Perry Erwin Adebar

This thesis focuses on the behaviour of thin and lightly-reinforced concrete walls subjected to axial and lateral in-plane force and displacement demands within high-rise cantilever and coupled shear wall type buildings. Many older and some new high-rise buildings employ thin wall elements of this type as a part of the main gravity system. A brief case study of a fictitious sample building is used to identify some shortcomings of these elements in design practice.Current North American design codes employ two main design methods to determine the in-plane, uni-axial capacity of thin bearing walls. Results of past wall tests are aggregated and compared to the empirical method and rational "moment magnifier" method of slender wall design. Comparisons of the design methods show that significantly different design axial load capacities are possible within the same design code. The results of this comparison are used to derive a new empirical bearing wall design formula which better corresponds with the results and design input parameters of the rational "moment magnifier" design method.Recent seismic events have also shown that these thin walls are subject to sudden compression failures when subjected large in plane lateral displacements. A database analysis of past wall tests is used to identify parameters which influence the drift capacity of these elements, and a new empirical relationship of overall wall drift capacity based on shear height and compression zone slenderness is derived. The database results are used to identify several low drift capacity elements for further analysis. An analysis of several previous wall tests and non-linear finite element models is used to determine the sectional and global response characteristics of these members. The results of this test specimen analysis shows that thin and lightly-reinforced wall elements show very little vertical spread of plasticity resulting in smaller than anticipated plastic hinge lengths, however sectional analysis methods produce good estimates of overall sectional properties. Finally, a model of in-plane shear displacements based on measured average vertical strains in the plastic hinge zone is validated for these types of elements.

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In residential high-rise buildings, flat plate slabs and columns are two of the primary structural member types. While codes provide prescriptive measures for their design, many decisions are still required by the design engineers. There are three primary objectives of this study. First, this study will discuss our current knowledge of construction practices, concrete material properties, and environmental conditions that impact slab design and deflections. Next, several current methods of estimating slab deflections will be reviewed, and the estimates will be compared with survey results from an actual high-rise. Finally, this study will identify optimum configurations of slabs and columns in residential high-rise buildings, in terms of cost.Through the review of the current literature and practice, it was found that there are many parameters that impact slab deflection and consideration must be taken to ensure that slabs do not have excessive deflection. This is especially true with residential towers with compressed construction schedules. The current finite element analysis programs are powerful tools but also can have significant drawbacks. The designer should understand the advantages and disadvantages of each method to ensure meaningful results. Several methods may need to be used to get an accurate understanding of a slab’s deflection behaviour. Through the cost analysis, it was determined that square columns with higher concrete strength and lower reinforcing ratios were the most cost effective. Moreover, for residential tower slab and column configurations, a layout that reduced the slab thickness with well distributed and smaller columns was found to be the most cost effective in the majority of circumstances.

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Reinforced concrete shear wall core structures are very common among high-rise buildings in Vancouver, and increasingly so elsewhere. While the flexural behaviour of these structures is well understood, the shear behaviour is not. Much of the research regarding the shear behaviour is related to the amplifications of demands due to higher mode seismic shear, however, there has been little research regarding the resistance of these structures to higher mode seismic shear demands. It is theorized that due to the lack of experimental data on which more complex shear models could be based on, structural engineers have resorted to using building models with complex non-linear fibre section flexural stiffness, but a linear elastic shear stiffness. Which may have lead to higher mode shear demands to be overestimated.Therefore, the goal of this thesis is to complete an experimental program in which scaled shear wall core specimens are tested under higher mode demands in the cantilevered direction. Through the experimental program, topics that are investigated include: the effect of the rate of loading on the shear resistance, the effect of existing flexural base yielding on the shear resistance, and the presence of a plastic hinge at the base on the shear resistance.In addition to the experimental program, a series of dynamic analyses were completed on a simplified model, in order to better understand the behaviour of a high-rise reinforced concrete shear wall structure to higher mode effects, and the results of the experimental program are also compared to predictions made using common analytical tools.

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The current research investigates the performance of commonly-used non-code-compliant stirrup detailing in concrete I-girder bridges, specifically when the lower hooks on the stirrups are oriented parallel to the longitudinal prestressing strands and are not bent around any longitudinal bars. Such detailing does not meet the specifications in the Canadian Highway Bridge Design Code CSA S6-06. An experimental investigation was conducted on full-scale partial sections of a concrete I-girder to evaluate the performance of such non-code-compliant stirrup anchorages by comparing their performance to the performance of code-compliant stirrup anchorages. An analysis of an example concrete I-girder bridge was conducted to determine the demands on the stirrup anchorage during the tests. In the tests, the flexural tension force was applied to the prestressing strand while a diagonal force was applied to the web of the test specimens at approximately 30° to the longitudinal axis of the specimen. Two pairs of stirrups were fixed to a support as the diagonal force was applied. The ratio of the slip of the stirrup to the strain along the exposed length of the stirrup, which equals to the debonded length, was monitored in order to observe the performance of the stirrup anchorage. After applying many cycles of the diagonal force, including about 100 cycles after yielding of the stirrups, the non-code-compliant hooks were found to perform adequately.

