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The Lightweight
Steel Frame
House Construction
Handbook
Chapter Two
Foundations
2-1
Foundations
Introduction
This chapter provides an overview of the fundamental issues relating to foundations supporting
lightweight steel framed houses. Typically foundations for lightweight steel framed houses are
constructed as they would be to accommodate a wood-framed structure and no special requirements
are necessary.
As with any foundation, it must be designed and constructed to resist all loads acting upon it.
Structural loads, occupant loads as well as wind and snow loads all must be transferred down through
the foundation onto the soil or rock below it. The foundation must also resist lateral loads such as
soil and water pressures that may be exerted against it. It must transfer all of these loads onto the
footing which rests on suitable soil or rock.
The proper design and construction of the foundation affects the rest of the structure that rests on it.
The foundation must provide a level surface to support the frame. A poorly built, out-of-square
foundation often translates into a poorly built, out-of-square home.
Foundations should not settle significantly over time or be affected by freezing and thawing of soils.
Controlling moisture and heat flow in basements should be considered in order to create a
comfortable living space.
In many instances, foundations are also expected to control potentially harmful soil gases, such as
radon, from entering the basement. In a number of regions in Canada specific measures must be
adopted. Refer to your local building code and construction guides for specific information
applicable to your area.
Footings
Footings must be designed to properly transfer and distribute the loads they support. Footings must
be located under all walls, pilasters, columns, piers, fireplaces and chimneys. Most building codes will
dictate a maximum supported span of floor joists and/or live loads beyond which the foundation
must be sized by a design professional.
In general, footing sizes depend upon the amount of load carried and the bearing capacity of the
soil. The larger the load or the weaker the soil, the larger the footings required. Figures 2.1 and 2.2
can be used to size conventional strip footings where the bearing capacity of the soil is at least 75
kPa (1500 psf ).
Building Science Note
Protecting basement wall tracks from freshly poured concrete
To prevent the bottom wall track of basement walls from coming into contact with
fresh concrete as the floor slab is poured, the track can be wrapped in polyethylene
(See Figure 2.2).
2-2 • The Lightweight Steel Frame House Construction Handbook
Foundations
Figure 2.1
Footing Sizes Under
Exterior Walls
The Lightweight Steel Frame House Construction Handbook • 2-3
Foundations
Figure 2.2
Footings Under
Interior Walls
2-4 • The Lightweight Steel Frame House Construction Handbook
Foundations
Once the footing width is determined, the footing thickness can be calculated. The thickness is never
less than the projection beyond the footing’s supported element. For column footings, the projection
is typically measured from the edge of the column’s steel base plate to the edge of the footing. See
Figure 2.3
Figure 2.3
Requirements
Common to all
Footings
It is important that the excavation is done to the correct elevation and that the bottom of the
excavation is level. Footing framing must also be level and square. Concrete strength for footings
must conform to all of the requirements of local codes. Typically concrete for house footings must
have a strength of at least 15 MPa (2200 psi) at 28 days, refer to NBCC article 9.3.1.6.
Foundation Walls
Foundation walls supporting steel frames are typically made of poured concrete or masonry but can
also be of insulated concrete forms (ICF) and preserved wood.
Foundation walls must safely support all loads transferred from the building above as well as
withstand any external loads from earth, water or earthquakes. When foundation walls are made of
poured concrete proper practice includes attending to proper curing time, appropriate use of
admixtures and proper care in extreme weather conditions.
Exterior foundation walls can be considered laterally supported or unsupported. A wall is considered
laterally supported if the floor system is anchored to the foundation wall regardless of floor joist
direction. The same wall would be considered laterally unsupported if large window or door openings
are installed. See Figure 2.4 for more specific information.
Whether foundation walls support a steel frame structure or a wood frame structure, the same issues
need to be addressed, namely: crack control, parging and finishing, dampproofing, foundation and
surface drainage, waterproofing, backfilling and soil gas control.
Inevitably irregularities at the top of the foundation may occur. Steel framing is usually able to
tolerate the same variations from level, plumb and square as wood framing. It is important,
nonetheless that exterior framing bear directly on the foundation. If a foundation is so uneven that
the sill gasket cannot accommodate it, it must be properly repaired with grout or chiseled and
chipped.
