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277 C RECOMMENDED ABC DESIGN SPECIFICATIONS 5.14.6 PROVISIONS FOR DESIGN OF PREFABRICATED SYSTEMS FOR ACCELERATED BRIDGE CONSTRUCTION 5.14.6.1 General The design of most modular systems for rapid renewal follows traditional LRFD Design Specifi cations. The requirements specifi ed herein shall supplement the require- ments of other sections of the LRFD Design Specifi cations for the design of prefabri- cated modular systems for rapid renewal. These requirements apply to precast concrete components and prefabricated composite steel girder systems. The design of bridges built using large-scale prefabrication is not specifi cally cov- ered in the AASHTO LRFD Bridge Design Specifi cations. When lifting prefabricated components, the location of the support points need to be identifi ed and accounted for in the design, including dynamic effects. 5.14.6.2 Design Objectives 5.14.6.2.1âRideability The provisions of LRFD 2.5.2.4âRideability shall be applicable with the follow- ing additions: Construction tolerances, with regard to the profi le and cross-slope of the fi nished deck, shall be indicated on the plans or in the specifi cations or special provisions. Where concrete decks without an initial overlay are used, consideration should be given to providing an additional minimum thickness of 0.5 in. to permit correction of the deck profi le by grinding, and to compensate for thickness loss due to abrasion. For precast decked concrete girder bridges, where the deck is part of the initial precast section, consideration should be given to either increasing this allowance or providing
278 INNOVATIVE BRIDGE DESIGNS FOR RAPID RENEWAL: ABC TOOLKIT a variable thickness deck to permit correction of the deck profile due to effects of camber. Overlay could be considered as an alternative to address the effects of camber. 5.14.6.2.2âDeformations Stresses and deflections shall be computed to control the integrity of the modular components during lifting and transportation. The engineer of record (EOR) shall define deformation controls suitable for each span. For steel or prestressed concrete modular systems, for the purposes of monitoring the structure under fabrication, lifting, transportation and setting in the final location, it is recommended that the EOR determine the anticipated deflection profile for the following conditions when spanning the temporary supports or pick points: ⢠Under the self-weight (and prestress) of the composite beams and diaphragms. ⢠Of the composite superstructure and with addition of superimposed dead load from barriers, parapets, medians or sidewalks. The above deflection conditions can be calculated using any appropriate calcula- tion technique based upon elastic analysis. For all the above, for precast prestressed or post-tensioned beams, take into account the age of the concrete at the time the opera- tion is assumed to take place. Under the initial lift condition, ensure that the anticipated flexural tensile stress induced in the top of the structural concrete slab for the assumed support locations is no greater than 0.125 ksi or 0.19âfâ²cm (ksi) where fâ²cm = anticipated strength of concrete at the time of the initial lift operation. If the above conditions cannot be satisfied, then it is recommended that the assumed locations of the lifting points be revised. 5.14.6.3 Loads and Load Factors 5.14.6.3.1âDefinitions ⢠Camber Leveling ForceâA vertically applied force used to equalize differential camber between prefabricated elements in a prefabricated modular structural sys- tem prior to establishing continuity or connectivity between the elements. ⢠Dynamic Dead Load AllowanceâAn increase or decrease in the self-weight of components to account for inertial effects during handling and transportation of prefabricated elements. 5.14.6.3.2âLoad and Load Designation CL = Camber leveling force (kip) C = Locked-in force effects due to load applied to erected prefabricated elements to correct misalignment due to differential camber prior to establishing continuity
279 INNOVATIVE BRIDGE DESIGNS FOR RAPID RENEWAL: ABC TOOLKIT 5.14.6.3.3âLoad Factors and Combinations When camber leveling forces, CL, are considered and they increase the critical effect in the design of the member, the load factor in all Service Load Combinations shall be taken as specified for DC in Table 3.4.1.1-1. Where camber leveling forces act to reduce the critical effect being considered, the load factor shall be taken as 0.0. 5.14.6.3.4âLoad Factors for Construction Loads This AASHTO LRFD Section 3.4.2 addresses the Strength Limit State and Service Limit State checks for construction loads. The following additional requirements for LRFD Section 3.4.2 are extended to apply to prefabricated elements and modular systems (concrete and steel composite). These additional requirements are invoked to guard against damage or permanent distortions to the modular system during handling and placement. 1. The Designer shall analyze spans on the assumed temporary/lifting supports based on the Strength I Limit State with a load factor equal to 1.25. 2. When investigating Strength Load Combinations I during construction, load fac- tors for the weight of the structure and appurtenances, DC and DW, as well as applied camber leveling load, CL, shall not be taken to be less than 1.25. 3. When evaluating prefabricated components or individual elements of modular sys- tems during construction, a dynamic dead load allowance of 15%, acting up or down, shall be applied to all dead load present at the time of handling and trans- portation. A reduced value may be used at the discretion of the Owner or when measures are taken to minimize inertial effects during transportation. 4. The Designer shall also check the spans to be brought into service for displace- ments based on Service I Limit State. Service stresses in the span while being handled and placed shall have a service load factor on dead load of 1.30 (handling impact factor). If a rigorous structural analysis allowing for the three-dimensional effects of inadvertent twist during transportation is undertaken and included, the service load handling impact factor may be reduced to 1.05. No factored loads shall be used for deflection calculations. 5. No permanent distortion (twist) as a result of handling and placement will be allowed. 6. Contract Documents shall include a completed table of âanticipated deflectionsâ as discussed in LRFD Section 3.4.2.2. 7. Plan notes for construction loads shall include âthe magnitude and locationâ of construction loads considered in the design as outlined in LRFD Section C3.4.2.1. 8. The bridge is not subject to seismic loadings (Extreme Event Limit State) while under construction. 9. The bridge is not subject to Service III limit state while under construction. Bridges analyzed carrying construction equipment shall utilize Service I with a 5% impact factor.
