Concrete Link Beams - Pour Sequence

[bookowski]

What is the preferred pour sequence/joint location for r/c link beams (beams linking shearwalls/cores). I am not referring to very large diagonally reinforced link beams, but the smaller traditionally reinforced type. Typically walls & columns are poured to underside of slabs and/or beams, then the slabs/beams, then again with walls/columns. With link beams this would create a joint at the slab depth below t.o.beam. It seems like there are the following options:
- Pour full link beams, leaving a ledge at sides with dowels out for the slab - you'd get a slab joint/crack along the beam.
- Block out the wall at the link beam including some depth back into the wall - this seems difficult to leave all the proper splices projecting
- Stop at b.o.slab and design all link beams as if they are shorter, thereby discounting the slab running over the top (reinf/ties could be held low as well). I've seen a variant of this detail where the link beam is held 2" low and the slab is poured thin over the beam - not sure if that is a good detail or not.
- Stop the entire wall low and then pour the upper wall + link beam + slab, this one would be a tough sell to the contractor.

 

[RHTPE]

bookowski:

If story height and the required link beam depth permits, cast the walls and link beams as a monolithic unit, holding the top of the walls & beam to the underside of slab elevation where necessary.

This could permit the wall forms to be utilized for the beam construction. Just make that clear on the contract drawings at bid time so that all bidders approach it the same way.

Casting a full depth beam over a previously cast wall is never an ideal situation, especially if one or more faces of that beam are on the shaft side of the wall. It may also make it difficult to utilize gang forms for the walls if the building's height justifies the use of gang forms.

When story height is restrictive, consider:
1) permitting a horizontal construction joint in the link beam such that the top bars are in the slab. Thicken the slab a bit if that helps, after all the amount of additional concrete due to a thicker slab within a core would be minimal. The beam can then be cast with the walls as suggested above.
OR
2) use mechanical couplers at the beam/wall interface and a minimal seat for the link beam. Cast the slab & link beams together after the walls are complete.

A serious consideration of the various contractors' means & methods is in order here, as this is probably the most difficult construction challenge in highrise cast-in-place concrete construction. it's such a small portion of a floor yet it's such high consumer of manhours compared to the rest of a floor.

Another somewhat far-out thought - precast the link beams with the bars projecting from each end. Set one face of the wall forms and drop the P/C link beams in place before tying the wall reinforcing. Obviously the link beams and wall must be the same thickness. Treat the top of the beam elevation as woukd be best for the project. Seating the slab into a shallow ledge on the beam or over-pouring the beams with 2" of concrete always results in a crack along the top of the slab. If the finishes hide it, so what. If they do not, then reconsider the situation.

Ralph
Structures Consulting
Northeast USA

 

[hokie66]

A sketch would help. What is preventing the casting of the link beams with the floor slab, just as for the other beams, which you indicated is typical?