Limit State Method Research Articles

Seismic behavior evaluation of zipper braced steel frames based on push-over and incremental dynamic analyses

PurposeAn analytical investigation is performed on zipper-braced frames. Zipper-braced frames are an innovative bracing system for steel structures. Conventional inverted-V-braced frames exhibit a design problem arising from the unbalanced vertical force generated by the lower story braces when one of them buckles. This adverse effect can be mitigated by adding zipper columns or vertical members connecting the intersection points of the braces above the first floor.Design/methodology/approachThis paper critically evaluates over strength, ductility and response modification factors of these structures. To achieve the purpose of this research, several buildings of different stories are considered. Static pushover analysis, linear dynamic analysis and nonlinear incremental dynamic analysis are performed by OpenSees software concerning ten records of past earthquakes.FindingsAlso, ductility factor, over strength factor and response modification factor, has been calculated for zipper-braced frames system. The values of 3.5 and 5 are suggested for response modification factor in ultimate limit state and allowable stress methods, respectively.Originality/valueThe fragility curves were plotted for the first time for such kind of braces. It should be mentioned that these curves play significant roles in evaluating seismic damage of buildings.

Assessing Seismic Performance of the Elliptic Braced Moment Resisting Frame through Pushover Method

The seismic performance of elliptic braced moment resisting frame (ELBRF) is assessed here and is found that the structural behavior is improved and is of free of architectural space. The demand for seismic performance of ELBRF is estimated through conventional pushover methods of 3, 5, 7, and 10-story ELBRF frames and they are compared with special moment resisting frames (SMRF) and X-Braced CBF and Inverted V-Braced CBF concentrically braced frames. The effective parameters in the seismic design of structures, like the ductility, overstrength and response modification factors are evaluated. The response modification factor for ELBRF in the design by ultimate limit state and allowable stress methods is proposed as 10 and 14.4, respectively. Finally, the process of forming plastic hinges in ELBRF is assessed and it is found that an increase in height makes the plastic hinges to be transmitted to the upper stories, allowing the structure to collapse at higher stories.

Evaluation of Geomembrane Effect Based on Mobilized Shear Stress due to Localized Sinking

Installing geosynthetic reinforcement at the bottom of the embankment will provide positive support for subgrade stability and settlement control, if there is a void or a weak foundation. In routine design work, the geomembrane effect must be well estimated and the tensile strain should be precisely predicted. Conventional analytical methods often adopt the limit state method to calculate the overlying load on the deflected geosynthetic. However, this assumption does not necessarily apply to all conditions, especially when the foundation soil can provide certain resistance. In this study, a semiempirical prediction method for evaluating the geomembrane effect of the basal reinforcement was proposed, and an iterative solution for calculating tensile strains of a deflected geosynthetic was deduced. In derivation, a virtual inclined slip surface and interaction between the geosynthetic and soil were quantitatively evaluated by coupling the arching effect and the geomembrane effect. Moreover, the development of shear stress along the slip surface can be considered, as well as different segments of the basal reinforcement. Then, the proposed method was validated by two large‐scale experiments. Comparison of the results of this method with measurements and results of other analytical models confirmed that this analytical method can take good care of the varying process of the localized sinking, regarding the overlying loads on the geosynthetic and the subsequent tensile strains.

Numerical method for analysis and design of isolated square footing under concentric loading

