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)

Part 1 Enclosed and Partially Enclosed Low-Rise Buildings
Part 2 Enclosed Simple Diaphragm Low-Rise Buildings

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 low-rise buildings using the envelope procedure.
  1. Part 1 applies to all low-rise buildings 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 low-rise buildings designated as enclosed simple diaphragm buildings as defined in Section 26.2.
A building whose design wind loads are determined in accordance with this section 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 28.1.2, 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.

Part 1 Enclosed and Partially Enclosed Low-Rise Buildings

The steps required for the determination of MWFRS wind loads on low-rise buildings are shown in Table 28.2-1.

User Note: Use Part 1 of Chapter 28 to determine the wind pressure on the MWFRS of enclosed, partially enclosed, or open low-rise buildings having a flat, gable, or hip roof. These provisions utilize the envelope procedure by calculating wind pressures from the specific equation applicable to each building surface. For building shapes and heights for which these provisions are applicable, this method generally yields the lowest wind pressure of all analytical methods specified in this standard.


Table 28.2-1 Steps to Determine Wind Loads on MWFRS Low-Rise Buildings
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 Fig. 26.5-1A, B, or C
Step 3: Determine wind load parameters: Step 4: Determine velocity pressure exposure coefficient, Kz, or Kh, see Table 28.3-1
Step 5: Determine velocity pressure, qz, or qh, Eq. 28.3-1
Step 6: Determine external pressure coefficient, (GCp), using Fig.28.4-1 for flat and gable roofs

User Note: See Commentary Fig. C28.4-1 for guidance on hip roofs.

Step 7: Calculate wind pressure, p, from Eq. 28.4-1


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 Kzt (Section 26.8),
  • 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 Kz or Kh, as applicable, shall be determined from Table 28.3-1.
      For a site located in a transition zone between exposure categories that is near to a change in ground surface roughness, intermediate values of Kz or Kh,, between those shown in Table 28.3-1, are permitted, provided that they are determined by a rational analysis method defined in the recognized literature.

Velocity Pressure Exposure Coefficients, Kh and Kz
Table 28.3-1


Height above level, z Exposure
ft (m) B C D
0-15 (0-4.6) 0.70 0.85 1.03
20 (6.1) 0.70 0.90 1.08
25 (7.6) 0.70 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


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/zg)2/α                   Kz = 2.01 (15/zg)2/α

    Note: z shall not be taken less than 30 feet in exposure B.
  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.
Velocity pressure, qz, evaluated at height z shall be calculated by the following equation:

qz  = 0.00256 KzKztKdV2 (lb/ft2) (28.3-1)

[In SI: qz  = 0.613 KzKztKdV2 (N/m2); V in m/s]

where
Kd = wind directionality factor defined in Section 26.6
Kz = velocity pressure exposure coefficient defined in Section 28.3.1
Kzt = topographic factor defined in Section 26.8.2
V = basic wind speed from Section 26.5.1
qh = velocity pressure qz calculated using Eq. 28.3-1 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 factor for a design application.
Design wind pressures for the MWFRS of low-rise buildings shall be determined by the following equation:

p = qh[(GCpf) - (GCpi)] (lb/ft2) (N/m2)          (28.4-1)

where
qh = velocity pressure evaluated at mean roof height h as defined in Section 26.3
(GCpf) = external pressure coefficient from Fig. 28.4-1
(GCpi) = internal pressure coefficient from Table 26.11-1
The combined gust-effect factor and external pressure coefficients for low-rise buildings, (GCpf), are not permitted to be separated.
The design wind pressure for the effect of parapets on MWFRS of low-rise buildings with flat, gable, or hip roofs shall be determined by the following equation:

pp = qp(GCpn) (lb/ft2) (28.4-2)

where
pp = combined net pressure on the parapet due to the combination 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
The positive external pressure on the bottom surface of windward roof overhangs shall be determined using GCp =  0.7 in combination with the top surface pressures determined using Fig. 28.4-1.
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.

Part 2 Enclosed Simple Diaphragm Low-Rise Buildings

The steps required for the determination of MWFRS wind loads on enclosed simple diaphragm buildings are shown in Table 28.5-1.

