Edge cover to vertical hook in slab top bar 1

[ajk1]

The following is asked with respect to our ongoing review of all of our standard typical details, and not with respect to any particular project.

1. What is the standard cover that would be specified to the vertical downward hook of top bars in one-way slab at the floor edge supporting spandrel beam?
In other words, should the slab vertical hook be placed to the outside of the supporting beam top bars (which may in some cases require moving the beam top bars more away from the exterior face of the beam (depending on the slab bar diameter), or inside of those bars?


Would that be shown in the ACI Detailing Manual (a copy of which I can't find at the moment locate).


3. If the slab bottom bars were extended to the exterior face of the supporting spandrel beam and hooked up vertically with a standard hook, would it be appropriate to specify the same cover as the top bars?

 

[ajk1]

First thoughts are sometimes best, and this is one of those cases. I do not think it should be on the inside of the beam cage because:
- the standard practice as far as I can see so far, is to put it on the outside, and I am not looking to change the construction industry to change their ways, because that is futile. If that is not the practice, please do let me know
-the ACI detailing manual shows it on the outside, and I would want a compelling reason to differ from that, for liability reasons
- If it is placed inside the beam cage the bar setting workers will want to place it so that it bends around a longitudinal bar, and in a 300± mm wide beam with 2 top bars, that would locate the hooked top slab bar towards the inside face of the edge beam, which would be a terrible place to put it from the point of view of strut-and-tie action to resist the radial forces at the bend
- although I have spent a good portion of my engineering career on rebar corrosion, and was for 10 years chairman of the technical committee on parking garage corrosion protection, I cannot identify any corrosion issue here (except in the very unusual case where the exterior face is exposed to chloride splash)
- I believe that hokie66 lives in Australia, perhaps where there is salt in the environment from the ocean spray, and is far hotter most of the year (accelerating the corrosion rate) than in Toronto, and in any event such an exposure condition should be dealt with by the measures in Canadian Standards Association (CSA) S413 Parking Structures, for which there may not be an equivalent Standard in Australia.
- if there is a corrosive environment, the the vertical legs of the stirrups would also be exposed to it, so they would need more cover too, and whatever side cover is used for the stirrups should be close to enough cover to protect the vertical leg of the slab top 15M bar -- if not, then increase the side cover to the stirrups

I conclude that KootK's first suggestion was soundly based (as usual) and correct, viz to place the vertical leg of the slab hook outside the beam cage.

 

[hokie66]

I am travelling, so didn’t respond sooner, but KootK explained my reasoning correctly, and I stand by it. I would give bar busters more credit than to place the cogs directly around an inboard corner bar, but if they did, they would learn quickly.

 

[ajk1]

I very much appreciate the time both of you have spent considering this question, as well as others that I have posted in the past. Nevertheless in this case I stand by my opinion, for the reasons that I stated, although I understand your reasoning. So this one time we will have to agree to disagree.

 

[hokie66]

ajk1,
It is difficult to argue against a long accepted detail like this, but I will persist with giving you my thoughts about the detail.

The specified cover to beam sides is normally used by detailers to size the width of stirrups. Therefore, if the slab top bars are of larger diameter than the stirrups, which is common, that in itself will violate the cover requirement. Also, pin size for bending the larger bar tails may be larger than the stirrup pin size, further pushing the tails into the cover. As well, for theoretically 90 degree bends, if the bend is just a bit incomplete, the ends of the tails can be even closer to the form.

They are the constructability issues which can reduce the desired cover. I believe there is rarely a technical reason to insist on the tails encompassing the outside beam bar, as the end span slab bars are nominal, not working very hard.

 

[ajk1]

To have corrosion there must be oxygen and water.
In Canada, the edge beam is located within the conditioned building envelope, because the vapour and air barrier and thermal insulation are located outside the structure (generally on the inside face of the exterior architectural cladding, or similar) Therefore I do not see a source of moisture to cause corrosion.

