Space Truss DesignEngineering Postgraduate Program Hasanuddin
University 15 B. OF DUCTILE STEEL STRUCTURES. Yoppy Soleman,
2005SPACE TRUSS DESIGN1. DESIGN SPECIFICATION1.1. Design Standard1)
The design basis of the tower applied is EIA Standard EIA-222-E
Structural Standards for SteelAntenna Tower and Antenna Supporting
Structure. The fabrication and materials of the tower willbe
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 andnut.1.2. Tower Structure Design
Condition1) 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 1.20diameter
at 35.0 m- 1 (one) Paraboloid grid antennas 1.20 diameter at 42.0
m4) Proposed antennas loading ( see the drawing attachment ) :- 1
(one) Paraboloid solid antenna 1.2 diameter at 38.0 m1.3. Loads1)
Dead loadDead load is weight of tower, antenna, ladder, platform
etc.2) Wind load on tower structureWind load calculation method on
the tower and appurtenances are as followsF = qz . GH . CF . AE and
not to exceed2 . qz . GH. AGqz = 0.613 . KZ . V2Kz = ( z / 10
)2/7GH = 0.65 + 0.60 / ( h / 10 )1/7CF = 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 / AGAE = DF . AFWhere :F = Horizontal wind force (
N )qz = Velocity pressure ( Pa )GH = Gust response factor ( 1.00 s
Kz s 1.25 )CF = Structure force coefficientAE = Effective projected
area of structural component in one face ( m2)AG = Gross area of
one tower face ( m2)Kz = Exposure coefficient ( 1.00 s Kz s 2.58 )V
= Basic wind speed for the structure location ( m/s )Space Truss
DesignEngineering Postgraduate Program Hasanuddin University 16 B.
OF DUCTILE STEEL STRUCTURES. Yoppy Soleman, 2005z = Height above
average ground level to midpoint of the section ( m )h = Ttotal
height of structure ( m )e = Solidity ratioAF = Projected area of
flat structural component in one face of the section(m2)DF = Wind
direction factor1.00 for square cross section and normal wind
direction1.00 + 0.75e for square cross section and 450wind
direction3) Wind load on AntennaWind load calculation method on the
parabolic antenna is as follow :Fa = Ca x A x Kz x GH x V2Fs = Cs x
A x Kz x GH x V2Kz = ( z /10 )2/7GH = 0.65 + 0.60 / (h/10)1/7Where
:Fa = Axial Force (lb)Fs = Side Force (lb)Ca = Wind load
coefficient for axialCs = Wind load coefficient for sideKz =
Exposure coefficient ( 1.00 s Kz s 2.58 )z = Height above average
ground level to midpoint of the section(m)h = Total height of the
structure (m)A= Normal projectedarea of AntennaV= Wind velocity (
m/s )4) Load combinationHerewith the following combinations are
used below :a) DL + WLat 0 degree direction (with weight of
existing antenna)b) DL + WLat 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 = Dead load weight
of the structure and appurtenances.WL = Design wind load on antenna
at above direction.1.4. Allowable unit
stressTheunitstressesinthestructuresmembersdonotexceedtheallowableunitstressesforthematerialsasspecified
in the AISC Standard (American Institute of Steel Construction
Standard)1. Tension : Ft = 0.60 Fy ( kg/cm2)2. Shear : Fv = 0.40 Fy
( kg/cm2)3. Compressioni) On the gross section of axially loaded
compression members when kl/r is less than Cc :(kl/r)2[ 1 -
----------] Fy2Cc2Fa =
----------------------------------------------- ( kg/cm2)5/3+
[3/8(kl/r)]/8Cc - [(kl/r)3/8Cc3]Space Truss DesignEngineering
Postgraduate Program Hasanuddin University 17 B. OF DUCTILE STEEL
STRUCTURES. Yoppy Soleman, 20052t2EWhere: Cc = ---------Fyii) On
the gross section of axially loaded compression members, when kl/r
exceeds Cc :12t2EFa =--------------- ( kg/cm2)23(kl/r)24.
