Table 1: The comparison of the natural frequencies, Hz.

RB standing aloneRB + TP, two basements fixedRB + TP, one basement fixedComments about modes
No.FrequencyNo.FrequencyNo.Frequency

13.16 𝐸 4 “Rigid” motion of portal
24.48 𝐸 4 “Rigid” motion of portal
38.78 𝐸 4 “Rigid” motion of portal
41.67 𝐸 3 “Rigid” motion of portal
12.4132Lumped masses on the top of portal move along Oy
14.044524.044554.0445RB moves in the Ozy plane
24.074734.064764.0593RB moves in the Ozx plane
39.213269.2057129.2137Torsion of RB around Oz
410.093710.0921410.092Vertical movement of RB