DESIGN OF SHALLOW FOUNDATIONS - FALMATASABA
Please purchase PDFcamp Printer on http://www.verypdf.com/ to remove this watermark.Foundation Engineering-IDesign of Shallow FoundationsCHAPTER FourDESIGN OF SHALLOW FOUNDATIONSTable of contentsPage No.4.1. General . . . 554.1.1. General Requirements of Foundations 554.1.2. Foundation Classifications . . . 574.1.3. Shear in Footings . . 654.1.4. Moment in Footings . 654.2. Elements of Reinforced Concrete Design . . . .664.2.1. General . . 664.2.2. Design Methods . . 674.2.3. Limit State Principles . . 684.3. The General procedure for the design of concentrically Load Isolated footing.774.4. Eccentrically Loaded Spread Footings . . 784.5. Combined Footings . . 814.6. Trapezoid Shaped Footings . . 844.7. Strap footings . 86- 54 -
Please purchase PDFcamp Printer on http://www.verypdf.com/ to remove this watermark.Foundation Engineering-I4.1Design of Shallow FoundationsGeneral4.1.1. General Requirements of FoundationsFor a satisfactory performance, a foundation must satisfy the following three basiccriteria:1. Location and depth criterion.2. Shear failure criterion or bearing capacity.3. Settlement.Location and Depth Criterion:As a general rule, any foundation should be placed at a depth where the soil stratum isadequate from the point of view of bearing capacity and settlement criteria.Minimum Requirements:FA foundation should be located at a minimum depth of 50cm below naturalground surface.FThe foundation must be placed below the zone of volume change, wherevolume change is expected. For example, in areas where there is expansivesoil the foundation should be taken below the active zone.FFoundations for structures in a river have to be protected from the scouringaction of the flowing-stream. The depth of foundation for a bridge pier or anysimilar structure must be sufficiently below the deepest scour level.Foundations Near Existing Structures:When footings are to be placed adjacent to existing structure, as indicated in figure4.1.1, the line from the base of the new footing to the bottom edge of the existingfooting should be 45?Or less with the horizontal plane. The distance m should be greaterthan Zf (fig.4.1.1(a)).Conversely, Fig.4.1.1 (b) indicates that if the new footing is lower than the existingfooting, there is a possibility that the soil may flow laterally from beneath the existingfooting.This may increase the amount of excavation somewhat but, more importantly, mayresult in settlement cracks in the existing building. This problem is difficult to analyze;however, an approximation of the safe depth Zf may be made for C-? soil.- 55 -
Please purchase PDFcamp Printer on http://www.verypdf.com/ to remove this watermark.Foundation Engineering-IDesign of Shallow FoundationsNote:FThe vertical pressure s 1 would include the pressure from the existing footing.FThe K in these equation is a lateral pressure coefficient of Ka K Kp.FIf the soil is sand (does not have cohesion) one cannot excavate to a depthgreater than that of the existing foundation.Figure 4.1.2 Illustrate how a problem can develop if the excavation for the foundation ofthe new structure is too close to the existing building. In this case the qNq term of thebearing capacity equation is lost, for most foundations below the ground surface this is amajor component of the bearing capacity.Fig. 4.1.1(a)Fig.4.1.1(b)- 56 -
Please purchase PDFcamp Printer on http://www.verypdf.com/ to remove this watermark.Foundation Engineering-IDesign of Shallow FoundationsFig.4.1.24.1.2. Foundation Classifications:Foundations may be classified based on where the load is carried by the ground,according to Terzaghi: Shallow foundations: termed bases, footings, spread footings,or mats. The depth is generallyD B 1 but may be somewhat more (fig. 4.1.3a)Deep foundations: piles, drilled piers, or drilled caissons.Lp B 4With a pileillustrated in figure 4.1.3bAs the column type members, transferring the superstructure load to the foundation soil,have higher strength than the soil the foundation will spread the load in a manner suchthat the soil limiting strength is not exceeded and the resulting deformations aretolerable.Figure 4.1.3 Definition of select terms used in foundation engineering- 57 -
