Grade beam modifications

[Ipetu]

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.

 

[lkjh345]

If the notch absolutely has to be cut into the top of the grade beam (and I would exhaust all possibilities to check if there is another solution), it is possible to dowel on a 'sister' beam to the side of the orginal beam to reinforce the exsting beam and carry load through the area where the notch will cut the top reinforcing.

 

[slickdeals]

Has the grade beam already been cast or are you putting a blockout for the notch?

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.

 

[Ipetu]

This is an existing grade beam. Sorry I did not state this in my original post.

 

[PMR06]

I like lkjh345's idea the best... but I would question the 24" oc stirrup spacing on the remaining spans. Without rigorously checking it, I would have expected d/2 or about 10" oc.

 

[slickdeals]

Good point PMR06. If you provide shear reinforcing, you are required to meet the d/2 requirements.

 

[larsacious]

Is the 5" notch for a frost wall stoop? If so, can you drill and epoxy the bars into the grade beam instead of a notch?

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.

 

[Ipetu]

A sketch of the existing and proposed grade beam is attached. The grade beam has to be notched to allow for barrier free access into the building. The slab on grade inside the building will also be dropped by 5".

 

[BAretired]

Is the proposed doorway in an interior or end span? Looks as if you have adequate reinforcement on the bottom to carry the load with the reduced effective depth and without compression steel. The stirrup spacing near the piles is questionable with or without a notch.

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

 

[hokie66]

The first step is to analyze the beam to determine whether it is deficient or not. Then you can decide what to do.

 

[larsacious]

Looking at it again, your beam will fail in shear without even removing the proposed 5 inches.

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.

 

[hokie66]

Perhaps you need to look at the structure above the floor, and find a way to take the load to the piles without relying on the grade beam.

 

[Ipetu]

hokie66, now that is outside the box thinking. I was trying to think outside the box but to no avail...I appreciate your suggestion.

 

[dik]

Conc shear stress is only about 80 psi and stirrups are likely for holding the top bars in place with 24" spacing.

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

 

[Ipetu]

dik, can you please expand on "not much you can do without overcutting the notch and epoxying embedded top bars at the notch"

Is this to essentially replace the top bars (to be cutoff) with new top bars but now located below the notch?

 

[JAE]

larsacious,
Why do you have a 1/2 factor on your shear capacity? [&Phi;]Vc/2 is only to check for required minimum stirrups, not strength checks.

[&Phi;]Vn = 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 > [&Phi;]Vc/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.

 

[dik]

YOu have to look at using a chipping hammer to remove the top 7 or 8 inches of beam as not to disturb the stirrups. The top bars have to be cut back and new top reinforcing added at a the new stepped level, wrapping the stirrups around the new dowels. You can use high strength dywidag bars for the dowels and Hilti Hy 150 hit system for anchorage. Then use an epoxy based cementitious grout to finish off the grade beam.

You have to check for flexure, etc. and transfer the udl across the door opening.

Dik

 

[larsacious]

JAE-

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.

 

[Ipetu]

The code is CAN/CSA A23.3. I think it is similar to ACI 318.

 

[BAretired]

Your notch is centered 7' from a pile center or 3' from midspan. The edge of opening is 5'-6" to the center of pile. Factored shear at edge of opening is 11.7k if you are in an interior span.

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