I created a finite element model of steel truss bridge and tried to do a modal analysis to generate the natural frequencies and mode shapes. Unfortunately, i don't get deformed shapes, but rather the SAP 2000 software will add more constraints at the middle spans of the bridge. Furthermore, negative Eigen values are also generated. Please i need help. Find the screen shot of the bridge before the modal analysis and after the modal analysis.
Dear Omoniyi Moses Tope
The negative eigenvalues represent buckling modes and refer to the reversed direction of loads. have you considered changing the load's directions?
Dear Mehran Shahpari,
Thank you for your timely response. I have not considered anything regarding the loads. The only loads I used for the trusses are the gravity loads and wind loads. Are to these what you are referring to sir?
For the truss element, if you discrete one of the truss girder into a series of elements, you should constrain all the rotational degrees for the nodes between the two end nodes. I am not sure if this is the reason for the error but you can check it.
Dear Haifeng He
Thank you for the response. Do you mean to do the discretisation for all the truss element in the section having this issue?
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