Limit State Capacity Research Articles

Machine learning-based prediction of seismic limit-state capacity of steel moment-resisting frames considering soil-structure interaction

Regarding the unpredictable and complex nature of seismic excitations, there is a need for vulnerability assessment of newly constructed or existing structures. Predicting the seismic limit-state capacity of steel Moment-Resisting Frames (MRFs) can help designers to have a preliminary estimation and improve their views about the seismic performance of the designed structure. This study improved data-driven decision techniques in Python software, known as supervised Machine Learning (ML) algorithms, to find median IDA curves (M-IDAs) for predicting the seismic limit-state capacities of steel MRFs considering Soil-Structure Interaction (SSI) effects. For this purpose, Incremental Dynamic Analyses (IDAs) were performed on the steel MRFs from two to nine-story elevations modeled in Opensees subjected to three ground motion subsets of Far Fault (FF), near-fault Pulse-Like (PL) and No-Pulse (NP) suggested by FEMA-P695. The result of the analysis confirmed that there is no specific model for predicting the M-IDA curve of steel structures; therefore, the best developed ML algorithms to reduce a complex modeling process with high computational cost using 128,000 data points were proposed. To provide convenient access to prediction results, Graphical User Interface (GUI) was developed to predict Sa(T1) of seismic limit-state performance levels with a large database based on prediction models.

Optimal retrofit strategy using viscous dampers between adjacent RC and SMRFs prone to earthquake-induced pounding

Nowadays, retrofitting-damaged buildings is an important challenge for engineers. Finding the optimal placement of Viscous Dampers (VDs) between adjacent structures prone to earthquake-induced pounding can help designers to implement VDs with optimizing the cost of construction and achieving higher performance levels for both structures. In this research, the optimal placement of linear and nonlinear VDs between the 3-story, 5-story, and 9-story Steel and RC Moment-Resisting Frames (SMRFs and RC MRFs) is investigated. It is shown that the pounding phenomenon can significantly affect the seismic performance capacities of buildings during earthquakes, and using VDs can improve the seismic limit-state capacities of buildings for retrofitting purposes. For this goal, the seismic limit-state capacities of both colliding structures were assessed using Incremental Dynamic Analysis (IDA) assuming Near-fault Pulse-Like, Near-fault No-Pulse, and Far-Fault seismic records suggested by FEMA-P695. To perform IDAs, structures were modeled according to the seismic codes using a developed algorithm in Matlab and OpenSees software with the ability to remove a collapsed structure during the analysis. The results present an optimal placement for using VDs between structures and also compare the possible conditions to implement VDs. Using these results, engineers can approximately predict the seismic performance levels of both structures prone to earthquake-induced pounding and their final performance after retrofitting. Finally, retrofitting modification factors were proposed to help designers to predict the limit-state performance levels of retrofitted colliding structures without involving complicated and time-consuming analyses.

Outsourcing Bureaucracy to Evade Accountability: How Public Servants Build Shadow State Capacity

The solution to weak bureaucratic capacity in developing countries is often presumed to be more accountability. This paper shows how accountability initiatives, intended to reduce corruption, can actually hinder the development of capable government agencies by making it harder for directors to recruit experts and spend their budgets. It further highlights a common way public servants escape the accountability rules that limit their effectiveness: outsourcing bureaucracies to nonstate organizations. This practice of outsourcing bureaucracy to avoid accountability rules creates what I call “shadow” state capacity and, paradoxically, it may help explain “pockets of effectiveness” among government social programs in developing countries. Drawing on in-depth interviews and descriptive statistics, I show how outsourcing was a critical factor in producing two of Brazil’s most vaunted social sector programs. However, I also suggest that outsourcing bureaucracy may ultimately limit state capacity, even if it helps to build capable programs in the short run.

Seismic fragility assessment for a RC building in seismically stable Brazil: A sensitivity analysis

While Brazil is located in a markedly seismically stable region, most buildings are designed with little to no consideration for seismic activity. For this reason, a thorough investigation of building fragility is a requisite for precise risk estimates. However, some constraints currently affect broad seismic fragility assessments in the country, causing inherent uncertainties in different steps of fragility estimation. This paper focuses on comparing how different variables stemming from such uncertainties may affect seismic fragility, in order to evaluate which may be more important for future assessments. Herein, a low-rise, open ground, reinforced concrete building is evaluated, and the following variables’ sensitivity to the fragility response are compared: ground motion record selection; intensity measure (IM) selection; structural model parameter values; failure mode and limit state capacity. A cloud-based approach is used, employing a surrogate demand model for non-collapse data and a Logistic model for the inclusion of collapse data. The sensibility analysis results indicate that, for this assessment, failure mode variability due to detailing practices may cause the largest global impact on fragility variability, with a greater effect on intermediate intensity values. On the other hand, column backbone parameter uncertainty can be the most important factor on some of the smaller intensity values, while record selection influence, conditioned on the number of records used and on IM sufficiency, can have a smaller impact than the other variables.

