TIA-222-G.pdf

TIA-222-G

(Revision of TIA-222-F)

August 2005

Structural Standard for Antenna Supporting Structures and Antennas

ANSI/TIA-222-G-2005

APPROVED: AUGUST 2, 2005

REAFFIRMED: DECEMBER 20, 2012

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(From Project No. 3-3278-RV7-RF1, formulated under the cognizance of the TIA TR-14.7 Subcommittee on Structural Standards for Steel Antenna Towers and Antenna Supporting Structures).

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ANSI/TIA-222-G

Structural Standard for Antenna Supporting Structures and Antennas

TABLE OF CONTENTS

OBJECTIVE 6

SCOPE 6

1.0 GENERAL 7

1.1 Strength Limit States 7

1.2 Serviceability Limit States 7

1.3 Analysis 7

1.4 Definitions 7

1.5 Symbols and Notation 8

2.0 LOADS 8

2.1 Scope 8

2.2 Classification of Structures 8

2.3 Combination of Loads 8

2.4 Temperature Effects 9

2.5 Dead Loads 9

2.6 Wind and Ice Loads 9

2.7 Earthquake Loads 26

2.8 Serviceability Requirements 38

3.0 ANALYSIS 59

3.1 Scope 59

3.2 Definitions 59

3.3 Symbols and Notation 59

3.4 Analysis Models 59

3.5 Displacement Effects 60

3.6 Wind Loading Patterns 60

3.7 Mast Shear and Torsion Responses for Guyed Masts 62

4.0 DESIGN STRENGTH OF STRUCTURAL STEEL 66

4.1 Scope 66

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4.2 Definitions 66

4.3 Symbols and Notations 67

4.4 General 69

4.5 Compression Members 70

4.6 Tension Members 75

4.7 Flexural Members 77

4.8 Combined Bending and Axial Forces 80

4.9 Connections 83

5.0 MANUFACTURING 102

5.1 Scope 102

5.2 Definitions 102

5.3 Symbols and Notations 102

5.4 Material, Structural Steel 102

5.5 Fabrication 103

5.6 Corrosion Control 103

6.0 OTHER STRUCTURAL MATERIALS 106

6.1 Scope 106

6.2 General 106

6.3 Loads 106

6.4 Analysis 106

6.5 Design Strength 106

7.0 GUY ASSEMBLIES 106

7.1 Scope 106

7.2 Definitions 106

7.3 Cables 107

7.4 End Attachments 107

7.5 Guy Dampers 108

7.6 Design 108

7.7 Manufacture 109

7.8 Installation 110

8.0 INSULATORS 110

8.1 Scope 110

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8.2 Design 110

8.3 Manufacture 111

9.0 FOUNDATIONS AND ANCHORAGES 111

9.1 Scope 111

9.2 Definitions 111

9.3 Site Investigation 111

9.4 Design Strength 111

9.5 Displacements 112

9.6 Seismic Considerations 112

9.7 Frost Depth Considerations 112

9.8 Submerged Conditions 112

10.0 PROTECTIVE GROUNDING 113

10.1 Scope 113

10.2 Definitions 113

10.3 General 113

10.4 Electrical Resistance 113

10.5 Grounding Materials 113

11.0 OBSTRUCTION MARKING 114

12.0 CLIMBING FACILITIES 114

12.1 Scope 114

12.2 Definitions 114

12.3 General 115

12.4 Strength Requirements 116

12.5 Dimensional Requirements 117

12.6 Climber Attachment Anchorages 117

12.7 Platforms 117

13.0 PLANS, ASSEMBLY TOLERANCES, AND MARKING 119

13.1 Scope 119

13.2 Plans 119

13.3 Tolerances 119

13.4 Marking 120

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14.0 MAINTENANCE AND CONDITION ASSESSMENT 120

