Long term strain gage monitoring of the cylindrical construction of the silo PC1

Lukáš Kowalski1

Annotation

Thin walled steel shell structures have considerable utilization in various industrial

applications – pipelines, various chemical applications, tanks, silos, etc. In ideal state of loading and

ideal geometry without any dents, shell theory can be applied – in construction occur only axial

forces. With increasing slenderness, problems of losing the stability come to prior. Usage of

various stiffening systems is one of the possibilities how to increase buckling resistance of the

structure. Among mentioned constructions belongs even construction of the silo PC1 in company

Holcim. Construction of the silo PC1 (serves for storing clinker) is the main subject of the article.

On originally not stiffened construction of the silo were noticed various failures that led to, at the

first stage to repair of the failures, in the second step led to overall reconstruction of the silo

structure through application of the orthotropic stiffeners. Article is focused on experimental

monitoring using the strain gages of the walls of the silo construction. Results of the monitoring are

being compared with results of the mathematical modeling of the construction using finite element

method in program ANSYS.

Introduction

In past years, significant number of silo failures was noticed. This fact raises serious concerns about

safety and reliability of the silo constructions. From research, common behavior of the collapsed

constructions was noticed, in most cases, wrong usage of the constructions was set as a origin of the

failures. Among such a reasons belong nonsymmetrical filling/discharge, wrong estimation of the

material characteristics (flow properties) of the stored material, the other utilization errors and poor

quality workmanship, unauthorized design changes, etc.. During usage of the silo PC1 serious

1 Kowalski, Lukáš, Ing., Department of steel and timber structures, Faculty of civil engineering, Slovak university of technology, Radlinského 11, 813 68, Bratislava, e-mail: lukas.kowalski@stuba.sk, 00 421 903 719 302

deformations even with the fracture of the bearing horizontal foundation stiffening ring occurred.

Application of the various types of the stiffeners is one of the possibilities of reconstruction of the

silo structures and of the increasing of the buckling resistance of the thin walled shell structures. In

the set of standards and in the literature, formulas and processes just for some basic types of

geometry and loading states are mentioned. General recommendations for design of the stiffeners

are not presented; just appeal to solution using FEM is mentioned. In recent years, research was

focused on the impact of the imperfections on the construction (Singer, Abramovic, 1995), buckling

of the not-stiffened cylinder shells (Galletly, 1987, Shen, Chen, 1991), stiffened cylindrical shells

(Agelidis, 1982, Miller, Vojta, 1984, Croll 1985). Impact of the discrete vertical restraints was

researched by Eggwertz and Samuelson (1991), impact of local imperfections as a consequence of

the hit was researched by Krishnakumar, Foster (1991).

Object: Silo PC1 in Holcim, Rohožník

In site Holcim, 2 same silos that serve for storage clinker, PC1 and PC2, are placed. Silos

were designed by PIO Keramoprojekt in 1973. Diameter of the silo PC1 is 36m, height of the

cylindrical part is 41,4m and overall height to the top of the silo is 50,525m. Storage capacity of the

silo is 60 000t of clinker. Constructional material of the silo walls with variable thickness (design

thickness 33mm – 13mm, real thickness from diagnostic overview 29mm – 10mm) is steel S275.

Construction of the silo is bolted to the foundation through welded T restraint (T330x70-250x25).

On the east side of the silo is lift to top of the silo. 8 hoppers are placed in two rows (2x4). Improper

use of the silo (nonsymmetrical charge, discharge) led to significant failure of the construction

(deformations, fracture of the bottom bearing ring), therefore resolution about upcoming

reconstruction were done. In the first stage, fractures were fixed, in the second stage system of

orthotropic restraints were designed and applied.

Fig. 1 View at the reconstructed structure of the silo PC1 with installed strain gage aparature

Experiment: long term strain-gage monitoring of the silo PC1

In 2009 was determined to examine an experimental long term strain gage monitoring of the

reconstructed construction of the silo PC1. Aim of the experiment was to track changes of the

stresses on silo walls from the change of the loading state (height of the clinker, change of the

temperature of the wall). Minimal length of duration of the experiment was set to 1year due to

lasting of the one cycle between ordinary shut down of the clinker furnace. Minimum height of the

storage clinker in the silo was reached on measurement no. 35 (3.12.2010, 4,995m). Before shut

down, silo was fully filled (measurement no. 40, 28.1.2011 approximately 46,59m). Measured

stresses serve as a basement for comparison with numerical results (ANSYS) of calculation of the

stresses on construction of the silo.

