Foundation Analysis And Design

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5Foundation Analysis and DesignMichael Valley, S.E.Contents5.1SHALLOW FOUNDATIONS FOR A SEVEN-STORY OFFICE BUILDING, LOSANGELES, CALIFORNIA. 35.1.1Basic Information . 35.1.2Design for Gravity Loads . 85.1.3Design for Moment-Resisting Frame System . 115.1.4Design for Concentrically Braced Frame System . 165.1.5Cost Comparison . 245.2DEEP FOUNDATIONS FOR A 12-STORY BUILDING, SEISMIC DESIGN CATEGORY D. 255.2.1Basic Information . 255.2.2Pile Analysis, Design and Detailing . 335.2.3Other Considerations . 47

FEMA P-751, NEHRP Recommended Provisions: Design ExamplesThis chapter illustrates application of the 2009 Edition of the NEHRP Recommended Provisions to thedesign of foundation elements. Example 5.1 completes the analysis and design of shallow foundations fortwo of the alternative framing arrangements considered for the building featured in Example 6.2.Example 5.2 illustrates the analysis and design of deep foundations for a building similar to the onehighlighted in Chapter 7 of this volume of design examples. In both cases, only those portions of thedesigns necessary to illustrate specific points are included.The force-displacement response of soil to loading is highly nonlinear and strongly time dependent.Control of settlement is generally the most important aspect of soil response to gravity loads. However,the strength of the soil may control foundation design where large amplitude transient loads, such as thoseoccurring during an earthquake, are anticipated.Foundation elements are most commonly constructed of reinforced concrete. As compared to design ofconcrete elements that form the superstructure of a building, additional consideration must be given toconcrete foundation elements due to permanent exposure to potentially deleterious materials, less preciseconstruction tolerances and even the possibility of unintentional mixing with soil.Although the application of advanced analysis techniques to foundation design is becoming increasinglycommon (and is illustrated in this chapter), analysis should not be the primary focus of foundation design.Good foundation design for seismic resistance requires familiarity with basic soil behavior and commongeotechnical parameters, the ability to proportion concrete elements correctly, an understanding of howsuch elements should be detailed to produce ductile response and careful attention to practicalconsiderations of construction.In addition to the Standard and the Provisions and Commentary, the following documents are eitherreferenced directly or provide useful information for the analysis and design of foundations for seismicresistance:ACI 318American Concrete Institute. 2008. Building Code Requirements andCommentary for Structural Concrete.BowlesBowles, J. E. 1988. Foundation Analysis and Design. McGraw-Hill.CRSIConcrete Reinforcing Steel Institute. 2008. CRSI Design Handbook. ConcreteReinforcing Steel Institute.ASCE 41ASCE. 2006. Seismic Rehabilitation of Existing Buildings.KramerKramer, S. L. 1996. Geotechnical Earthquake Engineering. Prentice Hall.LPILEReese, L. C. and S. T. Wang. 2009. Technical Manual for LPILE Plus 5.0 forWindows. Ensoft.Rollins et al. (a)Rollins, K. M., Olsen, R. J., Egbert, J. J., Jensen, D. H., Olsen, K. G.and Garrett,B. H. (2006). “Pile Spacing Effects on Lateral Pile Group Behavior: Load Tests.”Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 132,No. 10, p. 1262-1271.Rollins et al. (b)Rollins, K. M., Olsen, K. G., Jensen, D. H, Garrett, B. H., Olsen, R. J.and Egbert,J. J. (2006). “Pile Spacing Effects on Lateral Pile Group Behavior: Analysis.”5-2

Chapter 5: Foundation Analysis and DesignJournal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 132,No. 10, p. 1272-1283.Wang & SalmonWang, C.-K. and C. G. Salmon. 1992. Reinforced Concrete Design .HarperCollins.Several commercially available programs were used to perform the calculations described in this chapter.SAP2000 is used to determine the shears and moments in a concrete mat foundation; LPILE, in theanalysis of laterally loaded single piles; and spColumn, to determine concrete pile section capacities.5.1SHALLOWFOUNDATIONSFORASEVEN- ‐STORYOFFICEBUILDING,LOSANGELES,CALIFORNIAThis example features the analysis and design of shallow foundations for two of the three framingarrangements for the seven-story steel office building described in Section 6.2 of this volume of designexamples. Refer to that example for more detailed building information and for the design of thesuperstructure.5.1.1BasicInformation5.1.1.1 Description. The framing plan in Figure 5.1-1 shows the gravity load-resisting system for arepresentative level of the building. The site soils, consisting of medium dense sands, are suitable forshallow foundations. Table 5.1-1 shows the design parameters provided by a geotechnical consultant.Note the distinction made between bearing pressure and bearing capacity. If the long-term, service-levelloads applied to foundations do not exceed the noted bearing pressure, differential and total settlementsare expected to be within acceptable limits. Settlements are more pronounced where large areas areloaded, so the bearing pressure limits are a function of the size of the loaded area. The values identifiedas bearing capacity are related to gross failure of the soil mass in the vicinity of loading. Where loads areapplied over smaller areas, punching into the soil is more likely.5-3