14.1 Scope 120

14.2 Maximum Intervals 120

15.0 EXISTING STRUCTURES 120

15.1 Scope 120

15.2 Definitions 120

15.3 Classification 121

15.4 Changed Conditions Requiring a Structural Analysis 121

15.5 Structural Analysis 121

15.6 Exemptions 122

15.7 Modification of Existing Structures 123

16.0 INSTALLATION 123

ANNEX A: PROCUREMENT AND USER GUIDELINES (NORMATIVE) 124

ANNEX B: U.S. COUNTY LISTINGS OF DESIGN CRITERIA (NORMATIVE) 128

ANNEX C: DESIGN WIND FORCE ON TYPICAL ANTENNAS (NORMATIVE) 146

ANNEX D: TWIST AND SWAY LIMITATIONS FOR MICROWAVE ANTENNAS

(NORMATIVE) 146

ANNEX E: GUY RUPTURE (NORMATIVE) 146

ANNEX F: PRESUMPTIVE SOIL PARAMETERS (NORMATIVE) 146

ANNEX G: GEOTECHNICAL INVESTIGATIONS (NORMATIVE) 146

ANNEX H: ADDITIONAL CORROSION CONTROL (NORMATIVE) 146

ANNEX I: CLIMBER ATTACHMENT ANCHORAGES (NORMATIVE) 146

ANNEX J: MAINTAINANCE AND CONDITION ASSESSMENT (NORMATIVE) 146

ANNEX K: MEASURING GUY TENSIONS (NORMATIVE) 146

ANNEX L: WIND SPEED CONVERSIONS (NORMATIVE) 146

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ANNEX M: SI CONVERSION FACTORS (NORMATIVE) 146

ANNEX N: REFERENCES (NORMATIVE) 146

APPENDIX 1: DESIGN CRITERIA MAPS (NORMATIVE) 146

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Structural Standard for Antenna Supporting Structures and Antennas

OBJECTIVE

The objective of this Standard is to provide recognized literature for antenna supporting structures and antennas pertaining to: (a) minimum load requirements as derived from ASCE 7- 02, “Minimum Design Loads for Buildings and Other Structures”, and (b) design criteria as derived from AISC-LRFD-99, “Load and Resistance Factor Design Specification for Structural Steel Buildings” and ACI 318-05, “Building Code Requirements for Structural Concrete”. The information contained in this Standard was obtained from available sources and represents, in the judgment of the subcommittee, the accepted industry minimum structural standards for the design of antenna supporting structures and antennas. While it is believed to be accurate, this information should not be relied upon for a specific application without competent professional examination and verification of its accuracy, suitability, and applicability by a licensed professional engineer. This Standard utilizes loading criteria based on an annual probability and is not intended to cover all environmental conditions which could exist at a particular location.

When this Standard is adapted for international use, it is necessary to determine the appropriate basic wind speeds (3-second gust), wind on ice loads, and earthquake accelerations at the site location based on local data.

Equivalent International System of Units (SI) are given in square brackets [ ] throughout this Standard. SI conversion factors have been provided in Annex M.

Annex A provides procurement and user guidelines to assist in specifying the requirements for a specific structure. The user is cautioned that site-specific loading requirements, if known, take precedence over the minimum requirements of this Standard. Site-specific data and requirements differing from those contained in this Standard are to be included in the procurement specifications for the structure.

This Standard is intended to cover the requirements for most structural antennas and antenna-supporting structures, but recognizes that structures that are unusual for their height or shape, or for the shape and size of individual members, or located at sites having unusual geological or climatic conditions may require additional considerations. In these cases, a rational design based on theory, analysis, knowledge of local conditions and sound engineering practice, shall be used. The design shall be carried out by an engineer qualified in the specific design methods and materials to be used, and shall provide a level of safety and performance equal to or better than that implicit in this Standard.

SCOPE

This Standard provides the requirements for the structural design and fabrication of new and the modification of existing structural antennas, antenna-supporting structures, mounts, structural components, guy assemblies, insulators and foundations.

This Standard is based on limit states design. It is applicable mainly to steel structures but may also be applied to other materials, when required, so as to provide an equivalent level of reliability.

The appropriate standards should be referenced for structures that support antennas but that are primarily intended for other applications, such as water towers, electrical transmission line

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and to the serviceability limit states appropriate for structures that support antennas.

Annexes are to be considered normative elements that are necessary to comply with this Standard.

1.0 GENERAL

1.1 Strength Limit States

A structure designed to this Standard shall have sufficient strength and stability such that the design strength, φRn, defined in Section 4.0 equals or exceeds the required strength, ∑αiQi, defined in Section 2.0 as expressed by the following relationship:

φRn ≥ ∑αiQi

1.2 Serviceability Limit States

A structure designed to this Standard shall have sufficient rigidity such that the limit state deformations defined in 2.8.2 are not exceeded under the service loads defined in 2.8.3.

1.3 Analysis

Load effects on individual structural members shall be determined by methods of structural analysis that take into account equilibrium, general stability, geometric compatibility and material properties in accordance with Section 3.0.

1.4 Definitions

Antenna supporting structure: a structure, including guy assemblies, guy anchorages and substructures that support antennas or antenna arrays.