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The bending displacement capacity of elongated wall-like gravity-load columns subjected to lateral displacements due to earthquake demands on a high-rise building is of considerable concern. The long cross-sectional dimension makes these members much less flexible compared to square columns. Elongated gravity-load columns are popular because they can be hidden in walls and because they reduce the span of floor slabs, which means the thickness of the floor slabs can be reduced. No previous tests have been done on elongated gravity-load columns subjected to simulated earthquake loading. In the current study, five half-scale specimens including four column specimens and one wall specimen were subjected to constant axial compression and reverse cyclic lateral load to determine the displacement capacity of the members. The cross-sectional width-to-length ratios of the four columns were 1:1 (square), 1:2, 1:4, 1:8 and the wall specimen was 1:8. The load-deformation responses of the specimens were predicted using two nonlinear programs Response2000 and VecTor2, as well as hand calculation procedures. The predictions were used to design the test setup and were compared with the test results in order to better understand the significance of the test results. The predicted load capacities of all specimens were found to be similar to the observed maximum loads; but the displacement capacities of all specimens were significantly higher than predicted. Slip of the vertical reinforcing bars from the column foundations contributed to a large part of the increased displacement capacity of the columns. Only the elongated columns with a cross-sectional width-to-length ratios of 1:4 and 1:8 and the wall specimen suffered complete collapse during the test.

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The experimental research was completed to investigate the strain capacity of thin concretewalls. 45 wall elements that were either 24 in. (610 mm) or 36 in. (914 mm) high had 5.5 in.(140 mm),6 in. (152 mm), 8 in. (203 mm) or 10 in. (254 mm) thickness and variousreinforcement arrangements. The concrete wall elements were subjected to concentric axialcompression load. The strain capacity of the wall elements was measured. In many of thetests the strain was kept uniform and in some of the testing no effort was made to keep strainequable. The test results and observations indicated that the strain capacity of thin concretewalls could be as low as 0.0015. Low strain capacity was observed when the slenderspecimens (with height-to-thickness ratio of 4.5) did not contain lateral confinement. Inslender specimens that had straight horizontal reinforcement and no lateral ties, concretecover tended to separate. Moreover, diagonal cracks were formed through the location oflateral reinforcement. In some cases the cracks coalesced at the middle section of thespecimen into a larger crack and split the specimen in the middle. An analytical study wasconducted to investigate how the compression strain capacity could influence the axial loadcapacity of bearing walls. Maximum compression axial load was calculated for variousbearing wall lengths of 2 to 60 ft while keeping the strain demand on the bearing walls withincertain limits.

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The nonlinear shear behaviour of cantilever reinforced concrete shear walls is complex and not fully understood. Design assumptions often oversimplify the wall response and can yield results which do not reflect the true response of the shear wall. One such assumption is analyzing the wall ignoring the effects from multiple floor slabs connected to the wall over its height. Floor slabs can provide a significant increase in wall shear capacity. This thesis examines the nonlinear shear response of walls, including the effect of floor slabs as a wall-slab system, through state-of-the-art nonlinear finite element analysis. Finite element slab models were developed to emulate the 3D slab effect within a 2D analysis environment: a high-end pseudo 3D model for in-depth slab analysis and a simple 2D slab layer model for typical wall analysis. The slab effects are explored through a parametric study varying the wall size, concrete strength, axial load, horizontal steel ratio and the slab dimensions parallel and perpendicular to the wall. The slabs were found to act like large external stirrups which provide additional tension capacity for the slab and limit shear cracking and failure. The slabs can significantly increase the shear capacity of lightly-reinforced walls. Using the developed slab models, the bounds of the slab effect were investigated by a parametric study with lightly to heavily-reinforced walls, with and without axial load, as well as varying the slab spacing. Within this study, the nonlinear response of isolated walls is compared to nonlinear uniformly-loaded membrane models. It is determined that although code-based shear capacity equations are fairly accurate, the membrane models can underestimate the shear stiffness and over predict the ductility. This study also reveals that tightly spaced slabs can increase up to 3 times the isolated wall capacity for walls with minimum horizontal steel, whereas there is little effect for walls with horizontal steel above 1%. Finally, methods were developed to predict the nonlinear shear stress-strain response of isolated walls and the peak shear capacity of wall-slab systems.

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This thesis addresses the issue of seismic shear force demand in reinforced concrete cantilever walls, and specifically the dynamic amplification of shear force due to higher mode effects. Current design codes either do not account for this phenomenon, or vary considerably in the approach they take. In order to determine the reason for the variation, previous research is first examined, and it is found that the conclusions reached are not consistent with each other. It is identified that a major source of the problem is a lack of a comprehensive analysis of the problem.To attempt to address this issue, the analysis portion of the thesis starts by performing response spectrum analysis on structures with heights ranging from 5 to 70 storeys. It is shown that when a pin is placed at the bottom of the structure to simulate yielding, the moment is limited but the shear can still increase. A simple relationship between the fixed base and pinned base shear is found. Reduction in the shear stiffness, possible yielding higher in the structure, and the effect of the spectrum are also issues examined.The next two chapters deal with both linear and nonlinear time history analyses performed using OpenSees. Linear time history analysis is used to demonstrate the issues with ground motion scaling in tall structures. It is then shown that the shear at the base of the structure from a nonlinear analysis is more than the code predicts, as is the moment higher up in the structure. Furthermore, the shear at the base of the structure remains relatively constant no matter how the rest of the structure yields. A possible model is then proposed which adds the pinned response spectrum analysis results to the reduced shear. This model is compared to the nonlinear results and it is found to agree well.Finally, a chapter is devoted to factors which may complicate the results. They are separated into two sections: choice of hysteretic model, and the influence of the shear capacity on the demand. It is determined that further research is needed in these areas.

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