Mojtaba Dyanati

Cost-benefit evaluation of self-centring concentrically braced frames considering uncertainties

Abstract Self-centring concentrically braced frame (SC-CBF) systems have been developed to reduce post-earthquake damages in braced frames. However, due to special details required by the SC-CBF system, the construction cost of an SC-CBF is expected to be higher than that of a conventional CBF. In this study, the seismic performance and economic effectiveness of two prototype buildings utilising SC-CBFs are assessed and compared with buildings utilising conventional CBFs by evaluating the annual probabilities of exceeding various damage levels, expected annual losses, life cycle costs (under seismic hazard) and economic benefit of using SC-CBFs considering prevailing uncertainties. The results of this study show that the SC-CBF buildings have lower drift-related losses but higher acceleration-related losses. The SC-CBF is found to be beneficial for the 6-storey configuration, but not for the 10-storey configuration. For the 6-storey buildings studied here, if the construction cost of the SC-CBF is assumed to be twice that of the CBF, the pay-off time is expected to be 12 to 21 years, with a probability of 68%, considering the uncertainties in the demand, capacity, loss parameters and initial construction costs. Finally, appropriate probabilistic engineering demand parameter model formulation is critical for generating accurate loss analysis results.

Structural and Nonstructural Performance Evaluation of Self- Centering Concentrically Braced Frames Under Seismic Loading

Self-centering concentrically braced frame (SC-CBF) systems have higher drift capacity than conventional CBF systems prior to damage. To fully demonstrate the effectiveness of SC-CBF systems, a comparison study is performed considering both structural and nonstructural seismic performance. First, two prototype buildings are designed with the same configuration but different lateral load resisting systems (CBF or SC-CBF exclusively). Nonlinear dynamic analysis is conducted using finite element numerical modeling to obtain seismic responses under a suite of earthquake records with various hazard levels. The numerical results are then utilized to build probabilistic demand models of inter-story drift, floor acceleration, and roof acceleration. Finally, fragility curves for structural and nonstructural components are generated for various performance levels with demands and capacities. The comparison of the fragilities of the two structural systems reveals that the SC-CBF has better seismic performance than the conventional CBF; however, nonstructural components must be designed for higher force demands in the SC-CBF.

Life cycle cost-benefit evaluation of self-centering and conventional concentrically braced frames

Self-centering concentrically braced frame (SC-CBF) systems have been developed to increase the drift capacity of braced frames prior to structural damage. To achieve the improved seismic performance of SC-CBF system, the construction cost of an SC-CBF is expected to be higher than that of a conventional CBF. In this study, economic effectiveness of using an SC-CBF instead of a CBF in one prototype building is calculated to indicate the time that initial SC-CBF construction costs are compensated by lower earthquake-induced losses in the lifetime of the building (pay-off time). The results of this study show lower business interruption is the most significant component of the economic benefit of the SC-CBF compared to the CBF. Moreover, the pay-off time increases dramatically if the initial construction cost of the SC-CBF is more than 4% higher than the CBF. Earthquake-induced damages of buildings can cause social and economic disturbances. Selfcentering concentrically braced frame (SC-CBF) systems (Roke et al. 2009) have been developed to address the limitations of conventional CBFs by increasing drift capacity of the structure prior to damage and decreasing residual drift; thus SCCBF can mitigate losses due to earthquakes. This improved seismic performance of the SC-CBF system has been found experimentally and numerically (e.g., Roke et al. 2009; Dyanati et al. 2014, 2015). However, the construction cost of an SC-CBF is expected to be higher than that of a conventional CBF due to the special details and elements required by the SC-CBF. Therefore, it is necessary to investigate if the higher construction cost of SC-CBF system would be offset by lower earthquake-induced losses (due to better seismic performance of SC-CBF) during the life time of the building, which would demonstrate the economic effectiveness of SC-CBF systems compared with conventional CBF systems. Life cycle cost assessment has been used as a measure of the economic effectiveness of a structure. Wen & Ang (1991) and Wen & Shinozuka (1998) developed a life cycle cost formulation to investigate the cost effectiveness of an active control system in structures during earthquakes. Goda et al. (2010) used life cycle cost assessment to investigate the cost effectiveness of a seismic isolation technology. Kang & Wen (2000) used the minimum life cycle cost concept to develop an optimal design for structures under single and multiple hazards. Padgett et al. (2009) developed a retrofit strategy for bridges based on cost-benefit analysis using life-cycle cost to determine the most costeffective retrofit method that differs based on the seismic hazard characteristics of the location. Available software such as HAZUS (FEMA 2014) and PACT (FEMA 2012) have been used in the seismic performance and loss evaluation of buildings (Erberik & Elnashai 2006, Parvini sani & Banazadeh 2012) and can also be used for life 12 International Conference on Applications of Statistics and Probability in Civil Engineering, ICASP12 Vancouver, Canada, July 12-15, 2015 2 cycle cost estimation. However, there is a major drawback in both software packages: they both define engineering demand parameter (EDP) models as a function of only one seismic intensity measure (IM), pseudo spectral acceleration (PSA), which may not be accurate for CBF and SC-CBF structures (as studied by Dyanati et al. 2015), eventually leading to inaccuracy in the loss estimation. In this study, the economic benefit of the SC-CBF will be studied using life cycle cost formulation. The economic benefit of the SCCBF, which is the difference between life cycle cost of SC-CBF and CBF structures, will clarify if the higher construction cost of SC-CBF will be compensated by better performance of SC-CBF. 1. SC-CBF SYSTEM The general configuration of an SC-CBF is shown in Figure 1(a). There are two sets of columns in the SC-CBF: SC-CBF columns and adjacent gravity columns. As shown in Figure 1(b), the SC-CBF columns are allowed to uplift at the base, causing a rocking response under higher levels of lateral force. Vertically oriented post-tensioning (PT) bars and gravity loads are used to resist column uplift and provide selfcentering (i.e., reducing residual drift). The rocking behavior softens the lateral force-lateral drift response of the system, thereby permitting larger lateral displacements while limiting the member force demands, avoiding yielding or buckling in the braces. 2. SEISMIC LIFE-CYCLE COST-BENEFIT MODEL Life cycle cost of a building system subjected to seismic hazard includes three components (Kang & Wen 2000): initial construction cost, including structural and non-structural component costs (C0); earthquake-induced losses or life cycle loss of the building (e.g., repair cost, business interruption, injuries) (LCL); and operation/maintenance costs during the life cycle of the building (Cm), as shown in Equation (1).         x x x x m C t LCL C t LCC    , , 0 (1) where LCC = life cycle cost of the structure; t = life time of the structure; and x = vector of design variables for the structure. Figure 1: (a) Configuration of SC-CBF; (b) Rocking behavior of SC-CBF; (c) configuration of CBF. Construction cost estimation is straightforward and can be generally estimated using expert opinions or tools such as R.S. Square Foot Costs (RS Means 2013). Maintenance and operation costs are highly related to the occupancy of the building, rather than the structural system, and can be estimated using handbooks and standards such as Facilities Maintenance & Repair Cost Data Online (RS Means 2013). The life cycle loss (LCL) estimation, on the other hand, involves more complex procedures including hazard, response, damage, and loss analysis for calculating the losses from earthquakes. If the expected annual loss (EAL) from earthquakes is known, the expected value for life cycle loss (E[LCL]) can be evaluated as follows (Porter et al. 2004):     EAL e t LCL E t      1 ] , [ x (2) where e -γt = discounted factor over time t and γ = constant discount rate per year, which is used to calculate the present value of the future losses. Assuming C0,SC-CBF = a C0,CBF (a = relative cost coefficient and a > 1) and equal maintenance/operation costs for CBF and SCCBF systems, the expected economic benefit of using an SC-CBF instead of a CBF in a building, E[BSC-CBF], can then be calculated as follows: 12 International Conference on Applications of Statistics and Probability in Civil Engineering, ICASP12 Vancouver, Canada, July 12-15, 2015 3         CBF SC CBF t CBF CBF SC

