Grade Beam Reinforcement Requirements
Grade Beam Reinforcement Requirements
(OP)
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.
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.






RE: Grade Beam Reinforcement Requirements
Perhaps they were considering loading on the weak axis too, though, as from soil pressure to resist sliding?
Mike McCann, PE, SE (WA)
RE: Grade Beam Reinforcement Requirements
BA
RE: Grade Beam Reinforcement Requirements
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.
RE: Grade Beam Reinforcement Requirements
RE: Grade Beam Reinforcement Requirements
RE: Grade Beam Reinforcement Requirements
RE: Grade Beam Reinforcement Requirements
RE: Grade Beam Reinforcement Requirements
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.
RE: Grade Beam Reinforcement Requirements
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)
RE: Grade Beam Reinforcement Requirements
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
RE: Grade Beam Reinforcement Requirements
RE: Grade Beam Reinforcement Requirements
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?
RE: Grade Beam Reinforcement Requirements
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.
RE: Grade Beam Reinforcement Requirements
RE: Grade Beam Reinforcement Requirements
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.
RE: Grade Beam Reinforcement Requirements
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.
RE: Grade Beam Reinforcement Requirements