Design Guide for Bridges for Service Life (2013)

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587 Expansion joints are one of the main causes for high maintenance costs in bridges. A new seamless bridge system was envisioned within SHRP 2 Project R19A that should result in bridges with long service lives by eliminating the joints over the entire length of the bridge, approach slab, and a segment of the roadway (Ala and Azizinamini in press a and b). The system is similar to a system developed in Australia for use with continuously reinforced concrete pavements (CRCP) (Bridge et al. 2005). Proposed modifications have been made to the Australian system to adapt it to U.S. practice, in which most pavements are either jointed plain concrete pavement (JPCP) or flexible pavement (Ala 2011). Although pavement within a particular roadway may be jointed or flexible, the segment of roadway containing the bridge and the proposed seamless transition is similar in nature to CRCP. Therefore, transition details would be similar to those used when transitioning from CRCP to jointed or flexible pavements. The key factor is establishing an effective longitudinal force transfer mechanism from the transition slab to the base soil that minimizes the length of the transition. The goal is achieving limited end movements, a predictable and controlled crack pattern, and controlled axial forces in the system. The system developed to meet these needs is shown in Figure E.1. The transition slab is connected to a secondary slab that is embedded below. The two slabs are con- nected by a series of small piles. The secondary slab increases the stiffness of the transi- tion region, resulting in the desired short transition length. A similar system without the transition slab may lose its effectiveness after multiple cycles as a result of the compaction of the soil surrounding the small piles. A special reinforcement reduction detail is used over the length of the transition zone to achieve a controlled crack pattern when the bridge system is in tension. The system behavior in tension (temperature reduction–bridge contraction) is an important factor because the crack pattern plays a major role in design life and maintenance E DESiGN STEpS FOR SEAMLESS BRiDGE SySTEM DEvELOpED By ShRp 2 pROJECT R19A

588 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE costs. Figure E.2 shows a transition in which the reinforcement detail helps to main- tain the desirable crack pattern (Jung et al. 2007). The reinforcement is reduced over the length of the transition region as the force is reduced. TransitionBridge Road pavement Transition Zone Approach Zone Soil-nails Embeded Slab Bridge Approach Abutment Transition Zone JPCP Secondary Slab Small Piles Figure E.1. Schematic and rendering of the recommended practice for bridge–roadway interface. CRC PavementSaw Cuts or Induced Design Crack 100% Steel Zone 60% Steel Zone 30% Steel Zone Transition Transition JC Pavement Figure E.2. Gradual transition continuously reinforced to jointed pavement. Source: Jung et al. 2007.

589 Appendix E. DESiGN STEpS FOR SEAMLESS BRiDGE SySTEM DEvELOpED By ShRp 2 pROJECT R19A Analysis, design, and construction of a seamless bridge and approach slab system are similar to other bridge structures. However, some new components are involved in the system that are not typically seen in other bridge systems. The new compo- nents include a transition slab, secondary slab, small piles, and the connection of the small piles to the transition concrete slabs. Figure E.3 shows an example from the W10x49 4.0" 6. 0" 3. 0" 2. 0" PL16x16x1.0" 4.0" 2.5" CL 2.5" 5/8" Both Flange 3/8" 4.0"6.0" 16" 16" 58.0" W10x49 4.0" 6. 0" 3. 0" 2. 0" PL16x16x1.0" 4.0" 2.5" CL 2.5" 5/8" Both Flange 3/8" 4.0"6.0" 16" 16" 58.0" Figure E.3. Small piles to be used in the test to connect the upper and lower slabs.

590 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE developmental phase of the concept of the small piles used to connect the transition slab to the secondary slab. The initial system design is an iterative process in which the length of the transition and secondary slabs; the shape, size, and spacing of small piles; and the embedment depth of the secondary slab are determined via structural analyses of various sys- tem configurations. Demand in all components is determined during the initial design phase. The various parts of the system may then be designed according to the applica- ble LRFD Bridge Design Specifications (LRFD specifications) (AASHTO 2012). The reduced cracked stiffness of the system in tension may be neglected in the initial design. Relevant pavement design loadings are the longitudinal strains (thermal effects, creep, and shrinkage) and the out-of-plane effects caused by traffic wheel loads, settlement of approach embankments, and rotational effects transferred from the bridge deck. E.1 StructurAL AnALySiS Until further research is completed to develop a simplified analysis approach, the seam- less bridge should be analyzed as a holistic system with all components incorporated into the analysis. To account for the effect of temperature changes in design of the transition region of the seamless bridge system, only the effect of uniform temperature change needs to be considered. The calculation of uniform temperature change should be in accordance with LRFD specifications Article 3.12.2. The interaction between the soil and the small piles can be modeled in the structural analysis by using springs. The spring stiffness around the small piles highly depends on the relative density of the compacted soil material (geomaterial) surrounding the small piles and the confinement pressure. Because the soil material is manually compacted, the relative density of the compacted soil needs to be measured during the compaction process, and this compac- tion should be related to the soil stiffness. The connection of the small piles to the slabs can be assumed rigid for analysis purposes. The structural analysis should take into account the effects of longitudinal stiffness reduction due to cracking of the transition slab in tension (temperature reduction– bridge contraction). Iterative structural analyses of the seamless bridge and roadway system in conjunction with cracked section analyses are required. For the first itera- tion, the tensile forces in the structure are assumed equal to the compressive forces due to thermal expansion. Cracked section analyses are carried out for various segments of the transition slab, and the axial stiffness of the slab segments are modified. The structure is analyzed with the modified in-plane stiffness to determine the in-plane ten- sile axial forces in the system. This process is repeated until convergence of the axial forces is achieved. E.2 deSign oF SyStem comPonentS Once the initial system design has been completed, design of the individual system components can be performed.

