End Slabs Research Articles

Склеп “македонського типу” з некрополя Німфею

Склеп “македонського типу” з некрополя Німфею

Behaviour of composite high performance concrete slab on steel I-beams subjected to static hogging moment

This paper investigates the behavior of composite girders made of high performance concrete (HPC) slab connected to steel I-beam through stud shear connectors subjected to hogging moment. Such girders are widely used in bridge construction. However, reinforcing steel bars in the negative moment region contribute significantly to the moment resistance and is found to be ineffective due to cracking of ordinary concrete. This research proposes the use HPCs containing fibers (with better crack resisting and strain hardening characteristics) in the deck slab to enhance both cracking and yield load carrying capacity of such girders at the negative moment region. Four composite girder specimens made of different types of concrete and having variable slab continuity were tested to failure under monotonic loading simulating negative moment. Three types of concrete were employed in the first three girder specimens, namely: Normal Concrete (NC), Steel Fiber-Reinforced Concrete (SFRC) and engineered cementitious composite (ECC). The fourth girder specimen was constructed using two SFRC end slabs connected together with an Ultra-High Performance Fiber-Reinforced Concrete (UHPFRC) closure strip located at the maximum negative moment mid span region. This specimen represents the case of full width -full depth deck slabs resting over steel girders with transverse joints filled with UHPFRC used to accelerate bridge construction. The SFRC and ECC composite girders showed better performance in terms of cracking characteristics (multi-cracking and smaller crack width), higher pre-cracking linear stiffness, higher cracking load, higher concrete strain development during cracking and higher strength compared to their NC counterparts.

Nonlinear Behavior of Self -Compacting Reinforced Concrete Two-Way Slabs with Central Square Opening under Uniformly Distributed Loads

This research is carried out to investigate the behavior of self-compacting concrete (SCC) two-way slabs with central square opening under uniformly distributed loads. The experimental part of this research is based on casting and testing six SCC simply supported square slabs having the same dimentions and reinforcement. One of these slabs was cast without opening as a control slab. While, the other five slabs having opening ratios (OR) of 2.78%, 6.25%, 11.11%, 17.36% and 25.00%. From the experimental results it is found that the maximum percentage decrease in cracking and ultimate uniform loads were 31.82% and 12.17% compared to control slab for opening ratios (OR) of 11.11% and 6.25% respectively. Also the results showed that as OR is increased from 0.00% to 11.11%, a signifacant increase in deflection was occured. While the increase of OR from 11.11% to 25.00%, a slighlty decrease in deflection was occured compared to control slab within the entire range of loading starting from first cracking load up to ultimate load. The theoretical part of this research is adopted for both simply supported and clamped ends square slabs according to yield line theory. For simply supported slabs, the results showed a decrease in ultimate uniform loads for OR ranging between 0.00% and 25.00%. While beyond this value, an increase in the ultimate uniform load is occured. In addition, it is found that as OR was increased; the total ultimate load is decreased. Also from the theoretical analysis for clamped end slabs it is found that as OR was increased, both the ultimate uniform load and the total ultimate load were increased.
 

Source of Inclusion in IF Steel End Slabs

The source of inclusion in IF steel end slabs after reducing the casting remnant were studied by means of total oxygen and nitrogen content analysis, original position analyzer for metal (OPA), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Slims. The results show that: reducing the casting remnant have little influence on reoxidation in continuous casting process, the size of micro-inclusions are mostly smaller than 5 μm. The size of large-sized inclusions in end slab are mainly trapped in mold, they are mostly from 140 μm to 300 μm and over 300 μm, which accounted individually for 19.94% and 73.21% of total large-sized inclusions. In the end of a cast, decreasing the casting speed with slower speed changing rate can decrease the slag entrainment index of hot rolled coils from end slabs.