On silo was planned to install 16 strain gages, linear HBM LY11 and T rosettes HBM XY11. Strain

gages LY11 are placed near restraints and on the “L” shaped restraint. Strain gages XY11 are

placed between restraints. Horizontally, strain gages were placed in 3 sets – A, B, C. Vertically,

strain gages are glued at heights: +1,000m, +5,525m, 6,675m, +8,025m, +10,525m. Height of the

filling by storied clinker was measured by two radar rangefinders Siemens Sitrans LR400.

Measurement of the temperature of the silo wall was done from measurement no. 30 by non-contact

thermometer CEM DT-812 (1.10.2010).

Outputs of strain gage measurement are noticed in diary of the experiment. In diary are stored data:

change of the stress for appropriate strain gage, change of the height of the filling, weather

conditions, date and time of the measurement, progress of the work on long term monitoring. On

additional list are tracked data about temperature of the silo walls.

Then data from linear strain gages are calculated through formula

(1)

Data from T rosette strain gages are calculated to planar state of loading through formulas

(2)

(3)

Fig. 2 Schematic position of the strain-gage device

Fig. 3,4 View at installed strain gage

HBM XY11 on height +8.025mm

-80

-60

-40

-20

0

20

40

60

80

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53

number

chan

ge o

f st

ress

[M

Pa]

-50

-40

-30

-20

-10

0

10

20

30

40

50

hei

ght

of f

illi

ng

[m]

Fig. 5 Output from strain gage on height +8,025m, meridional and circumferential stresses, height

of the filling

HBM LY11 on height +5.525mm

-200

-150

-100

-50

0

50

100

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53

number

chan

ge o

f str

ess

[MP

a]

-100-90-80-70-60-50-40-30-20-1001020304050

heig

ht o

f fill

ing

[m]

Fig. 6 Output from strain gages on height +5,525m, meridional stresses, height of the filling

height of the filling of the silo PC1

05

10152025

3035404550

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53number

hei

gh

t o

f th

e fi

llin

g [

m]

Fig. 7 Height of the filling of the silo PC1

Finite element method simulation (model) of the silo PC1

Finite element method simulation (model) is part of theoretical analysis of the silo

construction. Model was created in FEM program ANSYS. In analysis, quadratic approximation

elements are used:

SHELL93 (8 node shell element) for silo walls

BEAM189 (3 node beam element) for restraints

SOLID95 (20 node volume element) for volume of storage clinker

Fig. 8 Elements used in

analysis, SHELL93, SOLID95, BEAM189

Interaction between construction of the silo and storage clinker was created through pair of

the surface contact elements TARGET170 (silo wall)/CONTACT174 (storage clinker). Mass of the

clinker is modeled as cylinder with appropriate height ranged from 5m (cylinder) to 47m (cylinder

+ cone) with step of 5m. Steel is described by bilinear model with stiffening. Value of yield stress is

fy=275MPa, modulus of elasticity E=210.109Pa for primary wave, E=210.107Pa for secondary

wave. For clinker, Drucker-Prager model for particle materials was used. Input data for clinker was

taken from standards STN (r=1500 kg/m3, f=30°) and EC (r=1800 kg/m3, f=40°). Modulus of

elasticity of clinker is E=10MPa. Coefficient of friction is m=0.56, cohesion is c=0kPa. System is

loaded by self weight of the construction and storaged material. In analysis, non uniform

diskretisation to final elements is used with increased density at lower part if the construction.

Connection of the silo to the foundation is pinned. Solution was done using small deformations

method.

