SDC D Precast diaphragm without topping slab

[Prestressed Guy]

I have an EOR that is calling for a precast roof in an SDC D structure to be built with prestressed slabs and no topping. The inside dimensions of the building are 114' x 30' with no interior walls. That gives a span to depth ratio of 3.8:1.

The documents show schematic shear plate joint connections with a note stating "FOR REFERENCE ONLY. ACTUAL PRECAST DESIGN AND DETAIL MAY VARY DESIGN OF CONNECTIONS BY PRECAST MANUFACTURER"

That drops the responsibility of the design on me as the Precast SSE. I responded in an email that as follows
[ul]
[li]Per ACI 318 R12.5.3.6 “This Code does not provide provisions for untopped diaphragms assigned to SDC D, E or F." Based on this, I do not know of a way to not have a cast-in-place topping unless there is some provision that I am unaware of.
[li] Section 18.12.6 states that the topping slab must be 2” thick if composite with the precast or 2.5” minimum if not composite. Based on past experience I feel we will be at least 2.5” thick. If we can use composite action with the precast that can be helpful.[/li]
[li] Preliminary design indicates that an 8” solid slab with 2.5” cast-in-place topping will work.[/li]
[/ul]

They came back with a comment about a new provision in the ACE 550.5 for untopped diaphragms in D, E & F but my old 2018ACI Collection does not have 550.5.
Do any of you know anything about this new procedure? The comments on 318 R18.12.11.1 do not make it sound like a very good idea.

 

[bones206]

It looks like ACI 550.5 is referenced in IBC starting with the 2021 version. There was an ESR-3010 that covered this for earlier code versions but it looks like it was withdrawn.

 

[Prestressed Guy]

Thanks for the info.

Since it is covered by an ESR it must use a proprietary connector. I tried to find the ESR-3010 but as you indicated, it is not available. Does that mean that is no longer available? Do you have any idea as to what the connection is and who makes it?

This is still sounding like a bad idea with no real upside. This is a large wall project with just this small area of roof, and it still seems that just a simple topping diaphragm is the better choice, especially given the large span to depth.

In reading this, I would say that there is no question that my reading is correct that untopped PC diaphragms are a no-go in D, E & F with the exception of this possible proprietary connection system.

 

[bones206]

ESR-3010 was actually for the design methodology itself, not for a connector. Here is an article when it was first released:

https://www.concreteconstruction.ne...-3010-issued-for-precast-concrete-diaphragm_o

And another when it was withdrawn in OCtober 2022:

https://lsc-pagepro.mydigitalpublic...783870&article_id=4519107&view=articleBrowser

This is all new to me as well. I just started researching it after you posted the question. Sounds like there has been significant effort put into codifying this diaphragm system/methodology in recent years, so it will take time for industry to catch up. What is the code of record for your project?

 

[lexpatrie]

Isn't 318 really saying there's no specific provisions to satisfy in SDC D, E, F, and it's on the engineer (i.e. there's nothing specific in the prescriptive sense to waive engineering analysis)? That's not the same as a ban on an untopped diaphragm.

 

[Prestressed Guy]

@bones206
This is a CBC 2019 project SDC D

@lexpatrie
That is not how I read it. the comment says that the chapter 14 provisions do not apply to D, E & F and refer you to chapter 18. Chapter 18 says composite and non-composite topping diaphragms only.

 

[bones206]

It sounds like the EOR is referencing a codified procedure (ACI 550.5-18), that is not referenced in the CB 2019 code of record. This may require a code official to weigh in, to say if the procedure is permitted or not. I don't see why it would be an issue for a code official since it's been adopted in IBC 2021.