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Performance Objectives
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Today, we'll talk about performance objectives in seismic design. Can anyone tell me what performance objectives are?
Are they the goals we set for how a structure should behave during an earthquake?
Exactly! We categorize them into three levels: operational safety, life safety, and collapse prevention. Can anyone explain what operational safety means?
It means the building should function normally after a minor earthquake, right?
Correct! Now, life safety aims to prevent loss of life even if the building suffers moderate damage. What do you think collapse prevention focuses on?
It’s about ensuring the structure doesn’t collapse during a major quake, keeping the occupants safe.
Exactly! Let’s remember these with the acronym OLC: Operational, Life safety, Collapse prevention. Great job, everyone!
Demand-Capacity Ratios
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Now, let's discuss demand-capacity ratios. Who can explain what this concept involves?
Is it about comparing how much demand a structure faces and what it can actually handle?
Exactly! We estimate the demands through the design spectrum. Why is it important to assess these ratios?
To ensure the building can withstand seismic forces without failing.
Right! If the demand exceeds the capacity, we need to rethink our design to meet our performance objectives. Can someone remember the tools we use to evaluate our capacity?
Pushover analysis!
Correct! Let's solidify that knowledge with the mnemonic 'DCR' for Demand-Capacity Ratio. Nice work, class!
Introduction & Overview
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Quick Overview
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It elaborates on performance objectives such as operational safety, life safety, and collapse prevention, while discussing how design spectra can be adapted based on these objectives. Demand-capacity ratios are introduced as a method to estimate and assess structural responses against predefined capacities.
Detailed
Use of Design Spectrum in Performance-Based Design
In the context of seismic engineering, design spectra play a crucial role in defining performance objectives for structures. These objectives, ranging from ensuring operational safety to achieving full collapse prevention, guide engineers in designing structures that can withstand seismic events with varying intensities.
Performance Objectives
Performance objectives can be categorized into three main levels:
1. Operational Level: This includes designs that allow minor damage to buildings so they can function normally after a small earthquake.
2. Life Safety Level: Structures designed at this level may sustain moderate damage but are expected to prevent loss of life during significant earthquakes.
3. Collapse Prevention Level: At this stage, the primary goal is to avoid structural collapse despite extreme seismic forces, ensuring safety for occupants.
Demand-Capacity Ratios
The demand-capacity ratio is a tool used to compare the demands placed on a structure (estimated through the design spectrum) with its capacity to resist those demands (assessed through methods like pushover analysis). By evaluating these ratios, engineers can determine if the structure meets the defined performance objectives, providing a systematic approach to performance-based design.
This section underscores the importance of adapting design spectra to meet specific performance criteria, ensuring both safety and functionality in the face of dynamic seismic loads.
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Performance Objectives
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33.16.1 Performance Objectives
- Operational (minor damage)
- Life safety (moderate damage)
- Collapse prevention (major damage)
Design spectra are modified based on performance levels and hazard exceedance probabilities.
Detailed Explanation
Performance objectives set the goals for how a structure should behave under seismic loads. They are categorized into three main levels:
1. Operational - This implies that the structure experiences minor damage that does not affect its functionality. An example could be a school where small cracks appear but students can continue classes.
2. Life Safety - At this level, moderate damage is acceptable, but the structure must protect the occupants from serious harm. For example, an office building may require minor repairs after an earthquake but is safe for occupancy.
3. Collapse Prevention - This is the most critical level, where the structure must remain standing during a severe earthquake to prevent loss of life, even if it becomes unusable afterward. Think of a hospital where ensuring that the building is still standing during an earthquake is vital for emergency services.
The design spectrum is modified to ensure that these performance levels are met based on the probability of a certain earthquake happening in the area, indicating how often these performance levels should be able to withstand seismic events.
Examples & Analogies
Imagine a children's playground designed for safety. The swings might be intended to be safe for everyday use (operational), the climbing frame should be resilient enough for children to play without risk (life safety), and structures like tall slides must be designed to tolerate extreme weather or seismic events without collapsing (collapse prevention). Just like the playground is designed with safety in mind at various usage levels, buildings are designed with different performance objectives depending on their use and the likelihood of earthquakes.
Demand-Capacity Ratios
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33.16.2 Demand-Capacity Ratios
- Spectrum used to estimate demand
- Compared against structural capacity curves (pushover analysis)
Detailed Explanation
Demand-capacity ratios are crucial for assessing a structure's ability to handle seismic loads. This process involves two main concepts:
1. Demand - This refers to the seismic forces that the structure is expected to experience, represented by the design spectrum. Engineers use this spectrum to analyze how much load the building might endure during an earthquake.
2. Capacity - This represents the ability of the structure to resist these forces, determined through analysis methods like pushover analysis. This method involves subjecting the structure to increasing lateral forces until it reaches its limit, allowing engineers to see where it might fail.
To ensure safety, the demand placed on the structure during an earthquake must not exceed its capacity. Engineers often express this relationship as a ratio: if the demand-capacity ratio is above 1, the structure may not withstand anticipated seismic events and require enhancements.
Examples & Analogies
Think of a tightrope walker trying to perform acrobatics with a safety harness. The 'demand' is the stress experienced by the rope when the performer shifts their weight, while the 'capacity' is the weight limit of the harness. The tightrope walker aims to stay well within the limits of the harness to ensure safety. Similarly, in building design, engineers need to ensure that the forces expected during an earthquake do not exceed the structural limits to keep the building safe.
Definitions & Key Concepts
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Key Concepts
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Performance Objectives: Guidelines for structural behavior during and after an earthquake.
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Demand-Capacity Ratios: Method of evaluating if structures can withstand expected seismic demands.
Examples & Real-Life Applications
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Examples
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An office building designed for operational performance may allow minor repairs after a small earthquake, allowing it to remain functional.
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A hospital built under life safety objectives ensures that even in a major quake, it can be evacuated safely.
Memory Aids
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🎵 Rhymes Time
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When the quake shakes, keep the stakes; operational means minor aches.
📖 Fascinating Stories
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In a busy city, a hospital stands strong after every quake, ensuring that patients are treated even when the ground shakes. They designed it for life safety, ensuring no one is lost in the chaos.
🧠 Other Memory Gems
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OLC for performance: Operational, Life safety, Collapse prevention.
🎯 Super Acronyms
DCR
- Demand over Capacity Ratio.
Flash Cards
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Glossary of Terms
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Term: Performance Objectives
Definition:
Goals set for how structures should behave during seismic events, categorized typically into operational safety, life safety, and collapse prevention.
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Term: DemandCapacity Ratio
Definition:
A comparison of the demand placed on a structure versus its capacity to withstand that demand.