1、外文原文 I： A fundamental explanation of the behaviour of reinforced concrete beams in flexure based on the properties of concrete under multiaxial stress M. D. Kotsovos Department of Civil Engineering, Imperial College of Science and Technology, London (U. K.) The paper questions the validity of the ge
2、nerally accepted view that for a reinforced concretestructure to exhibit ductile behaviour under increasing load it is necessary for the stressstrain relationships of concrete to have a gradually descending post-ultimate branch.Experimental data are presented for reinforced concrete beams in bending
3、 which indicate the presence of longitudinal compressive strains on the compressive face in excess of 0.0035. It is shown that these strains, which are essential for ductile behaviour, are caused by acomplex multiaxial compressive state of stress below ultimate strength rather than postultimate mate
4、rial characteristics. The presence of a complex stress system provides a fundamental explanation for beam behaviour which does not affect existing design procedures. 1. INTRODUCTION The plane sections theory not, only is generally considered to describe realistically the deformation response of rein
5、forced and prestressed concrete beams under flexure and axial load, but is also formulated so that it provides a design tool noted for both its effectiveness and simplicity 1. The theory describes analytically the relationship between load-carrying capacity and geometric characteristics of a beam by
6、 considering the equilibrium conditions at critical cross-sections. Compatibility of deformation is satisfied by the plane cross-sections remain plane assumption and the longitudinal concrete and steel stresses are evaluated by the material stress-strain characteristics. Transverse stresses and stra
7、ins are ignored for the purposes of simplicity. The stress-strain characteristics of concrete in compression are considered to be adequately described by the deformational response of concrete specimens such as prisms or cylinders under uniaxial compression and the stress distribution in the compres
8、sion zone of a cross-section at the ultimate limit state, as proposed by current codes of practice such as CP 110 1, exhibits a shape similar to that shown in figure 1. The figure indicates that the longitudinal stress increases with the distance from the neutral axis up to a maximum value and then
9、remains constant. Such a shape of stress distribution has been arrived at on the basis of both safety considerations and the widely held view that the stress-strain relationship of concrete in compression consists of both an ascending and a gradually descending portion (seefig. 2). The portion beyon
10、d ultimate defines the post-ultimate stress capacity of 宁波工程学院毕业设计（论文） 1 the material which, Typical stress-strain relationship for concrete in compression. as indicated in figure 1, is generally considered to make a major contribution to the maximum load-carrying capacity of the beam. However, a re
11、cent analytical investigation of the behaviour of concrete under concentrations of load has indicated that the post-ultimate strength deformational response of concrete under compressive states of stress has no apparent effect on the overall behaviour of the structural forms investigated ( 2, 3). If
12、 such behaviour is typical for any structure, then the large compressive strains (in excess of 0.0035) measured on the top surface of a reinforced concrete beam at its ultimate limit state (see fig. 1), cannot be attributed to post-ultimate uniaxial stress-strain characteristics. Furthermore, since
13、the compressive strain at the ultimate strength level of any concrete under uniaxial compression is of the order of 0.002 (see fig. 2), it would appear that a realistic prediction of the beam response under load cannot be based solely on the ascending portion of the uniaxial stress-strain relationsh
14、ip of concrete. In view of the above, the work described in the following appraises the widely held view that a uniaxial stress-strain relationship consisting of an ascending and a gradually descending portion is essential for the realistic description of the behaviour of a reinforced concrete beam
15、in flexure. Results obtained from beams subjected to flexure under two-point loading indicate that the large strains exhibited by concrete in the compression zone of the beams are due to a triaxial state of stress rather than the uniaxial post-ultimate stress-strain characteristics of concrete. It i
16、s shown that the assumption that the material itself suffers a completeand immediate loss of load-carrying capacity when ultimate strength is exceeded is compatible with the observed ductile structural behaviour as indicated by load-deflexion or moment-rotation relationships. 2. EXPERIMENTAL DETAILS
17、 2.1. Specimens Three rectangular reinforced concrete beams of 915 mm span and 102 mm height x 51 mm width cross-section were subjected to two-point load with shear spans of 305 mm (see fig. 3). The tension reinforcement consisted of two 6 mm diameter bars with a yield load of 11.8 kN. The bars were bent back at the ends of the beams so as to provide compression reinforcement along the whole length of the shear spans.Compression and tension reinforcement along each shear span were linked by seven 3.2 mm diameter stirrups. Neither compression reinforcement nor stirrups were
