Serviceability Limit State Design Research Articles

Serviceability limit state of incrementally launched steel bridge I-girders

Bridge girders are subjected to repeated patch loading during incremental launching (i.e. travelling over supports). Plastic deformations are likely to occur in the girder already at the first patch loading. They influence the girder’s structural response and bring about a decrease in the ultimate resistance at subsequent patch loadings. This problem is analysed in the present paper through the serviceability limit state (SLS) of incrementally launched bridge I-girders (ILBGs). The SLS of ILBGs is a current problem since only a few studies, with contrasting conclusions and certain limitations, have been recognised in the literature. Research is limited on plate girders without longitudinal stiffeners made of structural steel. Flat slide-type launching shoes are considered in this paper.The serviceability requirement (repeatable behaviour of ILBGs) and criterion (that effective membrane strains should remain in the elastic range) are adopted from the literature. So, the serviceability resistance is adopted as a load at the start of the effective membrane plastic strains in the girder. In order to propose a practical and simple SLS design procedure, the serviceability resistance is normalised against the ultimate resistance, so a serviceability correction factor (ksls) is defined. The paper presents a parametric numerical analysis of the factor (ksls). A finite element model previously developed and validated by the author is employed to simulate a nonlinear response of patch-loaded girders. The parametric study includes 599 girders. ksls is inversely proportional to the web thickness and directly proportional to the flange thickness and the yield strength of the steel material. The load length, web depth and flange width do not influence ksls. The influence of the spacing between transverse stiffeners on ksls is negligible. Finally, by regression analysis of the results of parametric study, an original and reliable serviceability correction factor is proposed as the main objective of the research. The range when the SLS of ILBGs should be checked is also defined.

Influence of wave spreading on offshore wind turbine design: IEA 15-MW scenario

Offshore ocean waves are directionally spread, whereby the propagation of energy travels in different directions. Despite this multi-directionality, the use of 3-dimensional wave models for loading on fixed offshore wind turbines has been limited. This is partially due to the common assumption that uni-directional sea-states are conservative within a design philosophy. This may not always be true given that in operating conditions the amount of aerodynamic damping in the side-side direction is much lower than the fore-aft direction. This paper aims to address this issue by providing the influence of wave spreading on various offshore wind turbine design scenarios: fatigue, ultimate and service limit state design. This study demonstrates that wave spreading indeed results in more fatigue damage for operating load cases. Despite this, the overall fatigue and ultimate limit state utilisation is still reduced when a wave spreading is adopted.

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Serviceability Limit State Assessment of Semi-Submersible Floating Wind Turbines

Abstract The design of a floating wind turbine (FWT) should satisfy the serviceability limit state (SLS) requirement for an efficient and safe operation throughout the entire work life. The SLS requirements are introduced by the owner/developer of the wind turbine facility to achieve serviceability (production of power) or an efficient operation of the facility or a “first step” towards ensuring safety. Currently, there is limited information about SLS requirements in design standards. This study deals with an assessment of current methods, criteria, and procedure for the SLS design check with an emphasis on tilt/pitch and nacelle accelerations in view of power production and its fluctuations. Moreover, other criteria, on the borderline between serviceability and safety criteria, e.g., relating to clearance, are briefly discussed. The criteria relating to power production are illustrated in a case study with a 10-MW semi-submersible FWT considered for an offshore site in the Northern North Sea. Simplified static/dynamic analysis methods for use in the global design phase and high fidelity integrated, dynamic analysis methods for detailed design in terms of serviceability are presented, discussed, and applied in the case study. A good understanding of wind turbine dynamic performance associated with serviceability is essential to facilitate design decision-making. The relative contribution of wind and wave loads to the different SLS criteria is investigated. Finally, the main conclusions are summarized. In lieu of the current state of the art regarding SLS requirements for FWTs, we hope that this study provides a basis for improving design standards and guiding research and engineering practice for the semi-submersible floater design of FWTs.

