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analysis of the structure of the telecommunication tower

Space Truss Design

SPACE TRUSS DESIGN

1. DESIGN SPECIFICATION 1.1. Design Standard 1) The design basis of the tower applied is EIA Standard EIA-222-E Structural Standards for Steel Antenna Tower and Antenna Supporting Structure. The fabrication and materials of the tower will be according to the relevant Indonesian Standard. 2) The self supporting tower has square cross sections. 3) All the legs and bracings are made of equals legs angles steel. 4) All the connections in the field are made with Steel Bolts, each fitted with one spring washer and nut. 1.2. Tower Structure Design Condition 1) Tower height : 42.0 meter ( location : Limboto, North Sulawesi ) 2) Maximum wind velocity (V) : V = 120 km/hour = 33.33 m/sec. 3) Existing antennas loading ( see the drawing attachment ) : 2 (two) Planar type antennas at 42.0 m 1 (one) Planar type antennas at 38.0 m 1 (one) Paraboloid grid antennas 1.20 diameter at 35.0 m 1 (one) Paraboloid grid antennas 1.20 diameter at 42.0 m 4) Proposed antennas loading ( see the drawing attachment ) : 1 (one) Paraboloid solid antenna 1.2 diameter at 38.0 m 1.3. Loads 1) Dead load Dead load is weight of tower, antenna, ladder, platform etc. 2) Wind load on tower structure Wind load calculation method on the tower and appurtenances are as follows F qz = qz . GH . CF . AE and not to exceed 2 . qz . GH. AG = 0.613 . KZ . V2

Kz = ( z / 10 )2/7 GH = 0.65 + 0.60 / ( h / 10 )1/7 CF = 4.0 e2 5.9 e + 4.0 ( square cross section ) CF = 3.4 e2 4.7 e + 3.4 ( triangular cross section ) e = AF / AG AE = DF . AF Where : F qz = Horizontal wind force ( N ) = Velocity pressure ( Pa )

GH = Gust response factor ( 1.00 Kz 1.25 ) CF = Structure force coefficient AE = Effective projected area of structural component in one face ( m2 ) AG = Gross area of one tower face ( m2 ) Kz = Exposure coefficient ( 1.00 Kz 2.58 ) V = Basic wind speed for the structure location ( m/s )

Engineering Postgraduate Program Hasanuddin University

15

B. OF DUCTILE STEEL STRUCTURES. Yoppy Soleman, 2005

Space Truss Design

z h e

= Height above average ground level to midpoint of the section ( m ) = Ttotal height of structure ( m ) = Solidity ratio

AF = Projected area of flat structural component in one face of the section (m2) DF = Wind direction factor 1.00 1.00 + 0.75e for square cross section and normal wind direction for square cross section and 450 wind direction

3) Wind load on Antenna Wind load calculation method on the parabolic antenna is as follow : Fa = Ca x A x Kz x GH x V2 Fs = Cs x A x Kz x GH x V2 Kz = ( z /10 ) 2/7 GH = 0.65 + 0.60 / (h/10) 1/7 Where : Fa = Axial Force (lb) Fs = Side Force (lb) Ca = Wind load coefficient for axial Cs = Wind load coefficient for side Kz = Exposure coefficient ( 1.00 Kz 2.58 ) z = Height above average ground level to midpoint of the section (m) h = Total height of the structure (m) A = Normal projected area of Antenna V = Wind velocity ( m/s ) 4) Load combination Herewith the following combinations are used below : a) DL + WL at 0 degree direction (with weight of existing antenna) b) DL + WL at 45 degree direction (with weight of existing antenna) c) DL + WL at 0 degree direction (with weight of existing + proposed antenna) d) DL + WL at 45 degree direction (with weight of existing + proposed antenna) Where : DL WL 1.4. Allowable unit stress The unit stresses in the structures members do not exceed the allowable unit stresses for the materials as specified in the AISC Standard (American Institute of Steel Construction Standard) 1. Tension 2. Shear 3. Compression i) On the gross section of axially loaded compression members when kl/r is less than Cc : (kl/r)2 [ 1 - ----------] Fy 2Cc2 Fa = ----------------------------------------------5/3 + [3/8(kl/r)]/8Cc - [(kl/r)3/8Cc3] ( kg/cm2 ) : Ft = 0.60 Fy ( kg/cm2 ) : Fv = 0.40 Fy ( kg/cm2 ) = Dead load weight of the structure and appurtenances. = Design wind load on antenna at above direction.

