(Introduction, Syllabus, 1.Prelims, 1-4Precon, 2. Excavation, 3.Foundation, 4.Framing, 5.Roof, 6.Close, 7.MEP, 8.Finish)

With the site cleared for the next step in the foundation’s construction, a stem wall is needed to support the floor framing. Stem walls make it possible to raise the floor level for storm cellars, basements, or mechanical crawl space.

A raised foundation also changes the profile of a building, reducing potential moisture penetration, mold and mildew, insects and insecticide contamination, and makes it easier to add or modify the house as mechanical requirements change over time. See the website for the Southern Forest Products Association.

Foundation Phase – Insitebuilders

The stem walls and piers could be built using cast-in-place concrete with rigid or insulated forms. Cast concrete walls have the advantage of being a strong monolithic structure in wet or difficult soil and are required by code for earthquake engineering, heavy loads, and some soil, ground water, or site conditions

Checking the footing
The foundation walls for this building are constructed with standard concrete masonry units (CMU). The concrete blocks are laid using common tools and reinforced with horizontal and vertical steel that is embedded in grout to form an integrated structural system.

Before beginning the wall’s construction, the site is cleared for safety, string lines are replaced on the batter board, and the foundation is checked for compliance with the construction documents prior to finalizing preparations for the work. Verification of existing conditions is especially important when the masonry contractor is not the builder who installed the concrete footing.

Foundation Phase – Insitebuilders

The workpoint is used to check the elevation of the footings and mark the corners of the foundation walls, verifying the string line intersections (see Part 10 Foundation Footings).

Foundation Phase – Insitebuilders

The foundation stem walls extend from the top of the concrete footing up to the bottom of the sill plates that support the structural framing. That means the elevation of the footings is directly related to the reference height of the stem walls and the corresponding elevation for the floor, wall, and roof framing.

Staging the Masonry
To prevent future conflict, some builders require each subcontractor to sign off on an existing installation prior to allowing them to move material and equipment onto the jobsite and start their work. This makes subsequent specialists solely responsible for each step in every phase of the construction.

 

Foundation Phase – Insitebuilders

Staging for the masons includes lifting pallets of masonry into the excavation and setting up a mixer along with grout/mortar and sand as required. Access to water is of course critical and power may be necessary for cutting tools and rebar bending equipment. Erosion control and safety concerns for the workers are also important.

Foundation Phase – Insitebuilders

As you’ll see in the next post, the walls for this building are designed using standard block sizes and masonry dimensions to minimize cutting and fitting. This means all horizontal and vertical dimensions are specified in eight inch (8”) increments.

(To be continued…)

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The material presented in this series has been taken from our book, “How a House is Built: With 3D Construction Models” The book includes annotated illustrations, captioned text, videos, models, and the 2D Preliminaries.

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(Introduction, Syllabus, 1.Prelims, 1-4Precon, 2. Excavation, 3.Foundation, 4.Framing, 5.Roof, 6.Close, 7.MEP, 8.Finish)

Once the concrete is in place, the site should be cleared of tools, equipment, and any debris that accumulated during the pour. Fencing, barriers, and safety controls should be replaced where they’ve been removed and the batter boards and workpoint checked for damage.

Strip the formwork
It’s also important to remove the formwork as soon as possible, but only as quickly as the weather will allow. In general, stripping immediately makes it easier to remove and clean the forms and allows the concrete to cure evenly. Waiting too long can weaken the concrete because of uneven curing and make it difficult to reuse the forms.

Foundation Phase – Insitebuilders

Unless it’s extremely cold, a chemical reaction occurs that prevents the concrete from freezing, but the footing’s surfaces can also dry out if it cures too quickly. This makes timing critical and it takes experience to know when it’s best to strip the forms.

Foundation Phase – Insitebuilders

Hands-on builders consider the weather and feel the warmth of the concrete before making a judgment as to the best way to protect the concrete as it cures.

A Different Approach
All of this can be avoided with an interesting innovation in concrete formwork. Of course, it takes a long time in the construction industry to change established methods, but some innovations suggest a particularly logical approach to conventional practice.

Foundation Phase – Insitebuilders

Fabric forms use tensile membranes instead of rigid members. Concrete is contained by the geometry of the fabric, forming structural curves as pure tensile elements and ideal parabolic shapes for foundation footings.

Foundation Phase – Insitebuilders

The fabric also adapts to uneven ground and uses far less material to hold the wet concrete in place. The permeable material also allows the concrete to cure evenly with a smooth finish, reducing labor, transportation, and storage. Better yet, stripping is reduced to stakes, spreaders, and runners that are easily removed and reused in the framing as blocking.