591 Appendix E. DESiGN STEpS FOR SEAMLESS BRiDGE SySTEM DEvELOpED By ShRp 2 pROJECT R19A E.2.1 Approach Slab and Bridge Deck Extra reinforcement may be required in the approach slab for crack control under the tensile in-plane forces resulting from thermal expansion. The bridge deck also has to be checked for cracking. The approach slab should be checked for compressive thermal stresses to avoid concrete crushing. The approach slab should be designed for the differential settlement of the bridge abutment and the transition system. Embank- ment settlement is another important criterion to check for the approach slab. The bridge approach embankments should be designed to achieve a long-term settlement of less than 3/4 in. to minimize traffic comfort issues on the motorway pavement. To account for the probable geotechnical and construction variations, however, a more conservative approach embankment settlement of 1.5 in. should be assumed for the seamless pavement design (Thomas Telford Service Ltd. 1993). E.2.2 transition Slab The main objective of providing a transition slab is to provide a means for controlling the movement of the system to the point at which no expansion devices are needed where the transition slab meets the pavement. The design items for the transition slab include designing against compressive force created by thermal expansion; achieving a uniform cracking pattern in the transition zone during contraction, preventing punch- ing shear failure at the pile-to-slab connection area; and ensuring adequate flexural capacity at these locations. The thickness of the transition slab should be determined on the basis of (1) the punching shear requirements, (2) the connection requirements for developing the moment introduced from the small piles, and (3) the in-plane hori- zontal stiffness of the system to reduce the movement of the end joint. Reinforce- ment of the transition slab should be determined from cracked section analysis under tensile in-plane forces. The transition slab should be checked for the maximum bend- ing moments between the rows of small piles. Stirrups (tie bars) may be required for the connection to the slab around the ends of the small piles. The transition slab should also be designed for the design truck-axle load exerted at the midspan of the slab between the small piles. Both slabs should be designed for punching shear and one-way shear. Detailed design provisions are provided in VTrans (2009) and also Ala and Azizinamini (in press a). E.2.3 Secondary Slab The length of the secondary slab should be greater than or equal to the length of the transition slab. Likewise, the secondary slab thickness is designed for punching shear and requirements to develop the moment introduced from the small piles into the slab (the secondary slab thickness will most likely be equal to the transition slab). The sec- ondary slab should also be designed for the bending moment due to the soil pressure underneath. This slab should be designed for punching and one-way shear.

592 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE E.2.4 Small Piles The stiffness, number, and arrangement of small piles connecting the transition and secondary slabs should be determined to control the longitudinal movement of the transition slab at the end where it meets the pavement. This limit eliminates expansion joint devices at these locations. Increasing the stiffness of the small piles will reduce the longitudinal movement of the transition slab at the end of transition zone. However, piles with high flexural stiffness will also create high stresses (tension or compression) in the transition slab and bridge deck, in addition to the secondary slab. Therefore, the design of small piles should consider a balance between longitudinal movement at the end of transition slab and the maximum longitudinal force that can be accommodated in the transition slab and bridge deck. Small piles with high stiffness will demand more sophisticated connection details to the transition and secondary slabs. The maximum longitudinal movement at the end of the transition slab, where it meets the pavement, should be limited to about 0.25 in. E.2.5 Connection of Small Piles to Slabs The connection design for attaching the small piles to the top (transition) and bottom (secondary) slabs should use high factors of safety and ensure that they stay elastic when the weak element of the entire system fails. This design concept is similar to the philosophy used in seismic design in which some of the bridge elements are protected and remain elastic while plastic hinges form in other parts of the structure. The con- nection should be designed for cyclic loading, as the system will be subjected to daily and seasonal temperature fluctuation. Figure E.4 shows one possible connection detail that was used during the experimental phase. The experimental results indicated that the area around the connection could have a larger thickness or, alternatively, could use advanced materials such as ultrahigh-performance concrete. Research is needed to develop more economical connection details. Concrete SlabStirrups Bars Studs Baseplate Figure E.4. Recommended small pile–concrete slab connection.

