Connection of Simple-Span Precast Concrete Girders for Continuity (2004)

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C-1 APPENDIX C PROPOSED REVISIONS TO THE AASHTO LRFD BRIDGE DESIGN SPECIFICATIONS INTRODUCTION A major goal for this research project was to develop pro- posed revisions to the AASHTO LRFD Bridge Design Spec- ifications addressing the design of simple-span precast gird- ers made continuous. Existing Article 5.14.1.2.7 specifically addresses this type of construction. The proposed revisions incorporate many of the existing provisions and provide addi- tional requirements to more fully address design issues and to implement the findings of research. VERSION OF SPECIFICATIONS USED AS BASIS FOR REVISIONS The base text for the proposed revisions to Section 5 includes revisions approved at the 2001 and 2002 meetings of the AASHTO Subcommittee on Bridges and Structures and any subsequent minor revisions. PRESENTATION OF REVISIONS The proposed revisions are presented in a format as simi- lar to the actual specifications (including the commentary) as possible. Since the existing Article 5.14.1.2.7 is brief and the proposed revisions are much longer, the proposed revisions are presented in their final format without typographical con- ventions identifying the additions and deletions from the cur- rent article.

PROPOSED REVISIONS SECTION 5—CONTENTS Replace the current headings for the subarticles of Article 5.14.1.2.7 with the following: 5.14.1.2.7 Bridges Composed of Simple Span Precast Girders Made Continuous 5.14.1.2.7a General 5.14.1.2.7b Restraint Moments 5.14.1.2.7c Material Properties 5.14.1.2.7d Age of Girder when Continuity is Established 5.14.1.2.7e Degree of Continuity at Various Limit States 5.14.1.2.7f Service Limit State 5.14.1.2.7g Strength Limit State 5.14.1.2.7h Negative Moment Connections 5.14.1.2.7i Positive Moment Connections 5.14.1.2.7j Continuity Diaphragms 5.3 NOTATION Revise or add the following definitions: Ac = area of core of spirally reinforced compression member measured to the outside diameter of the spiral (IN2); gross area of concrete deck slab (IN2) (5.7.4.6) (C5.14.1.2.7c) As = area of nonprestressed tension reinforcement (IN2); total area of longitudinal deck reinforcement (IN2) (5.5.4.2.1) (C5.14.1.2.7c) Atr = area of concrete deck slab with transformed longitudinal deck reinforcement (IN2) (C5.14.1.2.7c) Ec deck = modulus of elasticity of deck concrete (KSI) (C5.14.1.2.7c) fpsl = stress in the strand at the SERVICE limit state. Cracked section shall be assumed. (KSI) (C5.14.1.2.7i) fpul = stress in the strand at the STRENGTH limit state. (KSI) (C5.14.1.2.7i)
dsh = total length of extended strand (IN) (C5.14.1.2.7i) n = modular ratio between deck concrete and reinforcement (C5.14.1.2.7c) εeffective = effective concrete shrinkage strain (IN/IN) (C5.14.1.2.7c) εsh = concrete shrinkage strain at a given time (IN/IN); unrestrained shrinkage strain for deck concrete (IN/IN) (5.4.2.3.3) (C5.14.1.2.7c) C-2

Replace the current subarticles of Article 5.14.1.2.7 with the following: 5.14 PROVISIONS FOR STRUCTURE TYPES 5.14.1 Beams and Girders 5.14.1.2 PRECAST BEAMS 5.14.1.2.7 Bridges Composed of Simple-Span Precast Girders Made Continuous 5.14.1.2.7a General When the requirements of Article 5.14.1.2.7 are satisfied, multi-span bridges composed of simple-span precast girders with continuity diaphragms cast between ends of girders at interior supports may be considered continuous for loads placed on the bridge after the continuity diaphragms are installed and have cured. Continuity diaphragms shall fill the gap between ends of precast girders and shall connect adjacent lines of girders. The connection between girders at the continuity diaphragm shall be designed for all effects that cause moment at the connection, including restraint moments from time-dependent effects, except as allowed in Article 5.14.1.2.7. The requirements specified in Article 5.14.1.2.7 supplement the requirements of other sections of these Specifications for fully prestressed concrete components that are not segmentally constructed. Multi-span bridges composed of precast girders with continuity diaphragms at interior supports that are designed as a series of simple spans are not required to satisfy the requirements of Article 5.14.1.2.7. If a negative moment connection is provided between precast girders and the continuity diaphragm is installed prior to placement of the deck slab, the girders may be considered to be continuous for the dead load of the deck slab and for any other applied loads placed on the noncomposite girder after continuity is established. C-3 C5.14.1.2.7a This type of bridge is generally constructed with a composite deck slab. However, with proper design and detailing, precast members used without a composite deck may also be made continuous for loads applied after continuity is established. Details of this type of construction are discussed in Miller et al. (2004). The designer may choose to design a multi-span bridge as a series of simple spans but detail it as continuous with continuity diaphragms to eliminate expansion joints in the deck slab. This approach has been used successfully in several parts of the country. Where this approach is used, the designer should consider adding reinforcement in the deck adjacent to the interior supports to control cracking that may occur from the continuous action of the structure. Positive moment connections improve the structural integrity of a bridge, increasing its ability to resist extreme events and unanticipated loadings. These connections also control cracking that may occur in the continuity diaphragm. Therefore, it is recommended that positive moment connections be provided in all bridges detailed as continuous for live load. See Article 5.14.1.2.7h. SPECIFICATIONS COMMENTARY

