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Seismic and Thermal Resistance in Slab-on-Grade Foundations with Turned-Down Footings (Monolithic Slabs)

Scope

Externally insulated concrete slab-on-grade foundation with a turn-down footing, showing anchorage of the wall to the foundation for seismic resistance
Externally insulated concrete slab-on-grade foundation with a turn-down footing, showing anchorage of the wall to the foundation for seismic resistance

When constructing homes with slab-on-grade foundations with turned-down footings in areas known for earthquakes, connect the house walls to the foundation and connect the structural elements to each other to withstand seismic activity:

  • Provide wall-to-slab-on-grade foundation diaphragm anchors
  • Install building bracing
  • Install control layers to improve the energy efficiency and durability of the structure, including a rain control layer, an air control layer, a vapor control layer, and a thermal control layer.

See the Compliance Tab for links to related codes and standards and voluntary federal energy-efficiency program requirements.

Description

Seismic Design Approach

Most of the damage to buildings during earthquakes is caused by lateral movements, which disconnect the house from its foundation. These lateral loads (“shear”) due to ground movement need to be transferred to the ground. New homes and existing homes need to be connected to their foundations and the structural elements should be connected to each other to withstand seismic activity. Additionally, the foundation should be insulated to be thermally efficient. This guide describes options for providing seismic and thermal resistance in slab-on-grade concrete foundations with turn-down footings.

The home’s walls should be connected to its foundations by means of anchor bolts or approved connectors that connect the sill plates to the foundation as required by code (See Figure 1). Attaching the exterior wall sheathing to the sill plates (as shown in Figures 1, 2, 3, 4, and 6 below) will further strengthen the attachment of the walls to the slab foundation. According to the International Residential Code (IRC), anchor bolts should be at least ½-inch diameter and should be embedded at least 7 inches into the foundation concrete. They should be spaced no more than 6 feet apart. Any wall section over 24 inches long should have at least two bolts, located at least 3.5 inches but no more than 12 inches from the ends of the sill plate. Every bolt should have a tightened nut and washer. See the Compliance tab and the IRC for more details. Local code requirements may vary. Consult your local code, a code official, and/or a licensed architect or engineer when designing and constructing for seismic resistance.

Anchor bolts should be at least 1/2 inch diameter and should be embedded at least 7 inches into the foundation concrete, spaced not more than 6 feet apart, and between 3.5 and 12 inches from each end of the sill plates
Figure 1. Anchor bolts should be at least 1/2 inch in diameter and should be embedded at least 7 inches into the foundation concrete, spaced not more than 6 feet apart, and between 3.5 and 12 inches from each end of the sill plates (Source: City of San Jose, CA).

 

Connection of Building to Slab-on-grade Foundation

The following illustrations show five options for connecting and insulating an exterior wall where it attaches to a slab foundation with a turn-down footing. Consult a licensed architect or engineer for guidance for your specific project.

Figure 2 illustrates a concrete slab-on-grade foundation with a turn-down footing that is insulated along the exterior edge of the slab. Note the connection of the continuous exterior structural wood sheathing (plywood or oriented strand board “OSB”) on the exterior wall framing to the frame wall’s bottom plate.  Further, note the connection of the frame wall bottom plate to the concrete slab-on-grade foundation by means of an anchor bolt. In this manner, the wall is connected to the foundation diaphragm. Building bracing is provided by the use of the continuous exterior structural wood sheathing (plywood or OSB) in this and the following cases. 

Externally insulated concrete slab-on-grade foundation with a turn-down footing, showing anchorage of the wall to the foundation for seismic resistance
Figure 2. Externally insulated concrete slab-on-grade foundation with a turn-down footing, showing anchorage of the wall to the foundation for seismic resistance (Source: Building Science Corporation).

 

Figure 3 illustrates a concrete slab-on-grade foundation with a turn-down footing that is insulated on its top surface. Note the connection of the exterior wall’s structural wood sheathing (plywood or OSB) to the bottom plate as well as to the bearing plate (via attachment to the subfloor). The frame wall bottom plate is connected to the foundation via the anchor bolt. According to the IRC, interior bearing wall sole plates that are not part of a braced wall panel shall be positively anchored with approved fasteners (IRC R403.1.6). In Seismic Design Categories C, D0, D1, and D2, interior braced wall plates and interior bearing wall sole plates shall have anchor bolts spaced not more than 6 feet on center and within 12 inches of the ends of each plate section where supported on a continuous foundation (see IRC R403.1.6.1 for more information).