280 INNOVATIVE BRIDGE DESIGNS FOR RAPID RENEWAL: ABC TOOLKIT 5.14.6.4 Analysis LRFD Section 4.5 Mathematical Modeling provides general guidance for math- ematical modeling of bridges. The following additional requirements are extended to apply to prefabricated concrete and steel composite modular systems: 1. Prefabricated elements and modular systems are to be analyzed based on elastic behavior for handling and placement. Inelastic analysis will not be permitted. 2. The analysis may consider the influence of continuous composite precast barriers and rails on the behavior of modular systems during handling and placement. 3. Analysis of modular systems may be based on approximate or refined methods in accordance with AASHTO LRFD Bridge Design Specifications. 4. Contract Plans shall state that all formwork for the deck shall be supported from the longitudinal girders similar to conventional construction methods. Shored con- struction shall not be assumed. Decked girder systems shall be designed to accom- modate future deck replacement without the use of shoring during deck removal and replacement operations. 5.14.6.5 Control of Cracking (Non-Prestressed Components) LRFD Section 5.7.3.4âControl of Cracking by Distribution of Reinforcement addresses requirements for all reinforced concrete members. It is extended to apply to prefabricated elements and systems. 1. Provisions specified in LRFD Article 5.7.3.4 for the distribution of tension rein- forcement to control flexural cracking shall apply to all prefabricated elements and systems at the Service I Limit State. 2. The longitudinal reinforcement in the deck and superimposed attached items like sidewalks, parapets and traffic railings shall be analyzed. 5.14.6.6 Lifting and Handling Stresses (Non-Prestressed Components) Specify maximum tensile stress in non-prestressed precast concrete components during transportation, handling and erection under the Service I load combination. A 30% handling impact factor on dead loads shall be assumed. As an alternate, we can specify that precast components be handled in a manner that restricts the crack widths to acceptable limits. The lifting inserts should be so arranged that when the element is lifted it remains stable and the bottom edge remains horizontal. The positions of lifting inserts are calculated to limit lifting stresses and to ensure that the precast element hangs in the correct orientation during lifting. Check the potential for lateral instability during transportation and erection. Analysis of lifting and handling stresses shall be based on the recommended lifting points shown on the plans. The minimum concrete strength at which precast elements can be lifted should be specified on the plans.
