Grade beam modifications
Grade beam modifications
(OP)
There is a 10" x 24" deep concrete grade beam reinforced with 2-30M (#9) bars top and bottom and 10M (#4 - 0.16 sq. in) stirrups at 24" o.c. The grade beam is supported on concrete piles (piers) at 20'-0" on center. Total length of grade beam is 100'-0". The total factored uniformly distributed load on the grade beam is 38 kN/m (2.6 klf). The yield strength of the reinforcing steel is 400 MPa (58 ksi) and the 28 day compressive strength of the concrete is 30 MPa (4.4 ksi). Concrete cover is approx. 2".
Problem: A 5" deep x 36" wide notch (for a new door way) is required on the top of the grade beam at a location 7'-0" away from the pile center line. The center line of the 5" x 36" notch is 7'-0" from the pile center line.
Does anyone have experience with this sort of a challenge? What are the options available? Thanks in advance for the help.
Problem: A 5" deep x 36" wide notch (for a new door way) is required on the top of the grade beam at a location 7'-0" away from the pile center line. The center line of the 5" x 36" notch is 7'-0" from the pile center line.
Does anyone have experience with this sort of a challenge? What are the options available? Thanks in advance for the help.






RE: Grade beam modifications
RE: Grade beam modifications
If the beam has not been cast, then you may want to hook the #9 top bars at the notch, provide straight bars below the notch and extend them (development length + some) beyond the notch. Check for the shear in the beam in the notched segment, increase shear steel if necessary.
This is all assuming that the beam has not be cast already. If it has been cast, then the options are more limited.
RE: Grade beam modifications
RE: Grade beam modifications
RE: Grade beam modifications
RE: Grade beam modifications
Also, are you in a frost zone? If so, the 24" deep grade beam seems to shallow and could be exposed to frost heave.
It might be worthwile to attach a sketch.
RE: Grade beam modifications
RE: Grade beam modifications
If you are expecting a net uplift from frost, you will need to find a way of making the top steel continuous as suggested in earlier posts.
BA
RE: Grade beam modifications
RE: Grade beam modifications
Vu=2.6 klf x (20'-2'(piers assumed))/2 = 23.4 kips
1/2*phi*Vn=1/2x.75x2(4400)^(1/2)x10"x(24"-3.5")/1000
=10.2 kips
For a simple span, you have a 230% overstress in shear per ACI 318. For multiple spans it will be even worse.
RE: Grade beam modifications
RE: Grade beam modifications
RE: Grade beam modifications
It's nice to have a 5" step at door openings... keeps the rain from coming in <G>.
Not much you can do without overcutting the notch and epoxying embedded top bars at the notch... the stirrup ties can be exposed by using a chipping hammer and bent over the new top dwls.
Dik
RE: Grade beam modifications
Is this to essentially replace the top bars (to be cutoff) with new top bars but now located below the notch?
RE: Grade beam modifications
Why do you have a 1/2 factor on your shear capacity? ΦVc/2 is only to check for required minimum stirrups, not strength checks.
ΦVn = 0.75 x 2 x sqrt(4400) x 10 x 20.5 = 20.4 kips
Shear d from face of support:
with 24" dia. piers - distance from centerline to applicable Vu point = 12" + 20.5" = 32.5" from center of support.
(2.6 k/ft)(20/2) - (32.5/12)*2.6 = 18.95 kips
Granted, Vu > ΦVc/2 so minimum stirrups would be required, which they are not.
But also, this may not be an ACI 318 application. The OP didn't specify what code it falls under.
RE: Grade beam modifications
You have to check for flexure, etc. and transfer the udl across the door opening.
Dik
RE: Grade beam modifications
Since the shear reinforcing is spaced at 24" which is greater than d/2 you effectively have an beam that is not reinforced for shear. 1/2*phi*Vc is the nominal shear capacity of concrete beam without shear reinforcing.
RE: Grade beam modifications
RE: Grade beam modifications
Even better, get the architect to shift the door to the middle of the bay. That way, shear is no more critical than it was without the opening. All of the other grade beams are equally critical, so why worry about this one?
Since you are using CSA A23.3, I assume you have frost in the winter. You should have a voidformer under the beam.
Forget about trying to replace top reinforcement across the opening. Instead, sawcut the opening to the exact size required, cutting the top bars and stirrups. There is zero shear for the length of the notch, so the code is satisfied in that location.
Add a galvanized plate under the notch on each face of beam and fasten with Hilti bolts each side of the opening. This will provide for nominal uplift.
BA
RE: Grade beam modifications
I think I'd reinforce it...
Dik
RE: Grade beam modifications
Shear in a beam does not generate tension in the top fiber. Frost heave generates tension in the top fiber. Without frost heave, the top fiber at midspan is in compression.
The only way to get rebar across the door recess is to cut deeper into the beam, drill and embed bars each side of the recess, then fill with concrete, a questionable strategy in my opinion. It is better to reinforce externally as I suggested in my earlier post.
BA
RE: Grade beam modifications
There's no way I would use code tricks to take the design right to the edge when shear is the failure mode holding up multistory brick and load bearing wall. I still think sistering on a properly reinforced grade beam is the way to go. I just believe in getting the foundation right from the start.
RE: Grade beam modifications
In the remaining bays, you could add one pile at midspan in order to reduce the shear stress.
BA