### Arch Bridge..!! When? Where? & Why?

Want to travel across a valley? Want to cross an ocean? Want to span from one end to another? Whatever might be the need, bridge is required for that need. But it is a preliminary decision taken on the basis of budget, location, type of soil and surrounding to built a particular type of bridge. This can result in a most commonly used simply supported bridge or a suspension bridge or a cable stayed bridge or an arch bridge. Today we will be discussing about arch bridges. How they are designed? Where they shall be used? Why they are used?

First let us ask a basic question to ourselves. What is an arch? Arch is a member which takes majority of the stresses in compression while spanning a gap. The definition of a column and arch is very near to each other except the last words "spanning a gap". (Yeah, these last two words created majority of the complications in structural engineering). It can be of any shape, a parabolic, catenary, straight lines, etc. But how can we decide the shape?

What happens when you load a cable? It takes all the load in pure tension and it is because of it's flexibility. This gives the most efficient design possible. So that is why if we give a similar shape to arch for a particular loading you will find only compression acting in the arch. Isn't that awesome? We will not be dealing with bending and shear forces anymore..!! But wait that is not possible. We cannot give any desired shape to an arch. Why? Because it is in compression. Take the example of case 1. A point load. What will be a funicular shape? Two straight lines connected at the location of the load. Now imagine the length of the member if the bridge is spanning for 100m. The effect of slenderness will come into picture. So you will not build such shape in such cases except modifying them by adding braces and everything. Well it all depends on the span that the bridge need to cover.

Now, where can we can see one important reason that why arches aren't built everywhere. Arches exhibit a horizontal reaction at it's ends and this reaction is immense. In order to resist this reaction we need strong foundation, like a rocky mountain. If you are planning to design it on a river, than you need strong piles which can resist shear equivalent to the horizontal reactions and this task becomes pretty tedious. So that is why arch bridges are found across very deep valleys or places with strong rocks at the ends. This is where we construct the arches.

Well this was about where they are used and why. Next time we will learn how to design an arch. Thank you.

### Possible types of failures in a steel structure

We, structural engineers design all the members of a building, whether it might be a column, beam, a tie member or a strut anything, but we design it to resist certain forces. We predict a load, calculate forces in different members and design them member to resist a particular load. But sometimes because of some undetermined or unpredicted load the forced in certain members increase to a value which it cannot withstand and the LObmember fails. But what are the different possibilities of failure? How can a member fail? Don't worry, here is what we are going to talk about. The possible types of failures in steel structures. Steel is a ductile material and to build a structure using steel is like setting up a huge Jigsaw puzzle. You have 1000 different members and you need to connect them and tada..!! Your structure is up. But it is not as simple as it is visible. Steel being a very strong material  leads to slender members. Now you can imagine the difficulties associated with it

### 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

### Ductility and Elasticity

Ductility and elasticity,the two most important terms that are discussed frequently in structural engineering. Elasticity defines about how much the material is elastic, that is to which extent the deformations are proportional to the forces applied on the material. While ductility defines the capability of the material to get itself stretched beyond the elastic zone. Let me explain this by taking a real life example. Take a two different material, a rubber band and a very thin steel or copper wire.  Pull the rubber with your hands by applying the force in exactly opposite direction, and force means a tiny amount of pull. You will notice that the mount of deformations caused by the small pull is very large, but when you leave the rubber band it will come back to it's original position. This means that the rubber band is elastic in nature. Oh, now you got something in your bucket. But wait, here comes the question. Till what magnitude of force can rubber band behave in such