Please purchase PDFcamp Printer on http://www.verypdf.com/ to remove this watermark.Foundation Engineering-IDesign of Shallow FoundationsThe choice of the appropriate type of foundation is affected by:FType of superstructure to be supported: function and load that it transfers tothe foundation.FSubsurface condition and/or type of soil.FCost of foundation.A safe foundation design provides for a suitable factor against, Shear failure of the soil: by not exceeding the bearing capacity of the soil. Excessive settlement (both uniform and differential settlement)A) Shallow Foundations:Shallow/spread footings are the most widely used type among all foundations becausethey are usually more economical. Construction of footings requires a least amount ofequipment and skill and no heavy or special equipment is necessary.Shallow foundations are usually used when the soil at a shallow depth has adequatecapacity to support the load of the superstructure. For reasons of economy, shallowfoundations are the first choice unless they are considered inadequate.1. Wall or Continuous Footings:As the name implies, a wall footing supports a load-bearing wall. Continuous footings arethose footings that carry a series of closely spaced column loads along a row. The width,B, of such foundations is much less than their length (L).2. Footings:Footings belong to shallow foundation and their purpose is to transmit the load from thestructure to soil or rock. Included in the category of footings are those that support asingle column, referred to as: Isolated or spread footings: can be a square,rectangular or circular in shape depending on the relative magnitude of the moments Mxand My from the superstructure (square or rectangular footings), and the shape of thesuperstructure to be supported.Footings that support two or more columns are classified as combined footings.Rectangular, Trapezoidal and strap combined footings are special versions of combinedfootings, patterned to meet certain conditions or restrictions; so are mat foundations.- 58 -
Please purchase PDFcamp Printer on http://www.verypdf.com/ to remove this watermark.Foundation Engineering-IDesign of Shallow FoundationsPile caps are special footings needed to transmit the column load to a group or cluster ofpiles.Figure 4.1.4Typical configurations for various types of footings- 59 -
Please purchase PDFcamp Printer on http://www.verypdf.com/ to remove this watermark.Foundation Engineering-IDesign of Shallow FoundationsConcrete is almost always the material used in footings. It is strong, durable, and is eandadaptabletofieldconstruction and requirements. Concrete footings may be plain or reinforced, withreinforcement running in one (one way) or two (two way) directions, depending on thedirection of flexure.Footing shapes usually vary with specific requirements and design needs. For spread orisolated footings, square shapes are common and usually most economical, butrectangular shapes are used if space is limited in one direction, or when loads areeccentric in one direction. The typically desired objective is to select the footing shapethat makes the soil pressure (bearing pressure) as uniform as possible. Furthermore,footings may be of uniform thickness or may be sloped or stepped. Stepped or slopedfootings are most commonly used to reduce the quantity of concrete away from thecolumn where the bending moments are small and when the footing is not reinforced.Figure 4.1.5 Typical footings. (a) Single or spread footings; (b) stepped footing; (c) sloped footing;(d) Wall footing; (e) footing with pedestal.A steel base plate is used to spread the load from the steel column to the concrete, andthus ensure against crushing of the concrete. Pedestals are short concrete columns usedto interface steel columns with spread or wall footings that are located at the depth in- 60 -
Please purchase PDFcamp Printer on http://www.verypdf.com/ to remove this watermark.Foundation Engineering-IDesign of Shallow Foundationsthe ground. This prevents possible corrosion of steel through direct contact with the soil(fig. 4.1.5e).Spread footings are designed to satisfy a combination of flexure, shear, and bearing. Thesafety factor for allowable bearing capacity ranges from 2 to 5 for cohesionless materialsdepending on density, effects of failure and consultant caution. The value may rangefrom 3 to 6 for cohesive materials, with the higher values used where consolidationsettlements might occur over a long period of time.The pressure distribution under a footing depends, among other things, on footingrigidity, shape, footing depth, and soil properties. Generally, for ordinary spread footingsresting on cohesionless formations, the pressure distribution is as shown in fig. 4.1.4a.For