Footing is an important part of a structure, because its repair is extremely difficult and cumbersome. Therefore, all structural parameters should be carefully considered when designing a footing. An isolated footing needs sufficient depth, when considering the fixed base assumption. Extensive research has previously been conducted to define and formalize the depth of a rigid footing, i.e., the depth required, such that footing behaves as a rigid. Alternatively, working stress method (WSM) and limit state method (LSM) provide a lower depth for low subgrades and a higher depth for high subgrades than that required by rigid condition if a unit width footing is considered for the design. This paper presents a simple approach for calculating the depth to meet the rigid condition under static loading. The proposed calculation method produced a better rigidity than the existing approaches and it correlated well to the finite-element method (FEM) for low subgrades. The reinforcement distribution is function of the bending moment (BM). Steel is uniformly embedded throughout the length or width of conventional footing methods, but this is inappropriate, because the bending moment is not uniform along the length or width of the footing. This paper proposes solutions to this by redefining the placement of steel in the central zone of the footing. The effective zone for reinforcement was based on the FEM results. This simple procedure was developed for calculating the maximum moment using the Diagonal Strip Method (DSM). DSM is a substitute for FEM, and it has been shown to correlate well. The BM at central zone as well as at the edges can be calculated to define the spacing of the reinforcements.

Parametric study on cylindrical water tanks by varying their aspect ratios

Safe drinking water is one of the basic elements for humans to sustain healthy life. Reinforced concrete overhead water tanks are widely used to provide safe drinking water. Most water supply systems in developing countries, such as India, where urbanizing is increasing day by day, rely on overhead storage tanks and hence there is need to construct more number of water tanks. Earlier design of water tanks was done using the working stress method given in IS: 3370 1965. This method leads to thicker and heavily reinforced sections. The use of limit state method of design was then adopted in the revised code IS 3370: 2009 and provision for checking the crack width was included in the code. This study is carried out to analyze the cost of overhead water tanks of a fixed capacity, having different heights and diameters so as to determine the most economical height to diameter (H/D) ratio to be adopted in the design of the tank. To optimize the results and check the accuracy of design, six circular water tanks of 350 kL, with top and bottom dome pattern, were designed by varying H/D ratio from 0.50 to 0.75 in STAAD.Pro. After assuring the safety of all the structures, further analysis is done to calculate the cost-effectiveness of the structures by comparing the approximate total cost of materials. It was found that the aspect ratio (H/D) of 0.60 led to the most efficient design.

A systematic methodology for design of retrofit actions with longevity

This article presents a method for the design of retrofit actions with focus on energy savings that permits a systematic and thorough assessment of potential failures, anticipated maintenance and the expected durability of the retrofit actions. The proposed method combines the use of failure mode and effect analysis (FMEA) to permit identifying likely failure modes from which maintenance actions could be planned and the limit states (LS) method to assess the durability of the given retrofit action. One case study was completed to illustrate the application of: (1) the FMEA and LS method and (2) the proposed method for a retrofit action of an internal insulated solid wall of masonry bonded with lightweight aggregate concrete and floor division of concrete. It was evident that FMEA is useful regarding failure-mode identification and maintenance planning, and the LS method has its strength in durability assessment. Combining the use of both the FMEA and LS methods allowed improved design of new energy-saving retrofit actions, given that a thorough risk assessment was possible that included a decision-making process on maintenance planning, durability assessment and decision on potential redesign of retrofit actions.

Reliability Analysis of Retaining Walls Subjected to Blast Loading by Finite Element Approach

Conventional design methods adopt factor of safety as per practice and experience, which are deterministic in nature. The limit state method, though not completely deterministic, does not take into account effect of design parameters, which are inherently variable such as cohesion, angle of internal friction, etc. for soil. Reliability analysis provides a measure to consider these variations into analysis and hence results in a more realistic design. Several studies have been carried out on reliability of reinforced concrete walls and masonry walls under explosions. Also, reliability analysis of retaining structures against various kinds of failure has been done. However, very few research works are available on reliability analysis of retaining walls subjected to blast loading. Thus, the present paper considers the effect of variation of geotechnical parameters when a retaining wall is subjected to blast loading. However, it is found that the variation of geotechnical random variables does not have a significant effect on the stability of retaining walls subjected to blast loading.