FEMA P-751, NEHRP Recommended Provisions: Design 2"177'-4"1'-2"NFigure 5.1-1 Typical framing planBecause bearing capacities are generally expressed as a function of the minimum dimension of the loadedarea and are applied as limits on the maximum pressure, foundations with significantly non-square loadedareas (tending toward strip footings) and those with significant differences between average pressure andmaximum pressure (as for eccentrically loaded footings) have higher calculated bearing capacities. Therecommended values are consistent with these expectations.Table 5.1-1 Geotechnical ParametersParameterValueMedium dense sandBasic soil properties(SPT) N 20γ 125 pcfAngle of internal friction 33 degrees5-4

Chapter 5: Foundation Analysis and DesignTable 5.1-1 Geotechnical ParametersParameterValue 4,000 psf for B 20 feetNet bearing pressure (to controlsettlement due to sustained loads) 2,000 psf for B 40 feet(may interpolate for intermediate dimensions)2,000B psf for concentrically loaded square footings3,000B' psf for eccentrically loaded footingsBearing capacity (for plasticequilibrium strength checks withfactored loads)where B and B' are in feet, B is the footing width and B' isan average width for the compressed area.Resistance factor, φ 0.7[This φ factor for cohesionless soil is specified inProvisions Part 3 Resource Paper 4; the value is set at 0.7for vertical, lateral and rocking resistance.]Earth pressure coefficients:Lateral properties§ § § Active, KA 0.3At-rest, K0 0.46Passive, KP 3.3“Ultimate” friction coefficient at base of footing 0.65Resistance factor, φ 0.7The structural material properties assumed for this example are as follows:§ f'c 4,000 psi§ fy 60,000 psi5.1.1.2 Seismic Parameters. The complete set of parameters used in applying the Provisions to design ofthe superstructure is described in Section of this volume of design examples. The followingparameters, which are used during foundation design, are duplicated here.§ Site Class D§ SDS 1.0§ Seismic Design Category D5-5

FEMA P-751, NEHRP Recommended Provisions: Design Examples5.1.1.3 Design Approach. Selecting Footing Size and Reinforcement. Most foundation failures are related to excessivemovement rather than loss of load-carrying capacity. In recognition of this fact, settlement control shouldbe the first issue addressed. Once service loads have been calculated, foundation plan dimensions shouldbe selected to limit bearing pressures to those that are expected to provide adequate settlementperformance. Maintaining a reasonably consistent level of service load-bearing pressures for all of theindividual footings is encouraged since it will tend to reduce differential settlements, which are usually ofmore concern than are total settlements.Once a preliminary footing size that satisfies serviceability criteria has been selected, bearing capacity canbe checked. It would be rare for bearing capacity to govern the size of footings subjected to sustainedloads. However, where large transient loads are anticipated, consideration of bearing capacity maybecome important.The thickness of footings is selected for ease of construction and to provide adequate shear capacity forthe concrete section. The common design approach is to increase footing thickness as necessary to avoidthe need for shear reinforcement, which is uncommon in shallow foundations.Design requirements for concrete footings are found in Chapters 15 and 21 of ACI 318. Chapter 15provides direction for the calculation of demands and includes detailing requirements. Section capacitiesare calculated in accordance with Chapters 10 (for flexure) and 11 (for shear). Figure 5.1-2 illustrates thecritical sections (dashed lines) and areas (hatched) over which loads are tributary to the critical sections.For elements that are very thick with respect to the plan dimensions (as at pile caps), these critical sectiondefinitions become less meaningful and other approaches (such as strut-and-tie modeling) should beemployed. Chapter 21 provides the minimum requirements for concrete foundations in Seismic DesignCategories D, E and F, which are similar to those provided in prior editions of the Provisions.For shallow foundations, reinforcement is designed to satisfy flexural demands. ACI 318 Section 15.4defines how flexural reinforcement is to be distributed for footings of various shapes.Section 10.5 of ACI 318 prescribes the minimum reinforcement for flexural members where tensilereinforcement is required by analysis. Provision of the minimum reinforcement assures that the strengthof the cracked section is not less than that of the corresponding unreinforced concrete section, thuspreventing sudden, brittle failures. Less reinforcement may be used as long as “the area of tensilereinforcement provided is at least one-third greater than that required by analysis.” Section 10.5.4 relaxesthe minimum reinforcement requirement for footings of uniform thickness. Such elements need onlysatisfy the shrinkage reinforcement requirements of Section 7.12. Section 10.5.4 also imposes limits onthe maximum spacing of bars. Additional Considerations for Eccentric Loads. The design of eccentrically loaded footingsfollows the approach outlined above with one significant addition: consideration of overturning stability.Stability calculations are sensitive to the characterization of soil behavior. For sustained eccentric loads,a linear distribution of elastic soil stresses is generally assumed and uplift is usually avoided. If thestructure is expected to remain elastic when subjected to short-term eccentric loads (as for wind loading),uplift over a portion of the footing is acceptable to most designers. Where foundations will be subjectedto short-term loads and inelastic response is acceptable (as for earthquake loading), plastic soil stressesmay be considered. It is most common to consider stability effects on the basis of statically applied loadseven where the loading is actually dynamic; that approach simplifies the calculations at the expense ofincreased conservatism. Figure 5.1-3 illustrates the distribution of soil stresses for the variousassumptions. Most textbooks on foundation design provide simple equations to describe the conditions5-6

