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Which buildings will survive earthquake, Low rise or High rise?

Someone asked me a question on Quora about the survival of high rise vs a low rise building during a seismic event. I tried to do my best to explain which one can fall over and in the end, it depends on the design and analysis type.  The probability of failure of a structure depends on the way it was engineered. It is not as easy as it sounds or feels. Designing a structure to resist earthquakes lies under a lot of assumptions. Straight from the analysis approach, assumptions in code and probability of earthquakes. A building might collapse if: 1. Ground motion or ground acceleration or intensity of an earthquake is higher than what is calculated. A design level earthquake is the one that has the probability of exceedance of 2% in 50 years. Why 50 years? Because that is the assumed life of a building. If we have an earthquake stronger than design level earthquake, then there are chances that a building might collapse. 2. Low ductility in the building. If the building

What do Opera singers and Structures have in common?

Have you ever heard or watched an opera singer perform on stage? The way they break a wine glass just with the help of sound waves is amazing. Wait, I just said waves, and you know what kind of properties waves have. One is the frequency of wave and the other is amplitude. Now frequency of the waves will tell me how many waves will pass through a point in one second. If you want to visualize it, then take a tub of water and throw a small stone in it with one of your finger far away in the tub from where you threw the stone. Now, count the number of waves that pass next to your finger in a second. This number of waves passing through the finger per second is known as frequency of waves. Wine shown for illustrative purposes only. In reality there is no wine and empty glass only as wine will dampen the vibration of glass. Before we proceed I would recommend you to watch this video explaining resonance, damping and dynamic amplification. Now let us go back to sound waves and

What is a Response Reduction factor?

In our previous blogs we discussed about  Response Spectrum Analysis ,  Earthquake and Energy Dissipation  as well as  Ductility demand in structures during seismic loading . In response spectrum analysis topics like mode shapes, modal mass participation factors, derivation of response spectrum we discussed. In earthquake vs energy dissipation blog, we talked about energy dissipated from buildings through strain energy, inelastic energy, hysteresis, damping and ductility. In ductility demand we discussed about importance of ductile detailing and how it helps a building to work during earthquakes just like a marathon runner during long runs.  Generally inelastic energy dissipation, damping energy, ductility demand and ductility capacity, hysteresis loops are all captured when a nonlinear model is built, and time history analysis is performed for the structure. But to do nonlinear time history analysis, it takes a long time to build a model. The performance evaluation and result ve

Response Spectrum Analysis - II

In the blog post about Response Spectrum Analysis I , I tried to describe response of the system, but one comment on the post rightly pointed out as the concept of spectrum was not described clearly, so in this post we will be talking about the concept spectrum and how it is generated, what it signifies and how is it helpful for engineers to understand the dynamic forces acting on the building. Now, if you have not read the previous part of response spectrum analysis, I would strongly recommend you to read it. In that part we discussed about what is response of a system under free vibrations and forced vibrations. In this post, I will be talking about spectrum and what to do with this spectrum. Now what actually is a spectrum? Let us go back to 6th or 7th grade when we were studying about rainbows and prim, how light is made of different colors. That band of color is known as spectrum if you recall. Spectrum defines a broad variety. Similarly the word  Response Spectrum  defi

Ductility In Structures

Have you ever participated in a marathon? Or have you ever practiced for one? The only way you can finish a marathon is high stamina and a constant comfortable pace. If you run too fast, the energy in your body will drain out quickly and you will get tired within first 10% of the run. But if you maintain your own comfortable speed, where you heart beat is not too high then you could finish the marathon without even getting too tired. The key is glycogen stored in the body. On an average a person can store up to 2000 calories of energy that is readily available and then once this glycogen is consumed body starts burning fats to get energy which leads to fatigue. The key in marathon is to consume this glycogen efficiently and so you have to run at a comfortable pace.  Image credits: HALHIGDON Similar to this concept, in structural engineering, a building has to perform under large forces without breaking itself to pieces. As we discussed about different sources of energy d

Earthquake vs Energy Dissipation

 In this blog post, I will try to explain in simplest words possible as to what happens to a building during an earthquake.  Imagine this: You are travelling in a car which I am driving. I applied a sudden brake. You will experience a serious "jerk" and most probably curse me after that. But that sudden "jerk" also called as deceleration is experienced because your body changed the velocity from 100 feet/second to dead zero in 3 seconds. That is equivalent to gravitational acceleration applied in horizontal direction. This is called inertial force. There was a change in state of motion of your body.  "A body at rest stays at rest and a body in motion stays in motion unless an external force is applied to change the state of body" - Newton's first law of motion.  A force experienced by a body is equal to: Force = Mass x Acceleration When earth shakes, it gives these impulses at the base of the building. Let us look at an accelerat