1_spreadfoundations

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Foundation Systems 1,spread foundations

Whats the point?

The foundations primary purpose is:

Distribute building loads to the soil

Prevent frost uplift of the building (act asfrost barrier)

Some foundation systems also providea level, controlled surface from whichthe buildings walls are raised.

What types?

Spread (aka strip) footings with stemwall foundations

Pad footings with column foundations

Pile footings with caps

Caissons (aka drilled piers) with grade beams

Mat (aka raft, slab) foundation with stemwalls


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Spread Footings

Bottom of footing seton undisturbed earth orengineered fill andbelow frost line

The rule of thumb forproportioning (alwaysuse a professionalengineer!) is the footingis twice as wide as thefoundation walltransferring load to it.

Its thickness is at leastequal to the width ofthe foundation wall.

W

= 2W

= W

Characteristics

Light loads

Low rise

Stable soils

Common in residential and light commercial

Also used as a frost barrier with heavier foundation

systems

A shallow foundation system (usually within 25 feet ofgrade)


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Strip footings

Most common for low rise,low load conditions.

A strip of reinforced concreteready to accept a wall ofsome kind on top.

Dowels, Reinforcing barsprojecting from the top of thefooting connect to wallreinforcing to limit sideslip ofthe stemwall.

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Strip footings

An alternative to dowels is tomake a slight depression inthe top of the footing called akeyway.

The keyway works when it isfilled with concrete from thewall pour above. Theconcrete acts as a shear keypreventing sideslip of thestemwall.

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Spread footing process Layout strings from

batter boards - markoutside of walls or

outside of footing Project lines down to

bottom of excavationwith plumb lines andmark with paint orlime

Excavate for footing

If rain is expected- (to within a fewinches of bottomelevation - handdig the rest afterthe rain)

If no rain isexpected - dig tofinal elevation,

Spread footing process2 Set formwork for

footing,

Set to square andlevel with transit

double checkoutside walllocation tooutside of footing

stake to grade tobrace

Cross brace topof form

Set reinforcing onchairs or bricks tokeep it up off thebottom of thefoundation -

longitudinals firstthen transverse

Spread footing process2 Pour the footing

form full ofconcrete.

Set reinforcing barsas dowels in thetop of the concreteto tie into the walls

Estimating guidespriced this footingat approx. $24.00per linear foot in1998

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Spread footing process3 With the forms

stripped, thefoundation walls

made of block orconcrete can beconstructed on the

footing

If concrete blockwalls are used,horizontal jointreinforcing isplaced betweenalternate coursesto provideadditional lateralstability.

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Spread footing process4 Anchorage for sil l

plates isaccomplished byfilling block coreswith concrete andJ shaped anchorbolts are inserted.

Additional verticalreinforcing, ifrequired isaccomplished byplacing thereinforcing bar in avertical block coreand filling withconcrete.

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Cast in Place stemwallfoundations

Where high lateral forces,high hygroscopic pressuresor high masonry costs arefound, foundation walls areoften made of cast in placeconcrete

Its a premium foundationwall - often most expensive

Cost is high because webuild a wood wall (forms) fillit with a steel wall (rebar) andcover that with a concretewallthen throw the woodwall away!


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Begin withForms

Formwork is usually builtfrom the outside - in.

The exterior form(plywood) is constructedon a layer of vertical studs(to brace the plywood)these vertical studs arethen braced with horizontalstuds (walers) to make theform stiffer

Then the reinforcing isinstalled, wired together ateach vert/horiz intersection

Begin withForms

Formwork is usually builtfrom the outside - in.

The exterior form(plywood) is constructedon a layer of vertical studs(to brace the plywood)these vertical studs arethen braced with horizontalstuds (wales) to make theform stiffer

Then the reinforcing isinstalled, wired together ateach vert/horiz intersection

Ties thatbindand separate!

To keep the forms theproper distance apart,wire ties with plasticcones are insertedbetween forms. Thecones keep the inside andoutside form boards apart.

The loops in the ends ofthe forms allow the ties tobe tensioned so the formsdont spread apart.

They push and pull at thesame time


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The wet concrete mix is prettyheavy, the forms have to bebraced both horizontally andvertically to resist the hydraulicload imposed by the mix

Horizontal 2x4s (wales) areattached to the back of theplywood formwork to be thefirst bracing layer

Then vertical 2xs areclamped to the horizontalsto become the secondbracing layer

Finally, diagonal braces areadded and staked to theground, now the formbracing is complete

Forming apour joint

The pour joint begins withcontinuity of the reinforcing

Sometimes an additional dowelis placed to insure the joint willresist lateral displacement

To stop the pour, a plywood capis made by drilling holes in apiece of plywood, and attaching itto the plywood forms on eachside of the wall, making sure therebar passes through the holes

A 2x4 is often nailed to the insideof this plywood cap to displaceconcrete and form a shear key

Often, the foundationcannot be poured all atonce. To stop a pour, wedetail a pour jointthatwill let the reinforcingpass through and becontinuous (so the twopours structurallybehave like one pour )

The combination ofcontinuity of reinforcingand a vertical shear keyprevents lateralmovement of the secondpour relative to the first.


