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)

Part 1 Enclosed, Partially Enclosed, and Open Buildings of all Heights
Part 2 Enclosed Simple Diaphragm Buildings with h ≤ 160 ft (48.8m)

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

This chapter applies to the determination of MWFRS wind loads on enclosed, partially enclosed, and open buildings of all heights using the directional procedure.
  1. Part 1 applies to buildings of all heights where it is necessary to separate applied wind loads onto the windward, leeward, and side walls of the building to properly assess the internal forces in the MWFRS members.
  2. Part 2 applies to a special class of buildings designated as enclosed simple diaphragm buildings, as defined in Section 26.2, with h ≤ 160 ft (48.8 m).
A building whose design wind loads are determined in accordance with this chapter shall comply with the following conditions:
  1. The building is a regular-shaped building or structure as defined in Section 26.2.
  2. The building does not have response characteristics making it subject to across-wind loading, vortex shedding, or instability due to galloping or flutter, or it does not have a site location for which channeling effects or buffeting in the wake of upwind obstructions warrant special consideration.
The provisions of this chapter take into consideration the load magnification effect caused by gusts in resonance with along-wind vibrations of flexible buildings. Buildings not meeting the requirements of Section 27.1.2, or having unusual shapes or response characteristics, shall be designed using recognized literature documenting such wind load effects or shall use the wind tunnel procedure specified in Chapter 31.
There shall be no reductions in velocity pressure due to apparent shielding afforded by buildings and other structures or terrain features.
The wind load to be used in the design of the MWFRS for an enclosed or partially enclosed building shall not be less than 16 lb/ft2 (0.77 kN/m2) multiplied by the wall area of the building and 8 lb/ft2 (0.38 kN/m2) multiplied by the roof area of the building projected onto a vertical plane normal to the assumed wind direction. Wall and roof loads shall be applied simultaneously. The design wind force for open buildings shall be not less than 16 lb/ft2 (0.77 kN/m2) multiplied by the area Af.

Part 1 Enclosed, Partially Enclosed, and Open Buildings of all Heights

The steps to determine the wind loads on the MWFRS for enclosed, partially enclosed, and open buildings of all heights are provided in Table 27.2-1.

Table 27.2-1 Steps to Determine MWFRS Wind Loads for
Enclosed, Partially Enclosed, and Open Buildings of All
Heights
Step 1: Determine risk category of building or other structure, see
Table 1.5-1
Step 2: Determine the basic wind speed, V, for the applicable risk
category, see Figure 26.5-1A, B, or C
Step 3: Determine wind load parameters:
➢ Wind directionality factor, Kd, see Section 26.6 and Table
26.6-1
➢ Exposure category, see Section 26.7
➢ Topographic factor, Kɀt, see Section 26.8 and Figure 26.8-1
➢ Gust-effect factor, G, see Section 26.9
➢ Enclosure classification, see Section 26.10
➢ Internal pressure coefficient, (GCpi), see Section 26.11 and
Table 26.11-1
Step 4: Determine velocity pressure exposure coefficient, Kɀ or Kh, see
Table 27.3-1
Step 5: Determine velocity pressure qz or qh, see Eq. 27.3-1
Step 6: Determine external pressure coefficient, Cp or CN:
➢ Fig. 27.4-1 for walls and flat, gable, hip, monoslope, or
mansard roofs
➢ Fig. 27.4-2 for domed roofs
➢ Fig. 27.4-3 for arched roofs
➢ Fig. 27.4-4 for monoslope roof, open building
➢ Fig. 27.4-5 for pitched roof, open building
➢ Fig. 27.4-6 for troughed roof, open building
➢ Fig. 27.4-7 for along-ridge/valley wind load case for
monoslope, pitched or troughed roof, open building
Step 7: Calculate wind pressure, p, on each building surface:
➢ Eq. 27.4-1 for rigid buildings
➢ Eq. 27.4-2 for flexible buildings
➢ Eq. 27.4-3 for open buildings

User Note: Use Part 1 of Chapter 27 to determine wind
pressures on the MWFRS of enclosed, partially enclosed or an
open building with any general plan shape, building height, or
roof geometry that matches the figures provided. These
provisions utilize the traditional "all heights" method (direc-
tional procedure) by calculating wind pressures using specific
wind pressure equations
applicable to each building surface.
The following wind load parameters shall be determined in accordance with Chapter 26:

  • Basic wind speed, V (Section 26.5),
  • Wind directionality factor, Kd (Section 26.6),
  • Exposure category (Section 26.7),
  • Topographic factor, Kɀt (Section 26.8),
  • Gust-effect factor (Section 26.9),
  • Enclosure classification (Section 26.10), and
  • Internal pressure coefficient, (GCpi) (Section 26-11).
  • Based on the exposure category determined in Section 26.7.3, a velocity pressure exposure coefficient Kɀ or Kh, as applicable, shall be determined from Table 27.3-1. For a site located in a transition zone between exposure categories that is near to a change in ground surface roughness, intermediate values of Kɀ or Kh, between those shown in Table 27.3-1 are permitted, provided that they are determined by a rational analysis method defined in the recognized literature.
    Velocity pressure, qɀ, evaluated at height ɀ shall be calculated by the following equation:
    qz = 0.00256KɀKɀtKdV² (lb/ft²) (27.3-1)

    [In SI: qɀ = 0.613KɀKɀtKdV²(N/m²); V in m/s]

    where

    Kd = wind directionality factor, see Section 26.6
    Kɀ = velocity pressure exposure coefficient, see Section 27.3.1
    Kɀt = topographic factor, see Section 26.8.2
    V = basic wind speed, see Section 26.5
    qz = velocity pressure calculated using Eq. 27.3-1 at height ɀ
    qh = velocity pressure calculated using Eq. 27.3-1 at mean roof
    height h.

