[Paper ID] PERFORMANCE LEVEL EVALUATION OF TELECOMUNICATION TOWER

USING PUSHOVER ANALYSIS

Nadya Nor Azila1, Atika Ulfah Jamal 2, Rr Nur Ratri Purnama Dewi 3

ABSTRACT: Nowadays, telecommunication become an important part in human life. According to Indonesia National Standard (SNI) 03-1726-2012, telecommunication tower include as vital building. Telecommunication tower has a functions as a place for antenna in order to provide telecommunication signal for customers around the tower area. As we know, Indonesia is one of the country that is prone to earthquakes, so it necessary to evaluate the performance level of the tower structure when the earthquake occurred. As a vital building, telecommunication tower not allowed to be damaged, because telecommunication plays an important role in a process of emergency response. This evaluation also include as mitigation process because it is done in order to identify the risks that possibly arise in the event of disaster. Therefore, telecommunication tower buildings in Indonesia can be planned well to serve the needs of community well although disasters occur. The telecommunication tower that analyzed in this study is a tower with a height of 42 m. The analysis of performance level evaluation of the telecommunication tower structure using coefficient displacement methode according to FEMA 356 and pushover analysis done using software SAP2000. This reasearch is aimed to determine the location of the plastic hinge on tower structure so it can know the location of the damage if affected from the earthquake loads. Moreover, this study also to know the performance level of telecommunication tower structure when the earthquake occured. The result shown that structure still on elastic condition when affected by earthquake loads. in addition, the location of plastic hinges occured on the bottom part of the tower, while from the analysis it known that performance level of tower structure is Immediate Occupancy level.

Keywords: Performance evaluation, performance level, telecomunication tower, pushover analysis

1. INTRODUCTION

Telecommunication tower is one of the public facilities that need to design safely. Telecommunication tower has different seismic design philosophy if compare with common building, which should be remain completely elstic to against earthquake loads. This is because of the antenna that installed on the tower is very sensitive to rotation and displacement. In other hand, according to building codes, common building allow the element to having non-linier behaviour during strong ground motion.

Beside that, the different between telecommunication tower and common building is on the performance level of the structure. Telecommunication tower should be on the fully operation level because it plays important roles in handling post disaster management. The smoothness of telecommunications networks becomeone of the important points in disaster risk reduction. Therefore, it is very important to have safe telecommunications tower so it does not interfere with the performance of public service although disasters occurs.

1 Student of Earthquake Engineering, Magister of Civil Engineering, Faculty of Civil Engineering and Planning, Universitas Islam Indonesia. Email: nadyanor.azila@yahoo.co.id

2 Lecture of Building Structure, Civil Engineering Department, Faculty of Civil Engineering and Planning, Universitas Islam Indonesia,. Email: atika.ulfah@uii.ac.id

3 Civil Engineer, PT. Handasa Konsultan, Indonesia. Email: rr.nurratri@gmail.com

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[Paper ID] The purpose of this research is to determine the performance level of a telecommunication tower also to know the location of the plastic hinge that occurs so that the location of the possible damage can be predicted. It can also be used to check the design of the telecommunication tower, whether it has been designed to be completely elastic or not.

2. BACKGROUND

As we know that Indonesia is one of the country in the world which has a high rate of earthquake occurrence. This is because the geographical location of Indonesia which became the confluence site of three plates and have a lot of active volcanoes spread across almost the entire island in Indonesia. Based on these facts, it is very important to have a secure building in order to reduce the risks caused by the earthquakes, tellecommunication tower is one of them.

Research on tower is still not popular in Indonesia, whereas according to Indonesia National Standard (SNI) 03-1726-2012, tower include as a vital building so it must be designed safely. However, there are many overseas studies that have been discuss about telecommunications tower, one of the first studies conducted by Kimura and Konno. Their study was discussed about the effects on steel tower caused by Tokachi Earthquake on 1968. The result was full scale measurements and earthquake response analysis on actually existing steel tower for microwave antennas that erected on the roof of buildings by Nippon Telegraph and Telephone Public Corp. It shown that resonant phenomenon between steel tower and building has wide range of frequency. It also proved that seismic forces that occurred in the tower due to strong earthquake might be exceed the wind force.

