# Basement Block Question



## TBALL (Dec 30, 2005)

I have 12 inch block layed on a new basement below grade. However I think after the grade work I should have went maybe another 3 courses of 12's. This lot is below the road and not the best lot to build on but its a custom home I didn't buy it and it will need some fill dirt for backfill. 
My question is will the brick and the 8 inch block be ok if I have to go maybe 2-3 courses up on the brick or should I use a 4 inch cap block and bring it up to 3 more courses? Is this as strong a 12 inch block.
I am going to fill the 12 inch with concrete for added strenght.

thanks 
Tim


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## thom (Nov 3, 2006)

Block walls are sized based on height, length, intermediate intersecting support walls and some other stuff. This wall is holding back the weight of the dirt which, when saturated puts a substantial amount of pressure on the wall. Soil conditions play a role so I can only give a rough example. Use some assumptions. weight of saturated earth @ 75# per cubic foot and loading at 45degrees. The increase in loading is geometric meaning each additional foot of earth resting against the block adds more load than the previous foot of earth.

At 24" high (three blocks) = 150#
At 48" high (six blocks) = 600#
At 72" high (nine blocks) 1,350#
At 96" high (twelve blocks) 2,400#

This (horizontal) load builds quickly, which is why many places require engineers stamps.


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## concretemasonry (Dec 1, 2006)

How many courses of 12" block do you have now? - What kind of soil do you have.

You are not concerned with the vertical load, but the horizontal load. If you had no soil, 6" block would be adequate and many times stronger than the frame walls.

I have seen 12" basements with 13 courses of unreinforced block built as a matter of routine in good soil with drain tile.

If the top of the wall is still open, you could easily add some vertical steel for additional lateral strength. You really do not have to fill all the cores, but it is probablu just as easy to do it.

Bring the 12" block up as high as you can before you build the brick ledge. If you want to have more than one course of 8" block with brick that is OK, but bring the rebar to the top of the wall.


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## laybrick (Jul 2, 2006)

lay a bond-beam course with 2 continuous horizontal rebar, overlapping rebar by 16 inches. Put verticle rods every 2 feet,(+ corners) sticking out the top 14 inches or so. Plug remaining cores under the bond beam
Fill verticle rods,then bond beam (lift rebar so it's surrounded by concrete).Lay 2 more courses. Fill rest of verticle rods.


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## concretemasonry (Dec 1, 2006)

The critical part of the project is to reinforce is the bottom portion of the wall. Make sure your vertical steel goes all the way to the bottom of the wall and hits the footing. 2' spacing is a little too close for a 12' wall unless you have a silt or clay soil behind the wall. Sand and gravel is much better. In any event, you will end up filling most of the wall.

Adding a bond beam immediately on top of your wall is not necessary. Just add all but one of the courses you want to add using normal block. For the last course you use the bond beam block. Depending on the units available locally, you might be able to get a combination of solid bottom bond beams on open bottom units. Insert the vertical steel in where required. Wherever you have an open bottom with no vertical rebar, fill the core with crumpled newspaper jammed in tight.

Fill the cores with grout slowly and use grout that is wetter than typical concrete to make sure you fill all the cores. Because your block cores do not line up, some grout will leak into the adjacent cores, so fill with grout slowly and allow it to consolidate and settle. After filling the cores, fill the bond beam. This is one of the few times it is good to have a slight excess of water in a concrete mix.

Do not forget to put anchor bolts or strap anchhors into the bond beam. If you are required to use long anchor bolts set them in the areas where the vertical steel is.

All you are really doing is adding the courses you want, adding vertical rebar, putting a course of bond beam block on and filling the necessary block. If you were planning on filling the entire wall, this is not any extra grout (fine concrete).

Depending on what you have for an elevation and brick ledge, you may have to alter the details slightly, but the principal is the same.


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## laybrick (Jul 2, 2006)

bond beam at grade level where there's pressure. top locked by anchor bolts/ sill plate. Either way, strong enough. six one half dozen the other


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## concretemasonry (Dec 1, 2006)

If you use a bond beam it is at the top of the wall to tie everything together and provide a solid bearing surface. Intermediate bond beams are usually unnecessary and may be used at midpoints of a wall, but cannot replace the main bond beam in either partially or fully reinforced/grouted construction.

Much of the complication is because you have added to the wall height and the design tables do usually not cover walls 15 courses high. Filling the wall with concrete definitely helps increase the strength, but you will be hard pressed to show it works on paper. I have spent 35 years in the block/masonry industry and as a structural engineer found it difficult to prove things like that on paper.

A bond beam at the grade, wherever it may be (sloping?) today is unnesessary, a waste of money and complicates construction and reinforcing. There is little pressure at grade except for the minor weight of the house.

You should assume that the grade may be raised someday since the home is currently below street level. When you backfill, do not use "fill dirt", especially with the possibility of water draining toward the house. Use clean sand or gravel for backfill to reduce the load on the walls. Remember - the excavator dug a swimming pool that the mason contractor built a basement in for the General Contractor to sell as a dry basement when he doesn't bother to use drain tile.


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## ch0mpie (Nov 30, 2005)

thom said:


> Block walls are sized based on height, length, intermediate intersecting support walls and some other stuff. This wall is holding back the weight of the dirt which, when saturated puts a substantial amount of pressure on the wall. Soil conditions play a role so I can only give a rough example. Use some assumptions. weight of saturated earth @ 75# per cubic foot and loading at 45degrees. The increase in loading is geometric meaning each additional foot of earth resting against the block adds more load than the previous foot of earth.
> 
> At 24" high (three blocks) = 150#
> At 48" high (six blocks) = 600#
> ...


I know your only giving an example, but these assumptions are very unconservative, especially for saturated soil, and should not be used for any loading estimates.


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## laybrick (Jul 2, 2006)

been in the field for 25 years. lotta things look good on paper


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## concretemasonry (Dec 1, 2006)

Thr figures given by thom are definitely on the low side and not realistic. If you have the ideal backfill and not the usual dirty sand, durt, silt and clay, they are very misleading and could cause a building failure if used for any reasonable engineering design.

If you want some good empirical information (based on actual successful walls) on wall thickness and reinforcement look at ACI 530, which I believe was adopted by the new codes. - Not a design method, but tables for different heights of soil behind the wall.


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## jshuatree8 (Feb 1, 2007)

gravel between your cmu wall and your back fill will hold back the dirt


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## PHI (Feb 9, 2007)

You may need to speak to your architect, but if your at 12" block do not down size the block unless your going to do a brick ledge


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