5.14.1.2.7b Restraint Moments The bridge shall be designed for restraint moments that may develop because of time-dependent or other deformations, except as allowed in Article 5.14.1.2.7d. C-4 C5.14.1.2.7b Deformations that occur after continuity is established from time-dependent effects such as creep, shrinkage, and temperature variation cause restraint moments. Restraint moments are computed at interior supports of continuous bridges but affect the design moments at all locations on the bridge (Mirmiran et al., 2001 a, b). SPECIFICATIONS COMMENTARY Figure C5.14.1.2.7b-1—Development of Restraint Moments at Interior Support for Two-Span Continuous AASHTO Type III Girder Bridge (Miller et al., 2004) Figure C-1 shows the predicted development of restraint moments at the interior support for a typical bridge with continuity established at a girder age of 28 and 90 days. The figure demonstrates that the age of the girder when continuity is established has a significant influence on the development of restraint moments. Positive restraint moments have the most significant effect on bridges of this type because they may cause cracking at the bottom of the continuity diaphragm and they may cause stresses to exceed the stress limits at critical locations in the span. Analysis predicts that positive restraint moments do not develop for all precast girders made continuous. Analysis also predicts that precast girder bridges with composite deck slabs that are made continuous will develop negative restraint moments because of differential shrinkage between the girders and the deck. The older the girders are at the time the deck is cast, the more severe the predicted negative moment development. However, the consequences of negative restraint moments on these bridges are not generally as significant as for positive restraint moments. Data from various projects (Miller et al., 2004 ; Russell et al., 2003) does not show the effects of differential shrinkage of the decks. Therefore, it is questionable whether negative moments due to differential shrinkage form to the extent predicted by analysis. Since field observations of significant negative moment distress -600 -400 -200 0 200 400 600 800 0 2000 4000 6000 8000 Girder Age (Days) R es tr ai nt M om en t (k -ft ) 28 Days 90 days

Restraint moments shall not be included when computing design quantities that are reduced when combined with the restraint moment. 5.14.1.2.7c Material Properties Creep and shrinkage properties of the girder concrete and the shrinkage properties of the deck slab concrete shall be determined from either: • Tests of concrete using the same proportions and materials that will be used in the girders and deck slab. Measurements shall include the time-dependent rate of change of these properties. • The provisions of Article 5.4.2.3. The restraining effect of reinforcement on concrete shrinkage may be considered. C-5 have not been reported, negative moments caused by differential shrinkage are often ignored in design. Even if the negative moment formation in Figure C1 is ignored, the curves still show that the later continuity is formed, the lower the predicted values of positive moment that will form. Therefore, waiting as long as possible after the girders are cast to establish continuity and cast the deck appears to be beneficial. Several methods have been published for computing restraint moments (Oesterle et al., 1989; Mirmiran et al., 2001 a, b). While these methods may be useful in estimating restraint moments, designers should be aware that these methods may overestimate the restraint moments—both positive and negative. Existing structures do not show the distress that would be expected from the moments computed by some analysis methods. Since estimated restraint moments are highly dependent on actual material properties and project schedules, the computed moment may never develop. Therefore, a critical design moment must not be reduced by a restraint moment in case the restraint moment does not develop. C5.14.1.2.7c The development of restraint moments is highly dependent on the creep and shrinkage properties of the girder and deck concrete. Since these properties can vary widely, measured properties should be used when available to obtain the most accurate analysis. However, these properties are rarely available during design. Therefore, the provisions of Article 5.4.2.3 may be used to estimate these properties. Because longitudinal reinforcement in the deck slab restrains the shrinkage of the deck concrete, the apparent shrinkage is less than the free shrinkage of the deck concrete. This effect may be estimated using an effective concrete shrinkage strain, εeffective, which may be taken as: εeffective = εsh (Ac/Atr) (C5.14.1.2.7c-1) where: εsh = unrestrained shrinkage strain for deck concrete (IN/IN) Ac = gross area of concrete deck slab (IN2) Atr = area of concrete deck slab with transformed longitudinal deck reinforcement (IN2) = Ac + As(n − 1) As = total area of longitudinal deck reinforcement (IN2) n = modular ratio between deck concrete and reinforcement = Es / Ec deck Ec deck = modulus of elasticity of deck concrete (KSI) SPECIFICATIONS COMMENTARY