Chapter 5: Foundation Analysis and Designshown in Parts b, c and d of the figure; finite element models of those conditions are easy to develop.Simple hand calculations can be performed for the case shown in Part f. Practical consideration of thecase shown in Part e would require modeling with inelastic elements, but that offers no advantage overdirect consideration of the plastic limit. (All of the discussion in this section focuses on the common casein which foundation elements may be assumed to be rigid with respect to the supporting soil. For theinterested reader, Chapter 4 of ASCE 41 provides a useful discussion of foundation compliance, rockingand other advanced considerations.)Outside face of concretecolumn or line midwaybetween face of steelcolumn and edge ofsteel base plate (typical)PM(a)Critical sectionfor flexure(a)Loading(e M P)BL(b)Elastic, no upliftP e qm ax 1 6 B L L e L6extent of footing(typical)(b)Critical sectionfor one-way sheard(c)Elastic, at uplifte L6(d)Elastic, after uplift2Pqma x L 3 B e 2 L Lʹ′ 3 e 2 L e L26L'(e)Some plastification(c)Critical sectionfor two-way sheard/2(all sides)x(f)Plastic limitPx Bq c(MR P L 2 x2Figure 5.1-2 Critical sections for isolated footings)Figure 5.1-3 Soil pressure distributions5-7

FEMA P-751, NEHRP Recommended Provisions: Design Examples5.1.2DesignforGravityLoadsAlthough most of the examples in this volume do not provide detailed design for gravity loads, it isprovided in this section for two reasons. First, most of the calculation procedures used in designingshallow foundations for seismic loads are identical to those used for gravity design. Second, a completegravity design is needed to make the cost comparisons shown in Section 5.1.5 below meaningful.Detailed calculations are shown for a typical interior footing. The results for all three footing types aresummarized in Section Demands. Dead and live load reactions are determined as part of the three-dimensional analysisdescribed in Section 6.2 of this volume of design examples. Although there are slight variations in thecalculated reactions, the foundations are lumped into three groups (interior, perimeter and corner) forgravity load design and the maximum computed reactions are applied to all members of the group, asfollows:D 387 kipsL 98 kips§ Interior:§ Perimeter: D 206 kipsL 45 kips§ Corner:D 104 kipsL 23 kipsThe service load combination for consideration of settlement is D L. Considering the loadcombinations for strength design defined in Section 2.3.2 of the Standard, the controlling gravity loadcombination is 1.2D 1.6L. Footing Size. The preliminary size of the footing is determined considering settlement. Theservice load on a typical interior footing is calculated as:P D L 387 kips 98 kips 485 kipsSince the footing dimensions will be less than 20 feet, the allowable bearing pressure (see Table 5.1-1) is4,000 psf. Therefore, the required footing area is 487,000 lb/4,000 psf 121.25 ft2.Check a footing that is 11'-0" by 11'-0":Pallow 11 ft(11 ft)(4,000 psf) 484,000 lb 484 kips 485 kips (demand)OKThe strength demand is:Pu 1.2(387 kips) 1.6(98 kips) 621 kipsAs indicated in Table 5.1-1, the bearing capacity (qc) is 2,000B 2,000 11 22,000 psf 22 ksf.The design capacity for the foundation is:φPn φqcB2 0.7(22 ksf)(11 ft)2 1,863 kips 621 kips5-8OK

