Non-ductile Reinforced Concrete Buildings Research Articles

A method for automated development of model and fragility inventories of nonductile reinforced concrete buildings

The nonductile reinforced concrete building (NDRCB) stock—typically, pre-1974 structures in the U.S.—is a well-known high-risk group for seismic hazards. Prior studies indicate that there are approximately 1500 NDRCBs in Los Angeles. Through various ordinances, the owners are currently required to choose between demolition and, when appropriate, seismic retrofitting. Because fulfilling these ordinances will take decades, the potential risk of major losses will persist. In this study, a method for automated development of structural analysis models and damage fragilities for non-ductile moment-resisting frames is established. This capability enables seismic risk assessment at a regional scale using relatively limited building metadata and the era-specific seismic design code. The approach is used first to develop archetypal models in OpenSees, verified through static pushover and nonlinear time-history analyses against prior detailed studies. Fragility curves for discrete damage states are developed through a probabilistic seismic demand model. Additional investigations are carried out to consider the influence of soil-structural interaction effects and to determine the most suitable seismic intensity measures to quantify the seismic damage levels of NDRCB frames. The sensitivity of the proposed modeling method to variations/uncertainties in building configurations and properties is also examined through parametric studies. The method is limited to a particular subcategory of NDRBCs—namely, moment-resisting frames—but extensions to other types appear straightforward.

Effectiveness of the Seesaw System as a Means of Seismic Upgrading in Older, Non-Ductile Reinforced Concrete Buildings

This work investigates and discusses the effectiveness of the seesaw system when installed in an older, non-ductile reinforced concrete (RC) building for seismic upgrading purposes. In particular, two different configurations of the seesaw system are assumed in a two-storey 3D RC framed building which was designed according to older seismic provisions and, thus, is susceptible to flexural and shear failures. To check if there is any merit in employing the seesaw system in this RC building, non-linear time-history (NLTH) analyses are conducting using 11 seismic motions. Peak values for inter-story drift ratios (IDR), residual inter-story drift ratios (RIDR) and floor accelerations (FA) are computed, and the sequence and cause (i.e., due to surpass of flexural or shear strength) of plastic hinge formations are monitored. Leaving aside any issues related to fabrication and cost, interpretation of the results obtained by the aforementioned seismic response indices for the RC building under study leads to the conclusion that the seesaw system can be a retrofitting scheme for the seismic upgrading of older, non-ductile RC framed buildings.

Seismic Collapse Risk for Non-Ductile Reinforced Concrete Buildings According to Seismic Design Categories

Seismic Collapse Risk for Non-Ductile Reinforced Concrete Buildings According to Seismic Design Categories

Seismic Fragility Assessment of Non-ductile Reinforced Concrete Buildings in Australia

ABSTRACT This study aims to assess the seismic performance of typical reinforced concrete buildings constructed in Australia before the enforcement for seismic design. The response of both the primary lateral load resisting system and the gravity load resisting system is considered. A brief description of the modelling approaches adopted to simulate the response of the buildings’ components is provided. Nonlinear time history analyses are conducted and “cloud analysis” is used to develop the probabilistic seismic demand models. The results from the fragility assessment suggest that the low-rise buildings located on soft sites are the most vulnerable buildings.

Seismic vulnerability of non-ductile RC buildings in Pakistan through stochastic analysis of design/construction deficiencies

Reinforced concrete frame structures constitute a major portion of the building stock of Pakistan. In 2005, Kashmir earthquake exposed the current deficient local design and construction practices in Pakistan. It is hypothesized that inhomogeneity of concrete in general and faulty construction practices in Pakistan cause huge uncertainties in parameters affecting strength, serviceability and seismic assessment results for structures in this country. The aim of this study is to assess the vulnerability of existing reinforced concrete (RC) building stock of Pakistan with respect to a stochastic analysis of the parameters affecting the built infrastructure. For this purpose, typical building characteristics with uncertainties are identified. The probability density functions of material and geometric parameters are defined for these building characteristics. These functions are then used to account for the uncertainty in modelling of structural configurations. Structural response is evaluated through static pushover analysis and vulnerability curves for different structural configurations are derived using the framework developed for deficient RC buildings. The performance of buildings is evaluated in different seismic zones as classified in the new Building Code of Pakistan (BCP-SP 2007). Selected buildings performed satisfactorily in Zones 1, 2A and 2B. Buildings in Zones 3 and 4 are found to be more vulnerable.

