Natural Frequency of a Steel Chimney using Excel Spreadsheet
Natural Frequency of a Steel Chimney using Excel Spreadsheet
(OP)
I require a method to calculate the natural frequency of a steel chimney with multiple diameters using an excel spreadsheet. All the methods I have used to date work well for constant diameter and tapered structures but do not provide accurate results for stepped structures. I am considering using a dynamic approach but solving for eigenvalues is difficult without resorting to VBA.
Any advice would be much appreciated.
Thanks,
Any advice would be much appreciated.
Thanks,






RE: Natural Frequency of a Steel Chimney using Excel Spreadsheet
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RE: Natural Frequency of a Steel Chimney using Excel Spreadsheet
| n |
| SUM (wi * (deltai)2 |
T = 2 * pi * SQRT| i=1 |
| ___________________ |
| n |
| g* SUM (fi * deltai) |
| i=1 |
ASCE defines fi as "any lateral force distribution in accordance with structural mechanics." And the deltai are the elastic deflections "calculated using the applied lateral forces fi." Although it doesn't define it in this section, wi are the weights attributed to each level.
RE: Natural Frequency of a Steel Chimney using Excel Spreadsheet
Whether you are dealing with a tapered chimney or a stepped chimney, the method and results should be very similar. You have discretize your structure into lumped mass for either tapered or stepped and can fashion your stiffness parameters for each of the lumped masses.
Also, what is your intended use of this value. If it is for earthquake loading, then the first 3 modes are likely to be sufficient for your purpose. If your purpose is to investigate stability of the sections then you will require higher modes that will cause vibration in the walls themselves rather than the whole structure acting as a cantilever.
More information would be helpful.
Regards,
![[pipe] pipe](https://www.tipmaster.com/images/pipe.gif)
Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Natural Frequency of a Steel Chimney using Excel Spreadsheet
I currently calculate the moment due to self weight from the tip at each section of the stack and use the data to generate a third order polynomial. That equation is then intergrated twice to provide an equation that relates the lateral displacement of the stack as a function of elevation. This method is fine for straight structures but does not work when there is a change in diameter as there is no equation that can follow the step changes accurately.
I require a different method to define the deflection of the stack as a function of elevation.
Qshake, I require the natural frequency (1st and 2nd) to calculate the gust factor for wind loading and stack stability calculations.
RE: Natural Frequency of a Steel Chimney using Excel Spreadsheet
You could also probably use Holzer's method.
However I'd agree with Qshake, a tapered beam solution would be a good enough approximation given the many unknowns in reality.
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Natural Frequency of a Steel Chimney using Excel Spreadsheet
I am not sure if this makes sense as written. It is much more clear in my head. Maybe I will see if I can sketch this better and upload it later today...