Sap2000 base reaction vs joint reaction

[zeppelin_me]

thread801-230670
Answer: Response-spectrum base reactions will not match the sum of individual joint reactions because their formulations differ. Base reactions are calculated for each mode before modes are combined using the CQC or SRSS modal-combination rule. Joint reactions, on the other hand, are calculated using modal combinations which are applied to each individual joint.
For example, consider a structural system with four joints and two modes. The SRSS formulation is used. Base and point reactions are generated as follows:
Mode 1:

Point 1 reaction = R(1,1)
Point 2 reaction = R(2,1)
Point 3 reaction = R(3,1)
Point 4 reaction = R(4,1)
Mode 2:

Point 1 reaction = R(1,2)
Point 2 reaction = R(2,2)
Point 3 reaction = R(3,2)
Point 4 reaction = R(4,2)
Base reaction:

Mode 1: SUM mode 1 = R(1,1) + R(2,1) + R(3,1) + R(4,1)
Mode 2: SUM mode 2 = R(1,2) + R(2,2) + R(3,2) + R(4,2)
Base Reaction = √((SUM mode 1)2 + (SUM mode 2)2)
Point reactions:

Point 1 reaction = √(R(1,1)2 + R(1,2)2)
Point 2 reaction = √(R(2,1)2 + R(2,2)2)
Point 3 reaction = √(R(3,1)2 + R(3,2)2)
Point 4 reaction = √(R(4,1)2 + R(4,2)2)

 

[JoshPlumSE]

Yes, but the results should be very close if your base reactions are all at the same level. For sloped grade where the foundations are at multiple different elevations, I could see a significant difference.

The previous post suggested an order of magnitude difference which shouldn't be possible.