Simple Dynamic Analysis Problem
Simple Dynamic Analysis Problem
(OP)
I have a simple dynamic problem that I'm trying to do in SAP2000 to make sure I know what I'm doing before modeling a more complicated structure. I cannot get the correct first modal frequency from SAP. Here is the problem:
A lamp post is 12m tall, fixed at the base, with a 7.5kg lamp at the top. The post is a hollow steel tube, outer diameter is 140mm and wall thickness is 3mm. I used 205GPA as modulus of elasticity and 7850kg/m3 as density. The answer is supposed to be 0.87Hz but I keep getting 0.46Hz. Am I doing something wrong or is this the closest answer I can expect from SAP?
Thank you!
A lamp post is 12m tall, fixed at the base, with a 7.5kg lamp at the top. The post is a hollow steel tube, outer diameter is 140mm and wall thickness is 3mm. I used 205GPA as modulus of elasticity and 7850kg/m3 as density. The answer is supposed to be 0.87Hz but I keep getting 0.46Hz. Am I doing something wrong or is this the closest answer I can expect from SAP?
Thank you!





RE: Simple Dynamic Analysis Problem
RE: Simple Dynamic Analysis Problem
jatfuentes
RE: Simple Dynamic Analysis Problem
I don't have the hand calculations for the answer because I got the problem online and the answer was in brackets at the end. I will try it by hand using a distributed mass system and see what I get.
Hi Jatfuentes,
Good idea. How do I change the discretization in SAP?
Thank you.
RE: Simple Dynamic Analysis Problem
What I allways do, is divide the members if it is a bar element ( I think so). Select all the post, go to edit, and then subdivide members. You can subdivided the element in 3, 4 or five elements, whatever you need.
Sap has the capability of convert loads into masses, then you do not need to apply any mass.
jatfuentes
RE: Simple Dynamic Analysis Problem
Dividing the members causes the analysis to converge to 0.64 Hz, which is closer.
I have the gravity turned on (to get the post mass) and the lamp applied as a dead load at the top. I have my mass source set to be from element and any additional masses or loads with the dead load x1 as the load. Any other suggestions on how to get closer?
alannakate
RE: Simple Dynamic Analysis Problem
RE: Simple Dynamic Analysis Problem
RE: Simple Dynamic Analysis Problem
RE: Simple Dynamic Analysis Problem
C. E Beards BSc, PhD, C Eng, MRAeS, MIOA
Arnold
See attachment.
RE: Simple Dynamic Analysis Problem
RE: Simple Dynamic Analysis Problem
RE: Simple Dynamic Analysis Problem
I have been away and so I just had a chance to look at what you have written and try it out. My model now gives 0.86Hz! Thank you so much for all of your help, I really appreciate it. My biggest problem was that I had the lamp applied as a load instead of a joint mass and I only had it in the z direction.