Anchor Bolts Spacing Basement Walls 1

[bylar]

I am looking for your feedback regarding the anchor bolt spacing on residential basement walls (North Carolina). I know that lateral load issue has been discussed but I see no reference to the anchor bolt spacing issue.

The NC Building code (adapted from the IBC) in Section 1610 Soil Lateral Loads states “Basement walls and other walls in which horizontal movement is restricted at the top shall be designed for at-rest pressure” and “Exception Basement walls extending not more than 8 feet below grade and supporting flexible floor systems shall be permitted to be designed for active pressure” The table gives the at-rest values for sandy gravelly soils as 60 psf and clay type soils at 100 psf.

Using the above with a wall 10’ (most of the walls exceed the 8’ number) from ftg. to sole plate with 9’-4” retained soil that gives a load at the top of the wall of app. 812 lbs.

Using the National Design Spec for wood members a ½” bolt and a Southern Pine sole plate of 1-1/2” thick has a value of 410# that would mean that ½” bolts would have to be 6” o/c. The code calls for anchor bolts at 6’ max.

I know that there is also friction resisting value but that is somewhat minimal especially if the floor framing is parallel to the foundation wall.

In our area for years thy never even used anchor bolts and the wall did not collapse.

Please give me your insight.

 

[msquared48]

That's why the floor joists are dropped and hung off the wall sill plate - to allow the floor joists to take the lateral load to the floor diaphragm and not the anchor bolts. When you drop the joists, you can go with 1/2" @6' OC except at the shearwall locations requiring more.

FYI, I believe that the UBC required only 30 psf/ft minimum on non-yielding basement walls, and 50 psf/ft on yielding basement walls. These higher values seem excessive.

Mike McCann
MMC Engineering

 

[BAretired]

Mike, I think you got it backwards...shouldn't it be 30 psf/ft for yielding and 50 psf/ft for non-yielding? A yielding wall would be a cantilever retaining wall. A non-yielding wall is laterally braced at the top.

One of the mysteries of geotechnical engineering is the magnitude of lateral pressure on foundation walls. It is hard to say how they come up with these, but if your local code says 60 psf/ft for granular and 100 psf/ft for clay, then it would seem advisable to backfill with granular material even when the native soil is clay...and be sure to provide good drainage at the footing level.

Hanging joists off the sill plate is not standard practice in my area, and we have predominantly clay soil. It is also not usual to have a 9'-4" height of backfill on a 10'-0" high wall. Raising the main floor above grade will help to reduce the horizontal reaction and offers better illumination in the basement.

Walls running parallel to the joists are even more difficult. Solid blocking between joists adequately nailed to decking is required to resist the top reaction.

BA

 

[msquared48]

You're right. I remembered 15 minutes ago what I said, and hit myself. That's why I'm here. Thanks for catching it.

Mike McCann
MMC Engineering

 

[DaveAtkins]

It looks like you have conflicting Code provisions. If you can be comfortable with using anchor bolts @ 6' oc, do that. You will raise a lot of eyebrows if you ask for anchor bolts @ 6" oc.

DaveAtkins

 

[bylar]

Thanks for the input so far.
In my opinion the soil pressures are not arbitrary they are spelled out in the code.
The NC code is basically the IBC code and I assume that many of you are using that code.
This in my eyes is a test of engineering values. If actual construction works with ½ “bolts at greater than 6” then the assumptions (or code values) used are in error. Personally, if the code specifies a soil pressure I have no choice but to use it and if the calculations require ½” bolts art 6” that is what I will cal for or in actuality I spec larger bolts at a greater spacing.

As far as hanging joist from the wall with laterally resistance, I have never seen this done in the 40 years of construction.

The actual construction is with floor trusses. I could use top chord bearing and then brace the wall off the top chord. Again I have never seen this done (wall bracing). The problem is exaggerated at the trusses that are parallel to the wall.

In our area it is quite common to have a wall that is 10’ high footing to plate with a minimum of 8” from wood to soil (9’-4”).
With 4” of rock or sand and 4” of concrete at the floor that gives a 9’ floor to ceiling which allows for drop ceiling etc.

 

[BAretired]

I don't think that the use of bolts at close spacing is a good answer because you still have to develop the same horizontal force between the joist and plate, so in effect, you are carrying the load twice.

