## Posts

### 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 opera singers…

### What is a Response Reduction factor?

In our previous blogs we discussed about Response Spectrum AnalysisEarthquake 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 verification…

### 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 defines the bro…

### 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 dissipation (wh…

### 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 acceleration time history. (A r…

### Response Spectrum Analysis - I

"Response" is what an object, living or nonliving, gives to a certain situation. It can be passive response or active response but it doesn't matter. What matters is how the buildings respond to earthquakes. Let us say for an example, you take a pendulum and you pull it to a certain height and leave it for free oscillation. What you will see is the mass attached at the end of the pendulum moving back and forth constantly, such motion is simple to understand as it is clearly visible to your eyes. The time a pendulum takes to complete one oscillation is called the time period of a pendulum. This factor, "time period" is very important characteristic of a building, it can tell you everything about the building, every ins and outs. Coming back to pendulum and time period, when you plot the motion of pendulum with respect to time it will give you a sinusoidal curve called the response of the pendulum to the disturbance. It will look something like this:

Angle descr…

### Taiwan Earthquake 2016

Every time an earthquake strikes, we learn something new from it. As a structural engineer it is always difficult to explain people as why a building collapsed. The only reason is we cannot predict earthquakes. But wait I am not done here, yet..!! Let us take a deep breath and jump into the possibilities of failure of buildings in the city of Tainan which is located just at a distance of 48km (30 miles) from the epicenter. Let us take a look at USGS map of the affected zone.

Now the earthquake was a very strong earthquake of category 7 which describes the wave velocity of 20 cm/s and a peak acceleration of 0.22g. One more thing to keep in mind is that the earthquake was a shallow earthquake which means that the intensity of the earthquake might have amplified.
Talking about the building collapses in Tainan, we cannot directly blame at design engineers and the only reason I am saying this is because Taiwan has been famous for earthquakes and not small intensity but the large intensit…