Livaoglu SSI

7/29/2019 Livaoglu SSI

http://slidepdf.com/reader/full/livaoglu-ssi 1/8

Heat Effects on Dynamic Response of 52m-Steel Chimney via in-situ Modal

Testing

R. Livaoğlu

Gümü şhane University, Department of Civil Engineering, Gümü şhane, Turkiye

ABSTRACT:

In this study, heat effects on dynamic behaviour of a 52m-steel chimney in Yesilyurt township of Samsun City inTurkey was studied via in-situ modal testing,. For this purpose before commissioning of the chimney, a series of

test was realized and after the fabric get to work same tests were repeated for the same sensor locations to realizehow the heat affect on the dynamic response of chimney. The ambient and forced vibration tests are proven to befast and practical procedures to identify the dynamic characteristics of such kind of structures. Dynamic testingof the towers promises a widespread use as the identification of seismic vulnerability of lifeline structures

becomes increasingly important. The data presented in this study is considered to be useful for the researchersand engineers, for whom the heat effect on steel chimney is a concern. The present study also aims to provide the

designer with the material examples about the influence of heat on the seismic performance of steel chimney bymeans of reflecting the changes in dynamic behavior with the consideration of heat.

Keywords: Steel chimney, Dynamic response, Heat effect, ambient vibration testing, In-Situ Modal Testing

1. INTRODUCTION

Chimney structures are special-type structures which have different features than many other types of

structures and are subjected to different loading conditions during their service life. The design of

these slender structures becomes even harder considering the combined effects of local site conditions,

the type of foundation, seismicity of the region, the magnitude and characteristics of wind loading,

machine loading and loading which arise due to internal and external surface temperature difference.

All the above factors necessitate a detailed work in the design of these type of structures. In particular,

due to the fact that the dimensional proportion of the slender chimney structure in the longitudinal

direction is high as compared to other dimensions, the soil-structure interaction effects in chimney-

type structures should be carefully considered. In general design approach, the base of the structure is

assumed to be fully fixed and due to the reasons stated in this assumption may therefore lead tounjustified errors in design. Besides the peculiarities regarding the slender geometry and the

foundation behavior possible adverse local site conditions may further complicate the interaction and

may completely alter the dynamic behavior of the structure. One other important factor which affects

the behavior is the high levels of temperatures the chimney structure is subjected to during its service

life. Particularly, chimneys built-up using steel plate assemblies are more prone to such temperature

effects. As well known, steel suffers a progressive loss of strength and stiffness at increased

temperatures. The change can be seen at temperatures as low as 300°C. Although melting does not

happen until about 1500°C, only 23% of the ambient-temperature strength remains at 700°C. It is

considered that the dynamic behavior of the structure would be further altered under the effects of both

soil-structure interaction and loading due to temperature change.

Tall and slender steel structures are susceptible to wind action which is, in general, critical to their

design (Cheng and Li, 2009). Besides common failure causes for these types of structures, such as



wind load, foundation settlement and earthquakes, chimneys are subject to high chemical loads,

temperature loads, vortex shedding and ring oscillation ovalling (Simonović et all, 2008; Tranvik, P. and

Alpsten, 2002). Diverse loading of the chimney makes it prone to numerous different modes of failure,

such as mechanical overload, force or temperature induced elastic yielding, fatigue, corrosion, stress

concentration, buckling, wear, vibration etc. (Simonović et all, 2008).

In accordance with the above mentioned considerations, an existing 52 m long steel chimney structure

was selected in Samsun Yeşilyurt Ironworks to examine its dynamic characteristics. On-site modal test

methods were employed both under operating and non-operating conditions. It is noted that under

operational conditions outside temperature of the chimney is around 150-170oC while the inside

temperature may reach around 300oC. Detailed field investigations were carried out to identify the

material and structural properties of the chimney. Using the data obtained numerical models were

produced and analyzed assuming both fixed-base boundary condition. So it is aimed to understand

effect of the temperature loading on the dynamic behavior of the steel chimney structure, using the test

results and numerical analysis estimations.

2. DETAILS OF TEST AND CHIMNEY

This section provides a brief description of testing equipment, gives the details of the 52m-chimney

and the soil deposit underlying it, and summarizes the arrangement of instrumentation. The details of

the vibration tests and corresponding analytical solutions are also presented in this section.