Could you place joist hangers around the ends of the joists and nail them to a vertical nailer flush with the inside face of foundation wall? In other words, the joist hangers are placed to carry a horizontal load from end of joist to inside face of wall. You would need a similar detail where walls are parallel to joists only the hangers would be attached to solid blocking between joists.

If you use top bearing trusses, the same detail could work. Don't forget to tell the truss supplier the magnitude of the extra compression going into his top chord.

BA

 

[RHTPE]

Is the soil pressure 60 psf over the entire wall height or is it 60 psf per foot of depth (resulting in a triangular loading diagram)?


Ralph
Structures Consulting
Northeast USA

 

[kslee1000]

Ralph:

It is equivalent fluid pressure, should vary with depth.

 

[PSlem]

I'm working from memory, but I think the 2006 IRC has loads of 30 PSF for sand, 45 for silt and 60 for clay. It also decreased anchor bolt spacing where it is about 8" c. to c. for an 8' wall in silt. I brought this up to the county inspector who said go with the 6' as always.

 

[DaveAtkins]

I have always been comfortable using 30 psf equivalent fluid pressure for residential construction. The residential codes of the past that I have seen were all set up this way. To justify this, I think the basement wall deflects at midheight sufficiently to result in active pressure (yes, I know, the wall should deflect at the top to result in active pressure, but this is how I get comfortable with using 30 psf).

DaveAtkins

 

[BAretired]

PSlem: The county inspector should not be directing the engineer of record to "go with 6' as always" (for bolt spacing). His advice was evidently contrary to the code you were required to satisfy. The final responsibility for the design rests solely with the engineer.

DaveAtkins: Your practice is in agreement with NBC 2006 for walls supporting drained earth. If the earth is not drained, NBC requires that walls be designed for the additional fluid pressure of the surcharge.

Any time an engineer deviates from the code, he is putting himself at risk. If anything goes wrong, he will have a difficult time convincing the judge that he acted reasonably if he failed to follow the code.

BA

 

[msquared48]

bylar:

I have been using this detail for 30 years - either raising the wall or lowering the floor joists as suits the condition - to avoid spacing the anchor bolts too close. It is a simple solution and avoids the problem. I have never had a structural peoblem with this - even the venting can be done.

I do use 50 psf/ft for restrained walls, and have developed tables up to 12 foot wall heights with earth backfills from 4 to 11 feet. In the table, normally I do not have to hang the joists from the wall plate, but I will if I haved to.

The other obvious solution here though is to use an unrestrained wall and 30 psf/ft or more if the soils report warrants it. This way too, 6 feet on center is normal unless at shear walls.

Mike McCann
MMC Engineering

 

[CTSeng]

This issue has been around for a while and like many issues we (engineers) are damned if you do, damned if you don’t. Geotechs I work with provide the rotation required to make use of active or passive pressures and it is often a very small amount at the top of a wall (1/16” or less). So I only use passive with concrete floor systems. I’ve taken a hard line specifying anchors down to 1’ on center, using a bolted angle to bear on the joists, on occasion dropping the framing or even using a retaining wall design in some extreme cases. However, Simpson has recently come out with a great new product that has made everybody happy:

http://www.strongtie.com/products/connectors/FWAZ-FWANZ.asp

Of course, I also take pleasure in writing the occasional insurance inspection report for this failure.

 

[InDepth]

Design the wall as a cantilevered wall so you can avaoid soil load imposed into the structure. This also allows bacfilling to occurr earlier than having to wait for the lateral system to be installed. It also allows for the structure to be demoed and later replaced with a new structure without special wall bracing requirements.

 

[PSlem]

To follow up, here's how GA handled 2006 changes.
*Delete Table R404.1(1) ‘Top Reactions And Prescriptive Support For Foundations Walls’ without substitution.
(Effective January 1, 2007)
*Delete Table R404.1(2) ‘Maximum Plate Anchor-Bolt Spacing For Supported Foundation Walls’ without substitution.
(Effective January 1, 2007)
*Delete Table R404.1(3) ‘Maximum Aspect Ratio, L/W For Unbalanced Foundations’ without substitution.
(Effective January 1, 2007)