Zur Durchstanzbemessung von ausmittig beanspruchten Stützenknoten und Einzelfundamenten nach Eurocode 2

Punching Shear Capacity of Column‐Slab Connections with Moment Transfer and Footings According to Eurocode 2 The design provisions for punching of interior column‐slab connections without moment transfer are not sufficient for practical design issues. Very often, an additional moment transfer between slab and column has to be considered. While for interior columns moment transfer is caused by unequal spans or loadings, it is unavoidable for edge and corner columns. In addition, the determination of the punching shear capacity of footings is a very common problem. Compared to the German Building Code DIN 1045‐1, the Eurocode 2 provides different provisions to account for an interaction between moment and shear, and to determine the punching shear capacity of footings. In this paper, the Eurocode 2 provisions are critically reviewed and compared to the ultimate capacities of tests from literature.     

钢筋混凝土锥形基础的优化设计

以基础总造价为目标函数,以国家规范要求的地基承载力、抗弯和抗冲切强度等为约束条件,给出了钢筋混凝土锥形基础优化设计的数学模型;借助于MATLAB优化工具箱,通过编程,实现了此类基础的优化设计。算例表明,该优化设计方法经济效益显著,可推广应用于工程设计。     

Zur maximalen Durchstanztragfähigkeit von Flachdecken.

Maximum Punching Capacity of Flat Slabes Eurocode 2 assesses the maximum punching shear resistance by limiting the nominal shear stress at the periphery of the load to a value equal to the web‐crushing limit of beams with shear reinforcement. However, experimental investigations revealed that in most cases the failure occurs at a lower load level once the compression zone at the column face crushes. Furthermore, Eurocode 2 overestimates the maximum shear punching capacity for slabs including stirrups compared to the capacity of slabs including double‐headed studs according to construction approvals. To verify the maximum punching shear capacity according to Eurocode 2, at the Institute of Structural Concrete, RWTH Aachen University a punching test on a slab including stirrups as shear reinforcement was conducted and, in addition, tests from literature were evaluated.     

Effect of footing shape and load eccentricity on behavior of geosynthetic reinforced sand bed

This paper presents the results from a laboratory modeling tests and numerical studies carried out on circular and square footings assuming the same plan area that rests on geosynthetic reinforced sand bed. The effects of the depth of the first and second layers of reinforcement, number of reinforcement layers on bearing capacity of the footings in central and eccentral loadings are investigated. The results indicated that in unreinforced condition, the ultimate bearing capacity is almost equal for both of the footings; but with reinforcing and increasing the number of reinforcement layers the ultimate bearing capacity of circular footing increased in a higher rate compared to square footing in both central and eccentrial loadings. The beneficial effect of a geosynthetic inclusion is largely dependent on the shape of footings. Also, by increasing the number of reinforcement layers, the tilt of circular footing decreased more than square footing. The SR (settlement reduction) of the reinforced condition shows that settlement at ultimate bearing capacity is heavily dependent on load eccentricity and is not significantly different from that for the unreinforced one. Also, close match between the experimental and numerical load-settlement curves and trend lines shown that the modeling approach utilized in this study can be reasonably adapted for reinforced soil applications.     

钢筋砼板和基础的极限承载力计算及破坏类型判别方法

本文通过对国内外大量的钢筋砼板和基础试验资料的分析,划分了三种不同类型的破坏形态,即弯曲破坏、弯冲破坏及冲切破坏,建议了各类破坏形态的极限承载力计算公式,并提出了判别各种破坏类型的简便方法,计算结果与试验结果符合良好。     

Effects of Footing Size and Aspect Ratio on the Bearing Capacity of Sand Subjected to Eccentric Loading