5.14.1.2.7d Age of Girder when Continuity Is Established The minimum age of the precast girder when continuity is established shall be specified in the contract documents. This age shall be used for calculating restraint moments due to creep and shrinkage. If no age is specified, the girders shall be assumed to be 7 days old at the time continuity is established. The following simplification may be applied if the owner will permit this simplification to be used and if the contract documents require a minimum girder age of at least 90 days when continuity is established: • Positive restraint moments caused by girder creep and shrinkage and deck slab shrinkage may be taken to be as 0. • Computation of restraint moments shall not be required. • A positive moment connection shall be provided with a factored resistance, fMn, not less than 1.2 Mcr, as discussed in Article 5.14.1.2.7i. C-6 Equation C1 is based on simple mechanics (Abdalla et al., 1993). If the amount of longitudinal reinforcement varies along the length of the slab, the average area of longitudinal reinforcement may be used to calculate the transformed area. C5.14.1.2.7d Analytical studies show that the age of the precast girder when continuity is established is an important factor in the development of restraint moments (Mirmiran et al., 2001 a, b). These studies suggest that delaying continuity reduces or eliminates time-dependent positive restraint moments (see Figure C5.14.1.2.7b-1). The formation of positive restraint moments is largely the result of creep and shrinkage in the prestressed girder. The creep and shrinkage begin as soon as the prestressing force is applied, which may be less than 1 day after casting. If continuity is delayed, a larger fraction of the creep and shrinkage in the girder will occur prior to establishing continuity, so a lower positive moment will develop in the continuity diaphragm. In addition, allowing the girder to shrink before establishing continuity increases the differential shrinkage between the girder and deck. The differential shrinkage counteracts the formation of positive restraint moments. According to analysis, establishing continuity when girders are young causes larger positive moments to develop. Therefore, if no minimum girder age for continuity is specified, the earliest reasonable age must be used. Results from surveys of practice (Oesterle et al., 1989; Miller et al., 2004) show a wide variation in girder ages at which continuity is established. An age of 7 days was reported to be a realistic minimum. However, the use of 7 days as the age of girders when continuity is established results in a large positive restraint moment. Therefore, a specified minimum girder age at continuity of at least 28 days is strongly recommended. If girders are 90 days or older when continuity is established, the provisions of Section 5.4.2.3 predict that approximately 60% of the creep and 70% of the shrinkage in the girders, which could cause positive moments, has already occurred prior to establishing continuity. Since most of the creep and shrinkage in the girder has already occurred before continuity is established, the potential development of time-dependent positive moments is limited. Differential shrinkage between the deck and the girders, to the extent to which it actually occurs (see C5.14.1.2.7b) would also tend to limit positive moment development. Even if the girders are 90 days old or older when continuity is established, some positive moment may develop at the connection and some cracking may occur. Research (Miller et al., 2004) has shown that if the connection is designed with a capacity of 1.2 Mcr , the connection can tolerate this cracking without appreciable loss of continuity. SPECIFICATIONS COMMENTARY

5.14.1.2.7e Degree of Continuity at Various Limit States The connection between precast girders at a continuity diaphragm shall be considered fully effective if either of the following are satisfied: • The calculated stress at the bottom of the continuity diaphragm for the combination of superimposed permanent loads, settlement, creep, shrinkage, 50% live load and temperature gradient, if applicable, is compressive. • The contract documents require that the age of the precast girders shall be at least 90 days when continuity is established and the positive restraint moments are assumed to be 0 as allowed in Article 5.14.1.2.7d. If the connection between precast girders at a continuity diaphragm does not satisfy these requirements, the joint shall be considered partially effective. Superstructures with fully effective connections at interior supports may be designed as fully continuous structures at all limit states for loads applied after continuity is established. Superstructures with partially effective connections at interior supports shall be designed as continuous structures for loads applied after continuity is established