Wassim M. Ghannoum

Analytical Collapse Study of Lightly Confined Reinforced Concrete Frames Subjected to Northridge Earthquake Ground Motions

Review of older non seismically detailed reinforced concrete building collapses shows that most collapses are triggered by failures in columns, beam-column joints, and slab-column connections. Using data from laboratory studies, failure models have previously been developed to estimate loading conditions that correspond to failure of column components. These failure models have been incorporated in nonlinear dynamic analysis software, enabling complete dynamic simulations of building response including component failure and the progression of collapse. A reinforced concrete frame analytical model incorporating column shear and axial failure elements was subjected to a suite of near-fault ground motions recorded during the 1994 Northridge earthquake. The results of this study show sensitivity of the frame response to ground motions recorded from the same earthquake, at sites of close proximity, and with similar soil conditions. This suggests that the variability of ground motion from site to site (so-called intra-event variability) plays an important role in determining which buildings will collapse in a given earthquake.

Experimental Investigations of Loading Rate Effects in Reinforced Concrete Columns

Seismic loading rates can significantly affect the behavior of reinforced concrete (RC) elements. However, few data are available to quantify these effects. Shaking table tests allow the study of loading rate phenomena; however, they suffer from difficulties in assessing causality (direct assessment of causes on effects) and are expensive to conduct. An alternative is to test individual RC elements by directly imparting high-velocity loading protocols. However, multiactuator setups are necessary to achieve seismically representative loading and boundary conditions, which entails particularly challenging control requirements. This investigation makes use of recent advances in real-time testing hardware to study the effects of loading rates on the structural response of lightly confined RC columns. A pioneering test setup in which three actuators are controlled independently at high velocities was used to test a series of columns until axial collapse. The experimental challenges and column behavior are discussed.

COLLAPSE OF LIGHTLY CONFINED REINFORCED CONCRETE FRAMES DURING EARTHQUAKES

Post earthquake studies show that the primary cause of reinforced concrete building collapse during earthquakes is the loss of vertical-load-carrying capacity in critical building components leading to cascading vertical collapse, rather than loss of lateral-load capacity. In cast-in-place beam-column frames, the most common cause of collapse is failure of columns, beam-column joints, or both. Once axial failure occurs in one or more components, vertical loads arising from both gravity and inertial effects are transferred to adjacent framing components. The ability of the frame to continue to support vertical loads depends on both the capacity of the framing system to transfer these loads to adjacent components and the capacity of the adjacent components to support the additional load. When one of these conditions is deficient, progressive failure of the building can ensue. Post-earthquake reconnaissance of reinforced concrete buildings provides some insight into the prevalence of collapse among populations of heavily shaken buildings. Otani (1999) reports damage statistics of reinforced concrete buildings, with damage defined in three categories:

Behavior of Anchored Carbon Fiber-Reinforced Polymer Strips Used for Strengthening Concrete Structures

The anchorage of carbon fiber-reinforced polymer (CFRP) strips using CFRP anchors is gaining acceptance in strengthening applications of concrete members. CFRP anchors can fully develop the strength of CFRP strips when adequately detailed. However, parameters that influence the behavior and strength of CFRP strips and anchors are not well understood. In this study, 26 tests on concrete beams were conducted to study the influence of five key parameters on CFRP anchor effectiveness: 1) the width of the anchored CFRP strip; 2) the material ratio of CFRP anchor to CFRP strip; 3) the concrete strength; 4) the length/angle of anchor fan; and 5) the bond condition between a CFRP strip and concrete. Results indicate that narrow anchored CFRP strips developed higher stresses at fracture than wide strips and required smaller anchor material ratios to be fully developed. Test results provide valuable data for designing anchored CFRP strengthening systems.

Real-time hybrid simulation of a nonductile reinforced concrete frame

AbstractThis paper reports about a real-time hybrid simulation (RTHS) of a nonductile reinforced concrete frame that had previously been tested on a shake table (ST) at the University of California, Berkeley. This three-story, three-bay frame is numerically modeled with flexibility-based/layered nonlinear elements and over 400 degrees of freedom (DOFs), while one of the nonductile base columns is physically tested in the laboratory. RTHS is enabled through a new code developed by the authors, and these simulation results are compared with those obtained from the ST test. The comparison between ST tests and RTHS is encouraging, though still not acceptable (within 10%). Details of the simulation are provided, and preliminary results indicate that RTHS may be indeed provide the natural path for a gradual substitution of physical (and expensive) testing by numerical simulation. This contribution offers a small step in that direction.