Fragility analysis of structural pounding between adjacent structures arranged in series with various alignment configurations under near-field earthquakes

A major cause of local to total damages is related to structural pounding in a large number of past earthquakes. In general, these collisions take place as a result of differences in the dynamic characteristics of the colliding structures. To acquire a better perception of the behavior of structures, in this paper, three structures featuring different heights are modeled in series and with various configurations next to each other in OpenSees. To determine the collision effects of the structures, three different configurations of 4-, 8- and 12-story adjacent reinforced concrete special moment resisting frames were considered. Then, by conducting an incremental dynamic analysis, their structural seismic limit state capacities were assessed via 20 near-field record subsets recommended by FEMA-P695. At last, for the above adjacent buildings with various separation distances and configurations, the fragility curves were determined, and the probability of exceedance from the primary Hazus-MH failure criteria was estimated. In addition, the results were compared with those obtained when this phenomenon did not take place for buildings to have a better perception of the pounding phenomenon. The results of the analyses show that arranging adjacent structures in series greatly affects the collapse capacities of the colliding structures. In addition, in the case when the shorter structure is placed in the middle of two taller structures, it results in the most critical situation among all configurations, and in this case, a higher reduction is observed in the structural performance levels.

Seismic fragility assessment of building-type structures in oil refineries

A seismic fragility assessment methodology is presented for equipment-supporting reinforced concrete and steel buildings that are typically encountered in oil refineries. Using a suite of hazard-consistent ground motions and reduced-order models, incremental dynamic analysis is performed to obtain the seismic demand of the structural systems examined. Appropriate drift- and floor acceleration-sensitive failure modes are considered to define the limit state capacities of the supporting structure and the nested non-structural process equipment. Special care is exercised on the demand and capacity representation of structural and non-structural components, offering a transparent roadmap for undertaking analytical fragility assessment for equipment-supporting buildings typical to an oil refinery. The findings and the proposed methodology can be exploited by designers and facility managers for mitigating the risk of failure prior to the occurrence of an earthquake event, for designing the pertinent structures and their non-structural components by means of a risk-aware performance-based methodology, or as feed data in early warning systems.

The influence of coal gangue coarse aggregate on the mechanical properties of concrete columns

Using coal gangue as coarse aggregate is one of the solutions for the sustainable development of construction engineering. In this paper, the mechanical properties of coal gangue concrete (CGC) columns are investigated. The test results show that the failure modes of CGC columns under different replacement ratios are similar. In the loading process, the strain difference between CGC and reinforcement is small, indicating that there is sufficient bonding force between them. Coal gangue coarse aggregate can not effectively prevent the expansion of cracks, resulting in a decrease in the crack resistance of CGC columns. With the increase of coal gangue replacement ratio, the total deformation of the specimen increases, but the ductility decreases gradually. The reliability index of the bearing capacity limit state of CGC members should be improved in the design. Due to the large deformation of CGC, the stirrup is more important for the restraint of CGC column deformation, so increasing the stirrup ratio will be an effective method to improve the mechanical performance of CGC column. When calculating the bearing capacity of the CGC column, the parameters α and β of the equivalent rectangular stress block should be corrected according to the change of coal gangue replacement ratio.

Experimental study and theoretical analysis on the flexural performance of steel–bamboo composite beams strengthened with externally prestressed steel strands

The influence of prestress techniques on the flexure performance of steel–bamboo​ composite beams was investigated by testing twelve specimens, considering the prestressing scheme, and loading scheme as parameters. The experimental performances, failure modes, and mechanical characteristics were observed and analyzed. The effects of different parameters on deformation performance, failure modes, and load-bearing capacity were explored, and theoretical methods were proposed for estimating deformation and resistance. The results indicated that the failure of the test specimens was mainly material damage due to the tearing of bamboo flanges, interface debonding, and local buckling of steel sheets, depending on the choice of prestressing and loading schemes. The prestressed composite beams exhibited a relatively better deformation performance than ordinary beams. The equivalent bending stiffness increased by up to 30% in the serviceability limit state; therefore, the prestressed specimens generated smaller deflections under the same vertical load. Moreover, the two-point prestressed specimens exhibited better deformation performance than the one-point prestressed specimens. Finally, a comparison of the results indicated that the proposed methods provided feasible estimations of the deformation in the serviceability limit state and load-bearing capacity in the ultimate limit state, with relative errors of approximately 5% on average.

