Aman Mwafy

STATIC PUSHOVER VERSUS DYNAMIC COLLAPSE ANALYSIS OF RC BUILDINGS

Abstract Owing to the simplicity of inelastic static pushover analysis compared to inelastic dynamic analysis, the study of this technique has been the subject of many investigations in recent years. In this paper, the validity and the applicability of this technique are assessed by comparison with ‘dynamic pushover’ idealised envelopes obtained from incremental dynamic collapse analysis. This is undertaken using natural and artificial earthquake records imposed on 12 RC buildings of different characteristics. This involves successive scaling and application of each accelerogram followed by assessment of the maximum response, up to the achievement of the structural collapse. The results of over one hundred inelastic dynamic analyses using a detailed 2D modelling approach for each of the twelve RC buildings have been utilised to develop the dynamic pushover envelopes and compare these with the static pushover results with different load patterns. Good correlation is obtained between the calculated idealised envelopes of the dynamic analyses and static pushover results for a defined class of structure. Where discrepancies were observed, extensive investigations based on Fourier amplitude analysis of the response were undertaken and conservative assumptions were recommended.

CALIBRATION OF FORCE REDUCTION FACTORS OF RC BUILDINGS

A comprehensive study is undertaken to assess and calibrate the force reduction factors (R) adopted in modern seismic codes. Refined expressions are employed to calculate the R factors “supply” for 12 buildings of various characteristics represent a wide range of medium-rise RC buildings. The “supply” values are then compared with the “design” and “demand” recommended in the literature. A comprehensive range of response criteria at the member and storey levels, including shear as a failure criterion, alongside a detailed modelling approach and an extensively verified analytical tool are utilised. A rigorous technique is employed to evaluate R factors, including inelastic pushover and incremental dynamic collapse analyses employing eight natural and artificial records. In the light of the information obtained from more than 1500 inelastic analyses, it is concluded that including shear and vertical motion in assessment and calculations of R factors is necessary. Force reduction factors adopted by the design code (Eurocode 8) are over-conservative and can be safely increased, particularly for regular frame structures designed to lower PGA and higher ductility levels.

Wave Passage and Ground Motion Incoherency Effects on Seismic Response of an Extended Bridge

This paper investigates the implications of ground motion spatial variability on the seismic response of an extended highway bridge. An existing 59-span, 2,164-meter bridge with several bearing types and irregularity features was selected as a reference structure. The bridge is located in the New Madrid Seismic Zone and supported on thick layers of soil deposits. Site-specific bedrock input ground motions were selected based on a refined probabilistic seismic hazard analysis of the bridge site. Wave passage and ground motion incoherency effects were accounted for after propagating the bedrock records to the ground surface. The results obtained from inelastic response-history analyses confirm the significant impact of wave passage and ground motion incoherency on the seismic behavior of the bridge. The amplification in seismic demands exceeds 150%, whereas the maximum suppression of these demands is less than 50%. The irregular and unpredictable changes in structural response owing to asynchronous earthquake records necessitate in-depth seismic assessment of major highway bridges with advanced modeling techniques to realistically capture their complex seismic response. DOI: 10.1061/(ASCE)BE.1943-5592.0000155.

EFFECT OF BIDIRECTIONAL EXCITATIONS ON SEISMIC RESPONSE OF RC BUILDINGS

This paper investigates the effect of the horizontal and vertical components of ground motions (HGM and VGM, respectively) on the seismic response of Reinforced Concrete (RC) buildings designed to modern capacity design principles and located in the vicinity of active faults. Fiber-based analytical models are used to monitor the global and local response of twelve reference structures, including verifying the response modification factor and tracing the member shear supply-demand response using a ductility- and axial force-sensitive shear strength approach. The simulation models are subjected to near-field earthquake records with increasing severity up to collapse, including and excluding VGM. The results indicate that the lower the contribution of horizontal seismic forces to the seismic response, the higher is the significance of VGM. The fluctuation of axial forces in vertical structural members significantly increases when including VGM. This not only has direct consequences on tension and compression response but also has impact on shear capacity. The diverse range of buildings and performance criteria and large number of incremental dynamic analyses confirm the importance of including VGM in seismic design and assessment of contemporary RC buildings, and hence cast doubts on the reliability of pre-code structures located in the vicinity of active faults.

Analytical Assessment of Seismic Performance Evaluation Procedures for Bridges

The multi-limit state seismic design and evaluation procedure allows structures to satisfy different performance criteria against different levels of seismic excitation. To achieve the simultaneous satisfaction of the multi-level design approach, it is essential to employ accurate analysis procedures which can be consistently applied to various levels of ground motions. In this study, several analytical evaluation procedures are compared via the application of the methods to two bridge structures. In the first application, a bridge considered typical of the inventory in the Central and Eastern United States is analyzed. Inelastic Response History Analysis (IRHA), two Capacity Spectrum Methods (CSMs), two Elastic Response History Analysis (ERHA) approaches with different stiffness approximation, and single degree of freedom (SDOF) simulations are conducted. The second and more complex application, a 59-span irregular bridge crossing the Mississippi River is also analyzed in the elastic and inelastic ranges. Results from IRHA and simplified analysis procedures are compared to assess their applicability and limitations. It is concluded that the approximate methods have limited applicability, which depends on several parameters including intensity of ground motions and characteristics of bridge structures. The importance of inelastic and dynamic analysis in seismic assessment is emphasized, while cases where the simplified procedures yield acceptable response are presented.

