Rebar placement in below grade icf walls 1

[icfbunt]

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

 

[msquared48]

If the wall is retaining earth, I agree, but if not, then to resist wind and seismic forces, the center is best as those forces can swing either way.

Do the tables tell you what kind of loads the wall is reinforced for?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[icfbunt]

Mike, thanks for the response. See attached chart. Chart is for below grade foundation applications. I looked in a few other areas and found that some specify rebar towards tension side of wall and others do not specify (i guess it's to be assumed rebar goes in center of wall). In any case, wouldn't rebar in the center of the wall be good for horizontal shear only? Theoretically, the rebar would never be in tension or compression.

Thanks again!
Larry

 

[msquared48]

Perhaps the details in the manual show the placemnent of the rebar as it is not specified on the chart.

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

 

[msquared48]

A further consideration.

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

 

[BAretired]

The chart appears to be based on reinforcement being placed on the tension face with an effective depth of about 4.25".

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

 

[msquared48]

The use of a 5.5" wall here seems like it is pushing it. Even placed at 4.25", there is inadequate cover, especially with some of the bars #6's, which is ridiculous for a wall that height. You should only have to use #4's or #5's.

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

 

[BAretired]

Mike McC,

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

 

[icfbunt]

Thanks guys! BA, where are you getting the factored moment of 2344ftlb? Not sure what that# represents.

Thanks!!
Larry

 

[TXStructural]

The page you posted looks like it is from the original (old) HUD/NAHB Prescriptive Method. The current document is PCA100-2007; Table 3.6 lists the requirements for 6 nominal walls with grade 60 rebar. PCA100 specifies that the rebar be located at the centerline of the wall.

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.

 

[BAretired]

icfbunt,
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 1.25M_D + 1.5M_L.

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