Effect of postlimiting behaviour of isolated beam-to-column joints observed in force controlled experiments

It has been observed that steel and composite steel-concrete beam-to-column joints

exhibit in general three main regions of their behaviour, namely a quasi-elastic

region at which the joint initial stiffness can be conveniently measured, an inelastic

region at which the joint stiffness degrades from its initial value to that

corresponding to the joint quasi-plastic moment resistance, and finally a postlimiting

region at which the joint stiffness maintains its low value but sufficiently high

in order to allow for the joint to reach its ultimate moment resistance. The results

of experimental moment-rotation characteristics are used in the Continuous Stiffness

Degradation model (CSD model) in which the functions describing the joint

moment-rotation characteristic and its first derivative (tangent stiffness) are

continuous and take into account the effect of joint postlimiting behaviour modelling

of joint characteristics. The accuracy of such modelling is herein examined and

discussed. An analytical formulation is presented together with its limitations with

regard to adequacy of experimental data available from the force controlled tests

conducted at the Warsaw University of Technology. The scope of these tests

included three groups of specimens with isolated beam-to-column joints and with

a different reinforcement ratio in the hogging region of the reinforced concrete slab

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