Probabilistic Assessment and Cost-Benefit Analysis of Nonductile Reinforced Concrete Buildings Retrofitted with Base Isolation: Considering Mainshock–Aftershock Hazards

AbstractNonductile reinforced concrete (RC) frame buildings were found to be vulnerable to earthquakes. Thus retrofitting of these buildings is necessary, as addressed in the recent retrofit progra...

Seismic vulnerability assessment of high-rise non-ductile RC buildings considering soil–structure interaction effects

Traditionally fragility curves of reinforced concrete (RC) buildings are estimated with the assumption of fixed base structures. The objective of the present research is to study whether soil-structure interaction (SSI) and site effects may affect the seismic performance and vulnerability of reinforced concrete moment resisting frame (MRF) buildings and consequently modify the fragility curves. SSI is modeled applying the direct one-step approach considering either linear elastic or nonlinear soil behavior while site effects are inherently accounted for. To further examine the contribution of site and SSI effects, a two-step uncoupled approach is also applied, which takes into account site effects on the response of the fixed base structure, but neglects SSI effects. Additional analyses are performed investigating the influence of the soil depth and stratigraphy under nonlinear soil behavior on the seismic response and fragility of RC buildings. A 9-story RC MRF designed with low seismic code provisions is adopted as a reference structure. A comparative dynamic analysis is performed highlighting various trends in the seismic response of the considered SSI and fixed base systems. Fragility curves are derived as a function of rock outcropping peak ground acceleration for the immediate occupancy and collapse prevention limit states for the fixed base and SSI models based on the statistical exploitation of the results of incremental dynamic analysis (IDA) of the given structural systems. Results show the significant role of SSI and site effects under linear or nonlinear soil behavior in altering the expected structural performance and fragility of high-rise fixed base structures.

Numerical investigation of the affecting parameters on the shear failure of Nonductile RC exterior joints

In this paper, a parametric study has been carried out on reinforced concrete (RC) exterior beam-column joints without transverse reinforcement in the joint region. This type of joints belongs to a group of RC structures classified as non-ductile RC buildings. In the previous investigations, special attention is paid to the joint aspect ratio and beam longitudinal reinforcement ratio. These parameters have been shown by other researchers to govern the joint shear behavior, although the degree of influence was not scrutinized to the degree required. The results of the current study confirmed once again that these parameters have a significant influence on the shear failure in RC joints. Apart from that, the degree of influence has been quantified in great detail. The findings suggest that the correct selection of these parameters is extremely important in the design of RC buildings, and a wrong choice may adversely affect the behavior.

Simulation of Reinforced Concrete Frames with Nonductile Beam-Column Joints

The accurate prediction of shear strength and flexibility of beam-column joints without transverse reinforcement is essential to assess the seismic performance of nonductile reinforced concrete (RC) buildings characterized by having such unreinforced beam-column joints. In this study, a multilinear backbone curve to represent the moment-rotation relationship of an unreinforced corner beam-column joint is proposed. The modeling parameters of the backbone curve are estimated based on experimental results of four corner joint specimens recently tested by the authors. Furthermore, the proposed backbone curve is modified to be applicable to interior and roof beam-column joints. These backbone curves are validated by accurate reproduction of the force-drift responses of the four corner joint specimens and eight other exterior and interior joint specimens from literature. Using these backbone curves, nonlinear dynamic analyses are performed on three hypothetical building frames. The analytical results demonstrate the importance of joint flexibility for seismic assessment of nonductile RC buildings.

