Failure to process negative accidental eccentricity factor in Response Spectrum Analysis per IBC

First Affected Commercial Version: (v8i)
 First affected QA&R Version: (v8i)
Fixed and Released in Version:
 Area: Seismic analysis as per IBC 2012/2015/2018
 Issue #:A-1073141

Problem Description

When carrying out response spectrum analysis with IBC 2012/2015/2018, if the user-provides accidental
eccentricity factor (ECC) as negative, the program always considers a value of -0.05 in the calculation. In other words, the torsional responses are generated based on ECC = -0.05, for any negative value input for ECC. This may lead to unconservative torsional responses for cases where the input ECC < -0.05.

The issue will occur when ALL the following conditions are true:

1)a response spectrum analysis is performed with –
a.IBC 2012
b.IBC 2015
c.IBC 2018

2)the value of user-provided accidental eccentricity (ECC) factor is negative (except ECC = -0.05).

The bug does not lead to any error when the user-provided value of ECC is either a positive quantity or is equal to (-0.05). Refer to Table 1 for details.


No workaround is available as of now for this defect.


The defect will be addressed in the next scheduled release of STAAD.Pro ( 2023 version ) expected around Middle of 2023.