42

ngllrt 1. 10 Pite fouJKblloos aRllnlilallro u,ing apile dri'-cr. SUcllllS Ihis one.l1le pile is hfteoJ ntO lhe vcnkaJ >tioo. "hlC~ i. called Ihe luds. ,t..n drhen ;no!he: round " ith \he plle lIarnmcr. Thus.!he Plle dn"cl musl be shgtltly taller Ihan [he plle 10 be in .. allcd.

)"IKurt l.1 1 As p;ut of. )C,smlC retrofit prooecl. a des,gn enllinecr has caJled for in-~lalhntl4j().mm diarl\elcr. 9-m Ionll prc-strnscd concme plle fooooalionlllO be IIIstalled bellC.'31h Ihe ba~mem of atI c~iSlj !lg building This pile foundalion design IS un-buildable becau~ Ihe n:qu i..w pi lc_dri,-i ng eql"pmcnl would nOl til in !he bascmcm. and bec:all5C thc~ is r>OI coough room 10 50:1 lhe pile upngh t

Performance Aequiremenls

1" '"

EXISlmg Tllree V Office Buildln -~IOI)' g

Propost'tl Piles V fOl" Sci~m ic Retrofil

understand-ing of construction.

2.5 ECONOMIC REQUIREMENTS

Foundation designs are usually more conservative than tbose in tbe superstructure. This approach is justified for tbe following reasons:

Foundation designs rely on our assessments of tbe soil and rock conditions. These assessments a1ways inelude considerable uncertainty.

Foundations are not built witb tbe same degree of precision as the superstructure. For example, spread footings are typically excavated witb a backhoe and tbe sides of tbe excavation becomes tbe "fonnwork" for tbe concrete, compared to concrete members in the superstructure tbat are carefully fonned with plywood or other ma-terials.

The structural materials may be damaged when tbey are installed. For example, cracks and splits may develop in a timber pile during hard driving.

There is sorne uncertainty in tbe nature and distribution of tbe load transfer between foundations and tbe ground, so tbe stresses at any point in a foundation are not al-ways known with as much certainty as might be tbe case in much of tbe superstruc-ture.

The consequences of a catastrophic failure are much greater. The additional weight brought on by tbe conservative design is of no consequence,

because tbe foundation is tbe lowest structural member and tberefore does not affect tbe dead load on any otber member. Additional weight in tbe foundation is actually beneficial in tbat it increases its uplift resislance.

However, gross overconservatism is not warranted. An overly conservative design can be very expensive to build, especially witb large structures where tbe foundation is a greater portion of tbe total project cos!. This a1so is a type of "failure": tbe failure to produce an economical designo

The nineteentb-century engineer Arthur Wellington once said tbat an engineer's job is tbat of "doing well witb one dollar which any bungler can do witb two." We must strive to produce designs tbat are botb safe and cost-effective. Achieving tbe optimum balance between reliability (safety) and cost is par! of good engineering.

Designs tbat minimize tbe required quantity of construction materials do not neces-sarily minimize tbe cos!. In sorne cases, designs tbat use more materials may be easier to build, and tbus have a lower overall cost, For example, spread footing foundations are usually made of low-strengtb concrete, even tbough it makes tbem thicker. In tbis case, tbe savings in materials and inspection costs are greater tban tbe cost of buying more con-crete.

44 Chapler 2 Performance Requiremenls

SUMMARV

Major Points

1. The foundation engineer must determine Ihe necessary performance requirements befare designing a foundation.

2. Foundations must support various types of structuralloads. These can inelude nor-mal, shear, moment, andlor torsion loads. The magnitude and direction of Ihese loads may vary during Ihe life of Ihe structure.

3. Loads also are elassified according to Iheir source. These inelude dead loads, live loads, wind loads, earlhquake loads, and several olhers.

4. Oesign loads may be expressed using eilher Ihe allowable stress design (ASO) or Ihe load and resistance factor design (LRFD) melhod. It is important to know which melhod is being used, because Ihe design computations must be performed accard-ingly.

5. Strenglh requirements are Ihose Ihat are intended to avoid catastrophic failure. There are two kinds: geotechnical strenglh requirements and structural strenglh re-quirements.

6. Serviceability requirements are Ihose intended to produce foundations Ihat perform well when subjected to Ihe service loads. These requirements inelude settlement, heave, tile, lateral movement, vibration, and durability.

