Civil Engineering Association

Please guide me for fundamental frequency of Chimney. I got my staad file analyzed but now what..where to look...

If the chimney has constant ratios of change of the mean diameter and width, give me some details and I will calculate them to you.

D1 - mean diameter of the chimney shell at the base
D2 - mean diameter at the top
t1 - width of the chimney shell at the base
t2 - width of the shell at the top
H - height
E - modul of elasticity of the concrete

If you have to consider the rotational deformation of the foundation I need also (in case of circular foundation only):
ks - Winkler's constant of the soil
Jf - moment of inertia of the foundation

@ jacs127

Read Dynamics of Structures by Chopra about the vibration of structure with distributed mass and elasticity. Well there are simple formula for uniform distribution of mass and elasticity but not with variable one so do your math?..in case you want to go advance then find anything about Wittrick-Wiiliams Method, it is for exact vibration. Good Luck

Dear Friends,

Attaching the Staad file... i really poor at staad but with help from this great forum i will learn...the fundamental frequency from code based procedure shows significant diff from staad...


STAAD SPACE
START JOB INFORMATION
ENGINEER DATE 10-Jun-10
END JOB INFORMATION
INPUT WIDTH 79
UNIT METER KN
JOINT COORDINATES
1 0 -1.6 0; 2 0 0 0; 3 0 2 0; 4 0 4 0; 5 0 5 0; 6 0 10 0; 7 0 15 0; 8 0 20 0;
9 0 30 0; 10 0 40 0; 11 0 50 0; 12 0 60 0; 13 0 70 0; 14 0 80 0; 15 0 90 0;
16 0 100 0; 17 0 110 0; 18 0 120 0; 19 0 130 0; 20 0 140 0; 21 0 150 0;
22 0 160 0; 23 0 170 0; 24 0 180 0; 25 0 190 0; 26 0 200 0; 27 0 210 0;
28 0 220 0;
MEMBER INCIDENCES
1 1 2; 2 2 3; 3 3 4; 4 4 5; 5 5 6; 6 6 7; 7 7 8; 8 8 9; 9 9 10; 10 10 11;
11 11 12; 12 12 13; 13 13 14; 14 14 15; 15 15 16; 16 16 17; 17 17 18; 18 18 19;
19 19 20; 20 20 21; 21 21 22; 22 22 23; 23 23 24; 24 24 25; 25 25 26; 26 26 27;
27 27 28;
MEMBER PROPERTY AMERICAN
27 PRIS AX 17.33 IZ 412.95
26 PRIS AX 17.33 IZ 412.95
25 PRIS AX 17.33 IZ 412.95
24 PRIS AX 17.33 IZ 412.95
23 PRIS AX 17.33 IZ 412.95
22 PRIS AX 17.33 IZ 412.95
21 PRIS AX 17.33 IZ 412.95
20 PRIS AX 17.33 IZ 412.95
19 PRIS AX 18.94 IZ 486.57
18 PRIS AX 20.62 IZ 569.54
17 PRIS AX 22.69 IZ 691.45
16 PRIS AX 24.27 IZ 752.1
15 PRIS AX 26.43 IZ 960.08
14 PRIS AX 27.41 IZ 989.63
13 PRIS AX 29.32 IZ 1125.51
12 PRIS AX 31.17 IZ 1270.68
11 PRIS AX 33.54 IZ 1447.8
10 PRIS AX 35.98 IZ 1642.55
9 PRIS AX 39.49 IZ 1901.92
8 PRIS AX 41.39 IZ 2104.2
7 PRIS AX 42.7 IZ 2227.81
6 PRIS AX 43.4 IZ 2347.32
5 PRIS AX 45.85 IZ 2606.36
4 PRIS AX 45.99 IZ 2613.93
3 PRIS AX 46.13 IZ 2621.5
2 PRIS AX 46.56 IZ 2644.21
1 PRIS AX 46.82 IZ 2657.94
DEFINE MATERIAL START
ISOTROPIC MATERIAL1
E 3.5e+007
POISSON 0.2
ISOTROPIC MATERIAL2
E 3.35e+007
POISSON 0.2
ISOTROPIC MATERIAL3
E 3.2e+007
POISSON 0.2
ISOTROPIC CONCRETE
E 2.17185e+007
POISSON 0.17
DENSITY 23.5616
ALPHA 1e-005
DAMP 0.05
END DEFINE MATERIAL
CONSTANTS
MATERIAL MATERIAL1 MEMB 1 TO 7
MATERIAL MATERIAL2 MEMB 8 TO 13
MATERIAL MATERIAL3 MEMB 14 TO 26
MATERIAL CONCRETE MEMB 27
CUT OFF MODE SHAPE 30
SUPPORTS
1 FIXED
LOAD 1 LOADTYPE None TITLE DL
SELFWEIGHT Y -1 LIST 1 TO 27
CALCULATE RAYLEIGH FREQUENCY
LOAD 2
JOINT LOAD
*1 FX 7
2 FX 21.36
3 FX 28.97
4 FX 22
5 FX 44.47
6 FX 77.1
7 FX 83.8
8 FX 141
9 FX 204
10 FX 223
11 FX 240.92
12 FX 258
13 FX 270
14 FX 278.5
15 FX 285.6
16 FX 293.51
17 FX 305.2
18 FX 314.3
19 FX 319
20 FX 326.3
21 FX 339
22 FX 355
23 FX 370
24 FX 381
25 FX 393
26 FX 406.01
27 FX 420.6
28 FX 214.1
MODAL CALCULATION REQUESTED
SPECTRUM SRSS X 0.15 ACC SCALE 9.81 DAMP 0.05
0 0.12; 0.04 0.12; 0.08 0.12; 0.12 0.175; 0.16 0.175; 0.2 0.175; 0.3 0.175;
0.4 0.175; 0.5 0.175; 0.6 0.175; 0.8 0.146; 1 0.117; 1.2 0.097; 1.4 0.059;
1.6 0.073; 1.8 0.065; 2 0.058; 2.3 0.05; 2.6 0.045; 2.8 0.041; 3 0.039;
PERFORM ANALYSIS
FINISH

