Basement wall?

[psychedomination]

Hi there,

I am a recent graduate engineer and am currently working on a basement wall project. The 8" block basement wall is assumed pinned at the top, pinned at the bottom and restrained at the sides. I am trying to find out the flexural reinforcement requirement in the wall.

My main confusion is coming from my local building code. It states for retaining walls (not pinned at top) for an 8’ wall you can use 12” masonry block with T12s @ 8” centers. It then says for foundation walls where it is pinned at the top, you can use 12” block with T12s @ 24” centers.

I did a quick simplified calc for the design bending moment shown in the attached image. Is there something that I am missing in the calculations that would reduce the bending moment even further to match the requirements of the building code? It doesn't seem like changing the block size from 8" to 12" did anything for the bending capacity of the stem? I the assumed steel reinforcement was taking all of the bending.

After I finished the calcs the wall seemed to be ok with 8" block and T16 vertical bars at 16" centers. When I did the same calculations with the 12" block with T12s at 24" centers as shown in the local residential building code, the bending capacity of the wall was inadequate. Assuming no vertical load on these walls.

So not sure what is going on here?

Also one of the existing wall stems in this basement has reinforcement that is inadequate. How can I go about reinforcing the bending capacity of an existing wall stem?

I was thinking to add columns spanning about every meter along the wall to almost act like some sort of ribbing (like how soldier piles are). But not sure if that would be effective unless the spacing was less than a meter since the reinforcement design is done per meter strip?

Apologies for the various questions but knowing the answers would help me tremendously as I continue to work on harder projects. Any guidance would be appreciated.

 

[hokie66]

The bending capacity of a wall, simply put, is directly related to both the area of steel and the effective depth to that steel.

It is not surprising that an empirical residential building code provision does not give the same results as calculated by engineering principles. There is a lot of politics involved in building codes, but not in engineering analysis.

But in your case, the 8" wall will have about 60% greater flexural capacity than the 12" wall because the area of reinforcement is much greater, and the difference in effective depth is not large enough to compensate.

 

[psychedomination]

Thanks for clearing that up hokie66. I look forward to seeing some of the answers to the other questions posed also.

 

[XR250]

One thing to consider is that the 8" wall will be much more sensitive to the field placement of the bars. Works great on paper, but try moving the bar 1" and see if it still calcs out.
I always design 8" walls with the bars in the center but draw them on the edge.

 

[oldestguy]

The calculation sheet misses one important item for a typical basement wall. In order to be stable these blocks need a vertical load on them, the building. Place enough load and there won't be any tension requiring reinforcing. Specifically what exactly is a "pin" on the top of the wall? If there isn't any load, you don't get much shear resistance holding back that part of the wall. So add that vertical load to your calcs.

 

[psychedomination]

Works great on paper, but try moving the bar 1" and see if it still calcs out.

That's true, the 12" block would be easier to construct and have higher tolerances.

The calculation sheet misses one important item for a typical basement wall. In order to be stable these blocks need a vertical load on them, the building. Place enough load and there won't be any tension requiring reinforcing. Specifically what exactly is a "pin" on the top of the wall? If there isn't any load, you don't get much shear resistance holding back that part of the wall. So add that vertical load to your calcs.

Yes that is true, unfortunately the wall is built up to the floor slab but I do not think that it can be seen as taking any stabilising vertical dead load. Maybe it can be seen as taking some live load? It is not connected or doweled into the floor slab it is just built up to it and dry packed. It's restrained (pinned) by an external beam at the top.

 

[oldestguy]

If I was doing it I'd make sure there is load on top especially away from corners. Numerous ways are possible and not time consuming or costly. Two hardwood wedges facing each other is one way.

 

[psychedomination]

If I was doing it I'd make sure there is load on top especially away from corners. Numerous ways are possible and not time consuming or costly. Two hardwood wedges facing each other is one way.

That sounds like a good idea. Mind going in a little bit more detail on how that would work?

Is what I have drawn in the attached photo what you mean? I assume I can place these at 1m intervals along the wall and dry pack the 1m spaces in between?

How far in would the second wedge need to be hammered in before it can be assumed that the dead load is now active on the wall?

Out of curiosity what other cost effective methods can be used to get the dead load from above on top of the wall?

 

[oldestguy]

Seems like you are starting OK. I'd do something to keep the rear wedge from moving, such as a cast in spike or other stopper. A plank set back of the rear block (of course top blocks filled with grout where necessary). Dry back after wedging. I'd test or at least have a few different angles of the wedges. Your drawing possibly too much angle.

Another way is leave a block or two out (later placed). Use that opening to place a screw type jack (you can make one if necessary), do some light lifting while dry pack is placed elsewhere.

 

[hokie66]

oldestguy,
With respect, I consider that type advice to be unwise. It is practically impossible to cope with bending stress in residential basement walls without reinforcement. That is why we have so many queries on this site about broken basement walls. I think most of us who design masonry structures consider unreinforced walls to be non-structural.

 

[oldestguy]

hokie: who said no re bars?

 

[hokie66]

That's the way I took your post about a vertical load enough so that "there won't be any tension requiring reinforcing".

 

[oldestguy]

Load on top will help significantly to have that end of the "beam" to have that pin connection (similar to a house basement built from the ground up rather than it trying to be a retaining wall and no house. However, yes the compression in the blocks will assist the bending resistance. I'd still use re-bars even if they are difficult to install in this situation. Who knows how good a bond can be developed. I'd also like to know how the top course can be placed over those rods sticking up there (maybe located only at joints). Then comes getting the grout all the way down.

 

[oldestguy]

OG again. On this question.

Also one of the existing wall stems in this basement has reinforcement that is inadequate. How can I go about reinforcing the bending capacity of an existing wall stem?

I've seen some posts here with a pair of threaded rods and a turn buckle, with both ends of the rods secured to the wall and footing. as with clips, anchor bolts, etc.

 

[XR250]

In the past, I have cut out the face shells of the blocks and tapcon'ed plywood to the face and grouted in steps. For the top course, cut the face shell and install and then grout it back with a stiff mixture.
I have never found the axial stress to be significant enough to provide much offsetting of the flexural component.

 

[hokie66]

XR250,
Yes, it is a fool's errand. I did it myself years ago, using both mass and prestressing. Doesn't help enough to count it.