for strength and extreme event limit states only. C-7 This provision provides a simplified approach to design of precast-girder bridges made continuous that eliminates the need to evaluate restraint moments. Some states allow design methods where restraint moments are not evaluated when continuity is established when girders are older than a specified age. These design methods have been used for many years with good success. However, an owner may require the computation of restraint moments for all girder ages. C5.14.1.2.7e A fully effective joint at a continuity diaphragm is a joint that is capable of full moment transfer between spans, resulting in the structure behaving as a continuous structure. In some cases, especially when continuity is established at an early girder age, continuing upward cambering of the girders due to creep may cause cracking at the bottom of the continuity diaphragm (Mirmiran et al., 2001 a, b). Analysis and tests indicate that such cracking may cause the structure to act as a series of simply supported spans when resisting some portion of the permanent or live loads applied after continuity is established; however, this condition only occurs when the cracking is severe and the positive moment connection is near failure (Miller et al., 2004). Where this occurs, the connections at the continuity diaphragm are partially effective. Theoretically, the portion of the permanent or live loads required to close the cracks would be applied to a simply supported span, neglecting continuity. The remainder of the load would then be applied to the continuous span, assuming full continuity. However, in cases where the portion of the live load required to close the crack is less than 50% of the live load, placing part of the load on simple spans and placing the remainder on the continuous bridge results in only a small change in total stresses at critical sections due to all loads. Tests have shown that the connections can tolerate some positive moment cracking and remain continuous (Miller et. al., 2004). Therefore, if the conditions of the first bullet point are satisfied, it is reasonable to design the member as continuous for the entire load placed on the structure after continuity is established. The second bullet follows from the requirements of Article 5.14.1.2.7d, where restraint moments may be neglected if continuity is established when the age of the precast girder is at least 90 days. Without positive moment, the potential cracks in the continuity diaphragm would not form and the connection would be fully effective. Partially effective construction joints are designed by applying the portion of the permanent and live loads applied after continuity is established required to close the assumed cracks to a simple span (neglecting continuity). SPECIFICATIONS COMMENTARY

Possible cracking of the deck due to negative moments may be neglected in the analysis, as specified in Article 4.5.2.2. If the negative moment resistance of the section at an interior support is less than the total amount required, the positive design moments in the adjacent spans shall be increased appropriately for each limit state investigated. 5.14.1.2.7f Service Limit State Simple-span precast girders made continuous shall be designed to satisfy service limit state stress limits given in Article 5.9.4. For service load combinations that involve traffic loading, tensile stresses in prestressed members shall be investigated using the Service III load combination specified in Table 3.4.1-1. At the service limit state after losses, when tensile stresses develop at the top of the girders near interior supports, the tensile stress limits specified in Table 5.9.4.1.2-1 for other than segmentally constructed bridges shall apply. The specified compressive strength of the concrete, f’c, shall be substituted for the compressive strength of concrete at the time of prestressing, f’ci, in the stress limit equations. The Service III load combination shall be used to compute tensile stresses for these locations. Alternatively, the top of the precast girders at interior supports may be designed as reinforced concrete members at the strength limit state. In this case, the stress limits for the service limit state shall not apply to this region of the precast girder. A cast-in-place composite deck slab shall not be subject to the tensile stress limits for the service limit state after losses specified in Table 5.9.4.2.2-1. 5.14.1.2.7g Strength Limit State The connections between precast girders and a continuity diaphragm shall be designed for the strength limit state. The reinforcement in the deck slab shall be proportioned to resist negative design moments at the strength limit state. 