The current practice of estimating bearing capacity usually employs the conventional bearing capacity formula originally developed for strip footings under vertical central loading. In order to account for the effect of footing shape and eccentricity and inclination of loads, correction factors are introduced in the formula, which are derived based on a number of small-scale model test observations.This paper describes research on the bearing capacity of rectangular footings on sand subjected to vertical eccentric loading. Two aspects, namely the effects of footing size and of footing shape on the bearing capacity and deformation characteristics, are focused on. A series of loading tests was conducted in a centrifuge on rectangular footings with aspect ratios from 1 to 5, at two different centrifugal accelerations. In addition, finite element analyses were performed in which factors influencing the angle of shear resistance including stress level dependency, anisotropy and coefficient of intermediate principal stress, were taken into account.It was found that the shape factor of footing apparently increased with increasing footing width. This indicates that the shape factor used in the current practice underestimates bearing capacity of footings. This was also the case for failure locus in the M/B-V (moment-vertical) load plane. Normalized failure locus for wider footings with a smaller aspect ratio is considerably larger than that reported in the literature. The stress level dependency of the angle of shear resistance appeared to be responsible for the scale effects of footings on the failure locus.     

Flachdecken in Elementbauweise. Hinweise zum Durchstanznachweis nach DIN 1045‐1

Flat Slabs built with Semi‐Precast Elements. Advices to Punching Shear Verification according DIN 1045‐1 Flat slabs are increasingly built with precast slabs and insitu topping. The bearing behaviour of these semi‐precast slabs with lattice girders is similar to cast in one concrete slabs. In principle this is also applied for areas where punching failure is endangered. This was shown in full‐scale tests, which were taken as the basis for derivation of design rules. However during design of semi‐precast slabs some specific items have to be considered. The revision of technical approvals for punching shear reinforcements to the new German design standard for concrete and reinforced concrete DIN 1045‐1 took place during the introduction of new punching shear reinforcement for semi‐precast elements. The required verification of punching shear in element slabs are arranged and explained for different types of punching shear reinforcement.     

Punching shear tests on flat concrete slabs exposed to fire

Underground parking structures often consist of flat slabs connected by columns, for which punching shear is often the most critical design criterion. In fire conditions, the punching load can increase due to restraint of the thermal curvature of the slab or due to the expansion of the columns. This increase of the punching load is discussed in the paper by means of a literature review. On the other hand, during fire the punching resistance of the slab decreases due to a gradual reduction of the material properties. This reduction in bearing capacity is studied by means of real scale fire tests, consisting of 6 slabs measuring 3.2×3.5×0.25 m with a connected column stub and tested for punching shear with a specially designed loading frame. Two reference tests are executed at ambient temperature conditions and four slabs are submitted to ISO 834 curve for 120 min. Comparison of the test data with the expected increased axial load due to thermal restraint found in the literature, shows a potential danger for premature punching failure of flat slab-column connections exposed to fire.     

钢-超高性能混凝土组合板冲切性能试验研究和承载力计算

钢-超高性能混凝土(UHPC)组合板是将钢板与UHPC通过连接件组合成整体,具有高强、高延性、抗开裂、施工便利等特性,可应用于桥面板、防护工程等结构中.由于钢-UHPC组合板相对较薄且往往承受集中荷载,因此需要对其抗冲切性能进行重点研究.通过变化连接件参数、UHPC厚度、钢板厚度和加载区边长,完成了14块板件在集中荷载...     

非均质介质破坏机制及承载力的有限元分析

地基承载介质的破坏模式将直接影响到地基承载力计算方法的选择。为了有效地对非均质成层介质的承载力问题进行分析研究,编制了有限元程序对其破坏机制进行模拟计算。结合原位载荷试验,利用试验数据对有限元分析结果进行了验证,表明有限元计算结果是可信的。在此基础上对非均质成层介质地基在外部载荷作用下内部塑性变形和破坏面的形状进行计算分析,结果表明:非均质介质地基的破坏模式主要是整体剪切破坏和局部剪切破坏两种形式;塑性变形首先出现在基础边缘,随着外载荷的增大,塑性变形区域在非均质介质中逐渐扩展并最终形成一个贯通的塑性区。     