        The numerical coefficient 0.00256 (0.613 in SI) shall be used except where sufficient climatic data are available to justify the selection of a different value of this coefficient for a design application.
    Design wind pressures for the MWFRS of buildings of all heights shall be determined by the following equation:

    p = qGCpqi(GCpi) (lb/ft²) (N/m²) (27.4-1)

    where
    q = qɀ for windward walls evaluated at height ɀ above the
    ground
    q = qh for leeward walls, side walls, and roofs, evaluated
    at height h
    qi = qh for windward walls, side walls, leeward walls, and
    roofs of enclosed buildings and for negative internal
    pressure evaluation in partially enclosed buildings
    qi = qɀ for positive internal pressure evaluation in partially
    enclosed buildings where height ɀ is defined as the level
    of the highest opening in the building that could affect
    the positive internal pressure. For buildings sited in
    wind-borne debris regions, glazing that is not impact
    resistant or protected with an impact-resistant covering
    shall be treated as an opening in accordance with Section
    26.10.3. For positive internal pressure evaluation, qi, may
    conservatively be evaluated at height h(qi = qh)
    G = gust-effect factor, see Section 26.9
    Cp = external pressure coefficient from Figs. 27.4-1, 27.4-2,
    and 27.4-3
    (GCpi) = internal pressure coefficient from Table 26.11-1

    q and qi shall be evaluated using exposure defined in Section 26.7.3. Pressure shall be applied simultaneously on windward and leeward walls and on roof surfaces as defined in Figs. 27.4-1, 27.4-2, and 27.4-3.
    Design wind pressures for the MWFRS of flexible buildings shall be determined from the following equation:

    p = qGfCpqi(GCpi)(lb/ft²) (N/m²) (27.4-2)

    where q, qi, Cp, and (GCpi) are as defined in Section 27.4.1 and Gf (gust-effect factor) is determined in accordance with Section 26.9.5.
    The net design pressure for the MWFRS of open buildings with monoslope, pitched, or troughed roofs shall be determined by the following equation:

    p = qhGCN (27.4-3)

    where
    qh = velocity pressure evaluated at mean roof height h using the
    exposure as defined in Section 26.7.3 that results in the
    highest wind loads for any wind direction at the site
    G = gust-effect factor from Section 26.9
    CN = net pressure coefficient determined from Figs. 27.4-4
    through 27.4-7

        Net pressure coefficients, CN, include contributions from top and bottom surfaces. All load cases shown for each roof angle shall be investigated. Plus and minus signs signify pressure acting toward and away from the top surface of the roof, respectively.
        For free roofs with an angle of plane of roof from horizontal θ less than or equal to 5° and containing fascia panels, the fascia panel shall be considered an inverted parapet. The contribution of loads on the fascia to the MWFRS loads shall be determined using Section 27.4.5 with qp equal to qh.
    The positive external pressure on the bottom surface of windward roof overhangs shall be determined using Cp = 0.8 and combined with the top surface pressures determined using Fig. 27.4-1.
    The design wind pressure for the effect of parapets on MWFRS of rigid or flexible buildings with flat, gable, or hip roofs shall be determined by the following equation:

    pp = qp(GCpn) (lb/ft2) (27.4-4)

    where

    pp = combined net pressure on the parapet due to the com-
    bination of the net pressures from the front and back
    parapet surfaces. Plus (and minus) signs signify net
    pressure acting toward (and away from) the front
    (exterior) side of the parapet
    qp = velocity pressure evaluated at the top of the parapet
    (GCpn) = combined net pressure coefficient
    = +1.5 for windward parapet
    = –1.0 for leeward parapet

    Main Wind Force Resisting System – Part 1 All Heights
    Velocity Pressure Exposure Coefficients, Kh and Kz
    Table 27.3-1

    Height above
    ground level, z
    Exposure
    B C D
    ft (m)
    0-15 (0-4.6) 0.57 0.85 1.03
    20 (6.1) 0.62 0.90 1.08
    25 (7.6) 0.66 0.94 1.12
    30 (9.1) 0.70 0.98 1.16
    40 (12.2) 0.76 1.04 1.22
    50 (15.2) 0.81 1.09 1.27
    60 (18) 0.85 1.13 1.31
    70 (21.3) 0.89 1.17 1.34
    80 (24.4) 0.93 1.21 1.38
    90 (27.4) 0.96 1.24 1.40
    100 (30.5) 0.99 1.26 1.43
    120 (36.6) 1.04 1.31 1.48
    140 (42.7) 1.09 1.36 1.48
    160 (48.8) 1.13 1.39 1.55
    180 (54.9) 1.17 1.43 1.58
    200 (61.0) 1.20 1.46 1.61
    250 (76.2) 1.28 1.53 1.68
    300 (91.4) 1.35 1.59 1.73
    350 (106.7) 1.41 1.64 1.78
    400 (121.9) 1.47 1.69 1.82
    450 (137.2) 1.52 1.73 1.86
    500 (152.4) 1.56 1.77 1.89