Some provisions of calculations of hydraulic structures according to Russian norms and American standards

Ensuring the safety of hydraulic structures should be carried out, including calculations of the stress-strain state of the system “structure–base” by the method of limit States. A summary system of reliability coefficients, a combination of loads and operating conditions for different design cases is presented. Considered the stability condition of the concrete dam on the shift on the substrate. A comparison of the values of the generalized dam stability coefficients (reserve) according to Russian norms and American standards is performed. It is shown that when comparing the numerical values of the strength and stability coefficients regulated in the Russian norms and American standards, it is necessary to take into account the methods of determining the strength characteristics of materials, the practice of taking into account loads and effects, as well as the methods of computational studies of the stress-strain state of structures and their bases.

Harmonized European Standards for Proof of Competence of Cranes

With EN 13001-1 ff. a harmonized set of standards for safety of cranes was and is established. Due to harmonization the use of the standards leads to the assumption of conformity with the safety requirements of the machinery directive. A major argument for the application of the standards. The standards comprise new concepts of proof of competence in comparison to previous standards. Keywords of these new concepts are “Classification”, “Limit state method”, “Mass Distribution Class” and “Partial safety factors”. The article gives an overview to EN 13001-1, EN 13001-2 and EN 13001-3-1. This is the set of standards for proof of the structural parts of a crane. The main aspects of the standards are shown and discussed with regard to their impact on calculation.

Design of a Three-Storeyed R.C. Frame Building

In this project, we are going to plan, analyze and design a three storey multistoreyed building. The multi-purpose building includes a market place on the ground floor, a bank and post office on the first and a residential complex on the third. The site was identified and a suitable plan was prepared. All the necessary assumptions are made, and then the load calculation is done. The Frame is analyzed using Substitute Frame Method. A rigorous and in-depth analysis was carried out by moment distribution method to find the corresponding moments and shear forces in each member. For Earthquake Analysis, the method adopted here is the portal frame method. Finally, by checking all possible load combinations, we come to know the maximum factored loads in all members. Based on these factored loads, the design of various components such as slabs, beams, staircase etc. is done by Limit State Method of Design. For the Isolated Foundation, the method adopted is the Rigid Method. The detailing finally shows the schematic diagrams for the placement of reinforcement in the various components. IS Codes and Aids are used as per requirement.

Study on Flexural Behaviour of Ternary Blended Reinforced Self Compacting Concrete Beam with Conventional RCC Beam

The conventional concrete when used for structures having dense congested reinforcement, the problems such as external compaction and vibration needs special attention. In such case, the self compacting concrete (SCC) which has the properties like flow ability, passing and filling ability would be an obvious answer. All those SCC flow behavior was governed by EFNARC specifications. In present study, the combination type of SCC was prepared by replacing cement with silica fume (SF) and metakaolin (MK) along with optimum dosages of chemical admixtures. From the fresh property test, cube compressive strength and cylinder split tensile strength, optimum ternary mix was obtained. In order to study the flexural behavior, the optimum ternary mix was taken in which beam specimens of size 1200 mm x 100 mm x 200 mm was designed as singly reinforced section according to IS: 456-2000, Limit state method. Finally the comparative experimental analysis was made between conventional RCC and SCC beams of same grade in terms of flexural strength namely yield load & ultimate load, load- deflection curve, crack size and pattern respectively.

Economic viability of ultra high-performance fiber reinforced concrete in prestressed concrete wind towers to support a 5 MW turbine

Abstract The Ultra-High Performance Fiber-Reinforced Concrete is a material with remarkable mechanical properties and durability when compared to conventional and high performance concrete, which allows its use even without the reinforcement. This paper proposes the design of prestressed towers for a 5 MW turbine, through regulatory provisions and the limit states method, with UHPFRC and the concrete class C50, comparing the differences obtained in the design by parametric analysis, giving the advantages and disadvantages of using this new type of concrete. Important considerations, simplifications and notes are made to the calculation process, as well as in obtaining the prestressing and passive longitudinal and passive transverse reinforcement, highlighting the shear strength of annular sections comparing a model proposed here with recent experimental results present in the literature, which was obtained good agreement. In the end, it is estimated a first value within the constraints here made to ensure the economic viability of the use of UHPFRC in a 100 m prestressed wind tower with a 5 MW turbine.

