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ARCH 2614/5614 Lecture notes

Jonathan Ochshorn

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Wall sections: foundations

Substructure

Consists typically of footings and foundation walls

section showing foundation
Spread footings:

section showing footing
Bearing capacities (psf):

hard rock200,000
soft, broken bedrock20,000
gravel/ sand-gravel12,000
loose medium sand4,000
soft, broken shale, soft clay3,000

Generally, 1-2 story residential construction will not place more than about 1,500 psf load on the underlying soil.
loads from typical residential structure

Spread footings "spread out" the weight of the superstructure and foundation system on a larger surface area of soil or rock, thereby reducing the stress or pressure on the soil or rock.

spread footings

Matt foundations

Alternatively, a matt (or raft) foundation can be cast under the entire building footprint, reducing the tendency for differential settlement, and tying the structure together (useful in seismic zones).
matt footings
matt footings
Johnson Museum at Cornell (I.M. Pei) with schematic representation of matt footings.

In Ithaca's Collegetown, a new project also appears to utilize a matt foundation:

collegetown crossings construction sign, Aug. 2015
Here's the "artist's conception" at the construction site, Aug. 2015 (photo by J. Ochshorn)

collegetown crossings matt foundation reinforcement, Aug. 2015
Formwork and reinforcement is in place, and the concrete is being pumped into the fomrs, Aug. 2015 (photo by J. Ochshorn)

collegetown crossings matt foundation, Aug. 2015
The concrete is placed and the forms are removed, Aug. 2015 (photo by J. Ochshorn)

Foundation walls

foundation wall, Statler Hall addition
foundation wall, Statler Hall addition
Foundation wall for addition to Statler Hall, Cornell University (photos by J. Ochshorn)

Why is footing placed below grade?

  1. not a good idea to place building on top of organic matter (top soil), as it is not structurally adequate.
  2. soil below grade is already compressed/compacted by the weight of the soil above, and is therefore more likely to be able to carry the imposed loads of a building.
  3. avoid "frost heaving," where certain soils expand when the water in them freezes.
    footing depth below grade

Typical residential footing dimensions:
residential footing

Deep foundations (piles and caissons):

Where the soil immediately under the building is not structurally adequate, it is necessary to place the foundation system deeper. Two basic strategies exist:

  1. Caisson: like a spread footing, but deeper. The earth is removed by digging or drilling, after which concrete (and reinforcement) are placed in the hole.
  2. Pile: a pile is driven into the earth like a nail, but bigger. Piles can be driven all the way to bedrock (end-bearing pile) or may need to stop short and be designed as a "friction pile."

Piles are generally grouped together and consolidated at the top with a cast concrete pile cap which resembles a spread footing. The column (or wall) bears on the pile cap just as would bear on a spread footing; the difference is that in the case of a pile cap, the load is transferred through the piles down to a suitable stratum. See images below:
piles and caissons

Piles can be fabricated from wood, steel or reinforced (precast) concrete.
pile types
From Allen, Fundamentals of Building Construction

Excavation: To create a foundation, it is always necessary to excavate soil down to the level of the footings (or pile caps, or caissons). For ordinary spread footings or matt foundations, it is common to excavate under the entire building footprint in order to use the space for basement(s). Otherwise, it is only necessary to excavate where the actual footings will be placed. If the site is large enough (and the building is not at the edge of the site), one can slope back the excavated area, consistent with the angle of repose of the soil. However, in tight sites, especially urban sites, where the building is close to the property line, other techniques are needed to hold back the edge of the excavation. See images below:
excavation techniques

Notice the cross-lot bracing in image "A" below; then the soldier beams and lagging in the excavations in imagese "B" and "C":

Cross lot bracing, Hong Kong fast rail station
Image "A" showing cross lot bracing, Hong Kong high-speed rail terminal (photo by J. Ochshorn, Jan, 2017)

Cornell Statler addition excavation
Image "B" showing excavations at Statler Hall addition at Cornell University (Photo by J. Ochshorn)

Cornell Milstein HALL excavation
Image "C" showing Milstein Hall excavation at Cornell University (photo by J. Ochshorn)

Slurry walls: This technique for simultaneously casting foundation walls and preparing the site for excavation is used mainly in tight urban sites where the boundary condition at or near the property line goes down fairly deep, and convention excavation techniques are impractical. The idea is to excavate a 1-2' wide trench corresponding to the perimeter foundation wall. The trench is filled with a "slurry" that keeps it from collapsing as it is dug deeper. It is then filled with concrete (and reinforcement) which displaces the slurry; the slurry is collected and reused. After the concrete wall is completed, the excavation of the interior can begin.
slurry wall

WTC excavation image
WTC excavation image
Tie-backs in slurry wall in original WTC foundation (top); diagram showing earth/water pressure acting on slurry wall without tie-backs or bracing from buildings (bottom)

Retaining walls: Designed to hold back earth at vertical discontinuities. Crushed stone is placed behind the wall to allow water to escape, either through weep holes in the wall, or into drain pipes behind the wall.

retaining walls
retaining walls
Modes of failure for retaining walls and typical placement of reinforcement (top); failure of retaining wall, Ithaca, NY (bottom; photo by J. Ochshorn)

Foundation walls: At a residential (wood-frame type construction) scale, foundation walls have the following characteristic parts:
foundation walls

Foundation drainage:

foundation drainage options

Typical foundation wall drainage with perforated drain pipe and gravel (left); use of drainage mat (right)

Foundation insulation options:

foundation insulation options

(a) Batt insulation in stud walls and in foundation interior furring; (b) Batt insulation in stud walls with rigid insulation on exterior of foundation wall (protected with cementitious coating); (c) shallow foundation insulation (horizontal)

Foundation insulation options:

pressure-treated wood foundation
pressure-treated wood foundation
Pressure-treated wood foundations and diagram showing construction details (diagram source)

Foundation details

foundation detail
Working drawing foundation detail (detail source)

foundation, Ochshorn addition
See foundation case study of addition to my house.

Case study: Milstein Hall Substructure


See: video link or index of all Milstein Hall construction videos.