Why Performance-Based Seismic Design?

Earthquakes can easily disrupt a city in a matter of seconds. In the early 20th century earthquakes were not properly understood and there were not many building codes or even documentations that could help engineers in designing buildings. Look at 1906 Mexico earthquake, 1940 El Centro earthquake,1930 Irpinia earthquake in Italy and many more. We learnt from these earthquakes how they impact a structural system and tried to understood why a building collapses.

But even after that, if we look at 1985 Michoacan earthquake, 1994 Northridge earthquake and 2001 Gujarat earthquake there were so many structural collapses. The reason being in certain parts of the world, new failures were detected that caused structural collapses and in other parts of the world like India, we came to know how demanding a ground motion can be on the buildings! We continued to evolve as experts found new failure patterns in the buildings that we did not think of before! But why do earthquakes get so much attention? Because people die!

Now we are at a point where we know close to many things as in how 99% of the buildings behave with a 10% probability of collapse during a major earthquake. Why do we say 10% again? Because, a ground motion can be stronger than expected through the design life of a building. There are high chances that the building will never see a single earthquake during its entire life. So it does not make sense to design a building for an earthquake that can occur once in 10,000 years. We design buildings for an earthquake that has a return period of about 500 years, this reduces the design demands on buildings and the construction costs do not increase significantly.

In the green segmented line of the chart (for the Occupancy category I and II; e.g., common residential and office buildings), we can see that a structure is generally designed for life safety at the design earthquake intensity level (as depicted by the middle green square). The same building can be designed for collapse prevention for maximum considered earthquake intensity level (as depicted by the lower right side green square). However, if the building owner wants us to design the building for a life safety level (instead of collapse prevention level) and a maximum considered earthquake intensity, then it is also possible to accomplish this structure with Performance Based Design. The building will have a higher performance than intended by the building code. In fact, if the owner of the building wants us to design for an earthquake with a 5,000 year return period, it is also possible to do that with PBD.

Blue segmented line represents occupancy category III (e.g., schools, colleges, day care facilities) buildings and black segmented line represents occupancy category IV (e.g., hospitals, fire stations) buildings require more stringent criteria in case of an earthquake. For example, a common residential building is designed for life safety under design earthquake intensity level as discussed above, but a hospital will be designed for immediate occupancy for the same earthquake intensity. A PBD is also possible for the hospitals as well as schools and colleges with more stringent criteria and more rigorous review procedures. In short, it is possible to do Performance Based Design of any building of any height until it satisfies some basic code criteria.

All the provisions in the code are developed considering the minimum life safety factors associated to the building for design earthquake intensity level. But in case of a tall building (which falls outside of code provisions) if the design is only considering design earthquake and linear response spectrum analysis, engineers may miss out on many project specifics that would have been captured doing a nonlinear dynamic time history analysis.

Another challenge while designing tall building is the higher mode effects of the structure. Because the higher mode effects of a structure can be project specific and they are not appropriately captured using basic Response Spectrum analysis, it is important to analyze the buildings using more sophisticated numerical analysis methods. In other words, Performance based design means designing a structure according to its behavior and the acceptable level of performance criteria by doing nonlinear time history analysis. PBD is not only limited to tall buildings. If we find a very odd shaped structure or an innovative structure by an architect, we can definitely do performance based design for these “out of the box” structures too. In those cases, structural engineers make sure that the structure is stable and is designed using maximum considered earthquake (MCE) intensity level as shown in the cart above.

What difference does it make if we do performance based design?

Every building shape has different properties in terms of the lateral force resisting system (LFRS), building weight, soil properties as well as well as the location from the seismic fault. Because of these differences, the behavior of the structure will be different with varying earthquake intensity levels. Some earthquakes may shake tall buildings violently while other earthquake may shake shorter buildings severely. This nature of building response to an earthquake will be captured if we do nonlinear time history analysis. Identifying and assessing the capability of the building is an integral part of the performance-based design process. So here are some key points that are covered while doing a performance-based design:

• A building is tested to site specific ground motions which again depend on the soil properties of the site and seismicity of the region.
• Each structure is tested with 22-44 different ground motion pairs on the basis of near fault and far fault effects as well as the nature of source and hazard type is taken into account.

• Actual material strains and cracking of concrete is measured beyond the elastic limits.
• Building is tested under maximum considered earthquake.
• Building is tested under low intensity frequently occurring earthquakes also known as service level earthquake intensity and it is made sure that the building stays operational.
• In PBD, we not only test the building by performing nonlinear dynamic time history analysis, but we also do the code-based design under design level earthquake intensity to satisfy the criteria specified by building codes.
• If requested by the owner, the project team can discuss alternate building designs with more stringent criteria.

Are there any benefits to owners if they opt for Performance Based design?

Here is the thought, building codes are designed to encapsulated different types of building responses, in some cases such building design can be more conservative for seismic demands, and it is not the fault of the building code. Every time we try and generalize a concept to a wide variety of buildings, then yes, there will be some conservatism for certain types of buildings. Vertical rebars in tall buildings is also something similar. With Performance based seismic design we can also show that there is very little vertical reinforcement demands in the building down at the base, while the demands towards the top increases. This again is a result of higher mode effects. Doing these type of analysis in certain countries like India can reduce the construction cost of the buildings.

Not only that, but in countries like USA where tall buildings require perimeter moment frames which are expensive to construct and block beautiful city views, can be eliminated making a more simpler, lighter and cost effective buildings. The only thing it increases is the design and analysis complexity. But hey, that is why we are here for. We can do this rigorous analysis which is based on real life tests, with the right data added into the computer model. We do this type of analysis on the basis of scientifically proven research and make sure that the building will stand even after this high intensity earthquake.

Today, with the advancement of technology, everyone looks for quality of, not just the construction work, but also the engineering that goes behind it. Look at Amazon, it looks for customer satisfaction every single time and because of that it is growing fast and it is everyone's favourite. Same thing can be done in construction. Let us build buildings with enough reliability and accuracy that it not only reduces costs of construction but it also leads to a more efficient and safer tall building design in the seismic prone regions of the world.

Reach us out if you are interested in learning more about PBD and we can help you find a better solutions as well. In the end, we believe in a taller and a safer future.

Thank you for taking your time in reading this post. I hope you enjoyed our little talk. Have a nice day!