Engineering Postgraduate Program Hasanuddin University

16

B. OF DUCTILE STEEL STRUCTURES. Yoppy Soleman, 2005

Space Truss Design

22E Where: Cc = --------Fy ii) On the gross section of axially loaded compression members, when kl/r exceeds Cc : 122 E Fa = --------------23(kl/r)2 4. Bending Tension and compression on extreme fibers : Fb = 0.66 Fy ( kg/cm2 ) 5. Tension on bolts : Ft = 0.60 Fy ( kg/cm2 ) 6. Shear on bolts : Ft = 0.30 Fy ( kg/cm2 ) 7. Bearing on bolts : Ft = 1.20 Fu ( kg/cm2 ) 8. The maximum slenderness ratio (kl/r) are as follows : kl/r = 120 for compression members of legs kl/r = 150 for compression members of diagonals kl/r = 200 for tension members Notations : Ft = Allowable tensile stress ( kg /cm2 ) Fy = Minimum yield point ( kg /cm2 ) Fv = Allowable shear stress ( kg /cm2 ) Fa = Allowable compressive stress ( kg /cm2 ) k = Effective length factor l = Actual unbraced length of member ( cm ) r = Governing radius of gyration ( cm ) Cc = Column slenderness ratio E = Modulus of elascity of steel = 2,100,000 kg/cm2 Fb = Allowable bending stress ( kg /cm2 ) Fu = Minimum tensile strength ( kg /cm2 ) 1.5. Materials Steel materials to be used for the towers and appurtenances conform to the relevant Indonesian Standards and/or Japanese Industrial Standard. 1) Steel Structural Description Tensile Strength ( kg/cm2 ) Bj 41 SS 41 2) Bolts Description Ft Fv Friction Type Fv Bearing Type 4100 4100 Minimum Yield Point Fy ( kg/cm2 ) 2500 2500 ( kg/cm2 )

Engineering Postgraduate Program Hasanuddin University

17

B. OF DUCTILE STEEL STRUCTURES. Yoppy Soleman, 2005

Space Truss Design

( kg/cm2 ) A 325 Bolts 3) Concrete Design compressive strength of concrete (fc) at 28 days. K - 175 U - 24 1.6. Structural Analysis fc = 175 kg/cm2 Fy = 2400 kg/cm2 4) Reinforcement steel 3900

( kg/cm2 ) 1230

( kg/cm2 ) 1476

The purpose of the structural analysis is to find the joint translations and the design axial loads in all members of the tower. Load is applied and separate load cases combined to give the most severe design conditions at various section. The structural calculation is made using SAP 90 (Structural Analysis Program 90). The program will perform the static analysis of a space truss of arbitrary geometry by the stiffness method. The truss may be subjected to loads consisting of forces acting on the joints in any directions in space. The program output consists of the joint translations, the member forces and the support reactions. The program input contains : a. Structure title b. Loading system : number of static analysis that applied to the structure. c. Group of data corresponding to the properties of the mathematical model of truss and the applied joint load : Group 1 : Joint coordinates Group 2 : Support joint restraints Group 3 : Material and member data Group 4 : Joint loads Group 5 : Loading combinations

The location of the joints in any structure are expressed as coordinates in a global right hand othogonal XYZ coordinate system. For the space structures the Z axis is oriented in the vertical direction positive upward, with the X and Y axes oriented in the major directions of the structure. Z+ Y+

Global Axis 0 X+ All applied joint loads, joint displacement and reactions are expressed as component in the global coordinate system. Force component and translation components are positive if they act in the positive direction of an axis. The member forces and support reactions for both conditions, tower with existing antennas and tower with existing and proposed antennas, are attached in computer output. 1.7. Design Calculation Of Foundation The calculation of foundation consists of design and control of foundation. Control of foundation includes : 1) Control of stability for uplift force :

Engineering Postgraduate Program Hasanuddin University

18

B. OF DUCTILE STEEL STRUCTURES. Yoppy Soleman, 2005

Space Truss Design

Sf = W1 / T > 2.0 Where : W1= Weight of foundation and soil ( kg ) T 2.0 = Uplift force ( kg ) = Allowable safety factor

2) Control of bearing capacity of soil : Wt M F = -------- + --------------- < Q ( kg/m2) A 1/6.A . B Where : Wt = Total vertical load includes support reaction, weight of foundation and weight of soil (kg) M = Moment load ( horisontal loads x height of foundations ) ( kgm ) A = B = Q = Area of the foundation base ( width x length of foundation ) (m2) Width of the foundation base ( m ) Allowable bearing capacity of soil.

3) Control of sliding force : SF = Wt . / H > 1.5 Where : SF = Wt = and weight of soil (kg) = Coefficient of soil friction H = Horisontal loads ( kg ) 1.5 = Allowable safety factor 2. STRUCTURAL CALCULATION The structural analysis is made using SAP 90. Input and output program is shown as attachment. Safety factor Total vertical load includes support reaction, weight of foundation

Deflection, sway and twist are calculated as follows : a. Deflection : Dxn : Joint displacement at a point n Dxn : Joint displacement at a point n Dxn Dxn b. Sway angle = arc tan ( --------------------------------------------------------- ) Distance between point n and point n Dxn Dxn c. Twist angle = arc tan ( ---------------------------------------------------------- ) Distance between point n and point n 1) Tower without proposed antenna a. Deflection b. Dxn = 6.4177 cm = 5.2096 cm

Engineering Postgraduate Program Hasanuddin University

19

B. OF DUCTILE STEEL STRUCTURES. Yoppy Soleman, 2005

Space Truss Design

Dxn d Sway angle c. Dxn Dxn d Twist angle a. Deflection b. Dxn Dxn d Sway angle c. Dxn Dxn d Twist angle

= 5.8865 cm = 250 cm = arc tan (( 5.8865 5.2096 ) / 250 ) = 0.1551 degree = 5.8865 cm = 6.4173 cm = 300 cm = arc tan (( 6.4173 5.8865 ) / 300 ) = 0.1014 de