Serviceability Limit State Conditions Research Articles

Small-strain, non-linear elastic Winkler models for uniaxial loading of suction caisson foundations

Soils exhibit non-linear stress–strain behaviour, even at relatively low strain levels. Existing Winkler models for suction caisson foundations cannot capture this small-strain, non-linear soil behaviour. To address this issue, this paper describes a new non-linear elastic Winkler model for the uniaxial loading of suction caissons. The soil reaction curves employed in the model are formulated as scaled versions of the soil response as observed in standard laboratory tests (e.g. triaxial or simple shear tests). The scaling relationships needed to map the observed soil-element behaviour onto the soil reaction curves employed in the Winkler model are determined from an extensive numerical study employing three-dimensional finite-element analysis. Key features of the proposed Winkler model include: computational efficiency, wide applicability (it can be used for caisson design in clay, silt or sand) and design convenience (the required soil reaction curves can be determined straightforwardly from standard laboratory test results). The proposed model is suitable for small and intermediate caisson displacements (corresponding to fatigue and serviceability limit state conditions) but it is not applicable to ultimate limit state analyses.

Optimized Design of Cold‐Formed Steel Elements for Serviceability and Ultimate Limit States

AbstractCold‐formed steel (CFS) sections are gaining ever increasing popularity in modern construction practice due to their advantages, such as efficient material use and sustainability. While the manufacturing flexibility of CFS cross‐sectional shapes brings innovative and complex shapes within practical reach, obtaining optimum design solutions can be a challenging task due to the complex mechanical behaviour of CFS elements, which is controlled by local, distortional and global buckling modes, as well as the need to account for manufacturing and end‐use constraints. This study presents a practical framework for the development of optimised CFS beam sections with maximum flexural strength and minimum deflections in ultimate and serviceability limit state conditions, respectively, in accordance with Eurocode 3 (EC3). A population‐based Genetic Algorithm optimisation method is employed to obtain optimum solutions for ten different CFS cross‐section prototypes, while the EC3 geometric restrictions and a number of manufacturing and end‐use constrains are also considered. It is shown that sections optimised based on serviceability limit state (SLS) and ultimate limit state (ULS) requirements can provide up to 44% higher effective stiffness and 55% higher bending moment capacity, respectively, compared to standard commercial lipped channel sections with the same amount of material. The results also indicate that using optimised plain channels and folded‐flange sections generally leads to the best design solution in SLS and ULS optimization, respectively.

Resistance Factors at Serviceability Limit State Using the Texas Cone Penetrometer as the Predictive Model

This study presents the development and calibration of resistance factors for the serviceability limit state (SLS) condition (φSLS) used in the load and resistance factor design (LRFD) of deep foundations. The performance function was established based on load corresponding to tolerable displacement (Qδtol) and design load (Qd). A dataset of published full-scale load tests including projects from Texas, Missouri, Arkansas, Louisiana, and New Mexico was compiled and consisted of 60 load test cases comprising 33 driven piles and 27 drilled shafts. Resistance factors for SLS conditions were calibrated for tolerable displacements using both the Monte Carlo simulation (MCS) and the First Order Second Moment (FOSM) approaches. From the calibration study, resistance factors at SLS conditions were obtained ranging from 0.33 to 0.62 using FOSM method and 0.37 to 0.67 using the MCS for driven piles. In the case of drilled shafts, SLS resistance factors ranged from 0.37 to 0.77 following the FOSM method and 0.41 to 0.86 based on MCS.

Optimizing the steel girders in a high strength steel composite bridge

The use of high strength steel has the potential to reduce the amount of steel used in bridges and thereby, facilitate a more sustainable construction. In this study, an optimization of a composite steel and concrete bridge is presented. The study aimed at identifying the potential benefits of using high strength steel, compared to conventional steel, in terms of weight, material cost and environmental impact. The optimization routine followed what is outlined in the Eurocode and satisfied both ultimate and serviceability limit state conditions, with exception of fatigue. To study the influence of fatigue a separate verification was performed on the solutions found from the optimization. The study showed that substantial savings in weight, environmental impact and material cost could be made by implementing high strength steel. However, when the fatigue verification according to Eurocode was considered, the potential benefits of using steels of higher grade vanished completely.

