Hi All,
When you start work on Etabs modelling, many of us might wonder how to assign stiffness modifiers properly. Therefore, I would like to advise you how to basically assign structural Stiffness Modifiers for primary structural members in compliance with ACI318
Here is stiffness modifiers recommended by ACI 318:
- Beams........................................0.35*Ig
- Columns....................................0.70*Ig
- Walls-Uncracked.......................0.70*Ig
- Walls-Cracked...........................0.35*Ig
- Flat Plates & Flat Slabs.............0.25*Ig
How can we assign that into Etabs 2015 model:
- Beams..................................I22 = I33 = 0.35
- Columns..................................I22 = I33 = 0.70
- Walls-Uncracked.....................f11=f22=m11=mm2=mm12 = 0.70 if modeled as shell
- Walls-Cracked.........................f11= f22=m11=mm2=mm12 =0.35 if modeled as shell
(Please be noted that in practical, almost all designers do not prefer to design walls for out-of-plane bending to avoid excessive longitudinal reinforcement required. In this case, use a smaller modifier say 0.1 for m11, m22 and m12 so numerical instabilities could be addressed) - Flat Plates & Flat
Slabs.........f11= f22=
f12 = 0.25 if it is modeled as membrane or
f11=f22=f12=m11=mm22= mm12 = 0.25 if it is modeled as shell
(please remember that for both cases fij is not important if rigid diaphragm is assigned. For slabs where bending is always in the out-of-plane direction, modifiers m11, m22 and m22 are required to model cracking behavior)
Hopefully, that will be helpful to you. Please feel free to contact if you have any further question,
Best regards,
Kenny Pham, BE.Msc, CEng
Kenny Pham, BE.Msc, CEng
PS: Here is a link to that page for other structural documents:
https://www.facebook.com/pages/Advanced-Structural-Engineering-KP/1467728370205932