Design strength, φRn: the product of nominal strength and a resistance factor.

Factored load: the product of the nominal load and a load factor.

Limit state: a condition beyond which a structure or member becomes unfit for service and is judged to be no longer useful for its intended function or unsafe.-

Load effects: force and deformation responses produced in structures and their members by applied factored loads.

Load factor, αi: a factor that accounts for deviations of the actual load from the nominal load, for uncertainties in the analysis that transforms the load into load effects, and for the probability that more than one extreme load will occur simultaneously.

Nominal loads: the magnitudes of the loads specified in this Standard for dead, wind, ice, wind on ice, earthquake, and working and climbing facilities.

Nominal strength, Rn: the capacity of a structure or member to resist the effects of loads.

Required strength, ∑αiQi: the sum of the load effects due to applied factored loads and load combinations.

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Resistance factor, φ: a factor that accounts for the manner and consequences of failure and for unavoidable deviations from a calculated nominal strength.

Strength design: method of proportioning structural members such that the computed forces produced in the members by the factored loads do not exceed the design strengths of the members.

Structural antenna: a structure for radiating or receiving electromagnetic waves including reflectors, directors and screens.

1.5 Symbols and Notation

φRn = design strength (nominal strength multiplied by a resistance factor);

∑αiQi = required strength (load effects due to factored loads and load combinations).

2.0 LOADS

2.1 Scope

This section provides minimum load requirements for antenna supporting structures and antennas.

2.2 Classification of Structures

Structures shall be classified according to Table 2-1 for the purposes of determining nominal wind, ice and earthquake loads.

2.3 Combination of Loads

2.3.1 Symbols and Notation

D = dead load of structure and appurtenances, excluding guy assemblies;

Dg = dead load of guy assemblies;

Di = weight of ice due to factored ice thickness;

E = earthquake load;

Ti = load effects due to temperature;

Wo = wind load without ice;

Wi = concurrent wind load with factored ice thickness.

2.3.2 Strength Limit State Load Combinations

Structures and foundations shall be designed so that their design strength equals or exceeds the load effects of the factored loads in each of the following limit state combinations:

1. 1.2 D + 1.0 Dg + 1.6 Wo

2. 0.9 D + 1.0 Dg + 1.6 Wo

3. 1.2 D + 1.0 Dg + 1.0 Di + 1.0 Wi + 1.0 Ti 4 1.2 D + 1.0 Dg + 1.0 E

5. 0.9 D + 1.0 Dg + 1.0 E

Exceptions:

1. Temperature effects need not be considered for self-supporting structures.

2. Ice and earthquake loading need not be considered for Class I structures.

3. No load factor shall be applied to the initial tension of guys.

4. Load combinations 2 and 5 apply to self-supporting structures only.

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Notes:

1. A limit state conversion factor for ice load is applied to ice thickness in 2.6.8

2. Unfactored dead loads shall be used to determine earthquake loads, E, in loading combinations 4 and 5.

3. For foundation designs, the weight of soil and substructure shall be considered as dead load in all loading combinations.

2.4 Temperature Effects

The design tension of guys shall be based on an initial temperature of 60 degrees Fahrenheit [16 degrees C]. In the absence of more accurate site data, a 50 degree Fahrenheit [28 degree C] reduction in temperature shall be considered to occur with loading combinations that include ice.

2.5 Dead Loads

2.5.1 Definitions

Dead load: D, the weight of the structure, and appurtenances, excluding guy assemblies, and for foundation design, the weight of soil and substructure.

Guy assembly dead load: Dg, the weight of all guy assemblies, including guys, end fittings, and insulators.

2.6 Wind and Ice Loads

2.6.1 Definitions

Appurtenances: items attached to the structure such as antennas, antenna mounts, transmission lines, conduits, lighting equipment, climbing devices, platforms, signs, anti-climbing devices, etc.

Basic wind speed, V: 3-second gust wind speed at 33 ft [10 m] above the ground in exposure category C as defined in 2.6.5.1 for a 50-year mean recurrence interval.

Basic wind speed with ice, Vi: 3-second gust wind speed concurrent with the design ice thickness at 33 ft [10 m] above the ground in exposure category C as defined in 2.6.5.1 for a 50-year mean recurrence interval.

Design wind load, Fw: equivalent static force to be used in the determination of wind loads.

Design ice thickness, ti: the uniform radial thickness of glaze ice at 33 ft [10 m] above the ground in exposure category C as defined in 2.6.5.1 for a 50-year mean recurrence interval.

