11.4 Design considerations for isolated structures
4 min read•july 25, 2024
is a game-changer in earthquake engineering. It allows buildings to move independently from the ground, reducing earthquake forces. This approach requires careful design of the , superstructure, and substructure to work together effectively.
Key parameters like , damping, and displacement capacity are crucial. These factors, combined with soil-structure interaction and code requirements, shape the design process. While initially more expensive, seismic isolation can offer long-term economic benefits through improved performance and reduced damage.
Design Considerations for Isolated Structures
Design considerations for seismic isolation
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Top images from around the web for Design considerations for seismic isolation
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- Isolation system characteristics enhance seismic performance
- allows structure to move independently from ground motion
- reduces earthquake forces transmitted to superstructure
- supports gravity loads and resists overturning moments
- adapts to isolation effects
- Reduced seismic forces allow for lighter, more economical structural members
- Increased require flexible connections (expansion joints)
- accommodates isolation system
- include larger footings to resist overturning (spread footings)
- ensures proper load transfer and movement (bearing pads)
- Clearance and displacement considerations prevent damage
- allows for structure movement without impact (moat)
- Utilities and connections use flexible components to accommodate displacement (flexible pipes)
- influence long-term performance
- Temperature effects cause and contraction of isolators (elastomeric bearings)
- induce additional lateral forces on isolated structures (wind dampers)
- Long-term affects (UV radiation, ozone exposure)
Parameters of isolation systems
- Isolation period determines effectiveness of system
- Typical range: 2-4 seconds elongates natural period of structure
- Factors influencing selection include:
- Site conditions affect ground motion characteristics (soft soil, bedrock)
- Building height and mass impact dynamic response (tall buildings, heavy structures)
- reduces displacement and acceleration response
- Common values: 10-30% of critical damping
- Types of provide energy dissipation:
- use fluid resistance (hydraulic cylinders)
- rely on sliding interfaces (Friction Pendulum bearings)
- combine elastomeric isolation with lead core damping
- Displacement capacity ensures system functionality
- (MDD) accounts for design-level earthquake
- (TDD) includes additional safety margin (torsional effects)
- Factors affecting displacement:
- determines ground motion intensity (PGA, spectral acceleration)
- influence ground motion amplification (site coefficients)
- Isolator properties control stiffness and energy dissipation (hysteretic behavior)
- Isolation system stiffness impacts overall response
- () represents secant stiffness at design displacement
- () affects bilinear behavior of system (lead-rubber bearings)
Soil-structure interaction effects
- phenomena alter seismic response
- filters high-frequency ground motions (foundation embedment)
- modifies dynamic characteristics of structure (foundation flexibility)
- Influence on isolated structure response can be significant
- increases beyond isolation period (soft soil sites)
- Increased damping reduces overall system response ()
- decreases high-frequency content of input motion (large foundations)
- Soil conditions impact isolation effectiveness
- increases long-period ground motions (Mexico City effect)
- reduces effectiveness of isolation (rock sites)
- capture complex behavior
- use equivalent springs and dashpots (NIST GCR 12-917-21)
- Advanced numerical techniques employ (SASSI, OpenSees)
- affected by SSI require careful evaluation
- may increase or decrease depending on site conditions
- can be amplified or reduced (period elongation effects)
- may change due to modified dynamic characteristics
Codes for isolated structures
- provides general requirements
- Chapter 17: Special Inspections and Tests outlines quality control measures
- offers detailed design provisions
- Chapter 17: Seismic Design Requirements for Seismically Isolated Structures
- provides guidance and commentary
- Chapter 12: Seismically Isolated Structures explains design concepts and procedures
- Design procedures vary based on structure complexity
- for simple, regular buildings
- for more complex structures
- for critical or irregular buildings
- Testing requirements ensure system performance
- verifies isolator properties and behavior (full-scale tests)
- confirms production consistency (durometer hardness tests)
- establish minimum performance standards
- Isolator performance limits (maximum displacement, minimum restoring force)
- Structural response limits (story drift, floor acceleration)
Economic feasibility of seismic isolation
- Initial cost considerations may increase project budget
- Isolation system components add material costs (bearings, dampers)
- Additional structural elements accommodate isolation system (moat walls)
- Increased design and engineering efforts require more time and expertise
- Long-term benefits offset initial investment
- Reduced structural damage lowers repair costs after earthquakes
- Improved equipment and content protection preserves valuable assets (hospitals, data centers)
- Business continuity and downtime reduction maintain revenue streams (critical facilities)
- quantifies economic impact
- calculations account for future benefits
- estimation helps justify implementation
- Factors influencing economic viability vary by project
- determine potential benefits (essential facilities)
- Seismic hazard level affects likelihood of significant events (high seismic zones)
- and practices impact implementation feasibility
- Case studies and performance data support decision-making
- Post-earthquake assessments demonstrate real-world effectiveness (Christchurch Hospital)
- Comparative analysis with conventional structures highlights benefits (USC Hospital)
- Insurance implications can provide additional incentives
- Potential premium reductions due to lower risk profile
- Coverage enhancements for isolated structures (business interruption insurance)
Key Terms to Review (58)
Acceptance Criteria: Acceptance criteria are the specific conditions that must be met for a structure to be considered safe and satisfactory under certain loading conditions, particularly during seismic events. These criteria help in evaluating whether the design meets safety standards and performance objectives, guiding engineers in ensuring that structures can withstand earthquakes and other forces without excessive damage or failure.
