Wind Loading: American Standard ASCE 7

Buildings and their components must be designed to withstand code-specified wind loads. Wind load is the force exerted on a structure by the wind as it is an essential aspect of structural engineering that cannot be overlooked if we wanted to ensure the stability and safety of buildings against wind forces.

ASCE 7 provides methods for calculating the design wind pressure based on two categories of applications: the main wind force-resisting system (MWFRS) or components and claddings (C&C). By conducting wind load analysis, the safety and durability of buildings can be guaranteed regardless of harsh wind conditions, ASCE 7-22 will be used as the International Building Code (IBC) aligns with the most recent edition of ASCE7-22 with their latest advancements and research in structural engineering.

The process to calculate the wind load involves:

1. Determine the basic wind speed, V from the Basic Wind Speed Maps (ASCE 7-22 Figure 26.5-1A to 1D)

2. Determine site factors \(K_z\space and\space K_{zt}\) :

• Velocity pressure exposure coefficient, \(K_z\) (ASCE 7-22 Table 26.10-1) based on your exposure classification and your reference height above ground
• Topographic factor, \(K_{zt}\) (ASCE 7-22 Table 26.8-1) based on your distance to hills and escarpments.

❗The ground elevation factor, \(K_e\) is a new factor in ASCE 7-22. It is not considered in this calculator, though ASCE 7-22 permits \(K_e\) = 1 for all elevations and so the calculator is still valid.

3. Calculate the velocity pressure, qz (ASCE 7-22 Equation 26.10-1)
$$q_z=0.00256K_zK_{zt}K_eV^2(lb/ft^2);V,mi/h$$
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4. Determine the internal pressure coefficient, GCpi (ASCE 7-22 Table 26.13-1) based on your enclosure classification.

5. Calculate the design wind pressures, \(p\). The equation varies slightly depending on the method used and if you are considering the MWFRS or C&C. See below section for more details.

\( \textbf{1.1}\space\)Analysis Method

❗ASCE 7-10 provided two methods to calculate the design wind pressures: a simplified procedure and an analytical procedure. ASCE 7-22 has removed the simplified method. This calculator is based on the ASCE 7-10 analytical procedure that is still valid in ASCE 7-22.

ASCE 7-22 provides four separate chapters for determining the design wind pressures, based on application:

• Chapter 27: Wind loads on the main wind-force resisting system (MWFRS) using the Directional Procedure. This method applies to buildings of all heights in which it is necessary to separate applied wind loads onto the windward, leeward and sidewalls of the building to properly assess the internal forces in MWFRS members.sdf

• Chapter 28: Wind loads on MWFRS using the Envelope Procedure. This method applies to low-rise buildings that have a flat, gable or hip roof.

• Chapter 29: Wind loads on building appurtenances (such as roofs) and other structures (such as freestanding walls and chimneys) using the Directional Procedure. Note, not included in this calculator.

• Chapter 30: Wind loads on C&C. Includes Part 1 to 5 depending on your application: Part 1 for Low-Rise Buildings. Part 2 is for buildings with \(h\)> 60\(ft\), Part 3 is for Open Buildings, Part 4 includes Building Appurtenances, Rooftop Structures, and Equipment and Part 5 includes non-building structures like silos and tanks.

Wind loading analysis in all five sub-sections of this involve the following input parameters.‍

\( \textbf{2.1}\space\)Risk Category (or Building Classification): "I", "II", "III" or "IV"

Structures are classified with a "Risk Category" as per ASCE 7-22 Table 1.5-1, which is based on the risk to human life, health and welfare associated with their damage or failure by nature of their occupancy or use, for the purposes of applying flood, wind, tornado, snow, earthquake and ice provisions.

\( \textbf{2.1.1}\space\)Exposure Category: "B", "C" or "D"‍

Structures are classified with an "Exposure Category" as per ASCE 7-22 Section 26.7.3, for the purpose of assigning velocity pressure exposure coefficients, \(K_h\) and \(K_z\). They are defined as follows:

For each wind direction considered, the upwind exposure is based on ground surface roughness that is determined from natural topology, vegetation and constructed facilities. The surface roughness categories are defined as follows:

\( \textbf{2.2}\space\)Enclosure Classification: "Enclosed", "Open", "Partially Enclosed" or "Partially Open"

Rigid structures are given an "Enclosure Classification" as per ASCE 7-22 Section 26.2, for the purpose of assigning the internal pressure coefficient, GCpi. The classifications are defined as follows:

\( \textbf{2.3}\space\)Basic Wind Speed Maps, \(V\)

❗The wind speed maps in ASCE 7-22 and ASCE 7-10 differ slightly. Use the figures from ASCE 7-22 in this calculation, as provided below.

• Figure 26.5-1A: Basic wind speed, \(V\) for Risk Category I buildings and other structures

         Wind speeds correspond to approximately a 15% probability of exceedance in 50 years (Annual Exceedance Probability = 0.00333, MRI = 300 years).

• Figure 26.5-1B: Basic wind speed, \(V\) for Risk Category II buildings and other structures

          Wind speeds correspond to approximately a 7% probability of exceedance in 50 years (Annual Exceedance Probability = 0.00143, MRI = 700 years).

• Figure 26.5-1C: Basic wind speed, \(V\) for Risk Category III buildings and other structures

          Wind speeds correspond to approximately a 3% probability of exceedance in 50 years (Annual Exceedance Probability = 0.00058, MRI = 1,700 years).

• Figure 26.5-1D: Basic wind speed, \(V\) for Risk Category \(IV\) buildings and other structures

          Wind speeds correspond to approximately a 1.5% probability of exceedance in 50 years (Annual Exceedance  Probability = 0.00033, MRI = 3,000 years).

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\( \textbf{2.4}\space\)Topographic Factor, \(K_{zt}\)

The Topographic Factor, \(K_{zt}\), accounts for the wind speed-up effect over isolated hills and escarpments (ASCE 7-22 Section 26.8). Refer to Figure 26.8-1 for values of the topographic multipliers \(K_1\), \(K_2\)and \(K_3\).

Notation taken from ASCE7-22, page 276:

\(H\space\)= Height of Hill of escapment relative to the upwind terrain \(ft(m)\)
\(K_1\space\)= Factor to account for shape of topographic feature and maximum speed-ip effect
\(K_2\space\)= Factor to account for reduction in speed-up with distance upwind or downwind of crest
\(K_3\space\)= Factor to account for reduction in speed-up with height above local terrain;
\(L_h\space\)= Distance upwind of crest to where the difference in ground elevation is half the height of hill or escarpment, \(ft(m)\)
\(x\space\)= Distance (upwind or downwind) from the crest to the site of the building or other structure, \(ft(m)\)
\(z\space\)= Height above ground surface at the site of the building or other structure, \(ft(m)\)

\( \textbf{2.5}\space\)Wind Directionality Factor, \(K_d\)

❗In ASCE 7-22 the wind directionality factor, \(K_d\) has been moved from the velocity pressure equation, \(q\) to the design wind pressure equation, \(p\). The change is not reflected in this calculator, though the difference in results have assumed to be minor.

The Wind Directionality Factor, \(K_d\), is a load reduction factor to take into account it is very improbable that the maximum wind speed and the weakest direction of the building will coincide. Values for \(K_d\) provided in ASCE 7-22 is as per below.