combined and larger footings, the distribution may vary toward a more uniformshape near the middle two-thirds of the footing. On the other hand, for cohesive soils,the distribution appears to be opposite that for cohesionless; for this condition, theshape may approach that shown in fig. 4.1.4b. It is seldom that the engineer deals witha soil stratum that is totally cohesive or totally cohesionless; the more likely case is amixture of cohesive and cohesionless material. Thus, it is a widely accepted practice toassume a uniform pressure distribution rather than a variable one.Figure 4.1.6 Probable pressure distribution beneath a rigid footing. (a) On a cohesionless soil;(b) Generally for cohesive soils; (c) usual assumed linear distribution.- 61 -
Please purchase PDFcamp Printer on http://www.verypdf.com/ to remove this watermark.Foundation Engineering-IDesign of Shallow FoundationsCombined footings are used when:a) Columns are closely spaced and design/proportioning of isolated footings resultsin an overlap of footing areas and/or,b) When there is a property line/boundary line/restriction and there exists a columnalong the boundary line and use of isolated footing is not possible.Rectangular Combined footings are used:FWhen case(a) is encountered and the spacing between the columns is lessthan 6m-7m and/or,FWhen case (b) is encountered and the outer column, which is the onealong the boundary line, carries a larger load as compared to the innercolumn (the one to be combined with the outer column).Trapezoidal combined footings: are used when case (b) is encountered and the innercolumn carries a larger load as compared to the load carried by the column along theboundary line.Strap Footings:Strap (Cantilever) footings may be used in lieu of a combined rectangular or trapezoidalfooting when either case (a) or case (b) is encountered and the spacing between thecolumns is large (say greater than 6m-7m, for in this range it may be economical) and/or the allowable soil pressure is relatively large so that the additional footing area is notneeded. A strap footing should be considered only after a carefully analysis shows thatrectangular or trapezoidal combined footings- even if oversize- will not work. The extralabor and forming costs for this type of footing make it one use as a last resort. Thestrap serves the same purpose as the interior portion of a combined rectangular andtrapezoidal footing but much narrower to save materials.Figure: Sample problem on strap footings.- 62 -
Please purchase PDFcamp Printer on http://www.verypdf.com/ to remove this watermark.Foundation Engineering-IDesign of Shallow Foundations3. Mat Foundations:On grounds with very low bearing capacity or where excessive variations in groundconditions would lead to unacceptable differential settlement, mat foundations are usedas an alternative to the other types of shallow foundations.In the majority of the cases, mat foundations are used where the subsoil has lowbearing capacity. By combining all individual footings into one big mat, not only the unitpressures on the subsoil are reduced but also the bearing capacity is often increased(since bearing capacity increases with increasing width of foundation).A mat foundation may be used where the base soil has a low bearing capacity and/or thecolumn loads are so large that more than 50% of the area is covered by theconventional spread footings. It is common to use mat foundations for deep basementsboth to spread the column loads to a more uniform pressure distribution and to providethe floor slab for the basement. A particular advantage for basements at or below theGWT is to provide a water barrier. Depending on local costs, and noting that a matfoundation requires both positive and negative reinforcing steel, one may find it moreeconomical to use spread footings-even if the entire area is covered. Spread footingsavoid the use of negative reinforcing steel.Mat foundations may be supported by piles in the situations such as high ground water(to control buoyancy) or where the base soil is susceptible to large settlements.Note that the mat contact stress will penetrate the ground to a greater depth or havegreater relative intensity at a shallower depth. Both factors tend to increase settlementsunless there is stress compensation from excavated soil so that the net increase inpressure is controlled. In localities where the subsoil is very compressible and extends toa great depth, the so-called compensated design is used to the best advantage. In thisdesign, a deeper basement is made so that the net pressure (the total building loadminus the weight of the soil replaced by basement) at any depth in the subsoil isnegligible.Figure: sample problem on mat foundation- 63 -