5.14.1.2.7h Negative Moment Connections The reinforcement in a cast-in-place, composite deck slab in a multispan precast girder bridge made continuous C-8 Only the portion of the loads required to close the assumed cracks are applied. The remainder of the permanent and live loads would then be applied to the continuous span. The load required to close the crack can be taken as the load causing zero tension at the bottom of the continuity diaphragm. Such analysis may be avoided if the contract documents require the age of the girder at continuity to be at least 90 days. C5.14.1.2.7f Tensile stresses under service limit state loadings may occur at the top of the girder near interior supports. This region of the girder is not a precompressed tensile zone, so there is not an applicable tensile stress limit in Table 5.9.4.2.2-1. Furthermore, the tensile zone is close to the end of the girder, so adding or debonding pretensioned strands has little effect in reducing the tensile stresses. Therefore, the limits specified for temporary stresses before losses have been used to address this condition, with modification to use the specified concrete strength. This provision provides some relief for the potentially high tensile stresses that may develop at the ends of girders because of negative service load moments. This option allows the top of the girder at the interior support to be designed as a reinforced concrete element using the strength limit state rather than a prestressed concrete element using the service limit state. The deck slab is not a prestressed element. Therefore, the tensile stress limits do not apply. It has been customary to apply the compressive stress limits to the deck slab. C5.14.1.2.7g The continuity diaphragm is not prestressed concrete, so the stress limits for the service limit state do not apply. Connections to it are therefore designed using provisions for reinforced concrete elements. C5.14.1.2.7h Research at PCA (Kaar et al., 1961) and years of experience show that the reinforcement in a composite SPECIFICATIONS COMMENTARY

shall be proportioned to resist negative design moments at the strength limit state. Requirements of Article 5.7.3 applicable to the strength limit state shall be satisfied. Longitudinal reinforcement used for the negative moment connection over an interior pier shall be anchored in regions of the slab that are in compression at strength limit states and shall satisfy the requirements of Article 5.11.1.2.3. The termination of this reinforcement shall be staggered. All longitudinal reinforcement in the deck slab may be used for the negative moment connection. Negative moment connections between precast girders into or across the continuity diaphragm shall satisfy the requirements of Article 5.11.5. These connections shall be permitted when the bridge is designed with a composite deck slab and shall be required when the bridge is designed without a composite deck slab. Additional connection details shall be permitted if the strength and performance of these connections is verified by analysis or testing. The requirements of Article 5.7.3.4 shall apply to the reinforcement in the deck slab and at negative moment connections at continuity diaphragms. 5.14.1.2.7i Positive Moment Connections Positive moment connections at continuity diaphragms shall be made with reinforcement developed into both the girder and continuity diaphragm. Two types of connections shall be permitted: • Mild reinforcement embedded in the precast girders and developed into the continuity diaphragm. • Pretensioning strands extended beyond the end of the girder and anchored into the continuity diaphragm. These strands shall not be debonded at the end of the girder. Additional requirements for connections made using each type of reinforcement are given in subsequent articles. The critical section for the development of positive moment reinforcement into the continuity diaphragm shall be taken at the face of the girder. The critical section for the development of positive moment reinforcement into the precast girder shall consider conditions in the girder as specified in this article for the type of reinforcement used. Reinforcement for the positive moment connection shall be proportioned to resist the calculated positive moment, except that the positive moment connection shall be proportioned to provide a factored capacity (fMn) of at least 0.6 Mcr. C-9 deck slab can be proportioned to resist negative design moments in a continuous bridge. Limited tests on continuous model and full-size structural components indicate that, unless the reinforcement is anchored in a compressive zone, its effectiveness becomes questionable at the strength limit state (Priestly and Tao, 1993). The termination of the longitudinal deck slab reinforcement is staggered to minimize potential deck cracking by distributing local force effects. A negative moment connection between precast girders and the continuity diaphragm is not typically provided because the deck slab reinforcement is usually proportioned to resist the negative design moments. However, research (Ma et al., 1998) suggests that mechanical connections between the tops of girders may also be used for negative moment connections, especially when continuity is established prior to placement of the deck slab. If a composite deck slab is not used on the bridge, a negative moment connection between girders is required to obtain continuity. Mechanical reinforcement splices have been successfully used to provide a negative moment connection between box beam bridges that do not have a composite deck slab. C5.14.1.2.7i Positive moment connections improve the structural integrity of a bridge, increasing its ability to resist extreme events and unanticipated loadings. Therefore, it is recommended that positive moment connections be provided in all bridges detailed as continuous for live load. Both embedded bar and extended strand connections have been used successfully to provide positive moment resistance. Test results (Miller et al., 2004) indicate that connections using the two types of reinforcement perform similarly under both static and fatigue loads and both have adequate strength to resist the applied moments. Analytical studies (Mirmiran et al., 2001 a, b) suggest that a minimum amount of reinforcement, corresponding to a capacity of 0.6 Mcr, is needed to develop adequate resistance to positive restraint moments. These same studies show that a positive moment connection with a capacity greater than 1.2 Mcr provides only minor improvement in continuity behavior over a connection with SPECIFICATIONS COMMENTARY

