Analytical and Experimental Assessment of Seismic Vulnerability of Beam-Column Joints without Transverse Reinforcement in Concrete Buildings

ABSTRACT/nAnalytical and Experimental Assessment of Seismic Vulnerability of Beam-Column Joints without Transverse Reinforcement in Concrete Buildings/nby/nWael Mohamed Hassan/nDoctor of Philosophy in Engineering - Civil and Environmental Engineering/nUniversity of California, Berkeley/nProfessor Jack P. Moehle, Chair/nBeam-column joints in concrete buildings are key components to ensure structural integrity of/nbuilding performance under seismic loading. Earthquake reconnaissance has reported the/nsubstantial damage that can result from inadequate beam-column joints. In some cases, failure of/nolder-type corner joints appears to have led to building collapse./nSince the 1960s, many advances have been made to improve seismic performance of/nbuilding components, including beam-column joints. New design and detailing approaches are/nexpected to produce new construction that will perform satisfactorily during strong earthquake/nshaking. Much less attention has been focused on beam-column joints of older construction that/nmay be seismically vulnerable. Concrete buildings constructed prior to developing details for/nductility in the 1970s normally lack joint transverse reinforcement. The available literature/nconcerning the performance of such joints is relatively limited, but concerns about performance/nexist./nThe current study aimed to improve understanding and assessment of seismic performance/nof unconfined exterior and corner beam-column joints in existing buildings. An extensive/nliterature survey was performed, leading to development of a database of about a hundred tests./nStudy of the data enabled identification of the most important parameters and the effect of each/nparameter on the seismic performance./nThe available analytical models and guidelines for strength and deformability assessment of/nunconfined joints were surveyed and evaluated. In particular, The ASCE 41 existing building/ndocument proved to be substantially conservative in joint shear strength estimation. Upon/nidentifying deficiencies in these models, two new joint shear strength models, a bond capacity/nmodel, and two axial capacity models designed and tailored specifically for unconfined beamcolumn/njoints were developed. The proposed models strongly correlated with previous test/nresults./nIn the laboratory testing phase of the current study, four full-scale corner beam-column joint/nsubassemblies, with slab included, were designed, built, instrumented, tested, and analyzed. The/nspecimens were tested under unidirectional and bidirectional displacement-controlled quasi-static/nloading that incorporated varying axial loads that simulated overturning seismic moment effects./nThe axial loads varied between tension and high compression loads reaching about 50% of the/ncolumn axial capacity. The test parameters were axial load level, loading history, joint aspect/nratio, and beam reinforcement ratio. The test results proved that high axial load increases joint/nshear strength and decreases the deformability of joints failing in pure shear failure mode without/nbeam yielding. On the contrary, high axial load did not affect the strength of joints failing in/nshear after significant beam yielding; however, it substantially increased their displacement/nductility. Joint aspect ratio proved to be instrumental in deciding joint shear strength; that is the/ndeeper the joint the lower the shear strength. Bidirectional loading reduced the apparent strength/nof the joint in the uniaxial principal axes. However, circular shear strength interaction is an/nappropriate approximation to predict the biaxial strength. The developed shear strength models/npredicted successfully the strength of test specimens./nBased on the literature database investigation, the shear and axial capacity models developed/nand the test results of the current study, an analytical finite element component model based on a/nproposed joint shear stress-rotation backbone constitutive curve was developed to represent the/nbehavior of unconfined beam-column joints in computer numerical simulations of concrete/nframe buildings. The proposed finite element model included the effect of axial load, mode of/njoint failure, joint aspect ratio and axial capacity of joint. The proposed backbone curve along/nwith the developed joint element exhibited high accuracy in simulating the test response of the/ncurrent test specimens as well as previous test joints./nFinally, a parametric study was conducted to assess the axial failure vulnerability of/nunconfined beam-column joints based on the developed shear and axial capacity models. This/nparametric study compared the axial failure potential of unconfined beam-column joint with that/nof shear critical columns to provide a preliminary insight into the axial collapse vulnerability of/nolder-type buildings during intense ground shaking.

Advisor: Moehle, Jack P.