2.1. Testing Equipment The testing equipment consisted of an inertial shaker with force capacity of 250 N, a GW-300 W

power amplifier and a four-channel data analyzer (Data Physics DP Quattro). Inertial shakers are

permanent magnet devices, which may be sealed for short test operations. Cooling is necessary for

prolonged use of these devices, which can be achieved using a small cooling blower that circulates the

air through the inertial shaker. The power amplifier used can produce a sine signal source to provide

simple excitation for shaker. Alternatively, externally generated signals may be applied to theamplifier. Because of the type of structure as slenderness etc, however there was no reason to use

shaker system, so machine load working all around the chimney and the wind load exposed was

enough for the test. Two accelerometers located on perpendicular direction to each other (MMF

KB12VD 0.5g range, seismic) were used for data collection. Figure 1 shows the test setup and

accessories along with instrumentation equipment used in testing.

Figure 1. (a) Test setup and accessories (b) accelerometer.

(a)

(b)



2.2. Soil Profile A detailed geotechnical investigation of the test location was conducted. A total of nine boreholes

were drilled up to a depth of 20 m. Standard Penetration Tests (SPT) (ASTM D1586) were conducted

with 1.5 m intervals during drilling. A series of laboratory tests, such as Moisture Contest Test (ASTM

D4643 – 08), Atterberg Limits Test (ASTM D4318 – 05), Triaxial Test (UU) (ASTM D2850 - 03a)

and Direct Shear Test (ASTM D3080 – 04), were conducted on a variety of disturbed and undisturbed

samples recovered from the boreholes.

According to the results of the geotechnical investigation, the first 1.5 m of the soil profile was

classified as organic soil with very low bearing capacity. A simple relationship between N60 and low

strain shear modulus (G = 0.83 N60 ) was adopted in order to derive a reasonable approximation of

shear wave velocity profile (Imai and Yoshimura 1970). It is recognized that SPT correlations are

reliable only for granular material. However, using this correlation estimate dynamic stiffness of

clayey soil layers herein would have a small effect given for the limited thickness of the clay layer.

2.3. 52m-Chimney and Instrumentation Details

The chimney investigated in this study is located in the Yesilyurt township of Samsun City, Turkey

(See Figure 2). The location of the structure is classified as seismic zone 2, according to Turkish

Seismic Zoning. The structure is a steel chimney with a height 52 m from base. Chimney construction

consists of windshield (outer wall of the chimney) and separate flue duct inside windshield (inner

wall) that carries flue gases to the atmosphere and this layer is constructed to prevent heat effect on

steel as 70 mm of refractor concrete (see Fig.3).

Figure 2. (a) Image and (b) schematics of chimney.

The chimney is supported on reinforced concrete (R/C) boot as seen Fig.2 and has been built as 1.25 m

of inner radius with changing thicknesses material along to height between 20 mm to 8 mm. Chimneywalls are made of bolted connection steel part (6m-tubes) strengthen with welded 100 NPU at every

(a) (b)



1.5 m of level. Windshield sections are flanged together, while flue duct sections lean onto each other.

The R/C boot fixed the R/C foundation on 9-piles. Due to realized test in test piece taken from the

chimney, steel yielding strength was measured as 370 MPa. The steel Young’s modulus and unit

weight were interpreted as 2.1x105

MPa and 78kN/m3, respectively.

2. NUMERICAL MODEL OF 52M CHIMNEY

A series of numerical analyses was conducted to simulate the behavior extracted from the field tests.

In order to analyze both SSI system and the fixed base system, the finite element software ANSYS

(2006) was used. Modeling the chimney-foundation/soil system was performed using the finite

element model depicted in Figure 3. The main body of the structure and connection elements are

modelled using shell elements (six degree-of-freedom per node) for the supporting system and R/c

boot and foundation with solid element (eight node, three degree-of-freedom per node). Soil was

modeled using eight-node solid elements with three degrees of freedom in each node. Linear elastic

material model was assumed for soil and structural components of the tower.

Figure 3. Considered finite element model of the 52 m chimney-soil/foundations in this study

The simulation of the infinite medium in the numerical modeling of dynamic soil-structure interaction

problems is crucial. A general method of treating this problem is to divide the infinite medium into the

near field (truncated layer), which includes the irregularity as well as the non-homogeneity of the

foundation, and the far field, which is simplified as an isotropic homogeneous elastic medium (Wolf

and Song 1996). More appropriate approximations can be achieved using the artificial and/or

transmitting boundaries for preventing reflection and radiation effects of the propagating waves from

the structure-foundation interface. There are different types of boundaries for frequency and time

domains, including the viscous boundary (Lysmer and Kuhlemeyer 1969) and the damping-solvent

extraction (Wolf and Song 1996). In this study, viscous boundary is used for three dimensions

problems (see Fig 3). The 3D viscous boundaries were situated at two times of Rayleigh wavelength

from the centre of the structure. Maximum element size within the FE mesh was between 1/6th

and

1/8th

of the minimum Rayleigh wavelength in order to allow the higher frequency components of the

input motion to travel within the soil medium (Kramer 1996).

