PhD Oral Exam - Sk Amjad Hossain, Civil Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Reinforced Concrete (RC) shear wall is one of the common vertical elements of seismic force resisting system in mid-rise and high-rise that provides lateral strength and stiffness. Cantilever and couple shear walls are the two different categories of reinforced concrete (RC) shear walls commonly used. Building codes in various jurisdictions including Canada have moved towards performance-based design approaches. The National Building Code of Canada has been provided in an objective based code format which provides under the “Alternative Solution” category that a structure can be designed not only to have adequate strength, but also for the performance attributes, such as deformability. Despite all these developments in Building codes, still the reinforced concrete (RC) shear walls have shown some deficiencies in major seismic events in different parts of the world in brittle shear failure or damage due to multi-mode effects. The objectives of this thesis are to investigate the followings: (a) Effect of higher modes on seismic design principles and how to implement it on current practice of seismic design. (b) Parameters affecting ductility and plastic hinge in couple shear walls. (c) Dual hinge failure mechanism of moderate ductile shear walls.

To achieve the first objective, a set of buildings with four, six and eight storey heights, and with a simple configuration have been considered here. Here, a simple method has been proposed to estimate the shear demand and to enhance shear resistance of a structural wall economically to avoid unintended shear failure. To achieve the second objective, two separate investigations were carried out. The first one deals with plastic hinge length calculations in couple shear walls. Results from the present study show that inelastic curvatures are not uniform over the plastic hinge length and the Canadian requirement as per CSA A23.3-04 to calculate plastic hinge length is sometimes unconservative for couple shear walls and more critical for slender coupled shear walls. To achieve the other parts of the second objective, a study was performed on a set of 12 story coupled shear wall buildings with 14 different models. Inelastic rotational and curvature demand of six coupling beams on each building at different level which were modelled with conventional reinforcements are investigated and the results are compared with US building codes, ACI-318-19, ATC 40 and FEMA-273 and 356. Curvature ductility demands are calculated from those models which are the function of displacement ductility demand and also the plastic hinge length. In ductility approach calculations it was observed that the maximum flexural compression zone length limitation without confinement reinforcement as per (CSA A23.3 1994) is not adequate for slender coupled shear wall (tension or compression wall). Based on the study a modified formula has been suggested to calculate the plastic hinge length.

To achieve the third and last objectives of this thesis, a 12-story building with 28 different case studies was studied. The present building codes around the world do not have proper guidelines to design of dual plastic hinges in flexural walls. Formation of dual plastic hinges are observed to be formed in the mid or near mid height of the buildings along with the main plastic hinge near the base of the cantilever shear walls. It was observed that the increase in bending moment demands along the mid height of the walls in this 12- story cantilever shear wall building, which is possibly caused by higher mode of response. It was also observed that almost 40% of the maximum base moments were developed in the dual hinge locations. Investigation shows that if this dual plastic hinge is not properly designed with proper reinforcement detailing, structural failure could happen.