ASCE 7: ASCE 7 is the standard for minimum design loads for buildings and other structures, developed by the American Society of Civil Engineers. It provides essential guidelines for assessing the impacts of various loads, including seismic forces, which are crucial for ensuring safety and performance in the design of structures in earthquake-prone areas.
Base Shear: Base shear is the total horizontal force that a building or structure experiences during seismic events, primarily caused by ground motion. It is crucial for understanding how a structure will respond to earthquakes, as it influences the design and analysis of structures, ensuring they can withstand seismic forces while maintaining stability.
Base slab averaging: Base slab averaging is a design technique used in seismic isolation systems that helps distribute seismic forces more evenly across a structure's foundation. By averaging the stiffness and mass of the base slab, this method reduces the impact of localized stresses during an earthquake, ultimately enhancing the overall stability and performance of isolated structures. It plays a crucial role in the effective design of buildings in earthquake-prone areas, contributing to improved safety and resilience.
Building Importance and Occupancy: Building importance and occupancy refers to the classification of a structure based on its intended use and the potential consequences of its failure during an earthquake. This classification helps engineers determine the design parameters and level of seismic safety required for the building, ensuring that critical facilities can withstand seismic events and minimize risk to occupants.
Damping devices: Damping devices are mechanical systems used to reduce vibrations and oscillations in structures, particularly during seismic events. They help absorb and dissipate energy, thereby minimizing the amount of movement transferred to the building. By controlling vibrations, these devices enhance the safety and stability of isolated structures during earthquakes.
Damping Ratio: The damping ratio is a dimensionless measure that describes how oscillations in a system decay after a disturbance, indicating the relationship between the system's damping and its natural frequency. It provides insight into the stability and response characteristics of both single-degree-of-freedom and multi-degree-of-freedom systems under dynamic loading, including earthquakes. A higher damping ratio leads to reduced amplitude of vibrations, which is crucial for understanding how structures respond to seismic events and design safe buildings.
Displacement demands: Displacement demands refer to the amount of lateral movement or shift that a structure experiences during seismic events, which is crucial for evaluating its performance under earthquake loading. Understanding displacement demands is essential for designing structures that can adequately resist seismic forces and remain functional after an earthquake. This concept plays a significant role in developing innovative structural systems, designing isolated structures, and assessing how site conditions can amplify seismic effects.
Effective stiffness: Effective stiffness refers to the overall stiffness of a structural system, taking into account the behavior of seismic isolators and their interaction with the superstructure during seismic events. It plays a critical role in determining how structures respond to ground motion, influencing both design and performance during earthquakes. Understanding effective stiffness helps engineers assess how isolated structures can better absorb and dissipate seismic energy, leading to improved safety and stability.
Energy dissipation capacity: Energy dissipation capacity refers to the ability of a structure to absorb and dissipate energy during seismic events, minimizing the amount of energy transferred to the structure itself. This concept is crucial for ensuring that seismically designed structures can endure the dynamic loads imposed by earthquakes without suffering catastrophic failure. Effective energy dissipation mechanisms, such as damping systems or structural configurations, are essential for enhancing a structure's resilience and overall performance in seismic conditions.