The cracking moment, Mcr, is computed using Equation 5.7.3.6.2-2 with the gross composite section properties for the girder and the effective width of composite deck slab, if any, and the material properties of the concrete in the continuity diaphragm. The precast girders must be designed for any positive restraint moments that are used in design. Near the ends of girders, the reduced effect of prestress within the transfer length shall be considered. Additional positive moment connection details shall be permitted if the strength and performance of these connections are verified by analysis or testing. The requirements of Article 5.7.3.4 shall apply to the reinforcement at positive moment connections at continuity diaphragms. • Positive Moment Connection Using Mild Reinforcement The anchorage of mild reinforcement used for positive moment connections shall satisfy the requirements of Article 5.11 and the additional requirements of this article. Where positive moment reinforcement is added between pretensioned strands, consolidation of concrete and bond of reinforcement shall be considered. The critical section for the development of positive moment reinforcement into the precast girder shall consider conditions in the girder. The reinforcement shall be developed beyond the inside edge of the bearing area. The reinforcement shall also be detailed so that debonding of strands does not terminate within the development length. Where multiple bars are used for a positive moment connection, the termination of the reinforcement shall be staggered. C-10 a capacity of 1.2 Mcr. Therefore, it is recommended that the positive moment capacity of the connection not exceed 1.2 Mcr. If the computed positive moment exceeds 1.2 Mcr, the section should be modified or steps should be taken to reduce the positive moment. The cracking moment, Mcr, is the moment that causes cracking in the continuity diaphragm. Since the continuity diaphragm is not a prestressed concrete section, the equation for computing the cracking moment for a reinforced section is used. The diaphragm is generally cast with the deck concrete, so the section properties are computed using uniform concrete properties and the deck width is not transformed. Article 5.7.3.3.2 specifies a minimum capacity for all flexural sections. This is to prevent sudden collapse at the formation of the first crack. However, the positive moment connection that is being discussed here is not intended to resist applied live loads. Even if the positive moment connection were to fail completely, the system may, at worst, become a series of simple spans. Therefore, the minimum reinforcement requirement of Article 5.7.3.3.2 does not apply. Allowing a positive moment connection with lower quantities of reinforcement will relieve congestion in continuity diaphragms. The positive moment connection is designed to utilize the yield strength of the reinforcement. Therefore, the connection must be detailed to provide full development of the reinforcement. If the reinforcement cannot be detailed for full development, the connection may be designed using a reduced stress in the reinforcement. Potential cracks are more likely to form in the precast girder at the inside edge of the bearing area and at locations of termination of debonding. Since cracking within the development length reduces the effectiveness of the development, the reinforcement should be detailed to avoid this condition. It is recommended that reinforcement be developed beyond the location where a crack radiating from the inside edge of the bearing may cross the reinforcement. The termination of the positive moment reinforcement is staggered to reduce the potential for cracking at the ends of the bars. SPECIFICATIONS COMMENTARY

• Positive Moment Connection Using Prestressing Strand Pretensioning strands that are not debonded at the end of the girder may be extended into the continuity diaphragm as positive moment reinforcement. The extended strands shall be anchored into the diaphragm by bending the strands into a 90o hook or by providing a development length as specified in Article 5.11.4. • Details of Positive Moment Connection Positive moment reinforcement shall be placed in a pattern that is symmetrical, or as nearly symmetrical as possible, about the centerline of the cross section. Fabrication and erection issues shall be considered in the detailing of positive moment reinforcement in the continuity diaphragm. Reinforcement from opposing girders shall be detailed to mesh during erection without significant conflicts. Reinforcement shall be detailed to enable placement of anchor bars and other reinforcement in the continuity diaphragm. 