The pressure exerted by the wind is one of the important considerations in Structural Design. Once the wind passed through the building, a deflections perpendicular to the wind may also occur depending on its velocity. In some cases the load due to wind governs especially when you are considering a high or a tall structure, that is why wind loads should not be taken for granted. In fact when a building is too complex, a wind tunnel procedure can be considered. Nevertheless, the code set a standard in determining wind procedure that we require in our design.

**ASCE 7-10** provides two methods for wind load calculation: a simplified procedure and an analytical procedure. The simplified procedure is for building with simple diaphragm, roof slope less than 10 degree, mean roof height less than 30 feet (9 meters), regular shape rigid building, no expansion joints, flat terrain and not subjected to special wind condition. The analytical procedure is for all buildings and non-building structures. Each procedure has two categories: wind for **main wind force-resisting system (MWFRS)** and wind for **component and claddings (C&C)**.

Since most of our wind design considerations are for buildings other than the simplified procedure stated above, let us tackled the Analytical Procedure approach that can be applied both for buildings and non-building structure.

The objective of this article is to help you decide which wind load criteria is appropriate for your design as per the analytical procedure; here are the summaries of wind load analytical procedure approach as specified in ASCE 7-10. You are going to need a copy of the **ASCE 7-10** code for sections, figures and table references.

**Determine the Velocity Pressure, q**_{z}:

_{z}:

One of the important aspects _{z} equivalent at height z shall be calculated as

**q _{z} = 0.00256 K_{z} K_{zt} K_{d} V^{2} ** (lb/ft

^{2}) or

**q _{z} = 0.613 K_{z} K_{zt} K_{d} V^{2} ** (N/m

^{2}); V=m/s

where:

**K**_{z} is velocity pressure exposure coefficient

**K _{zt}** is the topographic factor

**K _{d}** is wind directionality factor

**V** is the basic wind speed

Velocity pressure exposure coefficients,
K_{z} are listed Table 27.3-1 of ASCE 7-10
or can be calculated as

**K _{z} = 2.01 (z/z_{g})^{2/α}**

from which, z is the height above ground and should not be less than 15 feet (4.5 meters) except that z shall not be less than 30 feet (9 meters) for exposure B for low rise building and for component and cladding. The parameters, ** α ** ** z _{g}** are taken as follows:

Exposure | α | z_{g }(ft) |
z_{g }(m) |

B | 7 | 1200 | 365.76 |

C | 9.5 | 900 | 274.32 |

D | 11.5 | 700 | 213.36 |

Topographic Factor, K_{zt}:

**K _{zt} = (1 + K_{1}K_{2}K_{3})^{2} **

where:

K_{1},
K_{2}, K_{3} are
determined from Figure 26.8-1 of ASCE 7-10
based on ridge, escarpment and hill. If site conditions and locations of
structures do not meet all the conditions specified in section 26.8.1 then K_{zt} =1.0

Wind Directionality Factor; K_{d} shall be determine from Table 26.6-1 and the basic wind speed, V is according to Figure 26.5-1 of ASCE 7-10.

## 1. Wind Load for Main Wind Force Resisting System (MWFRS)

**1.1 Rigid Building of All Height:**

The design wind pressure shall be calculated as

**P = q G C _{p} – q_{i} (GC_{pi}) **(lb/ft

^{2}) (N/m

^{2}) (27.4-1)

where:

**q = **q** _{z} **for windward walls evaluated at height z above ground.

**q = **q** _{h} **for Leeward walls, side walls, and roof evaluated at mean roof height h above ground.

**G** = 0.85 is gust response factor

**C _{p}_{ }**is the external pressure coefficient from Figures 27.4-1, 27.4-2 and 27.4-3 of ASCE 7-10.

Figure 27.4-1 is for gable, hip roof, mono-slope roof, and mansard roof

Figure 27.4-2 is for domed roof

Figure 27.4-3 is for arched roof

**G _{Cpi} **is internal pressure coefficient from Table 26.11 of ASCE 7-10.

**q _{i} **is internal pressure evaluated as follows:

**Enclosed building**:

**q _{i} = **q

**evaluated for windward walls, leeward walls, and side walls, and roof.**

_{h}**Partially enclosed building:**

**q _{i} = **q

**for negative internal pressure,**

_{h}**q**= q

_{i}_{z}for positive internal pressure at height z at the level of highest opening.

