Gravity Load Collapse Research Articles

Investigating the vulnerability of nonductile reinforced concrete columns in moderate seismic regions to gravity load collapse

SUMMARYReinforced concrete (RC) columns are important components of the lateral load‐resisting structural system of RC buildings. In moderate seismic regions, where stringent seismic reinforcement detailing requirements are usually not considered, RC columns are categorized as nonductile. Postearthquake studies have shown that gravity load collapse of RC columns can trigger the progressive collapse of RC buildings. The current study presents the seismic response behavior of nonductile RC columns in moderate seismic regions, with a particular focus on gravity load collapse. Six RC columns, three with lap splices and three without lap splices with variable aspect ratios, were tested under reversed cyclic loading. Experimental results show that the column failing in shear could tolerate the maximum drift in order of 2.7–3.5%, whereas the columns failed in flexural mode could achieve the maximum drift of 4.5%. For the columns with lap splice, the lateral strength was significantly degraded, but all spliced columns could sustain gravity load even when their displacements were more than 4% drift. This very large drift without axial collapse, observed in the current study, is associated with the splice slip causing the large rotation just above the splice region.

Collapse Time of Reinforced Concrete Buildings with Brittle Columns

Buildings with brittle columns are in danger of gravity load collapse and may be hazardous in the event of future earthquakes. To plan for building evacuation, the time required for such buildings to collapse must be known. This study thus uses dynamic analysis to examine the collapse time of reinforced-concrete (RC) buildings designed according to old Japanese codes. Model lateral load vs interstory drift relations are represented based on previous collapse tests of brittle columns. The results reveal that (1) in most cases, the time between shear failure and collapse is very short for individuals to safely evacuate a building, and (2) the collapse time can be long for earthquakes with long durations.

Gravity Load Collapse Behavior of Nonengineered Reinforced Concrete Columns

This paper aims at investigating gravity load collapse behavior of extremely poor quality reinforced concrete columns under cyclic loading. Such columns were usually constructed by local people and may not be designed to meet any of the standards. It was found that their concrete strength may be as low as 5 MPa and the amount of longitudinal reinforcement may be lower than 1%. This type of column is deliberately defined as “nonengineered reinforced concrete column,” or NRCC. During earthquake, the gravity load collapse of the NRCC columns caused a large number of death tolls around the world. In this study, four columns as representative of existing NRCC were tested under cyclic loading. The compressive strength of concrete in order of 5 MPa was used to be representative of columns with poor quality concrete. Two axial load levels of 6 and 18 tons were used to study the influence of axial load level on maximum drift at gravity load collapse. To investigate the effect of bar types on drift capacity, 9 mm round bars were used in two specimens and 12 mm deformed bars were used for the rest of the specimens. The maximum drift before gravity load collapse was very dependent on the axial load level. The maximum drift of the specimens subjected to high axial load (18 tons) was extremely low at approximately 1.75% drifts. The use of deformed bars (associated with larger amount of longitudinal reinforcement) caused the damage to severely dissipate all over the height of the columns. Such damage caused columns to collapse at a lower drift compared to those using round bars. Finally, the plastic hinge model was used to predict the maximum drift of the low strength columns. It was found that the model overly underestimates the drift at gravity load collapse.

Enhancement of seismic performance of reinforced concrete columns with buckling-restrained reinforcement

Reinforced concrete (RC) columns with light confinement prevalent in developing countries exhibit low ductility with brittle shear failure, especially when buckling of longitudinal rebars takes place. This study applies the buckling restraining concept widely used in seismic resistant steel structures to reinforcing bars. Two RC columns 270 mm×300 mm in cross section with a height of 1200 mm and minimum (non-seismic) transverse reinforcement were tested under cyclic lateral loading. Buckling-restrained reinforcement was provided over the critical zone. The buckling-restraining casing effectively prevented buckling of slender vertical bars under a substantially high axial load level, resulting in a more ductile mode of failure with the evident formation of plastic hinge at the base of the column. Prior to gravity load collapse, the drift capacities and the degraded concrete shear capacities of the specimens were significantly increased compared to their counterparts without casings.

Modeling progressive collapse in reinforced concrete buildings using direct element removal

AbstractThis paper presents a novel analytical formulation of an element removal algorithm based on dynamic equilibrium and the resulting transient change in system kinematics, by applying imposed accelerations instead of external forces at a node where an element was once connected. The algorithm is implemented into an open‐source finite element code, numerically tested using a benchmark structural system with simplified element removal criteria, and able to capture the effect of uncertainty in member capacity. Realistic element removal criteria are introduced for mode‐dependent gravity load collapse of seismically deficient and retrofitted reinforced concrete (RC) columns and unreinforced masonry (URM) infill walls. Two applications are conducted using structural systems of RC frames with URM infill walls. The first is a probabilistic study of a one‐story model subjected to an ensemble of 14 ground motion recordings from similar neighboring sites during an earthquake event. The study produces empirical probability curves for partial and complete collapse conditioned on different hazard levels, and concludes that the intra‐event variability is a major source of uncertainty affecting the outcome of progressive collapse simulations. The second application is a deterministic sensitivity study of progressive collapse response in a five‐story structural model to uncertainty in live load, stiffness, damping, and seismic hazard level, subjected to one ground motion record. The analysis identifies the time at incipient collapse as an adequate sensitivity measure, and the uncertainty in ground motion intensity as the most important, followed by the stiffness of the URM infill wall. Copyright © 2009 John Wiley & Sons, Ltd.

Gravity Load Collapse of Reinforced Concrete Columns with Brittle Failure Modes

During the 1995 Kobe earthquake, dozens of concrete buildings suffered complete collapse at an interme-diate story. Such collapse is believed to have occurred due to the gravity load collapse following shear failure of Steel Reinforced Concrete (SRC) or Reinforced Concrete (RC) columns at a certain intermediate story. To study this collapse tests of RC column specimens were conducted. Test parameters were axial load and loading history. The tests have revealed that: 1) collapse occurs when shear force decreases to about zero, 2) axial load level is decisive for the collapse behavior, 3) loading history occasionally has a significant effect on the collapse behavior including a failure mode, and 4) a change in vertical deformation increment vs. lateral drift increment relations until collapse can be explained by the flow rule in plastic theory considering reduced failure surface.