BendingTension 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 legskl/r = 150 for compression
members of diagonalskl/r = 200 for tension membersNotations :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 factorl = Actual
unbraced length of member ( cm )r = Governing radius of gyration (
cm )Cc = Column slenderness ratioE = Modulus of elascity of steel =
2,100,000 kg/cm2Fb = Allowable bending stress ( kg /cm2)Fu =
Minimum tensile strength ( kg /cm2)1.5. MaterialsSteel materials to
be used for the towers and appurtenances conform to the relevant
Indonesian Standardsand/or Japanese Industrial Standard.1) Steel
StructuralDescription Tensile Strength( kg/cm2 )Minimum Yield Point
Fy( kg/cm2 )Bj 41 4100 2500SS 41 4100 25002) BoltsDescription Ft Fv
FvSpace Truss DesignEngineering Postgraduate Program Hasanuddin
University 18 B. OF DUCTILE STEEL STRUCTURES. Yoppy Soleman,
2005Friction Type Bearing Type( kg/cm2 ) ( kg/cm2 ) ( kg/cm2 )A 325
Bolts 3900 1230 14763) ConcreteDesign compressive strength of
concrete (fc) at 28 days.K - 175 - fc = 175 kg/cm24) Reinforcement
steelU - 24 - Fy = 2400 kg/cm21.6. Structural AnalysisThe purpose
of the structural analysis is to find the joint translations and
the design axial loads in all membersof the tower. Load is applied
and separate load cases combined to give the most severe design
conditions atvarious section.The structural calculation is made
using SAP 90 (Structural Analysis Program 90). The program will
performthe static analysis of a space truss of arbitrary geometry
by the stiffness method. The truss may be subjectedto loads
consisting of forces acting on the joints in any directions in
space. The program output consists of thejoint translations, the
member forces and the support reactions.The program input contains
:a. Structure titleb. Loading system : number of static analysis
that applied to the structure.c. Group of data correspondingto the
properties of the mathematical model of truss and the applied
jointload :- Group 1 : Joint coordinates- Group 2 : Support joint
restraints- Group 3 : Material and member data- Group 4 : Joint
loads- Group 5 : Loading combinationsThe location of the joints in
any structure are expressed as coordinates in a global right hand
othogonal XYZcoordinate 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.Global
AxisAll applied joint loads, joint displacement and reactions are
expressed as component in the global coordinatesystem. Force
component and translation components are positive if they act in
the positive direction of anaxis.The member forces and support
reactions for both conditions, tower with existing antennas and
tower withexisting and proposed antennas, are attached in computer
output.1.7. Design Calculation Of FoundationThe calculation of
foundation consists of design and control of foundation.Control of
foundation includes :Z +X +Y +0Space Truss DesignEngineering
Postgraduate Program Hasanuddin University 19 B. OF DUCTILE STEEL
STRUCTURES. Yoppy Soleman, 20051) Control of stability for uplift
force :Sf = W1 / T > 2.0Where : W1= Weight of foundation and
soil ( kg )T = Uplift force ( kg )2.0 = Allowable safety factor2)
Control of bearing capacity of soil :Wt MF = --------
+--------------- 1.5Where : SF = Safety factorWt = Total vertical
loadincludes support reaction, weight of foundationandweight of
soil (kg)| = Coefficient of soil frictionH = Horisontal loads ( kg
)1.5 = Allowable safety factor2. STRUCTURAL CALCULATIONThe
structural analysis is made using SAP 90. Input and output program
is shown as attachment.Deflection, sway and twist are calculated as
follows :a. Deflection : Dxn : Joint displacement at a point nDxn :
Joint displacement at a point nDxn Dxnb. Sway angle = arc tan (
--------------------------------------------------------- )Distance
between point n and point nDxn Dxnc. Twist angle = arc tan (
----------------------------------------------------------
)Distance between point n and point n1) Tower without proposed
antennaa. Deflection = 6.4177 cmb. Dxn = 5.2096 cmSpace Truss
DesignEngineering Postgraduate Program Hasanuddin University 20 B.
