Hello all,
I posted earlier with this problem but I dont think I was clear enough in stating what my problem is. I am using version SAP 8.2.3 and I have a small, square, one storey structure (concrete unreinforced) and ran the following time histories using EL centro (just one lateral component) with no other loading not even gravity in one direction:
-linear modal time history
-linear time history using direct integration
-non-linear time history with direct integration with specified hinges in the frame members
The record I have is 40 seconds and I examined the structure 20 seconds after the record had finished (T=60)
My problem is that when I examine the sum of the reactions of this building at 40 seconds and even at 60 seconds for all 3 cases I end up with an imbalance of forces. That is, The sum of the forces in the lateral direction does not equal zero. Now I dont expect there to be no force in the base of the structure, however the sum of these reactions should be zero. If I take the resultant of the reactions in my structure at T=60 and divide it by the maximum reaction in my structure I get around 0.3% to 0.7% which isnt much but seems high. What really bothers me is that even the modal time history gives me a "residual" resultant in the structure a full 20 seconds after the record has finished. It just doenst make sense since I have no other loads what so ever not even gravity.
Any ideas what I can do to decrease this imbalance? I tried playing with tolerances in the analysis cases themselves and got very very little difference in the results. I used another record and still got an imbalance so it isnt just this record. However, the imbalance was a different number for this different record so the imbalance seems to depend on the record itself.
Thanks!
AR
I posted earlier with this problem but I dont think I was clear enough in stating what my problem is. I am using version SAP 8.2.3 and I have a small, square, one storey structure (concrete unreinforced) and ran the following time histories using EL centro (just one lateral component) with no other loading not even gravity in one direction:
-linear modal time history
-linear time history using direct integration
-non-linear time history with direct integration with specified hinges in the frame members
The record I have is 40 seconds and I examined the structure 20 seconds after the record had finished (T=60)
My problem is that when I examine the sum of the reactions of this building at 40 seconds and even at 60 seconds for all 3 cases I end up with an imbalance of forces. That is, The sum of the forces in the lateral direction does not equal zero. Now I dont expect there to be no force in the base of the structure, however the sum of these reactions should be zero. If I take the resultant of the reactions in my structure at T=60 and divide it by the maximum reaction in my structure I get around 0.3% to 0.7% which isnt much but seems high. What really bothers me is that even the modal time history gives me a "residual" resultant in the structure a full 20 seconds after the record has finished. It just doenst make sense since I have no other loads what so ever not even gravity.
Any ideas what I can do to decrease this imbalance? I tried playing with tolerances in the analysis cases themselves and got very very little difference in the results. I used another record and still got an imbalance so it isnt just this record. However, the imbalance was a different number for this different record so the imbalance seems to depend on the record itself.
Thanks!
AR