Draft

6.2TheCaseStudy 127

tower supportsandarermlyanchoredin bothbanksby hugeblocks of concrete,theanchors.

14 Becausethecablesaremuch longerthantheyarethick (large

I

L

), theycanbe idealizeda

perfectly
exiblememberswithnoshear/bendingresistancebutwithhighaxialstrength.

15 Thetowersare 578 ft tallandreston concretecaissonsin theriver.Becauseof ourassumption

regardingtherollersupportforthecables,thetowerswillbe subjectedonlyto axialforces.

6.2.2 Loads

16 Thedeadloadis composedof theweight of thedeck andthecablesandis estimatedat 390

and 400 psfrespectivelyforthecentralandsidespansrespectively. Assuminganaveragewidth

of 100ft, thiswouldbe equivalent to

DL= (390)psf(100)ft

k

(1;000)lbs

= 39k/ft (6.14)

forthemainspanand 40 k/ftforthesideones.

17 For highway bridges,designloadsaregiven by theAASHTO(Associationof AmericanState

Highway TransportationOcials). TheHS-20truck is oftenusedforthedesignof bridgeson

mainhighways,Fig.6.3. Eitherthedesigntruck withspeciedaxleloadsandspacingmustbe

usedortheequivalent uniformloadandconcentratedload.Thisloadingmustbe placedsuch

thatmaximumstressesareproduced.

Figure6.3:Truck Load

18 Withtwo decks,we estimatethatthereis a totalof 12 lanesor

LL= (12)Lanes(:64)k=ft=Lane= 7: 68 k/ft 8 k/ft (6.15)

We donotconsiderearthquake, or windloadsin thisanalysis.

19 FinalDLandLLare,Fig.6.4:T L= 39 + 8 = 47k/ft