Do I Need a New Foundation?
Not very likely. Foundations do two things: One, they hold a house up off the ground. Two, they need to hold up if subjected to earthquake forces.
In a retrofitted house all the earthquake forces transfer into the bolts. The bolts go through a soft piece of redwood (called the mudsill that sits on top of the foundation) into the concrete. These three things always work together. In an earthquake either the bolt is going to break, the redwood is going to split, or the concrete is going to shatter. Even very poor concrete is stronger than soft redwood, so the failure is always going to be in the redwood, not the concrete. This is why foundation condition is rarely a concern.
Practically all old foundations are cracked because as soil expands and contracts, the brittle concrete cracks. This happens even if the soil expands only an inch or two. Virtually every house we see has a few cracks in the foundation and are not anything to worry about.
What Do Foundations Do in Earthquakes?
The second thing a foundation does is resist earthquakes forces. If segments of the foundation are in bad shape, or parts are completely missing, it probably does not matter. This is the case so long as there is enough good foundation to attach the house to.
It is very rare for a foundation to be in bad enough shape to warrant replacement. Similarly, rarely does it make sense to replace a segment of deteriorated foundation. Most houses have an adequate amount of good foundation.
This is also borne out by tests done by the Structural Engineer’s Association which evaluated concrete with a quality rating as low as 1500 psi. Their tests concluded: “The difference in the strength of the concrete did not appreciably affect the performance of the foundation anchorage systems (Bolts) which work with new foundations just as effectively as with weaker concrete (psi). The predominant failure mechanism was the wood sill plate in both the good and weak concrete foundations.”
What about Brick Foundations?
I wrote this article in cooperation with one of the leading brick foundation experts in the world and it shows that for the most part you don’t have a lot to worry about.
The California Existing Building Code
The California Existing Building Code assumes old concrete is 1500 psi concrete, which is exactly what the structural engineers tested. Therefore, unless tests show otherwise, we can assume that the existing concrete will perform quite well in a retrofitted house. Please scroll down this page until you see section Table A4-A and look at item number 3.1.
More on Foundation Cracks
Extreme expansion and contraction of soil can cause severe foundation cracks. Floors become uneven and cracks appear in stucco and plaster walls. Doors often stick in different seasons, existing cracks in the plaster or sheet rock walls seem to get larger and smaller. Once repaired they simply return a few months later as the weather changes. In this article you can read more here about expansive soils. Historic building preservation structural engineer Nels Roselung wrote an excellent article on settling and foundation cracks.
If you see a half inch crack, it means the foundation heaved upward or sunk downward half an inch.
Expansive Soils Settling
Hold both ends of a pencil and snap it in two in an upward direction. This is similar to the upward expansion of soil under a foundation that causes it to crack. This is called soil expansion. Snap it in a downward direction. That is equivalent to downward pressure caused by the weight of the house on the foundation and is called foundation settlement. Foundation cracks occur in the same way. They are a normal part of settling and expansion.
This is How Foundations Resist Earthquakes
Earthquake forces put the foundation concrete in compression (pushing on the sides, not pulling up or pushing down). Concrete can resist a tremendous amount of force when put in compression. Imagine pushing a pencil on both ends towards the center. This is compression in action. No matter how hard you push, the pencil will not break.
Foundation Made with Salt Water
Some foundations in the San Francisco Bay Area have minimal amounts of mortar, used contaminated water from the bay, are made of brick, or have other very serious structural issues that need to be addressed before a retrofit is performed. In these cases the foundation must either be replaced or supplemented with segments of good foundation.
Old Foundations and Retrofit Foundation Bolts
Use some common sense. Do not put bolts near cracks.
Foundation settling and cracks are completely normal. 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 shrinks 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. The weight of the house itself is not always evenly distributed along the foundation such that a heavier portion, or a portion under softer soil, will sink.
The heaving and sinking 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.
Rotation is caused when the outer side of the foundation is saturated with moisture while the soil in the crawl space is dry. When the house was built, its weight pressed evenly on the inside and outside of the foundation. When the soil on the outside becomes soft and saturated the foundation starts to sink on the outside edge. This causes the outside edge of the foundation sink into the ground and the inside edge pull up. This has ZERO impact on a foundation’s ability to hold a house up off the ground or its ability to perform in an earthquake.
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.
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. Reinforcing steel has been a building code requirement since July of 1997 and gives concrete tension resistance. Tension strength is important when resisting the force of gravity or expansion and shrinking of the soil. 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.
A Retrofit Foundation-the concrete above ground is for more mass. Here we are looking at it from the end.
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 CONSTRUCTION a, b, c
|NUMBER OF FLOORS SUPPORTED BY THE FOOTING||WIDTH OF FOOTING (inches)||THICKNESS OF FOOTING (inches)|
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 stem walls. 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
Where required by this section, protection from decay and termites shall be provided by the use of naturally durable or preservative-treated wood.
2304.11.2 Wood Used Above Ground
Wood used above ground in the locations specified in Sections 2304.11.2.1 through 2304.11.2.7, 2304.11.3 and 2304.11.5 shall be naturally durable wood or preservative-treated wood using water-borne preservatives, in accordance with AWPA U1 (Commodity Specifications A or F) for above-ground use.
2304.11.2.1 Joists, Girders and Subfloor
Where wood joists or the bottom of a wood structural floor without joists are closer than 18 inches (457 mm), or wood girders are closer than 12 inches (305 mm) to the exposed ground in crawl spaces or unexcavated areas located within the perimeter of the building foundation, the floor construction (including posts, girders, joists and subfloor) shall be of naturally durable or preservative-treated wood.
2304.11.2.2 Wood Supported by Exterior Foundation Walls
Wood framing members, including wood sheathing, that rest on exterior foundation walls and are less than 8 inches (203 mm) from exposed earth shall be of naturally durable or preservative-treated wood.
2304.11.2.3 Exterior Walls Below Grade
2304.11.2.4 Sleepers and Sills
2304.11.2.5 Girder Ends
The ends of wood girders entering exterior masonry or concrete walls shall be provided with a 1/2-inch (12.7 mm) air space on top, sides and end, unless naturally durable or preservative-treated wood is used.
2304.11.2.6 Wood Siding
Clearance between wood siding and earth on the exterior of a building shall not be less than 6 inches (152 mm) or less than 2 inches (51 mm) vertical from concrete steps, porch slabs, patio slabs and similar horizontal surfaces exposed to the weather except where siding, sheathing and wall framing are of naturally durable or preservative-treated wood.
2304.11.2.7 Posts or Columns
Posts or columns supporting permanent structures and supported by a concrete or masonry slab or footing that is in direct contact with the earth shall be of naturally durable or preservative-treated wood.
- Posts or columns that are either exposed to the weather or located in basements or cellars, supported by concrete piers or metal pedestals projected at least 1 inch (25 mm) above the slab or deck and 6 inches (152 mm) above exposed earth, and are separated therefrom by an impervious moisture barrier.
- Posts or columns in enclosed crawl spaces or unexcavated areas located within the periphery of the building, supported by a concrete pier or metal pedestal at a height greater than 8 inches (203 mm) from exposed ground, and are separated therefrom by an impervious moisture barrier.