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Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?
4

Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

(OP)
I'm tinkering with some precast hollow core plank that has a 3" structural topping and is working pretty hard. My software reports that horizontal shear at the plank to topping interface is a problem. And, practically, there really aren't a lot of good options for remedying the situation. Reinforcement would be ridiculous in a dry cast system and changing system depth would be a disaster for the project at this stage.

So, out of desperation, I cracked open ACI to give it a go by hand. The related provisions are shown below. 17.5.4.1, meant for when you have ties, suggests that horizontal shear capacity should correspond to peak shear demand rather than averaged shear demand (composite steel) because redistribution potential is limited. There's also similar language in the old PCI Hollow Core Manual. By that standard, my hand calcs agree with my software output and I've got problems.

But wait! If you check out 17.5.4, it gives you an alternate method. And that one works for me. However, is the alternate method not essentially saying that I can average my horizontal shear demand? If that's not the case then I really don't know how to apply 17.5.4.

For what it' worth, my loading is dominated by two, equal, point loads located at about 0.4L & 0.6L. So my shear diagram is pretty flat and my moment diagram is pretty close to a perfect trapezoid. I've been attempting to apply the alternate method over a "segment" that I've delineated as the span from the end of the plank to the first point load.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

Was there a definition or limit on the "segment" in ACI?

lol - "my segment is 1/2 the span!!! Take that ACI!!!"

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RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

(OP)

Quote (JAE)

Was there a definition or limit on the "segment" in ACI?

Not that I could find. They seem to be relying on conventional English which is surely ill advised.

Quote (JAE)

lol - "my segment is 1/2 the span!!! Take that ACI!!!"

Yeah, precisely. Nothing to prevent that interpretation as far as I can see.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

It must be working hard; I can usually get just a raked concrete surface to be acceptable.

Just throwing out ideas but what about powder actuated fasteners used as horizontal shear reinforcement?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

(OP)
I'm willing but would need some vehicle for assessing the PAFS. I'm already assuming "intentionally roughened". The plank is actually working so hard that I need to solid grout two cores of each planks. Turns out there are some good reasons not to push things that far.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

Quote:

(KootK)

So, out of desperation, I cracked open ACI to give it a go by hand. The related provisions are shown below. 17.5.4.1, meant for when you have ties, suggests that horizontal shear capacity should correspond to peak shear demand rather than averaged shear demand (composite steel) because redistribution potential is limited.

And that is a good approach. A lot of research I have seen shows that a lot of the transfer is happening near the point of loading (in the case of point loads). There are also uplift forces to consider (that we typically don't think about) that can happen at highly concentrated locations.

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

Quote (KootK)

I'm willing but would need some vehicle for assessing the PAFS.

I've justified composite action with powder actuated fasteners using the concrete shear strength from the fasteners ESR report (such as Hilti X-U fasteners). They even call out details for hollow cores. Then they just set the fastener power so that the fastener projects proud of the plank equal to the embedment and you should get a shear capacity based on the ESR for that embedment in both the plank and topping.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

(OP)
Excellent TME, I'll look into that.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

ACI 318-14 is worded a little differently and references all locations for comparison.


16.4.5.1 As an alternative to 16.4.3.1, factored horizontal shear Vuh shall be calculated from the change in flexural compressive or tensile force in any segment of the composite concrete member, and Eq. (16.4.5.1) shall be satisfied at all locations along the contact surface:

φVnh ≥ Vuh (16.4.5.1)

Nominal horizontal shear strength Vnh shall be calculated in accordance with 16.4.4.1 or 16.4.4.2, where area of contact surface shall be substituted for bvd and Vuh shall be substituted for Vu. Provisions shall be made to transfer the change in compressive or tensile force as horizontal shear force across the interface.

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

(OP)
Thanks for that haynewp. I'm still perplexed though. Consider:

1) The bit about satisfaction at all locations does make things sound less pro-averaging.

2) If bv x d is still being swapped out for the segment contact area, that still sounds like averaging to me.