Generation of analytical fragility curves for Ghanaian non-ductile reinforced concrete frame buildings

This study is on seismic safety evaluation of non-ductile reinforced concrete (RC) frame buildings in Ghana. Generic 3-, 4- and 6-storey non-ductile RC buildings were characterised for assessment. The generic buildings were designed according to British Standard (BS) 8110. The fragility curve parameters were generated using inelastic time history analyses for seismic demand and inelastic pushover analyses for structural capacity of the buildings. As a result of lack of real time histories in Ghana, 3 suites of 100 synthetic time history records each were generated in order to generate lower and upper bounds for inelastic dynamic response. Parameters of collapse fragility curves with 50% probability of exceedance were established for the generic non-ductile Ghanaian frame buildings using short duration near-fault like records. All the curves represent the probability of exceeding the collapse limit state as a function of the peak ground acceleration (PGA) and spectral acceleration at the fundamental mode period of the generic buildings. Using the data from the nonlinear dynamic analysis of the generic buildings under all 300 synthetic time histories, a pair of three sets of fragility curves was developed. Results obtained showed that the generic non-ductile 3 to 6 storey RC frame buildings subjected to near-fault like ground motions may record high probabilities of collapse, if they are situated in 0.25 to 0.35 g PGA seismic zones.

Seismic performance of reinforced concrete buildings in the 22 February Christchurch (Lyttelton) earthquake

Six months after the 4 September 2010 Mw 7.1 Darfield (Canterbury) earthquake, a Mw 6.2 Christchurch (Lyttelton) aftershock struck Christchurch on the 22 February 2011. This earthquake was centred approximately 10km south-east of the Christchurch CBD at a shallow depth of 5km, resulting in intense seismic shaking within the Christchurch central business district (CBD). Unlike the 4 Sept earthquake when limited-to-moderate damage was observed in engineered reinforced concrete (RC) buildings [35], in the 22 February event a high number of RC Buildings in the Christchurch CBD (16.2 % out of 833) were severely damaged. There were 182 fatalities, 135 of which were the unfortunate consequences of the complete collapse of two mid-rise RC buildings. This paper describes immediate observations of damage to RC buildings in the 22 February 2011 Christchurch earthquake. Some preliminary lessons are highlighted and discussed in light of the observed performance of the RC building stock. Damage statistics and typical damage patterns are presented for various configurations and lateral resisting systems. Data was collated predominantly from first-hand post-earthquake reconnaissance observations by the authors, complemented with detailed assessment of the structural drawings of critical buildings and the observed behaviour. Overall, the 22 February 2011 Mw 6.2 Christchurch earthquake was a particularly severe test for both modern seismically-designed and existing non-ductile RC buildings. The sequence of earthquakes since the 4 Sept 2010, particularly the 22 Feb event has confirmed old lessons and brought to life new critical ones, highlighting some urgent action required to remedy structural deficiencies in both existing and “modern” buildings. Given the major social and economic impact of the earthquakes to a country with strong seismic engineering tradition, no doubt some aspects of the seismic design will be improved based on the lessons from Christchurch. The bar needs to and can be raised, starting with a strong endorsement of new damage-resisting, whilst cost-efficient, technologies as well as the strict enforcement, including financial incentives, of active policies for the seismic retrofit of existing buildings at a national scale.

Rehabilitation of RC frame joints using local steel bracing

Reinforced concrete (RC) structural frames that were built prior to the 1970s generally do not meet current design code requirements and may behave in a non-ductile manner. The lateral load carrying capacity of these structures is often insufficient due to non-ductile reinforcement detailing, which includes insufficient or no beam-column joint transverse reinforcement and inadequate anchorage for the beam's bottom reinforcement. Experience shows that such frames are prone to earthquake damage and often suffer shear and bond slip non-ductile modes of failure. Recent earthquakes have demonstrated that the beam-column joint safety is an important factor in keeping the integrity of the entire structure. Concentric steel bracing systems have been used for the rehabilitation of non-ductile RC buildings in a number of countries. However, the application of eccentric (local) steel bracing in the rehabilitation of RC structures still lags behind due to the limited research and information on the design, modelling and behaviour of such a combined concrete-steel system. In this paper a new beam-column joint rehabilitation technique using local steel brace members is proposed. Two full-scale specimens representing a standard joint and a rehabilitated joint were made and tested under reversed cyclic load. Their behaviour was compared to that of a non-rehabilitated specimen. It is shown that the rehabilitation technique was successful in enhancing the overall performance of the deficient joint and upgrading it towards a close to current standard performance.