CA3119158A1 - Reinforcing of solid round legs in telecom towers - Google Patents

Abstract

A method of reinforcing Solid Round (SR) leg in telecom towers (Guyed & Self-Support ), it increases the leg resistance in compression for over than 5096. This type of reinforcing is using the principal and definition of axial force in truss structure by keeping center of gravity of the new reinforcing members in the same location of existing SR. This type of reinforcing is using existing splice pads and bolts as a foundation and a platform to build the new reinforcing structure on it. The new reinforcing structure is a replacement of the bolting system with new threaded rods and couplers with high strength. The overstressed leg is beneficial the new additional rods by adding more cross-sectional area to the original section SR.

Description

Description Telecom towers are two types, Self-Support Tower & Guyed Tower. Engineers use SR legs in both types because of its good sectional properties being of a symmetrical shape and high torsional resistance. These towers are designed to support antennas, equipment and feeding cables and to resist wind pressure and ice. The telecom industry is growing so fast and requires changing antennas and their associated equipment frequently. These changes require structural engineers to check the structure in order to ensure the tower works within the allowable stress limits. In many cases some towers have issues with the legs. Leg's issue is being the compression resistance less than factored compression load C < Ci .
In this type of structures, we look at three capacities (resistance)for leg ,T &B r, sorted from least to highest, SR's compression, SR's tension and Bolt's tension.
Date Recue/Date Received 2021-05-19 Example -1 (Capacity of SR assembly) Calculate the resistance axial capacity of this assembly: 2" of SR (Grade 350W), with buckling length 24" & 4 0 1.0" bolts (Grade A235X).
o SR in compression (118 kips) Cr = (p. A.
Fy (1 + /1,231)¨lin eq. (1) o SR in tension (144 kips) Tr =
(p. A. Fy eq. (2) o Bolts in tension (225 kips) Br = 055 =
(Psoit = Fu = "rn = Ab eq. (3) Where:
K.1 F
.
E
Tt 3.14159265359 A Cross Sectional area.
= Factor = 1.34 = Radius of gyration.
1 Unsupported length of column.
= Column effective length factor = 1.0 for truss members F, Yield Stress.
Figure 1- SR Leg Assembly Fu Ultimate Strength.
= Modulus of Elasticity =29 x 106 psi = Steel resistance factor 0.9 (holt Bolt resistance factor 0.8 Then from the above, the compression resistance Cr governs the whole assembly.
Date Recue/Date Received 2021-05-19 SR tower (self-support or guyed) consists of welded sections. Each section is about (10 to 20) feet high. The three Legs are connected by bracing system (diagonals & horizontals). Bracing pattern could be one of these common shapes X, W, S, IS or K system. Sections are connected through splice connections (splice pads and bolts). Splice pads are welded to the leg at top & bottom.
Bolts connect sections through its splice pads.
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I ;111 :11 sommi _ _ Figure 2- SR Section, Before Reinforcing Date Recue/Date Received 2021-05-19 By-law a structural engineer is required to analyze the tower structurally for the loading:
1. Dead loads (Antennas, Equipment, Feeding cables and Mounts)

2. Live loads (Wind, Ice & Earthquake) The purpose of this structural analysis is to get the factored compression axial force Ci on each member and compare it to the member compression resistance Cr . It should be Cr Ci means, resistance loading. In some cases, due to the changes of loading or changing in wind & ice (CSA - S37-18) , the legs getting overstressed Cr < Ci, in this case, reinforcing leg is mandatory.
Date Recue/Date Received 2021-05-19 Reinforcing Leg in compression has (2) options as per (eq.1).
Option 1. Reduce the buckling length.
Option 2. Increase the cross-sectional area.
Example -2 (Option 1.) Calculate the compression resistance C., for SR in (Example -1) with buckling length 12".
Changing the buckling length in (Example-1) from 24" to 12" will change (1 + A2.11)-11n from 0.8215 to 0.9663 &Cr from 118kips to 139 kips Then the improvement due to the reduction of buckling length is 1796- in this example.
If this is the case that we have, Utilization ratio Ur = ¨ccf <1.17 and the bracing pattern can accept new horizontal member then it is good solution.
Date Recue/Date Received 2021-05-19 Reinforcing SR leg in Guyed Tower (Option 2.) The guyed tower is structurally a continuous vertical beam, and the cables are the supports for this beam in three u directions. We apply initial tension on cables to apply _) compression force on the tower and horizontal forces outward, this is to keep the tower structure standing vertical and ) ( - A '-stabilization. Wind pressure is the live load on tower, A
applying variable horizontal distributed loads. Due to this combination of loads we get normal force, shear force & bending moment on tower sections. However, our section is a Space k Truss, so the bending moment produces tension and compression in the three legs. This is the equation that gives tension or I 0 compression on each leg in the section. 4 1-10, 1:4 A