Mechanism of Bearing Capacity of Spread Footings Reinforced with Micropiles

The Hyogoken-Nambu Earthquake in 1995 caused extensive damages to the foundations of bridges. Ever since, methods to improve the bearing capacity of existing foundations have become an important aspect of foundation engineering in Japan. Micropiles are considered to provide promising solutions. The mechanism which enhances the bearing capacity of surface footings reinforced with micropiles is the subject of investigation in this study. As an initial phase, model tests were conducted to understand the load-displacement behavior of surface footings with and without micropiles on loose, medium dense, and dense layers of sands. Salient factors which influence the behavior of the footings were selected and their influence on bearing capacity was examined through a comprehensive series of model tests. Notable improvements in the bearing capacity of surface footings reinforced with vertical micropile groups were observed in the case of dense sand which is dilative during shear. To assess quantitatively the degree of improvement in the bearing capacity of surface footings reinforced with micropiles, an index R called “Network Effect Index” was introduced in this study. The index R of unity means that the bearing capacity of footings reinforced with micropiles is simply equal to the summation of the individual value of the surface footing and that of the micropile group. An index R of more than two is achieved in this study where surface footings reinforced with a group of vertical micropiles bear on a dense layer of dilative sand. By contrast, with loose and medium dense sand, which are contractive in nature, the index R is found to be less than unity.     

浅基础承载力离心模型试验研究

笔者利用容量为10gt的离心机对砂基上的浅基础进行了较为系统的试验,用以研究基础的尺寸、形状、埋深和砂土相对密度对浅基础承载力、承载力因数（Nr,Nq）、形状因数（ζr,ζq）及破坏型式的影响。笔者提出一种利用浅基础离心模拟试验资料确定浅基础的承载力因数和形状因数的方法。利用此方法确定的44T-4砂的承载力因数,形状因数与已有的各种理论解和试验解进行了对比。研究工作得出了一些有意义的结论。     

Prediction of bearing capacity of circular footings on soft clay stabilized with granular soil

The shortage of available and suitable construction sites in city centres has led to the increased use of problematic areas, where the bearing capacity of the underlying deposits is very low. The reinforcement of these problematic soils with granular fill layers is one of the soil improvement techniques that are widely used. Problematic soil behaviour can be improved by totally or partially replacing the inadequate soils with layers of compacted granular fill. The study presented herein describes the use of artificial neural networks (ANNs), and the multi-linear regression model (MLR) to predict the bearing capacity of circular shallow footings supported by layers of compacted granular fill over natural clay soil. The data used in running the network models have been obtained from an extensive series of field tests, including large-scale footing diameters. The field tests were performed using seven different footing diameters, up to 0.90 m, and three different granular fill layer thicknesses. The results indicate that the use of granular fill layers over natural clay soil has a considerable effect on the bearing capacity characteristics and that the ANN model serves as a simple and reliable tool for predicting the bearing capacity of circular footings in stabilized natural clay soil.     

Pile End-Bearing Capacity of Sand Related to Soil Compressibility

A simple method based on theoretical and experimental considerations is presented to predict the pile end bearing capacity and load-settlement curve in sands in relation to soil compressibility. The key to the method is in the assumption of a failure mode with a spherical cavity expansion pressure given as a function of the soil compressibility, shear stiffness and friction angle. The practical prediction of pile end-bearing capacity in sandy ground is discussed in terms of a compressibility factor, friction angle and shear stiffness. Further, the pile tip settlement behaviour is also discussed based on an empirical model incorporating the pile end bearing capacity from a cavity expansion analysis with a Kondner type hyperbolic function. The applicability of the proposed method is verified by comparing the predicted results with the results from a series of model pile load tests and a database for in-situ pile load tests in sands.     

Zum Durchstanzen von Flachdecken – Einfluß der Momenten‐Querkraft Interaktion und der Vorspannung

Punching of flat slabs considering prestressing and momentshear interaction. The bending moment of the framework effect causes an additional loading of the punching area of edge and corner columns. This leads to a significant moment‐shear interaction. A computational model was developed based on test results, theoretical investigations and on non‐linear three‐dimensional FE simulations to predict the punching resistance of edge and corner column slab connections taking into account the effect of eccentricity and prestressing. According to the initial calculation a load factor is presented resulting from the moment‐shear interaction of the edge and corner columns which can be used in DIN 1045‐1 and EC 2 instead of the former constant factors.     

