Tại sao sử dụng LRFD

Tại sao sử dụng LRFD - Tiếng Anh

Why the AASHTO Load and Resistance

Factor Design Specifications?

M Myint Lwin

‘heist of the new AASHTO Load and Resistance Facor Design

(RED) Bricge Design Speciteasns oes many uvanngee alee

Mages fo and benef of thse specfcadons we dctssed: The

‘AASHTO LRFD stased on new developmen in bide egies ‘ed pets and gai apraches to teste comnoee

ait, sevieaiiy, inspecably, economy an tntenes CRED

Incorporates th beat of working sues digs and Ions eệy tuc

fail pricing bridge engineer tis a eomprenensive, wet

organizes tod pacil pecdenon, wih commny 6 groves

3fivaistaodlaeEgrusdtiemnsden.Ney trưạn đen gasi ro

135 0 LREFD have he inherent vantage of «mere ene evel of

«sien ruling in low fee cot LRED aon ức úe dự ng,

‘thes in detian and analysis provides fey fr mining

fe ang access engineering paces oecutmizng aden ess

{ons and motivations to use LRFD Coles cases runing couse

sig example, workshop, nd compuer software we sow walls

‘es te iplenenason of AASHTO LAPD

‘tthe 1993 Anus! Meeting of he AASHTO Beige Subcommizee,

‘tember desively voted 4 0 $ to adoge the new AASHTO Land

‘nd Resistance Fastor Design (LRFD} Badge Devin ion and fer 7 year of concent 8 Commentary The vote ook place after expendig over efor by more han TD expers $2 mil

in bridge engineering and thousands of houn of review and eal

sigs fom sues, industry, consulta, sede tnd special

{83s In 1984, AASHTO peblishe heist edison of LRFD in

be customary (U'.) and St unis)

In deveioping the new LRFD specicaons, the philosophy, the

‘oplcailiy, ane he benefits wer subject te om extensive

‘view procedures inthe history ofthe Bridge subeommitee, snd

were tested by numerous ial signs performed by sate agencies

incase, academia private orgunizasons, dnd individu The al

sigs incu concre slab Bdge conc bea and prea

fiver bridges, ste! beam and ger Srdaes tum Drage, bop

‘Reval cence tide, wood bridges, calves and raining wale

‘The tal designs contribu sgnfcrly entry tat he ew

specications wee basicaly sound compreenst e810 Ue and

‘omparile ote AASHTO standart species Sice che fit ein

of LRFD was publi in 198, many sates,

‘uct s Florida Mihigan, Peasylvania,Temewee and Washing

tor fave been dgently developing plas and king sep to fly

‘plement LRFD, The Washiagon State Deparment of Tacspont

ton (WSDOT hasbeen agaressvely malig «eatin to ple,

‘mex the new spectcason WSDOT has provided bridge designers

with ang, ceveioped computer programs, snd pefomed despa dacs sing LRFD WSDOT saned applying

of presrested concrete ger biges because ay tides in Washingtn Sate are contucted-

TRENIA Wetem Resource Center 201 Mission Sse Sue 2100

“Sản Fanelee CA 94I05

of presmesed cone The frst LRFD bridge was designed ancon structed by using high performance concrete The bie nas opsncs

to aff in fll 1997, WSDOT has now congleed ptveued cọc

‘re, posensioned concrete pate girder tnd wel box pcre bridges using the new LRED specifeaton

‘Training counes textbooks, design examples, and computer po grams are now svalbleoelp with performing bridge eng tang LRED LRED cant implemented with confidence if Washinges Sut can do it eter tes can do ito,

way LRFD?

Jn 1986, se ge gen fom Califia, Clo, aia Michigan ana Washington bites eters Bota Soe site express tn eonce he AASHTO Sans Secs aon or Higny Bie wer fling ohn the na ss ftps td iconssteces eine ne seen arenes Seve concerns NCHRP ny was ined employ a, iy of developing ebb i sas pecan

