bending moments for stack

[grnblr]

I would like to know how, for a 45 mt chimney stack made up of 5 sections, the total Bending moment to wind loads, with section considered at the base of each stack piece is calculated. I only know that it is the algebric sum of all moments produced by the wind forces on one side of the section. It is also assumed that wind force is cting at the middle of the chimney height of each chimney piece. The folowing is the information given;

Chimney parameter/Chimney piece 1 2 3 4 5

Base dia in mts 1.5 1.2 0.9 0.6 0.6
Top dia in mts 1.2 0.9 0.6 0.6 0.6
length in mts, of each piece 10.0 10.0 10.0 10.0 5.0
thicknes in mm of each piece 14 14 14 14 14
surface area in sqmt 21.2 16.5 11.79 9.42 4.71
Wf=windforce in Nwtns 22671.9,17633.7,15512.5,13102.5,7770.3

My question is how Bending moment in Kgs mt is arrived at?

GRN

 

[dvd]

You can find a nice chapter on the subject of self supporting stacks in "Tubular Steel Structures - Theory and Design" by M.S. Troitsky, published byThe James F. Lincoln Arc Welding Foundation. The book is inexpensive. Call 216-383-2211.

 

[kafeel]

consider the chimney as a cantiliver beam of varying cross section. Solve and draw a Bending moment Diagram. you can tehn find the bending moment with respect to the base. Its a text book problem with complicated wordings. Any book on 'Strength of materials' should help you
All the best

Kafeel

 

[meca]

Using your numbers, I calculate the bending moment as follows:

Section # 1, 2, 3, 4, 5
Mid Pt Ht (m) 5, 15, 25, 35, 42.5
Mom @ Base(N-m) 113,360, 264,506, 387,813, 458,588, 330,238

Total Moment at Base (N-m)= 1,554,503

Maybe I am missing something, but there were some things that were unclear to me in your numbers:

1) Your calculated surface area appeared much higher than I expected. Do these areas include attachements (i.e. Ladders and Platforms)?

2) Have you applied a shape factor to the stack? Depending upon the code being used, usually 0.7 to 0.8 is used for the stack.

3) I ran this stack on my stack design software, using the information you provided. Perhaps I made some incorrect assumptions in my analysis, but it seems that 14 mm shell thickness is larger than required. I think this stack could be less expensive.

Thanks,

Chris
chris@mecaconsulting.com

http://www.mecaconsulting.com