You need to add one more primary load case naming NATURAL FREQUENCY and list all loads categorized as dead loads inside load case NATURAL FREQUENCY by substituting the global coordinate as GX. Here is an example.
STAAD SPACE HL5E HL5E01
* STAAD FILE FREQCHK.STD
UNITS FEET KIP
*****FOR UNCORRODED FREQUENCY CHECK
*** 22.97 FOOT DIAMETER 196.85 FOOT HIGH STACK
**** WIND VELOCITY = 117 MPH; EXP CAT = C; I = 1.15
JOINT COORDINATES
1 0.0 0.0 0.0 197 0.0 196 0.0
*
MEMBER INCIDENCES
1 1 2 196 1 1
UNITS INCH POUND
MEMBER PROPERTIES
138 TO 196 TA ST PIPE OD 276.2205 ID 275.5906
112 TO 137 TA ST PIPE OD 276.3780 ID 275.5906
60 TO 111 TA ST PIPE OD 276.6929 ID 275.5906
1 TO 59 TA ST PIPE OD 277.0079 ID 275.5906
*
*
*
*
*
*
*
*
CONSTANTS
E 29539300 ALL
*DENSITY 0.283 ALL
* SHELL DISTIBUTION
DENSITY 0.332 MEMBER 1 TO 3
* BASE RING DISTIBUTION
DENSITY 4.061 MEMBER 4
* SHELL
DENSITY 0.332 MEMBER 5 TO 8
* LOWER BREECH
DENSITY 3.116 MEMBER 9
* SHELL
DENSITY 0.332 MEMBER 10 TO 82
* UPPER BREECH
DENSITY 3.563 MEMBER 83
* SHELL
DENSITY 0.332 MEMBER 84 TO 91
* LOWER PLATFORM
DENSITY 4.570 MEMBER 92
* SHELL
DENSITY 0.332 MEMBER 93 TO 139
* INTERMEDIATE PLATFORM 1
DENSITY 2.012 MEMBER 140
* SHELL
DENSITY 0.332 MEMBER 141 TO 159
*INTERMEDIATE PLATFORM 2
DENSITY 4.387 MEMBER 160
* SHELL
DENSITY 0.332 MEMBER 161 TO 179
* TOP PLATFORM
DENSITY 5.738 MEMBER 180
* SHELL
DENSITY 0.332 MEMBER 181 TO 196
* DAMPER
DENSITY 3.406 MEMBER 94
* SILENCER
DENSITY 4.299 MEMBER 116
SUPPORTS
1 FIXED
UNITS FEET POUNDS
*****************************************************
LOADING 1 DEAD LOAD
*****************************************************
SELFWEIGHT Y -1
*
*
*
* INTERMEDIATE PLATFORM 1 MOMENT @EL. 140
MEMBER LOAD
140 CMOM GX 26365
* INTERMEDIATE PLATFORM 1 MOMENT @EL. 120
MEMBER LOAD
120 CMOM GX 26365
* INTERMEDIATE PLATFORM 1 MOMENT @EL. 100
MEMBER LOAD
100 CMOM GX 26365
* INTERMEDIATE PLATFORM 2 MOMENT
MEMBER LOAD
160 CMOM GX 61048
*
*
*
* INSULATION LOAD
MEMBER LOAD
1 TO 196 UNI GY -96.9
* PERSONEL PROTECTION LOAD
* ABOVE BASE
MEMBER LOAD
1 TO 8 UNI GY -72.1
* ABOVE LOWER PLATFORM
MEMBER LOAD
92 TO 100 UNI GY -72.1
* ABOVE INT. PLATFORM 1
MEMBER LOAD
140 TO 148 UNI GY -72.1
* ABOVE INT. PLATFORM 2
MEMBER LOAD
160 TO 168 UNI GY -72.1
* ABOVE TOP PLATFORM
MEMBER LOAD
180 TO 188 UNI GY -72.1
* UNDER LADDER
MEMBER LOAD
92 TO 180 UNI GY -3
*LADDER LOAD
* LADDER 1
MEMBER LOAD
92 TO 140 UNI GY -30
* LADDER 2
MEMBER LOAD
140 TO 160 UNI GY -30
* LADDER 3
MEMBER LOAD
160 TO 180 UNI GY -30
