Seismic strengthening of deficient exterior RC beam-column sub-assemblages using post-tensioned metal strips

Old reinforced concrete buildings are vulnerable to seismic actions as they were built in accordance with non-seismic code provisions and suffer from poor material quality and/or reinforcement detailing. Moreover, many buildings were constructed without even basic design code recommendations. Consequently, their structural components, in particular beam-column joints, suffer from a wide range of deficiencies. These joints may deteriorate severely under seismic actions leading to extensive damage and collapse.
The current study aims to develop an understanding of the behaviour of exterior beam-column joints with shear strength and anchorage deficiencies, and to examine a strengthening solution using post-tensioned metal strips in upgrading their performance.
A multiphase experimental programme was conducted including small and medium scale beams with inadequate lap splices loaded in tension and deficient isolated full-scale exterior beam-column joints subjected to quasi-static cyclic loading.
In the beam tests, deficient splices were investigated under different confinement conditions, namely, unconfined, internally confined by steel stirrups, and externally confined by metal strips. Test parameters included concrete cover, confinement ratios, concrete quality, and bar diameter. Providing post-tensioned external confinement had a considerable impact on the behaviour, and resulted in sizable enhancements in strength and ductility. Parametric studies were conducted to identify the parameters most influencing the contribution of external confinement to bond. A bond stress-slip model is proposed that can be used to predict and simulate the behaviour of splices strengthened by post-tensioned metal strips. This model was implemented in FE models of beams and showed good correlation with the measured response.
In the joint tests, four full scale exterior RC beam-column joints were tested under cyclic loading. The joints experienced severe cracking and damage including a shear mechanism in the panel zone. The joints failed prematurely at about 50% of their nominal flexural strength. Strengthening the joints with post-tensioned metal strips led to an improved performance, higher energy dissipation and more controlled shear failure along with moderate damage in the beam.
An enhanced ACI-based strut-and-tie joint model is proposed and verified against the current test results and results by others. The model can be used for strengthened specimens as well as unconfined exterior joints and it accounts for different beam anchorage lengths.
A quad-linear shear stress-strain model is proposed to simulate the behaviour of strengthened joints. The model was implemented in a finite element panel-zone scissors model. The scissors model was incorporated in nonlinear static and cyclic analyses. The simulated response was found to represent the joint behaviour reasonably well.
A full-scale two storey reinforced concrete framed building was designed and tested on a shaking table, in cooperation with different researchers and academic partners. The building was substandard with a multiple range of deficiencies in the joint regions and connecting elements. The bare building suffered severe damage under small seismic intensities. Upgrading the structure with schemes of post-tensioned metal strips led to a considerably enhanced performance.