3) Still really no definition regarding what an appropriate "segment" might be.

4) If the provision no longer allows for averaging, I don't see how the alternative method is really any different from the regular, peak stress method.

There's a distinct lack of clarity of intent on this one. Maybe this section is being stick handled by a summer intern or something.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

If you're thinking of going with P.A.F. I would rather utilize concrete screws.

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

(OP)
I was thinking that these patch pin things might be nice:

Quote (http://www.helifix.com/products/retrofit-products/...)


At the end of the day though, I'd be going with whatever I can find some kind of code approval for. I'm sure the precast erector would loath all of it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

Ooooh, potential safety issue with those patch pins. It looks like the face of those pins after they're driven is quite sharp. Obviously my nail idea isn't exactly something I'd want to run my face into or trip onto but I'd rather fall on a bunch of nail heads and get an interesting bruise pattern than fall on a bunch of twist drills.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

KootK, funny enough now I'm looking at bidding a job with a similarly heavily loaded prestressed solid plank but we do things the right way around here (tongue) and have hoops connecting our plank to the topping.

I have a calculation in my prestressed spreadsheet that the maximum spacing of horizontal composite shear reinforcement is per 16.4.7.2 (ACI 318-14) and 0.75h (total height of composite slab). I'm scratching my head to recall where the 0.75h is from as it's not shown in 16.4.7.2 (ACI 318-14). Since you've spent a bunch of time with this section of ACI 318 can you recall where that requirement is from?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

Regarding the 2 different ACI methods, it appears there is a diffeence in what is being compared. The first is Vu and the second is Vuh, the shear flow at the location of the horizontal plane being considered is the way I read it.

16.4.3.1 Design strength for horizontal shear transfer shall satisfy Eq. (16.4.3.1) at all locations along the contact surface in a composite concrete flexural member, unless 16.4.5 is satisfied:

φVnh ≥ Vu (16.4.3.1) where nominal horizontal shear strength Vnh is calculated in
accordance with 16.4.4.

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

This is waay out of my area of expertise (if I have any :>) but can you grout some steel into the cores?

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

(OP)

Quote (haynewp)

Regarding the 2 different ACI methods, it appears there is a diffeence in what is being compared.

This speaks to the crux of my concern. I don't see that there is much difference. Vu is horizontal shear averaged over a length equal to the flexural depth. Vuh is horizontal shear averaged over some other "segment" length. If Vuh needs to effectively be Vu, then the methods are identical. If Vuh can be different, then it seems that larger scale averaging is implied to be acceptable.

Quote (This is waay out of my area of expertise (if I have any :>) but can you grout some steel into the cores?)


You could so long as that steel reached up into the topping to engage it. My limited work with precast has taught me, in spades, something that I've heard TME echo in the past: once you compromise the repetitive, simple nature of precast framing, the gig is truly up. My client would absolutely loath any of the remedies that have been discussed in this thread. In part, that's why I've had them roll ahead with the "do nothing" shear averaging approach. These days, I like clients a good deal more than I like my rigidly held technical beliefs. Hopefully the tropical diving I expect to do this Christmas will give me a chance to wash off the icky patina of all my recent compromises.

Quote (TME)

Since you've spent a bunch of time with this section of ACI 318 can you recall where that requirement is from?

I've no idea I'm afraid. I like it though. I feel that "d" is sort of the scale at which these effects operate and should be addressed. Similar to how Plank's constant relates to all things quantum. While it almost never comes into play in hollow core, there might also be merit in envisioning the truss mechanics that would be involved if vertical shear reinforcement were to be truly engaged. Then, you would need to have horizontal shear addressed for each chunk of the topping as it rested on each individual truss strut. And that would take you back to a spacing again on the order of "d". Perhaps your spreadsheet was just written by someone wise, even if that someone was just a past iteration of yourself.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

Quote (KootK)

something that I've heard TME echo in the past: once you compromise the repetitive, simple nature of precast framing, the gig is truly up.

