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The 12.9 km Confederation Bridge, crossing the Northumberland Strait in eastern Canada, is one of the longest reinforced concrete bridges in the world. In the design of the bridge, the seismic hazard for the bridge location was represented by a seismic design spectrum. This spectrum was derived by applying spectral amplification factors to the peak ground acceleration, velocity and displacement, corresponding to the design life of the bridge of 100 years. The design forces and displacements due to seismic loads were computed using the modal response spectrum method. This paper describes results from a study on the dynamic behaviour of the Confederation Bridge due to seismic loads. The seismic hazard for the bridge location was represented by a uniform hazard spectrum corresponding to the most recent seismic hazard models. For the purpose of the seismic analysis of the bridge, a finite element model was developed using 3-D beam elements. The model was calibrated using measured data of the bridge vibrations during a dynamic load test. Dynamic time-history analyses were conducted by applying seismic excitation motions corresponding to the seismic hazard of the location. The bending moments and the displacements obtained from the seismic analysis were compared with the design values. It was found that the seismic effects used in the design are quite representative of the seismic hazard of the bridge location. (A)
Externally bonded fiber-reinforced polymer (FRP) composites have been widely adopted for the rehabilitation of aging bridge infrastructure in recent decades. The major reasons for the utilization of these materials in bridge rehabilitation include strength, durability, lightweight nature, design flexibility, and quick installation. However, deterioration of the bonded interface between the concrete substrate and the FRP—or between individual layers of a multilayer composite—can significantly impact the structural performance of the strengthened member or system. Therefore, it is necessary to evaluate the effectiveness and accuracy of condition assessment techniques to ensure satisfactory performance over the intended service life of the structure. This paper presents a visual inspection and detailed assessment of three full-size diaphragms from the Champlain Bridge in Canada. The diaphragms were strengthened with externally bonded carbon fiber–reinforced polymer (CFRP) sheets 5 years before the bridge was decommissioned as a result of extensive degradation after 57 years in service. The condition assessment includes nondestructive (acoustic tapping and infrared thermography) and semidestructive (direct pull-off) testing to identify surface and subsurface issues including CFRP delamination, material incompatibility, discoloration due to corrosion, interfiber cracks, and fundamental problems arising from the construction of the bridge diaphragms and installation of the CFRP. The results of 490 pull-off tests, comprising the largest single database of its kind to date, generally confirmed the results of nondestructive tests that aimed to locate hidden defects behind the strengthening layers. Microscopy of failed surfaces provides additional insights into the condition of the bond line in defective regions.
Spectrum-compatible seismic excitations are required when dynamic time-history analysis is used for determining the response of a structure. This paper presents results from a study on the effects of different types of spectrum-compatible excitations on the response of medium-height reinforced concrete frame buildings. Two six-storey buildings designed for Vancouver and a five-storey building designed for Montréal were used in the study. Nonlinear time-history analyses were conducted by subjecting the buildings to selected ensembles of spectrum-compatible excitations (i.e., accelerograms). The ensembles used in the study included spectrum-compatible artificial accelerograms, simulated stochastic accelerograms, and recorded earthquake accelerograms (i.e., real accelerograms) scaled to the design spectrum ordinate at the fundamental building period and to the area under the design spectrum within the predominant period range of the building. The responses of the buildings resulting from spectrum-compatible artificial accelerograms and those from scaled real accelerograms were found to be quite similar. Based on the results of this study, the scaling of real accelerograms to spectral area is preferred for obtaining spectrum-compatible accelerograms.Key words: seismic, excitation, response, spectrum, accelerogram, building, drift, curvature, ductility.
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A simplified nonlinear finite element analysis (NLFEA) based on enhanced inspection, material testing, and nonlinear sectional analysis is introduced as part of a semi-quantitative assessment approach of aged beam-columns. The focus is on the evaluation of the structural performance and residual capacities of slab-on-girder bridge columns subjected to combined external loads and reinforcement corrosion. NLFEA takes into account different levels of geometrical, material, and bond damage due to reinforcement corrosion. At each load step of the nonlinear analysis process, NLFEA establishes the instantaneous stiffness of the structure through effective transfer of the instantaneous axial and flexural rigidities from the sectional level to the element level. The model adopts a displacement field tuning convergence approach that involves single or multiple correction phases satisfying the equilibrium and any user-defined displacement tolerance. The efficiency and accuracy of the proposed NLFEA is verified by comparison with test and analytical results from previous studies conducted on undamaged and corrosion-damaged structural elements. NLFEA proves to have high numerical stability and fast convergence, establishing its adoptability in large structural analysis/assessment frameworks. For corrosion-damaged beamcolumns, it is found that critical design sections do not necessarily remain critical for the structural evaluation.
A mathematical model is developed to express the stress-strain relationship of high-strength concrete confined by transverse reinforcement. The model is applicable to both normal-strength and high-strength concretes, covering a strength range between 30 and 130 MPa. It incorporates all the relevant parameters of confinement that have been observed to play important roles in column tests. These parameters include the type, volumetric ratio, spacing, yield strength, and arrangement of transverse reinforcement as well as concrete strength and section geometry. Therefore, it can be used for concrete confined by spirals, rectilinear hoops, crossties, welded wire fabric, and combinations of these reinforcements. It has been verified extensively against data obtained from column tests under concentric and eccentric loads, as well as slow and fast strain rates.