    Notes:
    1. The velocity pressure exposure coefficient Kz may be determined from the following formula:

      For 15 ft. ≤ z ≤ zg   For z < 15 ft.            

      Kz = 2.01 (z/z/g)2/α Kz = 2.01 (15/z/g)2/α
    2. α and zg are tabulated in Table 26.9.1
    3. Linear interpolation for intermediate values of height z is acceptable.
    4. Exposure categories are defined in Section 26.7.



    Main Wind Force Resisting System – Part 1 All Heights
    Figure 27.4-1 External Pressure Coefficients, Cp Walls & Roofs
    Enclosed, Partially Enclosed Buildings




    Main Wind Force Resisting System – Part 1 All Heights
    Figure 27.4-1 (cont.) External Pressure Coefficients, Cp Walls & Roofs
    Enclosed, Partially Enclosed Buildings
    Wall Pressure Coefficients, Cp
    Surface L/B Cp Use With
    Windward Wall All values 0.8 qz
    Leeward Wall 0-1 -0.5 qh
    2 -0.3
    ≥4 -0.2
    Side Wall All values -0.7 qh

    Roof Pressure Coefficients, Cp, for use with qh
    Wind
    Direction
    Windward Leeward
    Angle, θ (degrees) Angle, θ (degrees)
    h/L 10 15 20 25 30 35 45 ≥60# 10 15 ≥20
    Normal
    to
    ridge for
    θ ≥ 10°
    ≤0.25 -0.7
    -0.18
    -0.5
    0.0*
    -0.3
    0.2
    -0.2
    0.3
    -0.2
    0.3
    0.0*
    0.4
    0.4 0.01 θ -0.3 -0.5 -0.6
    0.5 -0.9
    -0.18
    -0.7
    -0.18
    -0.4
    0.0*
    -0.3
    0.2
    -0.2
    0.2
    -0.2
    0.3
    0.0*
    0.4
    0.01 θ -0.5 -0.5 -0.6
    ≥1.0 -1.3**
    -0.18
    -1.0
    -0.18
    -0.7
    -0.18
    -0.5
    0.0*
    -0.3
    0.2
    -0.2
    0.2
    0.0*
    0.3
    0.01 θ -0.7 -0.6 -0.6
    Normal
    to
    ridge for
    θ < 10
    and
    Parallel
    to ridge
    for all θ
    ≤ 0.5 Horiz distance from
    windward edge
    Cp *Value is provided for interpolation
    purposes.

    **Value can be reduced linearly with area
    over which it is applicable as follows
    0 to h/2 -0.9, -0.18
    h/2 to h -0.9, -0.18
    h to 2 h -0.5, -0.18
    > 2h -0.3, -0.18
    0 to h/2 -1.3**, -0.18 Area (sq ft) Reduction Factor
    ≤ 100 (9.3 sq m) 1.0
    ≥ 1.0 > h/2 -0.7, -0.18 250 (23.2 sq m) 0.9
    ≥ 1000 (92.9 sq m) 0.8

    Notes:
    1. Plus and minus signs signify pressures acting toward and away from the surfaces, respectively.
    2. Linear interpolation is permitted for values of L/B, h/L and θ other than shown. Interpolation shall only be carried out between values of the same sign. Where no value of the same sign is given, assume 0.0 for interpolation purposes.
    3. Where two values of Cp are listed, this indicates that the windward roof slope is subjected to either positive or negative pressures and the roof structure shall be designed for both conditions. Interpolation for intermediate ratios of h/L in this case shall only be carried out between Cp values of like sign.
    4. For mono slope roofs, entire roof surface is either a windward or leeward surface.
    5. For flexible buildings use appropriate Gf as determined by Section 26.9.4.
    6. Refer to Figure 27.4-2 for domes and Figure 27.4-3 for arched roofs.
    7. Notation:
      B:  Horizontal dimension of building, in feet (meter), measured normal to wind direction.
      L:  Horizontal dimension of building, in feet (meter), measured parallel to wind direction.
      h:  Mean roof height in feet (meters), except that eave height shall be used for θ ≤ 10 degrees.
      z:  Height above ground, in feet (meters).
      G:  Gust effect factor.
      qz,qh:  Velocity pressure, in pounds per square foot (N/m2), evaluated at respective height.
      θ:  Angle of plane of roof from horizontal, in degrees.
    8. For mansard roofs, the top horizontal surface and leeward inclined surface shall be treated as leeward surfaces from the table.
    9. Except for MWFRS's at the roof consisting of moment resisting frames, the total horizontal shear shall not be less than that determined by neglecting wind forces on roof surfaces.
    #For roof slopes greater than 80°, use Cp = 0.8