Sensitivity analysis of seismic performance assessment and consequent impacts on loss analysis

The probabilistic framework for seismic performance evaluation developed by PEER has been widely used in the literature. This framework consists of four steps: hazard analysis, response analysis, damage analysis, and loss analysis. Typically the process involves ground motion selection for numerical analysis, probabilistic model development for engineering demand parameters (EDP), and an EDP hazard calculation approach. This study investigates the impact of three aspects (i.e., the ground motion suite selected, the EDP formulation chosen, and the hazard calculation formulation adopted) on the seismic performance of an office building located in downtown Los Angeles. The result of this study shows that these three aspects have significant impacts on the seismic performance in terms of dynamic responses, demand hazard, and expected annual loss. This study also suggests that more accurate and robust performance evaluation is obtained when: (1) using a ground motion suite that contains a rich intensity measure content, (2) using vector-valued demand models that provides more accurate predictions rather than scalar-valued demand models, and (3) using joint hazard formulations when two seismic intensity measures are involved.

Seismic Reliability of a Fixed Offshore Platform Against Collapse

As many jacket type steel platforms have been constructed in the highly active seismic area, seismic reliability evaluation of such structures is desirable. Ultimate limit state (ULS) with base shear capacity and demand can be used to estimate seismic performance of fixed offshore platform against collapse. Base shear capacity is evaluated from pushover analysis on a 3D finite element model of the offshore structure using different load patterns. Base shear demand is calculated from spectral acceleration at a given site and the total mass of the platform. Uncertainties are considered in both capacity and demand evaluations.With the limit state function, seismic fragility of a prototype structure is assessed using reliability analysis. The results indicate that various load patterns affect the seismic performance evaluation. It is also found that the steel yield stress is a critical parameter in the reliability of the steel jacket platforms.Copyright

Effect of site-specific soil nonlinearities and uncertainties on ground motion intensity measures and structural demand parameters

ABSTRACT This paper investigates the effects of site-specific uncertainties in soil strength and stiffness parameters on the commonly used ground motion intensity measures and the resulting engineering demand parameters of moment resisting frames of varying heights at a site in Los Angeles, California. Through a detailed site-specific uncertainty analysis, comparison with generic data, and an extensive parametric study, it is shown that underestimation or overestimation of the true soil uncertainties could significantly affect both the average values and variabilities of the Arias intensity and PGA, and that the uncertainty in the soil strength parameter has more profound effect on the intensity measures than the uncertainty in the soil stiffness parameter. It is also shown that depending upon the natural period of a structure, the uncertainty in both the soil strength and stiffness parameters could affect its peak floor acceleration and peak inter-story drift. Moreover, it is observed that the structure amplifies the soil uncertainty more than the seismological uncertainty.