Concrete slab-on-grade foundation with a turn-down footing insulated on its top surface, showing anchorage of the wall to the foundation for seismic resistance
Figure 3. Concrete slab-on-grade foundation with a turn-down footing insulated on its top surface, showing anchorage of the wall to the foundation for seismic resistance (Source: Building Science Corporation).

 

Figure 4 illustrates a similar approach to Figure 3 but with a brick veneer. Note the “seat” in the slab edge for the brick veneer. Two-piece structural brick ties would be used to connect the brick veneer to the exterior frame wall.

Brick veneer framed wall supported by a concrete slab-on-grade foundation with a turn-down footing insulated on its top surface, showing anchorage of the wall to the foundation for seismic resistance
Figure 4. Brick veneer framed wall supported by a concrete slab-on-grade foundation with a turn-down footing insulated on its top surface, showing anchorage of the wall to the foundation for seismic resistance (Source: Building Science Corporation).

 

Figure 5 is a modification of Figure 4 and illustrates a concrete slab-on-grade foundation with a turn-down footing and slab edge foundation where the brick veneer is supported by a concrete stem wall that is thermally broken from the slab-on-grade foundation. The stem wall is connected through the external slab edge insulation with fiberglass ties. Note the connection of the continuous exterior wall structural wood sheathing (plywood or OSB) to the frame wall bottom plate and the connection of the bottom plate to the concrete slab via the anchor bolt. Also, note the use of two-piece structural brick ties connecting the brick veneer to the exterior frame wall. Building bracing is provided by continuous exterior structural wood sheathing (plywood or OSB). 

Brick veneer is supported by a concrete stem wall thermally separated from the slab-on-grade foundation with turn-down footing which is also insulated on top; anchorage for seismic resistance is also shown
Figure 5. Brick veneer is supported by a concrete stem wall thermally separated from the slab-on-grade foundation with turn-down footing which is also insulated on top; anchorage for seismic resistance is also shown (Source: Building Science Corporation).

 

Figure 6 and Figure 7 illustrate a post-tensioned concrete slab-on-grade foundation with a turn-down footing that is insulated along the exterior slab edge. In Figure 7, note the connection of the exterior wall’s structural wood sheathing (plywood or OSB) to the bottom plate and the bottom plate’s connection to the slab-on-grade foundation. Again, in this manner, the wall is connected to the slab foundation diaphragm. 

 

Externally insulated post-tensioned concrete slab-on-grade foundation wall with a turn-down footing
Figure 6. Externally insulated post-tensioned concrete slab-on-grade foundation wall with a turn-down footing (Source: Building Science Corporation).
Externally insulated post-tensioned concrete slab-on-grade foundation wall with a turn-down footing showing anchorage of the wall to the foundation for seismic resistance
Figure 7. Externally insulated post-tensioned concrete slab-on-grade foundation wall with a turn-down footing showing anchorage of the wall to the foundation for seismic resistance (Source: Building Science Corporation).

 

Thermal Efficiency

Slab foundations in new and existing homes should be insulated to be thermally efficient, as required by code. The basis of thermal efficiency is environmental separation, which is provided in the home’s wall and foundation assemblies by continuous air, vapor, thermal, rainwater, and groundwater control layers.

All of the foundations depicted above show a continuous air and rainwater control layer that is attached to the exterior face of the structural sheathing on the walls to control air and rainwater entry. This air and water control layer could consist of house wrap, a fluid-applied barrier product, or a fully adhered membrane.

Vapor control of the walls is provided by controlling the temperature of the wall assembly’s condensing surface by installing continuous exterior insulation. Vapor control of the concrete slab is provided by installing sheet polyethylene under the concrete foundation, wrapping the sheeting under the turn-down footing, and extending it up to grade on the exterior of the footing.

Thermal control is provided by installing continuous exterior insulation and cavity insulation on and in the wall framing. Additionally, rigid insulation is installed either on the exterior edge of the slab (as shown in Figures 2, 5, 6, and 7) or on the top surface of the slab (as shown in Figures 3, 4, and 5).

These approaches work in all climates.

 

Ensuring Success

Consult a licensed architect or engineer to develop a detailed design and approach for the home to withstand seismic activity. Thermal efficiency should be incorporated as required by code.

Region

Earthquake Areas

The approaches to seismic control will work in all climates. However, check local building codes for specific requirements as seismic risk and requirements vary based on location; see map below. Insulation requirements for thermal efficiency are climate dependent; see the Compliance tab and consult local code for requirements. 

