Kutay Orakcal

Flexural Modeling of Reinforced Concrete Walls - Experimental Verification

This study presents detailed information on the calibration of a nonlinear wall macromodel by comparing model results with experimental results for slender reinforced concrete walls with rectangular and T-shaped cross sections. Test measurements were processed to allow for a direct comparison of the predicted and measured flexural responses. Responses were compared at various locations on the walls. Results obtained with the analytical model for rectangular walls compare favorably with experimental responses for flexural capacity, stiffness, and deformability, although some significant variation is noted for local compression strains. For T-shaped walls, the agreement between model and experimental results is reasonably good, although the model is unable to capture the variation of the longitudinal strains along the flange.

Modeling of Squat Structural Walls Controlled by Shear

Reinforced concrete squat walls are common in low-rise construction and as wall segments formed by window and door openings in perimeter walls. Existing approaches used to model the lateral force versus deformation responses of wall segments typically assume uncoupled axial/flexural and shear responses. A more comprehensive modeling approach, which incorporates flexure-shear interaction, is implemented, validated, and improved upon using test results. The experimental program consisted of reversed cyclic lateral load testing of heavily instrumented wall segments dominated by shear behavior. Model results indicate that variation in the assumed transverse normal stress or strain distribution produces important response variations. The use of the average experimentally recorded transverse normal strain data or a calibrated analytical expression resulted in better predictions of shear strength and lateral load-displacement behavior, as did incorporating a rotational spring at wall ends to model extension of longitudinal reinforcing bars within the pedestals.

Shear Strength of Lightly Reinforced Wall Piers and Spandrels

Between the 1950s and 1970s, a significant number of buildings were constructed using lightly reinforced perimeter walls with openings. Evaluation and rehabilitation of such buildings requires accurate assessment of the expected shear strength, stiffness, and ductility of the wall segments (wall piers and spandrels) that comprise the primary lateral load-resisting elements. Assessing wall shear strength is complicated by factors such as use of a single curtain of distributed reinforcement, lack of hooks, and use of weakened plane joints, which are all common in older construction. To address these issues, a database of existing test results was assembled and reviewed; and tests were conducted on lightly reinforced wall piers and spandrels to address significant gaps in the available test data. Observations indicate that the amount of boundary reinforcement provided, presence of axial load, and the location of a weakened plane joint on the wall are the most important factors in the assessment of nominal shear strength.

Shear-Flexure Interaction for Structural Walls

This paper proposes an analytical model that couples the flexural and shear responses of reinforced concrete (RC) structural walls. The proposed modeling approach involves incorporating RC panel behavior into a macroscopic fiber-based model. Results obtained with the analytical model are compared with test results for a slender wall and four short wall specimens. A reasonably good lateral load-displacement response prediction is obtained for the slender wall. The model underestimates the inelastic shear deformations experienced by the wall; however, shear yielding and coupled nonlinear shear-flexure behavior are successfully represented in the analysis results. The model captures accurately the measured responses of selected short walls with relatively large shear span ratios (e.g., 1.0 and 0.69). Discrepancies are observed between the analytical and experimental results as wall shear span ratios decrease (e.g., 0.56 and 0.35). Better response predictions can be obtained for walls with low shear span ratios upon improving the model assumptions related to the distribution of stresses and strains in a wall.

Ambient and Forced Vibration Testing of a Reinforced Concrete Building before and after Its Seismic Retrofitting

AbstractThis paper investigates the effects of seismic retrofitting on the modal characteristics of a 6-story RC building located in Istanbul, Turkey. Ambient vibration surveys were carried out before, during, and after the retrofitting work, which took place between June and December 2010. The building was retrofitted via jacketing of columns, addition of structural walls, and construction of a mat foundation. These studies were complemented with data from forced vibration tests performed with an eccentric-mass shaker after the retrofitting work was completed. During retrofitting, partitions were demolished; as a result, the first modal frequency of the building decreased by 11%, based on the results of the ambient vibration survey. The ambient vibration survey also showed that the modal frequencies after the seismic retrofitting increased by almost 96%. During the forced vibration tests, the building was excited around its modal frequencies using an eccentric-mass shaker. It was found that the modal dam...