Load envelope concept of offshore wind turbine monopile with the allowed inclined angle in sand

Monopiles are the economic choices in shallow water as the foundations of offshore wind turbines (OWTs). Maximum allowed rotation θ = 0.25° is set as the criterion in the serviceability limit state design. However, the allowed load combinations of monopiles in sand are not fully revealed and understood satisfying the criterion. In this paper, load envelop concept to ensure the allowed monopile rotation less than 0.25° was proposed to understand all possible load combinations on monopiles for the OWTs. Finite element models based on the sand hypoplastic model with inter-granular strain were established. After verification with the centrifuge results, different load–displacement curves were acquired by changing loading paths to acquire the envelope surfaces meeting the criterion. Finally, the most likely load combinations of the OWTs were particularly analysed. Under the condition with θ = 0.25°, the allowed horizontal loads and the bending moments are closely coupled. The calculated envelop curves composed of maximum allowed loads can be well fitted and described by the cubic function. This note provides a method to obtain load combinations with allowed inclinations, which is beneficial to understand the load states of monopile for OWTs.

Improved design shear method of the bolted cold-formed steel clip-angle for serviceability

This paper presents an experimental investigation on the three-bolted clip-angle (CA) connector under direct shear load. A total of 48 clip-angle connection tests were carried out with 27 different clip-angle configurations by varying the thickness, width and depth of the clip-angle. The present experimental results and the data from literature indicates that the existing design methods for the service limit state design of the clip-angle are inefficient. The present investigation proposes a novel and efficient design method for the service limit state after calibration against test data, for the low-grade steel clip-angle (fy=271MPa to 307 MPa) from the present research work and for the higher grade steel (fy=375MPa to 550 MPa) from the literature data. Hence the proposed design method has wider steel grade application from fy=271MPa to 550 MPa. Parametric studies were performed to evaluate the influence of the geometric variables on the service shear strength of the three bolted clip-angle. The proposed design method was also applied to the welded clip-angle configuration in the literature and corresponding design equations were developed. In addition, a comparative study between the present three-bolted and most recent welded clip-angle connection was conducted which indicated that the three-bolted clip-angle configuration has higher stiffness than that of the welded configuration. Reliability studies were performed and relevant design factors were determined according to the LRFD, LSD and ASD methods.

Experimental investigation of the service shear strength of the welded clip‐angle connector in a CFS beam‐to‐column connection

AbstractThe present research endeavours to experimentally evaluate the service shear strength of the cold‐formed steel (CFS) welded clip‐angle connector in a CFS beam‐to‐column connection. A total of 30 laboratory tests were conducted on the 27 configurations of the clip‐angle by varying its thickness, width, and depth. The governing design failure modes observed in the welded clip‐angle are (i) Local buckling and (ii) Distortional buckling. The existing traditional elastic design approach for the service limit state design of CFS connector was found to be highly conservative for most of the welded clip‐angle specimens. Hence a new design approach is developed for the welded clip‐angle configuration and the corresponding design shear equation is suggested. A comparative study was conducted between the present welded clip‐angle configuration and existing screwed configuration. The design recommendations were suggested for the efficient design of the connection for the welded clip‐angle configuration. The relevant design factors for the suggested shear equations of welded clip‐angle were evaluated from the reliability analysis.

Uncertainty analysis for drilled shaft axial behavior using CYCU/DrilledShaft/143

Drilled shaft axial capacities and their uncertainties were evaluated in this paper. A large load test database of drilled shafts labeled as CYCU/DrilledShaft/143 was compiled for this evaluation. All load tests were performed in the field under drained and undrained soil conditions. The database was made available at ISSMGE TC304 database sharing platform 304 dB. Drained and undrained analyses were conducted separately. These load test data were first analyzed using representative interpretation criteria to determine their “capacities”. The results of each of the interpretation criteria were next normalized by a standard interpretation criterion and a predicted capacity to establish the relationships between these various interpretation criteria and to characterize model factors, respectively. The analyses of 143 drilled shafts indicated that L1 and DeBeer interpreted loads can be recommended for the serviceability limit state design. The criteria van der Veen, 4%B, Terzaghi and Peck, slope–tangent, L2, and Fuller and Hoy yielded values that are at the transition region of the load–displacement curve, in which the “ultimate” capacity can be reasonably estimated. The Chin method is not conservative, because it produced a value larger than all of the above interpreted “ultimate” capacities with displacements exceeding 60 mm on the average and beyond the range of the measured load–displacement curve. The scatter in the load–displacement curves is studied by applying different normalization schemes and fitting the normalized curves to a hyperbolic model. The hyperbolic model (curve fitting) parameters are generally negatively correlated. Statistics of these hyperbolic model parameters are shown to be comparable to values reported in previous studies. This hyperbolic model is widely used for reliability-based design at the serviceability limit state. Hence, the statistics of the hyperbolic model parameters presented in this study are useful.

