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.

Introduction. The relevance of the topic under study is justified by the discrepancy of the operation data of a wide range of projects of steel structures of building roofs implemented according to the standard series with the instructions of the addendum to p. 15.4.6 of SК 16.13330.2017 "Steel Structures" (rev. 3) on the restriction of the use of purlins as bonding elements.

Aim. A computational and analytical substantiation of the mechanical process of work of the purlins as part of the roofing structures and the phenomenon of changes in its stress-strain state during loading for the actual conditions of connection with the chords of the trusses.

Materials and methods. Particular attention is paid to single-span purlins and their role as bracing elements of compressed chords of trusses when working in the direction from the plane of the truss. Для численного расчета конструкций прогонов использован метод конечных элементов, реализованный в программном комплексе ЛИРА САПР. Предварительно прогон рассматривался как как изгибаемый стержень (балка) на нагрузки от собственного веса, веса кровли и снега. For numerical analysis of purlin structures was used the finite element method implemented in the LIRA SAPR software package. The purlin was previously considered as a bending rod (beam) for loads from its own weight, the weight of roof and snow.

Results. The work of purlins as spacers dividing the compressed chords of trusses into design compartments is substantiated. It has been established that the fictitious force arising from compression of the upper chords of trusses does not have a decisive effect on the load-bearing capacity of purlins, but has an unloading effect when the work of the purlin is combined with the action of the transverse load.

Conclusions. The influence of the determining factors characterizing the inclusion of the purlin in the collaborative work with the roof structures, such as the blackness of the bolt hole, the tightening of the bolted joint force, the influence of the eccentricity of the support of the purlins to the chords of the truss, has been studied. The conducted study allowed us to draw the following conclusions: a purlin is an off-centre-loaded rod that supports loads from its own weight of structures, snow, wind and fictitious forces; by control parameters it was established that the inclusion of the purlins in the work as a bracing element occurs; the combination of the functions of the supporting structure and the spacer in one element increases the strength of the purlins.