5.14.1.2.7j Continuity Diaphragms The design of continuity diaphragms at interior supports may be based on the strength of the concrete in the precast girders. C-11 Strands that are debonded or shielded at the end of a member may not be used as reinforcement for the positive moment connection. There are no requirements for development of the strand into the girder because the strands run continuously through the precast girder. The following equations for the development length of hooked 0.5 IN diameter prestressing strands were developed by Salmons (1974, 1980), Salmons and May (1974), and Salmons and McCrate (1973). Strands used for a positive moment connection shall project at least 8 IN from the face of the girder before they are bent. The stress in the strands used for design, as a function of the total length of extended strand, shall not exceed: fpsl = (
dsh – 8)/0.228 (5.14.1.2.7i-1) fpul = (
dsh – 8)/0.163 (5.14.1.2.7i-2) where:
dsh = total length of extended strand (IN) fpsl = stress in the strand at the SERVICE limit state. Cracked section shall be assumed. (KSI) fpul = stress in the strand at the STRENGTH limit state. (KSI) Other equations are available to estimate stress in nonprestressed bent strands (Noppakunwijai et al., 2002). Tests (Miller et al., 2004) suggest that reinforcement patterns containing significant asymmetry may result in unequal bar stresses that can be detrimental to the performance of the positive moment connection. With some girder shapes, it may not be possible to install prebent hooked bars without the hook tails interfering with the formwork. In such cases, a straight bar may be embedded and then bent after the girder is fabricated. Such bending is generally accomplished without heating, and the bend must be smooth with a minimum bend diameter conforming to the requirements of Table 5.10.2.3-1. If the engineer allows the reinforcement to be bent after the girder is fabricated, the contract documents shall indicate that field bending is permissible and shall provide requirements for such bending. Since requirements regarding field bending may vary, the preferences of the owner should be considered. Hairpin bars (a bar with an 180o bend with both legs developed into the precast girder) have been used for positive moment connections to eliminate the need for post-fabrication bending of the reinforcement and to reduce congestion in the continuity diaphragm. C5.14.1.2.7j The use of the increased concrete strength is permitted because the continuity diaphragm concrete between girder ends is confined by the girders and by the continuity SPECIFICATIONS COMMENTARY

The continuity diaphragm shall be detailed to provide the required anchorage of reinforcement extending from the precast girders into the continuity diaphragm. The sequence for concrete placement in the continuity diaphragms and deck slab shall be shown in the contract documents. Precast girders may be embedded into continuity diaphragms. If horizontal diaphragm reinforcement is passed through holes in the precast beam or is attached to the precast element using mechanical connectors, the end precast element shall be designed to resist positive moments caused by superimposed dead loads, live loads, creep and shrinkage of the girders, shrinkage of the deck slab, and temperature effects. Design of the end of the girder shall account for the reduced effect of prestress within the transfer length. Where ends of girders are not directly opposite each other across a continuity diaphragm, the diaphragm must be designed to transfer forces between girders. Continuity diaphragms shall also be designed for situations where an angle change occurs between opposing girders. C-12 diaphragm extending beyond the girders. It is recommended that this provision be applied only to conditions where the portion of the continuity diaphragm that is in compression is confined between ends of precast girders. The width of the continuity diaphragm must be large enough to provide the required embedment for the development of the positive moment reinforcement into the diaphragm. An anchor bar with a diameter equal to or greater than the diameter of the positive moment reinforcement may be placed in the corner of a 90o hook or inside the loop of a 180o hook bar to improve the effectiveness of the anchorage of the reinforcement. Several construction sequences have been successfully used for the construction of bridges with precast girders made continuous. When determining the construction sequence, the engineer should consider the effect of girder rotations and restraint as the deck slab concrete is being placed. Test results (Miller et al., 2004) have shown that embedding precast girders 6 IN into continuity diaphragms improves the performance of positive moment connections. The observed stresses in the positive moment reinforcement in the continuity diaphragm were reduced compared with connections without girder embedment. The connection between precast girders and the continuity diaphragm may be enhanced by passing horizontal reinforcement through holes in the precast beam or attaching the reinforcement to the beam by embedded connectors. Test results (Miller