A10 Foundations RA1010-120 Spread Footings · 2020. 9. 11. · 452 A10 Foundations RA1010-120...

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A10 Foundations RA1010-120 Spread Footings General: A spread footing is used to convert a concentrated load (from one superstructure column or substructure grade beams) into an allowable area load on supporting soil. Because of punching action from the column load, a spread footing is usually thicker than strip footings which support wall loads. One or two story commercial or residential buildings should have no less than 1thick spread footings. Heavier loads require no less than 2thick. Spread footings may be square, rectangular or octagonal in plan. Spread footings tend to minimize excavation and foundation materials, as well as labor and equipment. Another advantage is that footings and soil conditions can be readily examined. They are the most widely used type of footing, especially in mild climates and for buildings of four stories or under. This is because they are usually more economical than other types, if suitable soil and site conditions exist. They are used when suitable supporting soil is located within several feet of the surface or line of subsurface excavation. Suitable soil types include sands and gravels, gravels with a small amount of clay or silt, hardpan, chalk, and rock. Pedestals may be used to bring the column base load down to the top of the footing. Alternately, undesirable soil between the underside of the footing and the top of the bearing level can be removed and replaced with lean concrete mix or compacted granular material. Depth of footing should be below topsoil, uncompacted fill, muck, etc. It must be lower than frost penetration (see local code or Table L1030-502 in Section L) but should be above the water table. It must not be at the ground surface because of potential surface erosion. If the ground slopes, approximately three horizontal feet of edge protection must remain. Differential footing elevations may overlap soil stresses or cause excavation problems if clear spacing between footings is less than the difference in depth. Other footing types are usually used for the following reasons: A. Bearing capacity of soil is low. B. Very large footings are required, at a cost disadvantage. C. Soil under footing (shallow or deep) is very compressible, with probability of causing excessive or differential settlement. D. Good bearing soil is deep. E. Potential for scour action exists. F. Varying subsoil conditions within building perimeter. Cost of spread footings for a building is determined by: 1. The soil bearing capacity. 2. Typical bay size. 3. Total load (live plus dead) per S.F. for roof and elevated floor levels. 4. The size and shape of the building. 5. Footing configuration. Does the building utilize outer spread footings or are there continuous perimeter footings only or a combination of spread footings plus continuous footings? COST DETERMINATION 1. Determine Soil Bearing Capacity by a known value or by using Table A1010-121 as a guide. Table A1010-121 Soil Bearing Capacity in Kips per S.F. Typical Allowable Bearing Material Bearing Capacity Hard sound rock 120 KSF Medium hard rock 80 Hardpan overlaying rock 24 Compact gravel and boulder-gravel; very compact sandy gravel 20 Soft rock 16 Loose gravel; sandy gravel; compact sand; very compact sand-inorganic silt 12 Hard dry consolidated clay 10 Loose coarse to medium sand; medium compact fine sand 8 Compact sand-clay 6 Loose fine sand; medium compact sand-inorganic silts 4 Firm or stiff clay 3 Loose saturated sand-clay; medium soft clay 2

Transcript of A10 Foundations RA1010-120 Spread Footings · 2020. 9. 11. · 452 A10 Foundations RA1010-120...

Page 1: A10 Foundations RA1010-120 Spread Footings · 2020. 9. 11. · 452 A10 Foundations RA1010-120 Spread Footings General: A spread footing is used to convert a concentrated load (from

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A10 Foundations RA1010-120 Spread FootingsGeneral: A spread footing is used to convert a concentrated load (from onesuperstructure column or substructure grade beams) into an allowablearea load on supporting soil.Because of punching action from the column load, a spread footing is usuallythicker than strip footings which support wall loads. One or two storycommercial or residential buildings should have no less than 1′ thick spreadfootings. Heavier loads require no less than 2′ thick. Spread footings maybe square, rectangular or octagonal in plan.Spread footings tend to minimize excavation and foundation materials, aswell as labor and equipment. Another advantage is that footings and soilconditions can be readily examined. They are the most widely used typeof footing, especially in mild climates and for buildings of four stories orunder. This is because they are usually more economical than other types,if suitable soil and site conditions exist.They are used when suitable supporting soil is located within several feetof the surface or line of subsurface excavation. Suitable soil types includesands and gravels, gravels with a small amount of clay or silt, hardpan, chalk,and rock. Pedestals may be used to bring the column base load down tothe top of the footing. Alternately, undesirable soil between the underside ofthe footing and the top of the bearing level can be removed and replacedwith lean concrete mix or compacted granular material.Depth of footing should be below topsoil, uncompacted fill, muck, etc. Itmust be lower than frost penetration (see local code or Table L1030-502

in Section L) but should be above the water table. It must not be at theground surface because of potential surface erosion. If the ground slopes,approximately three horizontal feet of edge protection must remain.Differential footing elevations may overlap soil stresses or cause excavationproblems if clear spacing between footings is less than the differencein depth.Other footing types are usually used for the following reasons:

A. Bearing capacity of soil is low.B. Very large footings are required, at a cost disadvantage.C. Soil under footing (shallow or deep) is very compressible, with

probability of causing excessive or differential settlement.D. Good bearing soil is deep.E. Potential for scour action exists.F. Varying subsoil conditions within building perimeter.

Cost of spread footings for a building is determined by:1. The soil bearing capacity.2. Typical bay size.3. Total load (live plus dead) per S.F. for roof and elevated floor levels.4. The size and shape of the building.5. Footing configuration. Does the building utilize outer spread footings or

are there continuous perimeter footings only or a combination of spreadfootings plus continuous footings?