Duration of extreme synoptic wind speeds for North America in a changing climate and its engineering implementation

In modern wind engineering practice, wind load effects are often defined as a function of a basic design wind speed, an aerodynamic force coefficient, several multipliers to consider surrounding terrain, topographic effect, and the directionality of the wind climate. Among these variables, the basic design wind speed is associated with an averaging time (also known as gust duration), e.g., 0.02-second, 3-second, 10-minute, or one hour. The design level aerodynamic force coefficients are defined as a specific percentile within a reference period, representing the generic time duration of wind events, i.e., wind duration, which is assumed to be one hour. A recent study showed that the hurricane wind duration could differ from this assumption. There has not been any study that investigated synoptic wind duration and its engineering implementations comprehensively. It is also unclear whether climate change impacts the synoptic wind duration. Historical records, often less than 50 years, are insufficient to calculate the wind duration for threshold wind speeds at a higher return period. Therefore, this study employed 15 long-term hourly wind speed simulations covering most of North America, each of which has 70 years in the past (1050 years in total) and 80 years into a future changing climate (1200 years in total), to investigate the wind durations in the current and future changing climate. It was found that the durations for winds of about 700-year return period are close to the traditionally assumed one hour and have limited effects on the wind loads for the strength design. However, wind durations can be much longer than one hour for the lower return period level wind speeds, which can impact the wind responses for serviceability limit state design and cladding and component (C&C) design.

Behavior and design of earthquake resistant concrete structure

Earthquake and seismic-resistant concrete structures are essential components and requirements of structural design. The performance and response of concrete structures during/after the earthquake event are significant to determine the condition of the structures as well as to identify the possibility of structural failure. The main objective is to investigate the appropriate method to evaluate the impact on the structure due to earthquakes based on closed-form solutions, and to determine its differences with the results of numerical modelling. The seismic behavior of structures is required to be designed in accordance with the minimum design requirements specified in the Standard. Seismic design, where ground behavior is considered, is presented as a significant part of design analyses, and ground movement and shaking, are associated with the ground deformation which is the primary seismic load consideration for the design of plain or reinforced concrete structures. The outcome of the longitudinal analysis provided the maximum axial and bending strain which is adopted to calculate the combined strain. These results of strain are considered with the design of concrete structures against Ultimate Limit State and Service Limit State design phases. Therefore, the results evaluate the effect of seismic design on the performance of the concrete structure.

Reliability-based serviceability limit state design of spread foundations under uplift loading in cohesionless soils

ABSTRACT The design of foundations is often governed by the serviceability limit state (SLS) requirements of the supported structure, particularly for large spread foundations. This paper aims to develop a reliability-based SLS design method for spread foundations under uplift loading in cohesionless soils. A probabilistic framework was adopted for the empirical characterisation of the compiled load-displacement curves and the quantification of the associated uncertainties. By using the obtained statistics of the curves, reliability analysis was carried out with Monte-Carlo simulations to calibrate the resistance factors within the load and resistance factor design (LRFD) framework. The calibration results showed that the embedment ratio of the foundation and the fitting errors of the empirical model, which were previously unaddressed in the literature, had notable effects on the calibrated SLS resistance factors. The relationship of the SLS with the ultimate limit state was assessed, including the governing limit state at each allowable displacement level, and the probability of ultimate failure of the foundation at the SLS condition. By considering the relationship between the limit states, the procedures for determining the design resistance factor and foundation capacity were proposed.

