Wind Load Calculation As Per Asce 7-05 !exclusive!

Understanding Wind Load Calculation as per ASCE 7-05 While newer versions of the ASCE 7 standard (like 7-10, 7-16, and 7-22) are now in use, ASCE 7-05: Minimum Design Loads for Buildings and Other Structures remains a foundational document in structural engineering. Many jurisdictions and existing building evaluations still reference this specific edition.

Calculating wind loads under ASCE 7-05 involves determining the pressure exerted by wind on a structure's surface, which is then used to design the Main Wind-Force Resisting System (MWFRS) and the Components and Cladding (C&C). 1. The Basic Wind Pressure Equation The core formula for calculating wind pressure ( ) in ASCE 7-05 is:

p=q×G×Cp−qi×(GCpi)p equals q cross cap G cross cap C sub p minus q sub i cross open paren cap G cap C sub p i end-sub close paren : Velocity pressure. : Gust effect factor. Cpcap C sub p : External pressure coefficient. GCpicap G cap C sub p i end-sub : Internal pressure coefficient. 2. Step-by-Step Calculation Process Step 1: Determine Basic Wind Speed (

Consult the wind speed maps in Figure 6-1 of ASCE 7-05. These speeds represent 3-second gust speeds in miles per hour (mph) at 33 feet above ground in Exposure Category C. Step 2: Determine Occupancy Category

Classify the building based on its use (Category I to IV). This determines the Importance Factor (

), which accounts for the hazard to human life and the need for the building to remain functional after a storm. Step 3: Determine Exposure Category (A, B, C, or D)

Exposure B: Urban/suburban areas with closely spaced obstructions.

Exposure C: Open terrain with scattered obstructions (the default). Exposure D: Flat, unobstructed areas and water surfaces. Step 4: Calculate Velocity Pressure (

This represents the kinetic energy of the wind converted into potential pressure:

qz=0.00256×Kz×Kzt×Kd×V2×Iq sub z equals 0.00256 cross cap K sub z cross cap K sub z t end-sub cross cap K sub d cross cap V squared cross cap I Kzcap K sub z

: Velocity pressure exposure coefficient (varies with height). Kztcap K sub z t end-sub : Topographic factor (for buildings on hills or ridges). Kdcap K sub d

: Wind directionality factor (typically 0.85 for buildings). Step 5: Determine the Gust Effect Factor (

For rigid structures, a simplified value of 0.85 is often used. For flexible (slender) structures, a more complex calculation is required to account for the dynamic response and vibration of the building. Step 6: Determine Pressure Coefficients ( Cpcap C sub p GCpicap G cap C sub p i end-sub External ( Cpcap C sub p

): These values depend on the wind direction and the building's geometry (e.g., windward wall, leeward wall, side walls, or roof). Internal ( GCpicap G cap C sub p i end-sub

): This depends on whether the building is "Enclosed," "Partially Enclosed," or "Open." 3. Analysis Methods

ASCE 7-05 provides three distinct methods for calculating wind loads:

Method 1 (Simplified Procedure): Used for "Regular" buildings with simple geometries and heights under 60 feet.

Method 2 (Analytical Procedure): The most common method, used for buildings of any height that don't meet the "Simple" criteria. This involves the step-by-step process outlined above.

Method 3 (Wind Tunnel Procedure): Used for complex, tall, or aerodynamically sensitive structures where standard equations are insufficient. 4. Key Differences: ASCE 7-05 vs. Later Versions wind load calculation as per asce 7-05

The most significant shift occurred in ASCE 7-10. In the 2005 version, wind speeds were Service Level (Allowable Stress Design). Starting in 2010, the maps shifted to Ultimate Strength (Load and Resistance Factor Design) wind speeds.

When using ASCE 7-05, ensure you are using the appropriate load combination factors ( 1.6W1.6 cap W for LRFD or 1.0W1.0 cap W for ASD) associated with service-level wind speeds.