Durability of Bridge Structural Elements

Due to environmental actions evocated by the atmospheric behaviour, the material degradation processes are started. The effects of material deterioration due to corrosion cause the loss of cross-sectional or member resistance, but the limit state method does not directly reflect design and verification allowing for failures caused by this influence. The time-variant loss of steel and reinforced concrete moment cross-sectional resistances were computed respecting specific corrosion models to express the time-dependent approximation function taking into account effects of corrosion degradation. The paper presents the general principles of structural elements design for durability based on the limit state concept and their application in the case of moment cross-sectional resistance respecting the effects of corrosion degradation.

Development of optimum design curves for reinforced concrete beams based on the INBR9

Structural optimization is one of the most important topics in structural engineering and has a wide range of applicability. Therefore, the main objective of the present study is to apply the Lagrange Multiplier Method (LMM) for minimum cost design of singly and doubly reinforced rectangular concrete beams. Concrete and steel material costs are used as objective cost function to be minimized in this study, and ultimate flexural strength of the beam is considered to be as the main constraint. The ultimate limit state method with partial material strength factors and equivalent concrete stress block is used to derive general relations for flexural strength of RC beam and empirical coefficients are taken from topic 9 of the Iranian National Building Regulation (INBR9). Optimum designs are obtained by using the LMM and are presented in closed form solutions. Graphical representation of solutions are presented and it is shown that proposed design curves can be used for minimum cost design of the beams without prior knowledge of optimization and without the need for iterative trials. The applicability of the proposed relations and curves are demonstrated through two real life examples of SRB and DRB design situations and it is shown that the minimum cost design is actually reached using proposed method.

MANNUAL AND SOFTWARE BASED NON LINEAR ANALYSIS OF RCC PORTAL FRAME

In recent days there is a lot of light thrown on behavior of structure post elastic limits and to understand the maximum permissible limits the structural members could be subjected before the structure reaches the limit where in it shall not be capable to with stand any more load and any repair or rehabilitation of structure stall not induce any structural strength. This report deals with Nonlinear Pushover analysis of rcc frame. A single bay portal frame of 6m length beam and 4m long column is subjected to manual analysis by slope deflection and kani’s method and is verified with software output values and the structural members are designed in accordance with Indian standard code in limit state method and its plastic moment values are validated.

Design and Analysis of HPFRCC Blast Resistant Structures

In the past few years, the increase in the number of terrorist attacks has shown that the effect of blast loads on buildings is a serious matter that should be taken into design consideration. These man-made disasters has created a challenge to structural engineers world over about the deficiency in the design process. Blast loads are extreme, instantaneous, unpredictive impulses acting over milliseconds. Due to this nature of blast loads, it is complicated to analyse the dynamic responses of the structures. Usage of advanced engineering materials for construction can solve these structural problems to an extent. This paper presents the design and analysis of an underground blast resistant shelter made up of high performance fiber reinforced cementitious composites (HPFRCC). This research focuses on an alternative section of cylindrical module of the shelter. The dynamic behavior of module under blast load is studied in finite element software Abaqus CAE 6.12. It is observed that the material stress-strain behavior is greatly influenced by strain rates of loadings. Shelter manually designed using codes in limit state method is verified with the analytical analysis.

Stress-strain characteristics and flexural behaviour of reinforced Eco-friendly coconut shell concrete

In this study the stress-strain behaviour of coconut shell concrete incorporating ground granulated blast furnace slag and manufactured sand was obtained and it was in good fit with popovics model (Mo et al., 2015). Coconut shell concrete of grade M20 was achieved using 401kg/m3 of cement by conceal curing. The flexural behaviour of under-reinforced and over-reinforced coconut shell concrete designed by limit state method using the actual stress-strain behaviour is analogous with the experimental values. The deflection and crackwidth of coconut shell concrete is comparable with the permissible values given by IS 456:2000, ACI-318 and EC 2:1992.