    Main Wind Force Resisting System – Part 1 All Heights
    Figure 27.4-2 External Pressure Coefficients, Cp Domed Roofs
    Enclosed, Partially Enclosed Buildings and Structures
    External Pressure Coefficients for Domes with a Circular Base.
    (Adapted from Eurocode, 1995)

    Notes:
    1. Two load cases shall be considered:
      Case A. Cp values between A and B and between B and C shall be determined by linear interpolation along arcs on the
      dome parallel to the wind direction;
      Case B. Cp shall be the constant value of A for θ ≤ 25 degrees, and shall be determined by linear interpolation from 25
      degrees to Band from B to C.
    2. Values denote Cp to be used with q(hD+f) where hD + f is the height at the top of the dome.
    3. Plus and minus signs signify pressures acting toward and away from the surfaces, respectively.
    4. Cp is constant on the dome surface for arcs of circles perpendicular to the wind direction; for example, the arc passing through B-B-B and all arcs parallel to B-B-B.
    5. For values of hD/D between those listed on the graph curves, linear interpolation shall be permitted.
    6. θ = 0 degrees on dome springline, θ = 90 degrees at dome center top point. f is measured from springline to top.
    7. The total horizontal shear shall not be less than that determined by neglecting wind forces on roof surfaces.
    8. For f/D values less than 0.05, use Figure 27.4-1.


    Main Wind Force Resisting System and Components and
    Cladding - Part 1
    All Heights
    Figure 27.4-3 External Pressure Coefficients, Cp Arched Roofs
    Enclosed, Partially Enclosed Buildings and Structures





    Conditions Rise-to-span
    ratio, r
    Cp
    Windward
    quarter
    Center
    half
    Leeward
    quarter
    Roof on elevated structure 0 < r < 0.2 -0.9 -0.7 - r -0.5
    0.2 ≤ r < 0.3* 1.5r - 0.3 -0.7 - r -0.5
    0.3 ≤ r ≤ 0.6 2.75r - 0.7 -0.7 - r -0.5
    Roof springing from ground level 0 < r ≤ 0.6 1.4r -0.7 - r -0.5

    *When the rise-to-span ratio is 0.2 ≤ r ≤ 0.3, alternate coefficients given by 6r - 2.1 shall also be used for the windward quarter.

    Notes:
    1. Values listed are for the determination of average loads on main wind force resisting systems.
    2. Plus and minus signs signify pressures acting toward and away from the surfaces, respectively.
    3. For wind directed parallel to the axis of the arch, use pressure coefficients from Fig. 27.4-1 with wind directed parallel to ridge .
    4. For components and cladding: (1) At roof perimeter, use the external pressure coefficients in Fig. 30.4- 2A, Band C with θ based on spring-line slope and (2) for remaining roof areas, use external pressure coefficients of this table multiplied by 0.87.






    Main Wind Force Resisting System - Part 1 0.25 ≤ h/L ≤ 1.0
    Figure 27.4-4 Net Pressure Coefficient, CN Monoslope Free Roofs
    θ ≤ 45°, γ = 0°, 180°
    Open Buildings



    Roof

    Angle

    θ
    Load

    Case
    Wind Direction, γ = 0° Wind Direction, γ = 180°
    Clear Wind Flow Obstructed Wind Flow Clear Wind Flow Obstructed Wind Flow
    CNW CNL CNW CNL CNW CNL CNW CNL
    A 1.2 0.3 -0.5 -1.2 1.2 0.3 -0.5 -1.2
    B -1.1 -0.1 -1.1 -0.6 -1.1 -0.1 -1.1 -0.6
    7.5° A -0.6 -1 -1 -1.5 0.9 1.5 -0.2 -1.2
    B -1.4 0 -1.7 -0.8 1.6 0.3 0.8 -0.3
    15° A -0.9 -1.3 -1.1 -1.5 1.3 1.6 0.4 -1.1
    B -1.9 0 -2.1 -0.6 1.8 0.6 1.2 -0.3
    22.5° A -1.5 -1.6 -1.5 -1.7 1.7 1.8 0.5 -1
    B -2.4 -0.3 -2.3 -0.9 2.2 0.7 1.3 0
    30° A -1.8 -1.8 -1.5 -1.8 2.1 2.1 0.6 -1
    B -2.5 -0.5 -2.3 -1.1 2.6 1 1.6 0.1
    37.5° A -1.8 -1.8 -1.5 -1.8 2.1 2.2 0.7 -0.9
    B -2.4 -0.6 -2.2 -1.1 2.7 1.1 1.9 0.3
    45° A -1.6 -1.8 -1.3 -1.8 2.2 2.5 0.8 -0.9
    B -2.3 -0.7 -1.9 -1.2 2.6 1.4 2.1 0.4