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The use of high-strength concrete (HSC) in seismically actively regions poses a major concern because of the brittle nature of material. The confinement requirements for HSC columns may be prohibitively stringent since they require proportionately higher confinement than columns of normal-strength concrete. An alternative to conventional confinement reinforcement is the use of fiber reinforced polymer (FRP) casings, in the form of a stay-in-place formwork, which can fulfill multiple functions of; i) formwork, ii) confinement reinforcement, and iii) protective shell against corrosion, weathering and chemical attacks. This paper investigates the use of stay-in-place FRP formwork as concrete confinement reinforcement for HSC columns with square and circular cross-sections. Large-scale specimens with 270 mm cross-sectional dimension and 90 MPa strength concrete, were tested under constant axial compression and incrementally increasing lateral deformation reversals. FRP casings were manufactured from carbon fiber sheets and epoxy resin. One of the square columns was equipped with internal FRP crossties, a new technique introduced by the authors, to provide well-distributed lateral restraints along the column face, thereby improving the mechanism of confinement. The results indicate that the deformation capacity of HSC columns can be improved significantly by using FRP casings. The results further indicate that the confinement effectiveness of FRP-confined square sections can be substantially increased with the use of FRP crossties.
A large proportion of existing building and bridge infrastructure across the world consists of seismically deficient non-ductile structural systems. Performance of structures during recent earthquakes have demonstrated seismic vulnerability of these systems, the majority of which were designed prior to the enactment of modern seismic codes, though some were designed more recently in areas where code enforcement provides challenges. These structures constitute considerable seismic risk, especially in large metropolitan centres. Because it is economically not feasible to replace a large segment of seismically deficient infrastructure with new and improved systems, retrofitting existing structures remains to be a viable seismic risk mitigation strategy. The objective of this paper is to highlight seismic retrofit strategies for deficient building and bridge infrastructures, with emphasis on experimental and analytical research conducted at the University of Ottawa. The retrofit strategies consist of structural upgrades at the system level, as well as at the element level. Non-ductile reinforced concrete frame retrofits, in the form of lateral bracing techniques, and concrete column and masonry wall retrofit methodologies are discussed. The use of innovative materials and techniques are presented.
A devastating earthquake and the resulting tsunami hit coastal areas of Indian Ocean on December 26, 2004. A team of Canadian engineers conducted a reconnaissance in Thailand and Indonesia. Their findings are presented in the paper in terms of performance of buildings, bridges, coastal structures and other physical infrastructure. It was observed that non-engineered reinforced concrete structures, low-rise timber frames and unreinforced masonry buildings suffered extensive damage due to hydrodynamic pressures generated by tsunami and impact forces induced by floating debris. Banda Aceh, Indonesia suffered extensive damage due to seismic excitations. A large number of non-engineered and engineered reinforced concrete frame buildings experienced partial or total collapse. A number of 3 to 5 story engineered reinforced concrete government buildings and shopping centers were damaged due to lack of proper seismic design practices. The engineering significance of disaster is presented in the paper with observations made during the reconnaissance visit.
Experimental investigation was carried out to verify the use of fiber reinforced polymer (FRP) grids as transverse reinforcement. The emphasis was placed on concrete confinement and seismic performance. Large-scale column specimens were tested under simulated seismic loading. The column reinforcement cages consisted of ordinary steel reinforcement as longitudinal bars and fiber reinforced polymer grids as transverse reinforcement. Grid spacing, grid pattern and the volumetric ratio of grid reinforcement, as well as the level of axial load were considered as test parameters. The results indicate improved deformability of columns when confined by properly designed grids. The grids provided easy cage assembly, savings in materials, and improved performance, while eliminating the congestion of column cage. Columns reinforced with fiber reinforced polymer grids developed at least 3% drift when current design practice was followed in terms of the amount and spacing of transverse reinforcement.
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Seismic resilience of new building has improved with the implementation of recent design codes and stringent quality of construction. However, vulnerability of existing buildings is apparent from recent earthquakes as a result of older design codes coupled with inherent area of weaknesses. Hence, there is a need for a simple procedure to identify critical structures for implementation of repair and upgrade. However, the impending challenge remains to be finding required funding from a limited resource. In this article, risk-based assessment technique is proposed to quantify the building vulnerability and consequently rank the critical building. The seismic hazard, building vulnerability, and consequence of failure are handled through hierarchical structures and the relative importance between each parameter is computed using the analytical hierarchic process. The input parameters are collected through a simple walk down survey, which is prone to subjective (vagueness) type uncertainty. Hence, the vagueness uncertainty is handled using fuzzy-based techniques and aggregated through fuzzy synthetic evaluation. The efficacy of the proposed method is illustrated with the 2003 Bingöl earthquake and 1994 Northridge earthquake data.