Please purchase PDFcamp Printer on http://www.verypdf.com/ to remove this watermark.Foundation Engineering-IDesign of Shallow FoundationsB) Deep Foundations:The most common used deep foundations are pile foundations. The following listidentifies some of the conditions that require pile foundations:FWhen the upper soil layer(s) is (are) highly compressible and too weak tosupport the load transmitted by the superstructure, piles are used totransmitted the load the underlying bedrock or a stronger soil layer. Whenbedrock is not encountered at a reasonable depth below the ground surface,piles are used to transmit the structural load to the soil gradually. Theresistance to the applied structural load is derived mainly from the frictionalresistance developed at the soil-pile interface.FWhen subjected to horizontal forces, pile foundations resist by bending whilestill supporting the vertical load transmitted by the superstructure. This typeof situation is generally encountered in the design and construction of theearth retaining structures and foundations of tall structures that are subjectedto high wind and/or earthquake forces.FIn many cases, expansive soils may be present at the site of proposedstructure; these soils may extend to a great depth below the ground surface.Expansive soils swell and shrink as the moisture content increases anddecreases, and swelling pressure of such soils can be considerable damage.However, pile foundations may be considered as an alternative when piles areextended beyond the active zone, which swells and shrinks.FFoundations of some structures, such as transmission towers, offshoreplatforms, and basement mats below the water table, are subjected touplifting forces. Piles are sometimes used for these foundations to resist theuplifting force.FBridge abutments and piers are usually constructed over pile-foundations toavoid the possible loss of bearing capacity that a shallow foundation mightsuffer because of soil erosion at the ground surface.- 64 -
Please purchase PDFcamp Printer on http://www.verypdf.com/ to remove this watermark.Foundation Engineering-IDesign of Shallow Foundations4.1.3. Shear in Footings: Design of footings for shear shall be in accordance with provisions for slabs. The location of the critical section for shear shall be measured from face of column,pedestal or wall for footings supporting a column, pedestal, or wall. For footings supporting a column or pedestal with steel base plates, the criticalsection shall be measured from halfway between face of column and edge of steelbase.4.1.4. Moment in Footings:The critical section for moment shall be taken as follows:(a)At the face of column, pedestal, or wall, for footings supporting a concretecolumn pedestal or wall.(b) Halfway between middle and edge of wall, for footing supporting a masonry wall.(c)Halfway between face of column and edge of steel base for footings supporting acolumn with steel base plates.Figure 4.1.7 Critical footing sectionsfor moment, and shear.- 65 -
Please purchase PDFcamp Printer on http://www.verypdf.com/ to remove this watermark.Foundation Engineering-IDesign of Shallow Foundations4.2. Elements of Reinforced Concrete Design4.2.1. General:The design of a structure for a specific function is usually a two-stage process, involvingfirst the selection of an appropriate type or form of structure and secondly the detaileddesign of the various parts of the chosen structure. In selecting the type or form ofstructure the question of the relative costs of different types of structures and ofdifferent methods of construction of the same structure will be of great importance. Inthis selection the designer must rely to a large extent on his experience judgment andintuition. A preliminary study of several types of structure may be necessary.Having selected the type of structure the designer then has to proceed with the detaileddesign of the chosen one, always bearing in mind the factors of safety considerationsand cost. In most cases the aesthetic requirements will have been substantially met inthe selection of the type of structure and will now be completely satisfied by thespecification of surface finishes, colour, etc.Fundamentally, then, the design processconsists of finding and