* TRANSITION DUCT WEIGHT
MEMBER LOAD
9 TO 83 UNI GY -289.3
****************************************************
LOADING 2 NATURAL FREQUENCY
****************************************************
SELFWEIGHT X 1.0
*
*
*
* INTERMEDIATE PLATFORM 1 MOMENT @EL. 140
MEMBER LOAD
140 CMOM GX 26365
* INTERMEDIATE PLATFORM 1 MOMENT @EL. 120
MEMBER LOAD
120 CMOM GX 26365
* INTERMEDIATE PLATFORM 1 MOMENT @EL. 100
MEMBER LOAD
100 CMOM GX 26365
* INTERMEDIATE PLATFORM 2 MOMENT
MEMBER LOAD
160 CMOM GX 61048
*
*
*
* INSULATION LOAD
MEMBER LOAD
1 TO 196 UNI GX 96.9
* PERSONEL PROTECTION LOAD
* ABOVE BASE
MEMBER LOAD
1 TO 8 UNI GX 72.1
* ABOVE LOWER PLATFORM
MEMBER LOAD
92 TO 100 UNI GX 72.1
* ABOVE INT. PLATFORM 1
MEMBER LOAD
140 TO 148 UNI GX 72.1
* ABOVE INT. PLATFORM 2
MEMBER LOAD
160 TO 168 UNI GX 72.1
* ABOVE TOP PLATFORM
MEMBER LOAD
180 TO 188 UNI GX 72.1
* UNDER LADDER
MEMBER LOAD
92 TO 180 UNI GX 3
*LADDER LOAD
* LADDER 1
MEMBER LOAD
92 TO 140 UNI GX 30
* LADDER 2
MEMBER LOAD
140 TO 160 UNI GX 30
* LADDER 3
MEMBER LOAD
160 TO 180 UNI GX 30
* TRANSITION DUCT WEIGHT
MEMBER LOAD
9 TO 83 UNI GX 289.3
CALCULATE NATURAL FREQUENCY
*******************************************************
LOADING 3 WIND
*******************************************************
MEMBER LOAD
138 TO 196 LIN Z 1047 1322
112 TO 137 LIN Z 917 1047
60 TO 111 LIN Z 646 917
1 TO 59 LIN Z 307 646
*
*
*
*
*
*
*
*
********************************************************
LOADING 4 SEISMIC
********************************************************
* SEISMIC FACTOR = 0·W
SELFWEIGHT X 0
* INTERMEDIATE PLATFORM 1 MOMENT
MEMBER LOAD
140 CMOM GX 0
* INTERMEDIATE PLATFORM 2 MOMENT
MEMBER LOAD
160 CMOM GX 0
*
*
*
* INSULATION LOAD
MEMBER LOAD
1 TO 196 UNI GX 0
* PERSONEL PROTECTION LOAD
* ABOVE BASE
MEMBER LOAD
1 TO 8 UNI GX 0
* ABOVE LOWER PLATFORM
MEMBER LOAD
92 TO 100 UNI GX 0
* ABOVE INT. PLATFORM 1
MEMBER LOAD
140 TO 148 UNI GX 0
* ABOVE INT. PLATFORM 2
MEMBER LOAD
160 TO 168 UNI GX 0
* ABOVE TOP PLATFORM
MEMBER LOAD
180 TO 188 UNI GX 0
* UNDER LADDER
MEMBER LOAD
92 TO 180 UNI GX 0
*LADDER LOAD
* LADDER 1
MEMBER LOAD
92 TO 140 UNI GX 0
* LADDER 2
MEMBER LOAD
140 TO 160 UNI GX 0
* LADDER 3
MEMBER LOAD
160 TO 180 UNI GX 0
* TRANSITION DUCT WEIGHT
MEMBER LOAD
9 TO 83 UNI GX 0
*****************************************************
LOADING 5 SPECTRUM LOAD
*****************************************************
SELFWEIGHT X 1
*
*
*
* INTERMEDIATE PLATFORM 1 MOMENT @EL. 140
MEMBER LOAD
140 CMOM GX 26365
