Essay: Displacement of towered buildings for different seismic zones

Communication towers
ABSTRACT
In today’s era the mobile sector is growing dynamically & the trend of mobile communication is increasing day by day. Generally for telecommunication purpose, the four legged supporting tower are used widely. In the last few years there has been tremendous increase in the communication industries which result in the installation of large number of tower for consistency of network and to increase the coverage area. The availability of land in urban areas is extremely limited for satisfying the ideal installation of tower thus giving no alternative but to adopt roof top towers. As tower plays a significant role for wireless communication network, the failure of such structure in a disaster is a major concern therefore utmost important has been given considering the seismic effect acting on the tower.
Firstly, the seismic effect is consABSTRACT
In today’s era the mobile sector is growing dynamically & the trend of mobile communication is increasing day by day. Generally for telecommunication purpose, the four legged supporting tower are used widely. In the last few years there has been tremendous increase in the communication industries which result in the installation of large number of tower for consistency of network and to increase the coverage area. The availability of land in urban areas is extremely limited for satisfying the ideal installation of tower thus giving no alternative but to adopt roof top towers. As tower plays a significant role for wireless communication network, the failure of such structure in a disaster is a major concern therefore utmost important has been given considering the seismic effect acting on the tower.
Firstly, the seismic effect is considered at various sizes of column of structure, then for different types of soils and in the last, we considered the position of tower at roof top of structure, and it is observed that the displacement is different at various height of structure i.e. the displacement is maximum at the top height of tower and minimum at building. The results obtained from the above analysis are tabulated, compared and conclusions is drawn that as the size of column of structure is increased, the displacement is decreased, also observed the displacement is minimum in soft soil, & displacement is minimum when the position of tower is at the centre of structure and increase the size of column in order to decrease the displacement.
idered at various sizes of column of structure, then for different types of soils and in the last, we considered the position of tower at roof top of structure, and it is observed that the displacement is different at various height of structure i.e. the displacement is maximum at the top height of tower and minimum at building. The results obtained from the above analysis are tabulated, compared and conclusions is drawn that as the size of column of structure is increased, the displacement is decreased, also observed the displacement is minimum in soft soil, & displacement is minimum when the position of tower is at the centre of structure and increase the size of column in order to decrease the displacement.
1
INTRODUCTION
1.1 General
For supporting parabolic antennas which are generally used for microwave transmission for communication and information are use for sending signals for radio, television signals to remote places, a tall structure named lattice tower is installed at specific height. These towers are categorized as three legged and four legged space trusses structure, which are self supporting. These self supporting structures are supported either on ground or on building and are generally square in plan or it may be triangular in plan. The major cause of failures of telecommunication tower throughout the world though still remains to be due to earthquake causes. These structures are generally designed to carry seismic load, which acts as cantilever trusses. As they cover less base area in towers, they are suitable in many situations but these structures demands more steel. The installation of these towers requires the availability of land depending and the location and availability of fund. The use of these towers is more in urban areas, but due to limited availability of land in urban areas, the concept of roof top tower is adopted in which tower is installed of the roof top of the building. Thus by this, the problem of land requirement can be removed and it serves efficiently.
A tower is a tall structure means taller than its width, often by a significant margin. Towers are distinguished from masts by their lack of guy wires and are therefore, along with tall buildings, self-supporting structures. Towers are generally built to take advantage of their height, and can stand alone on the ground, or as part of a larger structure or devices such as a fortified building.
1.2 Types of tower
The different types of communication towers are based on their structural action, their cross-section, the type of sections used and on the placement of tower.
1.2.1 Based on placement of tower
Based on this placement, communication towers are classified as follows:
1) Green field towers
2) Roof top towers
1. Green field tower
Green field towers are erected on natural ground with suitable foundation. Generally, the height of green field tower is 30-200m. Basically, it is placed in rural areas.
2. Roof top tower
Roof top towers are erected on existing building with raised columns and tie beams. Basically, height of the roof top tower is 9-30 m and it is placed in urban areas. It is more economical compare than to green field towers. Roof top tower is a tall skeleton steel structure with relatively small cross-section having larger ratio between height & maximum width. Such towers have their base or foundation on the roof of building or structures, therefore named as roof top tower.
1.3 Objective of the study
The main objectives of the present study are:
1) To study displacement of towered building for different seismic zone like
ZONE II, ZONE III, ZONE IV & ZONE V.
2) Parametric study of the seismic effect for towered building by changing different sizes of column of structure.
3) To analyze the displacement of towered building by changing different soil conditions like soft soil, medium soil & hard soil.
