Hollowcore Typical Large Opening Detail 1

[DayRooster]

Something that has bugged me regarding the typical hollowcore large opening detail is that shelf hangers/angles do not have any positive fasteners or tie-backs. The shelf angles support the cut planks and then just bear on adjacent hollowcore units with clip angles. Which just seems odd because just about every other type of opening in construction you see at least some means securing the support steel in place. Any thoughts from the community on this detail or am I just overthinking this one...

 

[jayrod12]

Friction? Honestly I feel you're likely overthinking it. It's been used in millions of square feet of hollowcore jobs and nothing bad has happened that I know of. The worst time would be from the time they place the hanger there, to the time the grout has cured. Once cured I don't see how it could go anywhere.

 

[DayRooster]

Yeah I figured it was held in place by friction. Just seems odd that all of forms of structural engineering you see so much attention around openings in roof diaphragms but not with hollowcore. I guess I should add that this thought came up with regards to me looking at an untopped hollowcore where the hollowcore would act as the lateral diaphragm and it has large openings in it...

 

[jayrod12]

Most people pay no attention to hollowcore diaphragms. But PCI has a good design manual for them. I'll see if I can find a link but they discuss how that all gets analyzed if I remember correctly.

 

[DayRooster]

Most structural engineers do not pay attention to the diaphragm design of hollowcore roofs? If that is the case then that’s wild. I guess I’m in the minority that care about building stability. As for PCI hollowcore manual, I am familiar with it and I have gone through it so no need to link it. That being said if I have skimmed over a section of it that discusses the influence of large openings with regards to diaphragm action then I would be all hears.

 

[jayrod12]

I don't remember seeing anything in there specific to large openings. Couldn't you just break it down into sub diaphragms and make sure your load can move around teh opening?

 

[KootK]

1) Disclaimer: I'm a part-time precast engineer.

2) Yeah, it's always also bothered me that the saddle hangers don't get tied down to the supporting members positively. As you've rightly, identified, there are requirements for minimum positive attachment of things for most kinds of construction and, in particular, heavy construction. ACI actually does have some requirements for positive connection of components but, frankly, the precast world plays a bit fast and loose with those requirements. Does grouting the keyways count? Does a topping with no tension tie rebar count? The industry answer to these questions often seems to be yes when, in some instances, my inclination would be to say no.

3) In a lot of situations, I'll use the detail shown below to positively tie the hanger lintel to the deck system. I don't believe that this is standard practice within the industry however.

4) There are two kinds of "large" openings from a precaster's perspective.

a) Large = wider than one standard plank. In this case, there's just no getting around the steel hanger being a permanent structural member of some importance.

b) Large = narrower than on standard plank. In this case, the steel hanger isn't a whole lot more than an erection aid. After keyway grouting, the supported plank spans laterally out to it's neighbors and, in some cases, it wouldn't even matter the hanger were removed. For this second kind of "large opening", the positive attachment of the steel hanger is obviously less critical.

5) Precast hollow core diaphragms are a weird thing. With these systems, it is a very easy thing to provide what I'd call shear panel diaphragm capacity. You know, an in plane shear capacity in both orthogonal directions. That said, a diaphragm is ostensibly incomplete without chords and collectors and that's where it gets tricky. Three cases arise in practice:

a) Rebar in the topping, if one exists, provides the chords and collectors. This is slick be can still be a problem if you've got two rebar collectors crossing in a relatively thin topping.

b) Without a topping, you're pretty screwed. You're basically stuck with weird steel strapping of some kind welded to embed plates in the top or of bottom of the plank deck. Frankly, this pretty much never gets done as it's very difficult to do and, sometimes, presents fire proofing issues.

c) Engineers will provide the shear panel diaphragm capacity that I mentioned and leave it at that. It's difficult to tell whether this constitutes, ignorance, negligence, or just a simple nod to pragmatism. In many buildings, openings will be relatively insignificant and the diaphragm boundary elements will be the walls themselves. Here, clearly, this approach has some justification.

 

[DayRooster]

KootK - I’m glad you chimes in. I was hoping you would, having read some of the past hollowcore posts. Also, I’m glad I’m not the only one to think about it! The detail you drew is exactly what I was thinking too. I figured that there should be 4 small weld plate at the corners just to tie everything together. Of course, I’m not positive that all hollowcore vendors can easily provide bottom plates. Often I hear that they are no big deal and other times all I hear is complaining about embedded plates. But that’s a completely different conversation there. Also the large opening I am referring to is what you described first, with the opening being larger than a plank width. As for the diaphragm action of the untopped hollowcore: I completely agree too. I am having to really dig into the fundamentals of engineering just to rationalize it and put calculations to it. Personally I am okay with the compression and shear across the hollowcore grouted joints but as for tension I’m not counting on it. So my end planks have an extra connector on them for suction. This building is small otherwise I would have just put a topping slab on it and called it good. Even though I have second guessed myself multiple times by not putting a topping slab on it because I can’t imagine the cost of a 2.5” slab with some #3 bars would be much at all...but at this point I’ve stuck with an untopped slab just because it seems customary for these small buildings...

 

[EZBuilding]

Personally I am okay with the compression and shear across the hollowcore grouted joints but as for tension I’m not counting on it. So my end planks have an extra connector on them for suction.

I have requested the same on my projects and have gotten substantial push back from contractors and owners alike. I agree with you, however, unless you have the single end plank performing as the entire diaphragm, you will end up with some net tension at that joint.

 

[DayRooster]

EZBuilding - Dang, well hopefully I have some better luck on my project when I request one extra connector (each side) for the end planks. Also, this might be a global hollowcore question but what is wrong with the hollowcore industry where they would fight over a few extra connectors? It has to be such a small amount of money...I mean the hollowcore industry makes working with wood diaphragms seem more reasonable

 

[KootK]

KootK - I’m glad you chimes in.

