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Building Foundations (Engeneering)

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This document downloaded from vulcanhammer.net since 1997, your source for engineering information for the deep foundation and marine construction industries, and the historical site for Vulcan Iron Works Inc. Use subject to the fine print to the right. All of the information, data and computer software ("information") presented on this web site is for general information only. While every effort will be made to insure its accuracy, this information should not be used or relied on for any specific application without independent, competent professional examination and verification of its accuracy, suitability and applicability by a licensed professional. Anyone making use of this information does so at his or her own risk and assumes any and all liability resulting from such use. The entire risk as to quality or usability of the information contained within is with the reader. In no event will this web page or webmaster be held liable, nor does this web page or its webmaster provide insurance against liability, for any damages including lost profits, lost savings or any other incidental or consequential damages arising from the use or inability to use the information contained within. This site is not an official site of Prentice-Hall, the University of Tennessee at Chattanooga, Vulcan Foundation Equipment or Vulcan Iron Works Inc. (Tennessee Corporation). All references to sources of equipment, parts, service or repairs do not constitute an endorsement. Don t forget to visit our companion site http://www.vulcanhammer.org ENCE 461 Foundation Analysis and Design Shallow Foundations General Considerations 1 Types of Shallow Foundations Shallow foundations are usually placed within a depth D beneath the ground surface less than the minimum width B of the foundation Shallow foundations consist of: Spread and continuous footings Square, Rectangular or Circular Footings Continuous footings Ring Foundations Wall footings Mats or Rafts 2 Footings A spread footing distributes column or other loads from the structure to the soil, where B < W < 10B A continuous footing is a spread footing where W > 10B. A wall footing is a long load bearing footing 3 Footings 4 Strap Footing 5 Mat Foundations A mat is continuous in two directions capable of supporting multiple columns, wall or floor loads. It has dimensions from 20 to 80 ft or more for houses and hundreds of feet for large structures such as multi-story hospitals and some warehouses Ribbed mats, consisting of stiffening beams placed below a flat slab are useful in unstable soils such as expansive, collapsible or soft materials where differential movements can be significant (exceeding 0.5 inch). 6 Dimension and Rubble Stone Footings Before 1800, most all footings were masonry, as shown Dimension stone footings Rubble stone footings Satisfactory for lighter structures, they were too heavy for the larger structures of the 19th century 7 Steel Grillage Footings Used first with the Montauk Block Building in Chicago (1882). First foundation type specifically designed for flexure. 8 Typical Concrete Footing 9 Methods of Construction of Concrete Footings Once form is made, before concrete is poured either anchor bolts or dowels are placed to enable connection of the foundation with the building. Formed footing 10 Mat Foundations 11 Conditions for Mat Foundations Structural loads require large area to spread the load Soil is erratic and prone to differential settlements Structural loads are erratic Unevenly distributed lateral loads Uplift loads are larger than spread footings can accommodate; weight of the mat is a factor here Mat foundations are easier to waterproof 12 Chase Tower, Houston (Built on Mat Foundation) Mat foundation is 3 metres thick and bottomed at 19.2 m below street level 13 Distribution of Bearing Pressure Distribution of bearing pressure depends on Eccentricity, if any, of applied load Magnitude of the applied moment, if any Structural rigidity of the foundation Stress-strain properties of the soil Roughness of the bottom of the foundation Spread footings are nearly rigid; effects of foundation/soil flexibility usually ignored Mat foundations are more flexible; flexibility an important factor 14 Bearing Pressure Distribution Concentric Loads Flexible foundation on clay Flexible Foundation on Sand Rigid foundation on clay Rigid Foundation on Sand Simplified Distribution 15 Bearing Pressure Distribution Concentric Loads Rigid foundation on clay Rigid Foundation on Sand Simplified Distribution 16 Computation of Bearing Pressure Bearing Pressure, Bottom of Shallow Foundation P W f q u D A q = bearing pressure P = vertical column load Wf = weght of foundation, including weight of soil above foundation, if any A = base area of foundation uD = pore water pressure at bottom of foundation 17 Pressure for Continuous Footings Load is usually expressed per unit length P Wf b b q u D B 18 Example of Bearing Pressure Calculation Given Find Foundation as shown, square footing Bearing Pressure Assumptions Unit weight of concrete = 150 pcf = 23.6 kN/m3 Assume concrete fills area above footing to surface 19 Bearing Pressure Example P W f q u D A