Portal Frame Analysis

[axle]

Hi there, I am after some advice on the analysis of an existing portal frame building using the structural analysis program ‘Microstran’. The portal frame is a standard portal frame consisting of I section columns and rafters. In addition, the portal frame also has a separate column approximately 600mm inside the main portal columns on each side of the portal frame to support an overhead traveling crane. The major x-axis of this crane support column is at right angles to the major x-axis of the main portal columns. I.e. the major x-axis of the crane support column is in the plane of the portal frame. This crane support column is braced back to the main portal columns via horizontal I section members spaced at approximately 1000mm centers that are fully welded to the flange of the main portal columns and also fully welded to the web of the crane support columns. This is where my question arises. When modeling this structure in ‘Microstran’ do I model the connection of these horizontal members to the main portal columns and the crane support columns as ‘pin connections’? Is it the same as with a truss? I.e. even though the connections at the nodes are fixed it is analyzed as though it is pinned. I have run the analysis of the frame through ‘Microstran’ with the connection of these horizontal members assumed as ‘fixed’ and I obtain very large axial loads in the members. If these members are to be assumed to have ‘pin connections’ at each end, why is this so when they are fully welded. Any help/advise would be greatly appreciated. Thanking you for your time. Axle.

 

[111213]

hi,

this is an construction quite common.

the portal frame gets the horizontal load at the
height of its rafter

this means that for global analysis you just connect the
portal frame at that point to the other structure

the other connections at the column are to avoid
problems of stability

please excuse my poor english