4) To consider the seismic effect of towered building by changing the position of tower over the roof of structure.
5) And conclusion based on parametric study will be done.
1.4 Organization of thesis
This study is presented in five chapters. This chapter contains introduction, importance and objectives of the study. The availability of land in urban areas is extremely limited for satisfying the ideal installation of tower thus giving no alternative roof top towers should be adopted. In second chapter, a detailed literature review pertaining to seismic evaluation & general critics is presented. In third chapter, proposed methodology is presented and lattice tower is designed using Indian standard code specifications. In fourth chapter, analyzed the sample building under different case & conditions. In fifth chapter, result and discussion is presented. In sixth chapter, conclusions based on analysis and future scope is presented.
1
INTRODUCTION
1.1 General
For supporting parabolic antennas which are generally used for microwave transmission for communication and information are use for sending signals for radio, television signals to remote places, a tall structure named lattice tower is installed at specific height. These towers are categorized as three legged and four legged space trusses structure, which are self supporting. These self supporting structures are supported either on ground or on building and are generally square in plan or it may be triangular in plan. The major cause of failures of telecommunication tower throughout the world though still remains to be due to earthquake causes. These structures are generally designed to carry seismic load, which acts as cantilever trusses. As they cover less base area in towers, they are suitable in many situations but these structures demands more steel. The installation of these towers requires the availability of land depending and the location and availability of fund. The use of these towers is more in urban areas, but due to limited availability of land in urban areas, the concept of roof top tower is adopted in which tower is installed of the roof top of the building. Thus by this, the problem of land requirement can be removed and it serves efficiently.
A tower is a tall structure means taller than its width, often by a significant margin. Towers are distinguished from masts by their lack of guy wires and are therefore, along with tall buildings, self-supporting structures. Towers are generally built to take advantage of their height, and can stand alone on the ground, or as part of a larger structure or devices such as a fortified building.
1.2 Types of tower
The different types of communication towers are based on their structural action, their cross-section, the type of sections used and on the placement of tower.
1.2.1 Based on placement of tower
Based on this placement, communication towers are classified as follows:
1) Green field towers
2) Roof top towers
1. Green field tower
Green field towers are erected on natural ground with suitable foundation. Generally, the height of green field tower is 30-200m. Basically, it is placed in rural areas.
2. Roof top tower
Roof top towers are erected on existing building with raised columns and tie beams. Basically, height of the roof top tower is 9-30 m and it is placed in urban areas. It is more economical compare than to green field towers. Roof top tower is a tall skeleton steel structure with relatively small cross-section having larger ratio between height & maximum width. Such towers have their base or foundation on the roof of building or structures, therefore named as roof top tower.
1.3 Objective of the study
The main objectives of the present study are:
1) To study displacement of towered building for different seismic zone like
ZONE II, ZONE III, ZONE IV & ZONE V.
2) Parametric study of the seismic effect for towered building by changing different sizes of column of structure.
3) To analyze the displacement of towered building by changing different soil conditions like soft soil, medium soil & hard soil.
4) To consider the seismic effect of towered building by changing the position of tower over the roof of structure.
5) And conclusion based on parametric study will be done.
1.4 Organization of thesis
This study is presented in five chapters. This chapter contains introduction, importance and objectives of the study. The availability of land in urban areas is extremely limited for satisfying the ideal installation of tower thus giving no alternative roof top towers should be adopted. In second chapter, a detailed literature review pertaining to seismic evaluation & general critics is presented. In third chapter, proposed methodology is presented and lattice tower is designed using Indian standard code specifications. In fourth chapter, analyzed the sample building under different case & conditions. In fifth chapter, result and discussion is presented. In sixth chapter, conclusions based on analysis and future scope is presented.
3
PROPOSED Methodology
3.1 General
Seismically deficient buildings and their rehabilitation are major problems in earthquake regions of India. Recently, the target of building rehabilitation and firmness has gained research attention and various techniques have been developed to achieve this. However, many of the techniques for strengthening raise problem for the occupants, who must have to vacate the building during renovation. In this study the proposed solution is to rehabilitate an existing structure which is seismically deficient. So it is necessary to analyze the behavior of towered building under seismic effect.
