Spalling consideration over stirrups in a 1000mm slab 1

[NunavutEng]

Hi all,

First post, I've been lurking threads for a long time but this one time my case is specific enough that a search did not produce any conclusive answers.

I'm on site building a slab on piles for an industrial facility, we're bending everything by hand due to the fast-track nature of our project and logistic concerns with rebar. The slab is 1000mm thick with piles every 3m c/c. There's sets of stirrups around piles to counter shear forces, and those stirrups are drawn with 135 degree hooks.

I've read other threads regarding hooks on stirrups and there's some debate regarding if 90 deg. hooks are sufficient or if 135 deg. hooks are needed even if the hook is not around a corner or if there's seismic consideration. We're building on the Canadian shield so there's no significant seismic consideration.

My question - is there spalling consideration to be taken of, if the stirrups are hooks under a contiguous surface with 50mm cover (IE not a corner)?

 

[dik]

What are your loads? I would think that a 1m slab with pile spacing @3m would not be a shear issue. Rather than stirrups, the problem might better be solved with a Strut and Tie approach... How large are your piles? waiting for KootK to take up the cudgel...

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

-Dik

 

[ggcdn]

Interpretation of CSA A23.3 clause 7.1.2 is the key here.

The standard stirrup and tie hooks shall have a bend of at least 135 deg unless the concrete cover surrounding the hook is restrained against spalling.

What does restrained against spalling mean, and how does one quantify that restraint? Can providing 3" of cover create the restraint? Some might argue that a footing cast against competent soil has the bottom cover restrained against spalling. But if this is a pile cap on some soft material, its debatable. If its a suspended slab wit typical covers, there's no restraint and 90s are out of the question.

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

 

[OldDawgNewTricks]

Not exactly the same situation, but this figure from ACI shows the hooks and spalling restraint issue pretty well.

 

[NunavutEng]

Load on slab are 40 kPa for containment areas, with process tanks in the 300-330 kPa range and piles are 406mm diameter (16"). Rebar for the mat is 25M and stirrups are 20M.

Disclaimer: I'm not the designer, I'm the client. Just trying to get more technical information to challenge the designer since it seem like they put 135 deg. hooks everywhere without much consideration if they're needed or not.

 

[NunavutEng]

What does restrained against spalling mean, and how does one quantify that restraint? Can providing 3" of cover create the restraint?

I read CSA A23.3 7.1.2 and that is why I made this thread. I have a hard time figuring out what qualifies as restraint.

FYI we're working with 50mm cover top of slab and 75mm bottom of slab. In areas where there's a base supporting process tanks we need to add an other 200-300mm of concrete over the stirrups, which would definitely make them restrained.

Otherwise as you might gather from my username, this structure is built on permafrost, therefore we can expect soil to settle significantly under the slab, which means the soil has no (long-term) bearing capacity in the design.

Not exactly the same situation, but this figure from ACI shows the hooks and spalling restraint issue pretty well.

Maybe it's because of my connection but I don't see a figure loading on my page.

 

[le99]

I am puzzled that how can a pile head embed in the mass concrete surrounding spall?

 

[NunavutEng]

I am puzzled that how can a pile head embed in the mass concrete surrounding spall?

The spall in question would not be caused by the piles and their embedment in concrete, but by the stirrups around them.

There's 3 sets of stirrups on each side of the piles (North-W-E-S) for a total of 12 sets of stirrups per pile. I've been told by the designer that these take the shear loads around the pile, which acts as a column since the soil has no bearing capacity.

The thread is me trying to educate myself on spalling around those stirrups and the necessity of the 135 deg. hooks.

 

[le99]

I still failed to capture the concern, can you show us a sketch?

 

[NunavutEng]

Here's a crude sketch of the design. Pictured here with 135 hooks.

 

[le99]

IMO, the seismic hook is to provide superior confining strength to prevent flexural elements (beams and columns) from damage caused by high compressive and twisting forces during dynamic events that would lead to buckling of the concrete core and dislocation of the longitudinal reinforcement. I don't think the pile cap would experience such forces in magnitude similar to the beams and columns. Therefore, I would accept 90 degrees hooks.

 

[rowingengineer]

If the building requires a 1m deep slab on piles at 3m, it has to have high loads, probably point loads from equipment. The industrial slab might have some nice equipment that cause large vibrations, which the code probably does specifically cover, who knows really, this is why I am sure the design engineer is the best placed to make this call.