Neural-based prediction of structural failure of multistoried RC buildings

Various vague and unstructured problems encountered the civil engineering/designers that persuaded by their experiences. One of these problems is the structural failure of the reinforced concrete (RC) building determination. Typically, using the traditional Limit state method is time consuming and complex in designing structures that are optimized in terms of one/many parameters. Recent research has revealed the Artificial Neural Networks potentiality in solving various real life problems. Thus, the current work employed the Multilayer Perceptron Feed-Forward Network (MLP-FFN) classifier to tackle the problem of predicting structural failure of multistoried reinforced concrete buildings via detecting the failure possibility of the multistoried RC building structure in the future. In order to evaluate the proposed method performance, a database of 257 multistoried buildings RC structures has been constructed by professional engineers, from which 150 RC structures were used. From the structural design, fifteen features have been extracted, where nine features of them have been selected to perform the classification process. Various performance measures have been calculated to evaluate the proposed model. The experimental results established satisfactory performance of the proposed model.

Role of suction stress on service state behavior of geosynthetic-reinforced soil structures

Abstract Several design guidelines adopt limit state methods (e.g., earth pressure or limit equilibrium slope stability methods) to estimate the reinforcement loads for geosynthetic-reinforced soil structures (GRSSs). Field measurements usually reveal lower tensile loads in the reinforcements than that predicted by conventional design methods. Consequently, limit state methods have been criticized for being conservative or inaccurate. However, these lower-than-expected loads are primarily due to redundancy in design, attributed to several factors such as toe resistance, soil volumetric dilation, underestimation of soil shear strength, and the effect of suction stress. While these factors commonly contribute to the performance of the GRSSs, they are not accounted for in design procedures. This disregard is accredited to complexities and uncertainties associated with reliable quantification of these factors during the life span of the structure. By properly quantifying the role of suction stress, this study aims to quantitatively explain a part of the discrepancy which exists between the tensile loads in the reinforcement, which are estimated by working-condition design methods versus those by limit state design methods. A suction stress-based formulation is presented for calculating the active earth pressures coefficient ( K a ) of unsaturated backfill under unsaturated steady flow conditions. The formulation is derived by implementing an analytical solution of one-dimensional steady flow into a limit equilibrium-based effective stress analysis. The proposed formulation is used in conjunction with the earth pressure method to illustrate the effect of suction stress on predicted reinforcement loads. Two backfills, referred to as marginal and high quality backfill, along with three representative annual rain events are examined. The results are compared with two classical earth pressure methods as well as two empirical methods. The results show that the suction stress contribution can lead to significantly lower reinforcement loads than that predicted by classic earth pressure methods for backfill types. Empirical methods lead to lower loads in some cases and higher predicted loads in others. The proposed formulation provides a mechanics-based approach to explain a part of the discrepancy which exists between the measured tensile loads in the reinforcement in the field versus those by limit state design methods.

Cálculo en agotamiento de secciones de fábrica reforzadas a flexión con láminas de FRP

Masonry structures can hardly resist tensile loads. For this reason these structures are vulnerable to accidental actions, such as earthquakes, or when they suffer damage that change their geometry conditions, for instead, due to differential soil settlements. This paper presents an ultimate limit state method for checking FRP-strengthened unreinforced masonry sections. The method is similar to the one used for reinforced concrete members, but it is adapted to masonry and FRP laminates particularities. In this proposal a bilinear strength-strain relationship is used and the FRP design strain is limited to take into account different aspects which cause the plate can’t reach its ultimate strength. In particular, it is proposed a “bond reduction factor”, derived from a large bending test database.