Dynamic Fragility Research Articles

Seismic performance assessment of code-conforming precast reinforced concrete frames in China

Precast reinforced concrete (RC) frames have been rapidly developed in structural engineering due to their various advantages, such as high efficiency and low environmental pollution. However, the construction quality of precast RC frames is difficult to guarantee, and the interface of new and old concrete and the bond-slip effects of additional steel bars may lead to different performance levels of precast RC frames. Therefore, it is necessary and important to investigate and assess the seismic performance of precast RC frames that adopt the conformation of Chinese code. In this paper, a numerical simulation method for precast RC frames is developed based on OpenSEES software, in which the Joint2D element is used to simulate the beam-to-column connections to consider the shear and bond-slip effects. Two prototype structures of precast RC frames are designed, and the nonlinear time history analysis is performed to explore the seismic performance of the precast RC frames according to the Chinese code. Meanwhile, the incremental dynamic analysis and fragility analysis are conducted to study the differences in seismic performance and collapse resistance between precast RC frames and cast-in-situ RC frames.

Seismic performance of offshore wind turbine in the vicinity of seamount subduction zone

The presence of seamount on the subducting interface of oceanic and continental tectonic plates has potential to alter the characteristics of ground motion due interface earthquakes. In this study, we address the influence of seamount presence on offshore wind turbine response, through a finite element simulation of earthquake and subsequently applying the recorded shaking from earthquake simulation as input to offshore wind turbines. The wind turbine response is computing by incremental dynamic analysis and fragility curves are derived for serviceability and ultimate limits of wind turbines. The effect of hypocenter being above or below the seamount, on the offshore wind turbine fragility is also addressed.

Seismic assessment and fragility analysis of traditional Chinese timber frame building considering degraded connections

ABSTRACT This paper studies the influence of degraded connections on dynamic characteristics, seismic response, and seismic fragility behaviors of traditional Chinese timber-framed buildings. Numerical models are developed based on the extended distinct element method, and calibrated with 1:3.52 scaled shake-table test data. The influence of fracture and wood aging on the performance of both mortise-tenon joints and Dougong brackets connections is discussed. A set of 20 ground motion records are selected for an incremental dynamic analysis, seismic behaviors, and fragility assessment. Results indicate that considering 0.75 times and 0.85 times degradation of mortise-tenon joints, natural frequencies are reduced by 8%, and 4%, respectively, and 4%, and 2% for Dougong brackets connections, respectively. The maximum acceleration response is reduced at most by 1.8% and 0.9%, respectively, and the maximum relative displacement is promoted by 7.3% and 5.3%, respectively. In addition, for a peak inter-structural layer drift of 1/442, 1/148, and 1/48, seismic intensities corresponding to median probabilities of exceedance of the structure with 0.75 times degraded performance of mortise-tenon joints are reduced by 32%, 16%, and 9%, respectively, and 14%, 6%, and 3% for Dougong bracket connections.

Tuning the Johari-Goldstein β-Relaxation and Its Separation from α-Relaxation of Poly(n-alkyl methacrylate)s by Small Molecule-bridged Hydrogen Bonds

Introducing small molecule-bridged hydrogen bonds (HBs) between polymer chains has been reported to effectively reduce the inter-chain cooperativity despite of strengthening the intermolecular interaction. Here, a systematic investigation on tuning the Johari–Goldstein β (βJG) relaxation by adding various low-molecular-weight phenols in poly(n-alkyl methacrylate)s is carried out to further clarify the anomalous dynamics. Given these small molecules capable of coupling the motion with pendent groups of host polymers due to forming at least two HBs per molecule, poly(n-alkyl methacrylate) mixtures exhibit rich dynamic changes in the βJG-properties and α, βJG separations. An increased loading of phenols with a small size and strong inter-HB strength (Δυi) clearly benefits for significant retardation and suppression of the βJG-relaxation, narrows the α, βJG separation and converges the βJG-peak with the α-peak, which demonstrates the alleviation of inter-chain topological constraints. However, small molecules with a relatively big size and weak Δυi are found to amplify the magnitude of the α, βJG separation of poly(butyl methacrylate), even though experimental results of changes in α-dispersion and dynamic fragility confirm a reduction of the coupling factor n in all of these hybrids. The counterintuitive phenomenon suggests that the crossover time tc in the Coupling Model is no longer a universal quantity if the inter-chain of polymers is strengthened by HBs. These compelling findings shed vital insights into the HB-induced anomalous dynamics, and provide essential guidance for tailoring the βJG behavior and designing glassy polymeric materials.