The ASCE 7-05 standard provides a comprehensive methodology for determining wind loads on structures. Unlike newer versions (like ASCE 7-10 or 7-16) that use "ultimate" wind speeds, ASCE 7-05 is based on service-level (nominal) wind speeds and relies on an Importance Factor ( ) to adjust for the risk category of the structure. Core Calculation Procedure

The standard primarily uses the Analytical Procedure (Method 2) for regular structures, which follows these logical steps: 1. Determine Velocity Pressure ( )

The foundation of wind load is the velocity pressure at a specific height , calculated using the formula:

qz=0.00256⋅Kz⋅Kzt⋅Kd⋅V2⋅I (lb/ft2)q sub z equals 0.00256 center dot cap K sub z center dot cap K sub z t end-sub center dot cap K sub d center dot cap V squared center dot cap I (lb/ft squared close paren

(Basic Wind Speed): The 3-second gust speed at 33 ft (10m) above ground, taken from ASCE 7-05 maps. Kzcap K sub z

(Velocity Exposure Coefficient): Accounts for height and terrain roughness. Kztcap K sub z t end-sub

(Topographic Factor): Accounts for wind speed-up over hills or ridges; typically for level ground. Kdcap K sub d (Wind Directionality Factor): Usually for buildings. (Importance Factor): Ranges from (low risk) to (essential facilities). 2. Calculate Design Wind Pressure ( ) Wind Load Calculation as per ASCE 7-16

To perform wind load calculations according to ASCE 7-05, the standard feature is the Method 2: Analytical Procedure, which determines design wind pressures ( ) or forces (

) using building-specific factors like velocity pressure, gust effects, and pressure coefficients. The design wind pressure is generally calculated as:

p=q⋅G⋅Cp−qi⋅(GCpi)p equals q center dot cap G center dot cap C sub p minus q sub i center dot open paren cap G cap C sub p i end-sub close paren : Velocity pressure ( for windward, for leeward/side/roof). : Gust effect factor (typically 0.85 for rigid structures). Cpcap C sub p : External pressure coefficient. GCpicap G cap C sub p i end-sub : Internal pressure coefficient. 1. Identify Occupancy and Risk Category

The first step is to determine the building's Occupancy Category (now often called Risk Category) from Table 1-1 of ASCE 7-05. This classification accounts for the importance of the structure and the potential hazard to human life in the event of failure. 2. Determine Basic Wind Speed and Importance Factor Find the Basic Wind Speed (

) using the wind speed maps in Figure 6-1 of the code. For ASCE 7-05,

is based on a 3-second gust at 33 feet (10m) above the ground. You must also select an Importance Factor ( ) from Table 6-1 based on your occupancy category. 3. Calculate Velocity Pressure ( The velocity pressure at height is calculated using the formula:

qz=0.00256⋅Kz⋅Kzt⋅Kd⋅V2⋅I (lb/ft2)q sub z equals 0.00256 center dot cap K sub z center dot cap K sub z t end-sub center dot cap K sub d center dot cap V squared center dot cap I (lb/ft squared close paren Kzcap K sub z (Exposure Coefficient): Determined by the height ( ) and the Exposure Category (B, C, or D). Kztcap K sub z t end-sub

(Topographic Factor): Accounts for wind speed-up over hills or ridges; it is typically 1.0 for flat terrain. Kdcap K sub d

(Wind Directionality Factor): Adjusts for the probability of the maximum wind coming from any one specific direction; typically 0.85 for buildings. 4. Determine Gust Effect Factor ( ASCE 7-05 Wind Load Calculations | PDF - Scribd Understanding Wind Load Calculation as per ASCE 7-05

Wind Load Calculation as per ASCE 7-05: A Comprehensive Guide

The American Society of Civil Engineers (ASCE) provides guidelines for calculating wind loads on buildings and other structures through its ASCE 7-05 standard. This standard, titled "Minimum Design Loads for Buildings and Other Structures," outlines the procedures for determining wind loads, which are a crucial consideration in building design. In this article, we will provide an in-depth look at wind load calculation as per ASCE 7-05.