Destructive test study of the flexural load capacity of a full-scale reinforced concrete T-girder strengthened by externally prestressed CFRP plates

With the upgrade of the transportation capacity of some existing freight tracks to heavy-haul railways in China, there is increased use of heavy-haul trains with larger axle loads and traction masses than in the past. The existing railway bridges not only bear the increased loads of heavy-haul trains but also experience much more loading times. This situation greatly affects the normal use and even the bearing capacity of reinforced concrete bridges and leads to concrete cracks and decreased flexural rigidity of the girders. As a result, the ultimate bearing capacity and operational safety of girders must be reasonably strengthened. Two reinforced concrete T-girders with a span of 12 m were demolished after 20 years of service. One of the T-girders, without any treatment, was selected for the destructive bending test, and the failure process and actual ultimate bearing capacity were obtained. The other girder was strengthened by the tensioning of prestressed carbon fiber reinforced plastic (CFRP) plates which showed the advantages of high tensile stress more than 2600 MPa, light mass, anti-corrosion and good resistance to durability. Then a destructive comparative bending test was performed to verify the strengthening effect. The research results showed that the actual ultimate flexural load capacity of the unstrengthened T-girder was 1.58 times the original design load capacity. After the T-girder was strengthened by prestressed CFRP plates, the cracking moment and actual ultimate flexural load capacity were 30.5% and 16.4% higher, respectively, which could decrease the cracking degree and deflection-span ratio of the T-girder under the operating conditions of heavy-haul trains. The strengthened T-girder showed a desirable serviceable limit state and ultimate flexural bearing capacity for KM98 trains with 300 kN axles.

Ultimate Limit State Scour Risk Assessment of a Pentapod Suction Bucket Support Structure for Offshore Wind Turbine

Scour risk assessment considering reaction force at foundation was proposed and applied to newly developed pentapod suction bucket support structures for a 5.5 MW offshore wind turbine under ultimate limit state environmental load. Scour hazard was obtained according to scour depth by using an empirical formula, which is the function of marine environmental conditions such as significant wave height, significant period, and current velocity. Fragility of the pentapod support structure was evaluated using the bearing capacity limit state criterion under ultimate limit state load case. Scour risk was assessed by combining the scour hazard and the fragility. Finally, scour risk of the developed pentapod suction bucket support structure under ultimate limit state has been assessed.

Full-range Experimental Analysis of Flexural Performance of an Existing PC Hollow Slab

Prestressed concrete (PC) hollow slab bridges are the most common type of short and medium span highway bridges in China. With the increasing load and environmental degradation factor, the flexural capacity of hollow slab bridges in service for many years can no longer meet the new safety regulations. Further research on the flexural capacity of these hollow slabs is necessary to ensure their safety. Experiments on the flexural bearing capacity of a full-scale damaged PC hollow slab are conducted. Elastic, cracking, re-cracking, and destruction tests are conducted to determine the deflection, strain, crack width, bearing capacity, stiffness, failure mechanisms, and dynamic characteristics of the slab. The strain measurements of the steel bar and prestressing tendons in the four-stage bending test indicate new failure characteristics of the PC hollow slab. After the steel bar yields, the concrete stress is redistributed, and the concrete on the top of the hollow slab exhibits tension, compression, and shear, providing science-based information to improve the ultimate bearing capacity of the slab. The cracking load (ultimate load) obtained from the full-scale bending test is 23% (72%) higher than the design value of the main slab’s internal force in normal service limit state (loading capacity limit state). The ultimate displacement in the midspan of the slab is 1/47 of the calculated span length, and the slab exhibits ductile failure characteristics, indicating very good ductility and sufficient strength of the slab after a 23-year service life. This study can provide a reference for the safety assessment and maintenance of numerous hollow slab bridges that have been in service for many years in China.