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Keep it drydampproofing Keeping the foundation wall dry is

accomplished in degrees.

The lowest level of protection is termeddampproofing

Dampproofing coatings are applied to theoutside face of the foundation.

Usually dampproof coatings do not have theability to bridge across cracks which developin the foundation

Dampproof coatings are carried down thefoundation wall and over the top of thefooting to minimize leakage through thefoundation/footing joint.

Dampproof coatings are usually brushapplied and can be cementitious (thoro seal)

or asphaltic

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Keep it drywater resistantcoatings Water resistant coatings are

the next step up in keeping thefoundation dry.

These coatings usually havethe ability to span cracks to amodest degree.

Water resistant coatings areusually applied with a trowel

and are often asphaltic orbituminous materials (blacksticky stuff)

Asphaltic and bituminouscoatings are UV sensitive,they need to be backfilled orprotected from the sun in lessthan 30 days or they begin tobecome brittle. (crack easily)

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Keep it drywaterproofcoatings Waterproof coatings offer the

highest level of water protectionfor the foundation wall.

Waterproofing systems fall intothree categories

Applied sheet membranes

Spray applied membranes

Attached board membranes

All offer superior waterproofingqualities when correctly applied.

All are fragile! When backfillingthey should be protected fromscuffing, scraping, bashing, orsimilar injury.

If the membrane is puncturedduring backfilling operations, thedamage wont be known until that

first rain.

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Keep it drywaterproofcoatings

Some popular membranes

Bituthene - a roll of heavypolyethylene backed with athick layer of sticky masticprotected by paper. With thepaper removed it sticks to thefoundation wall and spanscracks to keep water out.

Volclay panels - sheets ofcardboard filled with bentoniteclay mechanically fastened

(nail, screws) to thefoundation. In the presence ofwater, the clay expands to 15times dry volume forming animpervious clay layer outsidethe wall.

Spray applied modified bitumen- a spray on membrane,depending on thickness todevelop the ability to span.

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Keep it warminsulation Heat loss through the

foundation has been found tobe a significant source ofenergy consumption inresidential and smallcommercial buildings.

Rigid foam boards are themost common means ofinsulating the foundation wall.

Often the insulation is placedon the outside of thefoundation wall to keep themass warm

This also keeps the rocks,logs, disco boots, brokenbottles which may be part of

the backfill from poking a holein the waterproofing!

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Insulation flavors plain vanilla

Expanded polystyrene(beadboard or EPS) -available in

white

light grey

Open cells - Holds water(which reduces R value)

R value approx 3.5 per inch

Lowest price range


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Rainbow flavors

Extruded polystyrene(styrofoam) - available in

blue (dow)

Pink (Owens Corning)

Green (Amoco)

Yellow (Certainteed)

Closed cells - Wont holdwater

R value approx 5 per inch

Middle price range

Dirty yellow

Polyisocyanurate (polyiso) -available in

yellow

Closed cells - Wont holdwater

R value approx 10 per inch

highest price range

Keep it warminsulation Rigid board insulation comes in three

commonly used types

Expanded polystyrene R=3.5 per 1thickness (usually white, akabeadboard) holds water inside theboard between the beads, decreasingthermal effectiveness. Least expensive($ .61/sf)

Extruded polystyrene R= 5 per 1

thickness (usually blue, yellow, pink, limegreen) doesnt hold water inside,moderately expensive ($ .75/sf)

Polyisocyanurate R=7.5 per 1 thickness(usually a dirty yellow to orange color)can hold water, most expensive. ($.78/sf)

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The belt and suspendersdrainboard Additional insurance in the form of dry

foundation walls can be added byinstalling an air gap between the backfill

and the foundation wall assembly.

Making dirt conform to an air gap isdifficult, but a component called adrainboard makes it simple.

The drainboard is a plastic (often arecycled plastic) sheet which is vacuumformed into a surface of dimples andvalleys.

Attached to the top of the dimple is ageotextile fabric, also called a filterfabric. This fabric keeps large soilparticles from clogging the spacesbetween the dimples.

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Dropping waterdrainboard

A primary moisture moving mechanismin soil is capillary action. Water at ahigher vapor pressure moves betweensoil particles.

The drainboard allows the water throughthe filter, but then, unable to get acrossthe air gap, the water drops to thebottom of the drainboardand into awaiting drain tile.

DimplesGeotextile filter fabric

Getting rid of waterperimeter drain The drainboard is draped over another

recycled plastic product, the perimeteror footing drain.

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Getting rid of watersump pumps

This continuous flexible pipeis either sloped to run out ofthe excavation (run todaylight)

or is piped to a plastic bucket(sump)

which has a pump in it (sumppump)

connected to a pipe (ejectorpipe)

that either dumps the water inthe storm sewer or outside ongrade

.as long as the power is on,

the pump doesnt burn out,and the ejector pipe doesntfreeze up...

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