    Notes:
    1. CNW and CNL denote net pressures (contributions from top and bottom surfaces) for windward and leeward half of roof surfaces, respectively.
    2. Clear wind flow denotes relatively unobstructed wind flow with blockage less than or equal to 50%. Obstructed wind flow denotes objects below roof inhibiting wind flow (>50% blockage).
    3. For values of θ between 7.5° and 45°, linear interpolation is permitted. For values of θ less than 7.5°, use load coefficients for 0°.
    4. Plus and minus signs signify pressures acting towards and away from the top roof surface, respectively.
    5. All load cases shown for each roof angle shall be investigated.
    6. Notation:
      L : horizontal dimension of roof, measured in the along wind direction, ft. (m)
      h : mean roof height, ft. (m)
      γ : direction of wind, degrees
      θ : angle of plane of roof from horizontal, degrees


    Main Wind Force Resisting System - Part 1 0.25 ≤ h/L ≤ 1.0
    Figure 27.4-5 Net Pressure Coefficient, CN Pitched Free Roofs
    θ ≤ 45°, γ = 0°, 180°
    Open Buildings

    Roof
    Angle, θ
    Load
    Case
    Wind Direction, γ = 0°, 180°
    Clear Wind Flow Obstructed Wind Flow
    CNW CNL CNW CNL
    7.5° A 1.1 -0.3 -1.6 -1
    B 0.2 -1.2 -0.9 -1.7
    15° A 1.1 -0.4 -1.2 -1
    B 0.1 -1.1 -0.6 -1.6
    22.5° A 1.1 0.1 -1.2 -1.2
    B -0.1 -0.8 -0.8 -1.7
    30° A 1.3 0.3 -0.7 -0.7
    B -0.1 -0.9 -0.2 -1.1
    37.5° A 1.3 0.6 -0.6 -0.6
    B -0.2 -0.6 -0.3 -0.9
    45° A 1.1 0.9 -0.5 -0.5
    B -0.3 -0.5 -0.3 -0.7



    Notes:
    1. CNW and CNL denote net pressures (contributions from top and bottom surfaces) for windward and leeward half of roof surfaces, respectively.
    2. Clear wind flow denotes relatively unobstructed wind flow with blockage less than or equal to 50%. Obstructed wind flow denotes objects below roof inhibiting wind flow (>50% blockage).
    3. For values of θ between 7.5° and 45°, linear interpolation is permitted. For values of θ less than 7.5°, use monoslope roof load coefficients.
    4. Plus and minus signs signify pressures acting towards and away from the top roof surface, respectively.
    5. All load cases shown for each roof angle shall be investigated.
    6. Notation:
      L : horizontal dimension of roof, measured in the along wind direction, ft. (m)
      h : mean roof height, ft. (m)
      γ : direction of wind, degrees
      θ : angle of plane of roof from horizontal, degrees


    Main Wind Force Resisting System 0.25 ≤ h/L ≤ 1.0
    Figure 27.4-6 Net Pressure Coefficient, CN Troughed Free Roofs
    θ ≤ 45°, γ = 0°, 180°
    Open Buildings

    Roof

    Angle

    θ
    Load

    Case
    Wind Direction, γ = 0°, 180°
    Clear Wind Flow Obstructed Wind Flow
    CNW CNL CNW CNL
    7.5° A -1.1 -0.3 -1.6 -0.5
    B -0.2 1.2 -0.9 -0.8
    15° A -1.1 0.4 -1.2 -0.5
    B 0.1 1.1 -0.6 -0.8
    22.5° A -1.1 -0.1 -1.2 -0.6
    B -0.1 0.8 -0.8 -0.8
    30° A -1.3 -0.3 -1.4 -0.4
    B -0.1 0.9 -0.2 -0.5
    37.5° A -1.3 -0.6 -1.4 -0.3
    B 0.2 0.6 -0.3 -0.4
    45° A -1.1 -0.9 -1.2 -0.3
    B 0.3 0.5 -0.3 -0.4

    Notes:
    1. CNW and CNL denote net pressures (contributions from top and bottom surfaces) for windward and leeward half of roof surfaces, respectively.
    2. Clear wind flow denotes relatively unobstructed wind flow with blockage less than or equal to 50%. Obstructed wind flow denotes objects below roof inhibiting wind flow (>50% blockage).
    3. For values of θ between 7.5° and 45°, linear interpolation is permitted. For values of θ less than 7.5°, use monoslope roof load coefficients.
    4. Plus and minus signs signify pressures acting towards and away from the top roof surface, respectively.
    5. All load cases shown for each roof angle shall be investigated.
    6. Notation:
      L : horizontal dimension of roof, measured in the along wind direction, ft. (m)
      h : mean roof height, ft. (m)
      γ : direction of wind, degrees
      θ : angle of plane of roof from horizontal, degrees


    Main Wind Force Resisting System- Part 1 0.25 ≤ h/L ≤ 1.0
    Figure 27.4-7 Net Pressure Coefficient, CN Free Roofs
    θ ≤ 45°, γ = 90°, 270°
    Open Buildings

