ACI 318 Shear Provisions and Modified Compression Field Theory

[briancpotter]

Reading up on the failure of the Sleipner A drilling platform, much of the blame is not-so-subtley placed on the unconservativeness of the ACI 318-95 shear provisions - specifically, the ones that provide shear capacity for members under compression.

These provisions (eq. 11-6 in every version of ACI up to 318-11) are still the same today - unchanged since ACI 318-71, which is itself based on an ASCE committee report from 1962 - over 50 years old at this point.

We know these provisions are unconservative, and we know a more accurate method (modified compression field theory, used in the AASHTO code for over 20 years). Anyone have any insight as to why they haven't been updated? I'm sort of perplexed as to how a high profile, 100 million plus dollar failure didn't induce any.

Brian C Potter, PE
Simple Supports - Back at it again with the engineering blog.

 

[Deker]

I can't comment on why ACI has been slow to adopt the latest research, but it appears that change is on the horizon. The latest edition of Concrete International includes a brief history of the ACI shear provisions as well as six proposals for new provisions that will address known shortcomings--such as failure to account for the size effect--in the current equations. If approved, the changes will be adopted in the 2019 code.

 

[WARose]

We know these provisions are unconservative, and we know a more accurate method (modified compression field theory, used in the AASHTO code for over 20 years). Anyone have any insight as to why they haven't been updated?

I'm not sure either......but this isn't the first time I've heard this about the equations in ACI that take into account compression and shear. (In Wang & Salmon's concrete book (I've got the 6th edition) it (very subtly) indicates this as well.)

This is pure speculation on my part but: A lot of the guys on the committees are old school (at least the last time I looked).....and I've noticed most of the guys in that age bracket never use axial compression to help out with shear. So it may be a situation where they think: nobody is using it anyway. (Not to mention resistant to change.)

And I have to say: I can't imagine using a great deal of axial compression to help out with shear either. I've never done it. There are a lot of things in the code(s) that I don't think are good practice.