Sydorak D. Rational steel combined roof trusses

The main directions of development and improvement of steel trusses are minimization of mass and reduction of labor intensity. It was found that the development of new effective structural forms of roof trusses allows to reveal additional reserves of bearing capacity and to reduce the metal content of structures. An analysis of possible ways to improve the efficiency of combined steel roof trusses was carried out. It was established that the second effective method is the rational distribution of internal efforts. The main task of regulating the stress-strain state (SSS) in the combined truss is presented - obtaining a uniform structure, that is, the most rational system. It is established that for the development of new rational structural forms of steel trusses, it is necessary to use the principle of replacing traditional trusses with combined ones. The conditions for the construction of a new rational structural form of a combined steel truss (minimum mass) according to geometrical parameters (outline of the truss) and physical parameters (distribution of material between truss elements) are shown. The criterion of the rationality of the combined design is proposed - the simultaneous achievement by stresses of the caclulated strength of the material of the stiffening beam in the design sections of the stiffening beam for the main load. The general principles (minimization of material consumption and the principle of minimal technological costs) formation of new constructive combined systems - trusses and features of their use. The rational parameters of combined steel trusses with a span of 30 m were studied and obtained: a rational topology was chosen - a truss with parallel belts - a type of Pratt trusses with a lattice (N - truss) and resting on the upper belt; the ratio of the mass of the stiffness beam of the combined truss to 50% of the total weight of the truss; the angle of inclination of the truss racks is 78°-82°; a truss with the number of panels of the upper chord is six, and the upper chord of which has only five intermediate supports; calculation scheme - continuous upper and lower chords with a hinged connection of the grid; use of high-strength steel for braces. It is shown that the calculation of each type of such structures by known methods gives an uneven stress state along the length of the main element - the stiffness beam, which consists in a significant difference between support and span moments, and this makes them not always rational and economical. The problem of estimated regulation of SSS in steel combined trusses is formulated, the essence which consists in maximally ensuring the receiving uniform stress state in the design sections of the stiffening beam. It is stated that in order to improve the efficiency of steel combined trusses, minimize their mass and reduce labor intensity, in addition to calculation methods of SSS regulation, rational design is necessary. It was established that with the same loads and one span equal to 30 m, the mass of the truss according to DSTU B V.2.6-74:2008 is 2455.5 kg, the mass of the reference combined truss is 2022.1 kg, and the mass of the combined truss with calculated regulation ( eccentricity adjustment) – 1772.9 kg. It is stated that the mass of a combined truss with estimated regulation is 27.8% less than the mass of a truss according to DSTU and 12.3% less than a combined truss without SSS regulation. It is stated that the estimated regulation of SSS by changing the reference and of nodal eccentricities allows to make greater use of design reserves, without changing its reliability, in accordance with practically expedient values. The assessment of the effectiveness of SSS regulation by calculation was carried out method, which made it possible to use sections of a smaller area and reduce steel consumption from 12% to 17% for trusses with a span of 18 m, and from 20.5% to 25% for 24 m, respectively, from 12.5% to 18% for a span 30 m. A study of the operation of samples of combined steel trusses, an analysis of the SSS of the stiffening beam as the main element of the structure was carried out. The load-bearing capacity was checked according to groups I and II limit states in the elastic stage. It was established that the stress values in the sections do not exceed the calculated resistance values, the deflections do not go beyond the established limits. An evaluation of the technical, economic and environmental efficiency of rational combined steel trusses with SSS regulation was carried out. Four variants of trusses with different calculation schemes and strength characteristics are given. It is shown that in the rational construction of trusses the most it is advisable to use S420 grade steels to ensure the lowest mass, and the use of C345 high-strength steel and ordinary and C255 grade steels for some lattice elements ensures the lowest structural cost.