Foundation Phase – Insitebuilders

Trying something new depends on the type and volume of work, design of the foundations, and interests of the builders who might want to give an idea like this a try. For more information, see installation diagrams and videos at www.fab-form.com and an association website at www.fabricforming.org.

 (To be continued…)

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The material presented in this series has been taken from our book, “How a House is Built: With 3D Construction Models” The book includes annotated illustrations, captioned text, videos, models, and the 2D Preliminaries.


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(Introduction, Syllabus, 1.Prelims, 1-4Precon, 2. Excavation, 3.Foundation, 4.Framing, 5.Roof, 6.Close, 7.MEP, 8.Finish)

As soon as the rebar and the forms are ready, most building permits will require official approval before concrete can be placed. A code compliance inspector is then scheduled by the builder to visit the site, check the forms and rebar against the permit drawings, and authorize concrete placement with a stamp or signature.

Foundation Phase – Insitebuilders

Unless the foundation is unusually complicated, most inspectors will only take a cursory look at the formwork, spot check some rebar, and sign off without comment, after all most foundations are pretty much the same. But problems occur if the rebar is dirty, rusty, or oily, or the formwork somehow affects public property. Rarely is the inspector concerned about the foundation’s design or engineering, that’s a given.

PrePour Checklist
Good builders use a checklist to make sure the foundation is ready for inspection and anticipate any possible delay. Concerns include a final check of the formwork for gaps, weaknesses, or missing hardware, cleaning the forms and reinforcing, checking the position of sleeves and brackets, and making sure required insulation, expansion joints, and vapor barriers are installed.

Foundation Phase – Insitebuilders

Prior to ordering concrete, it’s important to make sure all the necessary tools and equipment are on site and ready to use. This includes shovels, water, vibrators, spreaders and trowels. High density lighting might also be required on short winter days, especially if there’s any possibility of a delay.

In addition, a safety officer should be assigned to check the weather, designate safe zones, truck routing and staging, mark power lines, check all equipment, and secure the wash down area. The entire site is then cleared of unnecessary tools, trash, and personnel.

Foundation Phase – Insitebuilders

Placing the Concrete
Once the concrete has been scheduled, a biodegradable release agent is applied to the forms to prevent bonding. A concrete pump makes it much easier to move concrete around the site, reducing risk and the number of laborers required for the work, as well as making it easier to spread and vibrate the material once in place.

Foundation Phase – Insitebuilders

The number of concrete trucks depends on the volume of material required, the supplier, and jobsite conditions. For large projects, more than one truck will mean staging the work according to volume calculations and site conditions in order to control joints, minimize waste, and prevent potential contamination from excess concrete runoff.

 (To be continued…)

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The material presented in this series has been taken from our book, “How a House is Built: With 3D Construction Models” The book includes annotated illustrations, captioned text, videos, models, and the 2D Preliminaries.

(Introduction, Syllabus, 1.Prelims, 1-4Precon, 2. Excavation, 3.Foundation, 4.Framing, 5.Roof, 6.Close, 7.MEP, 8.Finish)

Specialty contractors use reusable forms for their foundations, but for most small projects almost any skilled carpenter can quickly layout and assemble the formwork for a foundation footing. In this example, the footing formwork is assembled with standard lumber that will be reused in the floor and wall framing illustrated later in Phase 4.

Foundation Phase – Insitebuilders

Pier work
If soil conditions are right, the footings can also be cut directly into the soil as shown for the pier footing on the excavated shelf in the illustration above.

Forms for the rest of the pier footings are prefabricated on site so that they can be centered on the string line intersections marking the column locations, or positioned with a tape measure from the perimeter formwork. The center of the pier footing is the center of the column load coming from above.

Foundation Phase – Insitebuilders

Bridging is used to locate and suspend the concrete reinforcing (rebar) across the top of the open earthen formwork, or from the sides of the prefabricated forms. For heavily reinforced foundations, some contractors prefer to use light weight plastic “chairs” to hold the rebar up off the floor of the excavation (See Craig’s Concrete Specialties).

The size and location of the reinforcing varies with soil conditions and the weight to be distributed over the undisturbed surface of the excavation.

Quality control
It’s important to check the location and elevations of the footings for accuracy, as well as test soil density to make sure it meets the original engineering specifications. Loose or disturbed soil will sink or compress under the weight of the building.

Foundation Phase – Insitebuilders

Depending on building type and local conditions, some building departments will require certification for the location of the footing, along with penetration test results verifying soil density before authorizing concrete placement.

The workpoint is again the primary reference to check the layout and vertical elevations of the footings. It’s easy to see how a level and accurate excavation minimizes unforeseen material and labor costs for all future construction.