Environmental Factors: Environmental factors are conditions or influences that affect the physical, social, and economic aspects of a particular setting. In the context of isolated structures, these factors can include site-specific geological conditions, climate, surrounding infrastructure, and human activities that may impact the structure's performance and safety during seismic events.
Equivalent Lateral Force Procedure: The Equivalent Lateral Force Procedure is a simplified method used in structural engineering to estimate the lateral forces that a building or structure may experience during an earthquake. This approach translates the dynamic effects of seismic activity into equivalent static forces, allowing for easier analysis and design, particularly for isolated structures where traditional methods may not be as effective.
FEMA P-751: FEMA P-751, or 'NEHRP Recommended Provisions: Design Examples,' is a document published by the Federal Emergency Management Agency that provides guidelines and design examples for seismic-resistant structures. It aims to help engineers and architects design buildings that can better withstand earthquakes, incorporating principles of seismic isolation and energy dissipation to enhance structural performance during seismic events.
Finite element analysis: Finite element analysis (FEA) is a computational technique used to approximate solutions to complex structural engineering problems by breaking down structures into smaller, manageable elements. This method allows engineers to assess how structures respond to various loads and conditions, facilitating the design of safe and effective systems. FEA is particularly important in understanding ductile behavior, optimizing advanced numerical modeling techniques, and addressing design considerations for isolated structures.
Floor accelerations: Floor accelerations refer to the rate of change of velocity experienced by the floors of a structure during dynamic loading events, such as earthquakes. This measure is crucial as it influences how forces are transmitted through the structure and affects the overall response during seismic events. Understanding floor accelerations helps engineers design buildings that can withstand these forces effectively, leading to safer structures in earthquake-prone areas.
Foundation requirements: Foundation requirements refer to the essential conditions and specifications that must be met for the successful construction and performance of building foundations, particularly in seismic zones. These requirements ensure that the foundation can adequately support the structure above while minimizing potential damage during an earthquake, maintaining stability and safety.
Friction dampers: Friction dampers are devices used in structures to absorb and dissipate energy during seismic events by converting kinetic energy into thermal energy through friction. They enhance the performance of buildings by reducing the forces transmitted to the structure during earthquakes, making them crucial for improving safety and stability. The application of friction dampers helps to minimize structural damage and enhance the overall resilience of buildings against seismic activity.
Fundamental period elongation: Fundamental period elongation refers to the increase in the fundamental period of a structure when it is subjected to dynamic forces, such as those experienced during an earthquake. This elongation is important as it indicates how a structure's natural frequency can change in response to modifications, such as isolation systems, which help in reducing seismic forces by increasing the period of vibration.
Horizontal flexibility: Horizontal flexibility refers to the ability of a structure to deform laterally during seismic events without sustaining damage, allowing it to absorb and dissipate energy. This characteristic is crucial in seismic design, especially when using isolation techniques that aim to minimize the transmission of earthquake forces to the superstructure. Structures designed with adequate horizontal flexibility can better withstand lateral forces, which contributes to overall safety and performance during earthquakes.
Inertial interaction: Inertial interaction refers to the forces and movements that occur between a structure and the seismic waves generated during an earthquake. This interaction is crucial in determining how a building responds to ground motion, as it dictates how forces are transmitted through the structure and how it deforms or fails under seismic loads. Understanding inertial interaction is essential for analyzing structural failures, evaluating the performance of seismically designed structures, and ensuring the effectiveness of isolation techniques that mitigate earthquake impacts.
International Building Code (IBC): The International Building Code (IBC) is a set of regulations developed to ensure safe and sustainable construction practices across various types of buildings. It addresses structural integrity, fire safety, accessibility, and energy efficiency, promoting consistent building standards in different regions. The IBC plays a crucial role in the design and construction of isolated structures, as it provides guidelines that help mitigate risks associated with seismic activity and other environmental factors.
Isolation interface detailing: Isolation interface detailing refers to the specific design and construction practices used to create effective separation between a building structure and its foundation, particularly in seismic isolation systems. This detailing is crucial as it allows the building to move independently from ground motion during an earthquake, minimizing damage and improving safety. Key aspects include the materials used, connection methods, and the arrangement of isolators, all of which contribute to the overall performance of isolated structures under seismic loads.