Probabilistic Seismic Response and Capacity Models of Piles for Statewide Bridges in California

This study develops the probabilistic seismic response and capacity models for the broad collection of standard pile designs that are routinely incorporated into bridges in California. A review of the state bridge inventory indicates considerable variations in design details for different standard pile types used across multiple eras. For each pile type, fiber-section-based pile models attached with p-y soil springs are built to incorporate nonlinear behaviors of soil materials, a wide array of heterogeneous soil profiles, full-range damage states of piles, and realistic connection details between piles and footings (i.e., pile caps). Moreover, force-displacement responses of pile-soil systems under a large number of pushover analyses are regressed as response models consisting of five parameters [termed as response five parameter (R5P) models], which can capture all essential behaviors of laterally-loaded piles. Capacity damage states and limit state models are defined for different pile types by linking pile global responses to fiber-scale material behaviors at plastic hinge locations. Procedures are further provided to expand R5P models to pile foundation models at the regional scale, taking into account the pile group effect and capacities of pile cap backfills. R5P models for all distinct pile types are further summarized in an Excel workbook to facilitate their practical implementations. In general, this study provides a comprehensive and consistent set of response and capacity models to quantify the seismic damage potential of regional pile foundations, as well as to capture their dynamic interplays with other crucial bridge components, such as columns and abutment components. The proposed pile models are expected to significantly enhance the existing modeling capability toward improved seismic risk assessment of California’s bridge infrastructure.

Advanced corrosion-rate model for comprehensive seismic fragility assessment of chloride affected RC bridges located in the coastal region of india

In this study, a methodology is being proposed for performing seismic fragility analysis of corrosion affected bridges located in the coastal region of India. The study performed adopting this advanced corrosion rate model capturing nonlinearity that necessitates pitting corrosion as a realistic corrosion degradation mechanism rather than conventional uniform corrosion. With the application of advanced corrosion model, a distinct improvement over seismic fragility assessment achieved due to its capability to capture nonlinearity in the overall process of corrosion transport. The proposed model exhibits the corrosion rate accelerates after initial cracking of the concrete cover until at a phase, when severe cracking occurs leading to spalling of concrete. Based on the current degradation mechanism, the study estimates the capacity limit states of deteriorating reinforced concrete (RC) T-girder bridges as well as the impact of bridge degradation on the seismic response assessed using nonlinear time history analysis (NLTHA). However, the NLTHA of the finite element bridge models performed considering erraticism about material properties, geometries, ground motions and corrosion degradation parameters such that the uncertainties associated with the bridge population could be realistically taken into account as per the basic corrosion mechanism. Comparisons are drawn with respect to general and pitting corrosion as well as the adverse effect of pitting corrosion on the bridge service life has been identified. Thus, the output of the seismic fragility analysis would indicate some useful information in understanding the nonlinear crack growth of bridge vulnerability in marine environment, such that proper maintenance strategies could be framed for adequate safety of the highway infrastructure system for its enhanced design life.

Application on large-span reconstructed structure with external prestressing technology

The application of external prestressed reinforcement technology in large-span reconstruction projects is introduced through a project example of drawing columns to increase the use space of existing buildings. Simplified calculation method and midas gen finite element analysis software are used to analyze and calculate the bearing capacity limit state and normal service limit state of GHJL-1 and GHJL-2. In order to ensure the safety and applicability of the large-span reinforcement and reconstruction project, the static load method is used to inspect the serviceability limit state of the structure after the reconstruction. The results show that the deflection and crack width of GHJL-1 and GHJL-2 meet the requirements of the code limits.

Optimization-based record selection approach to incremental dynamic analysis and estimation of fragility curves

In order to estimate the seismic performance of structures, performing incremental dynamic analysis, and obtaining fragility curves are essential. Since producing these curves is time-consuming due to performing a large number of nonlinear dynamic analyses, the selection of appropriate earthquake ground motion records which give reliable responses with sufficient accuracy is important. Due to the lack of a solid framework, the selection of an appropriate ground motion recordset is still a challenging problem. In this paper, the primary goal is to select a suitable set of records from a general set of records in order to reach reliable limit-state capacities prediction of structures. To achieve this goal, incremental dynamic analysis is conducted for an equivalent single degree of freedom under a general set of records, and an optimization algorithm is employed to solve the problem by minimizing the error between the mean incremental dynamic analysis curves of each selected subset and the mean incremental dynamic analysis curve of a general set of records. The fragility curves obtained by all records and selected ones are compared and the results show that the fragility curves corresponding to the selected records estimate the target fragility curves appropriately.

Non-dimensional method of capacity limit states for the assessment of the safety level in the structures of large panel buildings.

In the article deals with the European safety issues of large-panel buildings from the point of view of exceeding the ULS. The proprietary assessment method was proposed under the name: Dimensional Limit States Method, which is dedicated to the assessment of the safety level in the structures of large-panel buildings. The work was based on many years of computer research conducted by the author. In the paper uses the proprietary construction model of the Wk-70 system building, presents the results for vertical reinforcement inserts.

