Odd results from SAP2000 non-linear direct time history

[tubsdua]

Hello All,

I am working on a project to build a numerical model of a power transmission line which is being monitored. The model has been provided with wind load time history. The mean wind component is as per EN and the turbulent part has been generated by a random process using Shinozuka approach.

I have applied the load time history and performed the non-linear direct time history analysis. I first idea was to compare the extreme value of axial force (found by appropriate statistical method for probability distribution) in the insulator to the axial force due to the quasi static loads provided by the EN. On completion of the analysis it was noticed in the time history of axial forces in insulator were mostly in compression. This is quite unexpected.

I am attaching a very simple model with 2 spans of 1 conductor which is supported in the middle via an insulator. In this simple model also the insulator is in compression.

Can someone explain how can this be possible when the wind load is acting in x direction and the conductors displacement suggests that the insulator will be in tension?

 

[tubsdua]

The time history response of the insulator for axial force is attached here. The portion above 0 (tension) is due to the the dead load.

 

[tubsdua]

Ok, working further on the problem I have simplified the model even more and reduced it to a beam member with pin support on the top and a time history of load on the free end. In this model also one would expect that the member will be in tension. To check this I applied a static live load at the free end and the member is in tension. But in case of time history the member is in compression.

Can someone make me understand what SAP2000 is actually doing here?