Semi-rigid behaviour of connections in precast concrete structures

Görgün, Halil (1997) Semi-rigid behaviour of connections in precast concrete structures. PhD thesis, University of Nottingham.

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Abstract

Multi-storey precast concrete skeletal structures are assembled from individual prefabricated components which are erected on-site using various types of connections. In the current design of these structures, beam-to-column connections are assumed to be pin jointed. This current research work focuses on the flexural behaviour of the beam-to-column connections and their effect on the behaviour of the global precast concrete frame.

The experimental work has involved the determination of moment-rotation relationships for semi-rigid precast concrete connections both in full scale connection tests and smaller isolated joint tests. This has been done using the so called "component method" in which the deformation of various parts of the connection and their interfaces are summated, and compared with results from full scale sub-frame connection tests. The effects of stress redistribution, shear interaction etc. are taken of by linear transformation in the results from the full scale tests, enabling parametric equations to be formulated empirically in order to describe the semi-rigid behaviour. Eight full scale column-beam-slab assemblages were tested to determine the (hogging) moment-rotation behaviour of double (balanced loading) and single sided in-plane connections. Two of the most common types of connection were used, the welded plate and the billet type. Proprietary hollow core slabs were tied to the beams by tensile reinforcing bars, which also provide the in-plane continuity across the joint. The strength of the connections in the double sided tests was at least 0.84 times the predicted moment of resistance of the composite beam and slab. The strength of the single sided connections was limited by the strength of the connection itself, and was approximately half of that for the double sided connection, even though the connection was identical. The secant stiffness of the connections ranged from 0.7 to 3.9 times the flexural stiffness of the attached beam. When the connections were tested without the floor slabs and tie steel, the reduced strength and stiffness were approximately a third and half respectively. This remarkable contribution of the floor strength and stiffness to the flexural capacity of the joint is currently neglected in the design process for precast concrete frames. Measurements of the extent of damaged zones near to the connection in full scale tests showed that, unlike steel connections, semi-rigid behaviour in precast concrete does not occur at a single nodal position. In general the double sided connections were found to be more suited to a semi-rigid design approach than the single sided ones.

Analytical studies were carried out to determine empirical design equations for column effective length factors β in unbraced and partially braced precast concrete frames. The main variables were the relative flexural stiffness α of the frame members, and the relative linear rotational stiffness Ks of the connection to that of an encastre beam.

The variation of β factors with Ks and α are presented graphically and in the form of design equations similar to those currently used in BS 8 110. The change in the response of a structure is greatest when 0< Ks <1.5 where β is found to be more sensitive to changes in Ks than α. When Ks >2 the changes in the behaviour are so small that they may be ignored within the usual levels of accuracy associated with stability analysis. This is an important finding because the experiments have found Ks to be generally less than 2 for typical sizes of beam. The results enable designers to determine β factors for situations currently not catered for in design codes of practice, in particular the upper storey of a partially braced frame. A design method is proposed to extend the concrete column design approach in BS 8110 and EC2, whereby the strength and semi-rigid stiffness of the connection enables column bending moments to be distributed to the connected beams. However, the suitability of each type of connection towards a semi-rigid design approach must be related to the stiffness and strength of the frame for which it is a part.

Item Type:Thesis (PhD)
Supervisors:Elliott, K.S.
Davies, G.
Uncontrolled Keywords:Structural engineering, Precast concrete construction, Joints
Faculties/Schools:UK Campuses > Faculty of Engineering > Department of Civil Engineering
ID Code:1294
Deposited By:Mrs K.J. Blore
Deposited On:24 May 2010 11:37
Last Modified:24 May 2010 11:37

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