Discrete appurtenance: an appurtenance modeled as a concentrated load.

Effective projected area, EPA: projected area of an object multiplied by a force coefficient (also called a drag factor) used in the determination of wind loads.

Escarpment: a cliff or steep slope generally separating two levels or gently sloping areas.

Glaze ice: ice accretion assumed to have a unit weight of 56 lb/ft3 [8.8 kN/m3].

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Height of structure, h: the height of a structure, including latticed or tubular poles mounted on the structure, but excluding lightning rods and similar appurtenances.

Hill: a land surface characterized by a strong relief in all horizontal directions.

Hurricane prone regions: areas vulnerable to hurricanes where the basic wind speed without ice is greater than 90 mph [40.2 m/s].

Importance factor, I: a factor that accounts for the degree of hazard to human life, damage to property and reliability of service.

Linear appurtenance: an appurtenance modeled as a distributed load.

Ridge: an elongated crest characterized by strong relief in two directions.

Symmetrical appurtenance: an appurtenance for which the effective projected area (EPA) is considered constant for all wind directions.

Velocity pressure, qz: equivalent static pressure used in the determination of wind loads.

Weight of ice, Di: the weight of factored ice accumulated on the structure, guys, and appurtenances.

2.6.2 Symbols and Notations

α = 3-second gust wind speed power law exponent;

ε = solidity ratio of the structure without appurtenances;

θ = relative angle between the azimuth of an appurtenance and the wind direction;

θg = angle of wind incidence to a guy chord;

Aa = projected area of an appurtenance;

Af = projected area of flat structural components;

Afs = projected area of flat components supporting a mounting frame;

Ag = gross area of one tower face or mounting frame;

Aiz = cross-sectional area of ice at height z, or mounting frame;

Ap = projected area of a pole structure;

Ar = projected area of round structural components;

Ars = projected area of round components supporting a mounting frame;

C = velocity coefficient for round, tubular and polygonal members;

Ca = force coefficient for a linear or discrete appurtenance;

Cas = force coefficient for a mounting frame;

Cd = drag factor for a guy;

Cf = force coefficient for a structure;

d = guy diameter;

D = diameter of a pole structure, a round structural component or a round appurtenance;

Dc = largest out-to-out dimension of a member;

Df = wind direction factor for flat structural components;

Di = weight of Ice;

Dr = wind direction factor for round structural components;

Ds = smallest projected dimension of a component;

(EPA)A = effective projected area of an appurtenance;

(EPA)FN = normal effective projected area of members supporting a mounting frame;

(EPA)FT = transverse effective projected area of members supporting a mounting frame;

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(EPA)MN = normal effective projected area of a mounting frame;

(EPA)MT = transverse effective projected area of a mounting frame;

(EPA)N = effective projected area associated with the windward face normal to the azimuth of the appurtenance;

(EPA)S = effective projected area of the structure;

(EPA)T = effective projected area associated with the windward side face of an appurtenance;

e = natural logarithmic base;

f = height attenuation factor;

FA = design wind force on appurtenances;

FG = design wind force on guys;

FST = design wind force on the structure;

FW = design wind load;

Gh = gust effect factor;

h = height of structure;

H = height of crest above surrounding terrain;

I = importance factor;

Ka = shielding factor for appurtenances (or wake interference factor);

Kd = wind direction probability factor;

Ke = terrain constant;

Kh = height reduction factor;

Kiz = height escalation factor for ice thickness;

Kt = topographic constant;

Kz = velocity pressure coefficient;

Kzmin = minimum value for Kz;

Kzt = topographic factor;

Lg = length of guy;

qz = velocity pressure;

Ra = ratio of projected area of attachments to projected area of structural member;

Rr = reduction factor for a round element in a tower face;

Rrf = reduction factor for a round element in a mounting frame;

ti = design ice thickness for site location;

tiz = nominal thickness of radial glaze ice at height z;

V = basic wind speed without ice;

Vi = basic wind speed with ice;

z = height above ground;

zg = nominal height of atmospheric boundary layer;

2.6.3 General

Antennas and antenna supporting structures have unusual shapes and response characteristics due to wind load. The provisions of this Standard take into consideration the load magnification effects caused by wind gusts in resonance with along-wind vibrations of self-supporting and guyed antenna supporting structures.

1. The basic wind speed without ice, V, the basic wind speed with ice, Vi, and the design ice thickness ti shall be determined from 2.6.4.