    Horizontal
    Distance from
    Windward Edge
    Roof Angle θ Load Case Clear Wind
    Flow
    Obstructed
    Wind Flow
    CN CN
    ≤ h All Shapes A -0.8 -1.2
    θ ≤ 45° B 0.8 0.5
    > h, ≤ 2h All Shapes A -0.6 -0.9
    θ ≤ 45° B 0.5 0.5
    > 2h All Shapes A -0.3 -0.6
    θ ≤ 45° B 0.3 0.3

    Notes:
    1. CN denotes net pressures (contributions from top and bottom surfaces).
    2. Clear wind flow denotes relatively unobstructed wind flow with blockage less than or equal to 50%. Obstructed wind flow denotes objects below roof inhibiting wind flow (>50% blockage).
    3. Plus and minus signs signify pressures acting towards and away from the top roof surface, respectively.
    4. All load cases shown for each roof angle shall be investigated.
    5. For monoslope roofs with theta less than 5 degrees, Cn values shown apply also for cases where gamma = 0 degrees and 0.05 less than or equal to h/L less than or equal to 0.25. See Figure 27.4-4 for other h/L values.
    6. Notation:
      L : horizontal dimension of roof, measured in the along wind direction, ft. (m)
      h : mean roof height, ft. (m). See Figures 27.4-4, 27.4-5 or 27.4-6 for a graphical depiction of this dimension.
      γ : direction of wind, degrees
      θ : angle of plane of roof from horizontal, degrees


    Main Wind Force Resisting System - Part 1 All Heights
    Figure 27.4-8 Design Wind Load Cases

    Case 1. Full design wind pressure acting on the projected area perpendicular to each principal axis of the
    structure, considered separately along each principal axis.
    Case 2. Three quarters of the design wind pressure acting on the projected area perpendicular to each
    principal axis of the structure in conjunction with a torsional moment as shown, considered
    separately for each principal axis.
    Case 3. Wind loading as defined in Case 1, but considered to act simultaneously at 75% of the specified
    value.
    Case 4. Wind loading as defined in Case 2, but considered to act simultaneously at 75% of the specified
    value.

    Notes:
    1. Design wind pressures for windward and leeward faces shall be determined in accordance with the provisions of 27.4.1 and 27.4.2 as applicable for building of all heights.
    2. Diagrams show plan views of building.
    3. Notation:
      PWX, PWY: Windward face design pressure acting in the x, y principal axis, respectively.
      PLX, PLY: Leeward face design pressure acting in the x, y principal axis, respectively.
      e (ex. ey) : Eccentricity for the x, y principal axis of the structure, respectively.
      MT: Torsional moment per unit height acting about a vertical axis of the building.
    The MWFRS of buildings of all heights, whose wind loads have been determined under the provisions of this chapter, shall be designed for the wind load cases as defined in Fig. 27.4-8.
    EXCEPTION: Buildings meeting the requirements of Section D.2 of Appendix D need only be designed for Case 1 and Case 3 of Fig. 27.4-8.
        The eccentricity e for rigid structures shall be measured from the geometric center of the building face and shall be considered for each principal axis (eX, eY). The eccentricity e for flexible structures shall be determined from the following equation and shall be considered for each principal axis (eX, eY):

    (27.4-5)

    where
    eQ = eccentricity e as determined for rigid structures in Fig.
    27.4-8
    eR = distance between the elastic shear center and center of mass
    of each floor

    I, gQ, Q, gR, and R shall be as defined in Section 26.9

        The sign of the eccentricity e shall be plus or minus, whichever causes the more severe load effect.

    Part 2 Enclosed Simple Diaphragm Buildings with h ≤ 160 ft (48.8m)

    The procedure specified herein applies to the determination of MWFRS wind loads of enclosed simple diaphragm buildings, as defined in Section 26.2, with a mean roof height h ≤ 160 ft (48.8 m). The steps required for the determination of MWFRS wind loads on enclosed simple diaphragm buildings are shown in Table 27.5-1.

    User Note: Part 2 of Chapter 27 is a simplified method for
    determining the wind pressures for the MWFRS of enclosed,
    simple diaphragm buildings whose height h is ≤ 160 ft (48.8
    m). The wind pressures are obtained directly from a table. The
    building may be of any general plan shape and roof geometry
    that matches the specified figures. This method is a simplifica-
    tion of the traditional "all heights" method (directional
    procedure) contained in Part 1 of Chapter 27.


    Main Wind Force Resisting System – Part 2 h ≤ 160 ft.
    Figure 27.5-1 Building Class
    Building Geometry Requirements
    Enclosed Simple Diaphragm Buildings



    Note: Roof form may be flat, gable, mansard or hip






    In addition to the requirements in Section 27.1.2, a building whose design wind loads are determined in accordance with this section shall meet all of the following conditions for either a Class 1 or Class 2 building (see Fig. 27.5-1):

    Class 1 Buildings:
    1. The building shall be an enclosed simple diaphragm building as defined in Section 26.2.
    2. The building shall have a mean roof height h ≤ 60 ft (18.3 m).
    3. The ratio of L/B shall not be less than 0.2 nor more than 5.0 (0.2 ≤ L/B ≤ 5.0).