If your argument were that the construction tolerances may result in the hook having less side cover than expected, then I would agree with you. Perhaps then we should require somewhat more side cover to the stirrups in edge beams. However I would be reluctant to try to change the general practice with regard to cover or to hook location, but will sleep on it for a few nights and discuss in office and our field inspectors.

Thanks.

 

[ajk1]

I have our internal office meeting Thursday to hopefully finalize some of the typical details that we are reviewing, including this question about the end cover to vertical face that we should show on our typical detail for top slab bar tail at edge supports in a normal non-corrosive environment.

I think we have established to-date that in such cases the standard is to show 1.5" side cover to beam stirrups in edge beams, as this is what the ACI Detailing Manual shows. The size of slab top bar might be 25M ± (Canadian, similar to American #8 imperial bar), as this might occur in a two-way flat slab column strip that extends perpendicular to the slab edge, where a certain area of top steel must be placed within a width of the support width + 1.5 t_slab each side I believe.

For a 25M top slab bar extending across an edge beam, 25 MPa concrete, non-corrosive environment,
l_hb = 500,
l_dh = 500 x 1.3 = 650 mm for > 300 mm concrete below (assuming such is the depth of edge beam),

if hook tail has 50 mm cover,
l_dh = 0.70 x 650 = 455

So for edge beams < 650 mm wide, the 50 mm cover to the hook tail seems to provide a significant benefit, although for beams less than 455 mm wide, there is still inadequate development l_dh even with 50 mm cover to the hook tail.

However, if the slab top 25M bars are to be specified with 50 mm end cover to the tail of the hook, then top longitudinal bars in the edge beam will have 50 + 25 = 75 mm side cover which would require the side cover to the vertical stirrups to be 75 - 10 = 65 mm (2.5"±) which is significantly different than shown in the ACI Detailing Manual.

So there seem to be two choices:

a) specify the stirrups in edge beams to have 65 mm cover (to each face of the beam for practicality)

or

b) place the hook inside the beam cage (which I think risks placing it too close to the inside face of beam which would be dangerous)

So I would tend to option a). What do you think?

I am interested in everyone's opinion, but particularly from our American friends.
Surely this is a common situation to which there has long ago been found a solution. Unless I am making a big deal out of nothing, or am failing to see something obvious...or perhaps ACI deals with it differently.

 

[MotorCity]

Here is my take. Cover is cover, whether its cover to a main bar, stirrup or whatever. If you detail the hook on the outside, cover is to the hook. If you detail the hook on the inside, cover is to the main bar. From there you simply determine the cover based on whether the bar is in tension or compression.

 

[KootK]

Given that this exercise is about tweaking standard details, why not do both? Show the two, alternate positions and say:

- At contractor's option. Choose your own adventure.
- If you put it on the outside, nail the cover.
- If you put it on the inside, shove it up next to the outermost longitudinal.

I'd not be fattening cover up all over the place for this reason alone. Those things tend to cascade on you and all of a sudden you've got:

- Less bars able to go into a row for a given beam width and;
- Beam bars that would normally pass outside column cages now interfering.
- Confusion from rebar detailers and placers who work with a lot of engineers all doing more or less the same thing.

 

[Settingsun]

I've looked up the Australian detailing manual. It specifically says not to have the cog/hook in the cover. It should be within the beam cage. Reason not stated.

If you're worried about the position being too close to the inside face of the beam, your detail could note that the cog is to be in contact with the beam's outer corner bar.

 

[ajk1]

Our internal office meeting decided yesterday to show the hook within the beam cage, wired to the exterior corner bar(this decision was made before seeing the June 27 comments here, including by steveh49). Our chief field engineer feels that it will be placed properly as shown. So it is gratifying to see that steveh49 finds that the Australian Code shows it this way. I believe that hokie66 (who is in Australia I believe) said this early on and kootk subsequently agreed with him, so thank you. The steveh49 suggestion to include a note that the cog is to be in contact with the beam's outer corner bar, is good.