User Note: Part 2 of Chapter 28 is a simplified method to determine the wind pressure on the MWFRS of enclosed simple diaphragm low-rise buildings having a flat, gable, or hip roof. The wind pressures are obtained directly from a table and applied on horizontal and vertical projected surfaces of the building. This method is a simplification of the envelope procedure contained in Part 1 of Chapter 28.
The following wind load parameters are specified in Chapter 26:
  • Basic wind speed V (Section 26.5),
  • Exposure category (Section 26.7),
  • Topographic factor Kzt (Section 26.8), and
  • Enclosure classification (Section 26.10).
A building whose design wind loads are determined in accordance with this section shall meet all the conditions of Section 28.6.2. If a building does not meet all of the conditions of Section 28.6.2, then its MWFRS wind loads shall be determined by Part 1 of this chapter, by the directional procedure of Chapter 27, or by the wind tunnel procedure of Chapter 31.
For the design of MWFRS the building shall comply with all of the following conditions:
  1. The building is a simple diaphragm building as defined in Section 26.2.
  2. The building is a low-rise building as defined in Section 26.2.
  3. The building is enclosed as defined in Section 26.2 and conforms to the wind-borne debris provisions of Section 26.10.3-1.
  4. The building is a regular-shaped building or structure as defined in Section 26.2.
  5. The building is not classified as a flexible building as defined in Section 26.2.
  6. The building does not have response characteristics making it subject to across-wind loading, vortex shedding, or instability due to galloping or flutter; and it does not have a site location for which channeling effects or buffeting in the wake of upwind obstructions warrant special consideration.
  7. The building has an approximately symmetrical cross-section in each direction with either a flat roof or a gable or hip roof with θ ≤ 45°.
  8. The building is exempted from torsional load cases as indicated in Note 5 of Fig. 28.4-1, or the torsional load cases defined in Note 5 do not control the design of any of the MWFRS of the building.
Simplified design wind pressures, ps, for the MWFRS of low-rise simple diaphragm buildings represent the net pressures (sum of internal and external) to be applied to the horizontal and vertical projections of building surfaces as shown in Fig. 28.6-1. For the horizontal pressures (Zones A, B, C, D), ps is the combination of the windward and leeward net pressures. ps shall be determined by the following equation:

ps = λ Kzt pS30 (28.6-1)

where
λ = adjustment factor for building height and exposure from Fig. 28.6-1
Kzt = topographic factor as defined in Section 26.8 evaluated at mean roof height, h
pS30 = simplified design wind pressure for Exposure B, at h = 30 ft (9.1 m) from Fig. 28.6-1

Main Wind Force Resisting System - Method 2 h ≤ 60 ft.
Figure 28.6-1 Design Wind Pressures Walls & Roofs
Enclosed Buildings
Notes :
  1. Pressures shown are applied to the horizontal and vertical projections , for exposure B, at h=30 ft (9.1 m). Adjust to other exposures and heights with adjustment factor λ.
  2. The load patterns shown shall be applied to each corner of the building in turn as the reference corner. (See Figure 28.4-1)
  3. For Case B use θ = 0°.
  4. Load cases 1 and 2 must be checked for 25° < θ ≤ 45°. Load case 2 at 25° is provided only for interpolation between 25° and 30°.
  5. Plus and minus signs signify pressures acting toward and away from the projected surfaces, respectively.
  6. For roof slopes other than those shown, linear interpolation is permitted.
  7. The total horizontal load shall not be less than that determined by assuming ps = 0 in zones B & D.
  8. Where zone E or G falls on a roof overhang on the windward side of the building, use EOH and GOH for the pressure on the horizontal projection of the overhang. Overhangs on the leeward and side edges shall have the basic zone pressure applied.
  9. Notation:
    a : 10 percent of least horizontal dimension or 0.4h, whichever is smaller , but not less than either 4% of least horizontal dimension or 3 ft (0.9 m).
    h : Mean roof height, in feet (meters), except that eave height shall be used for roof angles<10°.
    θ : Angle of plane of roof from horizontal, in degrees.
  10. For Load Case A, the roof pressure coefficient (GCpf), when negative in Zone 2 and 2E, shall be applied in Zone 2/2E for a distance from the edge of roof equal to 0.5 times the horizontal dimension of the building measured perpendicular to the ridge line or 2.5 times the eave height at the windward wall, whichever is less; the remainder of Zone 2/2E extending to the ridge line shall use the pressure coefficient (GCpf) for Zone 3/3E.

Simplified Design Wind Pressure, PS30 (psf) (Exposure Bath= 30 ft. with I= 1.0)



Adjustment Factor for Building Height and Exposure, λ
Mean roof
height (ft)
Exposure
B C D
15 1.00 1.21 1.47
20 1.00 1.29 1.55
25 1.00 1.35 1.61
30 1.00 1.40 1.66
35 1.05 1.45 1.70
40 1.09 1.49 1.74
45 1.12 1.53 1.78
50 1.16 1.56 1.81
55 1.19 1.59 1.84
60 1.22 1.62 1.87

Unit Conversions - 1.0 ft = 0.3048 m; 1.0 psf = 0.0479 kN/m2

The load effects of the design wind pressures from Section 28.6.3 shall not be less than a minimum load defined by assuming the pressures, ps, for zones A and C equal to +16 psf, Zones B and D equal to +8 psf, while assuming ps for Zones E, F, G, and His equal to 0 psf.
Resources