Improvement of the method of calculating the strength of reinforced-concrete elements on the basis of inclined sections

Abstract

Improvement of methods of calculating the strength of elements, particularly calculation on the basis of inclined sections as one of the most prevalent ones in the practice of designing structures, largely determining the total material and labor expenditures consumed for the needs of construction, should be based on current data of experimental and theoretical investigations, on design and construction experience, and also on the data on the work and state of operating structures.One of the main directions in the process of improvement should be a unification of the methods of calculating elements of hydraulic structures with the main principles of calculation presently adopted for other types of reinforced-concrete elements and structures being designed according to building code SNiP 2.03.01-84 [I].It should be noted that the method adopted in SNiP 11-56-77 [2] for calculating the strength of elements on the basis of inclined sections under the effect of a transverse force and bending moment, which is based on equilibrium of t-he limit forces in the inclined section, was an important step forward compared with the rather arbitrary method of calculations based on principal tensile stresses during elastic work of elements of these structures.The method of calculating the strength of reinforced-concrete elements based on inclined sections (formula (34) in [2]) mainly reflects the regularities of the change in the transverse force Qc being absorbed by the concrete of the compressed zone in the inclined section as a function of the coefficient of longitudinal reinforcement ~s = As/bho, change in the bending moment M and character of the application of the longitudinal forces N, and geometric parameters and strength characteristics of the concrete.In this case the use in calculations of the coefficient k = 0.5 +25, where $ = (As/bho)(Rs/Rp ) • (N/bhoRp) is insufficiently substantiated and primarily by virtue of the fact that failure of elements having ~ > 0.005 occurs along inclined cracks at stresses in the longitudinal reinforcement not exceeding its yield point, whereas the bearing capacity of lightly reinforced elements is mainly determined by exhaustion of the strength along a noraml section [3,4].It should be noted that calculations with the use in formula (34) of SNiP 11-56-77 of the design axial tensile strength of concrete Rpt and prism strength Rp for low grades of concrete gives a more substantial change in the relative strenth of the elements upon a change in the coefficient of longitudinal reinforcement Us (for example, K = 0.5+ 107u s for flexible elements for concrete MI50, reinforcement of class A-Ill, and tan B = i) than in the case of using concretes of higher grades (k = 0.5 + 43~ s for concrete M400, reinforcement A-Ill, and tan B = i).The given example indicates some contradiction of the initial premises of the method of calculating elements on the basis of inclined sections [2], which can lead to unsubstantiated results when evaluating the strength of structures intended for long service.