OF DUCTILE STEEL STRUCTURES. Yoppy Soleman, 2005Dxn = 5.8865 cmd =
250 cmSway angle = arc tan (( 5.8865 5.2096 ) / 250 ) = 0.1551
degreec. Dxn =5.8865 cmDxn = 6.4173 cmd = 300 cmTwist angle = arc
tan (( 6.4173 5.8865 ) / 300 ) = 0.1014 degree2) Tower with
proposed antennaa. Deflection = 6.5947 cmb. Dxn = 5.2534 cmDxn =
5.9388 cmd = 250 cmSway angle = arc tan (( 5.9388 5.2534 ) / 250 )
= 0.1570 degreec. Dxn = 6.4763 cmDxn = 5.9388 cmd = 300 cmTwist
angle = arc tan (( 6.4763 5.9388 ) / 300 ) = 0.1027 degreeSway and
twist at 120 km/hour wind velocity without proposed antennas as
follows :Actual AllowableDeflection (cm) 6.4177 42Sway angle
(degree) 0.1551 0.5Twist angle (degree) 0.1014 0.5Sway and twist at
120 km/hour wind velocity with proposed antennas as follows :Actual
AllowableDeflection (cm) 6.5947 42Sway angle (degree) 0.1570
0.5Twist angle (degree) 0.1027 0.5Space Truss DesignEngineering
Postgraduate Program Hasanuddin University 21 B. OF DUCTILE STEEL
STRUCTURES. Yoppy Soleman, 20053. FOUNDATION ANALYSIS3.1. Column
Anchorage Bolt Calculation1 1) Steel Bar Bj 37 --------- Fy = 2400
kg / cm22 2 ) Notation :Fy = Yield strength of steelFv = Allowable
shear strength of anchor boltFt = Allowable tensile stress of
anchor boltFts = Allowable tensile stress for bolt subject to
combine tension and stressFcv = Allowable bond stress of concretefv
=Actual shear stress of anchor boltft = Actual tensile stress of
anchor boltfc = Compressive strength of concreteA = Total area of
anchor boltP = Total compression of tower base per one legT = Total
uplift force at tower base per one legS = Total shear force at
tower base per one legLe = Required embeded length of anchor bolt
in concrete3 3 ) Maximum forces at tower basea. Tower with existing
antenna :T = 21110 488.09 = 20621.91 kgS = 2377 kgb. Tower with
existing and proposed antenna :T = 21110 488.09 = 20621.09kgS =
2387 kg1 4 ) Allowable tensile stress of anchor boltsFv = 0.3 Fy =
0.3 x 2400 = 720 kg/cm2Ft = 0.6 Fy = 0.6 x 2400 = 1440 kg/cm2a.
Tower with existing antenna :Number of anchor bolt = 6 | A = 6 x (
0.25t x 1.9052) = 6 x 2.85 = 17.1 kg/cm2fv = S / A= 2377 / 17.1 =
139.0 kg/cm2< Fv .Ok !Fts = 1.4 Ft 1.6 fv = ( 1.4 x 1440 ) ( 1.6
x 139.0 )= 2016 222.40 = 1793.60 kgFts > Ft ---------------- use
Ft = Fts = 1440 kg/cm2ft = T / A= 20621.09 / 17.1 = 1205.91
kg/cm2< Ft Ok !b. Tower with existing and proposed antenna
:Number of anchor bolt = 6 | ----- A = 17.1 cm2fv = S / A = 2387 /
17.1 = 139.59 kg/cm2< Fv . Ok !Fts = ( 1.4 x 1440 ) ( 1.6 x
139.59 ) = 2016 223.344 = 1792.656 kg/cm2Fts > Ft
---------------- use Fts = Ft = 1440 kg/cm2ft = T / A = 20621.09 /
17.1 = 1205.91 kg/cm2< Ft Ok !Keep using anchor bolt 6 |
Required embedded length of anchor bolt :Fcv = 0.53 fc = 0.53 175 =
7.0 kg/cm2Space Truss DesignEngineering Postgraduate Program
Hasanuddin University 22 B. OF DUCTILE STEEL STRUCTURES. Yoppy
Soleman, 2005Le = T / ( Fcv x 6 x t x d ) = 20621.09 / ( 7 x 6 x
3.14 x 1.905 ) = 82.1 cmUse Le = 85cm3.2. Column Base Plate1) Steel
: Bj 37 Fy = 2500 kg/cm2Concrete : K 175 Fp = 0,35 fc = 0,35 x 175
= 61.25 kg/cm22) The formula to calculate column base plate is
shown as follows :Ar = P / Fp ( m2)Ab > Ar then checkfp s FpAb =
B x Bt = ( 6M / Fb )Fb = 0.75 Fy = 0.75 x 2500 = 1875 kg/cm2Where :
P = Total compression at tower base per one leg ( kg )Ar = Required
area of column base plate ( m2)Ab = Designed area of column base
plate ( m2)B = Length of base plate ( cm )fp = Actual bearing
pressure ( kg/cm2)Fp = Allowable bearing strength stress ( kg/m2)tp
= Required thickness of base plate ( cm, mm)M = Moment at the edge
of base plate ( kgm, kgcm)Fb = Allowable bending stress of base
plate ( kg/cm2)Fy = Yield strength of steel ( kg/cm2)fc =
Compressive strength of concrete ( kg/cm2)m = Distance from steel
