Performance of exterior beam-column joints with u-shaped bars for different stirrups detailing : numerical study

Abstract EN

The relevant detailing of joint reinforcement plays a major role in ensuring adequate
performance of reinforced concrete frames in terms of strength and ductility.

The
reinforcement detailing of joints with U-shaped bars in most common codes is
achieved by providing a minimum stirrups reinforcement ratio and a maximum
spacing between them; however, the contribution of the cross ties configuration within
the joint core in the shear resisting mechanism is not considered.

The nonlinear finite
element analysis using ABAQUS software has been employed in this study to
numerically investigate the influence of different joint reinforcement configurations on
joint performance.

Four sets of beam-column connections with different joint stirrups
configurations have been modeled for achieving this purpose, considering two
validated experimentally tested beam-column connections as reference.

This study
reveals that achieving the codes minimum joint reinforcement ratio using both internal
cross ties and outer stirrups instead of outer stirrups only is more efficient in terms of
joint strength and deformation capacity which indicates that the contribution of the
cross ties considering its shape and location in the shear strength resistance should be
considered.

It has been observed that both the joint strength and deformation capacity
have been significantly improved with the use of two closed rectangular cross ties or
one polygon cross tie within the joint region.

Hence, those two layouts are considered
as the most practical solutions in which the maximum spacing between the stirrups can
be considerably increased without affecting the joint performance.

The results also
proved that the induced stress in the joint stirrups doesn't necessarily reach their yield
strength even at the instant of joint failure.

Hence, the yielding assumption for all
layers of joint reinforcement in some analytical models and different international
codes is unsafe and needs some revision.