Analytical justification of influence of purses on the work of steel roof trusses compressed chain belt

Introduction. Paper is devoted to analytical justification of purlins operation as struts based on the study of mechanical processes and stress-strain state. Paper presents additional factors characterizing the inclusion of purlin in joint work with the pavement structures, such as the blackness of the bolt hole, the tightening force of the bolt connection, the influence of the eccentricity of purlin resting on the top chord of truss. As a result of study it was established that pliability of purlin connection to the top chord of truss is not a reason to prohibit the use of purlin as a strut and does not deteriorate its mechanical properties. Methods. In order to assess the influence of joint pliability purlins’ displacements of when working as part of pavement structures were analyzed, including the eccentricity of the attachment, in spatial formulation of the problem, taking into account the value of bolt tightening force and the influence of friction forces. For numerical calculation of purlin structures used the finite element method implemented in the LIRA CAD software package. Preliminarily, the purlin was considered as a bending rod (beam) for loads from its own weight, roof weight, wind and snow. Results. The displacements of the upper chords in direction of horizontal load exceed the backlash (1 – 1.5 mm) and vary in range from 0.324 mm to 3.2 mm. Thus, displacements of truss girdles exceed the margin of pliability of connection and characterize operation of the rough hole, including in operation bolted connection of purlin to truss girdle. Inclusion of purlin in work of panel of the upper (compressed) girdle of truss occurs due to triggering of the backlash and the calculated initial imperfection exceeding its value. The calculation methodology for centrally compressed bars includes imperfections exceeding in value the black hole, thus characterizing the insignificance of the possible initial backlash. The friction force arising from uncontrolled tightening of the bolted connection of the purlin to the truss cannot completely prevent the shear forces, forcing the backlash to actuate and include the purlin in operation with the upper chords of the pavement trusses. Discussion. The support of the purlin on the top belt with vertical eccentricity has an insignificant effect on the displacement of the pavement structures out of their plane of operation, allowing this parameter not to be taken into account in the design scheme. The loading of the purlin with compression forces has a favorable effect on the load-bearing capacity of the purlin, reducing the span moment. The fastening of purlins, as well as of links, is carried out on bolts with the same accuracy (with the same hole blackness). The blackness of bolted connection hole triggers the operation of purlin in the same way as it triggers the operation of braces and struts.