Semester

Spring

Date of Graduation

2020

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Civil and Environmental Engineering

Committee Chair

Hota V.S. GangaRao

Committee Member

Udaya B. Halabe

Committee Member

P.V. Vijay

Committee Member

Ruifeng R. Liang

Committee Member

Karthik N. Ramanathan

Abstract

A review of post-earthquake reconnaissance studies revealed that reinforced concrete (RC) structures, designed and built before the development of Uniform Building Code (UBC) seismic design guidelines in 1976, have suffered complete collapse or severe damages due to the brittle failure of exterior beam-column joints. Over the past 50 years, several studies were conducted to strengthen exterior joints of in-service structures, with limited emphasis on developing simple, economic and durable repair strategies to improve energy absorption through large inelastic joint deformations. Even less emphasis was devoted to developing repair procedures that minimize stress-concentrations at joint corners and enhancing the strength, ductility, and energy dissipation capabilities of concrete structures with an emphasis on joint resistance improvements.

To address the above limitations, a novel approach of reinforcing vulnerable joints with filler-modules and fiber-reinforced polymer (FRP) composite wraps/gussets have been proposed and evaluated, herein. The proposed approach involved bonding filler-modules at the reentrant corners of a joint and securing them with reinforcing dowels to minimize corner stress-concentrations through smoother stress transfer in and around a joint. Additionally, bonding of FRP composite wraps or gussets on to the exposed beam-column faces was done to reinforce the joint core, thus enhancing the strength and energy absorption through joint confinement and reducing joint shear demand so that the plastic hinge could form away from the joint core.

To investigate the efficacy of the proposed approach in enhancing the joint structural capacities, twenty 2D RC exterior (T) joints were designed as per pre-1976 construction deficiencies and experimentally evaluated in control (as-built) and reinforced conditions through the variations in (i) filler-module geometric shape; (ii) filler-module material properties, (iii) FRP material, (iv) FRP wrap/gusset configurations, (v) confinement due to partial (U-anchors) versus complete (360o-anchors) diagonal wraps, and (vi) shear transfer through reinforcing dowels. The performance of the test specimens was recorded through numerical values of loads versus deformation and strains at the rupture of concrete, de-bond of FRP wrap from the concrete surface, yielding of steel rebar, shear failure of column or joint panel through diagonal tension and beam flexural failure phenomena. Test data evaluations measured up to the peak loads revealed that the proposed approach of reinforcing joints with filler-modules and FRP wraps/gussets is immensely useful in enhancing the strength and ductility by ~300%, and energy dissipation by about 1200%. Depending upon the reinforcing scheme(s), the magnitude of failure- loads and patterns varied in a controlled manner. Joints tested in “control” condition exhibited shear failure through diagonal tension and diagonal compression while the strengthened specimens failed in beam flexure or column shear, but in a ductile manner through yielding of the column- or beam- rebars. The strains measured on rebar surfaces at different locations of joint- and beam- sections revealed a significant reduction in strain progression towards the joint panel (beam-column overlap). It was also noted that the use of low-stiffness filler-modules such as syntactic foam and engineered wood coupled with FRP wraps has tremendously enhanced the structural response of reinforced joints. It was also observed that joints reinforced with filler-modules and FRP wraps or gussets exhibiting beam flexural failure had more energy dissipation capacity when compared to specimens that failed in column shear. Based on the experimental results of reinforced joint specimens, joint behavior is characterized into three zones, i.e., A (onset of filler-module cracking), B (idealized yield - defined as the point beyond which a truss mechanism primarily resists the forces), and C (Peak load - referred to as a highest numerical value of load recorded during the testing). Furthermore, limit states (principal tension and shear) for joint at the onset of filler-module cracking (i.e., point A) and idealized yield (i.e., point B) have been established as a function of the concrete tensile strength ( ).

The outcomes of this research have proven the ability of the proposed approach in strengthening concrete joints cost-effectively; thus, the overall structural integrity. Although the scope of this dissertation is limited to the evaluation of exterior beam-column joints designed before 1976, the concepts can be extended to other joint configurations, including timber and steel construction. Future research on the proposed approach must be directed towards evaluating the performance of joints with additional stiffness contributions from the slab, and transverse beams to establish joint curvature limit states. Furthermore, machine learning tools must be employed to train, evaluate, and develop strength prediction models after generating additional test data in a strategic sense with an understanding developed based on the current research. Besides, finite element analysis studies on joint inelastic behavior must be performed by incorporating material nonlinearity (post-cracking behavior of concrete joint or element) to arrive at the optimized shape of filler-modules and optimized fiber orientation of FRP wraps/gussets as a function of the substrate strength versus stiffness and bond strengths of glue lines of the substrate, filler-modules and FRP wrap or gusset.

Embargo Reason

Patent Pending

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