Is it required to hook the ends of the footing rebars?

[Penpendrum]

figure 1

figure 2

I have found some plans that is like figure 1 above that hooks the ends of the footing mat bars.. I was wondering in what section of the ACI(3-18) code that does require to hook the end of the footing mat bars and what is the size of the hooks?

do we really need to provide column ties with-in the footing and does the 2" spacing starts from the hook of the column longitudinal bar or does the 2" spacing start above the footing..?? in what sections of the ACI code proves this?

 

[JedClampett]

If the bars are very short and can't be developed through development length, a hook might be necessary. But that would a very weird footing (needing that much moment and not having the development length), that would be impractical. For instance, a #8 needs about 48 inches to develop. But you would need a huge load to create enough moment to need #8's.
I'd say for a normal footing, a hook isn't needed. Plus you might need to thicken the footing just to fit the hook in. #7's need a 14 inch hook. With cover you're up to about a 19 inches thick.

 

[weab]

Hooks are not required at the end of the horizontal bars as long as a straight bar can develop full strength where full strength is required, usually at the face of the column. That is, if the distance from the face of column to the end of the bar is less than the required development length, then a "standard hook" is required to reduce the development length. Standard hook is diameter dependent, see chapter 12 ACI 318-11 and earlier.

The column ties are usually required for seismic design. It increases ductility. Check the chapter for seismic requirements.

I am guessing but I believe the 2" is from the construction joint. I am at home and cannot point to a chapter other than we would generally want 2" cover for a column. A tie would be almost useless at the joint. I believe that the bottom tie location should be based on consistent spacing from the top down.

 

[hokie66]

The greater the pressure on the footing base, the more likely the cogs are necessary. Some consultants specify cogs as a matter of course. I was surprised by this when I started working in Australia, as it was not common practice in the US to show cogs.

Other than for seismic reasons, it is relatively common to show two ties within the footing, as it assists in locating the starters and keeping them where you want them during casting of the footing.

Not everything we do in concrete structures can be explained by reference to the ACI code.

 

[Penpendrum]

hello hookie66 what do you mean by "cogs"?

 

[Ingenuity]

hokie66 has been in Australia too long

Cogs = 90º hooks

 

[hokie66]

Yep, thanks, Ingenuity. I only pick the Australian lingo which I think makes good sense. Using "hook" for 180 and "cog" for 90 removes doubt, at least if folks are consistent.

 

[KootK]

Hooking is frowned upon in my area so I generally refrain. That said, I sympathize with those who choose to hook as a footing is essentially a cantilever member with a difficult to predict load distribution. If that same member were flipped upside down and installed 15 stories in the air, we'd all be hooking up a storm.

Your inclusion of top steel makes me wonder if this isn't a footing with an overturning moment. If that's the case, I would recommend hooking your bottom bars.

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.

 

[rapt]

I am in the cog camp, both technically and in terminology.

 

[JoshPlumSE]

I was told years ago, that if you had top and bottom steel, then hooking the bars was convenient.... Allowed for easier placement of the top bars, giving the contractor something to tie off to.

Though it was not a requirement unless you needed it for development length as others have stated.

 

[Agent666]

Funny, I've never seen a footing without the hooks. They are always included as a matter of course round these parts for the reasons joshplum pointed out. They also allow for the entire cage to be fabricated off site or at least out of the excavation.

I can't even imagine not having them, how on earth do you control the placement of the top bars?

 

[KootK]

I can't even imagine not having them, how on earth do you control the placement of the top bars?

Most footings in my area don't have top bars. And, in most cases were top steel is required, you'll see some standoff ties to create the separation. With larger footings, I imagine this would be required even with the bars hooked.

They also allow for the entire cage to be fabricated off site or at least out of the excavation.

For bottom steel only footings, this is even easier without the hook, particularly where transport is involved.

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.

 

[Penpendrum]

thanks for all your information KootK,JoshPlum,hokie66,and weab .

I just want to inquire one last thing . . if the plan states that ties spacing is 1 @ 50 mm,5@ 75 mm , 10 @ 100mm , rest @ 200mm .. where will the 50 mm spacing start? at the hook or above the footing ? if 50 mm starts at the top of the footing ,what will be the tie spacing within the footing.?

sorry if i'm just too slow to learn bros..

 

[rapt]

Ask the person you detailed the plan!

 

[hokie66]

That sounds like stirrup spacing for a beam, and the 50 would typically start from the supporting column or beam. Very unusual for a column to have that many different spacings.

 

[dik]

For small bars, 90deg hooks are not normally needed. The development length is normally adequate. Hooks are definitely not needed for the top bars unless you have substantial uplift and the bars cannot be developed.

Hooks from your piers should be turned the other way.

Dik

 

[Penpendrum]

thanks for the additional info #dik.. hello rapt and hokie66. . . Let me tell you the real story.. the truth is i live in the philippines (a third world country unlike yours) . this is a government plan and the system is too shaky that they could not rellay my inqueries in the their central office. they suggest that the 2" spacing starts from the column hook. but it does not seem to be right. that is why im asking help from people like you guys who understands the ACI code better.

 

[Agent666]

If in doubt, put them through at 2" throughout. The requirement is pretty vague unless it's backed up by some clarification on the drawings.

To kootk, on your reply to me, we end up with top reinforcement almost always due to punching shear and carrying some level of moment at the base of the columns in continuous concrete frames. With single bottom layer of reinforcement punching shear with moment is compromised, due to minimal effective depth on one side. There is always a moment, even if it's conveniently ignored in design/analysis of the superstructure, but you shouldn't ignore it in the detailing! (not round here with earthquakes)

 

[KootK]

@Agent666: I understand your position and agree with it to a large extent. That said, often the gravity load applied to a footing is such that the incidental moment at the column/footing joint can be handled with out inducing a hogging curvature into the footing and, therefore, without creating a punching shear demand for top steel. In my opinion, this would apply where:

1) Footings are supporting gravity only framing and;
2) Levels of expected seismic drift are small.

These limitations would be satisfied for most footings installed in North America. The situation is quite analogous to punching shear at roof slab joints where, despite the general demand for moment transfer between columns and slabs, it's rare to see a bottom mat of reinforcing that rivals the top mat in any meaningful way. Another feather in the cap for integrity steel I suppose.




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.

 

[AngDagat]

I am also in The Philippines. I was small building contractor before in Europe.

I am just starting my second 2 story building here. It is 2 story with part 3 story (stairwell to concrete roof). It is 300 sq M (excluding 4 balconies and roof of another 200 sqM)

I always consult with my engineer. Some of the 15no. footings for this project are 2.2M x 2.2M x 0.4M. There is only bottom rebar, typically 16mm matting at 150mm spacing. 17 each way in the case of 2.2m footing. The reason this is 2.2M is mainly due to the 4 cantilever balconies at the sea side, plus the 3 story stairwell (5.4m x 3.2m) and the concrete roof cantilevered 1.2m throughout.

Some of the columns (3) have 16no. x 16mm vertical bars, with stirrups (called rings here) in a 3 set configuration. That means one holding all 16 bars, the second holding 6 bars and the third holding 6 bar. These are sandwiched together as aset. Then we measure 50mm tho the next set. We use 4 sets @50mm. next 6 sets @75mm. next 6 sets @100mm. The remainder are at 150mm. For stirups not requiring sets the same distance above are used. This spacing is also used coming outwards of beams (including bottom tie beam).

The L's (or hooks) at the bottom of the column vertical bars are joined to the matting.

In answer to your last question. We start the stirrups at the L, so it is typically 2/3+ of the way down into the footing concrete.