Shell Stiffness Modifiers 1

[Murali27]

To All

As per ACI code, the initial stiffness modifiers are to be assingned for wall/slab/column/beam structural elements.

Slab - 0.25 Ig
Wall - 0.70 Ig
Beam - 0.35 Ig
Column - 0.7 Ig

In ETABS

Slab -> Assigned with shell element -> M11, M22 & M12 as 0.25

Wall -> Assigned with shell element -> f11, f22,f12 are related to in-plane forces.

Do we need to apply 0.7 factor to f11, f22 & f12 (axial & shear area) ? M11,M22 & M12 are insignificant since they affect out of plane bending forces.

Please clarify this point.

Thanks
Murali G

 

[johnjeed]

As I understand the intention of the stiffness modifiers is to account for reduced stifffness of cracked structural elements. I would have thought that the flexural stiffness of members (i.e. bending properties) only would need to be modified.

 

[Murali27]

Yes. Johnjeed.

You are correct. But in shear wall, the be section will undergo cracking due to in-plane moment.

How to assign stiffness modifiers for this condition? f11, f22 & f12 - 0.35Ig. But there are also related to axial stresses.

How to go about?

Thanks

 

[WillisV]

Typically I initially assign a 1.0 factor to f11, f22, and f12 along with a 0.01 factor to m11, m22, and m12 (so that the shearwall essentially has no out of plane stiffness and acts as a classical in-plane only resisting element). I then run the model and check axial stresses against the modulus of rupture to see if the assumption that the wall is uncracked (f11,f22,f12 = 1.0) was correct. For those portions of the wall that are cracked, I reassign with the lower 0.7 crack factor to f11,f22, and f12.

 

[1Quique]

Try and see. Model one slab of 240x240in, 4 beams and columns. Modify m22=.01, m12=.01 (don't use zero). Load slab with 0.4096 Kip-ft. Mesh it 6x6. Run model. See M33 on beams. See M11 vs. M22 on slabs. Good luck

 

[Murali27]

Hi IQuique

Once you change the m22 modifier, the load is going to be transfered in X direction i.e along local axis 1. Please let me know what is your point here.

Hi WillisV

For serviceability studies, lower factor 0.7 crack factor would be ok. But for ultimate state (design purpose), 0.35 would be appropriate.

Please explain with respect to ACI 10.11.1

Thanks
Murali

 

[WillisV]

Murali - agreed my crack factors shown are for the serviceability/deflection analysis of the walls.

 

[1Quique]

My point of view: Modifiers are very useful for driving the concrete behavior. Modifiers, concrete, steel bars, plans details and constructor technology are necessary guests to obtain the structure born in our mind. Then, initial modifiers suggested for ACI 10.11.1 must be adjusted when we know something about amount of bars on concrete. Another important question. Take in mind that walls are only connected to structure through 4 poits (or 3). In my example is impossible think in modifier m22=.01 because minimum fe2 requiered m22>.01. Forgive my english but my language is spanish. Bye