Fig. 9 Cross section of the silo structure and

storied clinker

Fig. 11 von Mises stresses on the silo

structure from loading of the self weight of

the storied material, height of the filling 5m

Fig. 10 Mass of the clinker, filling height 47m

Fig. 12 von Mises stresses on the silo

structure from loading of the self weight of

the storied material, height of the filling 47m

Output data from FEM analysis in program ANSYS

On the left side values of the stresses valid for location near restraint, on the right side between

restraints

0

5

10

15

20

25

30

35

40

-50 0 50 100 150

circumferential stress [MPa]

heig

ht [m

]

5m 10m 15m 20m 25m 30m 35m 40m 47m

0

5

10

15

20

25

30

35

40

-100 -80 -60 -40 -20 0 20

meridional stress [MPa]

heig

ht [m

]

5m 10m 15m 20m 25m 30m 35m 40m 47m

0

5

10

15

20

25

30

35

40

0 50 100 150 200von Mises stress [MPa]

heig

ht [m

]

5m 10m 15m 20m 25m 30m 35m 40m 47m

0

5

10

15

20

25

30

35

40

-40 -20 0 20 40 60 80 100 120 140

circumferential stress [MPa]

heig

ht [m

]

5m 10m 15m 20m 25m 30m 35m 40m 47m

0

5

10

15

20

25

30

35

40

-100 -80 -60 -40 -20 0 20

meridional stress [MPa]

heigh

t [m

]

5m 10m 15m 20m 25m 30m 35m 40m 47m

0

5

10

15

20

25

30

35

40

0 20 40 60 80 100 120 140 160 180

von Mises stress [MPa]

heig

ht [m

]

5m 10m 15m 20m 25m 30m 35m 40m 47m

Graph on Fig. 19 represents change of the meridional stress during increase of the filling

height from 4995mm to 46590mm (increase 41595mm). During that period, meridional stresses

were increased in range from 52,450MPa to 85,300MPa. Maximum amplitude of the measured

meridional stress was 105,400MPa. Output data from strain gage device are displayed as a square

marks (linear strain gage) and cross marks (T rosette). Lines (dot, solid) represent outputs from the

FEM analysis. Dot lines describe stresses from the filling the silo to

0

5

10

15

20

25

30

35

40

-120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120

change of the meridional stresses while filling of the silo structure [MPa]

hei

ght

[m]

A B C cross A cross B cross C EC stiff 47m

EC stiff 5m EC stiff 47-5m EC wall 47m EC wall 5m EC wall STN stiff 47m STN stiff 5m

STN stiff STN wall 47m STN wall 5m STN wall

Fig. 19 Comparison of measured changes of meridional stresses and data from FEM analysis

during filling process

Graph on Fig. 20 represents change of the meridional stress during discharge (change of the

filling height from 46590mm to 16140mm (decrease 30340mm). During that period, meridional

stresses were increased in range from -29,290MPa to -59,300MPa. Maximum amplitude of the

measured meridional stress was -69,000MPa.

From the graphs on Figures 19, 20 is obvious dispersion of the measured data of the changes of the

meridional stresses with increasing height of the installed strain gages. This effect is caused

probably due to decreasing of the stiffness with increasing height from the supports. On the strain

gages near horizontal “L” restraint was tracked the smallest dispersion of the measured data. It has

to be mentioned that input data of the loading for FEM analysis are characteristic values.

0

5

10

15

20

25

30

35

40

-120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120

change of the meridional stresses while discharge of the silo structure[MPa]

hei

gh

t [m

]

A B C cross A cross B cross C

EC stiff 47m EC stiff 15m EC stiff EC wall 47m EC wall 15m EC wall

STN VYSTUZ 47m STN VYSTUZ 15m STN VYSTUZ STN wall 47m STN wall 15m STN wall

Fig. 20 Comparison of measured changes of meridional stresses and data from FEM analysis

during discharge process

0

5

10

15

20

25

30

35

40

-120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120

amplitudes of the tracked meridional stresses [MPa]

hei

ght

[m]

A B C cross A cross B cross C

Fig. 21 Changes of the stresses on strain-gage devices during shut down of the clinker furnace,

measurements 40-43, 28.1.2011-25.2.2011

On Fig. 21 measured data during shut down of the clinker furnace are shown. During that

period were noticed only minor changes of the stresses on the silo walls. That measurement proved

sufficient stability of the strain-gage devices in rather long period. Again, dispersion of the

measured data was increasing with increasing height of the installed strain gages. Strain gages

installed near horizontal restraint proved the smallest dispersion of the measured data.

