The project is devoted to the study of the stiffness of the connections between the brace and the truss. In case of lightweight roof structures, the wind load can create compression in the bottom chords. Therefore, in such cases, additional bracing system (including vertical braces) is often designed to protect the compressed bottom chord against buckling. However, there is an alternative solution (described in several scientific publications since 2014, more advantageous in terms of economy and utility), consisting the use of appropriate stiffeners connected with the tensioned upper chord. The aim of the research project is to determine the rotational stiffness of the connection between the purlin (which is one of the bracing system elements) and the top chord of the truss. The proper design of this connection stiffness and the flexural stiffness of the purlin may result in a several-fold increase of the truss critical load (reduction of the buckling length of the compressed bottom chord by several dozen percent), which was so far the subject of numerous parametric analysis. In the first stage of the project, it is planned to create numerical models of construction nodes consisting two perpendicularly situated elements, i.e. section of purlin and the truss upper chord, connected with gusset plates and to conduct calculations (FEM, GMNIA). In the next step the biaxial strength testing machine will be used to carry out the experimental tests of the above described elements. Based on the numerical models, the load generated by the machine servomotors will cause the rotation of the purlins in relation to the chord and so the rotational stiffness of the connection will be determined. It is planned to measure strains and the angles of rotation for the selected points using sensors connected to the recording equipment. The results of the experimental tests will be used to validate the numerical models of structural nodes. At the last stage of the project, it is planned to create numerical models of braced trusses and carry out calculations (FEM, bar and shell elements, LBA, GMNIA), which will allow to determine the impact of the above described connection stiffness on the critical and limit load of selected roof trusses subjected to wind loading. The results of the preliminary tests described in the project are necessary to conduct further analysis aimed to the optimization of connections used in roof truss structures.