Seismic retrofit of a soft first story structure using an optimally designed post – tensioned PC frame

In this study, the seismic performance of a self-centering post-tensioned precast concrete (PC) frame with an enlarged beam end is evaluated for retrofit of a soft first story structure. A finite element model of a post-tensioned beam-column assembly with an enlarged beam end is validated by comparison with experimental results, and is used to validate an analytical model of the proposed system. Then the analytical model is used to retrofit a soft first story structure with only columns in the first story and infill walls above. A multi-objective optimization of the retrofit system is carried out using genetic algorithm. The seismic performance of the retrofitted structure is assessed by incremental dynamic analysis and fragility analysis. The analysis results show that the retrofit system calibrated by the optimization procedure is effective in increasing the seismic performance of the soft first story structure.

Evaluation of seismic damage to Iraqi educational reinforced concrete building using FEMA P-58 methodology

The building seismic performance assessment can be defined as a posterior phase of investigation in which a detailed analysis is executed in order to evaluation of seismic damages and quantify the sequences of the earthquake for a building, this approach could be probabilistic evaluation or deterministic one. The aim of this paper is to perform seismic damage evaluation for educational multi-storey reinforced Concrete Building in Al-Mustaqbal University College (MUC) in Al-Hilla City, Babylon governorate in the middle of Iraq by using FEMA P-58 methodology. A 3D mathematical modelling, pushover analysis by using SeismoStruct software. In addition, incremental dynamic analysis (IDA) and fragility curve according to FEMA P-58 are developed in order to obtain damage limits in term of performance levels of the case study building. Fragility curves are useful tools for showing the probability of structural damage due to earthquakes as a function of ground motion indices. IDA is performed for those simulated building using twelve ground motions with scaling peak ground acceleration increased every 0.1g until it achieved 1.5g to determine the drift capacity of the building in four performance levels, operational (OP), immediate occupancy (IO), life safety (LS) and near collapse (NC) as defined in FEMA P-58. Based on those capacities, fragility curves were developed in terms of peak ground acceleration (PGA) and elastic spectral displacement for drift levels with lognormal distribution assumption.

Analysis of plasticization and reprocessing effects on the segmental cooperativity of polylactide by dielectric thermal spectroscopy

In the present study, the dielectric properties of both neat and plasticized polylactide submitted to repeated extrusion and injection processes to simulate recycling were analysed. The dielectric relaxation spectrum, consisting of β and α relaxation, revealed the relevance of both acryl-PEG based plasticization and thermo-mechanical degradation induced by repeated extrusion and grinding-injection reprocessing cycles on polylactide (PLA). The β-relaxation has its origin in the intramolecular local motions of pendant groups of the PLA backbone, while the α-relaxation is representative of the intermolecular large-scale segmental motions of the PLA backbone. The addition of acryl-PEG to PLA (pPLA) induced an increment in the apparent activation energies and temperatures of the β-relaxation due to the different nature of the acryl-pendant group in pPLA. Moreover, the crystallization rate of pPLA was much faster compared to that of neat PLA, which hindered the movements of the acryl-pendant group, thus requiring more energy to orient the dipoles. The plasticizing effect of acryl-PEG effectively aids the large-scale motions associated to the glass transition, which are the origin of the α-relaxation. The effect of reprocessing in PLA is less relevant than the addition of the acryl-PEG plasticizer, in terms of dielectric behaviour. With increasing the number of processing cycles, the crystallization rate of PLA gradually increased, but the permittivity was still much lower than that of pPLA, for which crystallinity remained constant. Nonetheless, for the case of the α-relaxation of pPLA, the rearrangements produced by chain scission induced an enhancement of the thermal activation of the orientation of the dipoles but reduced the dynamic fragility and increased the embrittlement of pPLA after reprocessing. These differences found in the relaxation spectrum and the molecular dynamics in the environment of the glass transition for PLA and pPLA, together with the embrittlement of pPLA after reprocessing, is what will determine the final use of one or the other recycled polymer.

