grade beam detail question

huangyhg

grade beam detail question
i have always seen grade beams poured separately from the slab at grade level with dowels going from the grade beam into the slab. what is the disadvantage of pouring the grade beams integrally with the slab on grade (example 2'-0" deep grade beams)? i am figuring this it must be an ease of construction condition to do the pours separately but i am not sure why.
i know that elevated floors are required to have the beams cast integrally with the slab which i am more familiar with than grade beam supported slabs.

it depends in part on whether the slab is a structural slab where the "grade beams" need to be cast with the slab, but not necessarily monolithically, or a slab-on-grade, where the grade beam would work with the footings, separately from the slab.  
in the separated circumstance of the slab-on-grade, it is easier to deal with temperature and shrinkage issues in the slab.  
also, if this grade beam is a tension link, then it would be better to allow the slab to float on top of the grade beam with no connection between the two.
so the answer to your question is not so much economics, as how you model the interaction of the two in your structural solution to your foundation design.
mike mccann
mccann engineering
mike-
i had forgotten i just designed a 'ribbed' slab last year for a slightly expansive soil area and i detailed the slab to be cast monolithically with the grade beams. it did not require deep foundations.
the existing project i am looking at now was done in south louisiana so i am pretty sure it is expansive soils. it is a structural slab supported by hundreds of interior timber piles with grade beams only around the perimeter. and the slab is cast separately from the grade beams with dowels going from the beam into the slab.
there are tilt panels around the perimeter so i think the idea was to add some stiffness around the perimeter to carry the walls, and other than that the 8" structural slab was sufficient at the interior. i am not really sure why he didn't just cast the the grade beam monolithically with the slab around the perimeter though. by adding all the dowels, he did restrain the shrinkage of the slab at the perimeter to some extent, and there are no tension tie beams.
haynewp - maybe just an engineer second-guessing what the contractor would want to do (in terms of construction joints and sequencing concrete placements)?
really, in most concrete, with adequate reinforcing throughout the sections, whether you have a joint or not really doesn't always affect the behavior of the concrete structure once completed.
i do agree with mike above that maybe some shrinkage issue was thought about.
sometimes the slab will be poured last to avoid damage due to construction.
are the wall panels sitting on the perimeter beams or on the slab edge?
in tilt-up construction, usually there is a 4' wide +- "pour" or "closure" strip at the periphery of the slab next to the wall.  a lot of the slab shrinkage takes place before the the pour strip is placed.
due to the piling, there will be little if any settlement of the slab.  if the tilt-up walls are not on piling too, they could settle/heave differentially with respect to the slab. perhaps this is why they are not tied?
mike mccann
mccann engineering
i think in the builder's eyes it is just a constructability issue.  he wants to pour the beams as quickly as possible before the sides of the excavations start collapsing.  it is a lot easier to clean out a beam if you don't have the slab reinforcing in place.
an advantage to placing both the grade beams and slab at the same time is just that, the trucks and crew only have to be out there once. one way to proceed is that the thicker sections (ie grade beams)can be puddled early in the day and the slab concrete placed later in the day. you would have a cold joint at the top of the earlier placement, which may or may not be an issue.  
the tilt panels around the perimeter are on the grade beam which is spanning between lots of timber piles. there are timber piles everywhere, but the grade beams are only under the tilt up.