Question
Why are the member force results different when modeling a simple beam with plate and beam elements?
Answer
Unlike the beam element, the member forces of the plate elements are obtained as force per unit width.
Consider applying the same bending moment to the beam element and the plate element, as shown in the figure below.
In this case, the member force of the beam element is the resultant force at the centroid, calculated by integrating the stress over the entire cross-section.
On the other hand, the member force of the plate element is expressed as force per unit width obtained after integrating the stress in the direction of thickness.
In conclusion, the member force of the plate elements represents the member forces per unit width, regardless of the size of the plate element.