Basement wall analysis

[sflsan]

Sorry this is so long, I just want to make sure I don't leave out important details.

I am analysing a 10" x 9' basement wall with #4 18" horizontal and 36" vertical and a 8' imbalance. The wall is already in place. According to standards, 18" is the max rebar spacing you should use, but the IRC charts state basement walls can use greater spacings. When I anylize the wall as cantilevered, and no lateral support (the joists run parallel) it is a marginal failure with 36" verticals. There are 2 walls below the upper level garage that can be considered counterforts 12' in from the corners and 22' apart with a slab inbetween at the top of the basemetn wall. I am told they ate 9' tall and 4' long before stepping up, but they are not visible.

So what I'm wondering is, should I refrain from treating it like a standard cantilevered wall, and also, what impact the counterforts may have on the wall itself. Any input or even mere opinion would be appreciated.

 

[JAE]

Even with the floor members parallel to the wall, the normal practice is to take steps to secure the top of the wall to the floor structure and deliver the lateral top-of-wall thrust into the floor diaphragm.

Usually this is done by providing spaced struts, or blocking, perpendicular to the wall, which are extended into the floor system (between joists) and attached to the diaphragm floor.

Trying to use a cantilevered wall system can work, but you have to design the wall AND the foundation under the wall as a traditional cantilevered system - which probably isn't what has been done on your particular project.

The counterfort (perpendicular) walls do help - you end up with a wall that is connected at its base and vertical sides with a free edge along the top. This is more difficult to analyze - using finite elements or Roarks Formulae might be the way to go....but this is not usually done in most buildings.

 

[sflsan]

Thanks JAE, I will try that and see if it will work with minor adjustments and possibly additional support as you suggested. Sometimes it just takes another perspective. Thank you very much.

 

[COEngineeer]

how long is the span of wall? I use 36" o.c. basement wall for vertical all the time. I design it as fixed on both sides and pin at the bottom and design it like a tank. By doing this I can just use a standard 8" thick and 2' wide footing with minimal reinforcement. The horizontal reinforcement will do most of the work.

Never, but never question engineer's judgement

 

[sflsan]

The wall is just over 40' long. Have you ever experienced anything other than typical shrinkage cracks using 36" verticals on large walls?

 

[COEngineeer]

40' long for a basement..that is long! You have to use the footing and fix the bottom (increase the thickness of footing for hook embedment length, increase footing width for toe pressure). I am pretty sure your reinforcement is not enough (assuming you have 9-10' of soil).

When I have a wall this long then typically I have a wider and thicker footing, more vertical rebar, fewer horizontal rebar. Unless you have a counterfort or buttress on the middle of the span.

Never, but never question engineer's judgement