SPECIFICATIONS 1.3 DESIGN PHILOSOPHY General

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1 SPECIFICATIONS The attache ocment provies recommene moifications to Article.3 in Section I "Introction" of the AASHTO LFD Brige Design Specifications. The moifications provie a metho to incle system factors that accont for system ctility an renancy ring the esign an safety evalation of highway briges..3 DESIGN PHILOSOPHY.3. General Briges shall be esigne for specifie limit states to achieve the objectives of constrctability, safety an serviceability, with e regar to isses of inspectability, economy an aesthetics, as specifie in Article 2.5. egarless of the type of analysis se, Eqation shall be satisfie for all specifie force effects an combinations thereof..3.2 Limit States.3.2. General Each component an connection shall satisfy Eq for each limit state, nless otherwise specifie. For service an extreme event limit states, resistance factors shall be taken as.0, except for bolts, for which the provisions of Article shall apply, an for concrete colmns in Seismic Zones 2, 3, an 4, for which the provisions of Articles an b shall apply. All limit states shall be consiere of eqal importance. Q i i s n r (.3.2.-) where: s =system factor: relating to ctility, renancy an operational classification as specifie in Article.3.6 for the esign of strctral components for strength an extreme event limit states. For all other limit states, the system factors shall be taken as.0 =resistance factor: a statistically base mltiplier applie to nominal resistance, as specifie in Sections, 5, 6, 7, 8, 0, an 2 i =loa factor: a statistically base mltiplier applie to force effects n = nominal resistance r = factore resistance: s n Q i = force effect Service Limit State The service limit state shall be taken as restrictions on stress, eformation an crack with ner reglar service conitions. A.-

2 Fatige an Fractre Limit State The fatige limit state shall be taken as a set of restrictions on stress range e to a single fatige trck occrring at the nmber of expecte stress range cycles. The fractre limit state shall be taken as a set of material toghness reqirements of the AASHTO Material Specifications Strength Limit State Strength limit state shall be taken to ensre that strength an stability, both local an global, are provie to resist the specifie statistically significant loa combinations that a brige is expecte to experience in its esign life Extreme Event Limit States The extreme event limit state shall be taken to ensre the strctral srvival of a brige ring a major earthqake or floo, or when collie by a vessel, vehicle or ice flow possibly ner score conitions..3.3 Dctility The strctral system of a brige shall be proportione an etaile to ensre the evelopment of significant an visible inelastic eformations at the strength an extreme event limit states before failre. Energy-issipating evices may be sbstitte for conventional ctile earthqake resisting systems an the associate methoology aresse in these Specifications or in the AASHTO Gie Specifications for Seismic Design of Briges..3.4 enancy The strctral system of a brige shall be configre an its members esigne to ensre that it meets three system strength conitions: a) limite fnctionality an b) resistance to collapse if the strength of its most critical member is exceee, an c) ability to carry some level of live loa in a amage state. Therefore, mltiple-loapath, ctile an continos strctres shol be se nless there are compelling reasons not to se them. A system factor s shall be applie ring the esign of brige members to accont for a brige s system level of renancy as specifie in Article Operational Importance This Article shall apply to the strength an extreme event limit states only. The Owner may eclare a brige or any strctral component an connection thereof to be of increase operational priority. The Owner may also eclare a strctral component or connection to be amage-critical. A.-2