KT robailiy-ae nt sates specication ts commnesny oe Bil gap vơ cerct neeaiestes Tc uc icon te accep he comments tinued ates NCR ‘pectoton wit commen <n Jl 988 a cevelon + modern und compre og Mae 1993, fl df (2) was competed nd auido tte Beige Sbrormite for approval Lau year nes of thenöctonieeteeidcuirlyo set LRED l0 cong corprtesn be cai for AASHTO nun 994 ne ton of AASHTO frou rc spins tis, inet, evn 0 LRED have bese mu a nese LRFD wa paises in custom (US) cect swith ay sale caes wpa wi test gu aber ane en pen ef ovens laniepaconof changes dig oe cy ean ofS AASHTO and FHWA finde NCHRP 12-22 fo fronds ine tance rdchareenenoLBED Tang mm ta cmư bang vực rác L250 nnn drm ges

ile, an lel spc a enng comer al

‘Sricuniy peal, omy ands The AEBS Iospy conser wither major ie ceip co 12g being evlopedin Ata Cua sop, an ob pel es tdin oer amas of ssl ogre and lon acces (LED) td are ase evens soe LRED incoportes te tof working sees design (WSD) erases fons nt shipcollnon osu meule te te,

~~ Ghd linger peromare trae and ces Ae ne prbsbiny sued LRFD ued on sais sad espe Tag o tt time Ttưtconplenste xài vợ

pci speseation with ommenary 0 provide backETouDE

|nfomation ang exiastion

"Tee use of AASHTO LRFD hat many advanages, The major

changes ang benef are cuted nine fllowing section

THE LRFD EQUATION

‘The bateequton in LRED is given a follows

here

"—

2, = nominalreisadee;

4 factor elang i dusty, redundancy and operation i

porance

3 = fond factor, a iil based multiple: and

2 Seance factor stitial baved muller

“Te load and reisance factors have been exited by tl

signs io prove high ufo lve of safety inne bridges The

tater levels exprewed bya ria inden (), LRED provides 2

‘lab inden of about 3 fr deren rypes and fr điffret

Configurcion of Sige A reli ndex of = 3.8 ensures that

nly 2out of 10000 design elements or omponens, wil have he

Sn of the factored lone pene tan te factored resistance ding

‘hes of esac, Bridges designed according LRFD

ave e more uniform evel of safer, alls in Fague 1

Figur {eso sows tat he carrert AASHTO standard specis-

cations do not provide a eonssent and uniform safety level Foe

Sunes design’ according 19th 1989 AASHTO standard spe

ifexions te reality index (8) may bea low as 2.0 ora high as

sige 20-tontof#Odergn elements and components would

probably be overlcaded and would experience a problem a some

Ue curing the design fe ofthe strtre This vey bh rate of

` he oer fins te ceign would be very conservative an cosy

FOUR LIMIT STATES

LRED defines for limit rts thar te design must satis 12

ebieve safe) sees, and eansttabiiy Equant must

erated fo enc iit ate, The ou iit ses ae

fie

em tr SPANLENGTH fee

FIGURED Raabiiy oder = US

1 Service limit sae: Tis limit state imposes retitions on saris defomatin, and crack with under service conditions 18

‘miler to WSD in ensuring elastic behavior and litle need for -nimenaree ưng the service life ofthe sructres

3 Fanuc and rctr nit tate: This it state imposes resi dons on sess ange resling from design ruck moving sevoss he dpe and eausing a given numberof tress yeles Again its sim= Tar th fig design requirements in WSD and LFD in ensuring tha tere ro premature ftgue cracking or fare in he members tite vere

3 serengt iit stae: Tie it state spas the strength and scat egurements 0 resist the specified tistical significans Toad combinations expected tobe experienced by abridge overt eign if Itis similar fo LFD i ensuring adequate wlimat loud capacity

Exe event init sate: Thislimi sae ensures the sutra tui hi of «bride during # major earthquake, food or sur oF Sites clided by averel,vebll,o ce flow The designers ree (ued a consicer unique event to avoid major damage o colase ofthe beige

‘The lint wats serve a a systema approach l2 SH design {Denturetroublefee shor and longterm perfomance of bres [NEW LIVE LOAD MODEL

‘The LRFD lve lond mode, desigated HLS, was developed as 2

‘otal representation of shear and moment produced by group

‘Stabove ep lit vehicles routinely pemited onthe highways ip tovou ates ur te “granfater” exclusion provisions i sate Trea The vehicles wee bared on studpconduced by the Trans poration Reseach Board in 1990, The iv load model is calles vata! beware ir not itendedosepesnt ay panicular eck turproduces the sue exere load effecs ashe above legal-iit veces