The International Residential Code (IRC) takes a building’s seismic risk into account based on location. The IRC contours the United States into seismic design categories, from low risk to high risk as shown in Figure 1, which designates the categories by letter: A, B, C, D0, D1, D2, and E, with A designating the lowest risk and E designating areas with the highest risk. The IRC has design guidelines for categories A through D2 as well as scenarios for when a building in design category E can be reassigned to category D2. If a building located in design category E cannot be reassigned to category D2 then it must be designed using the International Building Code (IBC), not the IRC.

Seismic map of the 2018 International Residential Code adapted by FEMA to show Seismic Design Categories in color
Figure 1. Seismic map of the 2018 International Residential Code adapted by FEMA to show Seismic Design Categories in color (Source: FEMA 2020). 

 

Training

Right and Wrong Images

An uninsulated (or existing insulated) basement slab is retrofitted to reduce moisture transmission by sealing with epoxy paint.
An uninsulated (or existing insulated) basement slab is retrofitted to reduce moisture transmission by sealing with epoxy paint.
An uninsulated (or existing insulated) basement slab is retrofitted to reduce moisture transmission by sealing with epoxy paint.
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Source
Reference
Mass Save Deep Energy Retrofit Builder Guide, GM-1302
Author(s)
Neuhauser Ken,
Gates Cathy,
Pettit Betsy
Organization(s)
Building Science Corporation,
BSC
Description

Guidebook providing useful examples of high performance retrofit techniques for the building enclosure of wood frame residential construction in a cold and somewhat wet climate.

Monolithic slab with a grade beam
Monolithic slab with a grade beam
Monolithic slab with a grade beam
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Source
Stucco is installed over rigid insulation, which is installed over a drainage plane consisting of a drainage gap and building wrap layer over the sheathing
Stucco is installed over rigid insulation, which is installed over a drainage plane consisting of a drainage gap and building wrap layer over the sheathing
Stucco is installed over rigid insulation, which is installed over a drainage plane consisting of a drainage gap and building wrap layer over the sheathing
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Source
Courtesy Of
BSC
Right - Rigid foam insulation is installed along the exterior edge of an existing foundation slab.
Right - Rigid foam insulation is installed along the exterior edge of an existing foundation slab.
Right - Rigid foam insulation is installed along the exterior edge of an existing foundation slab.
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Source
Courtesy Of
BSC
This exterior insulated slab-on-grade monolithic grade beam foundation is protected from pests by termite shield at the sill plate, borate-treated framing, flashing at end of wall insulation, brick veneer over slab-edge insulation, and rock ground cover.
This exterior insulated slab-on-grade monolithic grade beam foundation is protected from pests by termite shield at the sill plate, borate-treated framing, flashing at end of wall insulation, brick veneer over slab-edge insulation, and rock ground cover.
This exterior insulated slab-on-grade monolithic grade beam foundation is protected from pests by termite shield at the sill plate, borate-treated framing, flashing at end of wall insulation, brick veneer over slab-edge insulation, and rock ground cover.
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Source
Courtesy Of
BSC
The foundation walls are ICF block providing two continuous layers (R-23) of insulation around the slab, which will sit on an additional 4 inches (R-20) of low-VOC EPS foam.
The foundation walls are ICF block providing two continuous layers (R-23) of insulation around the slab, which will sit on an additional 4 inches (R-20) of low-VOC EPS foam.
The foundation walls are ICF block providing two continuous layers (R-23) of insulation around the slab, which will sit on an additional 4 inches (R-20) of low-VOC EPS foam.
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Source
Wrong- The Anchor bolt washers overlap.
Wrong- The Anchor bolt washers overlap.
Wrong- The Anchor bolt washers overlap.
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Source

Videos

Compliance

ENERGY STAR New Homes and Apartments

DOE Zero Energy Ready Home

International Energy Conservation Code (IECC)

International Residential Code (IRC)

More Info

References and Resources

Homeowner’s Guide to Earthquake Safety

Publication Date
·
Author(s)
California Seismic Safety Commission
·
Organization(s)
SSC,
State of California
Description
Guidebook that provides information on earthquake hazards that can damage homes, the design and retrofit of earthquake resistant housing, and how to find more information on earthquake safety.

Concrete Solutions, BSI-118

Publication Date
·
Author(s)
Lstiburek Joseph W
·
Organization(s)
Building Science Corporation,
BSC
Description
Report covering insulating crawlspaces.

Residential Seismic Strengthening

Publication Date
·
Author(s)
City of Portland,
State of Oregon
·
Organization(s)
City of Portland,
State of Oregon
Description
Website providing resources for seismic retrofits for builders and homeowners as well as city permit requirements from the City of Portland OR.

*For non-dated media, such as websites, the date listed is the date accessed.

Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

Building Science Corporation

Pacific Northwest National Laboratory

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Disclaimer

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