ACI 318 Boundary Element

[BradGile]

Can anyone please help me understand the force-based boundary element requirement from ACI 318 shown as below?

My questions are as below,
1. Why do we use the gross section properties to determine the extreme fiber compressive stress even if the extreme fiber tensile stress is greater than the tensile strength of concrete, causing the concrete to cracked and making the extreme fiber compressive stress to further increase? Is this why the limit 0.2fc instead of fc?
2. What happens if the moment is so small that the section is fully in compression, does 0.2fc still apply in this case and why? i.e. Is boundary element required as long as N* is greater 0.2fcAg then?
Thank you.

 

[HTURKAK]

Boundary elements are required foe Special Shear Walls with boundary confinement at the ends of walls where the maximum compressive stress associated with flexural and axial loads are higher than 0.2fc . The use of gross section properties is for nominal check of the max. compression stress . THis para. copy paste from ACI 14 R18.10.6.3 :'....Recognizing that this loading condition may be repeated many times during the strong motion, the concrete is to be confined where the calculated compressive stresses exceed
a nominal critical value equal to 0.2fc′. The stress is to be calculated for the factored forces on the section assuming
linear response of the gross concrete section. The compressive stress of 0.2fc′ is used as an index value and does
not necessarily describe the actual state of stress that may develop at the critical section under the influence of the
actual inertia forces for the anticipated earthquake intensity....'


Boundary element is a requirement for SPECIAL SHEAR WALLS..which is code requirement for SDC D,E, and F. If the section is fully in compression, SDC should be A to C .. where ordinary shear walls applicable.

 

[ARKeng]

Boundary element is a requirement for SPECIAL SHEAR WALLS..which is code requirement for SDC D,E, and F. If the section is fully in compression, SDC should be A to C .. where ordinary shear walls applicable.

I don't think that's an assumption that can automatically be made. Just because a wall is "fully in compression" doesn't mean it's in a low seismic DESIGN CATEGORY, it just means is lightly stressed for overturning. There are plenty of walls with a low height-to-length ratio even in high seismic zones that are "fully in compression".

To answer OP's question #2, yes I think boundary elements are required if you're a special shear wall and your compressive stress is greater than 0.2' f'c. But if your wall is in that much compression, the most economical solution seems like it would be to increase the thickness to decrease the stress rather than providing special boundary elements throughout a significant length of wall.

 

[BradGile]

Thank you for your input.

Put SDC aside, my question #2 is related to my question #1. i.e. Is using the gross section properties the reason why 0.2fc is used instead of higher figure say 0.9fc? My understanding is that boundary element is required to restraint vertical bars to prevent from buckling when concrete fails in compression. So if concrete doesn't fail boundary element is not needed and concrete will not fail under 0.2fc stress (please correct me if I am wrong). So 0.2fc is used here because we are using gross section properties instead of cracked one when tensile stress develops in the section, reducing the neutral axis and increasing extreme fiber compressive stress to a value near fc. So if the section is fully in compression, the extreme compressive stress will be the same as we what we get using gross section and 0.2fc will not make concrete fail so buckling is not a concern for vertical bars in this case?
Also, does this 0.2fc apply to gravity load combinations? I don't think a wall with fcAg that is slight greater than 0.2N* a not economic solution and require ligs everywhere?

 

[HTURKAK]

I used the words ( SDC should be A to C and not SHALL.. ).. I could not imagine .. will you please show an example , where the N* is greater 0.2fcAg and bending moment is negligible and the SSW with hw/ℓw ≥ 2.0 ?

Pls read 18.10.6.2 , boundary elements are defined for Walls or wall piers with hw/ℓw ≥ 2.0 .


There are two options for determining need for boundary elements ;

i= Stress based = maximum compressive stress associated with flexural and axial loads are higher than 0.2fc

ii= Strain based = c ≥ ( lw /(600 δ/hw )


If the compression stress due to gravity loading is greater 0.2 Fc and bending moment is negligible boundary element is not necessary as per option ii . Pls look ACI 14 18.10.6.2 a and b .

 

[moments]

When you say 0.2fc stress, don’t forget that the load was determined with an R factor. The stress check option for is intended to make for an easy hand calc to answer a yes/no question whether special boundary elements are required, not represent the actual seismic behavior.