Very short concrete edge support

[bugbus]

I have a situation where I need to support some heavy precast concrete trough covers (lids) along a cast in place concrete wall. The covers are 200 mm (8") thick and will support pedestrian loading as well as some vehicle loads. They also need to be removable to inspect the trough below. Due to various constraints, I only have about 50 mm (2") of bearing width between the cover and the wall as shown below. Also, the clear space within the trough cannot be encroached on, so there is no possibility of supporting the covers on a ledge or bracket.

I have a few concerns: (1) bearing failure at the edge of the cover or wall with possibility of a diagonal crack forming; (2) anchorage of the reinforcement; (3) general wear and tear around the edges of the concrete as the lids are occasionally lifted and replaced and shifted around.

My concept is sketched below. I believe the only way of doing this would be to cast in some steel angles that would be anchored into the respective concrete elements with welded reinforcement. But this would require a lot of welding, which might be problematic.

Is there a better way of doing this that doesn't involve so much welding?

 

[Prestressed Guy]

As for bearing length on hollow core, we precast engineers "get away with it" based on lots of testing and many millions of square feet of past projects. Oh and then there is that little thing called the "code".
In SDC D and above you do need to increase the bearing length by and extra 2" per 18.14.4.1(d).
HC slabs are always designed as pin connected at the bearings so the moment is 0. They need to be designed for shear capacity which is the failure mode at the ends and often requires filling the some or all of the cores for a distance out from the bearing.

 

[bugbus]

In my mind, if you consider the support region to be a 'D'-region (to be analysed based on S&T principles), then the short (e.g. 2") bearing width cannot work in theory.

Even though the bending moment might be 0 at the support, there is still an immediate demand above the support for the tensile reinforcement to be anchored, to satisfy the diagonal shear (I am thinking of the truss analogy here). It is my understanding that this is the reason why various codes require reinforcement to be fully anchored a distance D from the theoretical point at which the bending is zero.

Obviously S&T modelling has limits, and cannot perfectly describe what's happening in practice. Whether we are unwittingly relying on the tensile capacity of the concrete in these end regions, or the anchorage of the strands/bars is enhanced by the bearing pressure, who knows?

 

[KootK]

As for bearing length on hollow core, we precast engineers "get away with it" based on lots of testing...

Show me any testing that addresses the anchorage of prestressed reinforcement at bearings in the sense that gusmurr and I have been describing it. I've been a part time precast engineer since 2016 and I've found none.

Oh and then there is that little thing called the "code".

The code references that you provided may cover bearing but, to my knowledge, they do not cover the anchorage of prestressed reinforcement at bearings. How could they as they say nothing at all about the actual reinforcement present? I could debond all of the strands at the end of a plank for the first 3' and still be compliant with those provisions.

HC slabs are always designed as pin connected at the bearings so the moment is 0.

Zero moment at the plank ends isn't the solution to reinforcement anchorage but, rather, the problem. If there were hogging moment at the ends and reinforcement provided for that, anchorage would be less of a problem.

They need to be designed for shear capacity which is the failure mode at the ends and often requires filling the some or all of the cores for a distance out from the bearing.

Shear capacity doesn't solve the reinforcement anchorage issue. That said, the sketch below is taken from one of the NZ seismic docs. It kind of suggests that core grouting creates a discontinuity at which flexural cracking might occur and then initiate the kind of reinforcement anchorage issue that we've been contemplating. It kind of depends on how much faith one has in the restraint relieving capacity of plank bearing strips I suppose.

...we precast engineers "get away with it" based on lots of testing and many millions of square feet of past projects.

Yeah. The historical performance is worth something, of course, but how much it's worth is always difficult to ascertain, particularly given events like Northridge that indicate that when things really get tested, what was assumed to work since time immemorial sometimes does not. I've always found it interesting that PCI has it's own sort of relaxation amendment to ACI: Link. Some of that stuff is backed by industry sponsored testing. Still, as a pseudo insider to that industry, I'm quite comfortable saying this:

1) Precast engineers are utterly beholden to the preferences of very cost conscious, fabricator clients. It creates the kind of pressure that starts to feel like a conflict of interest at times.

