Monopole Structures Current Issues.pdf

Copyright 2010 by Paul J. Ford and Company, all rights reserved.

2010 ASCE / SEI Structures Congress | Orlando, FL

Non-Building Structures | May 15, 2010 | D. Hawkins

Discussion of Current Issues Related to Steel Telecommunications Monopole Structures

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Discussion of Current Issues

Related to

Steel Telecommunications

Monopole Structures

David W. Hawkins, P.E., M.ASCE 1

1 Department Manager, Paul J. Ford and Company, Structural Engineers,

250 East Broad Street, Suite 1500, Columbus, OH 43215

PH (614) 221-6679; email: [email protected]





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About Paul J. Ford and Company

Founded in 1965 by Paul J. Ford

Employee-Owned (ESOP) Since 1994

Since 1976; Design of Self-Support and Guyed Towers for Cable TV and Long-Distance Telephone

Since 1985; Design of Telecom/Cellular Towers

Mid-1990s; Telecom Act of 96; Boom in Design of Telecom Towers and Monopoles

Yearly Average of 2,000 Analyses, Designs and/or Reinforcement of Towers and Monopoles





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About the Author

David W. Hawkins, P.E., M.ASCE

Education: BS (83), MS (89), The Ohio State University

Experience: 25 years with Paul J. Ford and Company,Structural Engineers

Member, TIA TR-14.7 Engineering Subcommittee

Member, TIA TR-14.7 Structural Reliability Task Group

Chair, TIA TR-14.7 Monopole Design Sub-Task Group





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Overview -- Types of Tubular Steel Poles:

Flagpoles

Electric Utility Poles

Traffic Signal Poles

Highway Light and Sign Poles

Commercial Sign Poles

Telecommunications Antenna Support Poles (Monopoles)





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Telecommunications Monopoles

Generally categorized into two main types:

(1.) Tubular Round Shafts with bolted flange splices

(2.) Polygonal Tapered Shafts with slip-joint splices and

bolted base plate connections.

(12 and 18-sided are common)





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PolygonalPoleFabrication





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PolygonalPoleComponents





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Telecommunications Design Reference Standards

TIA/EIA-222-F-96 (1996)

Referenced by IBC 2006 (Ch. 35)

Sections 1609.1.1, 3108.4

TIA-222-G-2005 (2005)

Referenced by 2007 Supplement

to the IBC (Ch. 35; Sect. 1609.1.1)

Referenced by IBC 2009 (Ch. 35)

Sections 1609.1.1, 3108.4





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Telecommunications Design Reference Standards

TIA-222-F (1996)

ASCE 7-88 Fastest Mile Winds

AISC ASD (elastic; 1/3 stress increase)

TIA-222-G (2005)

ASCE 7-02 3-sec Gust Winds

AISC LRFD (elastic limit states)

TIA-222-G-2 (2009)

AISC LRFD (plastic limit states)





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Compare TIA-222-G with G-2

TIA-222-G and G-1 (Addendum 1):

Factored Loads with

Elastic Limit States

TIA-222-G-2 (Addendum 2):

Factored Loads with

Plastic Limit States





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Compare TIA-222-G with G-2Table 4-8: Effective Yield Stress for Polygonal Tubular Members

TIA-222-G and G-1 (Addendum 1) Upper Limits:

Effective Yield Stress, Fy = Fy

Moment Capacity, Mn = Fy S (Elastic)

TIA-222-G-2 (Addendum 2) Upper Limits:

Effective Yield Stress, Fy = 1.27 Fy

Moment Capacity, Mn = Fy S

= (1.27 Fy) S

(where Z 1.27 S for 18-sided polygon)= Fy Z (Plastic)





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TIA-222-G Table 4-8 (18-sided Polygon)1.27 1.27 1.27 1.25

1.22

1.18

1.14

1.10

1.06

1.02

0.99

0.95

0.91

0.87

0.83

0.80

0.760.74

1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

0.96

0.92

0.89

0.85

0.82

0.78

0.75

0.72

0.680.67

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

F'y

/ F

y

(Fy/E)1/2 (w/t)

TIA-222-G-2 (Addendum 2)

TIA-222-G and G-1

Co

mp

act

Lim

it

Co

mp

act

Lim

it

0.7

59

1.1

7





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0

10

20

30

40

50

60

70

80

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

0.125 0.188 0.250 0.313 0.375 0.438 0.500 0.563 0.625 0.688 0.750

TIA-222-G Factored Flexural Strength, Mn

TIA-222-G-2 Factored Flexural Strength, Mn

TIA-222-G Table 4-8: Effective Yield Stress, f F'y

TIA-222-G-2 Table 4-8: Effective Yield Stress, f F'y

Shaft Wall Thickness, t (inches)

Comparison of TIA-222-G and G-1 (Addendum 1) with TIA-222-G-2 (Addendum 2)

Factored Flexural Strength (Shaft Moment Capacity) Chart

Example: Shaft 48 Diam; 18-Sided Polygon; Fy=65 ksi

Fa

cto

red

Fle

xu

ral

Str

en

gth

: S

ha

ft M

om

en

t C

ap

acit

y,

f

Mn

(f

t-k

ips)

Ta

ble

4-8

: E

ffe

cti

ve

Yie

ld S

tre

ss,

f

Fy





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Compare TIA-222-G with G-2

Summary:

Effective Shaft Flexural Strength for Polygonal Poles

Can be 10% to 27% Higher

With TIA-222-G-2

(Addendum 2)





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Fatigue and Vortex Shedding Issues

Criteria for fatigue and dynamic effects from vortex shedding are not currently addressed in TIA-222-G.