Coupled element and structural level optimisation framework for cold-formed steel frames

Optimisation of cold-formed steel (CFS) structures can be challenging due to the complex behaviour of thin-walled CFS sections affected by different buckling modes. In this paper, a coupled framework is presented for element and structural level optimisation of CFS portal frames, under serviceability limit state (SLS) and ultimate limit state (ULS) conditions, using Genetic Algorithm. First, CFS lipped-channel beam sections are optimised with respect to their flexural capacity determined in accordance with the effective width method specified in Eurocode 3 (EC3). The relative dimensions of the cross-section are considered as the main design variables, while the EC3 plate dimensions and slenderness limits and a number of manufacturing and end-use constraints are taken into account in the optimisation process. The results show that the optimum CFS sections exhibit significantly higher (up to 84%) ultimate capacity compared to the standard lipped channel sections with the same plate width and thickness. The structural level optimisation is then carried out to obtain the optimal design solution for a long-span CFS portal frame with knee braces under SLS and ULS conditions. Compared to conventional optimisation using standard cross-sections, it is shown that the proposed coupled framework leads to more cost-effective solutions (up to 20% less structural material) by using the more efficient CFS cross-sectional shapes optimised for generic applications. The results also indicate that optimising the frame geometry and knee brace configuration can noticeably improve the structural performance and reduce the required structural weight, especially when both ULS and SLS conditions are considered.

Development of optimum cold-formed steel beams for serviceability and ultimate limit states using Big Bang-Big Crunch optimisation

Cold-formed steel (CFS) elements are increasingly used as main structural members in modern construction practice. While flexibility of CFS cross-sectional shape allows achieving higher load carrying capacities by using more efficient shapes, obtaining optimum design solutions can be a challenging task due to end-use constraints and complex behaviour of CFS elements controlled by local, global and distortional buckling modes. This study aims to develop a practical methodology for optimum design of CFS beam sections with maximum flexural strength and minimum deflection under ultimate and serviceability load conditions, respectively, in accordance with Eurocode 3 by taking into account manufacturing and end-use design constrains. Population-based Big Bang–Big Crunch Optimisation method is employed to obtain optimum design solutions for twelve different CFS cross-sectional prototypes. To verify the flexural strength and stiffness of the optimum beam sections, detailed nonlinear finite element (FE) models are developed using ABAQUS by considering both material nonlinearity and initial geometrical imperfections. It is shown that the optimised sections based on serviceability limit state (SLS) and ultimate limit state (ULS) can provide, respectively, up to 44% higher effective stiffness and 58% higher bending moment capacity compared to a standard lipped channel beam section with the same plate width and thickness. Using plain channel and folded-flange sections generally leads to the best design solutions for SLS and ULS conditions, respectively. Finally, the results of detailed FE models are used to evaluate the adequacy of EC3 proposed procedures to estimate CFS beam capacity and deflection at ULS and SLS, respectively.

Preloading Effect on Strengthening Efficiency of RC Beams Strengthened with Non- and Pretensioned NSM Strips

The near surface mounted (NSM) technique has been shown to be one of the most promising methods for upgrading reinforced concrete (RC) structures. Many tests carried out on RC members strengthened in flexure with NSM fiber-reinforced polymer (FRP) systems have demonstrated greater strengthening efficiency than the use of externally-bonded (EB) FRP laminates. Strengthening with simultaneous pretensioning of the FRP results in improvements in the serviceability limit state (SLS) conditions, including the increased cracking moment and decreased deflections. The objective of the reported experimental program, which consisted of two series of RC beams strengthened in flexure with NSM CFRP strips, was to investigate the influence of a number of parameters on the strengthening efficiency. The test program focused on an analysis of the effects of preloading on the strengthening efficiency which has been investigated very rarely despite being one of the most important parameters to be taken into account in strengthening design. Two preloading levels were considered: the beam self-weight only, which corresponded to stresses on the internal longitudinal reinforcement of 25% and 14% of the yield stress (depending on a steel reinforcement ratio), and the self-weight with the additional superimposed load, corresponding to 60% of the yield strength of the unstrengthened beam and a deflection equal to the allowable deflection at the SLS. The influence of the longitudinal steel reinforcement ratio was also considered in this study. To reflect the variability seen in existing structures, test specimens were varied by using different steel bar diameters. Finally, the impact of the composite reinforcement ratio and the number of pretensioned FRP strips was considered. Specimens were divided into two series based on their strengthening configuration: series “A” were strengthened with one pretensioned and two non-pretensioned carbon FRP (CFRP) strips, while series “B” were strengthened with two pretensioned strips. Experimental tests illustrated promising results at ultimate and serviceability limit state conditions. A significant gain of the load bearing capacity, in the range between 56% and 135% compared to the unstrengthened beams, was obtained. Tensile rupture of the NSM CFRP strips was achieved, confirming full utilization of the material.