Te live load mode conse fs combiaton of the design suck orth design ane andthe desig lan od Te design ck ie funilar AASHTO HS20 muck The design ander isatandem of 40 (qual aes, ach 25 Kp an spaced at |2.m 4) The design ane Ted isthe fariar AASHTO uniform loud of 290 kp’ (640 I [To cvig ruck andthe design lane lous or tbe desig tandem ane ibe design lane lads combined o obtain te live loa effect, For eeu momen in continuous spans, 90 percent of the eect of

ee design cks place in wo ferent spans with headway ct tance oft lest 15.2 (30) ps a design an, wl be used if tis termination produces larger negative momen

NEW IMPACT FACTORS Impact factors in LRFD ae of consan:percenages ofthe stati tffec ofthe design tuck othe andem and are refered 038 i

‘pram load allowance The dynamic lad allowance for brida® tang i 3 perce for eck joins is 78 percent: and for fague desig itis 5 percent Theyre loud allowance doesn ply othe design lane and ie reuced 01S percent fr the ftgue Teac imi state, The pact factors are much simpler it ERED and are moc representative ofthe ees of ive loads one

— varios eres of te eg

DISTRIBUTION OF LIVE LOAPS

‘Avote! major change in LRED involves te ive loa distribution

factor for moment and shea Fer coneree decks on ttl or cone

crete bears, the dstibuion factors changed from spl S/S 10

‘more complex expression for moment sta smpler oe for het

For example in LRFD te dsirbution fcr for moment end

shear nan interior girder ofa pressed conree girder bride,

witha easrinpince concrete deck and wih vo ororeGesign anes

lade ceeds folowsFor mone

n0075-(8)"(" ca)"

Rarner

Sm er sping

peers

fDi fon —— and

WSDOT develsped QCoaBfge, sanpser program to facil

tae the caleultion of te live loud điemboton heCcxiecodng t2

‘he rew LRFD expression, This program, with reference mana id

‘onal canbe downloaded free of charge rom the WSDOT home

Page—hpw wsdouwa govlesfrige Other sates ay also

have in-house programs for determining ine live lad disbution

(set for various types of biden

‘With the new lve load ditbuton factors, the dynamic load

Allowances andthe oad factors the LRFD HL live load model

‘This method is based onthe variable angle trss analogy model and takes ito acecunt the inclination of diagonal seat cracks, the srin in logitinal steel, and the applied shear season te section

‘The nominal shear resistance of concre (Van he nomi hear resiane of vera ansvere reinforcement V7) ae expesied by the following

Ye = 003168, 75 “

‘These wo expression for V, and ¥, apply to both prestressed and nonpresvessed seeios, with he terms Band 8 depending on he ppd loading andthe properes of the section For nonpesresied

‘concrete sections containing the spec minimum amount of ears vere reinforcement he following valves may be uted: = 2.0 end

45 degrees These values can be substituted in Equations 3 and

4 to easily compute the nomial shea reitance of concrete and transverse enforcement However for pretrested cores see ions conning ransvere enforcement the vale off and have tobe determined through an eave proces using chars sven in LRFD The iterative proces is cumberrome and time or ales

‘sonsuming for longhand calculation Designers have developed spreadsheets todo the computation efficiently, The vereal compe nent of the pressing force also conmbutes tothe sear resistance

of retion, After using the chars and iratve process on several projec WSDOT conducted a paramere sy to find simplified way to determine the nominal shear ressance of pesmesed concrete wi transverse reinforcement Based on he parame sid, WSDOT discovered tha '=2.4and = 38 degrees wat adequate ore Wash

~ produces about the same ive toad effectrarur AASHTO HS2s-—— ington Stat sandardpresvesed corre ire Witter vauee, lousing, States curently using the AASHTO HS20 loading wil

‘experience higher live load demand ia thei designs The positive

news is that srucares designed according to LRFD will have no

problems meeting the current oad-ratingfeautements

FATIGUE LOAD

The fatigue loud consis of only one design tuck with a constant

‘pacing of m (0 ft) Sense he rear aes and dynamic lad atiomance of 15 percent repartee the ruse

of design les A, loud factor of 0.5 sued 1 reflec curvey fangs conlaing that

‘Re sverge ladetfct was about 7S perce: ofthe moment ened by

‘he cesign ruck The LRFD faigue provision i based on taf súc ‘vey and abridge design life

of 5 year The provision applicable

‘omdundant and nrwedundan member,

‘The fatigue lod specie inthe we sce ection of LRFD

race alowercalculted saessrange nha ofthe sr Eewlon for AASHTO HS25 eacng This represen a saving for spec

