Luis Ibarra

Effect of P-delta uncertainty on the seismic collapse capacity and its variability of single-degree-of freedom systems

This study assesses the effect of parameter uncertainty of non-deteriorating P-delta vulnerable single-degree-of-freedom systems on the median and dispersion of the collapse capacity. The post-yielding negative slope is a necessary condition of P-delta induced collapse that dominates the failure mode, and thus, it is the primary system parameter to be considered as a random variable. The parameter uncertainty on the collapse capacity is quantified with the first-order-second-moment method, and verified with the Latin hypercube sampling (LHS) technique. The total variability of the collapse capacity is estimated by combining the parameter uncertainty with record-to-record variability according to the square-root-of-sum-of-squares rule. Alternatively, the total variability of the collapse capacity is obtained from LHS realizations that simultaneously account for uncertainty of the post-yielding negative stiffness ratio and the earthquake excitation. The importance of uncertain post-yielding negative slope on the collapse capacity is underlined, and the main observations of the parameter uncertainty and total uncertainty of the collapse capacity are discussed.

Seismic Retrofit of a Three-Span RC Bridge with Buckling-Restrained Braces

AbstractThis study evaluated the potential benefits of using buckling-restrained braces (BRBs) to seismically rehabilitate straight bridges. For this purpose, a three-span RC box girder bridge was used as a case study. A three-dimensional model was developed using software to incorporate BRBs between bent columns. The BRB inelastic behavior was represented using the Menegotto–Pinto model to reproduce its isotropic and kinematic strain-hardening properties. Nonlinear time-history analyses were performed to assess the seismic performance of the BRBs and the existing RC bridge. The structure was evaluated under several performance limit states using far-field records, which were scaled to the maximum considered earthquake level at the site using an average interval scaling method. The results show that BRB components improve the seismic performance of bridges under serviceability and ultimate limit states by decreasing drifts in the bents and by reducing the steel and concrete strains of the original RC colu...

Threshold Chloride Levels for Localized Carbon Steel Corrosion in Simulated Concrete Pore Solutions Using Coupled Multielectrode Array Sensors

Threshold chloride levels for localized corrosion of carbon steel material have been studied in three types of simulated concrete pore solutions: sodium hydroxide-potassium hydroxide (NaOH-KOH) with pH 11.6, calcium hydroxide (Ca[OH]2) with pH 12.6, and sodium hydroxide-potassium hydroxide-calcium hydroxide (NaOH-KOH-Ca[OH]2) with pH 13.3. The nonuniform corrosion rates of carbon steel were measured with coupled multielectrode array sensors (CMAS) when the chloride concentration was changed from 0.0005 mol/L to 1 mol/L in each solution. Open-circuit potentials were also measured from the coupling joint of the CMAS probes and electrodes made of rebar specimens immersed in the simulated pore solutions to verify the results from the CMAS probes.

Optimal Spectral Acceleration-based Intensity Measure for Seismic Collapse Assessment of P-Delta Vulnerable Frame Structures

This study proposes an “optimal” spectral acceleration-based intensity measure (IM) to assess the collapse capacity of highly inelastic frame structures vulnerable to the P-delta effect. The IM is derived from the geometric mean of the spectral pseudo-acceleration over a certain period interval. The lower bound period of the averaging interval is related to the mode in which 95% of the effective modal mass is exceeded. The upper bound period is 1.6 times the fundamental period. This IM provides minimum, or close to the minimum, dispersion for frames with different fundamental periods of vibration, or number of stories.

Optimal intensity measure based on spectral acceleration for P-delta vulnerable deteriorating frame structures in the collapse limit state

This study proposes an “optimal” spectral acceleration based intensity measure (IM) to assess the collapse capacity of generic moment frames vulnerable to the P-delta effect. The IM is derived from the geometric mean of the spectral pseudo-acceleration over a certain period interval. The optimized IM includes for first time a flexible lower limit for the period interval, corresponding to the structural period associated with the exceedance of 95% of the total effective mass. This flexible lower limit bound provides an efficient IM, independently of the contribution of higher modes to the total response. The upper bound period is 1.6 times the fundamental period to account for period elongation due to inelastic deformations and gravity loads. In a parametric study on generic frames, structural parameters are varied to quantify the performance of this IM compared to classical benchmark IMs. The “optimal” IM provides minimum, or close to the minimum, dispersion for the entire set of frames with different fundamental periods of vibration, number of stories, and P-delta vulnerability.