New contributions from thermal and dielectric techniques to the understanding of the dynamic properties of medium to long chain poly (propylene glycols)

AbstractThe molecular mobility of poly (propylene glycols) with molecular weights between 2000 and 18,200 g mol−1 was studied in the amorphous solid state, in the region of the glass transition and in the rubbery state; the experimental techniques used were differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC). It was found that the glass transition temperature increases slightly with MW and tends to a limit value: TgDSC = −67°C, TgTSDC = −73°C.The different types of molecular mobility were analyzed: below Tg the secondary relaxations, and above this temperature the normal mode, were characterized in detail using TSDC; in the glassy transformation region, around Tg, the main relaxation was analyzed by both DSC and TSDC. It was found the dynamic fragility, m, behaves as independent of the molecular weight in the studied range: mDSC = 74 and mTSDC = 92. The TSDC results also show that, without influencing the kinetics of the segmental relaxation, the increase in molecular weight slows down the chain relaxation.

Nonlinear structural performance and seismic fragility of corroded reinforced concrete structures: modelling guidelines

The current paper provides modelling guidelines for nonlinear analysis and fragility assessment of corrosion-damaged reinforced concrete (RC) structures. A systematic guideline is proposed to model the adverse impact of corrosion on mechanical properties of steel and concrete materials including the negative impact of corrosion on inelastic buckling and low-cycle fatigue (LCF) degradation of reinforcing bars. The proposed fibre-based modelling technique is capable of simulating the spatial variability of localised corrosion and the influence of corrosion on damage states (DS). Corrosion-dependent pushover and cyclic analyses, incremental dynamic analyses (IDAs) and fragility analyses are carried out on three different case studies: (i) a cantilever square RC column, (ii) a cantilever circular RC column and (iii) a RC frame. The analyses results indicate that corrosion significantly affects the DS, sequence of failure mode, flexural capacity and energy dissipation capacity of RC structures. Moreover, alongside the corrosion, the geometry of RC components also affects their failure sequence, cyclic behaviour and energy dissipation capacity. It is shown that increasing degree of corrosion penetration results in the concentration of damage mainly in confined concrete.

Lifetime seismic performance assessment of high-rise steel-concrete composite frame with buckling-restrained braces under wind-induced fatigue

Under a severe environment of multiple hazards such as earthquakes and winds, the life-cycle performance of engineering structures may inevitably be deteriorated due to the fatigue effect caused by long-term exposure to wind loads, which would further increase the structural vulnerability to earthquakes. This paper presents a framework for evaluating the lifetime structural seismic performance under the effect of wind-induced fatigue considering different sources of uncertainties. The seismic behavior of a high-rise steel-concrete composite frame with buckling-restrained braces (FBRB) during its service life is systematically investigated using the proposed approach. Recorded field data for the wind hazard of Fuzhou, Fujian Province of China from Jan. 1, 1980 to Mar. 31, 2019 is collected, based on which the distribution of wind velocity is constructed by the Gumbel model after comparisons. The OpenSees platform is employed to establish the numerical model of the FBRB and conduct subsequent numerical computations. Allowed for the uncertainties caused by the wind generation and structural modeling, the final annual fatigue damage takes the average of 50 groups of simulations. The lifetime structural performance assessments, including static pushover analyses, nonlinear dynamic time history analyses and fragility analyses, are conducted on the time-dependent finite element (FE) models which are modified in lines with the material deterioration models. The results indicate that the structural performance tends to degrade over time under the effect of fatigue, while the influencing degree of fatigue varies with the duration time of fatigue process and seismic intensity. The impact of wind-induced fatigue on structural responses and fragilities are explicitly quantified and discussed in details.

Tuning the dynamic fragility of acrylic polymers by small molecules: the interplay of molecular structures.