Another example is a research from Soltanzadeh which discuss the seismic performance of self supporting telecommunication towers. In this study, they using ten existing 4-legged telecommunication towers with heights ranging from 18 to 67 meter and have been designed and installed in Iran. At first, nonlinear static analysis has been applied to all towers. For this purpose, three different vertical distributions of lateral load have been utilized. Then, both target displacement approach and capacity spectrum approach have been considered to calculate the seismic performance point of towers. At last, three equations have been presented to estimate towers yield base shear and base shear that corresponds to immediate occupancy level for 4-legged self-supporting telecommunication towers.

In other hand, Amiri investigated about earthquake amplification of telecommunication tower using eight sample of towers with ranging from 18 to 67. The entire samples are telecommunication towers located in iran. The objective of the research was to estimate the base shear and vertical response of the telecommunication tower. Strong motion earthquake are applied to these towers in both vertical and horizontal direction by using linier dynamic analysis. In other research, Amiri also investigate the seismic response of telecommunication tower using ten existing samples of towers in iran. The purpose of this study was to know the dynamic response of tower under seismic loading. These towers are under the effects of wind and earthquake loading. Wind effects treated as static load according to TIA/EIA code, while for the earthquake effect based on iranian seismic code of practice and normalized spectra of Manjil, Tabas, and Naghan.

3. RESEARCH OBJECT

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[Paper ID] There are two types of tower mainly known as greenfield and roof top. Greenfield tower also known as self-supporting tower, it categorized in to two groups of 4-legged and 3-legged. This research used 4-legged self supporting tower with height of 42 meter. This paper focuses on the performance level of existing telecommunications tower in Indonesia. The illustration of the tower can be shown on Figure 1.

Figure 1. 4-Legged Self Support Tower height 42 m

Tower as shown in figure 1 is the telecommunication tower that analyzed in this study. All connections ,materials, and sections were assumed to be in good working order and data provided by assumed to be correct. This tower has 42 m height and 4m for the bottom width. It consisted of four legs and 17 panels. Each member of the structural model is from Equal Angle Steel (EA) which has 245 Mpa yield strength (fy 245).

4. MODELING PARAMETERS AND ACCEPTANCE CRITERIA FOR NONLINEAR PROCEDURES

This research using SAP2000 as software for analysis purposes. All element selections are steel. In modeling the towers, all the members including the legs, the horizontal and diagonal members have been considered. Foundations of the tower are assumed perfectly rigid. Loads that used in this analysis are dead load which from the member weight, antenna and wind. In this research, all structures used nonlinier static procedure for pushover analysis according to the parameter of displacement coefficient FEMA 356.

According to FEMA 356 (2000), steel braced frames shall be defined as those frames that develop seismic resistance primarily through axial forces in the components. Steel braced frames act as vertical trusses where the columns are the chords and the beams and braces are the web members. In order to model nonlinear behavior in any structural element, a corresponding nonlinear hinge must be assigned in the structure model. Nonlinear hinges were assigned to the following structural elements expected to undergo inelastic deformation. The nonlinear load-deformation behavior of member of tower structures shall

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[Paper ID] be modeled as shown in Figure 2, with parameters a, b, c, as defined in Tables 5-6 and 5-7 at FEMA 356.

Figure 2. Generalized Force-Deformation Relation for Steel Elements or Components

Tabel 1. Modeling Parameters and Acceptance Criteria for Nonlinear Procedures

Component/Action

Modeling Parameters Acceptance Criteria

Plastic Deformation

Residual Strength

RatioPlastic Deformation

a b c IO LS CPBraces in Compression

0,5ΔC 9ΔC 0,2 0,25ΔC 5ΔC 7ΔC

Braces in Tension 11ΔT 14ΔT 0,8 0,25ΔT 7ΔT 9ΔT

ΔC is the axial deformation at expected buckling load.