2. A wind direction probability factor, Kd, shall be determined from Table 2-2.

3. An importance factor, I, shall be determined from Table 2-3 based on the structure classification listed in Table 2-1.

4. An exposure category and velocity pressure coefficient, Kz, shall be determined for the site location in accordance with 2.6.5.

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5. A topographic category and topographic factor, Kzt, shall be determined in accordance with 2.6.6.

6. A gust effect factor, Gh, shall be determined in accordance with 2.6.7.

7. The design ice thickness shall be escalated with height in accordance with 2.6.8.

8. The design wind force shall be determined in accordance with 2.6.9.

2.6.4 Basic Wind Speed and Design Ice Thickness

The basic wind speed without ice, the basic wind speed with ice and the design ice thickness shall be as given in Annex B except as provided in 2.6.4.1. Wind shall be considered to come from any horizontal direction. Ice shall be considered to be glaze ice.

Ice may be ignored for structures located in regions where the design ice thickness is less than or equal to 0.25 inches (6 mm).

2.6.4.1 Estimation of Basic Wind Speeds and Design Ice Thickness from Regional Climatic Data

For regions not included in Annex B, for the special wind or ice regions indicated in Annex B, and for sites where records indicate that in-cloud icing produces significant loads, extreme-value statistical-analysis procedures shall be used to establish design values consistent with this Standard from available climatic data accounting for the length of record, sampling error, averaging time, anemometer height, data quality, and terrain exposure.

2.6.5 Exposure Categories

2.6.5.1 General

An exposure category that adequately reflects the characteristics of ground surface irregularities at the site shall be determined. Account shall be taken of variations in ground surface roughness that arise from natural topography and vegetation as well as from constructed features. The exposure category for a structure shall be assessed as being one of the following:

1. Exposure B: Urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger. Use of this exposure shall be limited to those areas for which terrain representative of Exposure B surrounds the structure in all directions for a distance of at least 2,630 ft [800 m] or ten times the height of the structure, whichever is greater.

2. Exposure C: Open terrain with scattered obstructions having heights generally less than

30 ft [9.1 m]. This category includes flat, open country, grasslands and shorelines in hurricane prone regions.

3. Exposure D: Flat, unobstructed shorelines exposed to wind flowing over open water

(excluding shorelines in hurricane prone regions) for a distance of at least 1 mile [1.61 km]. Shorelines in Exposure D include inland waterways, lakes and non-hurricane coastal areas. Exposure D extends inland a distance of 660 ft [200 m] or ten times the height of the structure, whichever is greater. Smooth mud flats, salt flats and other similar terrain shall be considered as Exposure D.

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2.6.5.2 Velocity Pressure Coefficient

Based on the exposure category determined in 2.6.5.1, a velocity pressure coefficient (Kz) shall be determined as follows:

Kz = 2.01(z/zg)2/α

Kzmin ≤ Kz ≤ 2.01 where:

z = height above ground level at the base of the structure zg, α and Kzmin are tabulated in Table 2-4

2.6.6 Topographic Effects

2.6.6.1 Wind Speed-Up Over Hills, Ridges and Escarpments

Wind speed-up effects at isolated hills, ridges and escarpments constituting abrupt changes in the general topography, located in any exposure category, shall be included in the calculation of design wind loads under the following conditions:

1. The hill, ridge or escarpment is isolated and unobstructed by other similar topographic features of comparable height for a radius of 2 miles [3.22 km] measured horizontally from the point at which the height of the hill, ridge or escarpment is determined, and

2. The hill, ridge or escarpment protrudes by a factor of two or more above the average height of the surrounding terrain features within a 2 mile [3.22 km] radius, and

3. The slope (vertical to horizontal ratio) of the topographic feature exceeds 0.10, and

4. The height of the topographic feature is greater than or equal to 15 ft [4.57 m] for exposures C and D and 60 ft [18 m] for exposure B.

2.6.6.2 Topographic Categories

The topographic category for a structure shall be assessed as being one of the following:

1. Category 1: No abrupt changes in general topography, e.g. flat or rolling terrain, no wind speed-up consideration shall be required.

2. Category 2: Structures located at or near the crest of an escarpment. Wind speed-up shall be considered to occur in all directions. Structures located vertically on the lower half of an escarpment or horizontally beyond 8 times the height of the escarpment from its crest, shall be permitted to be considered as Topographic Category 1.