The Lightweight Steel Frame House Construction Handbook • 2-5
Foundations
Figure 2.4
Laterally Unsupported Walls
Attachment to Foundation Walls
There are various methods of floor system anchorage for a steel frame house:
1. Use a wood sill plate in the same manner as in a wood frame house; or
2. Attaching the closure channel directly to the foundation; or
3. Use a nested stud and track. This technique employs a piece of steel track in combination
with stud to replace the wood sill plate.
It is important to remember that any steel close to or in contact with concrete must be corrosion
resistant. With every type of system a sill gasket, a double bead of non-hardening caulking, or a
mortar bed must be provided under the sill plates (or under the closure channel if a sill plate is not
used) as required by local building codes, this prevents direct contact between the steel and concrete.
Foundation Anchors
Foundation anchors that connect the foundation and the house frame are usually sunk into the
concrete before it cures. If the foundation wall is made of concrete block the anchors are embedded
in the mortar which fills the top course of block. The size and spacing of anchor bolts must follow
the requirements of local building codes.
2-6 • The Lightweight Steel Frame House Construction Handbook
Foundations
Several different types of foundation anchors exist, see Figure 2.5. Critical for all types of anchor is
that a proper bond is achieved between the anchor and the foundation to resist ‘pull-out’. J-bolts and
threaded rods are embedded in the foundation with a threaded portion protruding for the attachment
of the sill plate or rim joist. Embedded into the foundation wall is either a J-section or a nut and bolt
that provide added resistance. Both J-bolts and threaded rods are commercially available.
Figure 2.5
Foundation Anchor
Types
Epoxied anchor bolts can be used if the contractor does not want to embed anchors. Once the
concrete has cured and the holes have been cleaned, appropriately sized holes are drilled into the
concrete and fitted with a threaded rod. An epoxy resin is injected into the hole to form a strong
bond.
Anchor ties, as described in later sections, are another way of fastening a steel frame to the
foundation. The size and spacing of the anchor bolts is specified by the building code as a minimum
diameter of 13 mm (1/2 in) bolts spaced at a maximum 2400 mm (94 in) o.c. Anchor bolt
placement is illustrated in Figure 2.6.
The Lightweight Steel Frame House Construction Handbook • 2-7
Foundations
Figure 2.6
Anchor Bolt and Joist
Spacing
Wood Sill Plate Anchorage
In the wood sill plate anchorage technique a wooden member, usually 38 x 89 mm (2 x 4 in), is
fastened to the foundation via anchor bolts. As depicted in Figure 2.7 a sill gasket, a double bead of
non-hardening caulking, or a mortar bed must be provided between the sill plate and the foundation.
The closure channel or rim joist must be securely fastened to the wood sill plate by means of a steel
plate, as shown in Figure. 2.8, or through the flange of the closure channel into the plate.
Figure 2.7
Wood Sill Plate
2-8 • The Lightweight Steel Frame House Construction Handbook
Foundations
Figure 2.8
Floor to Wood Sill
Connection
If the location of the anchor bolts interferes with the placement of the closure channel, the lower
channel flange can be notched to accommodate the bolt. In no case, however, should the anchor bolts
interfere with the connection of the joist to the closure channel: joists may never be notched. As an
alternative, the holding nuts can be counter sunk and the anchor bolts trimmed so that sill plate
surfaces remain smooth.
When using a wood sill plate it is recommended that the wood be kiln dried to decrease moisture
and shrinkage issues.
Direct Bearing Anchorage
Using direct bearing anchorage the closure channel/rim joist can be anchored directly to the
foundation without the use of a sill plate. This eliminates issues of shrinkage experienced with wood
but could be problematic if the foundation is not even. As with the wood sill plate method a sill
gasket, a double bead of non-hardening caulking, or a mortar bed must be provided between the
foundation and the rim joist.
There are two ways to connect the rim joist to the foundation: by means of a clip angle or with an
anchor tie.
A clip angle is used in conjunction with an anchor bolt. The clip angle is fastened with 8- #8 screws
to the rim joist and bolted to the foundation, they are spaced according to the required anchor bolt
spacing, at least every 2400 mm (94 in) o.c. See Figure 2.9.