3 .3.6 System Factor The system factor, s, is a mltiplier applie to the nominal resistances of the strctral components of a brige system or sbsystem to reflect the level of ctility, renancy an operational classification. For brige sperstrctres ner the effect of vertical loas, s, shall be taken as specifie in Article For brige sbstrctres ner the effect of horizontal lateral or longitinal loas, s, shall be taken as specifie in Article System Factors for Brige Sperstrctres ner Vertical Loas For the sperstrctres of briges classifie as being of increase operational priority an for trsses an arch briges, an for briges not covere in Tables throgh : s shall be calclate sing an incremental non-linear analysis following the provisions of Article For the sperstrctres of straight briges of types (a), (b), (c), (), an (k) as efine in Table having amage-critical components: s = min ( s, s ) For the sperstrctres of straight briges of types (a), (b), (c), () an (k) that are not amage critical: s = s where: s = system factor for sperstrctre fnctionality an resistance to collapse conitions. s =system factor for sperstrctre strength in amage state conition. s is applie to all members affecte by the vertical loa. A minimm vale of s =0.80 is recommene bt in no instance shol s be taken greater than.20. ecommene vales for s, for typical straight sperstrctres are specifie in Table for I-girer sperstrctres of type (a) an (k) an Table for sprea box girer briges sperstrctres of type (b) an (c) an Table for mlti-cell boxes of type (). ecommene vales for s, for typical straight sperstrctres are specifie in Table for I-girer sperstrctres of type (a) an (k) an Table for sprea box girer briges sperstrctres of type (b) an (c) an Table for mlti-cell boxes of type (). A.-3

4 Table System factors for straight I-girer sperstrctres of types (a) an (k) for system fnctionality an resistance to collapse conitions ner vertical loaing Brige cross section type System factor Continos steel I-girer briges with non-compact negative bening sections an All other simple span an continos I-beam briges s D s D 2.5 / 2 Table System factors for straight sprea box girer sperstrctres of types (b) an (c) for system fnctionality an resistance to collapse conitions ner vertical loaing Brige cross section type System factor Simple span box girer briges 24-ft wie s D Simple span box girer briges 24-ft wie Continos box girer briges 24-ft wie Continos steel box-girer briges with non-compact negative bening sections an.75 Continos box girer briges with compact negative bening sections s s s D 2.5 / 2 D 2.5 / 2 D 2.5 / 2 D 2.5 / s 4 2 Where: D/ = ea loa to resistance ratio for the member being evalate. = loa factor relate to the capacity of the system to resist the failre of its most critical member. D + + when.0 + L D = when.0 L (.3.6.-) = loa carrying capacity. D = ea loa moment effect. L = moment effect of applie live loa e to two sie-by-sie LFD esign trcks applie at the mile of the span or e to two trcks in one lane applie in each of two contigos spans. L D. F. LL D.F. = loa istribtion factor LL = effect of the LFD esign trck with no impact factor an no lane loa. The negative sperscript refers to negative bening an the positive sperscript refers to positive bening. A.-4

5 Table System factors for single cell an mlti-cell box girer sperstrctres of type () for system fnctionality an resistance to collapse conitions ner vertical loaing Brige cross section type System factor Single cell box girer briges s 0.80 Mlti-cell box girer briges s.00 Table System factors for straight I-girer sperstrctres of types (a) an (k) in amage state conition ner vertical loas. Brige cross section type enancy ratio System factor Simple span I-girer Briges S weight Continos steel I-girer briges with non compact sections in negative bening Continos prestresse concrete an steel I- girer briges with compact sections in negative bening S S s D 0.47 (0.47 ) A.-5

6 Table System factors for straight sprea box girer sperstrctres of types (b) an (c) in amage state conition ner vertical loas enancy ratio Brige cross section type System factor Fractre simple span steel box girer briges less than Non-renant s = ft wie Narrow simple span steel box girer briges less than 24-ft 0.46 with no torsional rigiity All other simple span box girer briges 0.72 Continos steel box-girer briges with non-compact negative bening sections an s D 0.47 (0.47 ) All other continos box girer briges Where = renancy ratio for amage brige systems S beam spacing in feet ( kip / ft) weight beam beam = total ea weight on the amage beam in kip per nit length. M kip. ft / ft M = combine moment capacity of the slab an members incling iaphragms expresse in kip-ft per nit slab with. Table System factors for single cell an mlti-cell box girer sperstrctres of type () in amage state conition ner vertical loas Brige cross section type System factor Single cell box girer briges s 0.80 Mlti-cell box girer briges s.20 A.-6

