Cover [PDF]

Standards [PDF]

Foreword [PDF]

Acknowledgements [PDF]

Dedication [PDF]

Contents [PDF]

Chapter 1 General

Chapter 2 Combinations of Loads

Chapter 3 Dead Loads, Soil Loads, and Hydrostatic Pressure

Chapter 4 Live Loads

Chapter 5 Flood Loads

Chapter 6 Reserved for Future Provisions

Chapter 7 Snow Loads

Chapter 8 Rain Loads

Chapter 9 Reserved for Future Provisions

Chapter 10 Ice Loads - Atmospheric Icing

Chapter 11 Seismic Design Criteria

Chapter 12 Seismic Design Requirements for Building Structures

Chapter 13 Seismic Design Requirements for Nonstructural Components

Chapter 14 Material Specific Seismic Design and Detailing Requirements

Chapter 15 Seismic Design Requirements for Nonbuilding Structures

Chapter 16 Seismic Response History Procedures

Chapter 17 Seismic Design Requirements for Seismically Isolated Structures

Chapter 18 Seismic Design Requirements for Structures with Damping Systems

Chapter 19 Soil-Structure Interaction for Seismic Design

Chapter 20 Site Classification Procedure for Seismic Design

Chapter 21 Site-Specific Ground Motion Procedures for Seismic Design

Chapter 22 Seismic Ground Motion Long-Period Transition and Risk Coefficient Maps

Chapter 23 Seismic Design Reference Documents

Chapter 24

Chapter 25

Chapter 26 Wind Loads: General Requirements

Chapter 27 Wind Loads on Buildings‒MWFRS (Directional Procedure)

Chapter 28 Wind Loads on Buildings‒MWFRS (Envelope Procedure)

Chapter 29 Wind Loads on Other Structures and Building Appurtenances‒MWFRS

Chapter 30 Wind Loads ‒ Components and Cladding (C&C)

Chapter 31 Wind Tunnel Procedure

Appendix 11A Quality Assurance Provisions

Appendix 11B Existing Building Provisions

Appendix C Serviceability Considerations

Appendix D Buildings Exempted from Torisional Wind Load Cases

Atmospheric ice loads due to freezing rain, snow, and in-cloud icing shall be considered in the design of ice-sensitive structures. In areas where records or experience indicate that snow or in-cloud icing produces larger loads than freezing rain, site-specific studies shall be used. Structural loads due to hoarfrost are not a design consideration. Roof snow loads are covered in Chapter 7.
Mountainous terrain and gorges shall be examined for unusual icing conditions. Site-specific studies shall be used to determine the 50-year mean recurrence interval ice thickness, concurrent wind speed, and concurrent temperature in
  1. Alaska;
  2. areas where records or experience indicate that snow or in-cloud icing produces larger loads than freezing rain;
  3. special icing regions shown in Figs. 10-2, 10-4, and 10-5; and
  4. mountainous terrain and gorges where examination indicates unusual icing conditions exist.
   Site-specific studies shall be subject to review and approval by the authority having jurisdiction.
   In lieu of using the mapped values, it shall be permitted to determine the ice thickness, the concurrent wind speed, and the concurrent temperature for a structure from local meteorological data based on a 50-year mean recurrence interval provided that
  1. The quality of the data for wind and type and amount of precipitation has been taken into account.
  2. A robust ice accretion algorithm has been used to estimate uniform ice thicknesses and concurrent wind speeds from these data.
  3. Extreme-value statistical analysis procedures acceptable to the authority having jurisdiction have been employed in analyzing the ice thickness and concurrent wind speed data.
  4. The length of record and sampling error have been taken into account.

FIGURE 10-2 Equivalent Radial Ice Thickness Due to Freezing Rain with Concurrent 3-Second Gust Speeds, for a 50-Year Mean Recurrence Interval
FIGURE 10-2 (Continued)
FIGURE 10-4 Fraser Valley Detail




