SAP2000 - Displacements for Reinforced Concrete Streuctures 3

[Hark88]

Hi everyone, could you help with a small inquiry? I'm trying to understand how SAP2000 works.

How does Sap2000 find the displacements of a reinforced concrete structure, let's say a concrete wall (shell)? As soon as the analysis is done sap gives you the displacements, right. But since you haven't specified the reinforcement (which actually you want to find by using sap) the stiffness will then be unknown. Hence, you cannot find displacements without the stiffness.

What I is that sap automatically uses the minimal reinforcement to give the displacements OR it includes the reinforcement that it has found after the analysis; still not clear!

Because in the end, when you get the displacements, how do you know which amount of reinforcement are those displacements relevant to?

 

[Hark88]

Or to put it in another way:

E(reinforcement)+E(concrete)=E(composite) - E is the modulus of elasticity

Only when you know E you can do static analysis (including finding displacements).

But if you do not know the amount of steel you cannot find the E(composite).

My question is: which is the amount of steel that sap uses for doing the analysis?

 

[PicoStruc]

Stiffness is function of EA/L or EI/L^3 only for frame and similar equation with thickness for shell (See finite element stiff formulation)

Rebar amounth is REALLY negligeable on stiffness (NOT resistance)

 

[IDS]

Rebar amounth is REALLY negligeable on stiffness (NOT resistance)

That's just wrong.

The flexural stiffness of a cracked concrete section is highly related to the reinforcement content.

I don't know SAP2000, so I can't comment on how it handles it, but if it is doing an automated design I presume it follows the process specified in the applicable code. If you want to get a reasonable estimate of deflections you need to use cracked section properties (where applicable), allowing for the effects of tension stiffening, shrinkage and creep, but that isn't necessary for strength design, so its a two stage process.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rapt]

I think you find that SAP2000, being an FEM analysis program, is simply calculating elastic deflections based on the concrete elastic modulus. It is not accounting for transformed section properties, cracking, creep or shrinkage.

 

[Hark88]

Thank you for your answers, it is always very helpful.

I think that that might be the case, sap2000 does just an elastic analysis of the structure and for that might use just the modulus of concrete. I also checked the .$2k file of sap and during the analysis only the E of concrete was used.