International Concrete Abstracts Portal

The design of beam-column joints in reinforced concrete moment frames is an area where USA and New Zealand standards have diverged for many years. USA design guidelines, and ACI 352 in particular, implicitly accept damage in the form of shear cracking, bar slip and possible column hinging for joints subjected to large lateral load reversals. Since the 1980’s, the New Zealand approach has been to minimize that type of damage and to concentrate the deformations in plastic hinges in the beams by careful detailing of the joint and adjacent beam regions, thus keeping columns essentially elastic. The recent February 22, 2011 Christchurch earthquake and its associated swarm present an excellent opportunity to contrast these approaches in terms of visual performance for a variety of New Zealand structures detailed and built before and after the newer, more stringent joint design guidelines came into effect. The main lesson from the Christchurch experience is the importance of providing both some degree of lateral resistance, e.g. via beam-column joint moment-resisting capacity, and an increased level of displacement capacity in secondary or gravity-frames in order to improve the overall building’s robustness and seismic resilience in response to earthquake demands beyond the code design level.

The paper presents an overview of the study carried out to compare the results of three research programs conducted on hooked bars anchored in beam-column joints in normal strength concrete (NSC) structures at the American University of Beirut (AUB). The specimen simulated the rigid connection of a cantilever beam to a column. In the first program, the beam-column joint was confined externally with carbon fiber reinforced polymer (CFRP) sheets. Confinement of the joint in the second program was provided internally by steel fibers of varying volume fractions incorporated in the concrete mix. Although stirrups were included in the beam and column elements of the specimens of the first two programs, however the column stirrups were not extended in the beam-column joint. In the third program, confinement of the joint was provided internally by different amounts of stirrups. Test results of the three programs indicated positive effect of the different confining modes on the bond strength of the anchored hooked bars and ductility of the load-deflection history of the tested specimens. The objective of the study reported in the paper is to perform a correlation between the positive effects of the three different confining modes of the beam column joint.

The main objective of this paper is to discuss the validity of seismic rehabilitation of concrete buildings in Japan based on the review of the behavior of seismically rehabilitated buildings during recent earthquakes. First, the state of seismic rehabilitation of concrete buildings is introduced. It is described that the seismic rehabilitation has been strongly promoted since 1) the enactment of the Law for Promotion of Seismic Rehabilitation of Existing Buildings in 1995 and 2) the declaration of the ten-year promotion plan of seismic rehabilitation as the government policy in 2005. Second, the behavior of rehabilitated buildings during recent earthquakes is introduced. It is described that 1) there were some insufficiencies in the early practice of seismic rehabilitation due to the lack of available guidelines, 2) the case of a rehabilitated building in Kobe that survived the 1995 Kobe Earthquake without any damage verified the validity of seismic rehabilitation, and 3) extensive nonstructural damage may result in the loss of building function, therefore, the target performance of “functional” may not be satisfied even if the performance of “life safety” is satisfied. Third, the behavior of un-rehabilitated local government buildings during the 2011 Great East Japan Earthquake is introduced. It is described that 1) extensive nonstructural damage may lead to shut down of the building even if structural damage is minor, 2) vulnerable essential buildings should be rehabilitated to satisfy the performance of “functional” as well as the performance of “life safety” and 3) essential building should be rehabilitated with highest priority.

The 2011 off the Pacific coast, Tohoku Earthquake (Mw=9.0) was the largest in the history of seismic observation in Japan. Seismology research community did not anticipate the possible occurrence of such a mega earthquake in the area. Main casualties were caused by tsunamis rather than by the collapse of buildings due to ground shaking. Structural engineering could not protect lives of building occupants if the building was buried under tsunami flood; good community planning is essential to mitigate the tsunami disaster in coast areas. Some low-rise reinforced concrete and steel buildings were moved or overturned by tsunami flood. Reinforced concrete structural walls failed by out-of-plane tsunami water pressure; floor slabs were lifted from floor girders by upward water pressure. Peak ground accelerations exceeding 1.0 g (g: acceleration of gravity, 9.81 m/s2 or 386 in./s2) were recorded at more than a dozen strong motion recording stations, but the destructive power of far-field earthquake motions was less for buildings than that of near-field earthquake motions. The seismic vulnerability of existing old buildings should be critically assessed, and vulnerable buildings should be retrofitted.

An experimental study was conducted in which 37 reinforced concrete deep beam specimens were tested. The specimens are some of the largest deep beams ever tested in the history of shear research. The data from the experimental program and from a database of 179 deep beam tests in the literature were used to assess the accuracy and conservatism of the strut and tie modeling provisions of ACI 318. In addition, the effects of the following variables were evaluated: distribution of stirrup legs across the web, triaxial confinement via concrete surrounding CCC and CCT nodal faces, quantity of web reinforcement, member depth, and a/d ratio. Based on the experimental findings of the study, suggested improvements to the ACI 318 STM provisions are discussed in this paper.