In order to achieve the above mentioned objective following methodology is adopted
1. Description and design of the tower.
2. Description and modeling of the towered building.
3. Analyze the behavior of towered building under the effect of earthquake.
4. Study of behavior of towered building under seismic zones.
5. Compare the behavior of towered building after changing the sizes of column.
6. Study of behavior of towered building after changing the types of soil.
7. Study of behavior of towered building after changing the position of tower at the roof of building.
3.2 Description of Tower
A 15m long four legged lattice telecommunication tower is proposed over a G+2 building. The tower is designed in three segments namely, segment 1, segment 2 and segment 3 with height of 6m, 6m and 3m respectively. To tower was designed to withstand lateral load due to wind action only. Wind loads were computed in accordance with IS 875 (part 3) 1987. For resisting lateral forces, the tower is provided with angle section and suitable bracing system. The bracing and angle system were designed in accordance to IS 800:2007 in accordance to the codal provisions. The design details of the tower are presented in Table3.1 and Figure 3.1. The analysis and design of the tower is presented in Appendix-A.
Table 3.1 Design details of the proposed tower
(Description of Tower by Bhosale et al, 2012)
SI No. Details Dimension
1 Height of tower 15m
2 Width of tower
at the top 1m
3 Weight of platform
at the 3m from the top 0.82 kN/m²
4 Weight of miscellaneous item 2.5kN/m
5 Weight of ladder & cage 10% of total weight of tower
6 Terrain category 2
7 Antenna Detail 4 nos. Microwave Antenna
8 Angle section: ISA 80 x 80 x 8
9 Bracing ISA 45 x 45 x 5
Figure 3.1 Lattice Tower
3.3 Description and modeling of the towered building
A two story residential building having an area of 81 sq. meters was selected. The floor to floor height was taken as 3m with external and internal walls having a thickness of 230 mm. Slab thickness was taken as 125mm and beam size was taken as 200mm x 200mm. The size of column varies from 200mm x 200mm to 300mm x 400 mm. Live load on roof was taken as 1.5 kN/m2 whereas, live load on floors were taken as 2.0 kN/m2. The slab load was taken as 4.625 kN/m2. Tower was placed at three different locations on the rooftop and are denoted as case 1, case 2 and case 3 respectively. The tower locations are shown in Figure 3.2-3.7.
The analysis was carried out for four earthquake zones namely ZONE-II, ZONE-III, ZONE-IV and ZONE-V and five column dimensions 200×200, 200×300, 250×350, 300×300, 300×400 were selected. Three soil types namely soft soil, medium soil and hard soil were considered. For all the combination displacement at different heights of towered building will be compared. The combination which gave least displacement is recommended.
Figure 3.2 Model of the G+2 building (case I)
Figure 3.3 Model of the (15m) tower mounted on G+2 building (case I)
Figure 3.4 Model of the G+2 building (case II)
Figure 3.5 Model of the (15m) tower mounted on G+2 building (case II)
Figure 3.6 Model of the G+2 building (case III)
Figure 3.7 Model of the (15m) tower mounted on G+2 building (case III)
3.4 Method of analysis of structures
3.4.1 Response Spectrum Method
A response spectrum is a plot of the peak or steady-state response (displacement, velocity or acceleration) of a series of oscillators of varying natural frequency that are forced into motion by the same base vibration or shock .The resulting plot can then be used to pick off the response of any linear system, given its natural frequency of oscillation. One such use is in assessing the peak response of buildings to earthquakes. The science of strong ground motion may use some values from the ground response spectrum (calculated from recordings of surface ground motion from seismographs) for correlation with seismic damage.
All design against seismic loads must consider the dynamic nature of the load. However, for simple regular structures, analysis by equivalent linear static methods is often sufficient. This is permitted in most codes of practice for regular, low- to medium-rise buildings. It begins with an estimation of base shear load and its distribution on each story calculated by using formulas given in the code (according IS1893 (part- 1): 2002).
4
ANALYSIS of towered building
4.1 General
In this chapter, displacement analysis, of G+2 building with a lattice tower of 15 m height and the grade of concrete used is M20 and steel for main and transverse reinforcement is Fe 415, based on seismic zone (ZONE II, ZONE III, ZONE IV & ZONE V), soil conditions and different size of column using codal provision IS 1893 (Part 1):2002. Comparison of displacement for different parameters of towered building is taken in consideration. Structure analysis (Linear structural analysis) and design are carried out as bureau of Indian standard Codes.
4.2 Details of load and load combinations
Equivalent static load is computed by the combination of various possible loads. Representation of maximum sagging, torsion & hogging is shown by different combinations of load. Variety of loads taken into consideration is as follows.
4.2.1 Gravity Load
Gravity load includes dead loads and live loads. A dead load is defined as the constant or permanent loads of the structure or equipment which will not change in its life-span. The dead load calculation is done as per provisions given in Indian Standard Specification (IS:875 (Part 1) 1987).
A live load is defined as the variable load which can change due to appliances & occupants. The live load / imposed load calculation is done as per provisions given in Indian Standard Specification (IS 875 (Part 2) 1987).