No sweat. I do like to talk hollow core and always enjoy the sound of my own voice.

...but at this point I’ve stuck with an untopped slab just because it seems customary for these small buildings...

Untopped is obviously inferior structurally but, at the same time, almost certainly the right call for a roof for economic reasons. More so than the material, it's the extra operation and the schedule impact as you know.

So my end planks have an extra connector on them for suction.

1) The monster diaphragm loads are often generated by seismic inertial loads. There, it makes a bit more sense to use the whole deck as the diaphragm.

2) For wind and stability purposes, I've been having some success with the detail shown below. I do this to create a wind / stability girt member out of however many planks I need to create a "diaphragm" with an aspect ratio no greater than 4:1 (my thing, not code). Depending on what I'm up to, sometimes this will be intermittent, weldable bars as shown or continuous bars across multiple planks. It's pretty easy to install the bars without holding up the erection schedule and the cost isn't too bad, especially if there's other welding happening anyhow as is often the case with precast jobs.

 

[KootK]

Also, this might be a global hollowcore question but what is wrong with the hollowcore industry where they would fight over a few extra connectors? It has to be such a small amount of money...

Hollow core, like structural engineering, is a commodity in the extreme. Economically, everything depends on simple, repeatable components getting pooped out of the extruder with as little attention from human beings as possible. Once you add the embeds, all of a sudden you've got:

1) A human spending a day walking down the bed to break open partially cured concrete, grout the cores, and place the embed.

2) Embeds getting placed incorrectly and all of the work that results from that.

You'll find that precasters will care about these kinds of costs much less if:

3) The costs made it into their bid for the job and;

4) The precaster is confident that their competitors had to price it the same way.

For these reasons, you'll minimize pushback by:

5) Putting the requirements in your drawings rather than your specs and;

6) Showing a suggested detail for which an approved alternate may be substituted. This way the precaster will have some degree of cost certainty and not fear that their margins for the job are likely to be tanked by some nutball EOR asking for weird stuff. I hate to say it but many EOR's don't actually know how to design diaphragms. I get asked to do crazy, nonsensical stiff with shocking regularity.

 

[jayrod12]

People that leave a bunch of stuff only in the specs drive me insane. There are actually a few local Architects that have all of their detail sheets in the spec. It's a nightmare.

 

[DayRooster]

KootK - I agreed with everything you stated and that detail you provided looks pretty reasonable. I might have to give it a shot on one of my projects. I do have a question about embed plates though. I generally specify bottom embedded plates mainly because I need them for the interior wall bracing (at the top of the wall). I steer clear of post installed anchors because all the anchors for hollowcore are rated for minimum 6000 psi and 1.375” to 1.5” bottom concrete flange requirements; which is odd since it excludes a bunch of hollowcore vendors from complying with them. So I stick with embed plates on the bottom for those interior wall details. That being said, I also steer clear of specifying top and bottom plates on a hollowcore plank because of the fear that a hollowcore vendor will complain about having to do both. So I generally stick with bottom only since I have a stronger argument for bottom plates. What are your thoughts on bottom plates, top plates, and bottom/top plates specified on the same plank?

 

[KootK]

My guys do dry cast extruded plank and prefer top side embeds. They tell me that it's a bit easier to knock out the top of the cores and insert embeds topside that it is do dig through the entire plank and dodge prestressing strand in order to install embeds bottom side. That said, most plank embeds are bottom side and my guys don't have a problem with that either. There's really no other way for something like a plank to supporting steel beam connection unless you want to get into exotic stuff like trying to place welded DBA's into keyway joints etc.

I've done combined topside and bottom side embeds and it can be done. I try to avoid that condition though, both because of congestion issues and because I feel that tends to create an unusually frail final connection. There's a dirty little secrete in hollow core connection design that nobody talks about: because the embeds get installed into little blobs of grout that are surrounded by void space, the appendix D anchorage provision are almost certainly unconservative.

 

[bones206]

Regarding hollowcore diaphragms, there is an ICC ESR out there for untopped diaphragms:

https://www.concreteconstruction.net/producers/esr-3010-issued-for-precast-concrete-diaphragm_o

 

[dik]

It's difficult to embed plates into extruded HC slabs... I've not secured the planks... no earthquake loading in this area... They have powdered activated fasteners that are used for HC slabs... I don't know if these would be suitable.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[DayRooster]

Bones - That is interesting I will need to dig deeper into that document and see if there is anything of benefit. At first glance though it looks a bit short, only 4 pages. Also it looks like they "punted on first down" with regards to diaphragm connections. All they do is define what a diaphragm connection is generically without getting into the details...which is generally where every other reference is lacking too. So I guess it is par for the course?

 

[DayRooster]

Dik - I do work all around the US...in some cases the diaphragm is seismic controlled and other places it is wind controlled. I assume wind still exists in your corner of the US? So in your situations are you designing your walls as free standing, not braced at the top, as defined by the PCI Hollow Core Manual? Also, send me over some cut sheets for those powder actuated fasteners. I generally have steered clear of them since most fasteners that have an ESR report, rated for "structural use", require thick bottom flanges and higher than normal concrete compressive strengths. Which I don't elect to use since I prefer to keep my hollow core bids as competitive as possible. But I am always open to more options to review!

 

[bones206]

The main thing that I get out of that document is that diaphragm connectors must have an ICC ESR Report unless they are generic rebar cast in a pour strip or topping slab. For example an embed plate with rebar welded across would technically require an ESR.

There are also some stipulations on diaphragm aspect ratio in there.