The Sliding Hinge Joint: Final Steps towards an Optimum Low Damage Seismic-Resistant Steel System

The Sliding Hinge Joint with Asymmetric Friction Connectors (SHJ), to give its full name, is a semi-rigid moment resisting joint used between the beams and columns of a moment-resisting steel frame and also at the column base between the column and the ground. It’s performance is intended to be as follows: 1) On completion of construction, rigid under serviceability limit state conditions, 2) During a severe earthquake, allowing controlled rotation between the column and the beam or foundation on designated friction sliding planes within the connection, then 3) Returning to its rigid in-service condition at the end of the severe shaking with the building returning to its pre-earthquake position (self-centering). During its development and proof of concept through large scale testing, the initial results showed that the SHJ as originally designed and detailed performs 1) and 2) very well, but the bolts in the friction sliding planes loose much of their original installed bolt tension during significant sliding, lowering the level at which rotation within the joint will occur post severe earthquake. A concerted research programme of component testing, analytical model development and numerical modelling in recent years has developed solutions to the bolt tension loss issue as well as enhanced the joint’s performance to deliver dependable self-centering capability for the building. This work marks the final steps towards developing an optimum low damage seismic-resisting steel moment frame system. This paper presents key findings from the research work and general recommendations for the optimum performing sliding hinge joint.

Reliability analysis of a Super-T prestressed concrete girder at serviceability limit state to AS 5100:2017

The level of safety or reliability index achieved by a design code is an important measure of its performance. Presently, such a measure is not readily known for the prestressed concrete (PSC) girders common in Australia – the Super-T girder – which is designed to the Australian bridge design code (AS 5100). To ascertain the performance of the latest edition, this paper determines the annual reliability indices of a Super-T PSC girder at serviceability limit state (SLS) designed using AS 5100 (2017) and compares the results to an equivalent design using the Eurocode. A single-span bridge deck consisting of five prestressed concrete Super-T girders spanning 33.5 m is used as the basis for comparison. Several design scenarios are conducted, considering both dead and traffic loads, for SLS conditions. The reliability analysis is conducted using first-order reliability method for the SLS limit state functions, using load and resistance random variables based on previous literature. Sensitivity analyses are conducted to consider both the relative contribution of these variables and the effect of their distributions on the annual serviceability reliability indices. It is found that a girder designed to AS 5100 has a larger design moment and requires more prestress to satisfy stress limits compared to a Eurocode design. Moreover, the annual serviceability reliability index of the girder designed to the Eurocode is slightly higher, particularly for compression stress at transfer and tension stresses at service. Service limit state functions are found to be particularly sensitive to traffic load, permissible stresses and prestress losses as well as their respective distributions. Hence, future revisions of the Australian Standard should focus on these aspects to achieve consistent levels of reliability performance.

Experimental and numerical research on the potentialities of layered reinforcement configuration of continuous sisal fibers for thin mortar panels

This work is dedicated to the assessment of the flexural capacity of thin mortar panels (about 12mm thick) reinforced with unidirectional continuous layers of sisal fibers. By adopting five stacked layers, each one involved in a mortar layer of about 1mm thickness, which constitute a fiber volume of 10%, a maximum flexural strength of 30MPa was obtained at a deflection of around 1/10 of the specimen’s span length. At this deflection the toughness is 46 times higher the elastic energy at crack initiation. Fiber pullout tests were carried out to assess the relevant properties of the fiber–matrix bond behavior. By adopting a multidirectional smeared fixed crack model to simulate the crack initiation and propagation in the cement matrix, considering the fibers like truss elements characterized by their elastic properties, and applying interface finite elements with a local bond-slip equation for modeling the sliding mode of the fiber–matrix interface, the two series of panels tested experimentally were simulated accurately. By adopting this numerical strategy, a parametric study was carried out to demonstrate that the load carrying capacity at serviceability limit state conditions can be significantly increased with the elasticity modulus of the fibers, which can be assured by applying a certain prestress level to the fibers, while the ultimate flexural capacity of these layered panels increases with the fiber–matrix bond strength.