et al., 2004; Salmons, 1974, 1980; Salmons and May, 1974; and Salmons and McCrate, 1973) show that such reinforcement stiffens the connection. The use of such mechanical connections requires that the end of the girder be embedded into the continuity diaphragm. Tests of continuity diaphragms without mechanical connections between the girder and diaphragm show the failure of connection occurs by the beam end pulling out of the diaphragm with all of the damage occurring in the diaphragm. Tests of connections with horizontal bars show that cracks may form in the end of the precast girder outside the continuity diaphragm if the connection is subjected to a significant positive moment. Such cracking in the end region of the girder may not be desirable. A method such as given in Article 5.6.3 may be used to design a continuity diaphragm for these conditions. SPECIFICATIONS COMMENTARY

REFERENCES Add the following references to the list of references currently appearing at the end of Section 5: Abdalla, O.A., Ramirez, J.A. and, Lee, R.H., “Strand Debonding in Pretensioned Beams: Precast Prestressed Concrete Bridge Girders with Debonded Strands—Continuity Issues”, Joint Highway Research Project, Indiana Department of Transportation/Purdue University, FHWA/INDOT/JHRP-92-94, June 1993, 235 p. Hanson, N. W., “Precast-prestressed Concrete Bridges, 2. Horizontal Shear Connections”, Journal of the PCA Research and Development Laboratories, Vol. 2, No. 2, May 1960, pp. 38–58. Also reprinted as PCA Bulletin D35. Kaar, P.H., Kriz, L.B. and Hognestad, E., “Precast-prestressed Concrete Bridges, 1. Pilot Tests of Continuous Girders”, Journal of the PCA Research and Development Laboratories, Vol. 2, No. 2, May 1960, pp. 21–37. Also reprinted as PCA Bulletin D34. Kaar, P.H., Kriz, L.B. and Hognestad, E., “Precast-prestressed Concrete Bridges, 6. Test of Half-Scale Highway Bridge Continuous over Two Spans”, Journal of the PCA Research and Development Laboratories, Vol. 3, No. 3, Sept 1961, pp. 30–70. Also reprinted as PCA Bulletin D51. Ma, Z., Huo, X., Tadros, M.K. and Baishya, M., “Restraint Moments in Precast/Prestressed Concrete Continuous Bridges,” PCI Journal, Vol. 43, No. 6, Nov/Dec 1998, pp. 40–56. Mattock, A.H and Kaar, P.H., “Precast-prestressed Concrete Bridges, 3. Further Tests of Continuous Girders”, Journal of the PCA Research and Development Laboratories, Vol. 2, No. 3, Sept 1960, pp. 51–78. Also reprinted as PCA Bulletin D43. Mattock, A.H. and Kaar, P.H, “Precast-prestressed Concrete Bridges, 4. Shear Tests of Continuous Girders”, Journal of the PCA Research and Development Laboratories, Vol. 3, No. 1, Jan 1961, pp. 19–46. Also reprinted as PCA Bulletin D45. Mattock, A.H., “Precast-prestressed Concrete Bridges, 5. Creep and Shrinkage Studies”, Journal of the PCA Research and Development Laboratories, Vol. 3, No. 2, May 1961, pp. 32–66. Also reprinted as PCA Bulletin D46. Miller, R.A., Castrodale, R., Mirmiran, A. and Hastak, M., NCHRP Report 519: Connection of Simple-Span Precast Concrete Girders for Continuity, National Cooperative Highway Research Program Report, Transportation Research Board, National Research Council, Washington, DC, 2004. Mirmiran, A., Kulkarni, S., Castrodale, R., Miller, R. and Hastak, M., “Nonlinear Continuity Analysis of Precast, Prestressed Concrete Girders with Cast-in-Place Decks and Diaphragms”, PCI Journal, Vol. 46, No. 5, September–October 2001, pp. 60–80. Mirmiran, A., Kulkarni, S., Miller R.A., Hastak, M., Shahrooz, B. and Castrodale, R.C., “Positive Moment Cracking in Diaphragms of Simple Span Prestressed Girders Made Continuous, SP 204, American Concrete Institute, August 2001. Noppakunwijai, P., Jongpitakseel, N., Ma, Z. (John), Yehia, S.A., and Tadros, M.K., “Pullout Capacity of Non- Prestressed Bent Strands for Prestressed Concrete Girders,” PCI Journal, Vol. 47, No. 4, July–August 2002, pp. 90–103. Oesterle, R.G., Glikin, J.D. and Larson, S.C., NCHRP Report 322: Design of Precast-Prestressed Bridge Girders Made Continuous, Transportation Research Board, National Research Council, Washington, DC, November 1989, 97 p. Priestley, M.J.N. and Tao, J.R., “Seismic Response of Precast Prestressed Concrete Frames with Partially Debonded Tendons”, PCI Journal, Vol. 38, No. 1, January–February 1993, pp. 58–69. Russell, H., Ozyildirim, C., Tadros, M. and Miller, R., Compilation and Evaluation of Results from High Performance Concrete Bridge Projects, Project DTFH61-00-C-00009 Compact Disc, Federal Highway Administration, Washington, DC, 2003. C-13

Salmons, J.R., Behavior of Untensioned-Bonded Prestressing Strand, Final Report 77-1, Missouri Cooperative Highway Research Program, Missouri State Highway Department, June 1980, 73 p. Salmons, J.R., End Connections of Pretensioned I-Beam Bridges, Final Report 73-5C, Missouri Cooperative Highway Research Program, Missouri State Highway Department, Nov. 1974, 51 p. Salmons, J.R. and May, G.W., Strand Reinforcing for End Connection of Pretensioned I-Beam Bridges, Interim Report 73-5B, Missouri Cooperative Highway Research Program, Missouri State Highway Department, May 1974, 142 p. Salmons, J.R. and McCrate, T.E., Bond of Untensioned Prestress Strand, Interim Report 73-5A, Missouri Cooperative Highway Research Program, Missouri State Highway Department, Aug. 1973, 108 p. C-14