UC Berkeley

ABSTRACT

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

by

Wael Mohamed Hassan

Doctor of Philosophy in Engineering - Civil and Environmental Engineering

University of California, Berkeley

Professor Jack P. Moehle, Chair

Beam-column joints in concrete buildings are key components to ensure structural integrity of

building performance under seismic loading. Earthquake reconnaissance has reported the

substantial damage that can result from inadequate beam-column joints. In some cases, failure of

older-type corner joints appears to have led to building collapse.

Since the 1960s, many advances have been made to improve seismic performance of

building components, including beam-column joints. New design and detailing approaches are

expected to produce new construction that will perform satisfactorily during strong earthquake

shaking. Much less attention has been focused on beam-column joints of older construction that

may be seismically vulnerable. Concrete buildings constructed prior to developing details for

ductility in the 1970s normally lack joint transverse reinforcement. The available literature

concerning the performance of such joints is relatively limited, but concerns about performance

exist.

The current study aimed to improve understanding and assessment of seismic performance

of unconfined exterior and corner beam-column joints in existing buildings. An extensive

literature survey was performed, leading to development of a database of about a hundred tests.

Study of the data enabled identification of the most important parameters and the effect of each

parameter on the seismic performance.

The available analytical models and guidelines for strength and deformability assessment of

unconfined joints were surveyed and evaluated. In particular, The ASCE 41 existing building

document proved to be substantially conservative in joint shear strength estimation. Upon

identifying deficiencies in these models, two new joint shear strength models, a bond capacity

model, and two axial capacity models designed and tailored specifically for unconfined beamcolumn

joints were developed. The proposed models strongly correlated with previous test

results.

In the laboratory testing phase of the current study, four full-scale corner beam-column joint

subassemblies, with slab included, were designed, built, instrumented, tested, and analyzed. The

specimens were tested under unidirectional and bidirectional displacement-controlled quasi-static

loading that incorporated varying axial loads that simulated overturning seismic moment effects.

The axial loads varied between tension and high compression loads reaching about 50% of the

column axial capacity. The test parameters were axial load level, loading history, joint aspect

ratio, and beam reinforcement ratio. The test results proved that high axial load increases joint

shear strength and decreases the deformability of joints failing in pure shear failure mode without

beam yielding. On the contrary, high axial load did not affect the strength of joints failing in

shear after significant beam yielding; however, it substantially increased their displacement

ductility. Joint aspect ratio proved to be instrumental in deciding joint shear strength; that is the

deeper the joint the lower the shear strength. Bidirectional loading reduced the apparent strength

of the joint in the uniaxial principal axes. However, circular shear strength interaction is an

appropriate approximation to predict the biaxial strength. The developed shear strength models

predicted successfully the strength of test specimens.

Based on the literature database investigation, the shear and axial capacity models developed

and the test results of the current study, an analytical finite element component model based on a

proposed joint shear stress-rotation backbone constitutive curve was developed to represent the

behavior of unconfined beam-column joints in computer numerical simulations of concrete

frame buildings. The proposed finite element model included the effect of axial load, mode of

joint failure, joint aspect ratio and axial capacity of joint. The proposed backbone curve along

with the developed joint element exhibited high accuracy in simulating the test response of the

current test specimens as well as previous test joints.

Finally, a parametric study was conducted to assess the axial failure vulnerability of

unconfined beam-column joints based on the developed shear and axial capacity models. This

parametric study compared the axial failure potential of unconfined beam-column joint with that

of shear critical columns to provide a preliminary insight into the axial collapse vulnerability of

older-type buildings during intense ground shaking.