7 Incremental Non-linear enancy Analysis for Briges ner Vertical Loas For trsses an arch briges, briges classifie to be of operational importance, an for briges not covere in Tables throgh , the system factor of Eqation for the strctral components of a system sbjecte to vertical loas shall be calclate from the reslts of an incremental analysis sing Eqation : f s min,, ( ) A minimm vale of s =0.80 is recommene bt in no instance shol s be taken as greater than.20. Where: = system reserve ratio for resistance to collapse conition f = system reserve ratio for the fnctionality conition, = system reserve ratio for amage state conition, f f, f, an are obtaine from the incremental non-linear analysis where: = vertical loa factor that cases the collapse of the sperstrctre f = vertical loa factor that cases the maximm vertical eflection of the sperstrctre to reach a vale eqal to span length/00. = vertical loa factor that cases the failre of a amage sperstrctre = vertical loa factor that cases the first member of the intact sperstrctre to reach its limit capacity A.-7

8 System Factors for Strctral Components of Brige Sbstrctres ner Horizontal Loas Design of abtments, piers an walls shall be investigate for strctral safety at the strength an extreme event limit states for each strctral component an joint sing Eqation for brige systems sbjecte to horizontal loas. Brige systems evalate sing the isplacement-base approach The isplacement base approach of the AASHTO Gie Specifications for LFD Seismic Brige Design may be se in lie of the force-base approach of Eq The isplacement-base approach compares the seismic isplacement eman to the seismic isplacement capacity sch that: eman s capacity ( ) where: s = 0.75, a system factor relating to ctility an renancy of strctral components evalate for seismic extreme event limit states. capacity = nominal seismic isplacement capacity of the brige sbstrctre element. eman = nominal seismic isplacement eman. For the sbstrctres of briges classifie as being of increase operational priority, capacity shall be calclate sing the incremental non-linear analysis approach provie in AASHTO (20) Gie Specifications for LFD Seismic Brige Design. For the sbstrctres of briges classifie to be amage-critical, the brige system mst also satisfy the provisions of the nonlinear analysis procere specifie in Article for vertical loaing. For the sbstrctres of all other brige systems evalate sing the isplacement base approach capacity shall be calclate sing the appropriate provisions in AASHTO (20) Gie Specifications for LFD Seismic Brige Design for seismic loaing. Brige systems evalate sing the force-base approach For the sbstrctres of briges classifie as being of increase operational priority an for briges not covere in Table , s shall be calclate sing the incremental non-linear analysis approach following the provisions of Article For the sbstrctres of briges classifie to be amage-critical, s shall be calclate sing the incremental non-linear analysis approach following the provisions of Article for horizontal loas. For systems with amage-critical colmns, an incremental non-linear analysis of the complete system ner vertical loaing shol also be performe following the provisions of Article A.-8

9 For the sbstrctres of straight briges with single-colmn an mlti-colmn bents of eqal heights ner lateral loa being evalate sing the force-base approach s F C tconf tnc tnc ( ) s is applie to all the brige members affecte by the applie loa. A minimm vale of s =0.80 an a maximm vale of s =.20 are recommene. where = isk factor specifie in Table F = mlti-colmn factor specifie in Table C = crvatre factor specifie in Table = ltimate crvatre at failre of the weakest colmn or connecting member in the system calclate sing Eq = constant crvatre for typical nconfine colmns specifie in Table tnc tconf = constant crvatre for typical confine colmns specifie in Table = crvatre rection factor for sbstrctre systems with eficient etailing calclate sing Eq The ltimate crvatre at failre is calclate from the ltimate plastic analysis of the colmn s cross section: c ( ) where = maximm strain at failre eqal to the concrete crshing strain or steel rptre strain c = istance from the fiber that fails first to the Netral axis. For the cases where the shear capacity or the etailing of the colmns or the capacity of the cap beams an pile caps are not sfficient to evelop a mechanism in the colmns bt the brige colmns reach their plastic moment capacity ner the effect of the applie loas, a correction factor is applie in Eq to rece the ltimate colmn crvatre Mavailable M p colmn if M M M M M colmn p colmn.0 otherwise colmn available p colmn ( ) A.-9

