Location of construction joints 5

[phuduhudu]

When casting continuous concrete beams it is normal to place construction joints at 1/3 points of the span on the basis that the moment is least there. Are minimum moments really the crucial factor though? Would it not be more prudent to place the joints at the point of minimum shear which would be at midspan? My reasoning is that shear across a poorly bonded day joint would be more dangerous than moment. With a moment the steel deals with the tension and any poor bonding in the compression zone will be improved by the compression.

Just wanted to question a normal procedure. Any thoughts? Carl Bauer

 

[macknwo]

Dear Carlbauer,

Your toughts are correct. I recently attended a concrete seminar two weeks ago and the speaker said the exact same thing that you are recomending, if you are going to have a construction joint it's better to have it near the point of no shear. This same recomendation holds true for plumbing and electrical tubes, if they have to cross a beam, it's recomended that it be in the middle third of the span.

 

[Ingenuity]

you do need to be mindful of the construction aspects of joints. for example i have seen formwork removed from construction pours (delay in next pouring) and the beam (or slab) has had to cantilever from the previous support to the new CJ. with a CJ at midspan that is a tall order, and the hence a preference for joints at 1/3 points.

also, most joints in beams and slabs are formed with a key, or cast-in sacrificial stop-ends that provide shear transfer in addition to rebar.

also, joints at midspan do not typically work for post-tensioned construction. you need to accomodate anchorages and at approx 1/3 span works best.

additionally, at 1/3 point where moment is reduced (or near zero), the shear is reduced (1/3 that at support for a UDL).

precast segmental bridges with dry joints also work based upon keys and prestress.

just my comments



mark

 

[phuduhudu]

Thanks for those comments. I wasn't thinking of prestressed construction as we don't do a lot of it here and I also hadn't considered depropping the formwork over part of the span as that would not be common practice. I am thinking of typical multi bay reinforced concrete frame buildings. In that case is the conclusion that midspan is the best place for the CJ?

Macknwo, you say the same holds true for penetrations through beams except that in that case I would be inclined to put them at 1/3 points as they may take out part of the compression zone of the beam so reducing the moment capacity. Carl Bauer

 

[dik]

Usually require that joints occur in the middle third, not necessarily at 1/3 point.

My preference is that beams be spliced closer to the third point and slabs at mid span; beams, for the reason that Mark mentioned, although I generally insist that the contractor provide reshores.

With any sort of alternate loading and for uniform spans, the bending moment is reasonably 'flat' in the middle third of a beam or slab, so for flexure it doesn't much matter, shear is the main concern and in the middle third, shear is generally light. For transfer girders, where a large point load occurs, particular care should be taken.

Usually take a 'gander' at shear design for splices and where it is a problem, usually detail it using shear friction or as a last resort, 2x4 keys.

Also, usually ask the contractor to provide a sketch showing where he would like to place pour joints.

We have several hundred standard details for just about anything (keeping track of them is another problem) and our standard detail for beam splices shows a keyway. The same standard detail has been used by several consultants and had originally been 'cribbed' from one prepared by one of our foremost consultants of yesteryear...

 

[JAE]

We generally follow the "middle third" concept, preferably at midspan. Also agree with dik on getting the contractor ALWAYS to send you his plan for layout of CJ to ensure that the joints stay at midspan. Some contractors come up with pretty wild joint plans due to their other operations.

The cantilever action doesn't matter in CIP concrete floors as the entire area is formed and we require the formwork to stay in place until concrete reaches 75% of f'c. Then, and only then, do we allow re-shoring of the floor.

In beam/slab or pan joist floors, when the CJ crosses a main girder at midspan, it is running parallel to the repetitious beams/joists. Here, we either allow a joint at the midspan of the slab (between the beams) or along the edge of a beam, creating a 1 1/2" wide shelf in the top corner of the beam to catch the slab. Of course, all of the reinforcing is always continuous through the joints.

 

[Gourile]

The point is that in the 1/3 span you usually got enough reinforcement to carry shear of that location, as it is always far lower than the maximum shear of the beam, but in the location of zero shear you will have maximum moment that you will design reinforcement exactly for that, then a joint would be danerous.

 

[dik]

Gourile:
Generally, at maximum moment(MML), there is enough 'clamping' friction to accommodate the small amount of shear, realizing that the PZS location is by differentiation the MML. I'm not saying that you don't concern yourself with it...

If you have problems with shear at maximum moment (likely due to alternate loading), then you likely have problems elsewhere, too.