ΔT is the axial deformation at expected tensile yielding load.

Parameters in displacement coefficient methode (FEMA 356) can be modified according to the field condition. In this analysis, as input data parameters for displacement method on SAP2000, using Response Spectra region Yogyakarta, Indonesia as demand spectrum definition with moderate soil type. This data was obtained from website of Public Works (http://puskim.pu.go.id/Aplikasi/desain_spektra_indonesia_2011/) and can be seen on Figure 3.

Figure 3. Spectrum response Yogyakarta, Indonesia with moderate type soil

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[Paper ID] Coefficient of seicmic reduction factor (R) and structure importance factor (I) that used in

correction of spectra ordinate are and characteristic period of respon

spectrum (Ts) at 0.2 sec, where Ts is the value of T at short period. Based on Table 3-1 FEMA 356, obtained a value of Cm = 0.9 for SBF (Steel Braced Frame) buildings type. Table 3-3 FEMA 356 shows that the value of C2 = 1,0 for buildings performance level on immediate occupancy (IO) and value of C3= 1,0 taken from engineering judgement based on base shear-deflection relationship on post-melting condition that shows increase curve (positive stiffness).

There are two steps of Loading type in pushover analysis, first is analysis which have not consider the nonlinear condition where the structure given the gravity loads, and the second steps was the analysis followed by providing lateral load pattern which given monotonic gradual with the lateral load pattern. Lateral load pattern is a seismic forces that occur at each level and have been calculated from static loading equivalent to the following equation:

With the value of the exponent k is related to the period of the structure. For a structure with

a period of ≤ 0.5 seconds, the value of k = 1.

5. ANALYSIS RESULT AND DISCUSSION

From the structure analysis, obtained the value of of Ti of the first mode (range of vibration) on the y-axis of building and second mode of x-axis of building was 0.4563 seconds. Natural vibration period which take into inelastic condition or effective vibration period (Te), can be obtained by the curve of pushover analysis result using displacement methode FEMA 356. The value of Natural vibration period early elastic (Ti) and the early stiffness of buildings on direction of interest (Ki), lateral stiffness of buildings (ke), efective vibration period (Te) and ratio of post-melt stiffness against effective elastic stiffness (α) from the analysis using displacement coefficient FEMA 356 shown on Table 2.

Table 2. The value of effective natural vibration period used displacement coefficient method (FEMA356)

Ti

(sec)Ki

(KN/m)Ke

(KN/m)Te

(sec)α

0,4075 437,9474 437,9474 0,4075 1

Pushover analysis result shown that the X-direction loading analysis stops in step (step) 12th, at the time of displacement of the control points according to determined value at 0.5 m and a base shear force of 161.630 kN. While the pushover analysis of the Y-direction, stops in step (step) 10th, which is when the control point displacement at 0.5 m and a base shear force of 161.093 kN.

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[Paper ID]

(a). X- direction (b). Y- direction

Figure 4 . Pushover Curve and performance point using displacement coefficient method (FEMA 356)

Figure 4, show that the base shear force and displacement occurs when displacement target is reached on pushover loading at X-direction and Y-direction, the value shown on Table 3.

Tabel 3. Displacement target by FEMA 356

Pushover Loading Direction

Base Shear Force (V1)

(kN)

Displacement Target (FEMA 356)

Vt(kN)

δt

(m)

X-direction 161,630 55,858 0,128Y-direction 161,093 55,858 0,128

From Table 3 can be seen the value of base shear force caused of lateral loads x-direction and y-direction were Vtx= 55,858 kN < V1x = 161,630 kN and Vty= 55,858 kN < V1y = 161,093 kN so based on displacement coefficient method (FEMA 356,2000) structure behaviour of X-direction and y-direction earthquake design still on elastic condition.