Multi-level nonlinear modeling verification scheme of RC high-rise wall buildings

Earthquake-resistant reinforced concrete (RC) high-rise wall buildings are designed and detailed to respond well beyond the elastic range under the expected earthquake ground motions. However, despite their considerable section depth, in terms of analysis, RC walls are still often treated as linear elements, ignoring the effect of deformation compatibility. Due to the limited number of available comprehensive experimental studies on RC structural wall systems subjected to cycling loading, few in-depth analytical verification studies have been conducted. Motivated by the increasing need for more accurate seismic risk assessment of high-rise buildings in multi-scenario seismic regions, a multi-level nonlinear modeling verification scheme is presented in this paper to investigate two different nonlinear modeling techniques for shear walls (2- and 4-noded fiber-based elements). The investigated modeling approaches and their key parameters are verified against the results of Phase I of uniaxial shaking table specimen tests (performed at the University of California, San Diego) on a seven-story full-scale RC shear wall structure under base excitations representing four earthquake records of increasing intensities. Three numerical models are created using two different tools (ZEUS-NL and PERFORM-3D). The results obtained from the numerical models are compared with the experimental results both on global and local response levels (top displacement, interstory drift, story shear force, story bending moment, period elongation and rebar tensile strain). The study reveals the superior performance of 4-noded fiber-based wall/shell element modeling approach in accounting for the 3D effects of deformation compatibility between lateral and gravity-force-resisting systems. The study also highlights the sensitivity of attained results to the stiffnesses assigned to the rigid links and 3D joints required to connect the shear walls to neighboring elements when a 2-noded element is used.

Assessment of Material Strength Implications on Seismic Design of Tall Buildings through Collapse Analysis

High-strength materials are widely utilized in multi-story buildings with shear walls to effectively utilize floor areas and control lateral drifts. To investigate the impacts of high-strength concrete on the seismic design coefficients of shear wall-supported structures, five different designs of 60-story buildings with varying concrete strength are considered. The reference structures are designed and detailed such that they have very close periods of vibration. The large number of inelastic analyses performed at different intensity levels using twenty earthquake records and detailed fiber-based simulation models enabled the effective verification of the seismic design coefficients. The results reflect the enhanced profits and safety margins of shear wall-supported structures with increasing concrete strength. There is a possibility for increasing the design coefficients, which has several economic advantages. This systematic study provides practical insights into the seismic response of high-strength shear wall-supported tall buildings at different performance limit states and enables the verification of essential coefficients used in seismic design.

Development of a multi-disciplinary graduate course on consequence-based earthquake risk management

Despite the recognition of the need for broad-based integrative education that prepares students for addressing complex problems in the real world, multi-disciplinary course offerings remain rare. The paper describes the development and offering of a multi-disciplinary graduate course covering earthquake loss assessment and mitigation ‘from source to society’. The course is built upon the comprehensive research, education, and outreach activities of the National Science Foundation-funded Mid-America Earthquake (MAE) Centre through the paradigm of consequence-based risk management (CRM). This is the first course of its kind in the field of earthquake engineering to expose students to the earthquake problem from source to society. The course was successfully offered at the University of Illinois at Urbana-Champaign, and will be offered at other universities. The CRM programme is a successful model for similar educational experiences. It opens new avenues in education of loss assessment and mitigation to mee...

Collapse Assessment of Substandard Concrete Structures for Seismic Loss Estimation of the Building Inventory in the UAE

Regional earthquake loss estimation systems describe the probability of losses that could happen by a seismic hazard to a certain region. In order to develop a loss estimation system for a region, the vulnerability characteristics of the exposed structures should be integrated with earthquake hazard and the inventory of the built environment. The accurate definition of structural collapse under earthquake loads is essential for deriving reliable vulnerability functions. In this study, the collapse of concrete buildings is described in terms of both global structural response and member failure, including shear failure modes. Experimentally verified shear strength models that effectively consider the reduction of shear strength with the concrete degradation under cyclic loading are implemented in a post-processor to monitor the shear supply-demand response of concrete structures under earthquake loads. A wide range of reference structures with diverse lateral force resisting systems and building heights is selected to represent substandard buildings in the UAE. Detailed fiber-based numerical models and a diverse set of earthquake records representing different seismic scenarios in the study region are employed in dynamic response simulations at various levels of ground motion intensities up to collapse. The effectiveness of the adopted shear strength models in predicting the brittle failure modes of substandard concrete buildings is demonstrated in this study. It is concluded that shear modeling is essential for the reliable earthquake loss estimation of pre-seismic code buildings. The advanced vulnerability functions confirm the need for mitigation strategies to reduce the earthquake losses of the substandard building inventory in the study area. This comprehensive study represents a step forward for the development of a reliable loss estimation system in the UAE and the surrounding region.

Implications of adopting different force reductions in seismic design of RC buildings

The implications of assigning different levels of force reductions by two seismic design codes on the relative safety margins of regular and irregular RC buildings are investigated in this study at various performance levels. Four moment-resisting frame structures are detailed according to two seismic design provisions; the Egyptian national design code in addition to the internationally recognised European design standard. The seismic demands of the buildings obtained from a large number of inelastic analyses are compared at the member and structure levels using detailed fibre-based simulation models. It is concluded that, in regions of medium seismicity, designing a building using more conservative force reduction factors has lower impacts on improving its seismic performance compared with the key role played by ductility and capacity design. The comparative study sheds light on the performance of structures when designed to different seismic forces but with adequate ductility, which helps in realising the differences between design standards.