3. Category 3: Structures located in the upper half of a hill. Wind speed-up shall be considered to occur in all directions. Structures located vertically on the lower half of a hill shall be permitted to be considered as Topographic Category 1.

4. Category 4: Structures located in the upper half of a ridge. Wind speed-up shall be considered to occur in all directions. Structures located vertically on the lower half of a ridge shall be permitted to be considered as Topographic Category 1.

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5. Category 5: Wind speed-up criteria based on a site-specific investigation.

2.6.6.3 Structures Supported on Buildings or Other Structures Wind speed-up shall not be used to account for the increased wind loads required due to height for structures supported on buildings or other structures. The height, z, above ground level shall be referenced to the ground level of the building or other supporting structure.

2.6.6.4 Topographic Factor

The wind speed-up effect shall be included in the calculation of design wind loads by using the factor Kzt:

2] hK tKeK [1ztK += where:

Kh = height reduction factor given by the following equation:

H zf( e e = natural logarithmic base = 2.718

Ke = terrain constant given in Table 2-4

Kt = topographic constant given in Table 2-5 f = height attenuation factor given in Table 2-5 z = height above ground level at the base of the structure

H = height of crest above surrounding terrain

Kzt = 1.0 for topographic category 1. For topographic category 5, Kzt shall be based on recognized published literature or research findings.

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2.6.7 Gust Effect Factor

2.6.7.1 Self-Supporting Latticed Structures

For self-supporting latticed structures, the gust effect factor shall be 1.00 for structures 600 ft [183 m] or greater in height. For structures 450 ft [137 m] or less in height, the gust effect factor shall be 0.85. The gust effect factor shall be linearly interpolated for structure heights between 450 ft [137 m] and 600 ft [183 m].

These conditions are expressed by the following equations:

3.0] h[0.150.85hG −+= h, in feet

3.0] 45.7 h[0.150.85hG −+= h, in meters

0.85 ≤ Gh ≤ 1.00 h = height of structure

Note: For structures supported on buildings or other structures, the height of structure, h, shall not include the height of the supporting structure.

2.6.7.2 Guyed Masts

For guyed masts the gust effect factor shall be 0.85.

2.6.7.3 Pole Structures

For pole structures the gust effect factor shall be 1.10.

2.6.7.4 Structures Supported on Other Structures

For cantilevered tubular or latticed spines, poles or similar structures mounted on guyed masts or latticed self-supporting structures, and for all structures supported on flexible buildings (height to width ratio greater than 5), the gust effect factor shall be 1.35. Gust effect factors for supporting guyed masts and latticed self-supporting structures shall be in accordance with

2.6.7.1 or 2.6.7.2 using the loads from the cantilever based on a 1.35 gust effect factor.

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2.6.8 Design Ice Thickness

The design ice thickness, ti, shall be escalated with height when calculating ice weight and wind on ice loads in accordance with the following equations:

tiz = 2.0 ti I Kiz(Kzt)0.35

Kiz = [ z ]0.10 ≤ 1.4 z, in feet

Kiz = [ z ]0.10 ≤ 1.4 z, in meters

2.0 = limit state conversion factor tiz = the factored thickness of radial glaze ice at height z ti = design ice thickness

I = importance factor for structure from Table 2-3

Kiz = height escalation factor for ice thickness z = height above ground level at base of structure

Kzt = topographic factor from 2.6.6.4

For purposes of calculating the additional projected area of ice, ice thickness shall be considered to accumulate with a uniform thickness around the exposed surfaces of the structure, guys and appurtenances (refer to Figure 2-1). The additional projected area of ice may be considered round when calculating wind on ice loads even though the bare projected area is flat.

For purposes of calculating the weight of ice, the cross-sectional area of ice shall be determined by:

)iztc(DiztπizA +⋅=

Aiz = cross-sectional area of ice at height z

Dc = largest out-to-out dimension of a member (refer to Figure 2-2)

The weight of ice shall be based on a unit weight of 56 lb/ft3 [8.8 kN/m3].

2.6.9 Design Wind Load

The design wind load shall include the sum of the horizontal design wind forces applied to the structure in the direction of the wind and the design wind forces on guys and appurtenances. All

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Strength design shall be based on the wind directions resulting in the maximum responses. For latticed structures, each of the wind directions indicated in Table 2-6 shall be considered for each face.

The horizontal design wind force for the strength design of appurtenances and their connections to supporting structures shall be determined using a gust effect factor of 1.0 and a directionality factor determined from Table 2-2. No shielding from the structure shall be considered (Ka= 1.0, refer to 2.6.9.2).

