its capacity in transmitting forces (that is, under its major actions) and the pertinent displacements, e.g. the relations between tensile forces and elongations (or crack openings), the bending moments and rotations which are defined by the force-displacement diagrams. It is vital to assess the effect of deformations of the connections, the possible restraints and details of ductile behaviors [25], which is followed by the attainable deformation capacity prior to the full degradation of the connection.
Precast structures must also be checked against probable chain collapses during the construction and service times. This type of failure, in particular could occur during the site construction stage, as a result of slipping and falling of precast elements over the simple supports due to cyclic or accidental loads, which indicates the high importance of developing interlocking precast connections.[26].
A certainly efficient component in practical utilization of the physically tested connections is the effect of scaling on them, but there are limited dimensionless formulations regarding precast connections, some of which are addressed in [27]. Mainly, there are two common assessing methods for precast connections: carrying out experimental work or using FE analysis, but for majority of the connections the results cannot be scaled up or down for the other dimensions of the similar geometries. Nonetheless, both finite element analyses [28] and experimental tests [29] are used for evaluation of the joints regarding multiple types of loads [24], [30]. These studies developed economic and simply constructible moment resistant precast concrete connections, which also confirmed the accuracy of the FE method [31]. These evaluations, which illustrate the performance of the connections, identify the type of the first failure (the crashing, cracking or rupturing) and the dimensions of the joint and its details. The geometric dimensions, the material properties and the type of the failure, for many geometries could be the adequate components for estimation of the failure loads, in further dimensionless calculations.
These elements can be made successfully by means of High precision formwork, from steel [32], plastic or wax [33]. Furthermore grinding [35] and high-pressure water jet cutting technology (CNC) [34] can assist in such a production.
Today, on one side the inability of connections to include all of the required joint types, their non-detachability and the compulsion of using steel in all connections, on the other side the advanced performance of the joints and the high capability of software for their analysis, necessitates study and design of new detachable connections to promote their mechanical behavior by post-tensioning and utilizing high-quality concrete. In this study, nine connections are adapted and redesigned for use in the construction of precast concrete structures with UHPFRC as the main material. The connections were load-tested, also, fully simulated by finite element numerical models. The basis of comparison of the current study are the measurements and the force–deformation diagrams obtained from the tests.
Section snippets
Experiments
The connections tested in this work are improved forms of some of the previously used joints, and could be employed in construction of concrete prefabricated buildings. It is desirable that the joint develops the full ultimate load capacity of the beam’s intact (i.e., uncut) section. The test specimens had delicate and complex geometries and hence, they could not be constructed by the commonly practiced methods, therefore, after designing the geometry of the joints through a series of
Numerical simulations
Tested connection models were numerically simulated in Abaqus as well, where the concrete body of the tested beams, was assumed as solid homogeneous material with the mechanical properties as per Fig. 5, with consideration to material plasticity, crack and crushing (Concrete Damage Plasticity, [36], [37]]). The Abaqus Quadratic element was utilised for meshing. The stress-strain diagrams and other plasticity parameters required for numerical modelling of Abaqus were obtained from the
Results of experiments and numerical analyses
In this section, the results of load tests on the specimens and numerical analyses by Abaqus are given. Along with the numerical and experimental results, the information regarding the geometry, rebars and supports of the specimens are given.
Conclusion
Dry, safe and robust joints are advantageous to construction of precast concrete structures with respect to time and management issues. For this research work, nine dry joints were developed mainly inspired from carpentry. The intricate forms of the selected joints with application to beam-to-beam or beam-to-column connections were carefully made with UHPFRC, load tested and simulated within finite element software, in order to evaluate their load capacity and to find out the weakness points.
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Declaration of Competing Interest
None.
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