This report studied changes in the dynamic fragility (m) of poly(butyl methacrylate) (PBMA) by introducing guest hindered phenols capable of forming two or three intermolecular hydrogen bonds (inter-HBs) per molecule with the host polymer. The small molecules effectively decrease the m value, even if they apparently increase the glass transition temperature (Tg) of mixtures. The reduction in m was confirmed by enthalpy relaxation in two aspects: adding the guest molecule leads to a stronger cooling rate dependence of the limiting fictive temperature together with an apparent increase in aging rate of PBMA hybrids at low concentrations. By varying the molecule size and steric hindrance of the hydroxyl group on the hindered phenols, we clarified that m is primarily governed by the strength of inter-HB interactions, while the Tg value of mixtures depends on a combined effect of additive bulkiness and HB interaction. The anomalous dynamics was further rationalized not only by the HB-induced flexibility balance between side groups and backbone, but also by the reduction of cooperative rearranging sizes and alleviation of long-chain connectivity in such HB-driven hybrids.

Self-centering BRBs with composite tendons in series:Tests and structural analyses

The self-centering system and yielding energy dissipation system are two main parts of self-centering buckling-restrained braces (SC-BRBs), which have important influences on brace performance. To improve the performance of the two parts, an SC-BRB with composite tendons in series (SC-BRB-CTS) is proposed by introducing a self-centering system in series that can improve the deformation capability of the brace, and the yielding energy dissipation system made of low-yield steel LYP160 with a strong energy dissipation capacity is adopted. The performance of the braces is studied by quasi-static tests, and the influence of the self-centering system in series and low-yield steel on the seismic performance of the structure is determined by nonlinear dynamic analyses and fragility analyses. The results show that the deformation capacity of the SC-BRB-CTS is approximately 44% higher than that of a traditional SC-BRB, and the collapse resistance of the structure is improved by avoiding or delaying tendon fracture. The use of LYP160 steel core plates can substantially improve the energy dissipation capacity and post-yielding bearing capacity of the brace, which is beneficial for reducing the seismic response of the structure.

Self-centering steel slotted friction device for seismic retrofit of beam-column joints

A new self-centering slotted friction device (SC-SFD) is introduced for seismic retrofit and upgrading of beam-column joints. The device can be used for existing or new structures. A framework utilizing a performance-based seismic design procedure combined with a genetic algorithm (GA) optimization is used to obtain the optimum design variables of the device. A 5-story building and one-story industrial structure are used as case studies. The effectiveness of the proposed retrofit is assessed through conducting non-linear time-history response analysis (NLTHA), incremental dynamic analysis (IDA), fragility analysis, and seismic life cycle cost (LCC) evaluation. The obtained results demonstrate that the proposed retrofit is effective in reducing the maximum inter-story drift ratio (MIDR) significantly and in eliminating the residual drift. Additional engineering demand parameters, such as the floor acceleration and the base shear have been investigated to prove the superiority of the proposed retrofit technique compared to the fully rigid joint alternative. A finite element method (FEM) is used to ensure that concrete stresses after retrofit are within the acceptable limits. The retrofitted models show high energy dissipating potential compared to the bare cases. The IDA and fragility analyses show significant improvement in the retrofitted structures in terms of the median collapse capacity and seismic fragility. The probabilities of exceeding different limit states and the LCC of the retrofitted structures have been reduced significantly compared to the bare cases. Based on these findings, the proposed retrofit is recommended for similar structures to improve their resilience against earthquakes and to reduce the total seismic LCC.

Influence of higher-order modes of slender tall pier bridge columns on the seismic performance of pile foundations

Numerous highway bridges have been constructed with slender tall piers over 40 m in Southwest China, due to the mountainous topography. Current researches on this type of bridges mainly focus on the higher mode effects on the seismic behavior of pier columns, while the responses of group pile foundations are generally neglected. The objective of this paper is to investigate how the dynamic demand of group piles are influenced by the higher-order modes of slender tall pier columns. To better understand the influence of higher order modes, two analysis scenarios, representing (1) the equivalent single-degree-of-freedom (SDOF) system neglecting higher mode effects of piers, and (2) the multi-degree-of-freedom (MDOF) system of the original bridge, are developed and analyzed. Fiber elements are utilized considering the potential nonlinear behavior of pier columns and piles, while nonlinear p-y, t-z and q-z springs are employed simulating the properties of soil layers in these numerical models. Using displacement of pile cap as damage index, the effects of higher-order modes on pile foundations are investigated through both incremental dynamic analysis (IDA) and fragility analysis procedures. The results show that the seismic demands of pile foundations could be significantly underestimated when the contribution of higher-order modes of columns are neglected. The damage of piles is observed prior to that of the pier columns, indicating that damage state of pile foundations should be carefully considered during seismic design, and only focusing on the pier columns would lead to unconservative results. Furthermore, p-y springs are shown to perform nonlinearly under strong excitations, which implies that modeling the soil layers with linear springs is insufficient and should be better considered with these nonlinear springs.