사각에 따른 RC슬래브교의 반력특성 분석

본 연구에서는 2경간 연속 RC 슬래브교를 대상으로 차량 이동하중 시간이력해석을 수행하여 사각에 따른 받침의 반력분포 특성을 분석하였다. 계측값과 해석값의 비교를 통해 적용 시간이력해석방법을 검증하였으며, 사각을 매개변수로 판 해석을 수행한 결과, 둔각부 최외측 받침에 반력이 집중되며, 사각이 감소함에 따라(사교의 정도가 커질수록) 반력의 편차가 커져 둔각부에 반력의 크기가 증가하는 것으로 나타났다. 또한, 반력이 집중된 최외측 받침의 인접 받침에서는 사각 <TEX>$70^{\circ}$</TEX> 이하에서 부반력이 발생하는 것으로 분석되었다. 따라서 사각을 갖는 RC 슬래브교에서는 받침의 용량 산정 시 반력의 편차와 부반력 발생에 유의하여야 한다고 판단된다. The support reactions of two-span RC slab bridges with various skews are studied through the time history analysis for moving vehicles. Dynamic behaviors of bridges are evaluated by using the commercial FEM code and the bridge is modelled using shell elements. The analysis method is verified by comparing measured values and analysis results. Parametric analysis are performed for various skew angles. The bearing below the end slab with an obtuse angle shows the largest reaction forces. As the skew angle is reduced(a skew angle of <TEX>$90^{\circ}$</TEX> means a straight bridge without skew in this study), the distribution of reaction force is growing uneven and the largest reaction force is increased. In case of the model having skew angles under <TEX>$70^{\circ}$</TEX>, the bearing located near the bearing with the largest reaction force shows the uplift force. Thus, it is concluded that the bearing in RC slab bridges with skew should be selected carefully by considering the variation of reaction force and uplift force.

Surface cleanliness evaluation in Ti stabilised ultralow carbon (Ti-IF) steel

The surface cleanness of Ti-IF steel, including head, transition, end and normal slabs, was studied by original position analyser. The content, number, size and porosity of inclusions on the surface were obtained. The results showed that: the contents of inclusions in head and end slabs were significantly higher than in normal or transition slabs and fluctuated significantly in non-steady state slabs within 3.5 mm of the surface; the porosity in 3.5 mm of slab surface was lower than other positions; the surface cleanliness order was normal slab>transition slab>end slab>head slab; a reasonable surface peeling thickness for non-steady state slabs was 3.5 mm. The energy dispersive spectroscopy showed that more lump Al2O3 existed in the head slabs, and slag entrapment inclusions also appeared frequently in the end slabs. The main effect factors for cleanness of the head and end slabs were reoxidation and slag entrapment.

Subcritical crack growth of edge and center cracks in façade rock panels subject to periodic surface temperature variations

This paper examines subcritical cracking in a rock panel or slab containing either a pre-existing edge or a center crack perpendicular to the panel surface. The panel is subject to periodic surface temperature variation on one side of the panel while the other is kept at a constant temperature. The thermally induced stress intensity factors are determined using superposition technique by employing the fundamental point load solution for an edge crack or a center crack in a slab of finite thickness. Rock panel is modeled as a long elastic strip with either a free or a fully constrained lateral end condition. The temperature variations versus time at various depths of the rock panel appear roughly as a sinusoidal function. The lateral thermal stress for the free end case is larger than the constrained end case; whereas stress intensity factors for both edge and center cracks in the constrained end slab are 1000 times larger than that of free end case. Subcritical crack propagation in rock panels on façade is then estimated as a function of time. This subcritical crack propagation continues until a critical crack size is attained and the rock panel will fail under wind load. This new theoretical framework provides a new paradigm to examine the mechanisms of time-dependent cracking in rock panels on façade of buildings.

Investigations of PCRV and nuclear reactor containments in China

This paper describes the state-of-the art of the research work on the mainland of China pertaining to the prestressed concrete reactor vessel and to nuclear reactor containment. The results of tests of 1 10 th scale model of a PCRV and its deep end slab are presented. The 3D linear and nonlinear stress analyses and ultimate load of these models are also described. A new type of containment, a steel-plate-concrete composite containment, is proposed and evaluated. At last, a survey of the Qin Shan Nuclear Power Plant is described.

An analytical method for three-dimensional non-linear responses of prestressed concrete nuclear reactor vessels

This paper describes the results of an investigation to develop an analytical method for assessing the non-linear behaviour and strength of three-dimensional concrete structures such as prestressed concrete nuclear reactor vessels (PCRVs) under monotonically increasing internal pressure. This three-dimensional finite element method uses a proposed material model for the non-linear response of concrete under a multiaxial stress state. Three numerical examples were analysed to investigate the feasibility of the analysis. The first was the calculation of the two-dimensional response of Kupfer's plain concrete. The second involved the analysis of a small-scale axisymmetric PCRV test model with a solid end slab. The third was the analysis of a three-dimensional PCRV test model with penetrations in the end slab. Good agreeement between experimental and analytical results was obtained.

Use of variational principles for solution of infinitely redundant continuum problem with special reference to containment vessels.

The calculation of time-dependent stresses in concrete structures operating at elevated temperatures is discussed. The method described is of a direct formulation technique and it is based on the principles of the calculus of variation. The paper mainly deals with the application of the method to a large and infinitely redundant continuum problem. The analytical procedure of the variational principle is also described and the mathematical expressions are developed for uniaxial and biaxial stress problems. The solution for the biaxial state of stress is carried out by a two-dimensional finite element stiffness analysis. A step-by-step method developed by the author using two-dimensional finite element stiffness analysis is also described in APPENDIX 3. Both methods are then applied to a real problem for which experimental data exist from ref. (1). Predicted analytical values obtained by both methods are compared with experimental results. The method is suitable for predicting the distribution of stress in the end slabs of containment vessels. These slabs are perforated to permit fuel loading by the charging machine.

Experimental and analytical studies of shear behaviour of slabs subjected to multiaxial stress conditions

Abstract Prestressed concrete pressure vessels include structural members which rely for their safety on the knowledge of the capacity of concrete to resist shear stresses. The shear forces developed on some of the structural components under operating conditions may be several times higher than those occurring in conventional structures. This paper reviews the experimental and analytical studies which have been initiated over the years by various organisations and research workers to examine the problem of high shear stresses and the ultimate load behaviour of end slabs for PCPV. The experimental programmes were carried out to assist in the development of various designs of PCPV and to introduce a more analytical approach for designs of deep members against shear failures.

An exploration of the design for a PCCV head slab with very large penetrations using a dynamic relaxation computer program

A prestressed concrete containment vessel for a Liquid Metal Fast Breeder Reactor has been developed. The current design for the head slab for this vessel differs geometrically from previous head slabs for other reactors. This head slab has a single very large central penetration which is surrounded by an annulus in which there are a number of large and small penetrations. The total area taken by penetrations is in excess of 50% of the total pressurised plan area. This paper describes a computer analysis carried out using a Dynamic Relaxation code to assess the structural behaviour of the head slab. The effects of variations in the geometry and boundary conditions have been examined. The mathematical models considered were based on one of a series of small scale physical models intended to validate the design of the containment structure. The dynamic relaxation technique adopted to analyse the models allows for non-linear behaviour of materials with the provision for the application of step by step increments of both pressure and prestress forces. Concrete is modelled by assuming that it has a limited tensile stress capacity and once this is exceeded it acts as an orthotropic material. In addition, a yield condition can be specified to allow for triaxial stress states and the possible plastic flow. The method predicts crack propagation, hinge positions if any, tendon forces and overall structural deformations. The analysis has demonstrated that for a very slender end slab design, the presence of large unlined penetrations reduces the strength of the slab. It has also been demonstrated that the use of an upstand ring beam appears to give a greater reserve of strength than flat slabs even in situations in which failure of concrete rather than the prestressing system is the cause of structural collapse. Comparison between the experimental model results and those of the theoretical analyses indicate general agreement in crack patterns and failure modes. However, there are some discrepancies in actual failure pressures. These differences may however be attributed to some uncertainties that exist in the material parameter values and assumptions in the computer code.