The horizontal design wind force for the strength design of a cantilevered tubular or latticed spine, pole or similar structure mounted on a guyed mast, latticed self-supporting structure, or flexible building shall be determined using a gust effect factor of 1.35 (refer to 2.6.7.4) and a directionality factor determined from Table 2-2 for the cantilevered structure.

Note: The directionality factor for determining the design wind load for the total structure, including the cantilever, shall be determined from Table 2-2 based on the type of supporting structure.

The design wind load, FW, shall be determined in accordance with the following:

FW = FST + FA + FG

FST = design wind force on the structure from 2.6.9.1

FA = design wind force on appurtenances from 2.6.9.2

FG = design wind force on guys from 2.6.9.3

The design wind forces, FS + FA, need not exceed the wind force calculated for a structure using a solidity ratio of 1.0 (solid-faced) plus the wind load on externally mounted appurtenances that are outside the normal projected area of the structure in the direction of the wind.

2.6.9.1 Design Wind Force on Structure

The design wind force, FS, applied to each section of a structure shall be determined in accordance with the following:

FST = qz Gh (EPA)S

FS = horizontal design wind force on the structure in the direction of the wind qz = velocity pressure from 2.6.9.6

Gh = gust effect factor from 2.6.7

(EPA)S = effective projected area of the structure from 2.6.9.1.1 or 2.6.9.1.2

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2.6.9.1.1 Effective Projected Area of Latticed Structures

The effective projected area of structural components for a section, (EPA)S, shall be determined from the equation:

(EPA)S = Cf [Df ΣAf + Dr Σ (Ar Rr )]

Cf = 4.0ε2 – 5.9ε + 4.0 (square cross sections)

Cf = 3.4ε2 – 4.7ε + 3.4 (triangular cross sections) ε = solidity ratio = (Af + Ar)/Ag

Af = projected area of flat structural components in one face of the section

Ar = projected area of round structural components in one face of the section including the projected area of ice on flat and round structural components in one face for loading combinations that include ice

Ag = gross area of one face as if the face were solid

Df = wind direction factor for flat structural components determined from Table 2-6

Dr = wind direction factor for round structural components determined from Table 2-6

Rr = reduction factor for a round element

= 0.57 - 0.14ε + 0.86ε2 - 0.24ε3 ≤ 1.0 when C < 32 [4.4] and for all iced conditions (subcritical flow)

= 0.36 + 0.26ε + 0.97ε2 - 0.63ε3 when C > 64 [8.7] for no-ice conditions

(supercritical flow)

C = [I KZ KZT]1/2 V D

I = importance factor from Table 2-3

KZ = velocity pressure coefficient from 2.6.5.2

KZT = topographic factor from 2.6.6.4

V = the basic wind speed for the loading condition under investigation, mph [m/s]

D = outside diameter of the structural component without ice, ft [m]

Notes:

1) The projected area of structural components shall include the projected area of connection plates in the face of a section.

2) In order for a structural component to be considered as a round structural component, the component must have a round profile on the windward and leeward sides of the

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3) Bracing members in adjacent faces and internal plan and hip bracing need not be included in the projected area of structural components.

4) For no-ice conditions, linear interpolation may be used when 32 [4.4] ≤ C ≤ 64 [8.7] to determine Rr. For iced conditions, Rr shall be based on subcritical flow for all values of C.

5) When attachments such as step bolts or similar irregularities are attached to a round structural member, the reduction factor for the round elements, Rr shall be calculated as follows:

(a) when Ra ≤ 0.1, the projected areas of the attachments may be ignored

(b) when 0.1 < Ra ≤ 0.2, the value for Rr shall be multiplied by 1.0 + 3(Ra – 0.1), and the projected areas of the attachments may be ignored

(c) when Ra > 0.20, or alternatively for any value of Ra, the value of Rr for subcritical flow shall be used. The projected areas of attachments shall be considered separately in addition to the structural member using appropriate force coefficients for appurtenances.

Where Ra is the ratio of the projected area of the attachments to the projected area of the structural member without the attachments for the portion being considered. For iced conditions, the ice thickness need not be included in the determination of Ra.

6) When attachments such as step bolts or similar irregularities are attached to a flat structural member, the projected areas of the attachments shall be considered separately in addition to the structural member using appropriate force coefficients except when Ra is less than or equal to 0.1, the projected areas of the attachments may be ignored.

