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# 1609.6 Alternate All-Heights Method

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The alternate wind design provisions in this section are simplifications of the ASCE 7 Directional Procedure.

As an alternative to ASCE 7 Chapters 27 and 30, the following provisions are permitted to be used to determine the wind effects on regularly shaped buildings, or other structures that are regularly shaped, that meet all of the following conditions:

- The building or other structure is less than or equal to 75 feet (22 860 mm) in height with a height-to-least-width ratio of 4 or less, or the building or other structure has a fundamental frequency greater than or equal to 1 hertz.
- The building or other structure is not sensitive to dynamic effects.
- The building or other structure is not located on a site for which channeling effects or buffeting in the wake of upwind obstructions warrant special consideration.
- The building shall meet the requirements of a simple diaphragm building as defined in ASCE 7 Section 26.2, where wind loads are only transmitted to the main windforce-resisting system (MWFRS) at the diaphragms.
- For open buildings, multispan gable roofs, stepped roofs, sawtooth roofs, domed roofs, roofs with slopes greater than 45 degrees (0.79 rad), solid freestanding walls and solid signs, and rooftop equipment, apply ASCE 7 provisions.

The following modifications shall be made to certain subsections in ASCE 7: in Section 1609.6.2, symbols and notations that are specific to this section are used in conjunction with the symbols and notations in ASCE 7 Section 26.3.

Coefficients and variables used in the alternative all-heights method equations are as follows:

For SI: 1 foot = 304.8 mm, 1 square foot = 0.0929m

C_{net} | = | Net-pressure coefficient based on K[(_{d} G) (C) - (_{p}GC)], in accordance with Table 1609.6.2._{pi} |

G | = | Gust effect factor for rigid structures in accordance with ASCE 7 Section 26.9.1. |

K_{d} | = | Wind directionality factor in accordance with ASCE 7 Table 26-6. |

P_{net} | = | Design wind pressure to be used in determination of wind loads on buildings or other structures or their components and cladding, in psf (kN/m^{2}). |

STRUCTURE OR PART THEREOF |
DESCRIPTION |
C_{ net } FACTOR |
||||

1. Main windforce-resisting frames and systems | Walls: |
Enclosed |
Partially enclosed |
|||

+ Internal pressure | - Internal pressure | + Internal pressure | - Internal pressure | |||

Windward wall | 0.43 | 0.73 | 0.11 | 1.05 | ||

Leeward wall | -0.51 | -0.21 | -0.83 | 0.11 | ||

Sidewall | -0.66 | -0.35 | -0.97 | -0.04 | ||

Parapet wall | Windward |
1.28 | 1.28 | |||

Leeward |
-0.85 | -0.85 | ||||

Roofs: |
Enclosed |
Partially enclosed |
||||

Wind perpendicular to ridge | + Internal pressure |
- Internal pressure |
+ Internal pressure |
- Internal pressure |
||