4.2.2 Seismic Load
To calculate seismic load, the towered building is to be situated on the earthquake zones of India. Seismic load calculation is done as per provisions given in Indian Standard Specification (IS 1893 (Part 1) 2002). Mathematical expression for the design of horizontal seismic coefficient (Ah ) as per clause no. 6.4.2 from IS 1893 (Part 1) 2002.
Ah = Z/2 I/R S_a/g
Table 4.1 Zone Factor, Z ( IS 1893 (part-I) 2002 clause 6.4.2)
Seismic Zone II III IV V
Seismic Intensity Low Medium Severe Very Severe
Z 0.1 0.16 0.24 0.36
Table 4.2 Importance Factor, I ( IS 1893 (part-I) 2002 clause 6.4.2)
SI No. Structure Importance Factore
1 important service and community 1.5
building, such as hospital, schools,
Monumental structures. Emergency
building like telephone exchange,
television stations, radio stations,
railway stations, fire station buildings,
large community halls like cinemas,
assembly halls and subway stations,
power stations
2 All other buildings 1
Table 4.3 Response Reduction Factor, R for Building System
( IS 1893 (part-I) 2002 clause 6.4.2)
SI No. Lateral Load Resisting System R
1 Ordinary RC moment frame (OMRF) 3
2 Special RC moment frame (SMRF) 5
4.2.3 Linear static analysis
A conventional analytical method for seismic evaluation or design of building systems is the equivalent base shear method. An approximate base shear is estimated with an equation of the form:
? V?_b=Z/2 I/R S_a/g W
Where Sa/g is the average response acceleration coefficient, W is the seismic weight of the structure and R is the response reduction factor that varies with the ductility of the system. The terms Z and I represent the seismic zone and importance of structure respectively.
4.2.4 Distribution of design force
The design base shear computed in 4.2.3 will be distributed along the height of the building as per the following expression:
? Q?_i =V_b (W_i h^2)/(?W_i h^2 )
Where Qi is design lateral force at floor i, Wi is the seismic weight of floor i and hi height of floor I measured from base, n number of storeys in the building is the number of levels at wich the masses are located.
4.2.3 Load combination
When earthquake forces are considered on a structure, these will be combined as per 6.3.1.1 and 6.3.1.2, IS 1893 (part I) 2002 where the terms DL, IL and EL stand for the response qualities due to dead load, imposed load, and designated earthquake load respectively, in the displacement analysis of different parameters of towered building , the following load combinations will be accounted for:
SI No. Load Combination
1 ELX
2 ELZ
3 DL
4 IL
5 1.5(DL+LL)
6 1.2(DL+LL+EQX)
7 1.2(DL+LL+EQ-X)
8 1.2(DL+LL+EQZ)
9 1.2(DL+LL+EQ-Z)
10 1.5(DL+EQX)
11 1.5(DL+EQ-X)
12 1.5(DL+EQZ)
13 1.5(DL+EQ-Z)
14 (0.9DL+1.5EQX)
15 (0.9DL+1.5EQ-X)
16 (0.9DL+1.5EQZ)
17 (0.9DL+1.5EQ-Z)
The combinations of load for serviceability used in the project as per the Indian Standard Specification (IS 456:2000) Clause 18.2.3.1
SI No. Load Combination
1 1(DL+LL)
2 1(DL+0.8 LL+0.8EQX)
3 1(DL+0.8LL+0.8EQ-X)
4 1(DL+0.8LL+0.8EQZ)
5 1(DL+0.8LL+0.8EQ-Z)
6 1(DL+EQX)
7 1(DL+EQ-X)
8 1(DL+EQZ)
9 1(DL+EQ-Z)
4.3 Procedure for static analysis
Designed the tower manually. Create the model, by selecting node, specifying the node distance, and no. of segments.
Assigned the section angle section and bracing in tower.
Using staad pro designed beams & columns building & also assigned support conditions for column.
Defined the design code as IS 800-2007.
Define the design code as IS 456-2000.
Define material properties as Concrete M-20 Grade and Steel Fe-415.
Define the properties of frame sections as rectangular with reinforcement for beams & columns and assign them to the respective beams and columns of the frame.
Define wall load and slab load as per the details.
Assigned the live load, dead load & the combinations of load for serviceability (according to IS 456-2000) to the building.
Assigned the seismic intensity, importance Factor is taken 1 and response reduction factor is taken 3 with respectively from table 4.1-4.3 & the combinations of load for earthquake (according to IS 1893 (part-1) : 2000) to the building.