COST DETERMINATION1. Determine Soil Bearing Capacity by a known value or by using Table A1010-121 as a guide.

Table A1010-121 Soil Bearing Capacity in Kips per S.F.Typical Allowable

Bearing Material Bearing CapacityHard sound rock 120 KSFMedium hard rock 80Hardpan overlaying rock 24Compact gravel and boulder-gravel; very compact sandy gravel 20

Soft rock 16Loose gravel; sandy gravel; compact sand; very compact sand-inorganic silt 12Hard dry consolidated clay 10Loose coarse to medium sand; medium compact fine sand 8

Compact sand-clay 6Loose fine sand; medium compact sand-inorganic silts 4Firm or stiff clay 3Loose saturated sand-clay; medium soft clay 2

ReferenceTables

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A10 Foundations RA1010-120 Spread FootingsTable A1010-122 Working Load Determination2. Determine Bay Size in S.F.

3. Determine Load to the Footing

Working LoadsType Load Reference Roof Floor Column Total

1. Total Load Floor System PSF PSF2. Whole Bay Load/Level (Line 1) x Bay Area � 1000 Kips Kips3. Load to Column [ ( Roof + (Floor x No. Floors ) ] Kips Kips Kips4. Column Weight Systems B1010 201 to B1010 208 Kips Kips5. Fireproofing System B1010 720 Kips Kips6. Total Load to Footing Roof + Floor + Column Kips

Working loads represent the highest actual load a structural member is designed to support.

4. Determine Size and Shape of Building

Figure A1010-123

5. Footing Configuration Possibilitiesa. Building has spread footings throughout (including

exterior and corners).b. Building has continuous perimeter footings to support

exterior columns, corners and exterior walls (as well asinterior spread footings).

Figure A1010-124

c. Building has both perimeter continuous footings for wallsplus exterior spread footings for column loads. If thereare interior spread footings also, use “b.”

1. Figure continuous footings from tables in Section A1010-110.2. Add total dollars for spread footings and continuous footings to

determine total foundation dollars.

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A10 Foundations RA1010-120 Spread FootingsTable A1010-126 Working Load DeterminationTabulate interior footing costs and add continuous footing costs from Systems A1010 210 and A1010 110.

Example: Spread Footing Cost DeterminationOffice Building . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 StoryStory Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12′Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75′ x 120′Bay Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20′ x 25′Soil Bearing Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 KSFFoundation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spread FootingsSlab on Grade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5″ ReinforcedCode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IBCConstruction . . . . . . . . . . . . . . . . . . . . . . . . From Example RB1010-112

Using concrete flat plate

Determine load to footings Table A1010-122 or Example RB1010-112.Working Loads

Type Load Reference Roof Floor Column TotalDetermine floor system System in Div. B1010 (Example: Flat Plate)1. Total Load System # B1010 223 146PSF 188PSF2. Whole Bay Load/Level Bay Area x Line 1/1000 73K 94K3. Load To Column Roof + (Floor x No. Floors) 73K 276K 449K4. Column Weight Systems B1010 201 to B1010 208 25K 25K5. Fireproofing N/A —6. Load to Footing Roof + Floor + Column 474K

1. Total load working from System B1010 223: 40# superimposed load forroof and 75# superimposed load for floors.

2. Bay Area 20′ x 25′ = 500 S.F. x Line 1500 x 146/1000 = 73K roof500 x 188/1000 = 94K floor

3. Roof + (floor x no. floors)73K + (4 x 94K) = 449K

4. Enter System B1010 203 with a total working load of 474K and 12′ storyheight. A 16″ square tied column with 6 KSI concrete is most economical.Minimum column size from System B1010 223 is 20″. Use the larger.Column weight = 394 #/V.L.F. x 12′ = approx. 5K.

5. No fireproofing is required for concrete column.6. Add roof, floors and column loads for a total load to each interior footing.

Footing Cost: Enter System A1010 210 with total working load andallowable soil pressure. Determine the cost per footing using

the closest higher load on the table. Add the total costs for interior, exterior,and corner footings.

Figure A1010-125

ReferenceTables

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A10 Foundations RA1010-120 Spread FootingsTable A1010-127 Typical Spread Footing, Square

EXTERIOR FOOTINGSExterior vs. Interior FootingsSystem A1010 210 contains the cost for individual spread interior footingsfor various loading and soil bearing conditions. To determine the loadsfor exterior footings:1. Multiply whole bay working load by 60% for half bay footings and by

45% for quarter bay (or corner) footings. This will give the approximatecolumn load for half and quarter bay footings.

2. Enter System A1010 210 and find cost of footing with loads determinedin step 1.

3. Determine the number of each type of footing on the job and multiplythe cost per footing by the appropriate quantity. This will give the totaldollar cost for exterior and interior spread footings.

Table A1010-128 Spread Footing CostFooting Type Working Load (Kips) Total Quantity System A1010 210 Unit Cost Total

Interior 474 10 $2,975 $29,750Exterior 284 14 1,665 23,310Corner 213 4 1,665 6,660Total $59,720

Example: Using soil capacity, 6KSFTotal working load from Table A1010-126 . . . . . . . . . . . . . = 474KHalf Bay Footings . . . . . . . . . . . . . . . . . . . . . 474K x 60% = 284KCorner Footings . . . . . . . . . . . . . . . . . . . . . . 474K x 45% = 213K

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