Rebar placement in below grade icf walls
Rebar placement in below grade icf walls
(OP)
I am a mechanical engineer and I have little knowledge about foundation design. My question has to do with rebar placement in icf walls (or most other concrete residential walls for that matter.) Why is it that when I look up vertical rebar replacement for below grade applications, the charts will say #5 rebar at 24" o.c. for example. The charts/details show the vertical rebar in the center of the concrete wall. It seems to me that placing the rebar here is least beneficial since it is at the neutrual axis of the wall. Shouldn't the rebar be placed towards the tension (inside) of the basement wall? I am just curious if anyone knows why they show the rebar in the centerline of the wall. At that location it would just prevent cracks from spreading, not necessarily taking the tensile load of the wall (since it is at the n.a.)
Thanks
Larry
Thanks
Larry






RE: Rebar placement in below grade icf walls
Do the tables tell you what kind of loads the wall is reinforced for?
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Rebar placement in below grade icf walls
Thanks again!
Larry
RE: Rebar placement in below grade icf walls
It is possible to design the wall for the steel in the center, but lacking a placement callout, I would place the steel to the inside face of the wall with 1.5" clearance.
Asw for the shear, any vertical and horizontal steel adds to the shear capacity of the system.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Rebar placement in below grade icf walls
The design should have been for a basement wall which would be acting as a simple beam, spanning between the foundation and floor diaphragm. Placement of the steel to the inside is optimal here. This places little impact to the footing size.
However, although very unlikely from the ICF manufacturer's perspective, the wall could have been designed as a cantilever, not requiring the presence of a floor diaphragm for lateral support, but necessitating a different footing arrangement, more steel, and placement of the steel at the soil face of the wall.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Rebar placement in below grade icf walls
For a 10' wall height and a 9' backfill height, the total load per foot is 1215#/'. The top reaction is 364.5#/'. Maximum moment occurs 5.93' from the top and has a value of 1562.5'#/' or a factored moment of 2344'#/'.
In order for this to work with any of the listed bar spacings, an effective depth of about 4.25" would be required, subtantially more than 5.5/2 = 2.75".
BA
RE: Rebar placement in below grade icf walls
The table is also for grade 40, with the spacing for grade 60 being 1.5 times that for grade 40. It would be better to hold the same spacing and reduce the bar size if possible if grade 60 is used.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Rebar placement in below grade icf walls
I agree that for a 10' high wall with 9' backfill, a 5.5" wall is pushing it a bit, but 3/4" cover over the reinforcement may be adequate, recognizing that there is an additional 2" of foam material outside the concrete cover.
My preference would be a 2" thicker wall.
BA
RE: Rebar placement in below grade icf walls
Thanks!!
Larry
RE: Rebar placement in below grade icf walls
There is also specific reinforcement for some seismic zones. (like minimum #5 @ 18", or #4 @ 12", for D)
Minimum reinforcement is #4 @ 48" for 6" ICF basement walls per US HUD ICF Prescriptive Method, 2nd edition, 2002 (as well as in PCA100.) The 2002 edition also specifies that vert and horiz reinforcement in ICF walls shall be in the middle third of the wall, except basement walls may be no closer than 3/4 inch from the interior face.
IBC 2006 doesn't list 6 inch basement walls, but does specify that reinforcement in foundations walls it does prescribe have 3/4"-1.25" clear cover to the inside face.
I don't have IBC 2009 or ACI 560 handy today.
Remember that axial loads in the beam-column strip moves the NA.
RE: Rebar placement in below grade icf walls
The factored load is deemed to be a live load and has a load factor of 1.5. If the actual moment is M, the factored moment is 1.5M. If the moment had been caused by a combination of dead and live load, the factored moment would be 1.25MD + 1.5ML.
In the good old days, we considered dead and live loads as equal. Now, we seem to believe that the magnitude of dead load is more precise that that of live load, so we assign a different load factor to each. Don't worry about it, it is not really important. For years, structural design was carried out on the basis of Working Stress Design (WSD) in Canada, or Allowable Stress Design (ASD) in the USA.
The factored load is 1.25D + 1.5L. In the case of lateral soil pressure, the load is deemed to be live load.
BA