Continuous reinforcing
Standard lengths for rebar are 20 feet, with diameters measured according to increments of 1/8”. In other words, a #4 bar is 4/8” and a #5 bar 5/8”. This means the rebar must be cut and bent in the field to fit in the formwork using a hydraulic tool (similar to this one supplied by Southern Tool).

Foundation Phase – Insitebuilders

Crossbars are installed to keep the formwork rigid and suspend the rebar. The steel reinforcing is tied to keep it away from the sides of the formwork and up off the ground. Again, chairs or similar spacers may be easier for some builders to install.

Clearances from the ground and sides of the formwork are necessary to protect the rebar and distribute the loads on the foundation once the concrete has cured.

Foundation Phase – Insitebuilders

The rebar must also be continuous at all intersections and around corners because any gap or break in the length may result in settlement cracking once the footing is under load.

Special attention is also required at any step or change in the footing elevation. Concrete stem walls are formed to fit this step, but for a masonry stem wall, the step should be located so that it fits standard masonry dimensions.

Foundation Phase – Insitebuilders

As you’ll see in a later phase, the stem walls require embedded vertical J-bars to tie reinforcing within the wall to the footing after the concrete is ready for the next step in the foundation’s construction.

 (To be continued…)

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The material presented in this series has been taken from our book, “How a House is Built: With 3D Construction Models” The book includes annotated illustrations, captioned text, videos, models, and the 2D Preliminaries.

insitebuilders.com
www.insitebuilders.com

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Part 10: Foundation; Footings

December 17, 2013

(Introduction, Syllabus, 1.Prelims, 1-4Precon, 2. Excavation, 3.Foundation, 4.Framing, 5.Roof, 6.Close, 7.MEP, 8.Finish)

The string lines are a quick reference and good enough for most projects, but experienced builders will use a transit level to cross check the layout of the foundation formwork based on vertical and horizontal distances and triangulated offsets from the workpoint.

Foundation Phase – Insitebuilders

In dense urban areas and zero lot line developments, a licensed surveyor is required to certify the location and depth of the foundation as a condition of the building permit. Zero lot lines occur when zoning restrictions allow construction right up to the property line.

Concrete Formwork
For some buildings, concrete is placed directly into a trench cut into the soil using the string lines as a reference. This method is commonly used for the perimeters and bearing walls of a slab or monolithic foundation.

For buildings supported by piers and continuous stem or basement foundation walls, spread footings are used to distribute the weight of the building to undisturbed soil.

Foundation Phase – Insitebuilders

General contractors hire subcontractors who specialize in foundations to install these footings. As masons or concrete workers, they clamp together reuseable forms, place the rebar and concrete, then disassemble the forms for their next job as soon as the concrete begins to set.

Foundation Phase – Insitebuilders

In contrast, hands-on builders assemble the footing formwork using standard lumber, setting aside the material for reuse in other parts of the building as blocking or non structural framing once the forms are stripped and cleaned.

The depth and size of the footing depends on the weight and total load of the building. Dimensions will vary with soil bearing capacity, moisture content, and in some regions the soil frost line.

Bearing capacity and soil

Because the footing is designed to distribute the bearing load from a column or stem wall above, it is offset so that the footing itself is centered on that load. In other words, the center of the wall or column above sits over the center of the footing (see the wall section in the previous post)

Foundation Phase – Insitebuilders

Centering the load in this way means the width of the footing extends out from both sides of the wall above. This means the outer portion of the footing extends beyond the face of the finished wall.

Foundation Phase – Insitebuilders

Technically, the footing might cross a setback line, but since it is below ground, it is most often ignored by building officials. However, when the footing lies against a zero lot line with no setback, the spread footing must be engineered so that the entire foundation remains within the property boundaries.

 (To be continued…)

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The material presented in this series has been taken from our book, “How a House is Built: With 3D Construction Models” The book includes annotated illustrations, captioned text, videos, models, and the 2D Preliminaries.
How a House is Built - insitebuilders.com

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(Introduction, Syllabus, 1.Prelims, 1-4Precon, 2. Excavation, 3.Foundation, 4.Framing, 5.Roof, 6.Close, 7.MEP, 8.Finish)

The horizontal member of all the batter boards should now be at the same elevation above sea level. This means the string lines attached to these batter boards can be used as a vertical reference to check the depth of the cut, in addition to locating the critical corners and bearing points for the foundation.

Excavation Phase – Insitebuilders 

Checking the depth of the cut
To save time, most builders use a fixed or transit/level to monitor progress and control the quality of the excavation. This survey instrument is set up over the workpoint so its scope remains level through 360 degrees of rotation (see this Tom Duffy video).

