grade beams for caissons

[bryson85]

Do caissons, 24" dia. need grade beams if they are supporting precast concrete walls?

 

[archeng59]

it is possible to eliminate the grade beams as long as the walls can span between the piers.

 

[COEngineeer]

I think grade beam will serve as a level surface. Not much area to play with w/o grade beam. Plus the spacing of the piers will dictate the size of your wall.

 

[bryson85]

My main concern is, if there a code issue for grade beams being required. I know that for piles grade beams are required unless there are three. But since caissons are larger, are they stable enough with the grade beams?

 

[jike]

All previous comments are very good. My concern is a practical one. It is pretty difficult to get the top elevation of caissons within 3" of the adjacent one, therefore, you might need to pour a cap or a grade beam to the proper elevation.

 

[jloo73]

A minimum would be a concrete cap to allow for proper elevation and redoweling/embedments.

Another consideration is frost. Colder climates we typically have a grade beam to below the frost line if an interior slab is poured next to the wall.

 

[csd72]

Also,

If you intend to put a caisson under each panel joint, remember that during installation the first panel installed will induce an eccentric load that needs to be taken out in the caisson.

Also if your walls have significant axial loads then you cannot rely on the full width of the wall to resist this axial load your axial force would be transfered at the ends something between a deep beam and an upside down concentrated load on masonry.

If you are relying on the concrete panels for shear walls, then the isolated caissons would need to be designed for lateral load whereas with a grade beam it acts more like a continuous portal frame and is much stronger.

I doubt any code will give any specific restrictions on theis, you just need to be confident that it will work.

 

[civilperson]

No grade beams are needed in frost climates or any other. The caisson is founded below the frost depth and that is sufficient for the walls and other supported elements. The method of connection to the other elements is paramount in importance as noted by csd72 in the portal frame example.

 

[STR04]

Keep in mind seismic considerations per IBC.

1808.2.23.1 Seismic Design Category C.

Where a structure is assigned to Seismic Design Category C in accordance with Section 1616, the following shall apply. Individual pile caps, piers or piles shall be interconnected by ties. Ties shall be capable of carrying, in tension and compression, a force equal to the product of the larger pile cap or column load times the seismic coefficient, SDS, divided by 10 unless it can be demonstrated that equivalent restraint is provided by reinforced concrete beams within slabs on grade or reinforced concrete slabs on grade or confinement by competent rock, hard cohesive soils or very dense granular soils.

 

[civilperson]

The definition of ties could include the precast wall panels and other structural connections. The intent is to hold the various piers in the same position relative to each other when subjected to seismic movement.

 

[jike]

Frost can still push up on the bottom of the precast panel if not below frost.

 

[STR04]

Civilperson,

I agree with using a precast wall to brace in-plane but this does not satisfy out-of-plane requirements.

STR04

 

[civilperson]

Slabs with reinforcement at the top of the piers/caissons can brace in all directions, caissons with precast walls at right angles can brace pier top.

 

[STR04]

Agree, but if the slab is floating and does not tie into a perimeter grade beam or deformed bar anchors at the wall then you need to find other means to brace for out-of-plane.