staad pro ce

Question

HELLO EVERYONE, 
 MY QUESTION IS WHEN I COMPARE TWO MODELS ONE WITH OFFSET COMMAND AND THE OTHER WITHOUT OFFSET AND IN BOTH MODELS I M GETTING DIFFERENT RESULTS WHY??? 
 1.0 Steel requirement in the top column is less in the offset model as compared to the without offset model. 
 2.0 Moment in the z-direction is less in the grid beam as compared to without offset model. 
 3.0 can we design the ring beam in staad if not then pls tell me for which moment we can go for a manual calculation. 
 Both the model attached belo 0363.225 kl 10 m stg R1.STD 0363.225 kl 10 m stg.STD w.

Suvadeep Acharjee · Answer

When offsets are assigned to a member, the centerline of the member gets shifted with respect to it's incident nodes by the offset amount. Analytically rigid links are created between the incident nodes and start/ends of the member. As such due to eccentricity and lever arm arising out of such eccentricity, secondary forces are induced in the structure when offsets are used. This is not the case when there are no offsets and as such no eccentricity to result in secondary forces. Thus it is expected that between two models one with offset and other without offsets difference in analysis results will be observed.

Suvadeep Acharjee · Answer

It depends on actual requirements in the structure, your modelling approach and engineering judgement. For example c enter line modeling approach is generally used in STAAD.Pro, so if heights are different for two entities and requirement is to make the tops flush then you may use the offset command to make the tops flush. So this is not a question about preference for analysis results but rather the modelling requirements.