3. DISCUSSION OF RESULTS

The results of the numerical analyses performed with fixed and flexible base assumptions are presented in this section along with the measurements of vibration tests. The results of the simulations

XY

Z

Z



performed using large strain excitations are then presented. The comparisons of the results of

numerical simulations with the field measurements are presented and a detailed discussion is provided

in order to reflect the importance of the consideration of foundation flexibility in seismic design of

these structures.

3.1. Fixed Base Model

The mode shapes of the fixed base model are shown in Figure 4. The first three vibration modes were

identified based on mass participation ratios. The first mode frequency was calculated as 1.372 Hz.

The modal frequencies for 2nd

and 3rd

modes were 6.142 Hz and 15.125 Hz, respectively. Mass

participation ratios for the first three modes were calculated as %87, %10 and %0.5, which constituted

over %98 of the total mass of the structure. It is worth to say here that in calculation of mass

participation ratios only steel chimney are taken into consideration and R/C boot mass are neglected.

So this part of structure behaves quite rigidly.

Figure 4. First three mode shapes and frequencies of fixed base model.

3.2. 52m-Chimney-Foundation/Soil Model

As mentioned earlier, the foundation soil stratum is composed of high plasticity soil. Therefore, soil-

structure interaction effects are expected to be substantial. Figure 5 gives the first three mode shapes

of the interacting soil-structure model. The results show that there are significant deviations of the

modal frequencies due to soil-structure interaction. The first mode frequency of soil-structure system

reduced from 1.372 Hz to 1.016 Hz. The typical period lengthening effect due to the flexible

foundation reduced the second and third mode frequencies to 4.553 and 15.695 Hz, respectively.

Figure 5. First three mode shapes and frequencies of chimney on flexible foundation.

Mod_1 (f=1.016 Hz) Mod_3 (f=15.695 Hz) Mod_2 (f=4.553 Hz)

XY

Z

XY

Z

XY

Z

Mod_1 (f=1,372 Hz) Mod_3 (f=15,125 Hz) Mod_2 (f=6.142 Hz)

XY

Z

XY

Z

XY

Z



3.3. Results of Vibration Tests

Figures 5 shows the normalized transfer functions of the system under ambient vibrations for three

different measurements from two perpendicular channels and their power spectrums. In the Fig 5.a the

result obtained before servicing of the chimney are given or ,in other worlds, dynamic behaviour for

chimney is illustrated for ignoring heat effect. Due to taken measurement after beginning to the service

of the chimney the power spectrum calculated is given in Fig 5b. 0.94, 4.595 and 1.14 Hz modal

frequencies were measured for the first three modes using ambient vibration for non-heat condition.

Similarly modal frequencies are occurred 0.90, 4.548 and 11.79 Hz under heat effect, respectively

Identical modal frequencies were similarly obtained under all loading conditions.

Figure 8. Power spectrum of the measured response to ambient vibration (a) for non-heat condition and (b)under heat effect..

3.4. Comparison of the Numerical Simulations and Field Measurements

As mentioned before, steel suffers a progressive loss of strength and stiffness at increased

temperatures. The change can be seen at temperatures as low as 300°C. Although melting does not

Mod_1 ( =0.90 Hz)

Mod_2 ( f =4.548 Hz)

Mod_3 ( f =11.79

Hz)

P o w e r a m p l i t u d e

f (Hz)

(b)

Mod_1 ( f =0.94 Hz)

Mod_3 ( f =11.14 Hz)

Mod_2 ( f =4.595 Hz)

P o w e r a m

p l i t u d e

f (Hz)

(a)



happen until about 1500°C, only 23% of the ambient-temperature strength remains at 700°C. So when

tests were realized in structure after servicing of the chimney, the measurement of heat at the bottom

surface of windshield (outer wall of the chimney) was about 150~200 °C due to wind direction. So

estimated response given in the table 3.1 prove that heat reduce the material strength and accordingly

the flexibility of the chimney getting higher. In this example, increases on heat was calculated as 200oC approximately and this cause 9% loss on stiffness of all structure.

