construction joint in concrete beam

[rittz]

Attached is a sketch of a portion of a foundation for a chemical storage shed. The Contractor wants to pour to the underside of the slab, remove the inside forms, and then pour the slab. Would that be acceptable?

 

[JAE]

rittz,
Here's a couple of thoughts on it:

1. Yes you can place the horizontal construction joint at the level aligned with the bottom of the slab. Specify a 1/4" roughened surface throughout.

2. If the beam is spanning and takes significant shear, the two legs of the stirrups must now serve a shear friction function across your new joint in addition to the shear in the beam. This will require additional stirrups most likely.

3. Since the stirrups are now working in shear friction, they must meet standard hook requirements vs. stirrup hook sizes. This means that you must develop the stirrup hook past your joint by the hook development length, ldh.

4. The void you have below the beam - if it is a product that deteriorates over time, then you will need some kind of fill retainer on either side of the beam to keep the soil on the sides of the beam from sluffing into the void space over time.

 

[slickdeals]

Agree with JAE on all the above. Look at the ACI code (if it applies) in Chapter 17 for composite construction requirements.

 

[BAretired]

Your stirrups should be closed and they should extend to the top of the beam. The short dowels may be eliminated.

If the pour ends at the bottom of the slab, make sure the surface is roughened.

BA

 

[JAE]

I would agree that the stirrups may have to extend further, vertically, but only if they need to so as to provide the hook development length above the bottom of the slab/joint.

But many times those 6" high curb extensions are tough to form with flying forms and it is easier to place the curbs later, after the beam/slab has been placed.

If you can't get your ldh within the slab thickness (which you probably can't) then yes, extend the stirrups higher.

I would agree with BAretired in that as a perimeter beam we would typically use closed stirrups - a "U" shaped stirrup with standard hooks (not stirrup hooks) and a stirrup cap.

 

[Teguci]

Just throwing this out there - I usually show the grade beam earth formed under the slab. That way the grade beam and the slab are poured together and the contractor doesn't have to remove any forms. Another possibility is to use rigid foam to form the inside face if you are concerned with the final shape of the grade beam (probably too late for this).

I'd show a construction joint between the curb and slab though with key and, if necessary, waterstop.

The goal is to cause as few pouring sessions as possible. I prefer to tell the contractor exactly where I want the construction joints on the drawings.

 

[csd72]

It is better to have a battered inside face to the beam anyhow as this provides (slightly) less resistance against shrinkage.