wood sill plate anchorage on concrete basement walls

[bjb]

I have a question regarding sill plate anchorage requirements. The 2000 IRC requires that the wood sill plate be anchored to the foundation with 1/2" anchor bolts at a maximum of 6' on center. What I am concerned about is a basement wall that retains earth. The sill plate is typically pressure treated southern pine, with the bolts loaded perpendicular to the grain. Because the load is permanent, Cd is 0.9. Therefore according to the NDS the strength of one of these bolts is 400 X 0.9 = 360 pounds. With an 8 foot basement wall retaining 6 feet of granular soil, the reaction at the top of the wall can be approximately 230 plf. Multiplying this by 6 feet gives you 1380 pounds, which is way more than the bolt capacity. My soil pressure is based on at-rest conditions, since the wall is prevented from yielding by the floor diaphragms. According to the NDS, even a 1" bolt is only good for 700 pounds before any adjustment factors.

I know that the code establishes a minumum requirement, but for most residences all I usually see is the 1/2" anchor bolt at 6 or 8 foot spacing, which appears to be inadequate.

Am I missing something here??

 

[SperlingPE]

Are you saying that the sill plate is securing the top of your retaining wall? If it is a cantilever retaining wall and the bolts start to pick up load from the soil pressure on the wall, then something bad is happening to your wall. The load that your sill bolts are transferring is lateral wind load or a seismic load. The bolts are not helping hold the wall in place.

 

[bjb]

It is not a cantilever retaining wall. It is a basement wall that is braced at the bottom of the wall by a footing, and at the top of the wall by the wood framing. Therefore, the wall spans vertically and the bolts between the concrete and the sill plate are transfering lateral soil pressure. This is typical residential type construction.

 

[CSEllc]

I have had the same concern for years. As you did, I "ran the numbers and the numbers don't work." Yet, every house constructed has the same (or similar) conditions.

Years ago I( started spacing the 1/2" bolts at 4'-0" oc. The numberes still don't work, but its a little better.

I think the friction force of the house is what helps a little and must make up the difference.

 

[bjb]

CSE, thanks for your reply. I agree that friction force is present, but I think that these bolts in service aren't functioning with the safety factors of the NDS, but are generally (but not always) loaded below their ultimate strength. Also, if the wall deforms a little bit because of deformation at the sill plate, some of the at-rest soil pressure will be relieved. Unfortunately, I don't think this is true for silt and clay backfill.

I would like to hear from some of the people from the ICC what the justification for the minimum anchorage requirement is.

 

[jike]

Often, small basements can function as open top boxes. The wall then acts as a panel that is pinned on the bottom, fixed at the sides and either free at the top or supported by springs.

If the walls are too long then they start to behave like you describe (pinned at the top and bottom).

I think this anchor bolt requirement is a holdover from the days when structures were relatively small and the walls were short. The prescriptive codes do not get updated as often as the engineered codes do and there is alot of momentum not to change from the Contractors.

 

[AlohaBob]

The code is wrong. There's a record of the inadequacy of the code written in the observed serviceability of basement walls. They are cracked, bulged, leak, blister paint, oftentimes poorly insulated. Residential scale is not typically subject to structural inspection.

We've got to focus instead on the most important issues. Like multiple permits, inspections and approvals to replace the water heater, or reshingle the roof, or replace an old toilet fixture.

 

[bjb]

alohabob, I agree that the code is wrong. Besides inadequate anchors at the top of the wall, I think that the use of vertical and horizonatl rebar should be mandatory in all basement walls. Many of the walls that I am familiar with that have bowed in and cracked would not have failed had there been some rebar in the wall. I know that the basement walls can be reinforced at a relatively small cost. The problem is that the contractor's scream bloody murder. Another problem is that I think the code people sometimes loose sight of the things that are really important. As you site, being more concerned about the bureucratic hurdles to make people jump through for non structural issues instead of making sure the structure is sound is a big problem.

 

[boo1]

An additional concern I have is when you place an anchor bolt in a 8" cmu for a 4" base plate. The anchor will be located ~1 3/4" from the face of the cmu. So whats the edge distance ~3/4". For the residential jobs i have been doing I use a continous bond beam at the top. So many foundation contractor are afraid of using a little bit more steel...

 

[Tmoose]

When we built our house I "looked" at some of these details. I spent some extra time thinking about the foundation wall parallel to the floor joists. At >>best<< it consisted of a continuous sill plate toenailed (Shear loads and toenailing - thread726-92155) to a single (typical) or double (code) 2X12 rim joist to which my deck is glued and nailed. I remember thinking that the double rim joist was useful to protect from buckling/overturning from the vertical wall load, but not to "beef up" the foundation wall.

The sill plate on the wall perpendicular to the joists has the benefit of a toenailed joist every 12-16 inches, plus the rim joist, plus friction.

As I may or may not recall correctly, I felt overall the plywood deck could play only a minor role in offering meaningful lateral support for the foundation wall.

The owner of a local excavation company did my backfilling.
He was a small old man, bowlegged and lame, but his machine was nimble and quick when he was at the controls. I remember watching him work and thinking he could have gently tied his granddaughter's shoelaces with the bucket. When I suggested we get the deck on before he did any backfiling, he said in his opinion it really didn't matter much.