Excavation Phase – Insitebuilders

Since the height above the workpoint is now constant, the scope can be aimed at a surveyor’s story pole, first to get an elevation for the existing grade at that point, then to check the depth of any cut by calculating the relative measurements on the story pole.

Excavation Phase – Insitebuilders

The idea is to avoid taking out more material than is absolutely necessary to comply with the requirements for the foundation on of the construction drawings. Removing too much material means it would have to be replaced as backfill or transported off the site an extra cost.

The foundation layout
The size and depth of the cut is based on the requirements specified on the construction drawings. These dimensions are shown as a sea level elevation on the civil engineering drawings and transferred as a reference zero point – usually set as the top of the finished floor.

Excavation Phase – Insitebuilders

A typical wall section shows the depth of the excavation from this zero point to the relative heights of foundation stem walls, footings, and the total thickness of the structure supporting the finished floor. The height of the structural framing, finished grades, walkways, and other site work are all referenced from this common zero point.

Excavation Phase – Insitebuilders

A plumb bob is used to transfer the location of the walls and bearing points from the string line intersections to the floor of the excavation and position the foundation formwork.

The depth of the excavation and the top of the formwork are then measured vertically from the string lines, and the corresponding horizontal members of the batter boards.

Excavation Phase – Insitebuilders

Important is that an accurately placed and level excavation sets the stage for masons and concrete workers, so that they can build the foundation according to their original contract estimates.

If the cut is misplaced, too deep, or irregular, it means additional labor and materials, schedule delays, and a possible change order.

 (To be continued…)

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The material presented in this series has been taken from our book, “How a House is Built: With 3D Construction Models” The book includes annotated illustrations, captioned text, videos, models, and the 2D Preliminaries.


(Introduction, Syllabus, 1.Prelims, 1-4Precon, 2. Excavation, 3.Foundation, 4.Framing, 5.Roof, 6.Close, 7.MEP, 8.Finish)

In an ideal world, construction starts once the general contractor has the site ready with temporary utilities, storage, recycling bins, and safety and environmental safeguards in place.

Excavation Phase – Insitebuilders

Layout foundation corners
An accurate layout is important, especially in high density areas when the building is near a setback or property line, because its final height and location must often be verified by a licensed surveyor prior to being granted a certificate of occupancy.

Excavation Phase – Insitebuilders

The layout begins by positioning a surveying instrument directly over the workpoint that was located during start-up planning. As noted earlier, this workpoint is a field reference for both an elevation above sea level and a precise location on the jobsite that corresponds with the elevations and dimensions shown on the construction drawings.

Excavation Phase – Insitebuilders

The sitework, foundation corners, floor and roof heights, are all located from this workpoint using either the geometric computations built into surveying instruments, or by triangulating the bearings and distance to the corners, framing, or ridgeline during construction. (See GIS surveying instruments)

Excavation Phase – Insitebuilders

For straightforward foundations on an open site, most builders would simply measure parallel offsets from a predetermined reference line sighted along the setback or property corners.

Excavation Phase – Insitebuilders

Measurements along the length of the reference line from the workpoint are taken from dimensions specified on the construction drawings. A light weight builder’s transit/level is then repositioning over stakes located at critical distances along that line to layout the walls or objects perpendicular to, or along arcs left or right of, this reference.

Excavation Phase – Insitebuilders

The layout is then checked with optical lasers, GPS, or by diagonal field measurements to square up the foundation corners.

Setting batter boards
Once the corner stakes are set and checked for accuracy, string lines are extended from temporary batter boards placed well outside the area to be excavated. Important is that the horizontal member for each of these batter boards is level, and that they all share the same elevation as referenced from the workpoint.

The batter boards allow builders to carefully position string line intersections over the center marker on the corner stakes. They will also be used to gauge the depth of the excavation and the height of the foundation walls and footings as the work continues.

Excavation Phase – Insitebuilders

The string lines are tied to nails on the upper surface of the batter boards. They locate the outside face of the foundation walls and any interior stem walls, special conditions, or changes in elevation within the excavated perimeter.

Excavation Phase – Insitebuilders

Once the string lines are checked and labeled, they can be untied and set aside in order to clear the area for the equipment needed to start the excavation. The strings can then be quickly reset during the excavation to check progress, verify the depth of the cut, and re-establish the foundation corners.

 

(To be continued…)

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How a House is Built - Insitebuilders
The material presented in this series has been taken from our book, “How a House is Built: With 3D Construction Models” The book includes annotated illustrations, captioned text, videos, models, and the 2D Preliminaries.

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