33.16.2. Demand-Capacity Ratios

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.

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.

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.

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.