593 Appendix E. DESiGN STEpS FOR SEAMLESS BRiDGE SySTEM DEvELOpED By ShRp 2 pROJECT R19A E.2.6 geomaterial Design of the geomaterial consists of the selection of the geomaterial type and the com- paction requirement. The required compaction depends on the stiffness requirement around the small piles. It is very important to achieve the required compaction (level and consistency) around the small piles, so stringent quality control is required during soil compaction. It is highly recommended to use granular material in this region for ease of compaction, which will result in smaller long-term settlement and smaller gap development around the piles caused by pile movements. Moisture density relation (compaction) tests, maximum and minimum density (relative density) tests, and in-place moisture content and density determinations dur- ing placement of the backfill (using a nuclear moisture density meter) are the recom- mended soil mechanics tests. E.3 crAcked Section AnALySiS Methods of determining the maximum probable crack width and stiffness reduction for an axially tensioned concrete member are explained in ACI Report 224.2R-92 (ACI 1997). The maximum probable crack width (Wmax) in a fully cracked member can be determined from Equation E.1: W f d A0.10 10 s cmax 3 3= × − (E.1) where dc = distance from center of bar to extreme tension fiber (in.); fs = service stress in the reinforcement (ksi); and A = effective tension area of concrete surrounding the tension reinforcement, having the same centroid as the reinforcement, divided by the number of bars (in.2). Figure E.5 demonstrates the calculation of A. S is the bar spacing, and H is the total thickness of the slab. Figure E.5. Determination of effective tension area of concrete surrounding the tension reinforcement for an axially tensioned concrete member.

594 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE As shown in Figure E.5, the parameter d Ac 3 can be determined to be d S d2c c 3 for both one and two layers of reinforcement. The crack width allowed is inserted in Equation E.1 to obtain the service stress and strain in the reinforcement. The LRFD specifications define an exposure factor (γe) that is 1.00 for a Class 1 exposure condition and 0.75 for a Class 2 exposure condition. The crack width associ- ated with Class 1 and 2 exposure conditions are 0.017 and 0.012, respectively. The amount of required reinforcing steel can be determined from the axial tensile force (P) in the member (determined from the structural analysis), as shown by Equa- tion E.2: P f A A P f .s s s s = ⇒ = (E.2) where As is the reinforcement steel area. The axial force in various segments of the transition slab (P) is determined from structural analysis. The transition slab may crack when it is in tension, which will result in reduction of axial stiffness. The reduced axial stiffness of the cracked transition slab should be used in structural analysis, requiring an iterative cracked section analysis. In this iterative analysis the section axial stiffness is modified on the basis of the axial force determined from the previous analysis. Next, the structure is analyzed using the modified axial stiffness. This process is repeated until convergence. For the first itera- tion, the slab can be assumed uncracked (the tensile force can be taken the same as the compressive force developed in the slab due to temperature increase). The following equations describe the method for determining the reduced axial stiffness of the concrete member in tension. ACI Report 224.2R-92 (1997) suggests using Equation E.3 for determining the direct tensile strength of the concrete ( ft′ ): f f0.33t c c 1 2γ[ ]′ = ⋅ ′ (E.3) where: From the LRFD specifications, for a given fc′ , the unit weight can be determined from gc = 0.14 + 0.0001 fc′ , and Ec can be determined from E K f33000c c c1 1.5γ= ′ . The stress in the reinforcing bars after the crack occurs fs cr,( )′ is determined from ACI Report 224.2R-92 (1997), as shown by Equation E.4: f f p n 1 1s cr t,′ = ′ − +     (E.4) where ρ is the reinforcing ratio (As/Ag), and n is the modular ratio of steel to concrete. The axial load that causes first cracking in the axially tensioned member is shown by Equation E.5: P f Acr s cr s,= ′ ⋅ (E.5)

595 Appendix E. DESiGN STEpS FOR SEAMLESS BRiDGE SySTEM DEvELOpED By ShRp 2 pROJECT R19A During the cracked section analysis, if the force in a segment of the slab is smaller than Pcr, the slab will not crack, and no modification will be required in the structural analysis. Otherwise, if the force in a segment of the transition slab exceeds the above Pcr, the segment will crack, and the modified axial stiffness of the segment should be determined and used for the next iteration. For a cracked section, the average strain in the tensile member can be calculated from the CEB-FIP Model Code (Thomas Telford Service Ltd. 1993), using Equa- tion E.6 to determine the modified axial stiffness: k f f 1m s s cr s , 2 ε ε= −             (E.6) where es = fs/Es, k = 1.0 for the first loading and 0.5 for repeated or sustained loading. For the seamless system, k = 0.5 should be used. Equation E.7 provides the effective modulus of elasticity of steel bars: E E k f f 1 sm s c cr s , 2 = −             (E.7) The effective axial cross-sectional stiffness of the tensile concrete member can be written as (EA)eff = EsmAs. The ratio term (EA)eff/(EA) is the section modification factor that should be used in the structural analysis to modify the axial stiffness of the mem- ber in tension. Figure E.6 shows the general flowchart for the cracked section analysis. Figure E.6. Cracked section analysis flowchart.