An implicit enumeration technique for the land acquisition problem

The effect of geometrical changes of symmetric rigid-jointed portal frames on their natural frequencies is analysed. Discussed are the following issues: (a) the sensitivity of the frequencies to selected shape variables; (b) the variation of the three lowest frequencies ω1, ω2 and ω3 as functions of the shape variables in a prescribed domain; and (c) the occurrence of the repeated lowest frequency ω1.