Leonardo M. Massone

Damage and Implications for Seismic Design of RC Structural Wall Buildings

In 1996, Chile adopted NCh433.Of96, which includes seismic design approaches similar to those used in ASCE 7-10 (2010) and a concrete code based on ACI 318-95 (1995). Since reinforced concrete buildings are the predominant form of construction in Chile for buildings over four stories, the 27 February 2010 earthquake provides an excellent opportunity to assess the performance of reinforced concrete buildings designed using modern codes similar to those used in the United States. A description of observed damage is provided and correlated with a number of factors, including relatively high levels of wall axial load, the lack of well-detailed wall boundaries, and the common usage of flanged walls. Based on a detailed assessment of these issues, potential updates to U.S. codes and recommendations are suggested related to design and detailing of special reinforced concrete shear walls.

Seismic Design and Construction Practices for RC Structural Wall Buildings

Reinforced concrete buildings utilizing structural walls for lateral load resistance are the predominant form of construction in Chile for buildings over four stories. Typical buildings include a large number of walls, with ratios of wall cross-sectional area to floor plan area of roughly 3% in each principal direction. Based on the good performance of RC buildings in the March 1985 earthquake, requirements for closely spaced transverse reinforcement at wall boundaries were excluded when Chile adopted a new concrete code in 1996 based on ACI 318-95. In recent years, use of three-dimensional linear models along with modal response spectrum analysis has become common. Since 1985, nearly 10,000 new buildings have been permitted. Although the newer buildings have similar wall area to floor plan areas as older buildings, newer walls are thinner and buildings are taller, leading to significantly higher wall axial load ratios.

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.

Shear-Flexure Coupling Behavior of Steel Fiber-Reinforced Concrete Beams

Although steel fiber-reinforced concrete (SFRC) has become a popular choice in construction due to its high-performance properties, the shear-flexure coupling behavior of SFRC beams has not yet been studied adequately. This study uses generalized modeling techniques to investigate shear-flexure coupling behavior of SFRC beams. Twelve SFRC beams were tested under low-to-high shear-to-moment ratios and with different quantities of longitudinal reinforcement and steel fibers. The study included both normal- and high-strength concrete with steel fibers. The results from the tests were used to validate the nonlinear modeling techniques developed for evaluating shear-flexure coupling effects in SFRC beams. Three different cases of transverse normal stress or strain profiles in the beam shear span were used in the analytical model, depending on the presence and extent of yielding of the longitudinal steel bars. The findings suggest that the shear-flexure interaction model accurately predicted the degradation associated with the shear behavior and ductility of SFRC beams. Directions for future research are discussed.

Confinement Behavior of Rectangular Reinforced Concrete Prisms Simulating Wall Boundary Elements

AbstractObservations following recent earthquakes, and from structural testing, indicate numerous brittle compression failures in reinforced concrete seismic-resisting walls. This is unexpected, as...

The Influence of RC Nonlinearity on p-y Curves for CIDH Bridge Piers

Abstract The p-y method is one of the most popular methods in pile design and has been calibrated for various boundary conditions using numerical and experimental studies during recent years. Most studies on reinforced concrete (RC) piles have included the impact of flexural nonlinearity, (e.g. nonlinear moment–curvature relations) but not considered associated pile shear deformations when deriving p-y curves from field data. Common p-y curves may be better applicable for piles with flexure dominated failures (e.g. piles with free- head boundary conditions). For piles with fixed head boundaries (i.e. rotation restrained piles) shear deformations could be of significant influence. To study this problem, a coupled shear flexure interaction model for axial-bending-shear behavior coded in OpenSees was applied to a 0.61 m (2 ft) diameter flagpole and a 0.61m (2 ft) diameter fixed head pile specimen to investigate the possible influence of shear deformations to the overall pile responses. The surrounding soil was represented by p-y curves derived from prior large scale testing on piles with similar boundary conditions. Analysis results show that for flagpole piles, shear forces and shear deformations were insignificant. Considerable contributions of pile shear displacements and forces were observed for the fixed head pile, with shear displacements contributing up to 40% of the total pile displacement. Results suggest that nonlinear shear deformations for reinforced concrete piles should be considered for fixedhead or similar conditions, and that currently used p-y curves may underestimate the actual lateral pile displacement and possibly lead to unsafe design for the particular boundary condition.

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.

Modal Parameter Variation of an Earthquake Damaged Building

On February 27, 2010 one of the largest magnitude earthquake ever registered occurred in Chile. Although, several tall buildings suffered damage without collapse, the response of these buildings, in general, has been considered a success. Several studies have been carried out to understand the seismic response of these damage buildings, in order to develop appropriate retrofit strategies. We have selected one building located in the coastal city of Vina del Mar for instrumentation. This area suffered strong shaking with peak ground acceleration close to 0.35 g and more than 180 s of motion. The structure is a residential shearwall building with 17 stories and one basement level. The main structural system was damaged at the basement and first floor levels. The main damage was concrete crushing of walls, along with longitudinal reinforcement buckling at wall boundaries and severe cracking of slabs and lintel beams. An array of 12 accelerometers was deployed in the building to evaluate its modal properties variations, recording five aftershocks. This publication presents the structural damage in the building and the preliminary system identification results from strong motion records. Variations of the modal parameters are correlated with motion amplitudes and compared with ambient vibration conditions.