Aramid/glass fiber-reinforced thermal break – thermal and structural performance

Energetically weak points in thermally insulated building envelopes are formed by thermal breaks that are implemented to structurally connect external balconies to internal slabs. Current thermal breaks comprise stainless steel bars that penetrate the insulation layer and thus cause significant thermal losses. A new thermal break composed of highly insulating aramid and glass fiber-reinforced polymer (AFRP and GFRP) components and aerogel granulate insulation materials was developed and the first prototypes of the load-bearing components were experimentally investigated. The use of AFRP leads to an excellent thermal performance with linear thermal transmittance values of below 0.15W/mK. The experimental prototype investigations confirmed the targeted ductile failure mode through concrete crushing in the component–concrete interfaces. The serviceability limit state conditions are met for a targeted balcony cantilever span of 4.0m. The material-tailored components can be manufactured by fully automated processes such as filament and tape winding and pultrusion to economically produce large quantities within a short time.

Prestress Losses in NSM‐CFRP Flexurally Strengthened RC Beams

AbstractThe use of prestressed near‐surface mounted fibre‐reinforced polymers (NSM‐FRP) has been long acknowledged to be a suitable approach to strengthen and retrofit existing reinforced concrete structures. The application of a certain amount of prestress to the FRP prior to its installation provides a number of benefits, mainly related to crack width and deflection requisites at serviceability limit state conditions. After transferring the prestress to a structural element, some of the existing cracks can be closed, decreasing the vulnerability of the element to corrosion, and a certain amount of deflection can be recovered due to the introduced negative curvature. However, these benefits can only be assured if the prestress is properly preserved over time. In this context, three series of reinforced concrete beams, in a total of 10 beams, were strengthened with a prestressed carbon FRP laminate (CFRP) and monitored for about 40 days. The data obtained from these tests is in this paper presented and analysed. The observed losses of strain in the CFRP laminate were found to be mainly located in the extremities of the bonded length, while in the central zone, most of the initial strain was well‐preserved over time. Additionally, the highest CFRP strain losses were observed in the first 6 to 12 days after prestress transfer, suggesting that the benefits of prestressed NSM‐FRP will not be considerably lost over time.

Mechanical behavior of a shelter system based on cable-strut structures

A shelter system based on cable-strut structures, consisting of compressive struts and high-tensile elements, is described in this paper. The deployment of the shelter is achieved by tightening inclined cables. Lower cables are used to terminate the deployment. The state of self-stress of the cable-strut structures in the fully deployed configuration is given, and the minimum strut length and the maximum load design of the shelter are discussed. The mechanical behavior of the system was studied under symmetrical and asymmetrical load cases. The results show that the shelter in the deployed configuration satisfies the ultimate limit and the serviceability limit state conditions. Finally, the stability of the cable-strut system is investigated, considering the effect of imperfections on the buckling of the shelter. We conclude that the influence of imperfections based on the consistent imperfection mode method is not significant.

Optimal design of an array of active tuned mass dampers for wind-exposed high-rise buildings

In this paper, a comprehensive procedure is developed for the optimization of a hybrid control system for tall buildings subjected to wind-induced vibrations. The control system is made of active tuned mass dampers (ATMDs) and is conceived to mitigate the flexural and torsional response in serviceability limit state conditions. The feedback information necessary to compute the control forces is provided by a limited number of accelerometers arranged over the building's height. To reduce the computational effort, subsequent optimization subprocedures are employed that take advantage of the genetic algorithm to find the solution of the nonlinear, constrained optimization problems. At first, the optimization of the ATMDs' number and positions over the top floor of the building is carried out. Then, the optimal location of the accelerometers over the building's height is obtained. The reduction of the flexural and torsional accelerations is chosen as target of the optimization problem. The technical limitations of the ATMDs, such as the actuators saturation and the limited stroke extensions, are the constraints to the problem. As an illustrative example, a control system is optimized for the response mitigation of a tall building subjected to wind load. Copyright © 2012 John Wiley & Sons, Ltd.

Parametric Study of the Use of Strain Softening/Hardening FRC for RC Elements Failing in Bending

Using a design-oriented model recently developed for prediction of the moment-curvature relationship of a cross section of beams made by strain-softening or strain-hardening fiber reinforced concrete (FRC) and that can also include a certain percentage of longitudinal steel bars, a parametric study is carried out to evidence the effect of relevant characteristics of the post-cracking behavior of these materials in the moment-curvature and force-deflection responses of this type of structural elements. The study also analyzes the influence of the reinforcement ratio of longitudinal steel bars, ρ, in order to show that for a certain content of fibers the benefits of fiber reinforcement, mainly at serviceability limit state conditions, decrease with the increase of ρ. Adopting the formulation of the CEB-FIB final draft Model Code, the moment-crack width relationship of FRC beams reinforced with steel bars are obtained and the predictive performance of this approach is assessed by comparing analytical and experimental results.