Modeling of Cyclic Shear-Flexure Interaction in Reinforced Concrete Structural Walls. I: Theory

AbstractExisting approaches used to model the lateral load versus deformation responses of reinforced concrete walls typically assume uncoupled axial/flexural and shear responses. A novel analytical model for RC walls that captures interaction between these responses for reversed-cyclic loading conditions is described. The proposed modeling approach incorporates RC panel behavior into a two-dimensional fiber-based macroscopic model. The coupling of axial and shear responses is achieved at the macrofiber (panel) level, which further allows coupling of flexural and shear responses at the model element level. The behavior of RC panel elements under generalized, in-plane, reversed-cyclic loading conditions is described with a constitutive fixed-strut-angle panel model formulation. The sensitivity of model results to various modeling parameters is investigated and results of the sensitivity studies are presented, whereas detailed information on calibration and validation of the proposed modeling approach is pr...

Modeling of Cyclic Shear-Flexure Interaction in Reinforced Concrete Structural Walls. II: Experimental Validation

AbstractThis paper presents the experimental calibration and validation of the analytical wall model that incorporates interaction between shear and flexural responses under cyclic loading conditions described in the companion paper. The model is calibrated and validated against detailed experimental data obtained from tests on five moderately slender reinforced concrete wall specimens that experienced significant levels of shear-flexure interaction. Test measurements were processed to allow for detailed comparisons between the predicted and measured wall responses at various locations and response levels. Response comparisons reveal that the proposed analytical model captures the experimentally measured nonlinear shear deformations and their coupling with flexural deformations throughout the cyclic loading history. In addition, the analytical results successfully represent various experimentally measured responses, such as lateral-load versus wall-top-displacement relations, magnitudes and distributions ...

The Effect of Lap Splice Length on the Cyclic Lateral Load Behavior of RC Members with Low-Strength Concrete and Plain Bars

Many existing reinforced concrete buildings in developing countries located in seismic areas do not possess sufficient strength and ductility characteristics to resist the effects of severe earthquakes. Among other deficiencies, improper detailing of reinforcement and poor quality of materials are major causes of poor seismic performance. More specifically, inadequate lap splice lengths provided at floor levels on plain column longitudinal bars, particularly when concrete strength is low, is a widespread deficiency that has not been investigated in detail to date. Information on the behavior of such columns under earthquake actions is extremely important for reliable assessment of the seismic safety of many existing structures with detailing deficiencies and poor construction quality. In this study, the effect of lap splice length on the cyclic lateral load behavior of low-strength RC columns with plain longitudinal bars (14 mm diameter) was investigated experimentally. The specimens were designed to represent columns with low axial loads. The geometric ratio of the longitudinal reinforcement and the volumetric ratio of the lateral reinforcement of the columns are 1% and 0.8%, respectively. The test program included five RC columns with lap splice lengths of 25, 35, 44 and 55 times longitudinal bar diameter, as well as a reference specimen with continuous longitudinal bars. Test results clearly demonstrated that presence of 180-degree hooks at the ends of the lap splice reduces the negative influence of the inadequate lap splice length on RC member performance, even in the case of low-strength concrete. All specimens reached their flexural strengths and did not experience considerable strength degradation until large drift levels. Test observations were supported with findings of a fiber-based analytical model, which also demonstrated the influence of hooks on improving the bond slip behavior along inadequate lap splices.

Performance Based Rapid Seismic Assessment Method (PERA) for Reinforced Concrete Frame Buildings

Recent destructive earthquakes have shown that many existing buildings, particularly in developing countries, are not safe against seismic actions. Since code-based seismic safety evaluation methods generally require detailed and complex structural analysis, the necessity for simplified, yet sufficiently accurate evaluation methods emerges for reducing cost and duration of assessment procedures. In this study, a performance based rapid seismic safety assessment method (PERA) is proposed for reinforced concrete buildings. The overall structural performance is determined based on the demand/capacity ratios of individual columns, as well as their failure modes (brittle/ductile), confinement characteristics, and levels of axial and shear stresses. The lateral drift of the critical story, calculated through a simplified approach, is also taken into account during determination of the global structural performance. The predictions of this method are compared with the results of conventional detailed seismic safety assessment analyses carried out for 672 different cases representing typical reinforced concrete frame buildings in Turkey. Good agreement is obtained between the predictions of the proposed algorithm and code-based structural performance assessment procedures. Finally, predictions of the proposed approach are compared with actual damages observed in 21 existing buildings in Turkey after destructive earthquakes that have occurred during the last two decades. These comparisons also point to an acceptable level of accuracy and sufficient conservatism for the methodology proposed.

