Design Guide for Bridges for Service Life (2013) / Chapter Skim
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Pages 578-588
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From page 578... ...
576 This appendix provides methods of estimating the restraining moment developed in prestressed girders when girders are made continuous over supports, as well as methods to mitigate the problem. D.1 bAckground In simple-span noncomposite bridges, time-dependent deformations result in little or no change in the distribution of forces and moments within the structure.
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From page 579... ...
577 Appendix D RESTRAiNT MOMENTS Deck Reinforcement M M = Positive Restraint Moment Positive Reinforcement M M Figure D.1.
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From page 580... ...
578 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE of the bridge may be reduced significantly with long-term and time-dependent loading effects and with thermal effects. In the experimental part of the jointless bridge research (Oesterle et al.
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From page 581... ...
579 Appendix D RESTRAiNT MOMENTS application of live load that would tend to produce a negative moment at the support, the crack at the bottom of the diaphragm concrete has to close before full negative moment develops.
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From page 582... ...
580 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE live load continuity in the NCHRP 12-53 study was assessed using change in reactions with application of live load. It is not clear how restraint moment present in the test specimen connection was considered.
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From page 583... ...
581 Appendix D
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From page 584... ...
582 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE to demonstrate that restraint moment is near zero if the girder age is at least 90 days when continuity is established did not include the effects of thermal gradient. Also, although the commentary to LRFD specifications Article 5.14.1.4.2 mentions temperature variation as a cause of restraint moments, Article 5.14.1.4 does not specifically address design considerations for thermal effects.
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From page 585... ...
583 Appendix D RESTRAiNT MOMENTS positive moment connection by the restraint of the abutment to longitudinal temperature movements.
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From page 586... ...
584 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Because of the uncertainty associated with calculations of positive continuity moments resulting from the variability of the creep and shrinkage effects, temperature gradient, differential coefficient of expansion effects, locked-in heat of hydration effects, settlement, and cracking, calculations to determine restraint moments are complex and probably unreliable. To eliminate the need to attempt to calculate restraint moments and to simplify the design, the following recommendations (Options 1 and 2 below)
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From page 587... ...
585 Appendix D RESTRAiNT MOMENTS that Mcr should be determined using the properties of the diaphragm concrete.
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From page 588... ...
586 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE The parametric studies in the FHWA jointless bridge project indicate that stresses in both the concrete deck slab and steel beams are not excessive under the combination of dead and live load forces combined with positive restraint moments. Consequently, explicit calculations considering positive restraint moments are not necessary.
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