Anchor ties are cast directly into the foundation and then fastened to the exterior side of the closure
channel, as shown in Figure 2.10. The ties must be spaced in accordance with the layout of the xbracing used for wind and racking resistance, see Chapter 4. Anchor ties are proprietary systems and
therefore information about spacing and fasteners is normally available from the manufacturer or
specified by a design professional. As with all fastening systems, the spacing may be no greater than
2400 mm (94 in) o.c. as per building code.
The Lightweight Steel Frame House Construction Handbook • 2-9
Foundations
Figure 2.9
Clip Angle without
Sill Plate
Figure 2.10
Anchor Ties
2-10 • The Lightweight Steel Frame House Construction Handbook
Foundations
Nested Track and Stud Sill Plate
This anchorage takes advantage of the ability of a steel sill plate to create a level surface while
avoiding the shrinkage issues of wood. A nested track and stud are used as a sill plate, anchored with
anchor ties. A closure channel is screwed onto the track and stud. Figure 2.11 illustrates this system.
Attachment of the closure channel to the sill plate and the sill plate to the foundation is the same as
described under the wood plate attachment requirements.
Figure 2.11
Nested Track and
Stud Detail
Other Foundation Types
Other foundation types may include proprietary systems such as insulated concrete forms. These
systems should include specific instructions for the attachment of steel framing.
Accommodating Window and Door Openings in Foundation Walls
Where openings occur in basement walls to accommodate windows, a header is required to support
the loadbearing walls and floor joists as shown in Figure 2.12. Note that in these areas the floor joists
must be shorter since they attach to the header rather than the closure channel. Header and header
selection are discussed in detail in Chapter 4.
Loadbearing basement window frames are available for small windows, which can eliminate the need
for a header. Refer to your window supplier.
The Lightweight Steel Frame House Construction Handbook • 2-11
Foundations
Figure 2.12
Framing Over
Basement Openings
Special Considerations
At this stage the final cut list should be considered. Although the cut list should be prepared before
construction, the final order should only be made once the foundation has been poured and field
measurements made. If the foundation is not placed as planned, joist lengths need to be adjusted in
order to accommodate the new dimensions. Double span joists can be adjusted by increasing their lap.
Looking Ahead
Anchor ties and X-bracing
If using anchor ties remember that their layout should correspond with the layout for the
x-bracing in the walls.
Looking Ahead
Exterior Finishes
Remember to plan your foundation wall with your exterior finish in mind. Sometimes it is
necessary that the foundation be wider than the required minimum in order to adequately
support a finish such as brick veneer.
2-12 • The Lightweight Steel Frame House Construction Handbook
Foundations
Foundations Checklist
Y
Top of
Foundation Walls
Wood Sill Plate
Direct Bearing
Anchorage
Nested Track and
Stud Sill Plate
Openings in
Foundation Walls
N
Top of foundation walls are level, or have been chipped and
grouted to provide a level surface
A sill gasket, mortar bed or double row of non-hardening caulking
has been installed between all framing members and the
foundation
Anchor bolts are a minimum diameter of 13 mm (1/2 in) bolts
spaced at a maximum 2400 mm (94 in) o.c.
If closure channel is fastened to the wood sill plate by means of a
steel plate, the steel plate is fastened with 4-#8 screws to the
closure channel and 4- 10d or 6-8d common nails connect the steel
plate to the wood sill
The closure channel can be fastened with screws through the
flange to the sill plate
If using anchor bolts and clip angles, the clip angle is fastened with
8- #8 screws to the rim joist and bolted to the foundation, they are
spaced according to the required anchor bolt spacing, at most
every 2400 mm (94 in) o.c.
If using anchor ties, they have been installed and spaced according
to manufacturer’s directions, at a spacing no greater than 2400 mm
(94 in) o.c.
Anchor bolts are a minimum diameter of 13 mm (1/2 in) bolts
spaced at a maximum 2400 mm (94 in) o.c.
If closure channel is fastened to the wood sill plate by means of a
steel plate, the steel plate is fastened with 4-#8 screws to the
closure channel and 4-#8 screws connect the steel plate to the sill
plate
The closure channel can be fastened with screws through the
flange to the sill plate
All openings have had a header designed from the appropriate
Member Selection Tables (Appendix A) or have structural windows
installed
The Lightweight Steel Frame House Construction Handbook • 2-13