2) The precast industry pushes very hard to trim as much fast as possible from their designs. This is evidenced by:

a) The practice document that I linked.

b) The kinds of questions that you see here from precast engineers. We're always the guys asking if:

i) columns can have fewer ties;

ii) column ties can have 90 degree hooks;

iii) if beam stirrup hooks can face longitudinally down the length of beams rather than transversely over the top steel;

iv) if beam ledges can be unreinforced;

v) if beam's loaded by ledges can use open stirrups.

And that's just the list of stuff that popped into my head because I've seen it in the last twelve months or so.

 

[dik]

Moment is zip, but there's a direct correlation between bond and shear. I suspect, with the high strength of the HC units, that plain concrete strength, plays a part of it until the strand can be utilised.

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

-Dik

 

[KootK]

That's my suspicion as well dik and precisely what I was getting at with this, earlier statement:

Another thing that I know about plank is that it almost never gets taxed in gravity applications. If it did, there would be cracks all over the place and frightened villagers running for their lives. And, perhaps, the odd catastrophic shear failure.

Potential problems with that:

1) In general, structural engineers are adverse to using plain concrete for serious things.

2) It makes the setup heavily reliant upon avoiding axial tension stresses in the plank.

3) If we are going to go down that path, we should say it out loud and develop an evaluation procedure for it.

 

[dik]

some structural engineers...


or understand what is actually happening... for the kazillion lineal feet of HC bearing installed, there has to be a pretty good reason for it working.

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

-Dik

 

[KootK]

some structural engineers...

Am I to infer from that comment that you are a big fan of utilizing plain concrete in high load applications with indeterminate levels of axial shrinkage restraint? Color me surprised.

...or understand what is actually happening...

In my opinion, understanding what is happening means figuring out a way to evaluate it. It's all just interesting chit chat until you've got a go/no-go criterion for designers work with.

It's something of a glaring discrepancy that ACI and CSA make a deliberate point of enforcing the bar anchorage business in CIP concrete that is, apparently, completely unnecessary in hollowcore plank.

It's another glaring discrepancy that the precast associations in other parts of the world do concern themselves with this issue while we in North America do not. Are we super smart and they're just a bunch of overthinking dummies? I doubt it. I managed to find the NZ thread on this. See the comments by rapt and Agent666 here: Link

 

[KootK]

This is the paper that Agent666 sometimes references on this topic: Link. In particular, the area around the diagram copied below.

 

[dik]

Not all the time... largest project was for a 6 storey parkade in Toronto, decades back... for a large part of the foundations...

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

-Dik

 

[rapt]

Kootk,

I would think the term "positive moment failure" in that diagram is incorrect.

More like longitudinal tension force/development failure leading to shear failure. I doubt Moment had a lot to do with it.

Even with no moment, there is a tension force that has to be transferred to the support, and pretensioned strand has no force developed in the last about 60-100mm and then gradually increasing from there as you said in an earlier post.

I have never understood how the PCI and industry has ever justified the design at the end of precast where they have short ledges for support.

 

[dik]

rapt... the force in the strand is 0 for the first 3" or 4" and then develops to full tension capacity. Do you have an idea of what the tension development is? about 2'? or more?

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

-Dik

 

[KootK]

I would think the term "positive moment failure" in that diagram is incorrect.

Meh, it's just a terminology thing... and not mine obviously. That said, I kind of get it. It is the particular shear/truss mechanism failure that you would only really have IF the bottom steel anchorage were insufficient and the member was in positive bending at/near the member end. Theoretically, you could also get a top steel anchorage failure in a hogging moment scenario of course. I think that the primary difference is that top steel is almost always well anchored by default, as a result of general continuity, whereas robust anchorage often takes a deliberate effort with the bottom steel.

 

[m_struct]

Is there any positive connection of the precast slab to the cast in place wall? If not, has deflection been consider to check against lose of bearing for lateral cases such as seismic MCE?

 

[dik]

Dowels between the joints is all that I've seen. As far as deflections go... I'd be cautious. I've seen long span HC slabe 30' or better, camber upwards with time, due to the high pretensioning required.

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

-Dik

 

[rapt]

dik

depends on strand size, concrete strength etc.

For .5" strand, about 5' (about 1.5m). Initial transmission length is about half of that, the rest is to develop stress above the initial prestress.

And it increases in tension zones, so could easily go up to 2m.

Where that crack was shown, the stress it could develop would be 0. So it is completely un-reinforced.