33.16.2 - Demand-Capacity Ratios
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Introduction to Demand-Capacity Ratios
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Today, we are discussing Demand-Capacity Ratios. Can anyone tell me what they think a demand-capacity ratio is?
Is it about comparing the forces acting on a structure to what the structure can actually withstand?
Exactly! The Demand-Capacity Ratio helps us understand if a structure can endure the demands placed on it during an earthquake.
How do we determine those demands?
Demands are typically estimated from seismic design spectra. We will elaborate on that shortly.
Establishing Structural Capacity
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To find the capacities of a structure, we often perform pushover analysis. Can anyone explain what that involves?
I think it’s a way to see how much a structure can take before collapsing, right?
Right! It involves applying loads to the structure incrementally until we reach its failure point.
So, once we have that capacity, we compare it to the demand?
Yes! This comparison results in the Demand-Capacity Ratio.
Interpreting Demand-Capacity Ratios
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Now, let’s talk about how to interpret the Demand-Capacity Ratio. If the ratio is less than one, what does that mean?
It means the structure should be safe under expected conditions?
Exactly! But if it's greater than one, that means the structure is at risk under those conditions.
So what can we do if the ratio is too high?
Good question! Engineers may need to design for higher capacities, alter the structural system, or improve the material used.
Practical Applications of Demand-Capacity Ratios
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Why do you think Demand-Capacity Ratios are vital in performance-based seismic design?
They help ensure structures can perform safely during earthquakes?
Exactly! It's a key element in performance-based design to ensure that buildings meet safety standards.
Are there set guidelines on what ratio is acceptable?
Yes, building codes provide guidelines for acceptable limits to ensure safety.
Introduction & Overview
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Quick Overview
Standard
This section explains Demand-Capacity Ratios as a method for estimating the demands that seismic loads impose on structures and compares these demands with the actual capacity of those structures, particularly through pushover analysis. It highlights the importance of understanding how structures can withstand earthquakes effectively.
Detailed
Demand-Capacity Ratios
In seismic design, understanding the relationship between the demands placed on a structure during an earthquake and the capacity of that structure to withstand those demands is critical for ensuring safety. The Demand-Capacity Ratio (DCR) provides a numerical value that describes this relationship. It is a measure obtained by comparing the estimated demand (derived from seismic design spectra) against the structural capacity curves derived from pushover analysis.
This approach enables engineers to assess whether a structure can adequately resist earthquakes. If the DCR is less than or equal to one, it indicates the structure can handle the expected demands; if greater than one, it points towards a potential failure under expected seismic loads. The use of DCR is particularly significant in performance-based design methods, where the structural response to different levels of seismic hazard is evaluated.
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Understanding Demand-Capacity Ratios
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Chapter Content
Spectrum used to estimate demand
Compared against structural capacity curves (pushover analysis)
Detailed Explanation
This chunk introduces the concept of Demand-Capacity Ratios (D/C ratios), which are essential in assessing the performance of structures during seismic events. The demand is represented by the spectrum, which estimates how much force or displacement the structure will experience during an earthquake. The capacity is determined by structural capacity curves obtained from pushover analysis, a method that describes how much load a structure can withstand before it fails.
Essentially, the D/C ratio is a measure of how much demand (expected forces or displacements) is placed on the structure compared to how much capacity (resistance to these forces) the structure has. If the demand exceeds capacity, there is a risk of structural failure during an earthquake.
Examples & Analogies
Imagine your car’s fuel gauge as the demand and the size of your fuel tank as the capacity. If the gauge indicates you're almost empty (high demand) but your tank is small (low capacity), you're likely to run out of fuel. In earthquake engineering, if the 'gauge' (demand from seismic forces) exceeds the 'tank' (capacity of the structure), it suggests the structure may fail under load.
Key Concepts
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Demand-Capacity Ratio: A measure indicating if a structure can withstand seismic loads compared to its capacity.
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Pushover Analysis: An established technique to analyze how structures respond to increasing loads until failure.
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Seismic Design Spectra: A crucial aspect of earthquake engineering, representing expected loads on a structure.
Examples & Applications
A building with a Demand-Capacity Ratio of 0.85 indicates it is adequately designed to withstand earthquake loads.
If a bridge's Demand-Capacity Ratio is 1.5, it suggests the structure requires reinforcement to avoid failure in seismic events.
Memory Aids
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Rhymes
When DCR's low, safety will glow; when it's high, with caution we sigh.
Stories
Imagine a bridge on a busy highway — it stands firm under the stress of cars (demands) while knowing it can handle everything with grace (capacity).
Memory Tools
DCR: Demand vs. Capacity Ratio – remember it's essential for safety.
Acronyms
DCR
Demand-Capacity Relationship in Seismic Safety.
Flash Cards
Glossary
- DemandCapacity Ratio (DCR)
A measure comparing the estimated seismic demand on a structure against its capacity to withstand such demands.
- Pushover Analysis
A method used to determine the capacity of a structure by applying a lateral load until failure occurs.
- Seismic Design Spectra
Graphs that represent how different structures will respond to seismic forces based on various parameters.
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