 

[BAretired]

Otherwise as you might gather from my username, this structure is built on permafrost, therefore we can expect soil to settle significantly under the slab, which means the soil has no (long-term) bearing capacity in the design.

Looks like this is a much greater concern than the stirrup question. What happens to the structure when the permafrost thaws?

BA

 

[NunavutEng]

If the building requires a 1m deep slab on piles at 3m, it has to have high loads, probably point loads from equipment. The industrial slab might have some nice equipment that cause large vibrations, which the code probably does specifically cover, who knows really, this is why I am sure the design engineer is the best placed to make this call.

I shared the loads in post #5. There's no equipment inducing vibrations in this building.

Looks like this is a much greater concern than the stirrup question. What happens to the structure when the permafrost thaws?

As mentioned in a previous post, the piles take all the loads of the slab. The soil has 0 bearing capacity in the design since permafrost degradation incurs settlements measured in feet, not inches. Piles are embedded in sound rock which is 10-12 m below concrete.

Something to note, we're an expanding a building with the same design that has been standing for 5 years, so I'm confident in the overall design. All I'm trying to do is to educate myself on stirrup hooks and spalling. Happy to answers questions though.

 

[le99]

The 135 degrees hook is sometimes called the "seismic hook", and is specifically required for the stirrups/ties of the main seismic resisting elements, and is essential for elements subject to high concentric compression that is prune to buckling. Your application and local do not possess such risks, so 90 degrees hook is sufficient.

However, from a constructability and engineering economic point of view, you should use either type of hook consistently throughout a project to avoid causing erection mistakes.

 

[dik]

Is there a need for your pile to be extended so far into the slab? and do you need closed stirrups?

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

-Dik

 

[BAretired]

Piles are embedded in sound rock which is 10-12 m below concrete.

I recall installing timber piles on the DEW line in the western arctic. They were steamed in place with the big end pointing down to prevent them from popping up from ice pressure. If your piles are embedded in rock, thawing of permafrost is not quite the same problem, so I retract my earlier comment.

Spalling of concrete is no more likely with 90 degree bends than with 135 degree bends as neither configuration prevents spalling.

BA

 

[NunavutEng]

However, from a constructability and engineering economic point of view, you should use either type of hook consistently throughout a project to avoid causing erection mistakes.

Agreed for a typical work, but I'm on site 7 days a week and one of my jobs is overseeing all the bending of rebar, which is done from straight bar here on site.

Thanks for the feedback on hook design le99, much appreciated.

Is there a need for your pile to be extended so far into the slab? and do you need closed stirrups?

Piles are not that deep into the slab (I think), just 100mm from bottom of the mat. The steel plate used to cap them just a bit larger than the diameter of the pile. As for the stirrups, they're closed yes.

I'm at the mercy of designer anyway for those elements.

Spalling of concrete is no more likely with 90 degree bends than with 135 degree bends as neither configuration prevents spalling.

Now I'll try to apply what we discussed so far in this thread - What you're saying here is what I understanding of CSA A23.3 clause 7.1.2. 135 degree bends have more concrete cover by the nature of their angle, so they're more resistant to spalling that 90deg hooks. But spalling should only occur in high torsion/seismic events, which shouldn't apply to my use case. So in essence, 135deg don't *prevent* spalling, you're right, but they're less prone to spalling than 90s in torsion/seismic/buckling events.

 

[dik]

Thanks... I generally embed them 3" from the bottom of the slab, for punching shear reasons, and would use open stirrups for bar placement, if I could...

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

-Dik

 

[BAretired]

7.1.2 Stirrups and ties
Stirrups and ties shall be anchored by standard stirrup and tie hooks or by heads of headed bars. The
standard stirrup and tie hooks shall have a bend of at least 135° unless the concrete cover surrounding the
hook is restrained against spalling, in which case a bend of at least 90° shall be permitted. Standard tie
hooks with a bend of at least 90° shall be permitted for ties in columns having a specified concrete
compressive strength equal to or less than 50 MPa. Stirrups and ties of size 20M and 25M shall have inside
bend diameters in accordance with Table 16 of CSA A23.1

A bend of 135^o appears to be the easiest way to satisfy the code. I question the validity of the red text above, but it doesn't appear to be worth disputing, particularly if a fast-track answer is needed.



BA