Stiffness modifier for beam, column, wall, slab

[structech08]

Hello,

What is the importance of stiffeness modifier for the beam, column, slab , wall etc to ETABS analysis and section design? What is the purpose of using the stiffness modifier?

Why is it that I get more reinforcement when no stiffness modification is applied to the columns, beams, wall and slab? When I put the stiffeness modifier as per ACI 318 requires, I got lesser area of steel reinforcement for the beams and columns.

Thank you for your response.

Structech08

 

[slickdeals]

Stiffness modifiers are used to account for cracking in concrete. It is the nature of concrete to crack and hence, it is important to modify its stiffness to accurately get deflections and P-Delta moments.

In a building with shear walls and beams/columns (which I presume is your case), cracking the columns/beams will reduce their stiffness and more moments/shears are transferred to the shear walls based on relative stiffness.

Stiffness attracts force. As a result, if your column/beam is relatively less stiff than the shear wall, it will have less moments and shears. This will result in lesser reinforcing.

Hope that makes sense. Post if you have other questions.

 

[structech08]

Hi slickdeals,

I really appreciate your response, your explanation is technically clear.

Thank you so much.

structech08

 

[zstone]

Dear friends

Regarding to fema 356
It is required to reduce also stifness properties of walls
For flexure 0.5 to 0.8 EI, and for shear 0.4EA,
My question is how to put this effect into walls by stifness modifiers,
Which parameters i need to reduce,
Because if i reduce f11, f22, or f12 i affect also axial rigidity ( which i dont like to affect axial rigidity because by fema 356 it will bee 1.0EA)
If i reduce m11, m22, m12 i affect out of plane bending stifness of plate, which also is not in my.

Kind Regards

 

[zstone]

Or i maybe need to reduce stifness only at beams and columns.

Many Thanks

 

[jenofstructures]

Is it based from the stiffness values stated on ACI?

Poems are made by fools like me, but only God can make a tree. engineers creates wonderful buildings, but only God can creates wonderful minds

 

[zstone]

Right
Both FEMA and ACI propose values for this purpose,
But how to include this values into walls,
For example
FEMA recomend to reduce Flexural rigidity of walls by 0.8EcIg
My question is: how can include this effect into ETABS through stifness modifiers,or any other way

maybe this values are available if you model walls as column elements ( onedimensional elements)

Regards

 

[slickdeals]

1. You can change Ig (if required) using the shell stiffness modifiers.

2. You can change Ec using material properties (specify 0.8E as your concrete Ec)

That way what you end up with is 0.8 Ec Ig

 

[zstone]

Dear friends, thank you for your response

It is a little bit a problem.
If i reduce modul of elasticity i affect all rigidities Flexural, shear and axial

Exactly im trying to do what fema 356 - chapter 6 - concrete - Table 6-5 - Effective Stiffness values recomend:

Walls uncracked ( on inspection)
Flexural Rigidity 0.8 EcIg, Shear Rigidity 0.4EcAw, Axial Rigidity EcAg

Walls - cracked -
Flexural Rigidity 0.5 EcIg, Shear Rigidity 0.4EcAw, Axial Rigidity EcAg

Now if i reduce modul of elasticity , i affect also axial rigidity and reducing values for flexure and shear are not same , which value to reduce E, with 0.8 or 0.4.

Regards
ZG