    Table 27.5-1 Steps to Determine MWFRS Wind Loads
    Enclosed Simple Diaphragm Buildings [h ≤ 160 ft. (48.8 m)]
    Step 1: Determine risk category of building or other structure, see
    Table 1.5-1
    Step 2: Determine the basic wind speed, V, for applicable risk
    category, see Figure 26.5-1A, B, or C
    Step 3: Determine wind load parameters:
    ➢ Exposure category B, C, or D, see Section 26.7
    ➢ Topographic factor, Kɀt, see Section 26.8 and Figure
    26.8-1
    ➢ Enclosure classification, see Section 26.10
    Step 4: Enter table to determine net pressures on walls at top and base
    of building respectively, ph, pO, Table 27.6-1
    Step 5: Enter table to determine net roof pressures, Pɀ, Table 27.6-2
    Step 6: Determine topographic factor, Kɀt, and apply factor to wall and
    roof pressures (if applicable), see Section 26.8
    Step 7: Apply loads to walls and roofs simultaneously.

    1. The topographic effect factor Kɀt = 1.0 or the wind pressures determined from this section shall be multiplied by Kɀt at each height ɀ as determined from Section 26.8. It shall be permitted to use one value of Kɀt for the building calculated at 0.33h. Alternatively it shall be permitted to enter the pressure table with a wind velocity equal to V where Kɀt is determined at a height of 0.33h.

    Class 2 Buildings:
    1. The building shall be an enclosed simple diaphragm building as defined in Section 26.2.
    2. The building shall have a mean roof height 60 ft < h ≤ 160 ft (18.3 m < h ≤ 48.8 m).
    3. The ratio of L/B shall not be less than 0.5 nor more than 2.0 (0.5 ≤ L/B ≤ 2.0).
    4. The fundamental natural frequency (Hertz) of the building shall not be less 75/h where h is in feet.
    5. The topographic effect factor Kɀt = 1.0 or the wind pressures determined from this section shall be multiplied by Kɀt at each height ɀ as determined from Section 26.8. It shall be permitted to use one value of Kɀt, for the building calculated at 0.33h. Alternatively it shall be permitted to enter the pressure table with a wind velocity equal to V where Kɀt is determined at a height of 0.33h .
    Refer to Chapter 26 for determination of basic wind speed V (Section 26.5) and exposure category (Section 26.7) and topographic factor Kɀt (Section 26.8).
    The design procedure specified herein applies to buildings having either rigid or flexible diaphragms. The structural analysis shall consider the relative stiffness of diaphragms and the vertical elements of the MWFRS.
        Diaphragms constructed of wood panels can be idealized as flexible. Diaphragms constructed of untopped metal decks, concrete-filled metal decks, and concrete slabs, each having a span-to-depth ratio of 2 or less, are permitted to be idealized as rigid for consideration of wind loading.
    Net wind pressures for the walls and roof surfaces shall be determined from Tables 27.6-1 and 27.6-2, respectively, for the applicable exposure category as determined by Section 26.7.
        For Class 1 building with L/B values less than 0.5, use wind pressures tabulated for L/B = 0.5. For Class 1 building with L/B values greater than 2.0, use wind pressures tabulated for L/B = 2.0.
        Net wall pressures shall be applied to the projected area of the building walls in the direction of the wind, and exterior side wall pressures shall be applied to the projected area of the building walls normal to the direction of the wind acting outward according to Note 3 of Table 27.6-1, simultaneously with the roof pressures from Table 27.6-2 as shown in Fig. 27.6-1.
        Where two load cases are shown in the table of roof pressures, the effects of each load case shall be investigated separately. The MWFRS in each direction shall be designed for the wind load cases as defined in Fig. 27.4-8.
        EXCEPTION: The torsional load cases in Fig. 27.4-8 (Case 2 and Case 4) need not be considered for buildings that meet the requirements of Appendix D.

    Main Wind Force Resisting System – Part 2 h ≤ 160 ft.
    Figure 27.6-1 Wind Pressures – Walls and Roof Application of Wind Pressures
    See Tables 27.6-1 and 27.6-2
    Enclosed Simple Diaphragm Buildings







    Main Force Resisting System – Part 2 h ≤ 160 ft.

    Application of Wall Pressures
    Table 27.6-1 Wind Pressures – Walls
    Enclosed Simple Diaphragm Buildings

    Notes to Wall Pressure Table 27.6-1:
    1. From table for each Exposure (B, C or D), V, L/B and h, determine ph (top number) and p0 (bottom number) horizontal along-wind net wall pressures.
    2. Side wall external pressures shall be uniform over the wall surface acting outward and shall be taken as 54% of the tabulated ph pressure for 0.2 ≤ L/B ≤ 1.0 and 64% of the tabulated ph pressure for 2.0 ≤ L/B ≤ 5.0. Linear interpolation shall apply for 1.0 < LIB < 2.0. Side wall external pressures do not include effect of internal pressure.
    3. Apply along-wind net wall pressures as shown above to the projected area of the building walls in the direction of the wind and apply external side wall pressures to the projected area of the building walls normal to the direction wind, simultaneously with the roof pressures from Table 27.6-2.
    4. Distribution of tabulated net wall pressures between windward and leeward wall faces shall be based on the linear distribution of total net pressure with building height as shown above and the leeward external wall pressures assumed uniformly distributed over the leeward wall surface acting outward at 38% of ph for 0.2 ≤ L/B ≤ 1.0 and 27% of ph for 2.0 ≤ L/B ≤ 5.0. Linear interpolation shall be used for 1.0 < L/B < 2.0. The remaining net pressure shall be applied to the windward walls as an external wall pressure acting towards the wall surface. Windward and leeward wall pressures so determined do not include effect of internal pressure.
    5. Interpolation between values of V, hand L/B is permitted.
    Notation:
    L = building plan dimension parallel to wind direction (ft.)
    B = building plan dimension perpendicular to wind direction (ft)
    h = mean roof height (ft.)
    ph, p0 = along-wind net wall pressure at top and base of building respectively (psf)