Isolation period: The isolation period is the time interval during which a structure, such as a building or bridge, remains disconnected from ground motion due to the use of isolation systems like base isolators. This period is crucial as it influences how the structure responds to seismic activity, allowing for reduced forces and increased safety by decoupling the building's motion from that of the ground.
Isolation system: An isolation system is a structural engineering technique used to reduce the transmission of seismic forces from the ground to a building or structure during an earthquake. By incorporating flexible elements, such as bearings or pads, between the structure and its foundation, these systems allow for relative movement that minimizes the impact of ground shaking. This method helps protect the integrity of the building and ensures the safety of its occupants.
Isolator displacements: Isolator displacements refer to the relative movements between a structure and its base isolation system during seismic events. These displacements are crucial for understanding how the structure can maintain stability and minimize damage, as they directly impact the effectiveness of the isolation system in decoupling the building from ground motions. Properly designed isolator displacements can ensure that the forces transmitted to the building are significantly reduced, ultimately protecting the structure and its occupants.
Isolator Properties: Isolator properties refer to the characteristics of devices used in seismic isolation systems that help protect structures from earthquake forces. These devices, such as rubber bearings or sliding isolators, are designed to allow a building to move independently from ground motion, thereby reducing the transmission of seismic energy. Understanding these properties is crucial for the effective design and implementation of isolated structures, ensuring they can withstand the impacts of seismic events while maintaining safety and functionality.
Kinematic Interaction: Kinematic interaction refers to the influence that the movement and deformation of one structure can have on another nearby structure during seismic events. This phenomenon can lead to additional forces and displacements in structures that are not directly subjected to ground motion, which can exacerbate damage and affect overall performance during earthquakes. Understanding kinematic interaction is crucial for analyzing past structural failures, assessing the effectiveness of seismic designs, and considering isolation strategies in engineering.
Lead-rubber bearings: Lead-rubber bearings are a type of seismic isolator designed to absorb and dissipate energy during an earthquake, allowing buildings to move independently from ground motion. These bearings consist of layers of rubber and a lead core, which enables them to provide flexibility and damping, reducing the amount of seismic forces transmitted to a structure. This unique combination of materials helps protect buildings from damage during seismic events, emphasizing their importance in modern earthquake-resistant design.
Lifecycle cost analysis: Lifecycle cost analysis is a method used to evaluate the total cost of ownership of a project or asset over its entire lifespan, including initial costs, operating costs, maintenance costs, and disposal costs. This approach helps decision-makers understand the long-term financial implications of their choices, particularly in the context of design and construction, enabling more sustainable and cost-effective solutions.
Local construction costs: Local construction costs refer to the expenses associated with building projects in a specific geographical area, influenced by factors such as labor rates, material availability, and regional regulations. These costs can significantly affect the overall budget of construction projects, especially for isolated structures that require unique design considerations due to their location and potential seismic activity. Understanding local construction costs is essential for project planning and risk management in earthquake engineering.
Material degradation: Material degradation refers to the deterioration of a material's properties over time due to various factors such as environmental conditions, mechanical stress, and chemical reactions. This process can significantly affect the structural integrity and performance of isolated structures, making it essential to consider when designing for earthquake resilience. Understanding how materials degrade helps engineers select appropriate materials and protective measures to enhance the longevity and safety of structures under seismic events.
Maximum design displacement: Maximum design displacement refers to the anticipated maximum lateral movement that a structure will experience during an earthquake, specifically calculated to ensure the safety and functionality of the building. This concept is crucial for designing isolated structures, as it helps engineers determine how much movement can be expected and how the building should respond under seismic loads. By incorporating this metric, engineers can devise systems that protect the structure from excessive displacement and potential damage.
Modeling approaches for ssi: Modeling approaches for soil-structure interaction (SSI) refer to the various methods used to analyze and predict how structures respond to seismic activities while considering the effects of the surrounding soil. These approaches help engineers understand the dynamic behavior of structures during earthquakes, ensuring that designs are robust and capable of minimizing damage. Accurate modeling is essential in designing isolated structures to optimize their performance and enhance safety during seismic events.
Net Present Value (NPV): Net Present Value (NPV) is a financial metric that calculates the difference between the present value of cash inflows and outflows over a specified time period. It helps in assessing the profitability of an investment or project by considering the time value of money, which means that a dollar today is worth more than a dollar in the future due to its potential earning capacity. In the context of isolated structures, NPV plays a crucial role in evaluating the economic feasibility and long-term benefits of implementing isolation technologies to reduce seismic risks.