DESIGN IMPLICATIONS OF A LARGE-SCALE SHAKING TABLE TEST ON A FOUR-STORY REINFORCED CONCRETE BUILDING

A full-scale, four-story, reinforced concrete building designed in accordance with the current Japanese seismic design code was tested under multi-directional shaking on the E-Defense shake table. A two-bay moment frame system was adopted in the longer plan direction and a pair of multi-story walls was incorporated in the exterior frames in the shorter plan direction. Minor adjustments to the designs were made to bring the final structure closer to U.S. practice and thereby benefit a broader audience. The resulting details of the test building reflected most current U.S. seismic design provisions. The structure remained stable throughout the series of severe shaking tests, even though lateral story drift ratios exceeded 0.04. The structure did, however, sustain severe damage in the walls and beam-column joints. Beams and columns showed limited damage and maintained core integrity throughout the series of tests. Implications of test results for the seismic design provisions of ACI 318-11 are discussed.

Analytical Element for Simulating Lateral-Strength Degradation in Reinforced Concrete Columns and Other Frame Members

An analytical element is proposed that is capable of simulating the lateral-strength degradation behavior of frame members subjected to seismic loading up to severe loss in lateral strength. Although element capabilities allow simulating the behavior of a wide range of frame members exhibiting loss of lateral strength, they were developed with the behavior of shear-critical reinforced concrete columns in mind. The element consists of a zero-length shear spring that connects in series with a beam-column flexural element. The proposed element can dynamically monitor beam-column elements for user-defined limiting forces and flexural deformations, and initiate degradation when either is reached. Upon initiation of degradation, the material model governing the behavior of the zero-length shear spring changes its constitutive properties to include pinching, strength degradation, and stiffness degradation. Cycle-, energy-, and displacement-based damage accumulation methods were implemented to provide users with the necessary tools to model a variety of frame members. A novel flexural-deformation compensation algorithm was implemented in the element that automatically adjusts the shear-spring stiffness and backbone curve such that a symmetric member response is achieved. The versatile element is shown to possess the necessary capabilities to simulate the nonlinear shear behavior and strength degradation of select reinforced concrete columns with only a limited number of parameters calibrated.

Rotation-Based Shear Failure Model for Lightly Confined RC Columns

AbstractA shear failure model for nonductile RC columns that sustain flexural yielding prior to shear failure is proposed. The model, which relates shear failure to column end rotation, was derived by performing a forward stepwise linear regression on a database compiled from column tests. Critical parameters that affect rotation capacity at shear failure were found to be axial load, spacing (rather than amount) of transverse reinforcements, and nominal shear stress. The model is suitable for use in nonlinear frame analyses using either a lumped-plasticity or fiber-section column idealization. The model also can be used to determine rotation limits for performance-based assessment of existing buildings.

Local deformation measures for RC column shear failures leading to collapse

A novel model capable of simulating shear strength degradation of pre 1970s reinforced concrete columns detailed for gravity load resistance and subjected to large seismic loads is presented. The model can account for both in-cyclic as well as cyclic shear damage up to complete loss of lateral strength and stiffness. The model has been implemented in the open source analysis software OpenSees and tested with a large column database. The proposed model utilizes localized column end-rotations, axial loads, and pertinent column properties to assess column shear capacity and trigger shear failure. Once shear failure is triggered, the analytical model modifies the column response by introducing softening and cyclic degradation in the constitutive properties of a zero-length shear spring element. The zero-length shear spring can be attached in series to any column element. Past investigitions have demonstrated that axial collapse in gravity detailed reinforced concrete columns will only occur after damage has become extensive and the shear strength is almost exhausted. Thus accurate column collapse assessment necessitates adequate modeling of shear strength degradation up to the critical limit that initiates axial collapse. The proposed shear failure model is capable of accomplishing this task and has been developed with the objective of improving the accuracy of collapse risk assessment efforts targeting pre 1970s reinforced concrete columns detailed for gravity load resistance.

INVERTED-T BEAMS: EXPERIMENTS AND STRUT-AND-TIE MODELING

Contrary to rectangular deep beams, inverted-T beams are loaded on a ledge at the bottom chord of the beam. This loading configuration induces a tension field into the web and the resulting complex strain distribution renders sectional design provisions inadequate. The applicability of strut-and-tie modeling (STM), developed for rectangular deep beams and simpler, two-dimensional designs, was evaluated. An experimental study was conducted in which 33 tests were performed on 22 large-scale reinforced concrete inverted-T beams and the effects of the following variables were investigated: ledge geometry, quantity of web reinforcement, number of point loads, member depth, and shear span-depth ratio. It was concluded that strut-and-tie modeling, although developed for much simpler structural components, offers a simple and accurate design method for the more complex strain distributions in inverted-T beams. The STM provisions developed for rectangular beams accurately captured both failure mode and ultimate capacity and are recommended for use in inverted-T beam design, as a major conclusion of this research.