Size effect factor in thick slabs 2

[TTomaz]

Hi there,

ACI 318-19 presentes throughout its new version a size effect factor, which directly reduces shear strength in thick slabs, for example.

In my case, I do have really thick slab subjected to high shear load.

The shear concrete strength (Vc) of my 150 cm (59'') slab 40 MPa (6000 psi) is only 350 kN/m due to a 0,58 size effect factor, which didn't exist before, which is a rather low considering the ultimate loads (around 1000 kN). Flexural reinforcement rate is the minimum (0,18% * Ag).

How have been your experiences with the new code with this factor?

 

[Agent666]

The so called size factor reflects reality. Its there for a reason.

Other codes around the world have similar factors, in the case of my own code the New Zealand concrete standard, which is loosely based on ACI318, it allows you to take this factor at 1.0 provided you have a minimum level of shear reinforcement. As the combination of a minimum level of flexural & shear reinforcement is intended to control crack widths which impact on the concrete shear mechanisms like aggregate interlock.

 

[JAE]

A short commentary I found about it:
[blue]As more large structures are designed to include thick slabs and other large members that support upper floors, shear provisions have been updated. ACI 318-19 sections on one-way shear and two-way shear (i.e., punching shear) consolidate what were previously a wide range of equations. They also provide a method to include size effect in shear design to avoid issues wherein increasing a member’s size can reduce the unit shear strength of a section. The new shear equations also allow the design engineer to take the effect of reinforcement ratio into consideration.[/blue]

 

[r13]

I think it (ACI) is taking shear deformation into account for deep plate members. similar to deep beam shear.

 

[JAE]

An ACI paper I read about the process of developing the Vc provisions for one-way slabs now found in 318-19, had this to say about the reasons for the adoption of a depth effect factor:

[blue]Many different concerns have been expressed with the ACI 318 one-way shear provisions over the past few decades, including: [/blue][green]
1) Vc does not consider a size effect in shear stress capacity for members without shear reinforcement; 2) Vc is the same for members with and without shear reinforcement;
3) Vc at nominal is taken as the diagonal cracking strength;
4) the effect of axial compression and prestressing on Vc is considered differently; 5) the angle of diagonal compression is fixed at 45 degree;
6) there are too many relationships for Vc for different conditions;
7) several influencing factors are not directly considered; and 8) design relationships are calibrated by laboratory test beams that are not representative of what is common in practice[/green]

Through tests and comparisons of many past tests, they found that the effect of shear stress
capacity decreased with increased depths. The depth effect factor apparently now deals with this – similar to other country’s codes, which they also included for comparison in the article.

Some other findings/observations: [green]
1. The use of minimum shear reinforcement negates any significant size effect on shear strength.
2. The limits used in ACI 318-14, such as fc′ ≤ 10,000 psi for no Av, fyt ≤ 60,000 psi, and Vs ≤ 8√fc′bwd, were maintained.
3. The 45-degree parallel chord truss model (originally used to develop 2xsqrt(f’c)) is effective and conservative.[/green]

The article:
Development of the One-Way Shear Design Provisions of ACI 318-19 for Reinforced Concrete
by Daniel A. Kuchma, Sihang Wei, David H. Sanders, Abdeldjelil Belarbi, and Lawrence C. Novak
July 2019.