ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/622975
AASHTO LRFD by Dr. Dennis R. Mertz 2016 Interim Changes Related to Concrete Structures, Part 1 At their 2015 annual meeting, hostedby the New York State Departmentof Transportation (NYSDOT) in SaratogaSprings, N.Y., in April, the American Associationof State Highway and Transportation Officials(AASHTO) Subcommittee on Bridges andStructures (SCOBS) considered and adoptedseven agenda items specifically related toconcrete structures. Technical Committee T-10,Concrete Design, developed agenda items 4through 9 and moved them to the subcommitteeballot for consideration in Saratoga Springs. Inconjunction with Technical Committee T-5, Loadsand Load Distribution, Technical CommitteeT-10 also developed agenda items 2 and 4 overthe past several years and moved them to thesubcommittee ballot. The agenda items representrevisions and additions to the 7th edition of theAASHTO LRFD Bridge Design Specifications.This column reviews the 2015 concrete-structuresagenda items, which are the 2016 InterimRevisions, that have been published and are nowavailable from AASHTO. Agenda Item 2 Agenda item 2 represents a major revision tothe wind-load provisions. It begins by makingrevisions to the descriptions of the limit-stateload combination dealing with wind, and theload factors for wind load, based upon new windloadprovisions applying a 3-sec. wind gust speedwith 7% probability of exceedance in 50 years(mean return period of 700 years). Wind loadprovisions in previous editions of the AASHTOLRFD specifications are based upon fastest-milewind speed measurements. The previous provisions allowed the use of a100-mph fastest mile base wind speed. This windspeed was used for the Strength III limit-state loadcombination. The Strength V, Service I, and ServiceIV limit-state load combinations were based uponconstant wind speeds. Instead of calculating thewind pressure for the specified constant windspeeds, the 100-mph wind speed is used and theStrength V, Service I, and Service IV limit-stateload factors for wind loads on the structure areadjusted to scale the resulting factored wind loadto the implied load factor and constant wind speedspecified for each load combination. In the revised provisions, the load factor forwind is 1.0 for all load combinations applied tothe wind pressure calculated for the wind speedspecified for each limit-state load combination.In addition, Article 3.8 titled “Wind Load: WL andWS” is replaced in its entirety by a new articledefining the new wind-load provisions. Theseprovisions provide consistent reliability acrossdifferent regions and locations unlike those thatthey replace. Although many typical bridges willnot see a change in design due to wind, thosestructures that fall between the typical range andthose needing site-specific considerations will bemore reliable through the application of the morerobust wind provisions. Finally, Article 5.14.2 is revised to make all ofthe load factors for wind during construction ofsegmentally constructed concrete bridges in Table126.96.36.199.3-1 equal to 1.0 (to be consistent with theprevious discussion). The revision further leavesthe specification of the wind speed for the variousservice limit-state load combinations to the owner,with the only exception being a specification forthe minimum wind speed of 70 mph for erectionstabilityanalysis of cantilever construction in lieu of abetter estimate by analysis or meteorological records. Agenda Item 4 Load factors for the Service III limit-stateload combination—the check of tensile stressin prestressed components—are addressedin agenda item 4. The calibration of theservice limit states for concrete components1concluded that typical components designedusing the refined estimate of time-dependentlosses method, which was incorporated in thespecifications in 2005 and includes the use oftransformed sections and elastic gains, have alower reliability index against flexural crackingin prestressed components. This is true whencompared against components designed usingthe prestress loss calculation method specifiedprior to 2005 based on gross sections andnot including elastic gains. For componentsdesigned using the currently specified methodsfor instantaneous prestressing losses and thecurrently-specified refined estimates of timedependentlosses method, an increase in the loadfactor for live load from 0.8 to 1.0 is required tomaintain the level of reliability against crackingof prestressed concrete components inherentin the system. Agenda item 4 inserts a tableinto Article 3.4.1 specifying a live-load loadfactor of 1.0 for prestressed concrete componentsdesigned using the refined estimates of timedependentlosses as specified in Article 188.8.131.52 inconjunction with taking advantage of the elasticgain and 0.8 for all other prestressed concretecomponents. The corresponding appropriaterevisions to the Manual for Bridge Evaluationwere also included in this agenda item. Agenda Item 5 Agenda item 5 integrates lightweight concrete(LWC) into the entirety of Section 5 in a moreconsistent and accurate manner based upon thework of Greene and Graybeal2-4 who presentedthese changes in greater detail in an article inthe Summer 2015 issue of ASPIRE.TM A reviseddefinition of LWC is provided to include concretewith lightweight aggregates up to a unit weightof 0.135 kip/ft3, which is considered the lowerlimit for normal-weight concrete. Also the terms“sand-lightweight concrete” and “all-lightweightconcrete” are removed in the proposed definitionto allow other types of LWC mixtures. The concretedensity modification factor, which has beenin earlier editions of the specifications, is nowdefined as λ and is introduced to modify varioustraditional resistance equations, stress limits, anddevelopment lengths. Finally, the shear strengthreduction factor, ϕ, for LWC has been set equal tothe factor for normal-weight concrete. The remaining concrete agenda items from the2015 SCOBS meeting, agenda items 6 through 9,will be discussed in a future column. References 1. Wassef, W. G., et al. 2014. Calibrationof LRFD Concrete Bridge DesignSpecifications for Serviceability, NCHRPWeb-only Document 201, TransportationResearch Board, National ResearchCouncil, Washington, DC. 2. Greene, G. G., and B. A. Graybeal.2013. Lightweight concrete :Mechanical Properties, Report No.FHWA-HRT-13-062, Federal HighwayAdministration, Washington, DC. 3. Greene, G. G., and B. A. Graybeal. 2014.Lightweight Concrete: Developmentof Mild Steel in Tension, Report No.FHWA-HRT-14-029, Federal HighwayAdministration, Washington, DC. 4. Greene, G. G., and B. A. Graybeal. 2015.Lightweight Concrete: ReinforcedConcrete and Prestressed Concrete inShear, Report No. FHWA-HRT-14-15-022,Federal Highway Administration,Washington, DC.