Hello there; First, enter the purlin between two consecutive beams with the r purlin command. Then, after entering the steel beam command and adjusting the section to work from the middle, use the element you entered with the r purlin as a base and pass over it with the steel beam command. In this way, your work will be easier as you can catch the nodal points. Then, you can delete the element you created with the R purlin command and duplicate the element created with the beam with the clone command according to the range you specify. In order to determine the locations on the main beam where you will copy the purlins, you can select the main beam and use the right click on the object special capture point option. Good work.