according to the invention fig1 shows in a perspective view a second embodiment of a fixing plate fig1 shows a second embodiment of locking the fixing plate to the beam fig1 shows in a top view the second embodiment the design of the invention and further advantages are described more in detail below in connection with the fig1 13 fig1 illustrates a recommended embodiment of a fixing device in the system according to the invention four fixing plates 1 of different kinds are in pairs holding two crossing beams 2 fixed between them by means of tightening bolts 3 and 9 from the sides of the fixing plate longitudinal strips 4 extend in the main directions of the beams fig2 illustrates the same montage of the fixing device seen in cross section the thrust in the bolts 3 and 9 causes fastening friction forces in the contact surfaces 21 between the fixing plates and the beams the resistance against side movements is strengthened by stop screws 5 according to a recommended design extends from de longitudinal strips 4 and provide a shape determined locking of the beams the stop screws are also used to temporarily keep the fixing plates 1 in position during the mounting the stop screws 5 are also used to adjust the side position of the fixing plates with respect to the beams fig2 further shows how the fixing plates 1 on each side of the beams can be tightened according to two alternatives the primary alternative is shown to the left in fig2 in this case the tightening bolts 3 engage the threaded sleeves 6 that are common to the two beams 2 and are located at the surfaces 30 opposite to each other of the two adjacent fixing plates these sleeves 6.