Note: The internal pressure shall be applied simultaneously on windward and leeward walls and both positive and negative pressures need to be considered. Therefore, it cancels each other for enclosed building except for roof. For partially enclosed building, internal pressure shall be added to leeward wall at the height of opening.

Wall pressure coefficient Cp for Gable, Hip roof (from figures 27.4-1, 27.4-2 and 27.4-3 of ASCE 7-10):

Surface | L/B | Cp | Use with |

Windward Wall | All values | 0.8 | q_{z} |

Leeward Wall | 0-1 | -0.5 | q_{h} |

2 | -0.3 | ||

≥4 | -0.2 | ||

Side Wall | All values | -0.7 | q_{h} |

**1.2 Low-Rise Building**

The design wind pressure for low-rise buildings shall be calculated as

**P = q _{h}[ (GC_{pf} ) – (GC_{pi})] **(lb/ft

^{2}) (N/m

^{2}) (28.4-1)

where:

**q _{h}** is velocity pressure at mean roof height h above ground.

**GC _{pf}** is external pressure coefficient from Figure 28.4-1 of ASCE 7-10.

**GC _{pi}** is internal pressure coefficient from Table 26.11-1 of ASCE 7-10.

Note: For wind pressures at edges and corners of walls and roof are higher than interior zone. Wind pressure at each zone needs to be calculated separately.

External pressure coefficient GC_{pf} (from Figure 28.4.1 of ASCE 7-10)

Roof Angle | Building Surface | |||||||

1 | 2 | 3 | 4 | 1E | 2E | 3E | 4E | |

0-5 | 0.4 | -0.69 | -0.37 | -0.29 | 0.61 | -1.07 | -0.53 | -0.43 |

20 | 0.53 | -0.69 | -0.48 | -0.43 | 0.8 | -1.07 | -0.69 | -0.64 |

30-45 | 0.56 | 0.21 | -0.43 | -0.37 | 0.69 | 0.27 | -0.53 | -0.48 |

90 | 0.56 | 0.56 | -0.37 | -0.37 | 0.69 | 0.69 | -0.48 | -0.48 |

### 1.3 Parapets

The design wind pressure for the effect of parapets on MWFRS of rigid or flexible buildings shall be calculated as

**P _{p} = q_{p }GC_{pn }**(lb/ft

^{2}) (27.4-4)

where

**P _{p}** is the combined net pressure on the parapet due to the combination of net pressure from front and back surfaces; ± signs signify net pressure toward and away from the exterior side of the parapet

**q _{p}** is velocity pressure at the top of parapet.

**GC _{pn}** is combined net pressure coefficient, +1.5 for windward parapet, -1.0 for leeward parapet.

### 1.4 Design Wind Load with Eccentricities:

Wind load design cases as defined in Figure 27-4-8 of ASCE 7-10

**Case 1:** Full wind loads in two perpendicular directions considered separately.

**Case 2:** 75% wind loads in two perpendicular directions with 15% eccentricity considered separately.

**Case 3:** 75% wind loads in two perpendicular directions simultaneously.

**Case 4:** 56.3% (75%x75%) of wind load in two perpendicular directions with 15% eccentricity simultaneously.

## 2. Wind Load for Component and Cladding (C&C)

### 2.1 Building 60 Feet (18 meter) or Lower (Low-Rise Buildings)

The design wind pressure shall be calculated as

**P = q _{h}[ (GC_{p} ) – (GC_{pi})] **(lb/ft

^{2}) (N/m

^{2})

**(30-4-1)**

where:

**q**_{h }is velocity pressure at mean roof height h above ground.

**GC _{p}** is external pressure coefficient given in:

Figure 30.4-1 (walls)

Figures 30.4-2A to 30.4-2C (flat roofs, gable roofs, and hip roofs)

Figure 30.4-3 (stepped roofs)

Figure 4-4 (multispan gable roofs)

Figures 30.4-5A and 30.4-5B (monoslope roofs)

Figure 30.4-6 (saw tooth roofs)

Figure 27.4-3, footnote 4 (arched roofs)

**GC _{pi} **is internal pressure coefficient from Table 26.11-1 of ASCE 7-10.

### 2.2 Building higher than 60 Feet (18 meters)

The design wind pressure shall be calculated as

**P = q (GC _{p}) – q_{i} (GC_{pi}) **(lb/ft

^{2}) (N/m

^{2}) (30.6-1)

where:

**q = **q** _{z}** for windward walls evaluated at height z above ground.

**q = **q** _{h}** for Leeward walls, side walls, and roof evaluated at mean roof height h above ground.

**q _{i} **is internal pressure evaluated as follows:

**Enclosed building:**

**q _{i} = **q

**evaluated at mean roof height for windward, leeward, and side walls, and roof.**

_{h}**Partially enclosed building:**

**q _{i} = **q

**for negative internal pressure,**

_{h}**q**q

_{i }=**for positive internal pressure at height z at the level of highest opening.**

_{z}**GC _{p} **is external pressure coefficient in:

Figure 30.6-1 for walls and flat roofs

Figure 27.4-3, footnote 4, for arched roofs

Figure 30.4-7 for domed roofs

Figure 30.6-1 Note 6 for other roof angles and geometries

**GC _{pi} **is internal pressure coefficient from Table 26.11-1 of ASCE 7-10.

Note: The internal pressure shall be applied simultaneously on windward and leeward walls and both positive and negative pressures need to be considered. Therefore, it cancels each other for enclosed building except for roof. For partially enclosed building, internal pressure shall be added to leeward wall at the height of opening.

**2.3 Wind Pressure on Parapets**

The design wind pressure for C&C of parapet surfaces for all building types and heights shall be:

**P = q _{p} (GC_{p}) – (GC_{pi}) **(30.9-1)

where

**q _{p}** = velocity pressure at the top of parapets.

**GC _{p}** is external pressure coefficient given in:

Figures 30.4-1, 30.4-2A to 30.4-2C, 30.4-3, 30.4-4, 30.4-5A and 30-5B, 30.4-6, 30.4-7, 30.6-1, 27.4-3 and 27.4-3 (footnote 4).

**GC _{pi}** is internal pressure coefficient from Table 26.11-1 based on the porosity of the parapet envelope.

Note: Two load cases shall be considered as per Figure 30.9-1 of ASCE 7-10.

**2.4 Wind Load on Open Building
and Other Structures**

The design wind load shall be calculated as

**P = q _{h}G CN_{ }**(30.8-1)

where

**q _{h}**= velocity pressure at mean roof height h using the exposure defined in Section 26.7.3

**G**= 0.85 as gust effect factor.

**C _{N}** is net pressure coefficients include from top and bottom surfaces given in

Figure 30.8-1 for mono sloped roof

Figure 30.8-2 for pitched roof

Figure 30.8-3 for troughed roof

**To Sum Up:**

The above wind load provisions are according to ASCE 7-10 Chapters that Sum Up as follows:

- Chapter 26: General Requirements for Wind Load Determination
- Chapter 27: Wind Load
Criteria for MWFRS using Directional Approach. - Chapter 28: Wind Load Criteria for MWFRS for Low-rise Buildings
- Chapter 29: Wind Load Criteria for MWFRS of Other Structures
- Chapter 30: Wind Load Criteria for MWFRS for Components and Cladding

A recommended reference book Guide to the Wind Load Provisions of ASCE 7-10 by *Kishor C. Metha *and *William L. Coulbourne *containing different wind analysis examples problems is worth to try.

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