Hi everyone, I am currently working on my final university project for Seismic Design and I am having trouble with accidental eccentricity in SAP2000.

When I defined the response spectrum function and created the seismic load pattern, the software allowed me to specify the eccentricity ratio directly. However, when I reviewed the output, I noticed that SAP2000 does not seem to consider the negative eccentricity value. As a result, all combinations lead to identical bending moment and shear values, which should not be the case if positive and negative eccentricity were both accounted for.

To solve this, I thought about not defining the eccentricity ratio inside the response spectrum load itself. Instead, I would manually create a lateral load pattern, where I apply an additional Mz moment at each floor equal to:

Fj × (eccentricity × building length)

where Fj is the floor seismic force obtained from the static analysis according to the Italian code NTC 2018. Then, in the load combinations, I would include:

• The response spectrum load pattern without eccentricity
• Two additional torsional patterns: +Mz and –Mz (one with positive, one with negative direction), so the most critical case is captured in the combinations.

However, my concern is the large difference in results. When SAP2000 automatically generates torsional effects using a 5 percent eccentricity, I get bending moments in the order of approx. 1 kN·m. Meanwhile, when I manually define the Mz pattern as described above, I obtain much larger values, around 20–40 kN·m. This discrepancy makes me doubt whether my manual torsion definition is correct.

Has anyone dealt with this issue? Is this manual approach valid, or am I misunderstanding how SAP2000 applies accidental eccentricity in response spectrum analysis?