 ## General Mechanical

#### Why is there a huge difference in Hand calculations and Ansys Mechanical results?

• Zabeeullah
Subscriber

Hello!

I have been doing modal analysis of a fixed beam with a point load acting at its centre. The natural frequency of the beam which i have calculated is aroung 23Hz and in ansys modal analysis it is 112Hz in bending mode. (2nd mode in modal analysis)

I am using a 170*80*4mm Rect Hollow beam of lenght 2m with both the ends fixed and a central load of 1000N acting on it. The mass of the beam per lenght is 5.45. the material is structural steel, E=210 GPA, Density=7405 kg/m^3, I=420565mm^4, y=35mm.

• peteroznewman
Subscriber
Please reply with details of the hand calculation. I haven't seen a hand calculation that included a pre-stress condition, but I haven't looked for it either.
If you create a Modal analysis without the Static Structural pre-stress, what are the first and second frequencies?
Use SpaceClaim (or DesignModeler) to draw a line body and assign it the correct material and cross-section, then mesh it with beam elements. What are the first and second frequencies?
• Sahil Sura
Ansys Employee

As numerical methods are based on result approximation, there will definitely be some deviation from the theoretical analytical methods. But this too can be minimized by considering apt boundary conditions, mesh and even theory being referred to while calculating, etc.
You might find this post helpful- Modal Analysis - Difference of Numerical and Analytical Results ÔÇö Ansys Learning Forum
Hope that helps!
Thanks Sahil.
How to access Ansys help links
Guidelines for Posting on Ansys Learning Forum
• Zabeeullah
Subscriber
Hi!
I have used the below two formules to calculate the natural frequencies of the beam. and my results were around 23Hz.
F= (1/2.pi) * (acceleration due to gravity / static deflection )^0.5
F= (1/2.pi) * (stiffness/mass)^0.5
I really dont understand where im getting it wrong.
• Zabeeullah 