Nonlinear time history analysis: Nonlinear time history analysis is a computational method used to evaluate the response of structures under dynamic loads, such as earthquakes, by accounting for material and geometric nonlinearity. This approach simulates real seismic events by applying recorded ground motion data over time, allowing for a more accurate prediction of how structures will behave during extreme loading conditions. It is particularly useful for isolated structures, as it helps to assess their performance and safety during seismic events while considering the complexities of nonlinear behavior.
Performance metrics: Performance metrics are quantifiable measures used to evaluate the efficiency and effectiveness of a system, process, or structure in achieving desired outcomes. In the context of isolated structures, these metrics help assess how well a design minimizes the impact of seismic events, ensuring safety and functionality during and after an earthquake. By utilizing these metrics, engineers can make informed decisions about the design and implementation of isolation systems to enhance resilience.
Post-yield stiffness ratio: The post-yield stiffness ratio is a measure that quantifies the stiffness of a structure after it has undergone yielding, indicating how much it can still resist deformation compared to its initial stiffness. This ratio is crucial in assessing the performance of isolated structures during seismic events, as it helps determine how well a structure can maintain stability and absorb energy after reaching its yield point.
Prototype testing: Prototype testing refers to the process of evaluating a preliminary version of a product or system to assess its performance, functionality, and design before full-scale production or implementation. This method is crucial in earthquake engineering as it helps identify potential weaknesses and optimize the design of isolated structures, ensuring that they can effectively reduce seismic forces during an earthquake.
Quality Control Testing: Quality control testing refers to the systematic processes used to ensure that materials, components, and systems meet specified requirements and standards throughout their production and implementation. This involves regular inspection and testing of materials and structures, especially in the context of isolated structures, to verify their performance under seismic loads and ensure they will behave as intended during an earthquake.
Radiation damping: Radiation damping refers to the phenomenon where the energy from a vibrating or oscillating system is lost to the surrounding medium, typically through the emission of waves or vibrations. This process plays a crucial role in understanding how structures respond to dynamic loads, particularly during earthquakes, by influencing the overall behavior and stability of soil-structure systems, assessing structural failures, and determining the effectiveness of isolation techniques.
Response Spectrum Analysis: Response spectrum analysis is a method used in seismic engineering to evaluate how structures respond to seismic ground motion. This technique generates a response spectrum, which represents the peak response (such as displacement or acceleration) of a series of oscillators of varying natural frequencies to a specific earthquake. It connects directly to various aspects of structural analysis, design methodologies, and compliance with seismic codes, highlighting how buildings and components behave during seismic events.
Return on Investment (ROI): Return on Investment (ROI) is a financial metric used to evaluate the efficiency and profitability of an investment, calculated by dividing the net profit of an investment by its initial cost. In the context of isolated structures, understanding ROI helps in assessing the benefits gained from implementing isolation techniques against their costs, emphasizing the importance of balancing safety and economic viability in structural design.
Seismic Gap: A seismic gap refers to a segment of a fault line that has not experienced an earthquake for a prolonged period, indicating a potential for future seismic activity. These gaps are significant as they suggest areas where stress is accumulating, making them critical in understanding earthquake risks and planning for structural safety. Identifying seismic gaps can help engineers and scientists assess the likelihood of future earthquakes and inform design strategies for structures that may be affected.
Seismic hazard level: Seismic hazard level refers to the assessment of the potential for ground shaking and related effects at a specific location due to earthquakes. This level is determined based on geological, seismic, and engineering factors, which help in understanding how likely an area is to experience seismic activity and the intensity of that activity. It plays a crucial role in designing structures that can withstand earthquake forces, especially for isolated structures that rely on specific design considerations for safety and performance.
Seismic isolation: Seismic isolation is a design technique used in earthquake engineering that involves the incorporation of flexible bearings or isolators between a building's foundation and its superstructure. This method allows the structure to move independently from ground motion, significantly reducing the amount of seismic energy transmitted to the building. The goal is to protect the building and its occupants during an earthquake by minimizing structural damage and enhancing safety.