Introduction

Wind loads are a significant factor in building design, particularly for tall buildings, long-span structures, and those located in areas prone to high winds. The ASCE 7-05 standard provides a framework for calculating wind loads, which helps engineers and architects design buildings that can withstand wind forces. The standard takes into account various factors, including building geometry, location, and terrain, to provide a comprehensive approach to wind load calculation.

Key Terms and Definitions

Before diving into the wind load calculation procedure, it's essential to understand some key terms and definitions:

ASCE 7-05 Wind Load Calculation Procedure

The ASCE 7-05 standard provides a step-by-step procedure for calculating wind loads. The following are the general steps:

  1. Determine the Basic Wind Speed (V): The basic wind speed is determined based on the building's location. The ASCE 7-05 standard provides a map of the United States with contours of basic wind speeds. The designer must determine the basic wind speed for the specific location of the building.
  2. Determine the Exposure Category: The exposure category is determined based on the terrain surrounding the building. The ASCE 7-05 standard defines three exposure categories:
    • Exposure B: Urban areas with numerous obstacles, such as buildings and trees.
    • Exposure C: Suburban areas with some obstacles.
    • Exposure D: Rural areas with few obstacles.
  3. Calculate the Height Factor (Kz): The height factor, Kz, accounts for the increase in wind speed with height. The ASCE 7-05 standard provides a table with Kz values for different heights and exposure categories.
  4. Calculate the Topographic Factor (Kzt): The topographic factor, Kzt, accounts for the effect of terrain features on wind speed. The ASCE 7-05 standard provides a procedure for calculating Kzt based on the terrain features.
  5. Calculate the Wind Speed (V): The wind speed at a specific height is calculated using the basic wind speed, height factor, and topographic factor:

V = V * Kz * Kzt

  1. Calculate the Wind Load: The wind load is calculated using the wind speed and the building's geometry. The ASCE 7-05 standard provides several methods for calculating wind loads, including:
    • Envelope method: A simplified method for calculating wind loads on rectangular buildings.
    • Directional procedure: A more detailed method for calculating wind loads on complex buildings.

Envelope Method

The envelope method is a simplified procedure for calculating wind loads on rectangular buildings. The method involves calculating the wind load on each face of the building and then combining them to determine the total wind load. The ASCE 7-05 standard provides a table with wind load coefficients for different building shapes and exposure categories.

Directional Procedure

The directional procedure is a more detailed method for calculating wind loads on complex buildings. The method involves calculating the wind load for each direction (e.g., north, south, east, and west) and then combining them to determine the total wind load. The ASCE 7-05 standard provides a procedure for calculating wind loads using this method.

Example Calculation

Let's consider an example calculation for a rectangular building located in an urban area (Exposure B). The building has a height of 20 meters (66 feet) and a plan dimension of 10 meters (33 feet) by 20 meters (66 feet).

  1. Basic Wind Speed (V): 45 m/s (100 mph)
  2. Exposure Category: Exposure B
  3. Height Factor (Kz): 0.925 (from ASCE 7-05 table)
  4. Topographic Factor (Kzt): 1.0 (flat terrain)
  5. Wind Speed (V): 45 m/s * 0.925 * 1.0 = 41.625 m/s
  6. Wind Load: Using the envelope method, the wind load is calculated to be 1,456 N/m² (31.4 psf)

Conclusion

Wind load calculation as per ASCE 7-05 is a critical step in building design. The standard provides a comprehensive framework for calculating wind loads, taking into account various factors such as building geometry, location, and terrain. By following the procedures outlined in ASCE 7-05, engineers and architects can ensure that buildings are designed to withstand wind forces and provide a safe and durable structure for occupants.