Structural and constructional problems in prestressed concrete reactor pressure vessels (Konstruktive und Ausführungstechnische Probleme bei Spannbeton-reaktordruckbehältern)

Prestressed concrete pressure vessels (PCPVs) for nuclear reactors differ ssentially from other civil engineering structures, namely in relation to their extraordinary dimensions and stress conditions and different components (liner, penetration liners, etc.). These peculiarities involve a series of structural and constructional problems. The voluminous dimensions and high demands on the quality of prestressed concrete pressure vessels raise the questions of specifying the optimum concrete quality and of the concreting units to important aspects. The orderly placement and compaction of concrete underneath the almost horizontal base of the liner leads to special technical difficulties. Some solutions pertaining to these problems are discussed. PCPVs are highly prestressed structures. The difficulties with regard to horizontal prestressing are quite apparent, whereas the vertical prestressing presents no special problems. The wire-winding prestressing system already represents great progress in comparison with single anchored cables. However, it is not an optimum possibility. Further improvements in circumferential prestressing techniques are presented. Special problems of structural layout arise in the transition regions between the cylindrical wall and the end slabs at the inside of the vessel. In these zones of high stresses several components such as the liner cooling system, its feeding pipes, liner anchorages, and a concentrated reinforcement arrangement in three or four directions meet. Placement of these different components and reinforcement in such a way that their installation remains practicable and such that proper compaction of concrete in these zones can be assured is a difficult task. The difficulties could be considerably reduced by using concrete reinforced with steel micro-reinforcement. The same benefits would result for the other zones of high concentration of reinforcement and components (for example standpipe regions and penetration zones) by using this wire-reinforced concrete. The advantages of this new material for the construction of entire PCPVs are discussed. Further important structural problems are the anchorage of the steel liner especially at the transition zone between the cylindrical wall and the end slabs and the anchorages of penetration liners. Different structural design possibilities and their respective design criteria are discussed. Their advantages and disadvantages are pointed out.

Prestressed concrete deep slabs with openings

A series of small-scale models of prestressed concrete reactor vessels, with and without perforations in the end slab, were loaded to failure. All vessels failed in shear. The presence of penetrations did not decrease the strength of the end slab. The behavior of the vessels is described. Using a finite-element model, the stress conditions in the end slab at the failure load are evaluated. A method for calculating the shear strength of a deep slab is described. The method consists of three consecutive steps. First, calculate the load at which the inclined crack is initiated. Second, determine the shape of the load-carrying dome that is carved out inside the slab. Third, calculate and evaluate the stress conditions in the dome.

Ultimate pressure analysis of deep end slabs for prestressed concrete pressure vessels

An analysis has been developed which described the possible ultimate strength of deep slabs under internal pressure. The analysis assumes a mode of failure which has been shown to occur experimentally under some conditions and it is demonstrated that this assumption gives reasonable bounds to the ultimate pressure for a considerable range of prestress. The analysis is based on an assessment of the strength of concrete under triaxial compressive stress and the data relating to strength under these conditions are reviewed.

Pressure tests on end slabs for prestressed concrete pressure vessels

With the present trend towards cylindrical shapes in the design of prestressed concrete pressure vessels it may be presumed that the majority of vessels for future reactors will involve the use of deep end slabs anchored to barrel walls by axial prestressing tendons. The importance of isolating the end slabs for experimental study has been pointed out by various reports on model tests of complete vessels. This article summarises the test results of a series of pressure tests on two types of slabs. The tests were designed primarily to study the influence of slab thickness and of prestress on the nature of the failure mode and on the magnitude of the ultimate pressure. Application of some of the conclusions reached to the design of end slabs for cylindrical PCPV are suggested.

Societies and Academies

Societies and Academies