detailing the most economical structure consistent with the safetyand serviceability requirements.In design the following points have to be taken into consideration:(i). variations in materials in the structure and in test specimens(ii). variations in loading(iii). constructional inaccuracies(iv). accuracy of design calculations(v). safety and serviceabilityFor (i) we know that the cube test is a reliable guide as regards quality of concrete fromthe mixer but does not guarantee that the concrete in the structure is the same. This iswhy we took a higher proportion of the cube strength as a permissible stress when wehave quality control. i.e. a design mix. The same applies to reinforcement, as tests arecarried out on small samples which may or may not be truly representative of the whole.For (ii) we must enquire how true the loading is. Constructional inaccuracies (iii) areprobably accidental. For (iv) designers can and do make mistakes in calculations butvery often in analysis they assume a structure will behave in a certain way or thatcertain conditions exist. Item (v) is dealt with quite arbitrarily in previous codes -if thestructure does not collapse it is deemed to be satisfactory.- 66 -
Please purchase PDFcamp Printer on http://www.verypdf.com/ to remove this watermark.Foundation Engineering-I4.2.2Design of Shallow FoundationsDesign Methods:Based on design load determination and the corresponding design strength of materials,different methods of design have been introduced.§ Permissible stress method: The ultimate st
Foundations may be classified based on where the load is carried by the ground, according to Terzaghi: Shallow foundations: termed bases, footings, spread footings, or mats. The depth is generally D B 1 but may be somewhat more (fig. 4.1.3a) Deep foundations: piles, drilled piers, or drilled caissons. Lp B 4 With a pile
ASCE 32-01, “Design and Construction of Frost-Protected Shallow Foundations”, contains several different, code approved, methods to design shallow foundations of various types. The reference booklet you downloaded for this course, the HUD “Revised Builder’s Guide to Frost Protected Shallow Foundations” contains design methods for the most
–ASCE 32‐01 20 What is a Frost Protected Shallow Foundation? American Society of Civil Engineers. 2001. Design and Construction of Frost‐Protected Shallow Foundations (32‐01). Reston, VA: American Society of Civil Engineers 21 FPSF Impact on Frost Depth 22 FPSFs in the 2009 IRC ‐Figure R403.3(1)
thickened (turned-down) edge slabs. The type of shallow foundation to be used will be based on the structure to be supported. The BDM includes the use of pile/drilled shaft supported footings; however, since the footing (shallow foundation) is supported by deep foundations see Chapter 16 for the design and analysis of the deep foundation.
does not offer dynamic well control methods of managing shallow hazards such as methane hydrates, shallow gas and shallow water flows. These negative aspects of "Pump and Dump" are in addition to the environmenta l impact, high drilling fluid (mud) . 3.2.2 Well Control "Modified Driller's Method" . .38 3.3 Dual Gradient .
compared with a conventional foundation. Briefly, a frost-protected shallow foundation relies on insulation strategically placed around the foundation to raise the frost depth around a building, thereby allowing foundation depths as shallow as 16 inches (0.4 meters) in the most severe U.S. climates.File Size: 1MBPage Count: 31
4.4 Frost Protection for Foundations Shallow foundations in section 4.2 are considered to be frost protected when placed at su cient depth to prevent supporting soils from freezing. Foundations in the perimeter of heated buildings where snow is not cleared are considered frost protected at 1.5 m depth (as having a soil cover of 1.5 m). Foundations
We acknowledge the American Society of Civil Engineers’ (ASCE) for permission to publish values that are contained in Tables 4 and A1 in SEI/ASCE 32-01, Design and Construction of Frost-Protected Shallow Foundations, 2001, authored and published by the ASCE.
The facts and extensive procedural history of Albert Woodfox’s case have been recounted time and again, but they bear repeatingsince they factored into theunconditional writ granted by the district court On April 17, 1972, . Correctional Officer Brent Millerof the Louisiana State Penitentiary in , Angola, Louisiana, was found murderedin the prison dormitory , havingbeen stabbed 32 times. The .
