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Imbalance of Forces

Imbalance of Forces

Imbalance of Forces

(OP)
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

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