In the American Steel Code AISC358, pre-approved fixed connections are introduced, some of which are given in the tenth topic of the Iranian National Building Code. Direct connection of a beam with a reduced cross-section, direct unreinforced welded connection, welded connection using header and skirt plates, bolted connection using header and skirt plates, four-bolt flange connection without using a lagging plate and four or eight-bolt flange connection using a lagging plate are such connections. In conventional designs, the type of connection is not affected by the modeling by the software. And the mentioned bending connections are all defined in the same way in the software. While the type of connection can play an important role in the stiffness and also the seismic requirement of the frame. In this research, we intend to obtain the lateral stiffness of these frames by laboratory prototyping of a bending frame and modeling in software for three types of unreinforced direct welded connections, welded connections using header and flange plates, and bolted connections using header and flange plates, and compare the obtained values with each other.