Tangentiniai įtempiai gelžbetoninėse tiltų konstrukcijose nuo išankstinio apspaudimo

Abstract

In many bridges of box cross-section erected by free cantilevering method in Lithuania and abroad excessive diagonal cracking of walls is observed [1,2,3]. This cracking can not be explained with application of standard methods for calculation of tangential and priciple stresses since values of these stresses do not exceed limiting values of code [4]. Detailed computer aided stress analysis of a recently constructed bridge (Fig. 1) in which diagonal cracks have appeared before service, has been performed in which the actual geometry of beams, permanent and variable load, prestressing forces in the upper and bottom tendons, temperature actions and their combination are modeled (Table 1). It was obtained that tangential stresses (Fig. 1, 4a; Table 1), especially principle stresses tmax = (σ1 - σ3)/2 in the cracking zones substantially exceed the allowable limit of 3.5 MPa according to [4] and values obtained by calculation according to formula t = Q · S / (b · J). The principle stresses also, though in lesser extant, exceed allowable values. It is shown that horizontal prestressing cable forces cause M = constant and not cause any shear force Q = dM/dx = 0, because diagram of tangential stresses due to prestress along the wall is of two signs and the sum of resultants of stresses equals to zero (Fig. 1c, 4). Thus, these tangential stresses due to prestress cannot be evaluated by formuls (1). It is suggested to calculate these stresses using computers models and in simple cases it is proposed to apply in Fig. 2 and in expressions (2-4) developed formula (5) for assessment of tangential stress due to prestress in cables. Accuracy of this formula was verified by comparing analytical results with results obtained from computer aided calculation (Fig. 4b). An example of calculation of tangential stresses according to formula (5) is presented.