Compute pore water pressure uD = (62.4)(4-3) = 62.4 psf Compute weight of foundation and soil Wf = (5)(5)(4)(150) = 15,000 lbs. q Compute Area 100 15 .0624 4.538 ksf 25 A = (5)(5) = 25 sq. ft. 20 Floating Foundations Type of mat foundation that relies partially or entirely on the weight of the soil/water combination it displaces to support the structure above it Although foundations can be made to float entirely, it many not be advisable due to heave or settlement due to changes in ground conditions Very useful for structures with hollow subterranean structures 21 Floating Foundation Example Given Foundation as shown, 50 m wide by 70 m long Sum of column and wall loads = 805 MN Find Average Bearing Pressure Increase in stress due to addition of foundation 22 Floating Foundation Example Compute weight of bottom mat Compute area of foundation Wf = (23.6)(50)(70)(1.8) = 148.68 MN A = (50)(70) = 3500 m2 Compute Average Bearing Pressure P W f uD q A u D 9.8 8.7 5 36.26 kPa 0.03625 MPa 805 148.68 q 0.03625 MPa 236.23 kPa 3500 23 Floating Foundation Example For increased pressure, use alternative method based strictly on buoyancy Compute weight of soil displaced by foundation Ws = (19)(50)(70)(8.7) = 578.55 MN Compute total structure load on foundation Wt = 148.68 MN + 805 MN = 953.68 MN 24 Floating Foundation Example Compute difference in weight displaced to structural load W = 953.68 MN - 578.55 MN = 375.13 MN Compute change in soil stress under mat 375.13MN/3500 m2 = 107.18 kPa 25 Floating Foundation Example Compute stress change using effective stress Compute effective stress at foundation depth before construction 'zD = H u = (19)(8.7) (9.8)(8.7-5) = 129.04 kPa Bearing Pressure after construction = 236.23 kPa Bearing Pressure Change = 236.23 129 = 107.19 kPa, so OK 26 Eccentric Loading Load is away from the centre of the foundation in the B direction only Non-continuous footings (actually e1) P b e1 e P b W f b Pe1 e P W f Continuous Footings 27 Moment Loading M b e P b W f b Load is away from the centre of the foundation in the B direction only Non-continuous footings M e P W f Continuous Footings 28 Variables for Eccentric and Moment Loading e = eccentricity of bearing pressure distribution P = applied vertical load P/b = applied vertical load per unit length of foundation M = applied moment load M/b = applied moment load per unit length of foundation e1 = eccentricity of applied vertical load Wf = weight of foundation Wf/b = weight of unit length of foundation 29 One-Way Loading One-way loading is loading along one of the centre axes of the foundation Three cases to consider e < B/6 e = B/6 e > B/6 30 Case 1: e < B/6 Maximum and Minimum Bearing Pressures P W f 6e q min u D 1 A B P W f 6e u D 1 q max A B 31 Case 2: e = B/6 Maximum and Minimum Bearing Pressures q min 0 P W f 6e u D 1 q max A B 32 Case 2: e > B/6 Since areas exist where pressure is less than zero, uplift will occur This case is not satisfactory and design must be altered so that e < B/6 33 Example of One Way Eccentricity Given Continuous foundation as shown Groundwater table at great depth Find Whether resultant force acts in middle third Minimum and maximum bearing pressures 34 Example of One Way Eccentricity Compute Weight of Foundation Wf/b = (5)(1.5)(150) = 1125 lb/ft Compute eccentricity M b 8000 0.61 ft. e P b W f b 12000 1125 B5 0.833 ft. 0.61 ft. 66 Thus, eccentricity is within the middle third of the foundation and foundation can be analysed further without enlargement at this point 35 Example of One Way Eccentricity Compute minimum and maximum bearing pressures P W f 6e q min u D 1 A B 6 0.61 12000 1125 0 1 703 psf q min 5 5 P W f 6e q max u D 1 A B 6 0.61 12000 1125 0 1 4546 psf q max 5 5 36 Two-Way Eccentricity Eccentricity in both L and B directions produces a planar distribution of stress 37 Kern of Stability Foundation stable against overturn only if resultant falls in the kern in the centre of the foundation Resultant in the kern if eB, eL = eccentricity in B, L directions 6e B 6e L 1 B L 38 Bearing Pressure at Corners Two-Way Eccentricity P W f 6e B 6e L q u D 1 a B L 39 Two-Way Eccentricity Example Given Grain silo design as shown Each silo has empty weight of 29 MN; can hold up to 110 MN of grain Weight of mat = 60 MN Silos can be loaded independently of each other 40 Two-Way Eccentricity Example Find Whether or not eccentricity will be met with the various loading conditions possible Eccentricity can be oneway or two-way 41 Two-Way Eccentricity Example One-Way Eccentricity Largest Loading: two adjacent silos full and the rest empty P = (4)(29) + 2(110) = 336 MN M = (2)(110)(12) = 2640 MN-m M P W f 2640 e 336 60 e 6.67 m e B 50 8.33 m 6.67 m 66 Eccentricity OK for one-way eccentricity 42 Two-Way Eccentricity Example Two-Way Eccentricity Largest Loading: one silo full and the rest empty P = (4)(29) + 110 = 226 MN MB = ML = (110)(12) = 1320 MN-m 1320 M 4.62m e B e L P W f 226 60 6e B 6e L 6 4.62 1.11 1 2 B L 50 Not acceptable 43 Two-Way Eccentricity Example Two-Way Eccentricity Solution to Eccentricity Problem: increase the size of the mat 6e B 6e L 6 4.62 2 1 B L B B L 55.4 m Necessary to also take other considerations into account (bearing failure, settlement, etc.) 44 Questions? 45

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