In building, slab is considered as a single rigid member during earthquake analysis. For that, all slabs are selected first and apply diaphragm action for rigid conditions.
Mass source is defined, 25% live load (of 2 KN/m2) is considered on all floor of building except at roof level, As per IS: 1893 (part 1) 2002.
Define static load as per IS 1893(part 1) 2002.
Analysis of displacement based on seismic zone for different parameters of towered building.
Several types of output can be obtained from the static analysis:
Maximum story displacement and lateral force at each story.
Comparison for different size of column of building.
Displacement check is carried for different earthquake zones.
Comparison for different types of soil.
Comparison for the different positions of tower on building.
The comparison is plotted graphically.
Output for the linear static analysis can be printed in a tabular form for the entire model or for selected elements of the models is shown in figure 4.1-4.3.
Figure 4.1 sway model of the 15m tower mounted on G+2 building (CASE I)
Figure 4.2 sway model of the 15m tower mounted on G+2 building (CASE II)
Figure 4.3 sway model of the 15m tower mounted on G+2 building (CASE III)
5
RESULTS AND DISCUSSION
5.1 General
In the present study modal analysis was carried out with the help of Staad pro software. The modal analysis helps in determination and analyzed earthquake effect on the towered building & found the displacement in different seismic zone according to codal provision IS 1893 (part-1) : 2000 (ZONE II, ZONE III, ZONE IV & ZONE V), Regardless of architectural and site constraints and parametric analysis of towered building. Following are the explanations for the sections of project:
Section I: Analyzed seismic effect on towered building for different column size under soil condition for various seismic zones.
Section II: Analyzed seismic effect on towered building for soil conditions (like soft soil, medium soil & hard soil) for various zones.
Section III: Analyzed seismic effect on towered building by changing the position of tower under soil conditions for various seismic zones.
5.2 Comparison of displacement of towered building at different size of column
In this section, the comparison of displacement of towered building at the different sizes of column (like 200mmX200mm, 200mmX300mm, 250mmX350mm, 300mmX300mm and 300mmX400mm) and size of beam 300mmX200mm are considered under different soils conditions for different seismic zones. In the analysis displacement obtained are shown in Table 5.1-5.27 and the comparison plot of displacement of towered building shown in figure 5.1-5.27.
6
Conclusion
6.1 General
In this study, seismic analysis of lattice tower on a rooftop was carried out using STAAD PRO software. Tower was placed at three different locations on the rooftop. The analysis was carried out for four earthquake zones, five column dimension and three soil types. For all the combination displacement at different heights of towered building were compared. The combination which gave least displacement is recommended. The following conclusions are drawn from the above analysis.
1. For all the earthquake zones, a column dimension and soil type, the displacement in the towered structure was found to increase with the height of structure. In all the cases, maximum displacement was found at 24m height of towered building.
2. For all the earthquake zones and soil type, the displacement at all height reduced with increasing the column dimension. Maximum displacement was observed for the column size 200x200mm whereas minimum displacement was observed for column size 300×400. When the column size was increased from 300x300mm to 300x400mm a marginal reduction in the displacement was observed therefore, a column size of 300x300mm is recommended.
3. For all the earthquake zones, column dimension considered, the displacement was maximum in hard soil which was followed by medium and soft soil. Therefore, in case of hard soil the position of tower and column dimension should be selected in such a way that the displacement is minimized.
4. For all the earthquake zones, column dimension, minimum displacement was observed when the tower was placed at the center of the roof i.e. CASE II. Therefore, the optimal position of the tower is at the center of the roof. In case of unavailability of the space at the center of the roof the tower can be provided at CASE III location. Maximum displacement was observed in CASE I location ( locations are shown in figure 5.1).
5. Increasing the size of column by jacketing could be provided for the existing building, which is not designed to withstand additional load due to tower construction and/or earthquake.
6. In the era to telecommunication, limited land availability requires antennas to be placed at the roof top. Therefore, the present studies address the important issue of optimal positioning of the towers on a building under different earthquake zones and soil types.
7. To the best of our no codal provision are available for the design of towered buildings. Further, no design criterions are available for the installation and design of towers in an existing building. The present analysis could provide a guideline for selection optimal locations of tower on an existing framed structure.
6.2 FUTURE SCOPE
• In case of high rise buildings additional displacements would occur due to wind load therefore, a detailed sway analysis of high rise towered structure under different load combination and tower position is required.
• Evaluation of additional measures to strengthen an existing building to carry the loadings due to tower construction is required.
• As the communication sector is progressing specially in urban areas, so the antennas are established on roof top due to the land availability is limited which will be a major challenge to face.