Effect of graphene nanoplatelets on the dielectric permittivity and segmental motions of electrospun poly(ethylene-co-vinyl alcohol) nanofibers

The influence of the addition of graphene nanoplatelets (GNPs) on the intra/inter – molecular segmental motions of poly(ethylene-co-vinyl alcohol) (EVOH) was assessed by means of dielectric thermal analysis (DETA). The relaxation spectra were studied in terms of the dielectric permittivity (ε′) and the dielectric loss tangent (tan δ) at wide ranges of frequency (from 10−2 to 107 Hz) and temperature (from -150 to 140 °C). Two relaxation zones were disthinguished. Below the glass transition temperature (Tg), two β-relaxations were observed, which are characteristic local modes of mobility of the EVOH side groups, and related to the influence of the different surroundings of ethylene or vinyl alcohol units. At higher temperatures, the dielectric α-relaxation in the vicinities of the glass transition of EVOH was determined. The thermal activation of the β-relaxations was explained by an Arrhenius model, and showed activation energies (Ea) around 55 and 80 kJ·mol−1. The α-relaxation was explained by the Vogel-Fulcher-Tammann-Hesse (VFTH) model. The study of the segmental dynamics showed an increase in the dynamic fragility parameters with the addition of GNPs. The permittivity was increased at preferential concentrations of GNPs. In particular, the addition of GNPs up to 0.5 wt% increased the dielectric permittivity of the electrospun EVOH/GNPs nanocomposite fibers, specially at low frequencies.

Structure, thermal properties and molecular mobility in cholesteryl hydrogen phthalate: Different approaches to the crystal, the glassy crystal and the mesophase

Cholesteryl hydrogen phthalate (CHP) is a good glass-former: cooling the isotropic liquid forms the cholesteric mesophase, and subsequent cooling leads to the formation of an orientationally ordered glass. The thermal behavior of CHP was studied by DSC and it was found that, once melted, the commercial crystalline form cannot recover through heat treatments: cold crystallization gives rise to different polymorphic forms; in contrast, recrystallization of the commercial crystal from an ethanol solution leads to the same crystalline form.The structure of the stable ordered polymorph (commercial) was resolved by single-crystal X-ray diffraction (SXRD) at room and low temperature (167 ± 2 K); it belongs to the monoclinic space group P21.The glass transformation of CHP was studied by DSC and thermally stimulated depolarization currents (TSDC). The values obtained for the glass transition temperature and the dynamic fragility indicate that CHP is a moderately fragile glass former.Concerning secondary relaxations, the results suggest that the mobility detected by TSDC has a Johari-Goldstein nature. On the other hand, the interpretation of the results above Tg raises difficulties eventually due to the non-isothermal nature of the TSDC technique together with the temperature dependent structure of the cholesteric mesophase.

BRB retrofit of mid-rise soft-first-story RC moment-frame buildings with masonry infill in upper stories

A high percentage of mid-rise soft-first-story (SFS) reinforced concrete (RC) buildings with masonry infill in upper stories collapsed in Mexico City during the SEPT/19/2017 earthquake. For this reason, in this work, an SFS building with similar structural characteristics to those damaged during such an event is subjected to several ground motions recorded at different Mexico City stations. As was expected, the peak drift demands are larger in comparison with the limit recommended by the Mexico City Building Code (2004). With an aim to improve the seismic performance of the building, three structural rehabilitation solutions are proposed and discussed. Then the seismic performance of the original building and the three rehabilitation solutions are compared via incremental dynamic analysis and structural fragility in terms of peak drifts. Among the three alternative solutions, it is shown that adding buckling restrained braces (BRBs) at the SFS of the building is a technique that leads to a structural fragility similar to that obtained when using a traditional rehabilitation technique; furthermore, when the BRB core area is reduced to an acceptable dimension, the forces transmitted to the foundation become lower, leading to an even better rehabilitation solution.

