Grade Beam Reinforcement Requirements 3

[JohnnnyBoy]

I just found something in the CSA A23.3-04 code that basically sizes a grade beam used for construction as well as needed reinforcement. I've attached the section of the code as well (21.11.3.2).

Basically it says that for a normal grade beam (24"x8") at least in my firm, needs reinforcing ties = 0.5 the least cross sectional dimension or 300mm (12"). In my case that would be every 4" c/c. This seems like a lot of reinforcement to me and I have never seen this much placed on site. As for the other stipulation I think it seems fairly reason as I do not normally span over 12'-0" and 144/20 = 7.2" which is okay.

I assume this reinforcement detail is for settlement of the piles and or lateral displacement of the pile causing a shear force in the section. Let me know what you guys this and if in this case there should be that much reinforcement. It also has no specification as to increase shear ties at the high shear locations (i.e close to the piles) and decrease at lower shear locations. If this was only specified for a certain length it would make much more sense to me.

 

[msquared48]

Seems like a lot to me too for gravity loads

Perhaps they were considering loading on the weak axis too, though, as from soil pressure to resist sliding?

Mike McCann, PE, SE (WA)

 

[BAretired]

There are a great many existing grade beams which do not comply with that requirement. I wonder what genius came up with that clause.

BA

 

[KootK]

I believe that provision is intended specifically for grade beams used as seismic ties between foundation elements. As such, I suspect that the tie spacing noted is about preventing buckling in the longitudinal rebar when the grade beams experience axial load demand. Perhaps some confinement benefit too although you'd think that the quantity of reinforcement would need to be specified for 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.

 

[wannabeSE]

ACI 318 has the same requirement in the seismic provisions when a grade beam acts as a tie between foundation elements (ACI 318-11 §21.12.3.2). I agree with KootK about the axial loading. It is probably cheaper to increase the 8" dimension and provide more concrete and less reinforcing.

 

[jayrod12]

I'm in the seismic tie group as well. I'm fairly certain chapter 21 is the seismic portion of the CSA code. I don't have mine handy to confirm.

 

[jdgengineer]

I don't have the ibc code in front of me but I believe there is a provision that does not require this tie spacing where the gradebeam is designed for the overstrength factor. I'm not sure if there is similar language in your code.

 

[jdgengineer]

Ok I looked it up on my phone. In 2012 IBC it is section 1810.3.12

 

[wannabeSE]

jdengineer
I see your point, but I interpret that section differently. When the overstrength factor is used, grade beams don't need to meet the special moment frame requirements. I don't know if it also applies to the ties between foundation elements. I'd like to here what other say.

 

[msquared48]

In reading the attachment it says "...Grade Beams NOT connected to a slab...".

This implies to me that there may be some consideration given to torsion on the unsupported beam section here.... The presence of a slab would negate any torsional effects.

Thoughts?

Mike McCann, PE, SE (WA)

 

[BAretired]

If the grade beams are not connected to a slab, the piles are not being tied in two directions as required in CSA 23.3.

Typically, in my experience, torsion is not a factor with grade beams but in situations where torsion is a factor, they should be designed in accordance with the torsion provisions of the code.

BA

 

[hawkaz]

I would think slenderness rather than torsion. If the grade beam takes axial loads and is not attached to the slab, its a long skinny column on its side. ACI7.10.5 has the a similar tie requirement for compression members

 

[jayrod12]

One thing as well OP. Is there a reason you're using the A23.3-04? As opposed to the -09 or the new -14 version? Surely wherever you're practicing the AHJ has at least enacted the NBCC2010 even if they haven't gotten around to approving the 2015 (mine is still dragging their heels on the 2015). However the wording of the clause in question is still identical.

Does an 8x24 grade beam qualify for a horizontal tie in your situation? The first part of that clause indicates the smallest dimension must be greater than or equal to the distance between columns divided by 20. Are your columns only 13ft apart that you're tying together?

 

[Canuck65]

I agree with the others above that the grade beams are being used as seismic ties between piles. If there were grade beams in both the N-S and E-W direction, would that not tie them together in both directions, with or without a slab?

An 8" wide grade beam seems awfully skinny, given the requirements for side cover x2, tie diameter x 2 and longitudinal rebar. Increasing the width will increase the tie spacing to something more reasonable and likely will result in an overall cheaper construction cost for the grade beams.

 

[JohnnnyBoy]

Thanks everyone for the input. Yes you were right this is for seismic regions only, I must have missed that when I was originally looking into it. As for the code I am not to sure since it is the 2010 print of the code although we have purchased the new 2014 code as well. And I would agree that since it is seismic restrain that the width of the grade beam would be the governing factor and the amount of ties accordingly should be correlated to the width of the grade beam for shear. As for tying in to the slab this make much more sense now as the slab would be resisting any differential lateral movement between the piles should everything be tied together.

 

[jdgengineer]

I agree 8" seems a bit narrow for 2 bars top & bottom with seismic ties. Seems like you would need at least 10" in width (preferably 12").

While I know this is a slight deviation from the original question as it references the IBC code, but isn't the only reference to the tie spacing in the IBC Section 1810.3.12? Seems to me this section pretty clearly states that you do not need to detail these elements to section 21.12.3 of ACI 318-11 where overstrength factors are used. 21.12.3.2 is the reference to the tie spacing, which seems would not apply where gradebeams are designed for the overstrength factor.

See attached for excerpts from code. I definitely could be misinterpreting this, but seems like I am reading it correctly.

 

[KootK]

I'd just like to add a little extra clarity to the situation here for all of you non-middle-Canadians. Which, I'm sure, is all but three of us. Here's how it is for us locals:

1) We are relatively aseismic and most design and construction is approached that way. I disagree with this philosophically but that's an issue for another day/thread.

2) As you Yankees have state birds and state flowers, we kind of have a "state grade beam". Provincial really but that just doesn't have the right ring to it. Our "state grade beam" is 8" wide and usually between 24" and 48" deep. It's the default market expectation for any work of a residential or light commercial nature.

Does it suck as a seismic tie? Yup. From a cover and tolerance perspective, is it unreasonable to pack it full of flexural reinforcing and closed stirrups as we do? Yup. Do I even have any idea how field tolerances are accommodated when, inevitably, 8" block or precast walls are stacked up on these 8" grade beams? Nope. In summary, we don't love our state grade beam but are kinda stuck with the darn thing.

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.

 

[sandman21]

Hey not all of us are Yankees......... But us Yanks have the same issue, while most of the seismic activity is on the West coast. Back has its own hot spot, but the exceptions for details dont allows align with good design and detailing. It gets real fun when they bring the same design philosophy out West.