Floating Columns.

 Just like the topic of the blog suggests, the columns float. Well, not literally. It's just that these types of columns do not follow the typical path of load transfer in a building. The columns are in-fact rested on a beam or slab. So, they look as if those columns are suspended or hanged. 

A floating column.

Principle of a Floating Column.


Well, the question then goes. Why do one need a column like that, isn't the simple geometry base plan enough for a building? The answer would be NO as per the Architects. What these columns give them is free reign over a room layout or any space in particular. Imagine a 3X3 grid column, a typical plan would consist of 4 small rooms, but if you remove the centre column you get one huge hall. I think I pretty much explained why Architects like it. They love it.

But as the quote goes, " An Architect's Dream is an Engineer's Nightmare ".

It's not exactly a nightmare, not the floating columns at least. But it's not typical either. The column is nothing special for us. It's the member which will support the column. You can see in the above pic it's rested on a Beam, a huge one in that. Those are called Transfer Beams.

Here's a preview. This is a shear reaction in a normal beam. 

Shear in the 1-2 Plane.

Min Shear (-ve) in the 1-2 Plane.


We can see that the shear reaction is larger at the end and decreases along the centre, as it generally is. The textbook example. The beam is constrained at the two ends which gives rise to shear. 


Now here's the reaction in a transfer beam. 

Shear in the 1-2 Plane.

Min Shear (-ve) in the 1-2 Plane.

Here we see an abrupt change in the shear distribution. One might even say there's an error in your model if they didn't know prior. This is due to the reaction transferred from the column. The column having no other support transfers all the loads into the beam. This leads to a steep change in the Shear force diagram in the beam. The point of magnitude change is where the column is planted. Now the beam has to be designed accordingly.


Comparison between the two beams.


It's not just the shear distribution diagram that has changed. Even the magnitudes have increased a ton, not literally. So the Design procedure changes compared to a normal Beam.  As the loads tend to be higher, the beam sizes increase too. It gives rise to Deep Beams. Technically, it has a definition but imagine a beam with depth more than 1.5 meters. 

For us Structural Engineers, this is the cool part. A Deep Beam does not follow the Euler-Bernoulli principle of Bending. The whole design philosophy of a normal beam depends on this principle. Thus it is logical to opt for different procedures. STRUT and TIE method can be used to design a deep beam. 
Strut and Tie method will be left for another blog.
In a software, it's easier to design. We design it for a particular set of loading combinations with the transfer member modifiers all set to UNITY.

Takeaway : Always check the reactions in the model for proper understanding of the behaviour of a structure. It's free and quite interesting too.

PS: My friend Aditya sent me those images from his model as I didn't have the license then. 




Comments

Post a Comment

Popular posts from this blog

Ritz and Eigen.

Image
Ritz, not Rizz and Eigen Vector are two ways of solving a Modal Analysis problem. It is what ETABS use while doing the Modal Analysis. Today's blog will be about the difference between these two. EIGEN-VECTOR : It determines the undamped free-vibration mode shapes and frequencies of the system. What free-vibration means in our term is that there is no external loads acting on it for any period of time, just the initial conditions. The idea of bending a ruler and allowing it to reverberate. The natural modes obtained through this provide an excellent insight into the behaviour of the structure.  But remember, it's the free-vibration response while the structures will be under various different loads. What should be done then? Here comes the Ritz Vector. RITZ-VECTOR : Research has indicated that the natural free-vibration mode shapes are not the best basis for a mode-superposition analysis of structures subjected to dynamic loading : Wilson, Yuan and Dickens, 1982. I stole this l...
Read more

Modifiers.

Image
This is one of the coolest concepts I have properly learnt at my job. I do know I will learn lot more on this. It's the Stiffness Modifiers.  It is a modification factor that is generally used in the properties of cross-section such as Moment of Inertia, Shear Area, Torsional Constant etc. It can also be used for Mass and Weight as per requirement. Modifiers for a typical Beam. Now why is it used? As the loads act on the structure, it will tend to generate cracks on the tension zone. Thus, reducing the area of cross section and stiffness of the member. 1 means the structure would take all of the loads, while 0 won't be taking any. It is kind of a paradox; the structure would resist all the loads if it is not cracked, and we know it will crack. In other way, we want the structure to not take the load as it will eventually crack. Concrete is weak in Tension. As per codes, IS Code ACI Code Now I have two great examples for putting my case. One is related to a torsion modifier I st...
Read more

Slab On Grade in ETABS.

Image
First let's know what an SOG is. The full form is Slab on Grade/Ground. One use for it to create an equal level for construction. Imagine a large area foundation with minimal area at a lower level. It is easier to provide reinforcement in a single plane; hence the lower level can be raised by filling it with soil or some other substances. Then it acts as one level foundation. I came across a scenario where I had to model an elliptical tank like structure, no columns in between. It was to be filled with soil. As you can see, I have attached an image, the slab span is quite large in between. The plan of the model. Imagine an ellipse with R1 = 37 m and R2 = 65 m. Now if you run the model as a normal slab, which it kinda is, one can easily guess the response. You can guess with experience that is, or a different case if you are a genius. I will tell you how the response would look like if you run it as a normal slab. You would get lots of local modes (I am assuming you know what local ...
Read more