Yep. A good analogy would be if a steel mill had to stop rolling wide flanges for a day and rig the machine to roll T sections. Could it be done; sure. But the headache this creates for the mill to make these sections means they'll just default to "you can't pay us enough to bother" as it takes their limited shop space/personnel/etc away from the things that make them more money and are in more demand.

Think of precast as an assembly line but with the ability to make runs of customized cars and trucks for each dealership; if you want tinted windows on one car we can do that. If you want us to build an airplane though...

So, could you put composite steel beams in the cores? Maybe, but the economics aren't there.

Quote (KootK)

Perhaps your spreadsheet was just written by someone wise, even if that someone was just a past iteration of yourself.

Yep, it was old me who clearly thought that he would never need to exceed that 0.75h spacing and didn't note where it came from. I can't find it in any of the obvious references (PCI, ACI, etc.) maybe it's buried in my Naaman prestressed textbook. He's way smarter than I am and I incorporated a number of his suggestions into my prestressed design spreadsheet.

Speaking of Naaman, I've been meaning to dig into that to see if he addresses your concern. I saw he had something regarding segment length but real work called and I had to table it. If I can find a second at the office I'll dig back into it and see if I can find any helpful nuggets for your problem, KootK.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

If the segment length is taken as 0.4L, the shear span, would the shear capacity be adequate to carry the average shear in that length?

BA

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

Quote (KootK)

Hopefully the tropical diving I expect to do this Christmas will give me a chance to wash off the icky patina of all my recent compromises.
I have gotten used to that. Keep those insurance premiums coming. Enjoy your trip!

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

[EDITED]
Regarding the ACI methods, the first, 17.5.3, calculates the horizontal interface shear force at each section of the beam which is taken equal to the average vertical shear on the section. Taking the average vertical shear (V/bd) accounts for non-linear redistribution at ultimate as apposed to the old linear elastic method (VQ/Ib) used for working loads.

The second method, 17.5.4, calculates the total shear force on the interface area. For a composite beam this shear force is due to the portion of axial compression component of the moment couple that has to be transferred into the deck/topping. The length of the interface is taken to be the distance from the point of zero moment where the axial compression transmitted is zero to the point of maximum moment where the transmitted compression peaks. For a simple span with a UDL this would be from the support to midspan. For dominant point loads it may be shorter (the shear span as BA says). The designer may also choose to design the interface for the axial capacity of the deck/topping instead of the actual shear load.

The second method, as far as I know, was developed for composite steel girders where there is sufficient slippage along the interface to mobilize all of the shear connectors equally. This doesn't happen with concrete on concrete which is why in the comments to 17.5.4 ACI says that any shear connectors should be distributed similar to the shear diagram which really takes you back to the first method. PCI and CPCI illustrates the second method in their design manuals but I think it's better suited to small interfaces like corbels or shear keys. In an unreinforced interface like hollowcore I'd be afraid that the relying on the second method could lead to a zipper failure. On the other hand I think topping failures on hollowcore are rare, though most would be uniformly loaded.

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

(OP)

Quote (BAret)

If the segment length is taken as 0.4L, the shear span, would the shear capacity be adequate to carry the average shear in that length?

It would and that's the route that I took. It's interesting. Zoomed out, the moment diagram looks quite tri-linear. Zoomed in, the slope of the moment diagram at the very end is clearly about 15% steeper than the "average over the segment". Presumably, that's the source of my difficulties.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Horizontal Shear in Composite Concrete Members Per ACI - Mixed Messages?

(OP)
So...

- horizontal shear wasn't working which has never happened to me before. Take the hint? Nope. Hand calc around it 'cause I'm clever like that dawg. So clever.

- vertical shear wasn't working so I had to grout every other core to make it go. Get the gist? Hell no. Just a little extra hatching in my CAD file.

So now, apparently, my guys can't cast this thing without the level of prestress required tearing their abutment things from the earth as if by the hand of god (non-denominational). And the plank needs to show up on site next week.

Me stubborn; me foolish; me sorry. So sorry.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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