Modeling Concrete Cracked Section Properties for Building Analysis P 2

BURJ KHALIFA - Dubai, UAE

Vertical Supporting Elements

First we will discuss the vertical supporting elements (columns and walls) in the structure under our consideration.

Concrete Columns

For concrete columns which are often modeled as frame element,  the flexural stiffness (EI) and the axial stiffness (EA) govern according to ACI 318-14 Recommendations  and there is no recommendation about modifying the shear stiffness (GA) or torsional stiffness (GJ).

From table table 6.6.3.1.1(a), to model cracking behavior of the column the frame property modifiers  should be as follows:

aN	1.00  Axial modifier
aS2 , aS3	1.00  Shear modifier
jO	1.00  Torsion modifier
i22 , i33	0.70  Bending modifier

Concrete Walls

For concrete walls (both piers and spandrels) which are often modeled as Shell element, based on the analogy with column element and depending on the orientation of the local axis:

• The flexural and axial behavior is controlled by either f11 or f22 modifiers.
• The shear behavior is controlled by f12 modifier.

If we consider the local axis 1-1 is horizontal and the local axis 2-2 is vertical which is the default when drawing walls in ETABS, to alter the flexural (EI)  behavior of the wall according to ACI 318-14 Recommendations for modeling cracking behavior we should apply the modifiers as follows: 

apply f11 modifier for spandrels.
apply f22 modifier for piers.

If you apply the modifier to both f11 and f22 it hardly affects the results. 

There is no recommendation about reducing the shear stiffness (GA). Where they expect deterioration of shear stiffness and want to be realistic in their modeling most engineers recommend to alter the shear behavior also, as follows:

apply f12 modifier for spandrels.
apply f12 modifier for piers.

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As a conclusion, to model cracking behavior of the walls and apply to the ACI 318-14 code and the best practices the shell stiffness modifiers  should be as follows:

1- For uncracked walls
In case the tensile stresses in the wall does not exceed the tensile strength of concrete.

f11 , f22 , f12	0.70  Membrane modifiers
m11 , m22 , m12	1.00  Bending modifiers
v13 , v23	1.00  Shear modifiers

2- for cracked walls
In case the tensile stresses in the wall exceeds the tensile strength of concrete.

f11 , f22 , f12	0.35  Membrane modifiers
m11 , m22 , m12	1.00  Bending modifiers
v13 , v23	1.00  Shear modifiers