Modeling Concrete Cracked Section Properties for Building Analysis P 1

BURJ AL ARAB - Dubai, UAE

Modeling concrete cracked section properties for building analysis is an important and critical topic especially when seismic analysis is required. So we will discuss this topic in detail and give the best practice at the end of discussion focusing on ACI 318-14 Recommendations and how to apply that using CSi ETABS Software.

CSi ETABS Element Model

Cracked-section properties are not automatically implemented in any type of analysis, though they may be manually input for Frame or Shell elements through the property modifiers.

Frame element has four types of stiffnesses:

• Axial stiffness (EAN) affected by the cross section area modifier: aN
• Shear stiffness (GAS) affected by the shear area modifiers: aS2 , aS3
• Torsional stiffness (GJO) affected by the torsion constant modifier: jO
• Flexural stiffness (EI) affected by moment of inertia modifiers: i22 , i33

Use the command Assign > Frame > Property Modifiers... to modify the stiffeness properties of the frame element as required.

Shell element also has two types of stiffnesses:
1- In-plane stiffness refers as:

• Membrane stiffness affected by modifiers: f11 , f22 and f12

2- out-of-plane stiffness refers as : 

• Bending stiffness affected by modifiers: m11 , m22 and m12
• Shear stiffness affected by modifiers: v13 , v23

Use the command Assign > Shell > Stiffness Modifiers... to modify the stiffeness properties of the shell element as required.

According to  the Figure below which shows the direction of local axes and their corresponding stiffnesses from CSi ETABS Analysis Reference.

ACI 318-14 Recommendations

In ACI 318-14 Recommendations Section 6.6.3.1.1 related to slenderness effects where flexural and axial deformations govern so, the code recommended to modifying the flexural stiffness (EI) and the axial stiffness (EA) by the factors in table 6.6.3.1.1(a) and no recommendations about reducing the shear stiffness (GA) or torsional (GJ) stiffness.

From the table above we see that the flexural stiffness (EI) and the axial stiffness (EA)  govern according to ACI 318-14 Recommendations and should be modified to model cracking behavior and There is no recommendation about reducing shear (GA) or torsional (GJ) stiffnesses.

In the next topics we will discuss how to apply these Recommendations to the vertical elements (columns and walls) and the horizontal elements (beams and slabs) of our structure under analysis in CSi ETABS software.