7. Settlement is oflen Ihe most important serviceability requirement. The response of structures to settlements is complex, so we simplify Ihe problem by considering two types of settlement: total settlement and differential settlement. We assign maxi-mum allowable values for each, Ihen design Ihe foundations to satisfy Ihese require-ments.

8. Ourability is anolher important serviceability requirement. Foundations must be able to resist Ihe various corrosive and deteriarating agents in soil and water.

9. Foundations must be buildable, so design engineers need to have at least a rudirnen-tary understanding of construction methods and equipment.

10. Foundation designs must be economical. Allhough conservatism is appropriate, ex-cessively conservative designs can be too needlessly expensive to build.

2.5 Economic Requirements 45

Vocabulary

Allowable differential Earthquake load Self-straining load settlement Economic requirement Serviceability requirement

Allowable total settlement Failure Settlement Allowable angular Fluid load Shearload

distortion Geotechnical strength Snow load Allowable stress design requirement Stream flow loads Braking load Heave Strength requirement Cathodic protection Impactload Structural strength Centrifugalload Lateral movement requirement Column spacing Live load Sulfate attack Constructibility Load factor Tilt

requirement Load and resistance factor Torsion load Deadload design Total settlement Design load Momentload Vibration Differential settlement Normal load Windload Durability Performance requirement Earth pressure load Resistance factor

COMPREHENSIVE QUESTIONS ANO PRACTICE PROBLEMS

2.9 A certain clayey soi! contains 0.30 pereent sulfates. Would you anticipate a problem witb con-crete foundations in this soil? Are any preventive measures necessary? E~plain.

2.10 A series of 50-ft long sleel piles are to be driven into a natural sandy soil. TIle groundwater table is at a depth of 35 ft below tbe ground surface. Would you anticipate a problem witb cor-rosioo? What additional data could you gather to make a more informed decision?

2.11 A one-story steel warehouse building is to be built of structural steel. TIle roof is to be sup-ported by steel trusses tbat will span tbe entire 70 ft width of tbe building and supported on columns adjacent to tbe exterior walls. These trusses will be placed 24 ft on center. No inte-rior columns will be presenl. TIle walls will be made of corrogated steel. There will not be any roll-up doors. Compute tbe allowable total and differential settlements.

2.12 TIle grandstands for a minor league baseball stadium are to be bui!t of structural steel. TIle structural engineer plans to use a very wide columo spacing (25 m) to provide the best specta-tor visibility. Compute tbe allowable total and differential settlements.

2.13 TIle owner of a lOO-story building purchased a plumb bob witb a very long slring. He selected a day witb no wind, and tben gentIy lowered tbe plumb bob from his pentbouse office win-

4' Chapter 2 Performance ReqUlrements

Flgun' 2.1 2 l'roposed d~partn",rl\ '>10'" for I'robl.m 2.1~.

dow. When ;1 rcachcd lhe ,i dcwal~. 11 was 1.0 m from Ihe sidc oflhe building. h thil bUIlding lilting eJlcessi,cly~ Explain.

2. 1" "h\(HlOry departrnenl

3

Soil Mechanics

Measure it with a micrometer, mark it with chalk, cut it with an axe.

An admonition to maintain a consistent degree olprecision throughout the analysis. design, and construction phases ola project.

Engineers classify earth materials into two broad categories: rock and soil. AIlhough bolh materials play an important role in foundation engineering, most foundations are sup-ported by soil. In addition, foundations on rock are often designed much more conserva-tively because of lhe rock' s greater strenglh, whereas economics prevents overconser-vatism when building foundations on soil. Therefore, it is especially important for lhe foundation engineer to be familiar wilh soil mechanies.

U sers of this book should a1ready have acquired at least a fundamental understand-ing of lhe principies of soil mechanies. This chapter reviews lhese principies, emphasiz-ing lhose lhat are most important in foundation analyses and designo Relevant principies of rack mechanics are included in later chapters wilhin lhe context of specific applica-tions. Geotechnical Engineering: Principies and Practices (Coduto, 1999), lhe compan-ion volume to this book, explores a11 of lhese topies in much more detail.

3.1 SOIL COMPOSITION

One of lhe fundamental differences between soil and most olher engineering materials is lhat it is a particulate material. This means lhat it is an assemblage of individual particles ralher lhan being a continuum (a continuous solid mass). The engineering properties of

47