F = - + - y AI Figure 3 Guyed Tower Due to the compression force coming from the tension in cable, we have N, high compression force on tower sections, gradually increases when going down to the base. In the equation above N is dominant up to the top guy level, as shown in leg load compression diagram.
The sections above the top cable are the cantilever part of the tower structure, where N is small due to the dead load only (antennas & equipment), so M is dominant, as shown in Leg Load Tension Diagram.
Date Recue/Date Received 2021-05-19 This is a guyed tower; in the leg load compression diagram, it shows overstressed sections. How to reinforce the SR leg to get Ur < 1.0 F.
g Figure 4 - SR Guyed Tower Date Recue/Date Received 2021-05-19 Cr -Cf Figure 5 - Leg Load Compression Diagram Date Recue/Date Received 2021-05-19 -Tr -Tf Figure 6 Leg Load Tension Diagram Date Recue/Date Received 2021-05-19 The steps of reinforcing a SR section.
Calculations Assume the section that we need to reinforce it as below:
Section Height =120"
Panel Height = 20"
SR size = 2", (Grade 350W) Number of bolts (m) = 4, Bolt 4)=0.75"
New bolting system (4) threaded rods 0 =0.75"
New Threaded Rods, (Grade A514 gr.100) Tie Node spacing = 15"
New capacity = Capacity of SR + Capacity of one Threaded Rod x number of Rods Cr = Crl Cr2 The new capacity must be > factored compression load.
Cf So, Cr Cf = Crl Cr2 Cr2 Cf Crl then Capacity of one threaded rod = Cr2 Now we have the required capacity of new rod and we had rod size, only the length is unknown (buckling length), using one of the three 1 , 2 & 3 graphs based on the strength (Yield Stress) of rod's material we get the length.
We use this length to define the required number of tie nodes to be pre-installed in mid assembly before to start.
We must adjust the spacing between tie nodes (buckling length) to avoid any interference with the bracing nodes. Based on the correct spacing we get the right number of tie nodes.
Capacity of SR 2" = 126 kips.
Capacity of Threaded Rod 0.75" = 14 kips.
Then the new capacity of reinforced leg = Cr = 126 -F 14 x 4 = 182 kips if Cf thenit's0K
Date Recue/Date Received 2021-05-19 Procedure 1. Replace splice bolts at top, one at a time with the new assembly in green, short threaded rod, two nuts below, one nut and one washer above the splice pad.
= ""il (11' Figure 7 Top Reinforcing Kit-in green color Date Recue/Date Received 2021-05-19 2. Replace splice bolts at bottom, one at a time with the new assembly in red, short threaded rod, two nuts below, one nut and one washer above the splice pad.
L) Figure 8 Bottom Reinforcing Kit-in red color.
Date Recue/Date Received 2021-05-19

3. Place the mid assembly in yellow between top & bottom (new replacement rods step 1 & 2) to be butt to butt.
=
Figure 9 Mid Reinforcing Kit in yellow Date Recue/Date Received 2021-05-19

4. Adjust the locknut at top of mid assembly in yellow to be away from the end by couple/2.
t , Ti Figure 10 locknut at coupler/2

5. Adjust the locknut at bottom of mid assembly in yellow to be away from the end by couple/2.
, Figure 11 locknut at coupler/2 Date Recue/Date Received 2021-05-19

6. Rotate the top coupler until it hits the locknut in step 4 in yellow.
1 II ) Figure 12 rotate coupler to hit yellow locknut.

7. Rotate the bottom coupler until it hits the locknut in step 5 in yellow.
and secure it with the bottom locknut in red.
[ 1-, , I
Figure 13 rotate coupler to hit yellow locknut.
Date Recue/Date Received 2021-05-19

8. Repeat the previous steps from 1 to 7 for the rest of old other splice bolts.

9. Connect the tie nodes around the SR leg.

10. Add the hose clamp above each set of tie nodes to 1, squeeze it all.
II
Ime i Figure 14 Connect tie nodes & add the

11. Tighten all the red bolts to secure the tie nodes. hose clamp.

12. Tighten the top coupler in green. using a wrench to the specified torque (T) & secure it with the top locknut in green.
The specified torque (T) is based on the residual loads on the leg at the location of this overstressed section.
"1 a Residual load (R)= initial condition of the tower where wind=0.0Pa & ice=0.0 mm.
Figure 15 Tighten the top coupler.
Residual load per leg = ¨

Assume area of one rod = a & total areas of (SR + threaded rods) = A
a Residual load per threaded rod (2)= X 71 (2 7trit +p) Torque T = Q r (2 7 r ¨ p) r = radius of threaded rod.
= friction coefficient.
p = pitch.

