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Slab On Grade in ETABS.

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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 ...
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Preconsolidation Pressure

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A lazy day for me at the office. Started to read topics I kept on hold. Stopped at the theory manual of Settle3. It's a software for Geotechnical Engineers. Not really my forte. Got into the topic of Preconsolidation Pressure.  Now what is it? One could define it as a memory pressure for the soil. It represents the maximum effective stress experienced by the soil in the past. It's the Newton's third law of motion.  Now you don't have to understand the graph completely. I don't get it too. But I am here to explain my part.  First understand what a Consolidation is. It is the gradual release of water in the soil when weight is applied to it, causing the soil particles to reorganize and settle.  It helps in creating a solid foundation stability for the structures, minimization of settlements and also improves the soil strength.  Consolidation causes the application of force (weight). Preconsolidation pressure is the reaction to it. It is the maximum effective press...
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Loads in Structures.

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Define Loads: The amount of weight carried by a structure, as per Cambridge Dictionary. Loads are what we design the structures for. So, it is imperative to work on it carefully. Defining loads is usually a one-time thing which is sorted out at the start of a project, but there could be changes. And as an Engineer, it's always important to understand the know-how.  In the Construction Industry, there are certain specified load patterns. These are Dead, Live, Wind, Snow, Earthquake and Special Loads. Now I won't go around explaining the different kinds of loads in this blog. It is quite vast. What I would be writing about is the way to incorporate these loads in the software.  First you have to define the Load Patterns . It looks as below. Load Patterns Here, you can define the types of loads. You can see there are lots. One should know the different load types while defining one. For example, Dead denotes the self-weight of the elements of a structure while Super Dead load con...
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Engineering vs True Strain

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As Engineers, we like to assume. I didn't appreciate it much earlier, but I will tell you why we do that.  Most of the formulas we study during our Bachelors, well I have only completed my Bachelors yet, there's always some assumptions mentioned before a formula, Always.  Thin Beam Model Critical Buckling Load Now why do we assume if it's not the actual response. It makes our life easier, by that I mean the calculation time and effort for getting into something. And it gets the job done. As I earlier once wrote in my blog : Acceptable within the Engineering world, due to certain circumstances. Does it mean we are wrong, NO! Most of the assumptions give the limits too. One such assumption is the Engineering Strain .  A comparison. Engineering Strain is a straight line which indicates that the strain in a structure varies linearly although that is far from actual scenario. In actual life, or per experimentation, the strain doesn't vary linearly. It sorts of curve down as ...
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Buckling of a Bucket

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A lazy Sunday afternoon on my apartment balcony, a friend sitting beside me on a bucket. Conversing rubbish most probably. As he got up to leave, the bucket he was sitting on crumbled. A normal occurrence with thin buckets. See, in my practice, that crumbling of bucket is called buckling. And rightly so, buckling came to my mind. So, what of it? Not that I have to design the bucket to resist my friend's weight. But you might agree with me on my intuition.  The formula for critical buckling load is quite eye catching. It's one of the easiest to remember, if you get what I mean. So, as you might have guessed it by now. My quest began to find the elastic modulus of the plastic.  First, I got a measuring tape to measure the dimensions. A bucket with a smaller diameter of 23 cm, large diameter of 32 cm and a Height of 32 cm. The bucket in question.  Those are actually the diameters and not radius as I wrote on my stickons. Taking pinned connection at the bottom, as it is not f...
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Debugging a Model.

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The ill-condition error is the one we dread the most. One which can take days to solve, if we ever find it. Now I won't be able to help you with clearing this particular error at a snap. There are certain modelling techniques I follow to ensure an error free model. While modelling the structure, connecting all the elements node to node is a must. This is an FEA software, you need the transfer of reactions whether it be loads, deflection or moments. As per my experience in RC structures, most of the errors/warnings can be solved through this. Minimize use of the option of all floors or use with caution. It helps when others work on your model too. There goes the modelling basics, now let's get to the part of debugging. There are various kinds of errors, I might not remember all of it while I am writing this blog but I will try to answer the best I can. Well for starters, standard solver would be the best. Although it takes much longer to analyse, it does give the error locations...
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Size Factor.

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As per tests, shear strength of a concrete does not increase in direct proportion with member depth. As per ACI, this phenomenon is called the Size Effect. Do mind that it is the contribution of Concrete only, and not including its reinforcement.  What this means is, a concrete block with X depth does not have twice the shear resistance of block with depth X/2.  As per ACI 318-19, How this acts in terms of one way and two shear can be seen from the code books. It's more dominant  in two way shear, a heads up. You can see from a simple calculation that the size effect factor of a 1000mm thick reduces the shear resistance of an element to around 63% of the original strength. But wait, if that's the case, won't the requirement for footings which take all the loads from the structure be too much. Clause 13.2.6.2 to the rescue. It permits the negligence of size effect factor for foundations. What a miracle. No it isn't. It's all calculated that's what I believe.  Now...
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Long Term Deflection due to creep.

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Structures are built to last. Most modern structures have a life expectancy much more than a regular person. So it's only natural to assume Engineers designed for it. We do design checks in this regard too.  It was not always a thing though, at least that's what I believe. And I will tell you why I think so. Civil Engineering is the oldest branch and has gone through some vey serious updates. My trade initially started with the Working Stress Method. It is also called the traditional method. In this, the design is based only within the elastic limit. Which means, the materials used for construction behaves in a linear elastic manner. You know how an elastic rubber works, it won't break unless stretched through its capacity. The philosophy is similar. As long as it is within the limit, it won't fail. But to make the structure not reach its limit, the materials required was much more. This, at times, leads in very un-conservative designs. This Method is good for critical ...
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Floating Columns.

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 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 typ...
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Ritz and Eigen.

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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...
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Draftsmen/Drafter.

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Draftsmen draft. Just kidding. They do draft but they are better than that, well! Some are. The need for a draftsmen might be meek in an ideal European sense, with all the automations and sort. Even without the automations, a Structural Engineer is expected to be able to do the drafting himself/herself.  But I don't live in Europe, do I? And the need also arises due to the sheer scale of some projects. The main task of Engineers is to design and analyse the structures. And it does take time. Hence, the need for draftsmen. Now what does a Draftsmen do. They draft. They are expected to be good in drafting softwares, whether it be 2D, 3D or even 4D. Though I have never been a part of a 4D project YET. Some general softwares can be the CAD lineup, Revit, Dynamo, Sketchup and the list goes on.  They should have the knowledge as how it all works. Some cases are not always general, and they have to come up with alternatives, just like Engineers in this case. For example, I had to imp...
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Modifiers.

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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...
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