Modifiers.

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 stumbled upon during Beam Design. There are two types of Torsion in a structure: Equilibrium and Compatibility Torsion. 

Equilibrium Torsion.

Compatibility Torsion.

From the two pictures above, one can easily say that the torsion in the case of Equilibrium torsion would be much more. Whereas, Compatibility distributes the torsion into other beams. So, we can say that the first case would have to resist more loads than the later. Hence, we give higher modifier to the former beam. My company uses a torsional constant of 0.1 for the later case, though I have to study why that is so.

Another is a case by the Steel guys, I am an RCC guy (Reinforced Cement Concrete) in the company. But I like to roam around whenever I am free. In their case they had to design a certain system of Truss. And their model had deck/slab over their trusses. You won't be walking on a truss now, would you? As the deck had been modelled, it will take some of the loads. It is also a path for the loads now. What they did was set the Shear Area Modifiers to Zero. Now the software will not allow the deck to take any Shear Force. This allows all the loads to pass only through the truss system. And it can be designed accordingly. Isn't that interesting?

Among the two, Steel guys play more with their Modifiers. It is all properly understood and calculated first though. The software will give results for any condition you provide, it's just a super fast calculator.

One other example is the PT Beams. PT stands for Post-Tension. Now we can't model a PT beam on ETABS and SAP. So what we do is give higher than 1 stiffness constant to the Beam, we assume it will be taking more loads than it can. Sounds absurd, right? But wait, we do so matching the deflection in that area with another software called RAM. RAM designs PT structures.

I took this image from the internet, but this is how it looks like for a PT beam.

Always open for discussions.

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