Indonesian National Standard 03-1726-2012 clause 10.2.6 states that the displacement requirement does not apply to non-building structures if a rational analysis proves that the allowed displacement can be exceeded without reducing the stability of the structure or the connecting object.

From pushover analysis, can be known the location of plastic hinges that occured in the tower building. The analysis result shown that the point of the performance of the structure in the X-direction pushover loading is reached at step 5th, there are three elements that have been intemperate IO (Immediate Occupancy) and the Y-direction pushover loading is reached at step 5th, there are five elements that have been intemperate IO (Immediate Occupancy). The position of the damaged structural elements (plastic joints) when the performance point is reached can be seen in Figure 5 for the loading of X-direction and Figure 6 for the Y-direction loading. From Figure 5 and 6 it can be seen that the first plastic hinge occurred on bottom part (panel 17th).

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[Paper ID]

Figure 5. Plastic hinge position at pushover loading X-direction

Figure 6. Plastic hinge position at pushover loading Y-direction

According to FEMA 356 (2000), determination of the level of performance of the structure (structural performance levels) based on structural drift ratio criteria obtained when the performance point is reached. Structural drift ratio obtained by the displacement that occurs at displacement control point, which on joint number 71.

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[Paper ID] Table 4. The value of structural drift ratio acording to displacement control point when the

structural performance level reached Pushover loading

direction

Elevation of control point (m)

FEMA 356δt

(m)Structural drift (%)

X- Direction 42,00 0,128 0,305Y-Direction 42,00 0,128 0,305

Table 4 shows that the value of structural drift ratio was smaller than 1%, so can be concluded that structural performance level according to FEMA 356 was Immediate Occupancy (IO). This is because the required drift-ratio limit from FEMA 356 (2000) for Immediate Occupancy (IO) performance level is 1%.

6. CONCLUSIONThe results obtained from this study can be summarized as follows :1. structure behaviour of X-direction and y-direction earthquake design still on elastic

condition when affected by earthquake loads.2. The first plastic hinge occurred on bottom part (panel 17th)3. Performance level of tower structure is Immediate Occupancy (IO)

7. REFERENCE

BSN, 2012, SNI 1726-2012, Standard of Earthquake Resilience for Building, Badan Standardisasi Nasional. Jakarta

FEMA 202, 1990, Earthquake Resistant Conctruction of Electric Transmission and Telecommunication Facilities Serving the Federal Government Report, Federal Emergency Management Agency, Washington, D.C

FEMA 356, 2000, Federal emergency Management Agency. Prestandard and Commentary for Seismic Rehabilitation of Building. Federal Emergency Management Agency, Washington, D.C

G. Ghodrati Amiri and et.al., 2007, Earthquake Amplification Factors for Self-supporting 4-Legged Telecommunication Towers , World Applied Science Journal 2 (6). P. 635-643.

G Ghodrati Amiri, S.R. Massah, 2007, ‘Seismic Response of 4-Legged Self-supporting Telecommunication Tower’, International Journal Engineering, Iran, Vol. 20. No. 2. P.107-126.

G. Ghodrati Amiri and et.al. ,2004, Seismic Behaviour of 4-Legged Self-supporting Telecommunication Towers, World Conference on Earthquake Engineering, Vancouver, Canada. No. 215.

G. Soltanzadeh and et.al., 2014, Seismic Performance of 4-Legged Self-supporting Telecommunication Towers, International Journal of Applied Sciences and Engineering Research, Vol.3, Issue 2. P. 319-322.

Konno T, Kimura E., 1973, Earthquake Effects on Steel Tower Structures Atop Buildings, Proceedings of the 5th World Conference on Earthquake Engineering, Rome, Italy, Vol. 1. P.184-193.

Research Institute for Human and Settlement, 2011, Desain Spektra Indonesia, (http://puskim.pu.go.id/Aplikasi/desain_spektra_indonesia_2011/) Acsessed on October 2, 2014

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