The results presented earlier in this study showed that the soil-structure interaction effects on the

studied structure is significant. It is well known that the tall and slender structures situated on soft soil

deposits are exposed to such critical period lengthening effects (Aviles and Perez-Rocha 1998). The

agreement between the results of numerical model and field tests shows that the numerical model is

representative of the in-situ soil-structure conditions. Table 3.1 summarizes the results of numerical

simulations and field tests. As can be seen from the table, the measured and simulated values differ by

only about 7% for the first mode. The second mode field behaviour differed by 1% only. One can

clearly see that first and second mode are enough to represent all dynamic behaviour of the structure.

So these variations are considered to be minor. Thus, the numerical model is representative of the

actual structure in small strain ranges.

Table 3.1. Comparison of Field Test Results and Numerical Simulations.

Mode

Fixed base

model(Hz)

SSI model

(Hz)

Field test

Non‐heat

(Hz)

Field test

Under heat

(Hz)

Error

Between FEM

and Test (%)

Differences

Between heat

condition (%)

1 1.372 1.016 0.940 0.900 7 4

2 6.142 4.553 4.595 4.548 ‐1 1

3 15.125 15.695 11.140 11.790 29 ‐6

5. SUMMARY AND CONCLUSIONS

The dynamic response of 52m-chimney situated on a soft soil deposit was studied by means of field

vibration tests and numerical simulations. Geotechnical investigations were carried out to define soil

strata. The ambient vibration tests were conducted to identify the SSI effects and the heat effect on the

small strain dynamic behavior of the structure. The following is the summary of the results and

conclusions arising from this study.

A total of three mode frequencies were obtained from the FEM of the systems under investigation.

First and second modes which are extracted from among a great number of modes can be evaluated as

sufficient because their contributions to total response are approximately 97% or over this value in all

analyses. Therefore, one may say that only four modes can be adequate to estimate the total response

of such a system investigated in this study.

As well known, steel suffers a progressive loss of strength at increased temperatures. The change can

be seen at temperatures as low as 300°C. Although the temperature reaches 200°C maximally in this

study, nearly 9% loss of stiffness were observed. As mentioned in well-known literature, getting lost

of steel elastic characteristic starts at almost 300°C, by all means, this study showed that lower

temperature than this value, the deterioration on dynamic characteristic of the investigated structure

taken place. So the effect of the heat through this temperature value must be consider by the designer

who must provide such a structure to heat effect.

The results of the vibration tests provide strong support for the finite element models presented in this

paper. Thus, it can be easily stated that the proposed finite element models themselves are the

meritorious approximations to the real problem, and this makes the models appealing for use incomprehensive investigations.



ACKNOWLEDGEMENT

I would like to give my very special thanks to Yeşilyurt fabric director M.E.Çetin LİVAOĞLU (M.Sc)

providing endless support and resources during test and I also want to thank Yeşilyurt fabric employee

for their support on preparation of the test setup.

REFERENCES

ANSYS. (2006). Theory Manuel; Edited by Peter Kohnke, Twelfth Edition . SAS IP , Inc, 1266.Aviles, J. and Perez-Rocha, E.L. (1998). Effect of Foundation Embedment During Building-Soil Structure

Interaction. Earthquake Engineering and Structural Dynamics, 27: 1523–1540.Cheng, J. and Li, Q.S. (2009). Reliability analysis of a long span steel arch bridge against wind-induced stability

failure during construction, Journal of Constructional Steel Research, v.97:3-4, 132-139

Imai, T. and Yoshimura, M. (1970). Elastic shear wave velocity and mechanical characteristics of soft soildeposits. Tsuchi to Kiso, 18:1, 17-22.

Kramer, S.L. (1996). Geotechnical earthquake engineering. Prentice-Hall Inc., Englewood Cliffs, N.J.Lysmer, J. and Kuhlmeyer R.L. (1969) Finite dynamic model for infinite media, ASCE. Engineering Mechanic

Division Journal, 95, 859–877.Simonović A.M., Stupar, S.N and Peković, A.M., (2008). Stress Distribution as a Cause of Industrial Steel

Chimney Root Section Failure. FME Transactions, v.36, 119-125Tranvik, P. and Alpsten G.: Dynamic Behaviour Under Wind Loading of a 90 m Steel Chimney, Alstom Power

Sweden AB, Växjö, 2002. Wolf J.P. and Song C.H. (1996). Finite element modeling unbounded media, The 11

thWorld Conference on

Earthquake Engineering , San Francisco 70, 1-9.

Documents

Livaoglu SSI