10 Where M available =moment capacity of the connecting elements sch as cap beams an pile caps or the rece moment that can be spporte by the colmn base on the available shear reinforcement, evelopment length, splice or connection etailing. M = plastic moment capacity of colmn, p colmn M colmn = ltimate overstrength moment capacity of colmn. For briges with nontypical configrations which are not covere in Tables , the incremental non-linear analysis approach of Article shall be se. For briges with one-colmn bents an those sbjecte to longitinal loaing with bearing connections between sperstrctres an sbstrctres: s ( ) A.-0

11 Table ecommene vales for renancy parameters for briges with one-colmn an mlti-colmn bents ner horizontal loa Variable Applicability ecommene vale, risk factor s, system factor for: One colmn bents Longitinal loaing of systems with bearing connections between sperstrctres an sbstrctres Systems where failre is controlle by shear. Systems with insfficient etailing to allow plastic moment capacity of the colmns to be reache. Systems connecte to components with plastic moment capacities weaker than those of the colmns. Systems evalate sing the isplacement base approach s, system factor for: Mlti-colmn bents ner lateral loa Longitinal loaing of systems with integral connections between sperstrctres an sbstrctres. Systems with sfficient etailing to allow plastic moment capacity of the colmns to be reache. Systems connecte to components with plastic moment capacities stronger than those of the colmns. Seismic hazars =0.75 All other lateral loas =0.85 Non-renant systems enant systems s F s C tconf tnc tnc F,mlti-colmn factor: For briges loae laterally, cont the nmber of colmns in each bent For briges loae longitinally with integral connections between sperstrctre an sbstrctre, cont the nmber of bents between expansion joints C, crvatre factor tnc crvatre tconf crvatre, typical nconfine colmn ltimate, typical confine colmn ltimate Two-colmn systems Three-colmn systems Systems with for or more colmns Constant for all systems Constant for all systems Constant for all systems F =.0 F =.6 F =.8 C =0.24 tnc =3.64 x 0-4 (/in) tconf =.55 x 0-3 (/in) A.-

12 Incremental Non-linear enancy Analysis for Briges ner Horizontal Loas For briges classifie to be of operational importance an for briges not covere in Table that are being evalate sing the force-base approach, the system factor of Eqation for the strctral components of a system sbjecte to horizontal loa shall be calclate from the reslts of a nonlinear pshover analysis sing Eqation : f s min,, ( ) where: = system reserve ratio for resistance to collapse conition, f = system reserve ratio for the fnctionality conition, = system reserve ratio for amage state conition, P, P f, P an P are obtaine from the incremental pshover analysis f Pf P P P p p P P p P = lateral loa that cases the failre of the sperstrctre P f = lateral loa that cases a maximm lateral eflection eqal to clear colmn height /50. P = the lateral loa that cases the failre of a amage sperstrctre P p = lateral loa that cases the first member of the intact sbstrctre to reach its limit capacity EFEENCES Ghosn, M. an Yang, J., (203) NCHP 2-86, BIDGE SYSTEM SAFETY AND EDUNDANCY, Transportation esearch Boar, Washington DC Ghosn, M., an Moses, F., (998). enancy in Highway Brige Sperstrctres. National Cooperative Highway esearch Program, NCHP eport 406, Transportation esearch Boar, Washington DC: National Acaemy Press. Li, D., Ghosn, M., Moses, F., an Neenhoffer, A., (200). enancy in Highway Brige Sbstrctres. National Cooperative Highway esearch Program, NCHP eport 458, Transportation esearch Boar, Washington, DC: National Acaemy Press. Bckle, I, Frielan, I, Maner, J., Martin, G,, Ntt,., Power, M. (2006), Seismic etrofitting Manal for Highway Strctres, Part Briges, FHWA-HT , Trner-Fairbanks Highway esearch Center, McLean, VA AASHTO (20) Gie Specifications for LFD Seismic Brige Design, 2n Eition, Washington, DC. A.-2