Leeward roof or flat roof | -0.66 | -0.35 | -0.97 | -0.04 | ||

Windward roof slopes: | ||||||

Slope < 2:12 (10°) | Condition 1 | -1.09 | -0.79 | -1.41 | -0.47 | |

Condition 2 | -0.28 | 0.02 | -0.60 | 0.34 | ||

Slope = 4:12 (18°) | Condition 1 | -0.73 | -0.42 | -1.04 | -0.11 | |

Condition 2 | -0.05 | 0.25 | -0.37 | 0.57 | ||

Slope = 5:12 (23°) | Condition 1 | -0.58 | -0.28 | -0.90 | 0.04 | |

Condition 2 | 0.03 | 0.34 | -0.29 | 0.65 | ||

Slope = 6:12 (27°) | Condition 1 | -0.47 | -0.16 | -0.78 | 0.15 | |

Condition 2 | 0.06 | 0.37 | -0.25 | 0.68 | ||

Slope = 7:12 (30°) | Condition 1 | -0.37 | -0.06 | -0.68 | 0.25 | |

Condition 2 | 0.07 | 0.37 | -0.25 | 0.69 | ||

Slope = 9:12 (37°) | Condition 1 | -0.27 | 0.04 | -0.58 | 0.35 | |

Condition 2 | 0.14 | 0.44 | -0.18 | 0.76 | ||

Slope = 12:12 (45°) | 0.14 | 0.44 | -0.18 | 0.76 | ||

Wind parallel to ridge and flat roofs | -1.09 | -0.79 | -1.41 | -0.47 | ||

Nonbuilding Structures: Chimneys, Tanks and Similar Structures: |
||||||

h/D | ||||||

1 | 7 | 25 | ||||

Square (Wind normal to face) | 0.99 | 1.07 | 1.53 | |||

Square (Wind on diagonal) | 0.77 | 0.84 | 1.15 | |||

Hexagonal or octagonal | 0.81 | 0.97 | 1.13 | |||

Round | 0.65 | 0.81 | 0.97 | |||

Open signs and lattice frameworks | Ratio of solid to gross area | |||||

< 0.1 | 0.1 to 0.29 | 0.3 to 0.7 | ||||

Flat | 1.45 | 1.30 | 1.16 | |||

Round | 0.87 | 0.94 | 1.08 | |||

2. Components and cladding not in areas of discontinuity—roofs and overhangs | Roof elements and slopes |
Enclosed |
Partially enclosed |
|||

Gable of hipped configurations (Zone 1) | ||||||

Flat < Slope < 6:12 (27°) See ASCE 7 Figure 30.4-2B Zone 1 | ||||||

Positive | 10 square feet or less | 0.58 | 0.89 | |||

100 square feet or more | 0.41 | 0.72 | ||||

Negative | 10 square feet or less | -1.00 | -1.32 | |||

100 square feet or more | -0.92 | -1.23 | ||||

Overhang: Flat < Slope < 6:12 (27°) See ASCE 7 Figure 30.4-2A Zone 1 | ||||||

Negative | 10 square feet or less | -1.45 | ||||

100 square feet or more | -1.36 | |||||

500 square feet or more | -0.94 | |||||

6:12 (27°) < Slope < 12:12 (45°) See ASCE 7 Figure 30.4-2C Zone 1 | ||||||

Positive | 10 square feet or less | 0.92 | 1.23 | |||

100 square feet or more | 0.83 | 1.15 | ||||

Negative | 10 square feet or less | -1.00 | -1.32 | |||

100 square feet or more | -0.83 | -1.15 | ||||

Monosloped configurations (Zone 1) | Enclosed |
Partially enclosed |
||||

Flat < Slope < 7:12 (30°) See ASCE 7 Figure 30.4-5B Zone 1 | ||||||

Positive | 10 square feet or less | 0.49 | 0.81 | |||

100 square feet or more | 0.41 | 0.72 | ||||

Negative | 10 square feet or less | -1.26 | -1.57 | |||

100 square feet or more | -1.09 | -1.40 | ||||

Tall flat-topped roofs h > 60 feet |
Enclosed |
Partially enclosed |
||||

Flat < Slope < 2:12 (10°) (Zone 1) See ASCE 7 Figure 30.8-1 Zone 1 | ||||||

Negative | 10 square feet or less | -1.34 | -1.66 | |||

500 square feet or more | -0.92 | -1.23 | ||||

3. Components and cladding in areas of discontinuity—roofs and overhangs (continued) | Gable or hipped configurations at ridges, eaves and rakes (Zone 2) | |||||

Flat < Slope < 6:12 (27°) See ASCE 7 Figure 30.4-2B Zone 2 | ||||||

Positive | 10 square feet or less | 0.58 | 0.89 | |||

100 square feet or more | 0.41 | 0.72 | ||||

Negative | 10 square feet or less | -1.68 | -2.00 | |||

100 square feet or more | -1.17 | -1.49 | ||||

Overhang for Slope Flat < Slope < 6:12 (27°) See ASCE 7 Figure 30.4-2B Zone 2 | ||||||

Negative | 10 square feet or less | -1.87 | ||||

100 square feet or more | -1.87 | |||||

6:12 (27°) < Slope < 12:12 (45°) Figure 30.4-2C | Enclosed |
Partially enclosed |
||||

Positive | 10 square feet or less | 0.92 | 1.23 | |||

100 square feet or more | 0.83 | 1.15 | ||||

Negative | 10 square feet or less | -1.17 | -1.49 | |||

100 square feet or more | -1.00 | -1.32 | ||||

Overhang for 6:12 (27°) < Slope < 12:12 (45°) See ASCE 7 Figure 30.4-2C Zone 2 | ||||||

Negative | 10 square feet or less | -1.70 | ||||

500 square feet or more | -1.53 | |||||

3. Components and cladding in areas of discontinuity—roofs and overhangs | Roof elements and slopes |
Enclosed |
Partially enclosed |
|||

Monosloped configurations at ridges, eaves and rakes (Zone 2) | ||||||

Flat < Slope < 7:12 (30°) See ASCE 7 Figure 30.4-5B Zone 2 | ||||||

Positive | 10 square feet or less | 0.49 | 0.81 | |||

100 square feet or more | 0.41 | 0.72 | ||||

Negative | 10 square feet or less | -1.51 | -1.83 | |||

100 square feet or more | -1.43 | -1.74 | ||||

Tall flat topped roofs h > 60 feet |
Enclosed |
Partially enclosed |
||||

Flat < Slope < 2:12 (10°) (Zone 2) See ASCE 7 Figure 30.8-1 Zone 2 | ||||||

Negative | 10 square feet or less | -2.11 | -2.42 | |||

500 square feet or more | -1.51 | -1.83 | ||||

Gable or hipped configurations at corners (Zone 3) See ASCE 7 Figure 30.4-2B Zone 3 | ||||||

Flat < Slope < 6:12 (27°) | Enclosed |
Partially enclosed |
||||

Positive | 10 square feet or less | 0.58 | 0.89 | |||

100 square feet or more | 0.41 | 0.72 | ||||

Negative | 10 square feet or less | -2.53 | -2.85 | |||

100 square feet or more | -1.85 | -2.17 | ||||

Overhang for Slope Flat < Slope < 6:12 (27°) See ASCE 7 Figure 30.4-2B Zone 3 | ||||||

Negative | 10 square feet or less | -3.15 | ||||

100 square feet or more | -2.13 | |||||

6:12 (27°) < 12:12 (45°) See ASCE 7 Figure 30.4-2C Zone 3 | ||||||

Positive | 10 square feet or less | 0.92 | 1.23 | |||

100 square feet or more | 0.83 | 1.15 | ||||

Negative | 10 square feet or less | -1.17 | -1.49 | |||

100 square feet or more | -1.00 | -1.32 | ||||

Overhang for 6:12 (27°) < Slope < 12:12 (45°) | Enclosed |
Partially enclosed |
||||

Negative | 10 square feet or less | -1.70 | ||||

100 square feet or more | -1.53 | |||||

Monosloped Configurations at corners (Zone 3) See ASCE 7 Figure 30.4-5B Zone 3 | ||||||

Flat < Slope < 7:12 (30°) | ||||||

Positive | 10 square feet or less | 0.49 | 0.81 | |||

100 square feet or more | 0.41 | 0.72 | ||||

Negative | 10 square feet or less | -2.62 | -2.93 | |||

100 square feet or more | -1.85 | -2.17 | ||||

Tall flat topped roofs h > 60 feet |
Enclosed |
Partially enclosed |
||||

Flat < Slope < 2:12 (10°) (Zone 3) See ASCE 7 Figure 30.8-1 Zone 3 | ||||||

Negative | 10 square feet or less | -2.87 | -3.19 | |||

500 square feet or more | -2.11 | -2.42 | ||||

4. Components and cladding not in areas of discontinuity—walls and parapets | Wall Elements: h ≤ 60 feet (Zone 4) ASCE 7 Figure 30.4-1 |
Enclosed |
Partially enclosed |
|||

Positive | 10 square feet or less | 1.00 | 1.32 | |||

500 square feet or more | 0.75 | 1.06 | ||||

Negative | 10 square feet or less | -1.09 | -1.40 | |||

500 square feet or more | -0.83 | -1.15 | ||||

Wall Elements: h > 60 feet (Zone 4) See ASCE 7 Figure 30.6-1 Zone 4 |
||||||

Positive | 20 square feet or less | 0.92 | 1.23 | |||

500 square feet or more | 0.66 | 0.98 | ||||

Negative | 20 square feet or less | -0.92 | -1.23 | |||

500 square feet or more | -0.75 | -1.06 | ||||

Parapet Walls | ||||||

Positive | 2.87 | 3.19 | ||||

Negative | -1.68 | -2.00 | ||||

5. Components and cladding in areas of discontinuity—walls and parapets | Wall elements: h ≤ 60 feet (Zone 5) ASCE 7 Figure 30.4-1 |
Enclosed |
Partially enclosed |
|||

Positive | 10 square feet or less | 1.00 | 1.32 | |||

500 square feet or more | 0.75 | 1.06 | ||||

Negative | 10 square feet or less | -1.34 | -1.66 | |||

500 square feet or more | -0.83 | -1.15 | ||||

Wall elements: h > 60 feet (Zone 5) See ASCE 7 Figure 30.6-1 Zone 4 |
||||||

Positive | 20 square feet or less | 0.92 | 1.23 | |||

500 square feet or more | 0.66 | 0.98 | ||||

Negative | 20 square feet or less | -1.68 | -2.00 | |||

500 square feet or more | -1.00 | -1.32 | ||||

Parapet walls | ||||||

Positive | 3.64 | 3.95 | ||||

Negative | -2.45 | -2.76 |

^{2}, 1 degree = 0.0175 rad.- Linear interpolation between values in the table is permitted.
- Some
*C*values have been grouped together. Less conservative results may be obtained by applying ASCE 7 provisions._{net }

When using the alternative all-heights method, the MWFRS, and components and cladding of every structure shall be designed to resist the effects of wind pressures on the building envelope in accordance with Equation 16-35.

Design wind forces for the MWFRS shall be not less than 16 psf (0.77 kN/m

Design wind forces for the MWFRS shall be not less than 16 psf (0.77 kN/m

^{2}) multiplied by the area of the structure projected on a plane normal to the assumed wind direction (see ASCE 7 Section 27.4.7 for criteria). Design net wind pressure for components and cladding shall be not less than 16 psf (0.77 kN/m^{2}) acting in either direction normal to the surface.The MWFRS and the components and cladding of every building or other structure shall be designed for the pressures calculated using Equation 16-35.

The MWFRS shall be investigated for the torsional effects identified in ASCE 7 Figure 27.4-8.

Velocity pressure exposure coefficient,

*K*shall be determined in accordance with ASCE 7 Section 27.3.1 and the topographic factor,_{z},*K*, shall be determined in accordance with ASCE 7 Section 26.8._{zt}- For the windward side of a structure,
*K*and_{zt}*K*shall be based on height_{z}*z*. - For leeward and sidewalls, and for windward and leeward roofs,
*K*and_{zt}*K*shall be based on mean roof height_{z}*h*.

For the design of the MWFRS and for components and cladding, the sum of the internal and external net pressure shall be based on the net pressure coefficient,

*C*._{net}- The pressure coefficient, C
, for walls and roofs shall be determined from Table 1609.6.2._{net} - Where C
has more than one value, the more severe wind load condition shall be used for design._{net}

When using the alternative all-heights method, wind pressures shall be applied simultaneously on, and in a direction normal to, all building envelope wall and roof surfaces.

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