    Table 27.6-1
    MWFRS – Part 2: Wind Loads – Walls
    Exposure B



    Table 27.6-1
    MWFRS – Part 2: Wind Loads – Walls
    Exposure C



    Table 27.6-1
    MWFRS – Part 2: Wind Loads – Walls
    Exposure D

    The effect of horizontal wind loads applied to all vertical surfaces of roof parapets for the design of the MWFRS shall be based on the application of an additional net horizontal wind pressure applied to the projected area of the parapet surface equal to 2.25 times the wall pressures tabulated in Table 27.6-1 for L/B = 1.0. The net pressure specified accounts for both the windward and leeward parapet loading on both the windward and leeward building surface. The parapet pressure shall be applied simultaneously with the specified wall and roof pressures shown in the table as shown in Fig. 27.6-2. The height h used to enter into Table 27.6-1 to determine the parapet pressure shall be the height to the top of the parapet as shown in Fig. 27.6-2 (use h = hp).

    Main Wind Force Resisting System - Part 2 h ≤ 160 ft.
    Application of Parapet Wind Loads - See
    Table 27 .6-1
    Table 27.6-2 Parapet Wind Loads
    Enclosed Simple Diaphragm Buildings





    Main Wind Force Resisting System - Part 2 h ≤ 160 ft.
    Application of Roof Pressures
    Table 27.6-2 Wind Pressures - Roof
    Enclosed Simple Diaphragm Buildings


    Notes to Roof Pressure Table 27.6-2:
    1. From table for Exposure C, V, h and roof slope, determine roof pressure ph for each roof zone shown in the figures for the applicable roof form. For other exposures B or D, multiply pressures from table by appropriate exposure adjustment factor as determined from figure below.
    2. Where two load cases are shown, both load cases shall be investigated. Load case 2 is required to investigate maximum overturning on the building from roof pressures shown.
    3. Apply along-wind net wall pressures to the projected area of the building walls in the direction of the wind and apply exterior side wall pressures to the projected area of the building walls normal to the direction of the wind acting outward, simultaneously with the roof pressures from Table 27.6-2.
    4. Where a value of zero is shown in the tables for the flat roof case, it is provided for the purpose of interpolation.
    5. Interpolation between V, hand roof slope is permitted.




    Main Wind Force Resisting System - Part 2 h ≤ 160 ft.
    Table 27.6-2 Wind Pressures - Roof Application of Roof Pressures
    Enclosed Simple Diaphragm Buildings


    Table 27.6-2
    MWFRS – Part 2: Wind Loads – Walls
    Exposure C



    Table 27.6-2
    MWFRS – Part 2: Wind Loads – Walls
    Exposure C



    Table 27.6-2
    MWFRS – Part 2: Wind Loads – Walls
    Exposure C



    Table 27.6-2
    MWFRS – Part 2: Wind Loads – Walls
    Exposure C



    Table 27.6-2
    MWFRS – Part 2: Wind Loads – Walls
    Exposure C



    Table 27.6-2
    MWFRS – Part 2: Wind Loads – Walls
    Exposure C



    Table 27.6-2
    MWFRS – Part 2: Wind Loads – Walls
    Exposure C



    Table 27.6-2
    MWFRS – Part 2: Wind Loads – Walls
    Exposure C



    Table 27.6-2
    MWFRS – Part 2: Wind Loads – Walls
    Exposure C



    Table 27.6-2
    MWFRS – Part 2: Wind Loads – Walls
    Exposure C



    Table 27.6-2
    MWFRS – Part 2: Wind Loads – Walls
    Exposure C



    Table 27.6-2
    MWFRS – Part 2: Wind Loads – Walls
    Exposure C

    The effect of vertical wind loads on any roof overhangs shall be based on the application of a positive wind pressure on the underside of the windward overhang equal to 75% of the roof edge pressure from Table 27.6-2 for Zone 1 or Zone 3 as applicable. This pressure shall be applied to the windward roof overhang only and shall be applied simultaneously with other tabulated wall and roof pressures as shown in Fig. 27.6-3.

    Main Wind Force Resisting System - Part 2 h ≤ 160 ft.
    Application of Roof Overhang
    Wind Loads – See Table 27.6-2
    Table 27.6-3 Roof Overhang Wind Loads
    Enclosed Simple Diaphragm Buildings




    Resources