structural to the edges of base plate ( cm )fb = Bending stress (
kg/cm2)The calculation is shown as follows below :a. Tower without
proposed antennasColumn base plate areaThe existing column base
plate : 600 mm x 600 mm x 25 mmMaximum compression force ( P ) =
26980 kgApplied load at support join = 488.09 kgP Total = 26980 +
488.09 = 27468.09 kgA = 60 x 60 = 3600 cm2fp = P / A = 26980 / 3600
= 7.494 kg/cm2< Fp . Ok !Column base plate thicknessUse m = (60
15) / 2 cm = 22.5 cmM = q m2= x 7.494 x 22.52= 1896.92 kgcmcheck
the stress : fb = ( 6M / tp2)= (6 x 1896.92 / 2.52)= 1821.042
kg/cm2 2.67t/m2.FailThedimensionof
foundationisdesignedbasedonthenomogram. Asshownin calculationabove,
thebearing capacity of soil is unable to support the existing
tower. In fact, the soil is bearable. Possibly this
isduetothedifferenceintypeanddimensionbetweentheexistingtower
foundationandthedesignedfoundation above.5) Factor of safety
against slidingWt = 71.970 tH = 2.377 tu = Coefficient of friction
= 0.45SF = Wt x u / H = 71.970x 0.45 / 2.377= 13.62 > 1.50 . Ok
!3.3.2. Tower with existing and proposed antennas1) Design load : H
= 2387 kg ( max horizontal reaction )V = 27200kg ( max vertical
reaction )T = 21160kg ( max uplift reaction )V1 = 488.09 kg ( dead
load at support join )P = V + V1 = 27200+ 488.09 = 27688.09 kgTt =
T V1 = 21160 488.09 = 20671.09 kg2) Check stability for uplift
forceConcrete volume ( Vc ) :Pedestal column : 0.80 x 0.80 x 2.15 =
1.376 m3Footing : 3.0 x 3.0 x 0.70 = 6.300 m3= 7.676 m3Soil volume
for anti uplifting ( Vs ) :Vs = (( 3.0 x 3.0 ) ( 0.80 x 0.80 )) x
1.95= 16.30 m3Weight of concrete and soil :W1 = W+ Ws = 7.676 x 2.4
+ 16.30 x 1.6 = 44.502 tS.F = W1 / T = 44.502 / 21.160 =
2.103>2.0 ..Ok !3 ) Bearing capacity of soilThe allowable
bearing capacity of soil is 0.267 kg/cm2= 2.67 t/m24 ) Check of
compressive forceWt = 44.502 + 27.688 = 72.190 tM = 2.387 x 2.85 =
6.803 tmZ = Section modulus of footing baseZ = 3.0 x 3.0 x 3.0 / 6
= 4.500 m3fe = Compressive stress of footing baseA = Area of
foundation base = 3.00 x 3.00 = 9.00 m2fe = 72.190/ 9.00 + 6.803 /
4.500= 8.022 t/m2> 2.67t/m2.Fail5) Factor of safety against
slidingWt = 72.190 tH = 2.387 tu = Coefficient of friction =
0.45Space Truss DesignEngineering Postgraduate Program Hasanuddin
University 25 B. OF DUCTILE STEEL STRUCTURES. Yoppy Soleman, 2005SF
= Wt x u / H = 72.190 x 0.45 / 2.387= 13,61> 1.50 . Ok !Space
Truss DesignEngineering Postgraduate Program Hasanuddin University
26 B. OF DUCTILE STEEL STRUCTURES. Yoppy Soleman, 20054. CONCLUSION
AND RECOMMENDATIONWe have carefully analysed the existing tower of
Limboto structure for the proposed additional antenna at 120km/hr
wind velocity. The following major conclusions have been drawn from
this analysis :- Theexistingtower hasstrengthenoughtosupport
theexistingconfigurationandtheproposedantennas at 120 km/hr maximum
wind velocity.- The anchor bolt and the base plate has strength
enough to resist the forces at support joint.- Additionalforce at
the towerbase (maximum)due tothe proposedantennas is less than
2.10% ofsupport reaction at tower without proposed antennas.- The
designed foundation has strength enough to resist the uplift and
shear forces.- The designed foundation has not strength enough to
resist the compressive force. It means that thebearing capacity of
soil is unable to support the structure. This is, possibly, due to
the difference intype and dimension between the existing tower
foundation and the designed foundation.- The minimum required of
bearing capacity to support the tower with existing and proposed
antennas isabout 8.0 t/m2.Luwuk, Januari 2001Yoppy SolemanSpace
Truss DesignEngineering Postgraduate Program Hasanuddin University
27 B. OF DUCTILE STEEL STRUCTURES. Yoppy Soleman,
200580020070019503000GLSoil 2850