2.6.9.1.1.1 Effective Projected Area of Latticed Leg Structures Latticed legs shall be considered as equivalent round members for the purpose of determining the effective projected area, (EPA)S, of structures with latticed legs.

The effective projected area of an individual latticed leg shall be determined in accordance with

2.6.9.1.1 with Rr based on subcritical flow and the direction factors, Df and Dr, equal to 1.0. The diameter of the equivalent round member shall be determined by dividing the (EPA)S of the individual latticed leg by the quantity of 1.2 times the length of the latticed leg. Gross area, Ag, of the structure shall be based of the full width of the structure including the width of the latticed leg and ice when applicable.

The reduction factor, Rr, for the equivalent round member shall be based on subcritical flow.

For loading conditions that include ice, the design ice thickness shall be considered uniformly distributed around each member of the latticed leg for determining effective projected areas (ice thickness need not be added to the equivalent round member). The weight of ice shall be determined by considering each member of the latticed leg in accordance with 2.6.8.

2.6.9.1.2 Effective Projected Area of Pole Structures

The effective projected area of a pole section, (EPA)S, shall be determined from the equation:

(EPA)S = Cf AP

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Cf = force coefficient for cantilevered pole structure from Table 2-7

AP = actual projected area based on the pole outside diameter (for rounds), the outside point-to-point diameter (for polygons), or overall width, including ice thickness for load combinations that include ice

Note: In the absence of a detailed transmission line layout and installation bend radii of the lines, the minimum diameter of a pole structure shall not be less than the diameter which results in 45% utilization of the cross-section for the placement of internal transmission lines.

2.6.9.1.3 Uniform Wind and Ice Applied to Structure

The design wind force and ice thickness applied to a section of a structure may be based on the velocity pressure and ice thickness at the mid-height of the section. The section length considered to have uniform velocity pressure and ice thickness shall not exceed the following:

a) 60 ft [18 m] for latticed structures

b) 20 ft [6 m] for pole structures

2.6.9.2 Design Wind Force on Appurtenances

The design wind force on appurtenances (either discrete or linear but excluding microwave antennas), FA, shall be determined from the equation:

FA = qz Gh (EPA)A qz = velocity pressure at the centerline height of the appurtenance from 2.6.9.6

Gh = gust effect factor from 2.6.7

(Note: see 2.6.9 for Gh for the strength design of appurtenances.)

(EPA)A = effective projected area of the appurtenance including ice for loading combinations that include ice.

The design wind force, FA, shall be applied at the centroid of the effective projected area of the appurtenance in the direction of the wind. For a linear appurtenance, the length considered to have uniform velocity pressure and ice thickness shall not exceed the section length specified in 2.6.9.1.3.

In the absence of more accurate data, the design wind force on microwave antennas shall be determined using Annex C.

In the absence of more accurate data specifying effective projected area values for each critical wind direction, the effective projected area, (EPA)A, of an appurtenance shall be determined from the equation:

(EPA)A = Ka[(EPA)N cos2(θ) + (EPA)T sin2(θ)]

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Ka = 1.0 for round appurtenances, regardless of location, when transitional or supercritical force coefficients are considered.

= (1- ε) for appurtenances when subcritical force coefficients are considered, entirely inside the cross section of a latticed structure or outside the cross section entirely within a face zone as defined in Figure 2-3, where ε is the minimum solidity ratio of the structure considering each face for the section containing the appurtenance. Ka need not exceed 0.6.

= 0.8 for antenna mounting configurations (when subcritical force coefficients are considered only) such as side arms, T-arms, stand-offs, etc. when 3 or more mounts are located at the same relative elevation (shielding from the mounting configuration and shielding of mounting members from antennas is excluded, refer to 2.6.9.4)

= 1.0 for other appurtenances unless otherwise specified in this section

(Notes: 1. Ka = 1.0 may be conservatively used for any appurtenance

2. The value of Ka is constant for all wind directions) θ = relative angle between the azimuth associated with the normal face of the appurtenance and the wind direction (refer to Figure 2-4).

(EPA)N = effective projected area associated with the windward face normal to the azimuth of the appurtenance.

(EPA)T = effective projected area associated with the windward side face of the appurtenance.

The larger value of (EPA)N or (EPA)T may be conservatively used for (EPA)A for all wind directions.

In the absence of more accurate data, an appurtenance shall be considered as consisting of flat and round components in accordance with the following:

(EPA)N = Σ(Ca AA)N

(EPA)T = Σ(Ca AA)T

Ca = force coefficient from Table 2-8

AA = projected area of a component of the…