Precast Connection Ductility to CSA A23.3

[Brad805]

I am designing a connection for a precast concrete project. This is a delegated design project, and the feedback from the EOR is not stellar. In many cases we are doing projects that are designed as tilt-up, but for any number of reasons the GC decides to precast the project. Due to that, most of these EOR's are far more familiar with tilt design req'ts. Clause 21.7 of the CSA tilt up section (below) has a special design requirement for the connections. There is not a similar clause in 21.8 for precast concrete. I have had discussions with others and some feel this clause applies and others believe it does not. I tend to err on the side of caution. The Rd=1.5, Ro=1.3 for this building. This clause requires that non-ductile connection should be designed for a force determined using an RdRo=1.3 instead of RdRo=1.95. The building is located in a very low seismic region.

Question:
1. Below you can see an image of the connection. This is a dry connection consisting of an embed plate, and A706 rebars. Design of the weld for RdRo=1.3 is easily accomplished. Would you design the embedded elements for RdRo=1.3 or 1.95? Obviously, the conservative approach would be to use the increased load, but when I do the connection is causing a great deal of difficulty.

For Koot's anchorage theory I have included a screenshot of the analysis model. This is an image of the connection with a shear load =200kN and axial =200kN. You can see the cracking starting in the shape as a breakout. I stopped the analysis at this point since it was no longer converging properly. I thought I would include this out of interest.

 

[ggcdn]

This is mostly thinking out loud, but I think that's an oversight on the precast provisions, which are overdue for an update. We see time and time again it's bad business to have non-ductile connections as the weak point in a structure. We know there is going to be shear and flexural overstrength in the panels, so the forces at the connection will increase beyond RdRo=1.95 forces, unless you put in some kind of (reliable) ductile yielding mechanism. I think there's a lot of good reasons to design for the amplified forces, and few good reasons not to.

-JA
try [link calcs.app]Calcs.app[/url] and let me know what you think

 

[Brad805]

I doubt you would be surprised who thought this clause did or did not apply. The CSA committee member I spoke with seemed somewhat dumbfounded by the question about the applicability of the clause to precast whereas the CPCI tech engineer basically indicated they do not open that section unless the Rd =2.0.

If one assumes the clause applies, the question would still remain if this type of connection would be considered non-ductile or ductile by other engineers. Non-ductile to me would be the failure of headed concrete anchors. Here the failure is not at all like that. The model is a work in progress, but below you can see the load v. deflection curve. The failure of this connection is not a brittle, but I am still on the fence.

 

[KootK]

I can think of no rational reason why precast wall panel connections ought to be held to a lower standard than tilt-up wall panel connections. That said, "what I would do" here in practice would be to not apply the ductility provision if that would save my client any meaningful amount of pain. That, because:

1) It's good business.
2) You've got the code "out"... for now.
3) It's a low seismic location.
4) Via your modelling, you have reason to believe that the connection is fairly ductile anyhow.

On this forum, we quite often hear stuff to the tune of "the code is just the minimum, sometimes we have to go beyond that!". I get it but, at the same, I also know precaster clients. Things that look like small inconveniences on paper can add up to big $$$ in a fabrication plant. And precasters will rarely batt an eye at swapping out your services for someone else's if theirs are more "convenient". Precasters also talk amongst themselves a fair bit which only compounds the resulting pressure that lands on us.

 

[KootK]

For Koot's anchorage theory I have included a screenshot of the analysis model.

What do you make of the clouded areas below? I don't imagine that even IdeaStatica models bar slip, right?

 

[Brad805]

I always appreciate your practical comments. Many of the uber smart have a hard time with that.

No, Ideastatica does not have much for bond slip material models. This is something I need to look at and discuss with them a bit more. I am using a Shima bond slip model, and honestly, some of these details I need a bit more assistance with. I have bars in the model near each A706 bar, but since the bars are modeled as lines the size of the mesh comes into play. There is a fine balance between mesh size and computation time.

This will be of interest to you with your anchorage/development discussion. In the graph you can see where the load drops off as different cracks start to open. Image #1 is the first group of cracking in a shallow arc around the embed group. After that crack opens, the rebar thru the crack starts to pickup load, and the load starts to increase again. Near the failure load you can see the whole block is pulling out. I stopped the analysis at this point since it starts to take far too long to converge as the model becomes more non-linear.

Crack Image #1

Crack Image #2

Crack Image #3

 

[TLHS]

How expensive is IdeaStatica? It looks amazingly fun every time I see it, but also looks like it costs a pretty penny.

 

[Brad805]

This was done in Diana FEA, but Koot knows we have IdeaStatica as well. If you are doing fairly regular concrete problems, IDEAStatica is better for a design firm since it includes the code check reports we want for our file. Diana FEA is a bit north of $10,000EU/year, but if you compare that to Abaqus, ATENA, or ANSYS it is not expensive. The time to learn and run the analysis is another big consideration. We licensed it for the year for some specific problems that I want to understand better. I hired them to assist with aspects of the analysis and modeling.

 

[Settingsun]

Out of curiosity, is the code clear on the difference between tilt-up and precast? ie anything stopping people classifying tilt panels as precast if it suits?

 

[Brad805]

If one reads the definition, yes, but I have chatted with a few different EOR's that do not agree. I wish the definitions were clear. If one asks the question I tend to find most simply err on the side of caution without giving it a second thought.

 

[Settingsun]

Arbitrary distinctions lead to arbitrary decisions. Just like the Australian code and walls vs columns.