* INTERMEDIATE PLATFORM 1 MOMENT @EL. 120
MEMBER LOAD
120 CMOM GX 26365
* INTERMEDIATE PLATFORM 1 MOMENT @EL. 100
MEMBER LOAD
100 CMOM GX 26365
* INTERMEDIATE PLATFORM 2 MOMENT
MEMBER LOAD
160 CMOM GX 61048
*
*
*
* INSULATION LOAD
MEMBER LOAD
1 TO 196 UNI GX 96.9
* PERSONEL PROTECTION LOAD
* ABOVE BASE
MEMBER LOAD
1 TO 8 UNI GX 72.1
* ABOVE LOWER PLATFORM
MEMBER LOAD
92 TO 100 UNI GX 72.1
* ABOVE INT. PLATFORM 1
MEMBER LOAD
140 TO 148 UNI GX 72.1
* ABOVE INT. PLATFORM 2
MEMBER LOAD
160 TO 168 UNI GX 72.1
* ABOVE TOP PLATFORM
MEMBER LOAD
180 TO 188 UNI GX 72.1
* UNDER LADDER
MEMBER LOAD
92 TO 180 UNI GX 3
*LADDER LOAD
* LADDER 1
MEMBER LOAD
92 TO 140 UNI GX 30
* LADDER 2
MEMBER LOAD
140 TO 160 UNI GX 30
* LADDER 3
MEMBER LOAD
160 TO 180 UNI GX 30
* TRANSITION DUCT WEIGHT
MEMBER LOAD
9 TO 83 UNI GX 289.3
* SEISMIC AG:0.22g
SPECTRUM CQC X 1 ACC SCALE 32.2 DAMP 0.02 LIN
0 0.253; 0.2 0.77; 0.6 0.77; 0.7 0.627; 0.8 0.517; 0.9 0.462;
1.1 0.396; 1.2 0.363; 1.4 0.319; 1.6 0.286; 1.8 0.275; 2 0.2728;
LOAD COMB 100 DEAD LOAD
1 1.0
LOAD COMB 101 DEAD+SPECTRUMX
1 1.0 5 1.0
UNITS FEET KIP
PERFORM ANALYSIS
PRINT SUPPORT REACTION
LOAD LIST 3
PRINT JOINT DISPLACEMENT LIST 83 197
STEEL TAKEOFF
FINISH

Take a look at LOADING NATURAL FREQUENCY. Then run the analysis and go to STAAD REPORT and search Natural Frequency.

Hi ,
I can Help you to get natural frequency by using Sap
follow the following steps
1- draw the model as a shell elemnts ( this is more accute rather than to draw as frame elemnts)

2- Run analysis

Sap automatically will generate the modal analysis
(modal analysis = dynamic analysis method to get the natural frequencies)
then you will get the first natural frequency then second then third
and so and so

usually we will be interested to get the first natural frequency in x direction
and first natural frequency in y direction
and because the cheminy is symmetric so you will get firsnatural freq in x-dir same as first natural frequency in y dirction.

from this analysis you will get time period of first mode T
this value is very important to get the seismic force

Hoping this will be useful for you

Hi mecheil.edwar, thanks for your post.
I wish you could spell out step by step in a detailed way where to go and what to do while modelling a chimney, i wd be so much obliged. I already got some suggestions from other helpful members but i would love to get more ideas in reagrd to the above concern.
Posting a model would do just fine.
Thanks