Hello Sir,
I am trying to study the theoretical and modeling correlation using Staad software. Here i am trying to model a portal frame with a beam of infinite stiffness and co-relate the results with theory.
Problem Statement:
Total Weight= 9 kN
Stiffness = 488 kN/m
Objective: to compare frequency and time period as obtained theoretically with the one by Staad.
Theory: We know the mass from the weight and the stiffness is already know. So from the basic equation we get the fundamental frequency of vibration = 0.73 rad/s (Omega = Sqrt(k/m) ). Thus the time period is 8.6s and frequency is 0.116 Hz.
Staad Model: I have considered a beam column portal frame with length and height of 2 m each. In this modeling, the each beam and column elements are 1m each. Total structure has 7 nodes and 6 elements. On the basis of the stiffness of the structure = 488 kN/m, the dimensions for column are 2000mm x 55mm x 55mm. On the baiss of weight of structure, the beam dimensions are 2000mm x 242mm x 242mm. Also, the material property of both the beam and column is steel with E as 2.05e8 kN/m2 and Density as 76.8195 kN/m3.
For staad i am getting time period of 0.554s and Frequency of 1.8 Hz. which is way off from the theory.
I fail to understand the reason. Seeking you kind inputs.