FIGURE 10-5 Columbia River Gorge Detail
Dynamic loads, such as those resulting from galloping, ice shedding, and aeolian vibrations, that are caused or enhanced by an ice accretion on a flexible structural member, component, or appurtenance are not covered in this section.
Electric transmission systems, communications towers and masts, and other structures for which national standards exist are excluded from the requirements of this section. Applicable standards and guidelines include the NESC, ASCE Manual 74, and ANSI/EIA/TIA-222.
The following definitions apply only to the provisions of this chapter.
   COMPONENTS AND APPURTENANCES: Nonstructural elements that may be exposed to atmospheric icing. Examples are ladders, handrails, antennas, waveguides, radio frequency (RF) transmission lines, pipes, electrical conduits, and cable trays.
   FREEZING RAIN: Rain or drizzle that falls into a layer of subfreezing air at the earth's surface and freezes on contact with the ground or an object to form glaze ice.
   GLAZE: Clear high-density ice.
   HOARFROST: An accumulation of ice crystals formed by direct deposition of water vapor from the air onto an object.
   ICE-SENSITIVE STRUCTURES: Structures for which the effect of an atmospheric icing load governs the design of part or all of the structure. This includes, but is not limited to, lattice structures, guyed masts, overhead lines, light suspension and cable-stayed bridges, aerial cable systems (e.g., for ski lifts and logging operations), amusement rides, open catwalks and platforms, flagpoles, and signs.
   IN-CLOUD ICING: Occurs when supercooled cloud or fog droplets carried by the wind freeze on impact with objects. Incloud icing usually forms rime, but may also form glaze.
   RIME: White or opaque ice with entrapped air.
   SNOW: Snow that adheres to objects by some combination of capillary forces, freezing, and sintering.
As = surface area of one side of a flat plate or the projected area of complex shapes
Ai = cross-sectional area of ice
D = diameter of a circular structure or member as defined in Chapter 29, in ft (m)
Dc = diameter of the cylinder circumscribing an object
fz = factor to account for the increase in ice thickness with height
Ii = importance factor for ice thickness from Table 1.5-2 based on the Risk Category from Table 1.5-1
Iw = importance factor for concurrent wind pressure from Table 1.5-2 based on the Risk Category from Table 1.5-1
Kzt = topographic factor as defined in Chapter 26
qz = velocity pressure evaluated at height z above ground, in lb/ft2 (N/m2) as defined in Chapter 29
r = radius of the maximum cross-section of a dome or radiusof a sphere
t = nominal ice thickness due to freezing rain at a height of 33 ft (10 m) from Figs. 10-2 through 10-6 in inches (mm)
td = design ice thickness in in. (mm) from Eq. 10.4-5
Vc = concurrent wind speed mph (m/s) from Figs. 10-2 through 10-6
Vi = volume of ice
z = height above ground in ft (m)
= solidity ratio as defined in Chapter 29
The ice load shall be determined using the weight of glaze ice formed on all exposed surfaces of structural members, guys, components, appurtenances, and cable systems. On structural shapes, prismatic members, and other similar shapes, the cross-sectional area of ice shall be determined by
(10.4-1)

Dc is shown for a variety of cross-sectional shapes in Fig. 10-1.


FIGURE 10-1 Characteristic Dimension Dc for Calculating the Ice Area for a Variety of Cross-Sectional Shapes
   On flat plates and large three-dimensional objects such as domes and spheres, the volume of ice shall be determined by
(10.4-2)
   For a flat plate As shall be the area of one side of the plate, for domes and spheres As shall be determined by
          (10.4-3)
   It is acceptable to multiply Vi by 0.8 for vertical plates and 0.6 for horizontal plates.
   The ice density shall be not less than 56 pcf (900 kg/m3).
Figs. 10-2 through 10-6 show the equivalent uniform radial thicknesses t of ice due to freezing rain at a height of 33 ft (10 m) over the contiguous 48 states and Alaska for a 50-year mean recurrence interval. Also shown are concurrent 3-s gust wind speeds. Thicknesses for Hawaii, and for ice accretions due to other sources in all regions, shall be obtained from local meteorological studies.


FIGURE 10-3 Lake Superior Detail



FIGURE 10-6 50-Yr Mean Recurrence Interval Uniform Ice Thicknesses Due to Freezing Rain with Concurrent 3-Second Gust Speeds: Alaska
The height factor fz used to increase the radial thickness of ice for height above ground z shall be determined by

(10.4-4)
In SI:
Importance factors to be applied to the radial ice thickness and wind pressure shall be determined from Table 1.5-2 based on the Risk Category from Table 1.5-1. The importance factor Ii shall be applied to the ice thickness, not the ice weight, because the ice weight is not a linear function of thickness.
Both the ice thickness and concurrent wind speed for structures on hills, ridges, and escarpments are higher than those on level terrain because of wind speed-up effects. The topographic factor for the concurrent wind pressure is Kzt and the topographic factor for ice thickness is (Kzt)0.35, where Kzt is obtained from Eq. 26.8-1.
The design ice thickness td shall be calculated from Eq. 10.4-5.