Einfluß der Deckenschlankheit auf den Durchstanzwiderstand nach DIN 1045‐1, SIA 262, Ö‐Norm B 4700(01) und Eurocode prEN 1992‐1‐1

Influence of the Slab Slenderness on the Punching Resistance according to DIN 1045‐1, SIA 262, Ö‐Norm B 4700(01) and Eurocode prEN 1992‐1‐1 In the last three years the new design codes [1] — [4] were established. The punching resistance of each code was developed on the state of the art. But the formulas show significant differences. Parametric studies documented, that the punching resistance of less slender flat slabs ( λ = length between the supports / effective depth = l/d = 20) is increased according to SIA 262 and Ö‐Norm B 4700(01) in comparison to DIN 1045‐1. EC prEN 1992‐1‐1 neglects the influence of the flat slenderness, however the maximum punching capacity of stirrups is on the design level of double headed studs or stud‐rails. This is contradictory to the current design experience and punching test results [5]. In this contribution the influence of the slab slenderness on the punching resistance with and without shear reinforcement is discussed. In addition, the extremely high maximum punching shear capacity according prEN 1992‐1‐1 is judged by the codes DIN 1045‐1, SIA 262 and Ö‐Norm B 4700(01).     

Behavior of closely spaced square and circular footings on reinforced sand

This paper describes an experimental investigation conducted to evaluate the ultimate bearing capacity, the settlement and the tilt of two types closely spaced footings, one having square shapes and the other having circular shapes, on unreinforced and reinforced soil. To decrease the objectionable influence of interference on the performance of the closely spaced footings, the foundation soil is reinforced by geogrid layers. The results of this reinforcement show both positive and negative effects, namely, a positive effect because there is a considerable increase in the ultimate bearing capacity, and a negative effect because there is an increase in settlement and tilt. Regarding the experimental results, the negative effect of interference can be decreased considerably through the use of soil reinforcements. The ultimate bearing capacity of the interfering footings increased by about 25–40%, whereas the settlement of the interfering footings at the ultimate load increased in the range of 60–100%. However, the closely spaced footings tilted by approximately 45% and 75% for reinforced sand with one and two layers of geogrid, respectively.     

Experimental investigation of edge restraint on punching shear behaviour of RC slabs

Current codes of practice usually do not consider the effect of edge restraint on the punching shear capacity of flat plate type reinforced concrete structures. As the punching shear provisions incorporated in various codes of practice are a direct result of the empirical procedures, they do not usually provide an accurate estimation of the ultimate punching load capacity of a slab with its edges restrained against rotation. This is because no account is taken of the enhancement of punching capacity due to the in-plane restraint in many types of reinforced concrete slab systems. A total of 16 model slabs with restrained and unrestrained edges have been tested in an effort to ascertain the influence of boundary restraint, thickness of the slabs on their structural behaviour and punching load-carrying capacity. Edge restraint has been provided by means of edge beams of various dimensions in order to mimic the behaviour of continuous slabs. The cracking pattern and load-deflection behaviour of the slabs tested have also been monitored closely.     

Total strain FE model for reinforced concrete floors on piles

A finite element (FE) model using a total strain material model has been developed to predict the behavior of warehouse reinforced concrete floors on piles. The material model (not the FE model itself) was calibrated to material tests. The FE model for the floor structure was checked with full‐scale experiments. For a warehouse, punching load optimization and surface crack control are important design factors. It is concluded that if calibrated material models are used, total strain‐based FE models are able to indicate surface crack width and punching strength for several types of reinforcement. Furthermore, it is concluded that it is possible to develop a total strain material model from material tests. Copyright © 2007 John Wiley & Sons, Ltd.     

各向异性软土地基极限承载力计算

将土的各向异性不排水剪强度应用到滑移线场理论中,得出不等向团结(K_0固结)粘土地基极限承载力系数的理论解.对不同试验条件下的极限承载力系数及其与K_O或(?)(有效内摩擦角)的关系进行分析,得到一些有益的结果.通过模型试验的计算结果与实测值的比较,证明本文理论的合理性和有效性.理论分析及计算结果表明,对于K_0固结粘土,实践中通常使用的极限承载力系数N_(cu)=5.14,如不经修正是不适宜的.最后通过现场载荷试验和工程实例对本文方法和常规各向同性方法进行了比较分析.