281 INNOVATIVE BRIDGE DESIGNS FOR RAPID RENEWAL: ABC TOOLKIT 5.14.6.7 Prestressed Components Requirements of LRFD Section 5.9.4âStress Limits for Concrete shall be modi- fied as follows for modular systems: Minimum compressive strength at time of handling f â²cm should be specified on the plans. 5.9.4.1â For Temporary Stresses Before Lossesâ Fully Prestressed Components 5.9.4.1.2âTension Stresses Modify second bullet of Table 5.9.4.1.2-1 for âOther Than Segmentally Con- structed Bridgesâ: 1. In areas other than the precompressed tensile zone and without bonded reinforcement, and in top flanges of noncomposite prestressed components that will serve as the riding surface in the finished bridge Add to Table 5.9.4.1.2-1 for âOther Than Segmentally Constructed Bridgesâ: 2. For handling stresses in the top flange of noncompos- ite prestressed components that will serve as the riding surface in the finished bridge 0.24 âf â²cm (ksi) 5.9.4.2â For Stresses at Service Limit State After Lossesâ Fully Prestressed Components 5.9.4.2.1âCompression Stresses This section addresses compression stresses in prestressed concrete members. It is extended to apply to prefabricated elements and systems. LRFD Table 5.9.4.2.1-1 the third bullet shall apply to prestressed girder ele- ments and modular systems during shipping and handling with a Ïw = 1.0. 5.9.4.2.2âTension Stresses This section addresses tension stresses in prestressed concrete. It is extended to apply to prefabricated elements and systems. Prestressing losses may be calculated by either the Approximate or Refined methods in AASHTO LRFD Articles 5.9.5.3 and 5.9.5.4. Service III is for tension limits subject to normal anticipated highway âtraffic loadingâ. These loadings do not include nor do they apply to construction vehicles. Use Service I for construction loadings. During design, the actual scheduling of construction is not known. Since the age of the members can have a significant effect on the stresses early on, conservative assumptions must be made to ensure that the design stresses are for the worst case scenario. Add to Table 5.9.4.2.2-1 for âOther Than Segmentally Constructed Bridgesâ: 3. For components subjected to locked-in effects due to application of camber leveling forces No tension
282 INNOVATIVE BRIDGE DESIGNS FOR RAPID RENEWAL: ABC TOOLKIT 5.11.5.3.1âLap Splices in Tension This section specifies a minimum of 12 in. length for lap splices in tension. The minimum length requirement may be waived if demonstrated by test results on a specimen representing the proposed joint design using UHPC. An experimentally determined development length may be used as the basis for the joint design. 5.14.6.8 Design of the Grouted Splice Coupler The AASHTO LRFD Bridge Design Specifications Article 5.11.5.2.2 requires that all mechanical reinforcing splice devices develop 125% of the specified yield strength of the bar. Several manufacturers produce grouted splice couplers that can meet and exceed this requirement. If this requirement is met, the coupler can be treated the same as a reinforcing lap splice. 5.14.6.9 Provisions for Joints The following sections modify applicable sections of Section 5 of the LRFD Bridge Design Specifications: 5.14.4.3.3dâLongitudinal Construction Joints For longitudinal joints designed as shear-flexure joints without transverse post- tensioning that are also required to resist forces due to differential camber between adjacent components, the key shall be filled with an approved concrete. Minimum compressive strength and time required to attain the minimum compressive strength shall be specified on the plans. The applied camber leveling force shall not be removed until the joint is capable of resisting shear due to differential camber. Grinding for profile or cross-slope correction shall not begin until the concrete has attained the specified minimum compressive strength. 5.14.4.3.3eâCast-in-Place Closure Joint Concrete in the closure joint should have strength comparable to that of the pre- cast components. The width of the longitudinal joint shall be large enough to accom- modate development of reinforcement in the joint. Where development sufficient for anchorage of the reinforcement can be demonstrated by test results on a specimen representing the proposed joint design, the width of the joint can be based upon an experimentally determined development length plus a clear distance between the joint reinforcement and the nearest concrete surface adequate for concrete placement in the joint. Otherwise, the joint width shall not be less than 12.0 in. 5.14.6.10 Provisions for Steel Composite Systems This AASHTO subsection addresses requirements for the design of composite steel modular systems. The following sections modify applicable sections of Section 6 Steel Structures of the LRFD Bridge Design Specifications: 6.7.4.1âDiaphragms and Cross Frames This section addresses the location of diaphragms and cross frames in steel structures. The following additional requirements are extended to apply to prefab- ricated elements and systems.
283 INNOVATIVE BRIDGE DESIGNS FOR RAPID RENEWAL: ABC TOOLKIT 1. In interior lift points for composite modular system shall be considered an interior support. 2. At interior supports provide either a diaphragm or a cross-frame with nec- essary stiffeners as appropriate for bracing, connections and local bearing. The designer should address suitable diaphragm or cross-frame details to provide the necessary compression flange stability under temporary handling conditions. 3. Investigation shall include the stability of compression flanges during han- dling and placement. Diaphragms or cross-frames required for the construc- tion condition may be specified to be temporary bracing. 6.10.1.1.1aâSequence of Loading This section addresses loads applied to a steel structure. The following addi- tional requirements are extended to apply to prefabricated steel modular systems. 1. Shored construction as allowed in the last sentence of this section is not al- lowed for spans assembled using steel modular systems. 2. Contract Plans shall state that forming and shoring of the deck shall be sup- ported from the longitudinal girders similar to conventional construction methods.