Calorimetric (DSC) and dielectric (TSDC) investigation on the thermal behavior and molecular mobility in the supercooled liquid and glassy states of bifonazole and lamotrigine

The thermal behavior of the two glass-forming drugs bifonazole and lamotrigine was studied by differential scanning calorimetry (DSC); we reported a bifonazole polymorph not yet described in the literature, as well as weak evidence for lamotrigine polymorphism; still by DSC we investigated the glass-forming ability, the tendency for crystallization from the glass (glass stability) from the metastable and equilibrium melt, for the two glass-formers under analysis. Finally, this technique was used to characterize the glass transition of the two active pharmaceutical ingredients by determining the activation energy of the structural relaxation, the dynamic fragility and the heat capacity jump associated with the glass transformation. Using thermally stimulated depolarization currents (TSDC) and, on the other hand, different aspects of the molecular mobility of these glass-formers were analyzed; the glass transition relaxation and the dynamic fragility, as well as the secondary mobility of the two glass-forming systems, were characterized by this dielectric technique. The absence of other dynamic studies on these glass-formers does not allow the comparison of our results with those of other techniques, namely with dielectric spectroscopy; however, despite this, attention was drawn to the general need to understand in their mutual relationship the TSDC and DRS results on the secondary relaxations.

Hydrogen bonding-induced anomalous dynamics of polyacrylates mixed with small molecules

Enhancing the intermolecular interaction generally leads to an increased topological constraint, thus resulting in a decreased non-exponential parameter βKWW of segmental cooperative rearrangement during glass transition. In this report, we illustrate a counterintuitive case when the inter-chain interaction is strengthened by small molecule-bridged hydrogen bonds. It was found that hindered phenols capable of forming double hydrogen bonds per molecule can effectively narrow the dielectric loss peak of poly(ethyl acrylate) and the βKWW increases monotonically with the intermolecular hydrogen-bonding strength even if the small molecule (AO300) has a glass transition temperature Tg much higher than the host polymer. Further experiments revealed an increased Tg but a decreased dynamic fragility m, accompanied by gradual merging of the suppressed secondary relaxation toward the primary peak, demonstrating that introduction of AO300-bridged hydrogen bonds appears to enhance the intermolecular interaction, but actually reduces the inter-chain cooperativity. This anomalous dynamics stemming from hydrogen bonding was rationalized in terms of small molecule-alleviated chain connectivity, flexibility of bulky pendant groups and changes in enthalpic contributions to segmental cooperative rearrangement.

Thermal behavior and molecular mobility studies in the supercooled liquid and glassy states of carvedilol and loratadine.

The thermal behavior of carvedilol and loratadine was studied by differential scanning calorimetry (DSC). The glass-forming ability, as well as the the tendency for crystallization from the glass (glass stability) and from the metastable and equilibrium melt were also investigated by DSC. In addition this technique was also used to characterize the glass transition of carvedilol and loratadine by determining the activation energy of the structural relaxation, the dynamic fragility, and the heat capacity jump associated with the glass transformation. Different aspects of the molecular mobility in carvedilol and loratadine were analyzed by Thermally Stimulated Depolarization Currents (TSDC), while in carvedilol the Dielectric Relaxation Spectroscopy (DRS) technique was also used. Carvedilol stands out for its high values of specific heat jump and dynamic fragility, which has been attributed to the particular mobility of this glass-former in the glass transformation region, a consequence of specific characteristics of its molecular structure. These molecular features are also at the origin of a relaxation above Tg that has been detected and characterized by TSDC; the DRS investigation allowed to better understand the molecular dynamics in carvedilol in the amorphous solid, in the metastable liquid state and in the glass transformation region. Finally, the secondary relaxations in loratadine were studied by TSDC, while those in carvedilol were studied by the two dielectric techniques and the results were compared and discussed.