Seismic Rehabilitation—Benefits of Component Testing

FEMA 356 backbone relations tend to provide conservative estimates of the available strengths and deformation capacities of reinforced concrete components, leading to costly seismic rehabilitation solutions for California hospitals. The conservatism is driven by the lack of test data for older, typically poorly detailed, structural components. Although FEMA 356 does provide for building specific component testing, this approach is not common due to cost concerns and/or schedule constraints. An assortment of large-scale wall segments, columns, and beam-column-joint assemblies were tested until substantial lateral strength degradation and loss of gravity load support was observed. Presented test results are compared with FEMA 356 backbone relations to highlight the advantages associated with building-specific test programs. In general, the test specimens revealed more strength and deformation capacity than assumed by FEMA 356, and more gradual strength deterioration. The test results, when coupled with the FEMA 356 Nonlinear Static Procedure (NSP) and Nonlinear Dynamic Procedure (NDP), enabled the development of more rational and substantially more economical rehabilitation solutions. Introduction Following damage to hospitals in the 1994 Northridge earthquake, California Senate Bill 1953 passed, requiring evaluation of pre-1973 acute care facilities with a timeline for rehabilitation or change in use. SB 1953 requires deficient facilities to be upgraded by 2013 to prevent collapse and loss of life; facilities must be upgraded to provide for continued operation after an earthquake by 2030. A review of existing California hospitals (OSHPD, 2001) reveals that 975 out of 2507 of pre-1973 buildings were rated SPC-1, buildings that pose a significant risk of collapse and must be upgraded by the 2013 deadline. A significant portion of these at-risk buildings are reinforced concrete construction. The FEMA 356 Pre-Standard (FEMA, 2000), recently updated as ASCE/SEI 41 (2006), is commonly used to evaluate and upgrade existing buildings. These documents provide a range of analysis approaches. However, among the most popular is the nonlinear static procedure (NSP). For the NSP, component modeling parameters and acceptance criteria are assigned for the components that contribute the lateral stiffness and strength of the building. For a given building, pushover analyses (NSP) are performed, target displacements are defined representing the expected displacement demand for the design earthquake event, and acceptance criteria are checked, for each of the structural components. The modeling parameters and acceptance criteria defined in FEMA 356 substantially impact the results of this process. In some cases, relatively little information exists to assign modeling parameters and FEMA 356 provides limited information. A good example of this condition is the one row of modeling parameters that exist for wall segments controlled by shear in FEMA 356 Table 6-19. In other cases, an abundance of information exists. However, the FEMA 356 provisions tend to provide a lower bound estimate to the observed test data (e.g., bond strength of §6.4.4 and the wall shear strength of §6.4.5) therefore, use of these provisions tends to produce rather conservative results. In other instances, the structural details used within the building may not fit into the predefined categories; several specific examples that fall into this category are provided later. In all of these cases, the conservatism built into the modeling parameters is likely to produce evaluation results that indicate existing buildings are excessively deficient and thus require costly and disruptive seismic rehabilitation solutions. As the 2013 deadline for upgrading SPC-1 rated buildings approaches, it is imperative that more economical seismic rehabilitation solutions be identified. This is particularly important given the significant rise in construction costs, which have greatly increased the overall estimated rehabilitation costs. One important attempt to address this issue is the recent update to the concrete provisions of ASCE/SEI 41, referred to as Supplement #1 (Elwood et al, 2007). Supplement #1 provides updates to modeling parameters for beams, columns, slab-column connections, and walls controlled by flexure and shear, as well as a number of other changes. The most significant change in Supplement #1 involves the changes to columns, where the deformation capacity at the collapse prevention limit state is substantially higher for many columns. However, even with the improvements provided by Supplement #1, substantial conservatism still exists. FEMA 356 and ASCE/SEI 41 include the option to derive modeling parameters and acceptance criteria based on testing in §2.8 provided that: (1) test subassemblies are identifiable with a portion of the structure and replicate construction details and boundary conditions, and (2) the test assemblies are subjected to reverse cyclic lateral loading at increasing displacement levels with the number of cycles and displacement levels based on the expected response of the structure to the design earthquake. The limiting strength and deformation capacities are determined from the experimental program using the average values of a minimum of three tests performed for the same design configuration and test conditions. Conducting building-specific test programs is not common due to the costs associated with the test program as well as scheduling constraints. However, modeling parameters derived from building-specific tests have the potential to substantially reduce the costs associated with seismic rehabilitation. This is particularly true for cases where the construction details are not consistent with predefined categories specified in the FEMA 356 document. Equally important, the tests can often be completed within a reasonable timeframe that does not cause problems for the design team or the client. Building-specific test programs also offer the advantage of providing greater confidence in achieving the desired performance level and help the design team communicate design objectives to the client.