References

FAQs

  1. What is the basic wind speed?: The basic wind speed is the wind speed at a height of 10 meters (33 feet) above the ground, measured over a distance of 1 kilometer (0.62 miles).
  2. What is the exposure category?: The exposure category is a classification of terrain that affects wind speed, including urban, suburban, and rural areas.
  3. What is the height factor (Kz)?: The height factor, Kz, accounts for the increase in wind speed with height.
  4. What is the topographic factor (Kzt)?: The topographic factor, Kzt, accounts for the effect of terrain features on wind speed.

By understanding the procedures and guidelines outlined in ASCE 7-05, engineers and architects can ensure that buildings are designed to withstand wind loads and provide a safe and durable structure for occupants.

9. References

  1. ASCE/SEI 7-05. (2006). Minimum Design Loads for Buildings and Other Structures. American Society of Civil Engineers.
  2. Mehta, K. C., & Coulbourne, W. L. (2013). Wind Loads: Guide to the Wind Load Provisions of ASCE 7-10. ASCE Press.
  3. Simiu, E., & Scanlan, R. H. (1996). Wind Effects on Structures (3rd ed.). John Wiley & Sons.

Disclaimer: This paper is for educational purposes. For actual structural design, always consult the full ASCE 7-05 standard and local building code requirements.

To develop a feature for wind load calculations based on ASCE 7-05, you should structure your tool to follow the Method 2 (Analytical Procedure) outlined in the standard. This method is the most robust for diverse building types and is widely used in engineering software. 1. Core Calculation Parameters

Your feature will need to take the following user inputs to determine the velocity pressure ( Basic Wind Speed (

): 3-second gust speed in mph (typically from ASCE 7-05 Figure 6-1). Importance Factor ( ): Based on the building's occupancy category (Table 6-1).

Exposure Category: A, B, C, or D depending on the surrounding terrain (Section 6.5.6). Topographic Factor ( Kztcap K sub z t end-sub

): Accounts for wind speed-up over hills or ridges (Section 6.5.7). Wind Directionality Factor ( Kdcap K sub d ): Generally 0.85 for buildings (Table 6-4). 2. The Governing Equations

The primary output of your feature should be the Design Wind Pressure ( ). Velocity Pressure ( ) Calculate the pressure at height using Equation 6-15:

qz=0.00256⋅Kz⋅Kzt⋅Kd⋅V2⋅Iq sub z equals 0.00256 center dot cap K sub z center dot cap K sub z t end-sub center dot cap K sub d center dot cap V squared center dot cap I Kzcap K sub z

: Velocity pressure exposure coefficient, which varies with height and exposure category. 0.002560.00256 : Conversion factor for air density and units. Design Wind Pressure ( )

For the Main Wind Force Resisting System (MWFRS) of rigid buildings, use Equation 6-27:

P=q⋅G⋅Cp−qi⋅(GCpi)cap P equals q center dot cap G center dot cap C sub p minus q sub i center dot open paren cap G cap C sub p i end-sub close paren : Gust effect factor (typically 0.85 for rigid structures). Cpcap C sub p : External pressure coefficient (from Figures 6-6 to 6-8). GCpicap G cap C sub p i end-sub

: Internal pressure coefficient based on enclosure classification (Enclosed, Partially Enclosed, or Open). 3. Implementation Roadmap

To make this feature useful for engineers, consider including these specific sub-tools: Design Wind Pressure P Equation 6 27 Asce 7 05 - NIMC

3. Gust Effect Factor, G

Step 5: (G = 0.85), (GC_pi = \pm 0.18) (enclosed).

2.1 Basic Wind Speed ((V))

D. Exposure Category

Defines the surface roughness of the terrain surrounding the building (Section 6.5.6).

6. Main Wind-Force-Resisting System (MWFRS)

For determining overall overturning, shear, and moments: