Calibration of AASHTO LRFD Concrete Bridge Design Specifications for Serviceability (2014)

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6 PROPOSED CHANGES TO AASHTO LRFD Various articles of AASHTO LRFD which would need to be modified to implement the calibrated SLS resulting from this research were identified. This Chapter contains the suggested modifications formatted in a form suitable for consideration by the affected Technical Committees as potential Agenda Items for the Highway Subcommittee on Bridges and Structures (HSCOBS). Since the various SLS revisions are independent of each other and could be implemented individually, the suggested provisions are presented in separate subsections for each SLS. The article numbering system used in AASHTO LRFD has been preserved. The proposed revisions are underlined and deletions are shown as strikethrough. 6.1 Cracking of Prestressed Concrete – Currently Service III 6.1.1 Proposed Revisions to Section 5 5.7.3.4—Control of Cracking by Distribution of Reinforcement The provisions specified herein shall apply to the reinforcement of all concrete components, except that of deck slabs designed in ……. C5.7.3.4 All reinforced concrete members are subject to cracking under any load condition, including thermal effects and restraint………….. . . . . . . . . . . . . . . . . . The requirements for skin reinforcement are based upon ACI 318-95. For relatively deep flexural members, some reinforcement should be placed near the vertical faces in the tension zone to control cracking in the web. Without such auxiliary steel, the width of the cracks in the web may greatly exceed the crack widths at the level of the flexural tension reinforcement. Such reinforcement may be included in strength computations if a strain compatibility analysis is made to determine stresses in the individual bars or wires. The reliability index for control of cracking by distribution of reinforcement in reinforced concrete decks using the conventional design methods and using Equation 5.7.3.4 was investigated in Wassef et. al. (2014). It was found that the equation gives a fairly uniform reliability index. 138

6.2 Cracking of Prestressed Concrete 6.2.1 Proposed Revisions to Section 3 3.4—LOAD FACTORS AND COMBINATIONS 3.4.1—Load Factors and Load Combinations The total factored force effect shall ……. Service I—Load combination relating to the normal operational use of the bridge with a 55 mph wind and all loads taken at their nominal values. Also related to deflection control in buried metal structures, tunnel liner plate, and thermoplastic pipe, to control crack width in reinforced concrete structures, and for transverse analysis relating to tension in concrete segmental girders. This load combination should also be used for the investigation of slope stability. Service II—Load combination intended to control yielding of steel structures and slip of slip-critical connections due to vehicular live load. Service III—Load combination for longitudinal analysis relating to tension in prestressed concrete superstructures with the objective of crack control and to principal tension in the webs of segmental concrete girders. C3.4.1 The background for the load factors…….. Compression in prestressed concrete components and tension in prestressed bent caps are investigated using this load combination. Service III is used to investigate tensile stresses in prestressed concrete components. This load combination corresponds to the overload provision for steel structures in past editions of the AASHTO Specifications, and it is applicable only to steel structures. From the point of view of load level, this combination is approximately halfway between that used for Service I and Strength I Limit States. Prior to 2014, the longitudinal analysis relating to tension in prestressed concrete superstructures was investigated using a load factor for live load of 0.8. The live load specified in these specifications This load factor reflected, among other things, the then-current exclusion weight limits mandated by various jurisdictions at the time of the development of the specifications in 1993. Vehicles permitted under these limits have been in service for many years prior to 1993. It was concluded at that time that, for longitudinal loading, there is no nationwide physical evidence that these vehicles have caused cracking in existing prestressed concrete components. The 0.8 load factor was applied regardless of the method used for determining the loss of prestressing. The statistical significance of the 0.80 factor on live load is that the event is expected to occur about once a year for bridges with two traffic lanes, less often for bridges with more than two traffic lanes, and about once a day for bridges with a single traffic lane. The calibration of the service limit states for concrete components (Wassef et. al. 2014) concluded that typical components designed using the Refined Estimates of Time-Dependent Losses method incorporated in the specifications in 2005 have a lower reliability index against flexural cracking in prestressed components than components designed using the prestress loss calculation method specified prior to 2005. For 139

Service IV—Load combination relating only to tension in prestressed concrete columns with the objective of crack control. components designed using the currently-specified methods for instantaneous prestressing losses and the currently-specified Refined Estimates of Time- Dependent Losses method, an increase in the load factor for live load from 0.8 to 1.0 was required to maintain the level of reliability against cracking of prestressed concrete components inherent in the system. Components which design satisfies all of the following conditions: • A refined time step method is used for calculating the time-dependent prestressing losses • The section properties are determined based on the concrete gross section, and, • The force in prestressing steel is determined without taking advantage of the elastic gain, were not affected by the changes in the prestressing loss calculation method introduced in 2005. For these components, a load factor for live load of 0.8 was maintained. Service I should be used for checking tension related to transverse analysis of concrete segmental girders. The principal tensile stress check is introduced in order to verify the adequacy of webs of segmental concrete girder bridges for longitudinal shear and torsion axial load, longitudinal moment, longitudinal shear and torsion. 140

Table 3.4.1-1—Load Combinations and Load Factors Load Combination Limit State DC DD DW EH EV ES EL PS CR SH LL IM CE BR PL LS WA WS WL FR TU TG SE Use One of These at a Time EQ BL IC CT CV Strength I (unless noted) γp 1.75 1.00 — — 1.00 0.50/1.20 γTG γSE — — — — — Strength II γp 1.35 1.00 — — 1.00 0.50/1.20 γTG γSE — — — — — Strength III γp — 1.00 1.4 0 — 1.00 0.50/1.20 γTG γSE — — — — — Strength IV γp — 1.00 — — 1.00 0.50/1.20 — — — — — — — Strength V γp 1.35 1.00 0.4 0 1.0 1.00 0.50/1.20 γTG γSE — — — — — Extreme Event I γp γEQ 1.00 — — 1.00 — — — 1.00 — — — — Extreme Event II γp 0.50 1.00 — — 1.00 — — — — 1.00 1.00 1.00 1.00 Service I 1.00 1.00 1.00 0.3 0 1.0 1.00 1.00/1.20 γTG γSE — — — — — Service II 1.00 1.30 1.00 — — 1.00 1.00/1.20 — — — — — — — Service III 1.00 0.80 γLL 1.00 — — 1.00 1.00/1.20 γTG γSE — — — — — Service IV 1.00 — 1.00 0.7 0 — 1.00 1.00/1.20 — 1.0 — — — — — Fatigue I— LL, IM & CE only — 1.50 — — — — — — — — — — — — Fatigue II— LL, IM & CE only — 0.75 — — — — — — — — — — — — Table 3.4.1-4—Load Factors for Live Load for Service III Load Combination, γLL Component γLL Prestressed concrete components designed using a refined time step method to determine the time-dependent prestressing losses in conjunction with the gross section properties and without taking advantage of the elastic gain 0.8 All other prestressed concrete components 1.0 141

6.3 Fatigue Only Fatigue I Limit State is applicable to concrete and reinforcement. Information on the Fatigue II Limit state is included for reference and they were based on work done on steel components by Kulicki et al. (2013). 6.3.1 Proposed Revisions to Section 3 3.4—LOAD FACTORS AND COMBINATIONS Table 3.4.1-1—Load Combinations and Load Factors Load Combination Limit State DC DD DW EH EV ES EL PS CR SH LL IM CE BR PL LS WA WS WL FR TU TG SE Use One of These at a Time EQ BL IC CT CV Strength I (unless noted) γp 1.75 1.00 — — 1.00 0.50/1.20 γTG γSE — — — — — Strength II γp 1.35 1.00 — — 1.00 0.50/1.20 γTG γSE — — — — — Strength III γp — 1.00 1.4 0 — 1.00 0.50/1.20 γTG γSE — — — — — Strength IV γp — 1.00 — — 1.00 0.50/1.20 — — — — — — — Strength V γp 1.35 1.00 0.4 0 1.0 1.00 0.50/1.20 γTG γSE — — — — — Extreme Event I γp γEQ 1.00 — — 1.00 — — — 1.00 — — — — Extreme Event II γp 0.50 1.00 — — 1.00 — — — — 1.00 1.00 1.00 1.00 Service I 1.00 1.00 1.00 0.3 0 1.0 1.00 1.00/1.20 γTG γSE — — — — — Service II 1.00 1.30 1.00 — — 1.00 1.00/1.20 — — — — — — — Service III 1.00 0.80 1.00 — — 1.00 1.00/1.20 γTG γSE — — — — — Service IV 1.00 — 1.00 0.7 0 — 1.00 1.00/1.20 — 1.0 — — — — — Fatigue I— LL, IM & CE only — 1.50 2.0 — — — — — — — — — — — — Fatigue II— LL, IM & CE only — 0.75 0.80 — — — — — — — — — — — — 142

6.3.2 Proposed Revisions to Section 5 5.5.3.2—Reinforcing Bars The constant-amplitude fatigue threshold, (ΔF)TH, for straight reinforcement and welded wire reinforcement without a cross weld in the high-stress region shall be taken as: ( ) min24 0.33THF f∆ = − (5.5.3.2-1) ( ) min19 0.26THF f∆ = − (5.5.3.2-1) The constant-amplitude fatigue threshold, (ΔF)TH, for straight welded wire reinforcement with a cross weld in the high-stress region shall be taken as: ( ) min16 0.33THF f∆ = − (5.5.3.2-2) ( ) min13 0.26THF f∆ = − (5.5.3.2-2) where: fin = minimum live-load stress resulting from the Fatigue I load combination, combined with the more severe stress from either the permanent loads or the permanent loads, shrinkage, and creep-induced external loads; positive if tension, negative if compression (ksi) The definition of the high-stress region for application of Est. 5.5.3.2-1 and 5.5.3.2-2 for flexural reinforcement shall be taken as one-third of the span on each side of the section of maximum moment. C5.5.3.2 Bends in primary reinforcement should be avoided in regions of high-stress range. Structural welded wire reinforcement has been increasingly used in bridge applications in recent years, especially as auxiliary reinforcement in bridge I- and box beams and as primary reinforcement in slabs. Design for shear has traditionally not included a fatigue check of the reinforcement as the member is expected to be uncracked under service conditions and the stress range in steel minimal. The stress range for steel bars has existed in previous editions. It is based on Hansen et al. (1976). The simplified form in this edition replaces the (r/h) parameter with the default value 0.3 recommended by Hansen et al. Inclusion of limits for WWR is based on recent studies by Hawkins et al. (1971, 1987) and Tadros et al. (2004). Coefficients in Equations 5.5.3.2-1 and 5.5.3.2-2 have been updated based on calibration reported in Kulicki et al (2013). Since the fatigue provisions were developed based primarily on ASTM A615 steel reinforcement, their applicability to other types of reinforcement is largely unknown. Consequently, a cautionary note is added to the Commentary. 143