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In the same structural system, there can be a difference in member forces depending on the change offset.

Question

In the same structural system, there can be a difference in member forces depending on the change offset.

 

Answer

The reason why there is a difference in member force due to changing 'Section Offset' is because the location of the node is different.

 Boundary conditions are created at the node point. Therefore, if the location of the node in the section changes due to the 'Section Offset', the location of the boundary conditions also changes accordingly.

 For example, there are two cases where 'Element Beam Load' is acting on the centroid of a cross-section in the x-axis, one with the 'Section Offset' = 'Center-Center' and the other with 'Center-Top'.

 The Dx degree of freedom (d.o.f.) of the boundary condition is restrained in the above figure, so a reaction force in the X direction is generated at the boundary condition due to the 'Element Beam Load'. In addition, the member forces such as My are output as results at the centroid position of the section, regardless of the location of the node.

 Therefore, when the 'Section Offset' is Center-Center (left image), the horizontal reaction passes through the centroid of the section, so there is no additional moment due to the reaction force.

 However, in the case of 'Section Offset' being Center-Top (right image), since the horizontal reaction does not pass through the centroid of the section, an additional moment occurs equal to 'horizontal reaction times the distance between the reaction and the centroid'.

 In conclusion, the reason why there is a difference in the results of the reaction depending on the 'Section Offset' is that the location where the boundary condition is created is different.

 Copy the node from two different 'Section Offsets' to the same location, and input the boundary condition. Then connect the node of boundary condition and the node on the element with a Rigid Link. As a result, the same reaction forces will be output even if the 'Section Offset' is different.

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