Wind Load Analysis to ASCE 7: A Guide for Structural Engineers

Introduction

Wind load analysis, which involves the computation of forces exerted by the wind on structures, is a crucial aspect of structural engineering. Wind can exert significant lateral forces on a structure, which can cause it to sway or even collapse if it is not properly designed to withstand these loads.

Table of Contents:

• Key Considerations of Wind Load Analysis
• Wind Load Analysis for MWFRS
• Wind Load Analysis for Components and Cladding

To ensure the integrity of the structure is maintained, structural engineers must take steps to accurately analyze and transfer these lateral loads using various structural elements. By doing so, they can help to ensure that the building remains stable and safe for its occupants, particularly in regions prone to severe weather conditions such as hurricanes or typhoons.

The American Society of Civil Engineers (ASCE) has established a set of standards known as ASCE 7 to ensure consistency, accuracy, and safety of built structures.

The guidelines provide a framework for accurate calculations based on geographical location, building characteristics, and wind speeds.

Key Considerations of Wind Load Analysis

Overall, wind load analysis begins with determining the risk category of the building (using Table 1.5-1 of ASCE 7-16).

Wind Speed

The most critical aspect of wind load analysis is obtaining the basic wind speed, $V$, where the building is located. Wind speed is typically sourced from related code-based maps, such as that provided by ASCE 7-16 Figure 26.5-1A to D and Figure 26.5-2A to D (see Figure 1), or local meteorological records.

Figure 1: Figure 1. An example of a basic wind speed map as per Figure 26.5-1A in ASCE 7-16 (Reference)

The process then identifies wind load parameters including wind directionality, exposure category, topography, and ground elevation.

Wind Directionality Factor

Wind directionality in wind load analysis refers to the consideration of how wind flows around and interacts with a building. It takes into account the shape, size, and height of the structure, as well as its surrounding terrain.

Wind directionality is crucial in determining the magnitude and distribution of wind loads on a building. For low-rise buildings, such as residential houses or small commercial structures, the simplified method to consider wind directionality is normally used, and the wind directionality factor, $K_d$, can be obtained from Table 26.6-1 of ASCE 7-16.

Exposure Category

We can determine the exposure category based on ground surface topography, vegetation, and built structures.

In ASCE 7-16 (section 26.7), the following exposure categories can apply in wind load calculations:

• Exposure B - Urban/wooded areas with many obstructions
• Exposure C - Open terrain with scattered obstructions
• Exposure D - Flat unobstructed terrain

When it comes to residential design, structural designers tend to take a cautious approach by considering the worst possible scenario when selecting an exposure category. For example, if you are constructing a building at the edge of a new development and one side is exposed to Category B while the other side faces Category C, the general practice is to opt for Category C as the conservative choice.

Topographic Factors

When building on hills, ridges, or escarpments as low as 15 feet, wind load calculations must consider the topographic factor, $K_{zt}$, which accounts for increased wind speeds due to local topography.

$K_{zt}=(1+K_1 K_2 K_3 )^2$

Where:

• $K_{zt}$ = Topographic factor
• $K_1$ = Factor to account for shape of topographic feature and maximum speed-up effect.
• $K_2$ = Factor to account for reduction in speed-up with distance upwind or downwind of crest.
• $K_3$ = Factor to account for reduction in speed-up with height above local terrain.

Ground Elevation

Areas with higher elevations have lower air density, resulting in reduced wind loads. In wind loads calculation, the ground elevation factor is used to adjust for air density, $K_e$, and can be obtained from Table 26.9-1 in ASCE 7-16.

Building Enclosures

Another important factor to consider in wind load analysis is the building enclosure. The principle behind this is to consider the difference in pressure between the inside and outside of the building.

ASCE 7 has classified building enclosures into four categories:

• Enclosed: Air generally doesn’t come in. Wind pressure outside.
• Partially enclosed: Air flows into the building, but can’t escape.
• Open: Air flows into the building, and can escape
• Partially open: Air flows through the building

Figure 2: Illustration of enclosed building and partially enclosed building and the effect wind direction on internal and external pressures. (Reference)

These parameters play a substantial role and can drastically influence the wind pressures experienced by a building and contribute to the subsequent steps of wind load calculations, such as internal pressure coefficient, $GC_{pi}$, external pressure coefficients, and wind pressure, $P$.

The wind pressures is determined with the following equation:

$p= q_h[(GC_{pf}) − (GC_{pi})](lb/ft^2)$

Where:

• $q_h$ = velocity pressure evaluated at mean roof height $h$
• $GC_{pf}$ = external pressure coefficients from Fig. 28.3-1 of ASCE 7-16 (see Figure 3)
• $GC_{pi}$ = internal pressure coefficient from Table 26.13-1 of ASCE 7-16

This article primarily focuses on applying wind load calculations for two commonly used systems in residential and light commercial projects: the main wind-force resisting system (MWFRS) and the components and cladding systems of structures.

