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# Staad & Concrete Analysis + Design - Stiffness?

## Staad & Concrete Analysis + Design - Stiffness?

(OP)
I understand that Staad uses the moment distribution method for beams, a stiffness matrix for frames, and FEM for plates.

I'm trying to better understand how Staad works for concrete.

My core Concrete questions are:

1) How does Staad arrive at it's default assumed isotropic Young's modulus of 435,600 ksf? Is this a composite value that assumes some percentage of steel is in the concrete?

2) It appears, that Staad uses the full 100% moment of inertia for concrete sections, but I have heard talk that the software makes adjustments by assuming the concrete is cracked and uses a reduced MOI. Is this true?

The company I work for pays for all sorts of training and support from Bentley as part of our Staad/Microstation license, but I can't make heads or tails of their website. I have looked on Bentley's Connection Client & "LearnServer"; along with various other product support pages. Their website is incredibly confusing though.

Thanks!

### RE: Staad & Concrete Analysis + Design - Stiffness?

#### Quote:

1) How does Staad arrive at it's default assumed isotropic Young's modulus of 435,600 ksf? Is this a composite value that assumes some percentage of steel is in the concrete?

I'm not sure. I use a user input value (calculated by the code equation).

#### Quote:

2) It appears, that Staad uses the full 100% moment of inertia for concrete sections, but I have heard talk that the software makes adjustments by assuming the concrete is cracked and uses a reduced MOI. Is this true?

Whenever I use reinforced concrete members in STAAD, I use user defined, "general" member properties. That lets you define the moment of inertia and other properties.

For slabs, I adjust the Elastic constant to reflect the cracked properties. (You can several different sets of material properties in the same model, for the same material.)

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