‘aes eure asng te HS25 lacing for devgn The figueprovie

sion of LRFD igh bridges are more referee fag lads experienced by

SHEAR DESIGN

[RED incorporates the modifies compretsion fel theory for

—sheardeszrof prerressed un ronprestsied cores memiben, We varous per of brie founsitons

the shear design fr pretreated girders i siplifed by providing stansard shar enforcement shown in Figue 2

FOUNDATION DESIGN

‘The service limi sate in foundation desig ie consoled by see ment, tral explacemens, and he coresponding bearing ress tance Senlementcrieria shouldbe based on sttira performance safety ieabliy and aesthetics Seniement itso 13102 54cm (to lin) ar peneraly used sat the service condition

‘The sength and exteme even limites re based on consi dien for bering resistance, exestve los of comact,sliing, loss of Tatra suppor and los of over sabiliy Bearing eis: tance is calculated according other fire instead of element,

‘Shear strength limited bering resistance may be hee one ies the setdement lite bearing resistance cepending ons pe and footing size, Figure 3 illsrates bearing resisance for an end ier on spread footing The smictural engineers sould work co0p-

‘ravely withthe geotechnical engineer fo ake advantage of te higher bearing resisance forthe suength and exreme lit states

to prsuce foundation desigs comparable a those prodsced by ti [AASHTO standard specifcntions

(Continuing researc and development wil provice a larger and beter dasbase fr enhancing the probably based LRFD founds: tion desig speciation and for reining he ressance factors for

GIRDER END ELEVATION (WSDOT WHGPS GIRDER)

FIGURE? Shear reinforcement (t CONTINUOUS IMPROVEMENT

‘The new AASHTO LRFD isthe result of continsous improvenents

since AASHTO adopted the frat nadonal bpe code in 1931 The

ane hanged from WSD to LFD in 97S and later tthe cures oud

{ed sistance fictor design (RFD), LRFD isa modern and com>

pehensive bridge peifiation, whieh pute United Stes pracice

{te ieading edge of bridge engineering LRFD allows the use of

vanced metods indesign and arlysis and provides lexi fr

‘munainng good and succes offie practices or customizing oud

ted resnance factors to meet project demands, Maay reasons and

‘evans exit for adopting the curent AASHTO LRFD Bridge

Design Speciation, College lasses, raining cours design ads,

workshops and computer software are now avaabero facili the

implementation of LRFD

ca

Capaciy

eva eee Ho

tive Fong With (1) FIGURE 3 Bearing capacity fran end pier

(in=03m,

stem

CONCLUSIONS + The AASHTO LRFD is based on technological advances in bridge enginering, sound scien principls and a systematic a proach io ensure consruciabliy, safer, serviceailiy.nspecabl (conor, and ashes,

The AASHTO LRFD has the inherent advantage of amor i= forrn eve of efey in new bridges, which means low mntenance and repair and low life-cycle cost * The AASHTO LRFD allows th engineers o take advange of the properies of new cnareson materials or expand the aplic~ sili of exiting materiale, For example, WSDOT sparmering with members ofthe Pacific Norwest Chaper of the Presmessed Con: (Gee Insite to cevelop new preszesed perfection that can span {46.8 to 6m (160 10 200 ft) by esing LRFD and hgh prforman The parallel commentary in the AASHTO LRFD helps bridge engineers understand te rasonale andthe background of the spectieatons [takes he “Slack box" ovto he specification requirements > The AASHTO LRFD fulfils vision to dexign and build quality iges witha high level of reiabii forthe 21st entry

REFERENCES

1 AASHTO LAFD Bridge Deven Speceaton it ed American Aso- ‘tion of Sut Hipvay snd Tsuponauon Offa, Washington Bie 194

2 alot M.Deeopmen (Commemar? Pal Repor NCHRP 13-3, 193 of Comgrehnsie Bride Speciewonsond

‘Plicaron fis paper sporsced by Conminee on Generel Sacre