Wind Load Analysis for MWFRS

The main wind-force resisting system, or MWFRS, is an ensemble of structural elements designed to take on lateral and vertical wind loads, including structural walls, columns, rigid frames, and bracing systems.

The MWFRS is vital as it forms the backbone of any structure, ensuring overall stability against wind forces by transferring these wind loads. The mechanism involves transmitting wind forces from their point of application, typically the external envelope of the building, through the structural system and ultimately to the ground, thereby ensuring the building remains upright and secure.

For the MWFRS wind loads calculation, ASCE 7-16 presents two methods: The Directional Procedure, which is widely used for all building types, and the Envelope Procedure, which specifically applies to low-rise buildings and is the primary focus of this article.

The following table outlines the steps required to determine the MWFRS wind loads using the envelope procedure:

• Step 1: Determine the risk category of the building
• Step 2: Determine the basic wind speed, $V$
• Step 3: Determine wind load parameters:
  • Wind directionality factor, $K_d$
  • Exposure category
  • Topographic factor, $K_{zt}$
  • Ground elevation factor, $K_e$
  • Enclosure classification
  • Internal pressure coefficients, $GC_{pi}$
• Step 4: Determine velocity exposure coefficient
• Step 5: Determine velocity pressure
• Step 6: Determine external pressure coefficient, $GC_p$, based on Figure 28.3-1 for flat and gable roofs
• Step 7: Calculate wind pressure, $p$

Figure 3: An example of how external pressure coefficients are calculated for MWFRS loads with enclosed and partially enclosed buildings with low-rise walls and roofs. (Reference)

Wind Load Analysis for Components and Cladding

Components and cladding systems pertain to the exterior elements of a structure, such as roofs and walls. These elements directly interact with the wind and are subject to different wind pressures.

While the MWFRS ensures overall structural stability against wind loads, the significance of components and cladding in wind load analysis arises from their crucial role in withstanding localized wind pressures, safeguarding the building's integrity, and protecting its occupants.

There are distinct differences in the design considerations for the MWFRS and components and cladding systems. While the MWFRS is designed to counter wind loads from any direction, ensuring overall structural stability, the components and cladding system are primarily evaluated for wind loads that are normal or perpendicular to the surface.

According to the ASCE 7 guidelines, similar to the MWFRS, calculating wind loads for components and cladding systems starts with determining the risk category and basic wind speed.

However, additional factors come into play in the case of components and cladding systems. The effective wind area, which is the area that the wind load is distributed over, is a key factor in these calculations.

The external pressure coefficients, $GC_p$, an example given in Figure 4, also come into consideration, determined by the specific component's shape, angle, and location on the building.

Figure 4. An example of the external pressure coefficients for components and cladding system with enclosed or partially enclosed walls and flat, gable, hip, or monoslope roofs. (Reference)

Similarly, the pressure coefficient values would differ for windows and doors, which often serve as pressure release points during extreme wind events. Their design must withstand not only the direct wind loads but also the rapid changes in pressure caused by gusty winds or the event of a sudden windstorm.

The following table outlines the steps required to determine the components and cladding wind loads:

• Step 1: Determine the risk category of the building
• Step 2: Determine the basic wind speed, $V$
• Step 3: Determine wind load parameters:
  • Wind directionality factor, $K_d$
  • Exposure category
  • Topographic factor, $K_{zt}$
  • Ground elevation factor, $K_e$
  • Enclosure classification
  • Internal pressure coefficients, $GC_{pi}$
• Step 4: Determine velocity exposure coefficient
• Step 5: Determine velocity pressure
• Step 6: Determine external pressure coefficient, $GC_p$
  • Walls
  • Flat roofs, gable roofs, hip roofs
  • Monoslope roofs
• Step 7: Calculate wind pressure, $p$

Conclusion

Wind load analysis is an essential aspect of structural engineering that cannot be overlooked if we want to ensure the stability and safety of buildings against wind forces.

In the United States, the ASCE 7 guideline is the go-to resource for wind load calculations. This guideline takes into account several factors, including wind speed, wind directionality factor, exposure category, topographic factors, ground elevation, and building enclosure.

For residential or light commercial projects, wind load analysis is typically used for two systems: the main wind-force resisting system (MWFRS) or components and cladding systems. By conducting a thorough wind load analysis, we can guarantee the safety and durability of buildings in the face of harsh wind conditions.

Frequently Asked Questions about ASCE 7

What is the difference between ASCE 7-16 and ASCE 7-22?

The ASCE frequently revises its guidelines to incorporate recent advancements in research, technology, and industry practices. The update from ASCE 7-16 to ASCE 7-22 reflects these changes.

The International Building Code (IBC) incorporates ASCE 7-16, which was published in 2016, in its latest 2021 edition.

Looking ahead to the upcoming 2024 edition of the IBC, it is highly likely that it will continue to adopt ASCE 7 as the primary reference for design loads. The IBC typically aligns with the most recent edition of ASCE 7-22 to incorporate the latest advancements and research in structural engineering.

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