Calibration of Temperature Gradient Load Factor for Service Limit State Design of Concrete Slab-on-Girder Bridges

Calibration of Temperature Gradient Load Factor for Service Limit State Design of Concrete Slab-on-Girder Bridges

Chronology of Service Limit State Development for Geotechnical Elements Designed Using AASHTO Load and Resistance Factor Design

The purpose of this paper is to provide a chronology of the development of the Service Limit State design process for geotechnical elements that are designed using the reliability-based Load and Resistance Factor Design (LRFD) approach in the Bridge Design Specifications of AASHTO. This chronology will help future researchers to better understand the basis of decisions made by AASHTO, with input from the FHWA, and thus will serve as an important benchmark for future work.

Reliability-Based Serviceability Limit State Design of Driven Piles in Glacial Deposits

Reliability-Based Serviceability Limit State Design of Driven Piles in Glacial Deposits

Behavior of flat slabs with partial use of high-performance fiber reinforced concrete under monotonic vertical loading

Reinforced concrete flat slabs are used worldwide in multi-story buildings. In these slabs, the design is often governed by punching shear and serviceability. The mitigation of these issues during design usually leads to increased raw material consumption and costs. Previous studies have shown that using Fiber Reinforced Concrete (FRC) or High-Strength Concrete (HSC) only at the vicinity of the column, while casting the rest of the slab with Normal Strength Concrete (NSC), can lead to an improved behavior under gravity loads in terms of both serviceability and ultimate capacity. Motivated by these results and the scarcity of previous tests, the present paper experimentally investigates the applicability of High-Performance Fiber Reinforced Concrete (HPFRC) as an alternative material that can be seen as an improvement over FRC and HSC, allowing a combination of ductility and strength. In addition, the HPFRC used in this paper is self-compacting, thus reducing the labor costs associated with concrete vibration. Five 150 mm thick flat slabs were tested under monotonically increasing punching load. The experimental variables were the flexural reinforcement ratio and the extent of the HPFRC zone. One of the specimens was cast only with NSC and served as a reference slab. Results show that the solution was effective for both flexural reinforcement ratios considered. Cracking load, maximum load, as well as the displacement capacity were increased significantly, even for a small extent of HPFRC (1.5 times the effective depth from the face of the column). Regarding the ultimate load capacity, it was observed an increase of 44% to 58% for the specimens with lower reinforcement ratio (0.64%) and between 15%–21% for the specimens with higher reinforcement ratio (0.96%). The results indicate that the use of HPFRC is a promising solution regarding both serviceability and ultimate limit state design of reinforced concrete flat slabs under gravity loading, with obvious advantages in material savings and labor costs.

Load and resistance factor design versus reliability-based design of shallow foundations

ABSTRACT The load and resistance factor design (LRFD) approach employed in geotechnical design codes is often calibrated using the reliability-based design (RBD) method. However, the LRFD may not achieve the target safety level exactly. This paper makes comparisons between the LRFD and RBD by considering the ultimate and serviceability limit state design of strip foundations. The RBD is carried out via the Random Finite Element Method. It is found that the failure probability and the resulting foundation width for ULS obtained using the RBD increase with the soil spatial correlation length, gradually reaching a plateau. The comparison between the LRFD and RBD results for ULS suggests that the LRFD is conservative for low to medium soil variability, particularly at smaller correlation lengths. The reliability-based SLS design is less dependent on the soil correlation length, particularly for lower coefficients of variation of the soil elastic modulus. However, a resistance factor of 1.0 for SLS is unconservative, and resistance factors of 0.65, 0.7 and 0.8 are better aligned with the RBD when the target failure probabilities are 1×10−3, 1×10−2 and 1×10−1. The current study can be used to guide the design of shallow foundations and the calibration of the LRFD approach.

