When you specify partial fixity by either of the `proportional' options, the spring stiffness is calculated from the section properties and length of the current member.
This is fine for single span members but overestimates the stiffness if the span is divided into sub-members because of incoming secondary beams etc.
The current version of the program does not `know' when the current member is only a segment of a longer full span. The members marked red in the illustration are only a small portion of the span between columns so a calculation based on their stiffness 4EI/L is incorrect . The parallel beams which are not so divided return correct values for spring stiffness based on proportional fixity.
You can easily adjust for this as follows using the Member Attributes dialog.
After inputting partial fixity as a proportion of member stiffness or as a proportion of fully fixed, select the `absolute stiffness' option. This causes the equivalent spring stiffness to be displayed in kNm/radian. Multiply this value by the local member length, then divide by the span length and input the result into the absolute stiffness field. We plan to upgrade the software to deal with this issue automatically.
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