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At the center of attention of participants of the international Symposium on Analytical Evaluation of Dam Related Safety Problems held on July 5, 1989 in Denmark (Copenhagen)* was, among others, the problem of increasing reliability of massive structural members of hydraulic structures and provision of their safety during operation [15]. One of the ways to solve these most important problems is based on the more complete consideration of the characteristics of the formation and development of cracks in reinforced-concrete members in a general evaluation of the strength of structures.Economic indices, remaining as usual dominant when comparing variants at the feasibility study stage of the structure being planned, should also optimally conform with the requirements of reliability and safety of the operating hydraulic structures.The role of these requirements is steadily increasing under conditions of the current ecological situation.For a long time the main criterion of the urgency of research was considered a guaranteed economy (scientifically unfounded in certain cases) of construction materials and labor expenditures as a result of the proposed realization of the results of the planned investigations.The practice of operating hydraulic structures showed that such a trend toward cost effectiveness (in a number of cases at the expense of reducing the safety factor of the members) had a negative effect when drawing up individual points of the design standards of reinforced-concrete members.Thus, the current standards do not take into account the possible formation and intense development along the longitudinal concreting block joint of secondary cracks separating the structural member and the transformation of the initial stresses into a secondary stress field accompanying such a phenomenon.At the same time, the investigations of Soviet and foreign specialists into the formation of secondary cracks showed their negative effect on the strength of individual zones of massive members and reliability of the structure as a whole as a consequence of insufficient transverse reinforcement [i, 2, 3, 4, 15]. in this connection, the approach to a calculated evaluation of transverse reinforcement of massive members of hydraulic structures on the basis of secondary stress fields is prospective [5].The principle of calculation on the basis of secondary stress fields consists in evaluating the new stress state of the member occurring after the formation in it of a system of initial (or primary) main cracks or after opening of the transverse block joints.The analytical dependences of such a method of calculation were developed by an experimental theoretical analysis of the results of calculations by numerical methods (finite-element method) of the stress state of tested beam-type reinforced concrete models [2,6,7, 14].A change from an elastic stress state to the real behavior of the member at the limit stage is most effective, since the limit state of massive members cannot be substantiated by experimental data.in addition to the...

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.