Chapter 5: Foundation Analysis and DesignFor use in subsequent calculations, the factored bearing pressure qu 621 kips/(11 ft)2 5.13 ksf. Footing Thickness. Once the plan dimensions of the footing are selected, the thickness isdetermined such that the section satisfies the one-way and two-way shear demands without the addition ofshear reinforcement. Demands are calculated at critical sections, shown in Figure 5.1-2, which depend onthe footing thickness.Check a footing that is 26 inches thick:For the W14 columns used in this building, the side dimensions of the loaded area (taken halfwaybetween the face of the column and the edge of the base plate) are approximately 16 inches.Accounting for cover and expected bar sizes, d 26 - (3 1.5(1)) 21.5 in.One-way shear: 11 1621.5 12Vu 11 ( 5.13) 172 kips12 2φVn φVc ( 0.75) 2 4,000 (11 12 )( 21.5)( ) 269 kips 172 kips11,000OKTwo-way shear:2Vu 621 ( 16 1221.5 ) (5.13) 571 kipsφVn φVc ( 0.75) 4 4,000 4 (16 21.5) ( 21.5)( ) 612 kips 571 kips11,000OK5.1.2.4 Footing Reinforcement. Footing reinforcement is selected considering both flexural demandsand minimum reinforcement requirements. The following calculations treat flexure first because itusually controls:2 11 16 112M u (11) (5.13) 659 ft-kips2 2 Try nine #8 bars each way. The distance from the extreme compression fiber to the center of the top layerof reinforcement, d t - cover - 1.5db 26 - 3 - 1.5(1) 21.5 in.T As fy 9(0.79)(60) 427 kipsNoting that C T and solving the expression C 0.85 f'c b a for a produces a 0.951 in.1 673 ft-kips 659 ft-kipsφ M n φT ( d a2 ) 0.90 ( 427 ) ( 21.5 0.9512 ) ( 12 )OKThe ratio of reinforcement provided is ρ 9(0.79)/[(11)(12)(26)] 0.00207. The distance between barsspaced uniformly across the width of the footing is s [(11)(12)-2(3 0.5)]/(9-1) 15.6 in.According to ACI 318 Section 7.12, the minimum reinforcement ratio 0.0018 0.00207OK5-9

FEMA P-751, NEHRP Recommended Provisions: Design Examplesand the maximum spacing is the lesser of 5 26 in. and 18 18 in. 15.6 in.OK5.1.2.5 Design Results. The calculations performed in Sections through are repeated fortypical perimeter and corner footings. The footing design for gravity loads is summarized in Table 5.1-2;Figure 5.1-4 depicts the resulting foundation plan.Table 5.1-2 Footing Design for Gravity ng Size and Reinforcement;Soil CapacityD 387 kipL 98 kip11'-0" 11'-0" 2'-2" deep9-#8 bars each wayP 485 kipPu 621 kipPallow 484 kipφPn 1863 kipD 206 kipL 45 kip8'-0" 8'-0" 1'-6" deep9-#6 bars each wayP 251 kipPu 319 kipPallow 256 kipφPn 716 kipD 104 kipL 23 kip6'-0" 6'-0" 1'-2" deep6-#5 bars each wayP 127 kipPu 162 kipPallow 144 kipφPn 302 kipCritical Section Demands andDesign StrengthsOne-way shear:Vu 172 kipφVn 269 kipTwo-way shear:Vu 571 kipφVn 612 kipFlexure:Mu 659 ft-kipφMn 673 ft-kipVu 88.1 kipφVn 123 kipTwo-way shear:Vu 289 kipφVn 302 kipFlexure:Mu 222 ft-kipφMn 234 ft-kipOne-way shear:Vu 41.5 kipφVn 64.9 kipTwo-way shear:Vu 141 kipφVn 184 kipFlexure:Mu 73.3 ft-kipφMn 75.2 ft-kipOne-way shear:

Chapter 5: Foundation Analysis and DesignCorner:6'x6'x1'-2" thickPerimeter:8'x8'x1'-6" thickInterior:11'x11'x2'-2" thickFigure 5.1-4 Foundation plan5.1.3DesignforMoment- ‐ResistingFrameSystemFraming Alternate A in Section 6.2 of this volume of design examples includes a perimeter momentresisting frame as the seismic force-resisting system. A framing

ACI 318 American Concrete Institute. 2008. Building Code Requirements and Commentary for Structural Concrete. Bowles Bowles, J. E. 1988. Foundation Analysis and Design. McGraw-Hill. CRSI Concrete Reinforcing Steel Institute. 2008. CRSI Design Handbook. Concrete Reinforcing Steel Institute. ASCE 41 ASCE. 2006. Seismic Rehabilitation of Existing .

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