Study on the performance of concrete-filled steel tube beam-column joints of new types

In this paper, the influence of axial compression ratio on the mechanical properties of new type joints of side span of rectangular concrete-filled steel tubular column-H-type steel beam is studied. Two new types of side-span joints of rectangular concrete-filled steel tubular column-H-type steel beam are designed and quasi-static tests of five new type joints with 1:2 scale reduction ratios are performed. The axial compression ratio of joint JD1 is 0.3, 0.4 and 0.5, and the axial compression ratio of joint JD2 is 0.3 and 0.5. In the joint test, different axial forces were applied to the top of the column according to different axial compression ratios, and low-cyclic reciprocating load was applied on the beam. The stress and strain distribution, beam and column deformation, limit state, failure process, failure mechanism, stiffness degradation, ductile deformation and energy dissipation capacity of the joint were measured and analyzed. The results show that: with the increase of axial compression ratio, the ultimate bearing capacity of the joint decreases slightly, the plastic deformation decreases, and the stiffness and ductility decrease. According to the energy dissipation curve of the specimen, the equivalent damping coefficient also increases with the increase of axial compression ratio in a certain range, indicating that the increase of axial compression ratio can improve the seismic performance of the joint to a certain extent. The finite element method is used to simulate the joint test, and the test results are in good agreement with the simulation results.

Seismic fragility assessment of ductile reinforced concrete building frames

Purpose Increasing awareness of the society and complying with design requirements of building codes for seismic safety of structures and inhabitants during severe earthquakes are the primary purpose of seismic analysis. This study aims to present the variability in seismic fragility functions for frames of different heights for the most vulnerable condition of structure using nonlinear time history analysis. Design/methodology/approach A total of 4, 8 and 20 stories reinforced concrete (RC) moment-resisting two-dimensional frames are considered for this study. Ground motions (GM) are selected as per the conditional mean spectrum and these are conditioned on a target spectral acceleration at the concern time period. RC frames are designed and detailed as per Indian standards. A concentrated plasticity approach is adopted for non-linear analytical modeling of the RC frames. Deterministic capacity limit states in terms of maximum inter-story drift ratio are considered for different damage states. Fragility functions have been derived following a lognormal distribution from incremental dynamic analysis curves. Finally, the maximum likelihood estimation of the response is obtained for fitting curves with observed fragility. Findings The fragility functions of the three structures reflect that under critical or extreme conditions of GM the taller buildings have higher fragility than the shorter buildings for each level of limit states even though both are designed to meet their code-level design forces. Research limitations/implications The study is conducted on the extreme scenario of GM conditioned on the fundamental time period of each building, whereas comparison can be developed by selecting various methodologies of GM set. The probabilistic capacity model can be developed for future studies to check the fragility variation with deterministic and probabilistic capacity. Originality/value The investigation endeavors to present a comprehensive fragility assessment framework by analytical method. The outcome will be useful in the development of a disaster management strategy for new or old buildings and the response of seismic force with a variation of the building’s height. The findings will also be useful for updating the earthquake-resistant building codes for the new building construction in a similar context.

Investigating the effects of structural pounding on the seismic performance of adjacent RC and steel MRFs

An insufficient separation distance between adjacent buildings is the main reason for structural pounding during severe earthquakes. The lateral load resistance system, fundamental natural period, mass, and stiffness are important factors having the influence on collisions between two adjacent structures. In this study, 3-, 5- and 9-story adjacent reinforced concrete and steel moment resisting frames (MRFs) were considered to investigate the collision effects and to determine modification factors for new and already existing buildings. For this purpose, incremental dynamic analysis was used to assess the seismic limit state capacity of the structures using a developed algorithm in OpenSees software including two near-field record subsets suggested by FEMA-P695. The results of this paper can help engineers to approximately estimate the performance levels of MRFs due to pounding phenomenon. The results confirm that collisions can lead to the changes in performance levels, which are difficult to be considered during the design process. In addition, the results of the analyses illustrate that providing a fluid viscous damper between adjacent reinforced concrete and steel structures can be effective to eliminate the sudden changes in the lateral force during collision. This approach can be successfully used for retrofitting adjacent structures with insufficient in-between separation distances.

A Simplified Framework for Seismic Fragility Assessment of Reinforced Concrete Columns under Different Failure Patterns

ABSTRACT A simplified framework is proposed in this study to assess the seismic fragility curves of reinforced concrete (RC) columns under different failure patterns in a unified manner. Within this framework, RC columns are simulated by an equivalent single-degree-of-freedom system, where a quasi-static experimental database is compiled to calibrate the seismic demand model and identify the limit state capacity. By comparing the seismic demand model with the limit state capacity, the seismic fragility curves of RC columns under different failure patterns are generated accordingly. Finally, empirical equations are proposed to predict the seismic fragility parameters for engineering applications.