AASHTO has fatigue criteria for highway structures, but may not be appropriate for telecommunications poles.

ASCE Manual 72 (1990) and ASCE 48-05 Standard (2006) mention vortex shedding and fatigue but do not provide specific criteria.

TIA TR-14.7 Subcommittee intends to address fatigue and vortex shedding criteria in future revisions of the TIA-222 Standard.

Additional research and testing is needed to develop fatigue design criteria for poles.





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Polygonal pole shaft with field bolted channel reinforcements. (Courtesy B. Reese) 16

Shaft Upgrades and Reinforcing

Main Types:

1. Field Welded

Risks: Fire, Corrosion

2. Field Bolted

Cons: Larger connectionsthan welded





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Discussion of Monopole Failures

Man-Caused:

Monopole Fires

Vandalism

Cracks in Poor Welds in Base Plates

Environmental Influence:

Extreme Wind with Debris

Vortex Shedding Fatigue





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(Courtesy B. Reese)

(Courtesy B. Reese)

18

Monopole Fires

Most fires are caused by field activities such as flame torching holes in the shaft and field welding. Heat from careless burning and

welding ignites debris, such as bird nests, in the pole which then ignites the plastic coated coaxial cables.

(Courtesy B. Reese)





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(Courtesy B. Reese)

19

Monopole Flange Connection Failures

Collapse of pipe poles

with bolted flange splices

(D. Hawkins)





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(D. Hawkins)

(D. Hawkins)





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(D. Hawkins) (D. Hawkins)





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Toe crack at top of weld at vertex in shaft to base plate connection. (Courtesy B. Reese)

22


Toe Cracks in Base Plate Welds at Shaft Vertex in Polygonal Poles

Toe crack at vertex in shaft. (Courtesy B. Reese)





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Ultrasonic (UT) TestingMagnetic Particle (MT)

23


NDT Testing for Toe Cracks

in Base Plate Welds

(Courtesy B. Reese)(Courtesy B. Reese)





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Base Plate and Flange Plate Connections

Base Plates Without Stiffeners

Base Plates With Gusset Plate Stiffeners

Flange to Shaft Connection Types:

(a) Socket Type with fillet welds

(b) Tee Joint Type with CJP Welds





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Base Plates Without StiffenersCurrent design methodologies assume linear elastic bending of the base plate. Typically used with AISC-ASD with TIA-222-F working loads.

TIA TR-14.7 Subcommittee researching new base plate design criteria:

Plastic yield line methods at limit states. Analogous to yield lines in concrete slabs, uses balanced energy approach.

Uses AISC-LRFD with TIA-222-G factored loads.

Base plate is proportioned for minimum thickness to eliminate prying forces on the bolts.

Plate ultimate capacity determined by plastic bending limits (FyZ).

Results indicate plate thickness should be at least equal to anchor rod diameter.

Preliminary FEA studies of unstiffened base plates show yield line distribution of stress.





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FEA model results for six bolt double flange connection on round pole shaft.






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FEA model results for six bolt flange connection. Round pole shaft elements hidden.






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FEA model results for six bolt flange connection.






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Base plate reinforcement with Tall gusset plate stiffeners, new anchor rods and brackets. (Courtesy B. Reese)

Base plate reinforcement with Tall gusset plate stiffeners. (Courtesy B. Reese)

29

Base Plates Reinforced With Gusset Plate Stiffeners





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Gusset Plate Stiffener Proportions

Short Stiffener with h:w = 1:1 aspect ratio(Not recommended)

w

6

6h

Chamfer (typ.)

Taper (typ.)

Tall Stiffener with h:w = 3:1 aspect ratio

(Recommended)

Chamfer (typ.)

Taper (typ.)

w

6

18h





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FEA of Stiffened Base Plates

No Stiffeners 3x3 Stiffeners

6x6 Stiffeners

6x18 Stiffeners

Dbl 6x18 Stiffeners





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FEA of Stiffened Base Plates

No Stiffeners 3x3 Stiffeners 6x6 Stiffeners

6x18 Stiffeners

Dbl 6x18 Stiffeners





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Further Research NeededIssues:

TIA TR-14.7 Subcommittee has voluntary members from engineering firms, tower

owners, fabricators and contractors.

There has been no significant funding available for research. Each group

contributes their own time and resources for any research work done.





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Further Research NeededOpportunities for Additional Research:

Develop base plate design methods using limit states and reliability based approaches.

FEA studies of unstiffened and stiffened base plates using non linear methods.

Full scale physical testing of unstiffened and stiffened base plates to correlate with FEA.

Fatigue research for base plate connections, with and without stiffeners.





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Additional Research Needed

Non-Rigid Base Plates with Bolt Prying

Square Base Plates (bolts in quadrants)

Base Plates with Stiffeners (incl. fatigue)

Using Nonlinear Material (Plastic) Methods

Parametric studies (plate thickness, bolt size, shaft type, bolt circle, flange diameter, etc.) to determine empirical relationships

FEA Simulations:





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Additional Research Needed(continued)

Full scale physical testing of base plates

Full scale physical testing of base plates with stiffeners

Physical Testing:





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THANK YOU

Documents

Monopole Structures Current Issues.pdf