Seismic Performance of Dry Casks Storage for Long- Term Exposure

This research will evaluate the mechanical performance of dry-cask storage (DCS) under seismic loading for mid-term operational periods. The spent fuel at nuclear power plants (NPPs) is initially stored in pools to control fuel-assemblies temperature. Thereafter, the spent fuel is transferred to DCSs at sites contiguous to the plant, known as Interim Spent Fuel Storage Installations (ISFSIs). DCSs are usually designed as free-standing structures resting on a reinforced concrete foundation pad, or casks anchored to a foundation pad. The casks can also be designed to be placed in concrete bunkers in horizontal or vertical position. Numerical seismic analyses of ISFSIs are usually performed assuming a compliance periods of 20 years. The consideration of DCSs for storing spent fuel for hundreds of years has created new challenges that have only recently started to be addressed. In the case of seismic hazard, longer-term operating periods result in i) very large horizontal accelerations, ii) destabilizing effects of vertical accelerations, and iii) aging material deterioration. The first two factors affect the overall seismic performance of DCSs, whereas material degradation reduces the capacity of the casks. Large earthquakes can lead to casks tipping over, impacting the concrete pad or adjacent casks. The casks may also slide and collide with other casks or structural components. The mechanical stresses and strains caused by these impacts are important even if the overpack does not breach. The DCSs performance should ensure fuel-rod integrity and canister integrity because eventually the spent fuel-rods need to be shipped either to a reprocessing plant or a repository. Experimental and numerical tests will be performed to evaluate the long-term seismic performance of DCSs. Numerical analyses using finite element programs with explicit time integration will be performed for freestanding, anchored, and vaulted casks considering several cask-pad-soil systems. Coupled effects of mechanical stresses caused by seismic events, and aging material degradation will be included in the simulations. The experimental tests will evaluate the dynamic seismic response of freestanding and anchored DCS prototypes on a six Degree-of-Freedom (DOF) Shaking Table. This will be the first time that an experimental seismic simulation of DCSs will include seismic accelerations in three orthogonal directions. The study will estimate the seismic accelerations that lead to tip over, the parameters that most contribute to DCSs damage, provide recommendations for optimal ISFSI designs for freestanding and anchored DCSs.

High-Strength Fiber-Reinforced Concrete Beam-Columns with High-Strength Steel

A series of reinforced concrete (RC) components with high-strength materials were tested under pure flexural monotonic and cyclic quasi-static loading protocols. The specimen materials included high-strength concrete (HSC), ultra-high-strength steel (UHSS), and steel fibers to create high-strength fiber-reinforced concrete (HSFRC) specimens. The replacement of conventional steel with UHSS increased the specimen’s ultimate flexural capacity more than 60%, but reduced its ductility. The addition of steel fibers to HSC specimens increased the peak strength by an additional 10%, and greatly reduced cracking and spalling of HSC specimens. However, the energy dissipation capacity of the specimens did not increase due to the presence of fibers.

Probabilistic Assessment of Seismic Risk of Dwelling Buildings of Barcelona. Implication for the City Resilience

The knowledge of seismic risk of buildings can contribute to increase the resilience of cities. In the present work a new assessment of the seismic risk of dwelling buildings of Barcelona was done. This assessment was performed according to a probabilistic methodology, which is summarized in the following steps: (1) performing a probabilistic seismic hazard assessment (PSHA) to obtain exceedance rates of macroseismic intensities; (2) performing a probabilistic seismic vulnerability assessment (PSVA) of each building in order to determine probability density functions that describe the variation of a vulnerability index; and (3) performing a probabilistic seismic risk assessment (PSRA) to generate seismic risk curves in terms of frequencies of exceedance of damage states. In the present work 69,982 dwelling buildings of Barcelona were assessed. According to the results the percentage of dwelling buildings of Barcelona that have a probability equal or greater than 1% of suffer partial collapse in the next 50 years is a value between 0% and 34.29%. A value of 0% corresponds to the results of seismic risk obtained for the case where regional vulnerability modifiers were not considered during the procedure to assess the seismic vulnerability of buildings and 34.29% correspond to the case where regional vulnerability modifiers were considered. For the same two options, the losses due to the physical damage of the dwelling buildings of Barcelona assessed for an exposure time of 50 years, could vary from 807.3 to 1739.4 millions of euros, respectively. Finally, possible uses of the seismic risk results computed in the present work are mentioned.

Risk Assessment of Structural Integrity of Transportation Casks after Extended Storage

This study is part of an investigation into the structural reliability of a spent nuclear fuel (SNF) cask subjected to normal or accidental conditions of transport, after a dry storage period of up to 300 years. A probabilistic approach is chosen for this assessment to account for the uncertainties related to the timedependent material degradation mechanisms acting on the cask components. Preliminary investigations revealed that the fuel rod cladding is expected to control structural failure of the cask. The probability of failure is likely increased by hydride-related material degradation in the cladding and due to low cladding temperature after long-term storage. Mechanical testing revealed a tendency towards a brittle response on pinching loads at low material temperature of cladding that contains an excessive amount of hydrogen, or after the application of high cladding hoop stresses (CHSs) at high cladding temperatures. Therefore, the hydrogen content (CH) of the cladding and the value of the peak CHS during vacuum drying of the rods were identified as possible controlling parameters of hydride-induced cladding embrittlement. In the scope of the probabilistic risk assessment (PRA), statistical methods are used to predict the expected fuel rod conditions for the moment of transport. This paper presents an exemplary assessment of the probability of hydride-related cladding embrittlement due to high CHSs during vacuum drying or due to excessive hydrogen incorporation in the cladding considering in-reactor corrosion and fission gas release (FGR). The analysis reveals that between 5 and 12% of the SNF rods likely have an offset strain capacity below 2%, and therefore, could react brittle under pinching loads.

Effects of Partial-Design-Strength Concrete on the Seismic Performance of Concrete-Filled Tube Columns in Accelerated Bridge Construction

This study evaluated the seismic performance of circular concrete-filled tube (CCFT) columns in accelerated bridge construction (ABC). In these projects, the bridge should be open to service after a couple of days. For this reason, this study evaluated the ability of CCFT columns to perform adequately under gravitational and seismic loading before the concrete reached its design strength at 28 days. A reduced seismic hazard that accounted for this temporal condition was implemented, and a performance evaluation was performed to obtain the CCFT column probability of failure. The performance of a CCFT column was compared with that of a circular reinforced concrete (RC) column of a Caltrans bridge. Probabilistic analyses using reduced seismic loads for the temporary condition indicated that CCFT columns with partial design concrete compressive strength can be used for ABC without resulting in an increased probability of failure.