A simplified model for the analysis of piled embankments considering arching and subsoil consolidation

An analytical model is presented for the design of geosynthetic-reinforced and pile-supported (GRPS) embankments in this paper. The originality of the proposed solution lies in the fact that it allows considering the influence of the subsoil consolidation on the soil arching and geosynthetic strain. A nonlinear function is implemented to describe the subsoil behavior with the consolidation process in a closed-form solution. A simplified approach is then presented to link the arching development with the subsoil consolidation. The arching theory is combined with the tensioned membrane theory and the soil-structure interaction mechanisms to provide a simple and suitable design approach that enables a realistic approximation for designing soil–geosynthetic systems. The analytical model is capable of performing an ultimate and serviceability limit state design of GRPS embankments. While current methods cannot fully address the important effects of the subsoil consolidation, the analytical results suggested that arching and differential settlements increase with an increase of the subsoil consolidation degree. The analytical model is compared to field measurements and five other design standards for several full-scale field tests to study its validity. The results showed a satisfactory agreement between the proposed model and measured data, and generally better results are obtained as compared with other design methods.

Safety margin from characteristic values in settlement calculations of embankments on clay

Settlement calculations in serviceability limit state (SLS) design of embankments require the estimation of characteristic values of soil properties to account for different sources of uncertainty. The upcoming revised version of EN 1997-1:2004 (Eurocode 7) presents a statistically based equation to determine characteristic values of soil properties with the aim of providing a more consistent safety margin. The aim of this study is to quantify the safety margin in settlement calculations using this equation and performing probabilistic analyses. Four different cases of embankments on soft soil with different soil conditions were considered, including significant non-linear stress–strain behaviour. This allowed evaluation of the consistency of the safety margin from statistically derived characteristic values for different soil conditions. Probabilistic analyses were performed using Monte Carlo simulations to determine the statistics of total settlements. The safety margin was then defined based on the distance between the settlement from characteristic values and the mean settlement value of the simulations. The quantified safety margin was not consistent among the cases, which may be related to varying levels of non-linear compressibility and degree of consolidation. Moreover, for some cases, the safety margin might be too conservative for SLS design of embankments.

Numerical simulation and data-driven analysis on the flexural performance of steel reinforced concrete composite members

The flexural stiffness and flexural capacity of composite member are decisive indexes for the serviceability limit state and ultimate limit state design. To deepen the understanding of the flexural behavior of Steel Reinforced Concrete (SRC) composite members, a computationally efficient numerical technique based on fiber section analysis is employed with discussion of concrete confinement effect and steel section placement location. A dataset of SRC flexural members from published literature, covering a wide spectrum of geometrical and material properties, was built and compared with numerical predictions. The comparison signifies that the proposed numerical model is able to reproduce the moment-deflection relation of SRC flexural members, giving close estimation of the flexural stiffness, flexural capacity and flexural ductility. Parametric study is then carried out using the validated numerical model by varying concrete class, steel grade, steel section size and other influential parameters. Based on the numerically generated database, Machine Learning (ML) analysis is performed through an Artificial Neutral Network (ANN) algorithm. Compared with current design codes, the ML technique efficiently predicts the flexural capacity and flexural stiffness with high level of accuracy and robustness. As a consequence, it can be employed to intelligently estimate the flexural characteristics of SRC flexural members.

Reliability-based design of rigid pipes installed by induced trench method with tire-derived aggregate inclusions

A reliability-based design (RBD) method for rigid pipes installed using induced trench with tire-derived aggregates (TDA) inclusion is introduced in this study. The design process is to find an optimal set of the TDA inclusion dimension (i.e., width B and thickness t) that satisfies the 0.01-inch crack serviceability limit state and ultimate failure limit state requirements of the pipe, and achieves a target probability of failure or target reliability index using Monte Carlo simulation. The uncertainty and load-dependent compressibility of TDA inclusion are considered. Two design examples are provided, which show that the serviceability limit state design controls the finial design of induced trench installation (ITI) pipes. Increasing the thickness of TDA inclusion is helpful to improve reliability of the pipes, while expanding the width of TDA inclusion might increase probability of failure. The sensitivity and flexibility of using different target reliabilities, parameter uncertainties and calculation models in the proposed RBD method are examined. The results show the importance of considering the variability and load-dependent compressibility of TDA in the proposed RBD method for the design of ITI pipes with TDA inclusions.