Browsing by Subject "reinforced concrete"
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Item Impact of seismic code provisions in the central U.S.: a performance evaluation of a reinforced concrete building(2009-05-15) Kueht, ErinThe close proximity to the New Madrid Seismic Zone and the significant population and infrastructure presents a potentially substantial risk for central U.S. cities such as Memphis, Tennessee. However, seismic provisions in currently adopted Memphis building codes for non-essential structures have a lower seismic design intensity level than the 2003 International Building Code (IBC) with broader acceptance nationally. As such, it is important to evaluate structures designed with these local seismic provisions to determine whether they will perform adequately during two different design-level earthquakes in this region. A four-story reinforced concrete (RC) moment frame with wide-module pan joists was designed according to current building codes relevant to the central U.S.: the 2003 IBC, the City of Memphis and Shelby County locally amended version of the 2003 IBC, and the 1999 Standard Building Code (SBC). Special moment frames (SMFs) were required for the IBC and SBC designs, but lower design forces in the amended IBC case study permitted an intermediate moment frame (IMF). However, the margin by which a SMF was required was very small for the SBC design. For slightly different conditions IMFs could be used. Nonlinear push-over and dynamic analyses using synthetic ground motions developed for Memphis for 2% and 10% probabilities of exceedance in 50 years were conducted for each of the three designs. The FEMA 356 recommended Basic Safety Objective (BSO) is to dually achieve Life Safety (LS) for the 10% in 50 years earthquake and Collapse Prevention (CP) for the 2% in 50 years earthquake. For the member-level evaluation, the SMF designs met the LS performance objective, but none of the designs met the CP performance objective or the BSO. However, the margin by which the SMF buildings exceeded CP performance was relatively small compared to that of the IMF building. Fragility curves were also developed to provide an estimate of the probability of exceeding various performance levels and quantitative performance limits. These relationships further emphasize the benefits of using an SMF as required by the IBC and, in this case, the SBC.Item Seismic fragility and retrofitting for a reinforced concrete flat-slab structure(Texas A&M University, 2004-09-30) Bai, Jong-WhaThe effectiveness of seismic retrofitting applied to enhance seismic performance was assessed for a five-story reinforced concrete (RC) flat-slab building structure in the central United States. In addition to this, an assessment of seismic fragility that relates the probability of exceeding a performance level to the earthquake intensity was conducted. The response of the structure was predicted using nonlinear static and dynamic analyses with synthetic ground motion records for the central U.S. region. In addition, two analytical approaches for nonlinear response analysis were compared. FEMA 356 (ASCE 2000) criteria were used to evaluate the seismic performance of the case study building. Two approaches of FEMA 356 were used for seismic evaluation: global-level and member-level using three performance levels (Immediate Occupancy, Life Safety and Collapse Prevention). In addition to these limit states, punching shear drift limits were also considered to establish an upper bound drift capacity limit for collapse prevention. Based on the seismic evaluation results, three possible retrofit techniques were applied to improve the seismic performance of the structure, including addition of shear walls, addition of RC column jackets, and confinement of the column plastic hinge zones using externally bonded steel plates. Finally, fragility relationships were developed for the existing and retrofitted structure using several performance levels. Fragility curves for the retrofitted structure were compared with those for the unretrofitted structure. For various performance levels to assess the fragility curves, FEMA global drift limits were compared with the drift limits based on the FEMA member-level criteria. In addition to this, performance levels which were based on additional quantitative limits were also considered and compared with FEMA drift limits.Item Seismic fragility estimates for reinforced concrete framed buildings(Texas A&M University, 2007-04-25) Ramamoorthy, Sathish KumarGravity load designed (GLD) reinforced concrete (RC) buildings represent a common type of construction in the Mid-America Region. These buildings have limited lateral resistance and are susceptible to story mechanisms during earthquake loading. Fragility estimates are developed to assess the seismic vulnerability of GLD RC buildings in the Mid-America Region. Fragility is defined as the conditional probability of reaching or exceeding a performance level for a given earthquake intensity measure. Five sample buildings of various story heights (1, 2, 3, 6, and 10 stories) are used to represent generic RC frame buildings of 1 to 10 stories tall. A Bayesian methodology is used to develop probabilistic demand models to predict the maximum inter story drift given the spectral acceleration at the fundamental period of the building. The unknown parameters of the demand models are estimated using the simulated response data obtained from nonlinear time history analyses of the structural models for a suite of synthetic ground motions, developed for Memphis, Tennessee. Seismic structural capacity values are selected corresponding to the performance levels or damage states as specified in FEMA-356 and as computed by nonlinear pushover analyses. For the sample buildings, fragility estimates are developed using the predicted drift demands and structural capacity values. Confidence bounds are developed to represent the epistemic uncertainty inherent in the fragility estimates. In addition, bivariate fragility estimates, formulated as a function of spectral acceleration and the fundamental building period, are developed from the fragility estimates of the individual buildings. The bivariate fragilities can be used to quantify the seismic vulnerability of GLD RC frame buildings of 1 to 10 stories. Using the Bayesian approach, a framework is developed to update the analytical fragility estimates using observed damage data or experimental test data. As an illustration of the updating framework, the analytical bivariate fragility estimates for the sample buildings in the Mid-America Region are updated using the damage data obtained from 1994 Northridge, California earthquake. Furthermore, to investigate and demonstrate the increase in seismic performance of the GLD RC frame buildings, the columns of the 2 and 3 story buildings are retrofitted by column strengthening. Fragility estimates developed for the retrofitted buildings show the effectiveness of the retrofit technique by the improved seismic performance of GLD RC frame buildings.Item Seismic vulnerability of older reinforced concrete frame structures in Mid-America(Texas A&M University, 2004-09-30) Beason, Lauren RaeThis research quantifies the seismic vulnerability of older reinforced concrete frame structures located in Mid-America. After designing a representative three-story gravity load designed reinforced concrete frame structure, a nonlinear analytical representation was used evaluate inter-story drift demands from simulated earthquake time histories that were representative for the region. Limit state story drift capacities were identified for FEMA 273 guidelines, nonlinear pushover analyses, and incremental dynamic analyses. Integrating these two quantities allowed for the creation of fragility curves which relates the probability of exceeding a particular limit state given an imposed spectral acceleration at the fundamental building period. These curves were then used to evaluate the seismic vulnerability of the representatively designed structure. The structure as originally designed was found to be inadequate to resist large lateral loading that would be typical for the Memphis area. So structural retrofit performed by increasing the column-to-beam strength ratio was evaluated by increasing the strength of the column members in the analytical model. The first retrofit raised the column-to-beam strength ratio to 1.2, which is currently required by the ACI code provisions. The second retrofit raised the column-to-beam strength ratio to 1.8, as suggested in previous studies. The story capacity, demand, and fragility curves were once again created for these retrofitted structures. Comparison of these fragility curves is discussed in relation to the retrofit strategy of column strengthening for older reinforced concrete frame structures in Mid-America.