Hasan, Sarakot
(2011)
Behaviour of discontinuous precast concrete beam-column connections.
PhD thesis, University of Nottingham.
Abstract
The study investigates experimentally and theoretically the behaviour of an internal precast concrete beam-column connection, where both the column and beam are discontinuous in construction terms. The aim was to modify the behaviour mechanisms within the connection zone by introducing a beam hogging moment resistance capacity under dead loads and limiting the damage within the connection. This is to offer permanent dead load hogging moments that could counterbalance any temporary sagging moment generated under sway loads, enhance the rotational stiffness, balance the design requirements for the beam-end and beam mid-span moments, provide efficient continuity across the column, and reduce the deflection at the beam mid-span.
Three full-scale beam-column connection tests subjected to gravity loads were conducted taking the connection reinforcement detail as the main variable. The configuration of the three main interfaces within the connection was based on the experimental results of small-scale tests. The results of the full-scale tests showed that, by using the strong connection concept, it was possible to produce equivalent monolithic behaviour, control the crack width within the connection zone, and force the final damage to occur outside of this zone, which comprises the interfaces and parts of the adjoining elements. The strong connection consisted of using additional short steel bars crossing the connection at the top of the beam, horizontal U-shaped links at the beam-ends, and additional column links.
In addition, the experimental programme included two full-scale tests to investigate the behaviour of the connection under sway loads using two different connection reinforcement details. The results of this study showed that the proposed modification in the reinforcement details was able to mobilise the beam sagging moment through the dowel action of the column main bars but it was also accompanied by large relative beam-column rotations (low rotational stiffness).
The evaluation of the behaviour of the connections was carried out by incorporating the experimental rotational stiffnesses in semi-rigid frame analyses using the ANSYS software package and a Visual Basic program based on the conventional semi-rigid analysis approach. In addition, a simplified technique has been validated against these two methods to replicate the semi-rigid behaviour. In the same respect, the study is proposing a new approach for classifying precast concrete beam-column connections as rigid by relating the connection fixity factor with the moment redistribution. It has been shown that the connection could be classified as rigid if the fixity factor is not less than 0.73 and the available moment redistribution from the midspan to the supports is not less than the required moment redistribution resulting from semi-rigid frame analysis.
In the theoretical part, an analytical tool has been calibrated to predict the rotational stiffness of the specimens with semi-rigid behaviour under gravity loads. The model showed a reasonable agreement with the experimental results. To help the modelling, two pull-out tests were conducted to determine the bond-slip relation of steel bars embedded in cement-based grout. Moreover, a finite element numerical simulation model using the ANSYS software package was carried out to replicate the experimental results of the semi-rigid specimens tested under gravity loads. In spite of providing results close to experimental values prior to yielding, the FE model was not able to predict the failure mode and consequently the correct ultimate load. This is due to the simplified way of modelling the interaction between the corrugated sleeves and the surrounding concrete as perfect bond.
The research concludes that the precast beam-column connection investigated in the current study can be treated as an emulative monolithic connection under gravity loads through using the strong connection concept; however, it is not suitable to resist beam net sagging moments. Besides, the study concludes that to consider a precast concrete beam-column connection as rigid, it is required to correlate the fixity factor with the moment redistribution.
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