This search bar will answer most questions

Foundations, Rebar, Cracks, and Foundation Settling

Foundation Settling and Cracking in Bay Area Homes

Most of the Bay Area, especially in the flat lands, has soil with a great deal of clay in it.  Clay soil expands when it is wet and retracts or settles when it is dry.  This expansion pushes up on the foundation causing the house to move upward slightly while dry soil causes the foundation to settle.  This heaving and contraction causes foundation cracks and over time the house settles into the clay.  If the settling on one side of the house is more than on another you will see uneven floors.  This can also cause cracks in the walls and ceilings, especially in the corner of door ways.  As you can imagine sinking of only one inch can put considerable strain on brittle plaster, causing it to crack, and put doors out of plumb enough to cause them to stick.

There is no viable solution to this besides jacking up the floor on the sunken side and increasing the height of the foundation until the floors are even.  This is an expensive enterprise and does not really address the problem clay soil.  Best to learn how to live with it.  From an earthquake resistance point of view it makes no difference at all.

Steel in New Foundations Prevents Cracking

The purpose of putting steel reinforcement into a foundation is to add tensile strength to the concrete. Steel has tremendous tensile strength.  Concrete is the other way around, it has tremendous compression strength (after all, it is basically composed of rock), but its tensile strength is weak.   In the image below “load” means weight of the house on the foundation.

If steel in placed in the foundation to prevent the concrete from bending, in other words it resists tension, the concrete should not settle or crack.

Tension Compression Forces on a House Foundation

Expansion and Settlement Cracks in a House Foundation

More on Reinforcing Steel

Long rods of steel are placed in the foundation ditch as shown by the red arrow before being filled with concrete and has been a building code requirement since July of 1997 and gives concrete compression strength.  Compression strength is important when the force of gravity or the heaving of expansive soil places pressure on the foundation.   Foundation bolts put the concrete in tension which is an inherent strength of concrete.  In other words, not having rebar in a foundation from a retrofit point of view is not a big deal.

R403.1.3 Seismic reinforcing.  Bottom reinforcement shall be located a minimum of 3 inches (76 mm) clear from the bottom of the footing.

 

 

Foundation

A Retrofit Foundation-the concrete above ground is for more mass 

Basic House Foundation Requirements

1809.4 Depth and width of footings.
The minimum depth of footings below the undisturbed ground surface shall be 12 inches (305 mm). The minimum width of footings shall be 12 inches.

1809.7 Prescriptive footings for light-frame construction. 

TABLE 1809.7 PRESCRIPTIVE FOOTINGS SUPPORTING WALLS OF LIGHT-FRAME CONSTRUCTIONa, b, c

NUMBER OF FLOORS SUPPORTED BY THE FOOTING WIDTH OF FOOTING (inches) THICKNESS OF FOOTING (inches)
1 12 6
2 15 6
3 18 8g

The ground under the floor shall be permitted to be excavated to the elevation of the top of the footing.

Here you will find the complete code citation.  

In order to figure out how much concrete you will need, you multiply the length of your ditch by the width and the depth using this conversion tool.  

 

R403.1.3.1 Foundations with stemwalls. Foundations with stem walls shall have installed a minimum of one No. 4 bar within 12 inches (305 mm) of the top of the wall and one No. 4 bar located 3 inches (76 mm) to 4 inches (102 mm) from the bottom of the footing.

Table R611.5.4.(1) requires Grade 60 rebar to have a lap splice as follows:

#4 Bar – Lap Splice shall be 30-inches

#5 Bar – Lap Splice shall be 38-inches

#6 Bar – Lap Splice shall be 45-inches

 

Preventing Decay to the Cripple Wall

SECTION R319 PROTECTION AGAINST DECAY

R319.1 Location required. Protection from decay shall be provided in the following locations by the use of naturally durable wood or wood that is preservative treated.

  1. All wood framing members that rest on concrete or masonry exterior foundation walls and are less than 8 inches (203 mm) from the exposed ground.
  2. Wood siding, sheathing and wall framing on the exterior of a building having a clearance of less than 6 inches (152 mm) from the ground.

R319.1.2 Ground contact. All wood in contact with the ground,

R319.1.1 Field treatment. Field-cut ends, notches and drilled holes of preservative-treated wood shall be treated in the field in accordance with AWPA M4.

R319.1.4 Wood columns. Wood columns shall be approved wood of natural decay resistance or approved pressure-preservative-treated wood.

R319.3 Fasteners. Fasteners for pressure-preservative and fire-retardant-treated wood shall be of hot-dipped zinc-coated galvanized steel, stainless steel, silicon bronze or copper.

Exceptions:

  1. One-half-inch (12.7 mm) diameter or larger steel bolts.
  2. Fasteners other than nails and timber rivets shall be permitted to be of mechanically deposited zinc- coated steel with coating weights in accordance with ASTM B 695, Class 55, minimum.

Areas Served

San Jose,Sunnyvale, Fremont,Oakland,Berkeley
And Surrounding Areas
408-664-6355
510-548-1111

Member

eeri-logo icc-icon
Structural Engineer's Association Icon

License Info

Works' Comp RWCC64393236
Contractors Lic #558462
Bond #SC6334450
Liability Ins PCA 1045011
Link to Contractor's License Board