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Foundation Systems 1,spread foundations

What’s the point?

• The foundation’s primary purpose is:– Distribute building loads to the soil– Prevent frost uplift of the building (act as

frost 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 set

on 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.It’s 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 of

grade)

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Strip footings• Most common for low rise,

low load conditions.• A strip of reinforced concrete

ready 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 to

make 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 oroutside of footing

• Project lines down tobottom 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 process…2• Set formwork for

footing,– Set to square and

level with transit– double check

outside 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 process…2• 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 process…3• With the forms

stripped, thefoundation wallsmade of block orconcrete can beconstructed on thefooting

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

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Spread footing process…4• Anchorage for sill

plates isaccomplished byfilling block coreswith concrete and“J” 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

• It’s 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 concretewall…then 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 thatbind…and 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 formsdon’t 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 2x4’s (wales) areattached to the back of theplywood formwork to be thefirst bracing layer

• Then vertical 2x’s 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 joint thatwill 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 dry…dampproofing

• Keeping the foundation wall dry isaccomplished in degrees.

• The lowest level of protection is termed“dampproofing”

• 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 dry…water resistant coatings

• Water resistant coatings arethe 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 troweland are often asphaltic orbituminous materials (blacksticky stuff)– Asphaltic and bituminous

coatings 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 dry…waterproof coatings

• Waterproof coatings offer thehighest 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 won’t be known until thatfirst rain….

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Keep it dry…waterproofcoatings

• Some popular membranes…– Bituthene - a roll of heavy

polyethylene 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 warm…insulation

• Heat loss through thefoundation 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 ofthe 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 - Won’t holdwater

– “R” value approx 5 per inch– Middle price range

Dirty yellow• Polyisocyanurate (“polyiso”) -

available in– yellow

– Closed cells - Won’t holdwater

– “R” value approx 10 per inch– highest price range

Keep it warm…insulation

• Rigid board insulation comes in threecommonly used types– Expanded polystyrene R=3.5 per 1”

thickness (usually white, aka“beadboard”) 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) doesn’t 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 suspenders…drainboard

• Additional insurance in the form of dryfoundation walls can be added byinstalling an air gap between the backfilland 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 water…drainboard

• 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 drainboard…and into awaiting drain tile.

DimplesGeotextile filter fabric

Getting rid of water…perimeter drain

• The drainboard is draped over anotherrecycled plastic product, the perimeteror footing drain.

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Getting rid of water…sump 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 (sump

pump)– connected to a pipe (ejector

pipe)– that either dumps the water in

the storm sewer or outside ongrade

– ….as long as the power is on,the pump doesn’t burn out,and the ejector pipe doesn’tfreeze up...