Lessons learned from usage of the long-term strain gage monitoring

During overall long term monitoring was strain-gage device working and react properly on

changes of usage conditions (height of filling, temperature of the wall, outside temperature). Usual

problem was that strain gage was not able to maintain “zero” reference value in long term period.

Mostly, this problem was dominant on strain gages installed on the “L” restraint. On these strain

gages, values of the changes of the circumferential stresses lowered down to the value

approximately -400MPa. Trends of behavior of the installed strain gages are similar.

-500

-450

-400

-350

-300

-250

-200

-150

-100

-50

0

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53

number

cha

ng

e o

f st

ress

[M

Pa

]

Fig. 22 Output from strain gages “L” restraint, circumferential stresses

Measurement in short time period proved that strain gages were working properly and

outputs from the strain gages followed data of the filling the silo (vertical channel) and data

describing the temperature of the environment and temperature of the walls of the silo (horizontal

channel).

-80

-70

-60

-50

-40

-30

-20

-10

0

25.0 25.2 25.4 25.6

number

chan

ge o

f st

ress

[M

Pa]

-140

-130

-120

-110

-100

-90

-80

-70

-60

26.0 26.2 26.4 26.6 26.8

number

chan

ge o

f str

ess [

MPa

]

Fig. 23 Output from strain gages “L” restraint, circumferential stresses, 1.7.2010, 25.7.2010

Application of the strain gages is strongly recommended when temperature exceed at least

15°C. By lower temperatures were noticed problems with setting of the glue that led to

impossibility of application of the strain gages. It is necessary to take special care about isolation of

the strain gage device against weather conditions, humidity, water, dust, etc.. Installation of the

strain gages in required quality were time consuming, therefore were maximum 4 strain gages

installed in one day (2-3 workers). Generally, it is possible to validate used strain gages as

conditionally sufficient for long term exterior monitoring of the construction that are exposed to

difficult weather conditions. It is recommended to install several strain gages to provide possibility

of comparing results.

References

[1] STN EN 1991-4, Eurokód 1: Zaťaženie konštrukcií. Časť 4: Silá a nádrže, Slovenský ústav

technickej normalizácie, 2006

[2] STN EN 1993-1-6, Eurokód 3: Navrhovanie oceľových konštrukcií. Časť 1-6: Všeobecné

pravidlá. Pevnosť a stabilita škrupinových konštrukcií, 2007

[3] CARSON, JOHN W., Handbook of powder technology, Toronto, Jenike & Johanson inc. ,

2001, 15 str.

[4] AGÓCS, Z., BRODNIANSKY, J., ÁROCH, R., SLIVANSKÝ, M.., Expertízne posúdenie

technického stavu oceľových konštrukcií slinkových síl PC1 a PC2. Návrh opatrení na

zabezpečenie prevádzkovej spoľahlivosti a bezpečnosti objektov posudzovaných síl v

závode HOLCIM, Bratislava, SvF STU, 2006

[5] Ansys release 11.0, Documentation for ANSYS, Ansys, Inc. , 2007, USA

[6] KŘUPKA, V., SCHNEIDER, P., Konstrukce aparátů, PC-DIR, 1998, 290 str., ISBN 80-

214-1124-4

[7] GOBRATOV, N., VALENTA, J., Statika skořepin a skořepinových konstrukcí, SNTL,

1972. Praha

[8] ECCS – European convention for constructional steelwork, Buckling of shells – European

design recommendations, 2008, ISBN 92-9147-000-92

[9] CARSON, J., HOLMES, T., Silo failures: Why do they happen?

www.inti.gov.ar/cirsoc/pdf/ silos /TQ407B-G.pdf

[10] DOGANGUN, A., KARACA, Z., DURMUS, A., SEZEN, H., Cause of damage and failures

in silo structures

http://www.inti.gov.ar/cirsoc/pdf/silos/ASCE_JPCF_Silos_3_2009.pdf

[11] TENG GUANG, J., Buckling of thin shells: Recent advances and trends,

http://shellbuckling.com/papers/1996bucklingsurvey.pdf

[12] ROMBACH, G., AYUGA, F., NEUMANN, F., VÁZQUEZ, E., Modelling of granular flow

in silos based on finite element method Ansys vs. Silo,

http://www.tu-harburg.de/mb/PDF-Dokumente/2005-powder-grains.pdf