                                 td = 2.0tIifz(Kzt) 0.35 (10.4-5)
Ice accreted on structural members, components, and appurtenances increases the projected area of the structure exposed to wind. The projected area shall be increased by adding td to all free edges of the projected area. Wind loads on this increased projected area shall be used in the design of ice-sensitive structures. Figs. 10-2 to 10-6 include 3-s gust wind speeds at 33 ft (10 m) above grade that are concurrent with the ice loads due to freezing rain. Wind loads shall be calculated in accordance with Chapters 26 through 31 as modified by Sections 10.5.1 through 10.5.5.
Force coefficients Cf for structures with square, hexagonal, and octagonal cross-sections shall be as given in Fig. 29.5-1. Force coefficients Cf for structures with round cross-sections shall be as given in Fig. 29.5-1 for round cross-sections with Dqz2.5 for all ice thicknesses, wind speeds, and structure diameters.
Force coefficients Cf shall be as given in Fig. 29.4-1 based on the dimensions of the wall or sign including ice.
The solidity ratio ∈ shall be based on the projected area including ice. The force coefficient Cf for the projected area of flat members shall be as given in Fig. 29.5-2. The force coefficient Cf for rounded members and for the additional projected area due to ice on both flat and rounded members shall be as given in Fig. 29.5-2 for rounded members with Dqz2.5 for all ice thicknesses, wind speeds, and member diameters.
The solidity ratio ∈ shall be based on the projected area including ice. The force coefficients Cf shall be as given in Fig. 29.5-3. It is acceptable to reduce the force coefficients Cf for the additional projected area due to ice on both round and flat members by the factor for rounded members in Note 3 of Fig. 29.5-3.
The force coefficient Cf (as defined in Chapter 29) for ice-covered guys and cables shall be 1.2.
The design temperatures for ice and wind-on-ice due to freezing rain shall be either the temperature for the site shown in Figs. 10-7 and 10-8 or 32°F (0°C), whichever gives the maximum load effect. The temperature for Hawaii shall be 32°F (0°C). For temperature sensitive structures, the load shall include the effect of temperature change from everyday conditions to the design temperature for ice and wind on ice. These temperatures are to be used with ice thicknesses for all mean recurrence intervals. The design temperatures are considered to be concurrent with the design ice load and the concurrent wind load.

FIGURE 10-7 Temperatures Concurrent with Ice Thicknesses Due to Freezing Rain: Contiguous 48 States



FIGURE 10-8 Temperatures Concurrent with Ice Thicknesses Due to Freezing Rain: Alaska
The effects of a partial ice load shall be considered when this condition is critical for the type of structure under consideration. It is permitted to consider this to be a static load.
  1. The nominal ice thickness, t, the concurrent wind speed, Vc, and the concurrent temperature for the site shall be determined from Figs. 10-2 to 10-8 or a site-specific study.
  2. The topographic factor for the site, Kzt, shall be determined in accordance with Section 10.4.5.
  3. The importance factor for ice thickness, Ii, shall be determined in accordance with Section 10.4.4.
  4. The height factor, fz, shall be determined in accordance with Section 10.4.3 for each design segment of the structure.
  5. The design ice thickness, td, shall be determined in accordance with Section 10.4.6, Eq. 10.4-5.
  6. The weight of ice shall be calculated for the design ice thickness, td, in accordance with Section 10.4.1.
  7. The velocity pressure qz for wind speed Vc shall be determined in accordance with Section 29.3 using the importance factor for concurrent wind pressure Iw determined in accordance with Section 10.4.4.
  8. The wind force coefficients Cf shall be determined in accordance with Section 10.5.
  9. The gust effect factor shall be determined in accordance with Section 26.9.
  10. The design wind force shall be determined in accordance with Chapter 29.
  11. The iced structure shall be analyzed for the load combinations in either Section 2.3 or 2.4.
This section lists the consensus standards and other documents that are adopted by reference within this chapter:

ASCE
American Society of Civil Engineers
1801 Alexander Bell Drive
Reston, VA 20191

ASCE Manual 74
Section 10.1.3
Guidelines for Electrical Transmission Line Structural Loading, 1991

ANSI
American National Standards Institute
25 West 43rd Street, 4th Floor
New York, NY 10036

ANSI/EIA/TIA-222
Section 10.1.3
Structural Standards for Steel Antenna Towers and Antenna Supporting Structures, 1996

IEEE
445 Hoes Lane
Piscataway, NJ 08854-1331

NESC
Section 10.1.3
National Electrical Safety Code, 2001
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