13. Remove the hose clamp in blue_ Date Recue/Date Received 2021-05-19 Reinforcing SR leg in Self-Support Tower (Option 2.) The self-support tower is structurally a cantilever vertical beam.
ri The wind pressure is live load on the tower, applying variable u horizontal distributed loads, and all antennas equipment are the " Fl dead loads. Due to this combination of loads we get normal force, -shear force & bending moment on tower sections. However, our _4 F -section is a space truss, so the bending moment produces tension and compression in the three legs. This is the equation that gives tension or compression on each leg in the section.11 Fl I
1r r y A I
Figure 16 Self-Support Tower N is small due to the dead load only (antenna equipment), and M is dominant, we have tension and compression in all sections; however, compression is always higher than tension.
Date Recue/Date Received 2021-05-19 This is a self-support tower; in the Leg Load Compression Diagram, it shows overstressed sections. How to reinforce the SR leg to get Ur <1.0 -D
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or 1 Figure 17 SR Self-Support Tower Date Recue/Date Received 2021-05-19 õ
, 5 , õ 5 , 5 , 5 õ 5 . 5 , 5 õ 5 , , ' , ..icf -Cr 5 i Figure 18 Leg Load Compression Diagram Date Recue/Date Received 2021-05-19 Tf Tr -Br Figure 19 Leg Load tension Diagram Date Recue/Date Received 2021-05-19 Tf -Br-on e Bolt Figure 20 Leg Load tension Diagram (Bolts-one Bolt"' Date Recue/Date Received 2021-05-19 The steps of reinforcing a SR section.
Self-Support is the same as guyed tower as a procedure, except one thing.
Before starting step 1 (Replacing splice bolts), we must look at the graph "Leg Load tension Diagram (Bolts-one Bolt" to make sure that while replacing one bolt at a time with the new threaded rod, still the rest of bolts are enough to carry the load.
Calculations Assume the section that we need to reinforce it as below:
Section Height =120"
Panel Height = 30"
SR size = 2", (Grade 350W) Number of bolts (m) = 4, Bolt 4)=1.00"
New bolting system (4) threaded rods 0 =1.00"
New Threaded Rods, (Grade AS14 gr.100) Tie Node spacing = 30"
New capacity = Capacity of SR + Capacity of one Threaded Rod x number of Rods Cr = Cr1 Cr2 1 The new capacity must > factored compression load.
Cr > Cf So, Cr Cf = Cr1 Cr2 Cr2 Cf Cr1 then Capacity of one threaded rod =2 Now we have the required capacity of new rod and we had rod size, only the length is unknown (buckling length), using one of the three 1 , 2 & 3 graphs based on the strength (Yield Stress) of rod's material we get the length.
We use this length to define the required number of tie nodes to be pre-installed in mid assembly before to start.
We must adjust the spacing between tie nodes (buckling length) to avoid any interference with the bracing nodes. Based on the correct spacing we get the right number of tie nodes.
Capacity of SR 2" = 104 kips.
Capacity of Threaded Rod 1.00" = 13 kips.
Then the new capacity of reinforced leg = Cr = 104 -F 13 x 4 =156kips if Cr Cf thenit's0K
Date Recue/Date Received 2021-05-19 Charts SR's Compression Resistance 300W fy =43510psi 40 i 1 \
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,I l Cr -i.......ko) Figure 21 Compression Capacity of SR, Yield Stress = 43510 psi r SR's Compression Resistance 350W fy =50760psi 4' I
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Figure 22 Compression Capacity of SR, Yield Stress = 50760 psi Date Recue/Date Received 2021-05-19 Threaded Rod's Compression Resistance A514 gr.80 fy =80060psi , ' \ ' :
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, ' cr (kipo) Figure 23 Compression Capacity of New Threaded Rod, Yield Stress = 80060 psi , :
Threaded Rod's Compression Resistance A514 ar.90 fy =89920p5i ' :
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õ...i Figure 24 Compression Capacity of New Threaded Rod, Yield Stress = 89920 psi Date Recue/Date Received 2021-05-19 Threaded Rod's Compression Resistance A514 gr.100 fy =100080psi , - -iiiiiiiiiiiiiiT
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Figure 25 Compression Capacity of New Threaded Rod, Yield Stress = 100080 psi Date Recue/Date Received 2021-05-19

Claims (4)

Claims Reinforcing solid round legs

1. By using existing splice bolts.

2. By using existing splice pads.

3. By using existing splice connections.

4. By bearing threaded rods (bars) on splice connections.
Date Recue/Date Received 2021-05-19