Simplified methods: Simplified methods refer to design techniques that streamline the analysis and design processes for structures, particularly in earthquake engineering, allowing engineers to estimate seismic responses without extensive computational modeling. These approaches are valuable because they reduce complexity, save time, and enable quicker assessments while maintaining adequate safety levels for isolated structures against seismic forces.
Soft soil amplification: Soft soil amplification refers to the phenomenon where seismic waves experience an increase in amplitude when they travel through soft, unconsolidated soils. This effect can lead to greater shaking intensity and damage during an earthquake, especially for structures built on such soils. The interaction between seismic waves and the mechanical properties of soft soils can significantly influence the behavior of buildings and infrastructure in seismic regions.
Soil conditions: Soil conditions refer to the physical and mechanical properties of soil that influence its behavior under loads, especially during seismic events. The characteristics of the soil, such as its composition, density, moisture content, and layering, significantly affect how structures respond to earthquakes. Understanding soil conditions is crucial for designing isolated structures that can effectively mitigate seismic forces and ensure stability.
Soil-structure interaction (SSI): Soil-structure interaction (SSI) refers to the complex relationship between a structure and the underlying soil during dynamic events, such as earthquakes. This interaction influences how both the soil and structure respond to loading, affecting structural performance and stability. Understanding SSI is crucial for the design of isolated structures, as it helps engineers determine how forces are transferred between the soil and the structure, impacting overall safety and resilience.
Stiff soil deamplification: Stiff soil deamplification refers to the phenomenon where seismic waves experience reduced amplification when they propagate through stiff or dense soil layers compared to softer soil. This effect is crucial in understanding how ground motion is affected by different soil types, especially when designing isolated structures that need to withstand earthquakes while minimizing forces transferred from the ground.
Substructure Design: Substructure design refers to the planning and engineering of the part of a building or structure that is below ground level, including foundations and any supporting elements. It plays a critical role in ensuring stability and load transfer from the superstructure to the ground, especially in isolated structures where minimizing seismic forces is essential for performance during earthquakes.
Superstructure design: Superstructure design refers to the process of designing the upper portion of a structure, which includes all components above the foundation. This aspect of construction is critical as it ensures that the building can support its intended loads and withstand various forces, including seismic activity. The design must consider factors like materials, architectural features, and the interaction with the underlying foundation system to create a stable and resilient structure, particularly for isolated buildings that aim to minimize seismic forces.
Thermal expansion: Thermal expansion is the tendency of materials to change their dimensions when exposed to changes in temperature. As the temperature increases, the particles within a material move more vigorously, causing them to occupy a larger volume. This phenomenon is crucial in engineering design, particularly in ensuring that structures can accommodate movements without leading to damage or failure.
Total Design Displacement: Total design displacement refers to the maximum horizontal movement that a structure is designed to accommodate during seismic events, taking into account both the inherent flexibility of the isolation system and the expected ground motion. This concept is crucial for ensuring that isolated structures can absorb and dissipate energy generated by earthquakes, preventing structural damage and enhancing safety.
Utilities connections: Utilities connections refer to the essential services such as water, electricity, gas, and telecommunications that are linked to a structure. These connections are crucial for the functionality and livability of a building, especially in the context of isolated structures where minimizing disruption during seismic events is paramount. Proper design and integration of utilities connections ensure that they do not compromise the structural integrity or performance during an earthquake.
Vertical rigidity: Vertical rigidity refers to the ability of a structure to resist vertical deformations and deflections under load. It is a crucial characteristic that affects the structural behavior of buildings, particularly in how they respond to forces such as gravity and seismic activity, influencing design decisions for isolated structures.
Viscous dampers: Viscous dampers are devices designed to dissipate energy from structural vibrations, particularly during seismic events, by converting kinetic energy into thermal energy through the flow of a viscous fluid. They play a crucial role in enhancing the performance of structures by reducing motion and stresses, thereby contributing to overall stability and safety during earthquakes. By absorbing energy, they help protect both the structure and its occupants from excessive movements that could lead to failure.
Wind loads: Wind loads refer to the forces exerted on structures due to wind pressure and are critical for ensuring the stability and safety of buildings and other structures. Understanding wind loads is essential for structural design, as these forces can significantly impact the integrity and performance of isolated structures during extreme weather events. Engineers must consider factors such as wind speed, building height, and shape to accurately assess the wind loads acting on a structure.