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C-1 APPENDIX C PROPOSED REVISIONS TO THE AASHTO LRFD BRIDGE DESIGN SPECIFICATIONS INTRODUCTION of the AASHTO Subcommittee on Bridges and Structures and any subsequent minor revisions. 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 18.104.22.168.7 specifically PRESENTATION OF REVISIONS addresses this type of construction. The proposed revisions incorporate many of the existing provisions and provide addi- The proposed revisions are presented in a format as simi- tional requirements to more fully address design issues and lar to the actual specifications (including the commentary) as to implement the findings of research. possible. Since the existing Article 22.214.171.124.7 is brief and the proposed revisions are much longer, the proposed revisions are presented in their final format without typographical con- VERSION OF SPECIFICATIONS USED AS BASIS FOR REVISIONS ventions identifying the additions and deletions from the cur- rent article. The base text for the proposed revisions to Section 5 includes revisions approved at the 2001 and 2002 meetings
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C-2 PROPOSED REVISIONS SECTION 5--CONTENTS Replace the current headings for the subarticles of Article 126.96.36.199.7 with the following: 188.8.131.52.7 Bridges Composed of Simple Span Precast Girders Made Continuous 184.108.40.206.7a General 220.127.116.11.7b Restraint Moments 18.104.22.168.7c Material Properties 22.214.171.124.7d Age of Girder when Continuity is Established 126.96.36.199.7e Degree of Continuity at Various Limit States 188.8.131.52.7f Service Limit State 184.108.40.206.7g Strength Limit State 220.127.116.11.7h Negative Moment Connections 18.104.22.168.7i Positive Moment Connections 22.214.171.124.7j 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) (126.96.36.199) (C188.8.131.52.7c) As = area of nonprestressed tension reinforcement (IN2); total area of longitudinal deck reinforcement (IN2) (184.108.40.206.1) (C220.127.116.11.7c) Atr = area of concrete deck slab with transformed longitudinal deck reinforcement (IN2) (C18.104.22.168.7c) Ec deck = modulus of elasticity of deck concrete (KSI) (C22.214.171.124.7c) fpsl = stress in the strand at the SERVICE limit state. Cracked section shall be assumed. (KSI) (C126.96.36.199.7i) fpul = stress in the strand at the STRENGTH limit state. (KSI) (C188.8.131.52.7i) dsh = total length of extended strand (IN) (C184.108.40.206.7i) n = modular ratio between deck concrete and reinforcement (C220.127.116.11.7c) effective = effective concrete shrinkage strain (IN/IN) (C18.104.22.168.7c) sh = concrete shrinkage strain at a given time (IN/IN); unrestrained shrinkage strain for deck concrete (IN/IN) (22.214.171.124.3) (C126.96.36.199.7c)
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C-3 SPECIFICATIONS COMMENTARY Replace the current subarticles of Article 188.8.131.52.7 with the following: 5.14 PROVISIONS FOR STRUCTURE TYPES 5.14.1 Beams and Girders 184.108.40.206 PRECAST BEAMS 220.127.116.11.7 Bridges Composed of Simple-Span Precast Girders Made Continuous 18.104.22.168.7a General C22.214.171.124.7a When the requirements of Article 126.96.36.199.7 are This type of bridge is generally constructed with a satisfied, multi-span bridges composed of simple-span composite deck slab. However, with proper design and precast girders with continuity diaphragms cast between detailing, precast members used without a composite deck ends of girders at interior supports may be considered may also be made continuous for loads applied after continuous for loads placed on the bridge after the continuity is established. Details of this type of construction continuity diaphragms are installed and have cured. are discussed in Miller et al. (2004). 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 188.8.131.52.7. The requirements specified in Article 184.108.40.206.7 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 The designer may choose to design a multi-span continuity diaphragms at interior supports that are designed bridge as a series of simple spans but detail it as continuous as a series of simple spans are not required to satisfy the with continuity diaphragms to eliminate expansion joints in requirements of Article 220.127.116.11.7. 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. If a negative moment connection is provided between See Article 18.104.22.168.7h. 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.
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C-4 SPECIFICATIONS COMMENTARY 22.214.171.124.7b Restraint Moments C126.96.36.199.7b The bridge shall be designed for restraint moments Deformations that occur after continuity is established that may develop because of time-dependent or other from time-dependent effects such as creep, shrinkage, deformations, except as allowed in Article 188.8.131.52.7d. 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). 800 Restraint Moment (k-ft) 600 400 200 0 -200 28 Days -400 90 days -600 0 2000 4000 6000 8000 Girder Age (Days) Figure C184.108.40.206.7b-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
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C-5 SPECIFICATIONS COMMENTARY 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. Restraint moments shall not be included when Since estimated restraint moments are highly computing design quantities that are reduced when dependent on actual material properties and project combined with the restraint moment. 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. 220.127.116.11.7c Material Properties C18.104.22.168.7c Creep and shrinkage properties of the girder concrete The development of restraint moments is highly and the shrinkage properties of the deck slab concrete dependent on the creep and shrinkage properties of the shall be determined from either: girder and deck concrete. Since these properties can vary widely, measured properties should be used when · Tests of concrete using the same proportions and available to obtain the most accurate analysis. However, materials that will be used in the girders and deck these properties are rarely available during design. slab. Measurements shall include the time-dependent Therefore, the provisions of Article 22.214.171.124 may be used rate of change of these properties. to estimate these properties. · The provisions of Article 126.96.36.199. The restraining effect of reinforcement on concrete Because longitudinal reinforcement in the deck slab shrinkage may be considered. 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, effective, which may be taken as: effective = sh (Ac /Atr) (C188.8.131.52.7c-1) where: sh = 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)
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C-6 SPECIFICATIONS COMMENTARY 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. 184.108.40.206.7d Age of Girder when Continuity Is Established C220.127.116.11.7d The minimum age of the precast girder when continuity Analytical studies show that the age of the precast is established shall be specified in the contract documents. girder when continuity is established is an important factor This age shall be used for calculating restraint moments in the development of restraint moments (Mirmiran et al., due to creep and shrinkage. If no age is specified, the 2001 a, b). These studies suggest that delaying continuity girders shall be assumed to be 7 days old at the time reduces or eliminates time-dependent positive restraint continuity is established. moments (see Figure C18.104.22.168.7b-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. The following simplification may be applied if the owner If girders are 90 days or older when continuity is will permit this simplification to be used and if the contract established, the provisions of Section 22.214.171.124 predict that documents require a minimum girder age of at least approximately 60% of the creep and 70% of the shrinkage 90 days when continuity is established: in the girders, which could cause positive moments, has · Positive restraint moments caused by girder creep and already occurred prior to establishing continuity. Since most shrinkage and deck slab shrinkage may be taken to be of the creep and shrinkage in the girder has already as 0. occurred before continuity is established, the potential · Computation of restraint moments shall not be development of time-dependent positive moments is limited. required. Differential shrinkage between the deck and the girders, to · A positive moment connection shall be provided with a the extent to which it actually occurs (see C126.96.36.199.7b) factored resistance, fMn, not less than 1.2 Mcr, as would also tend to limit positive moment development. discussed in Article 188.8.131.52.7i. 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.
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C-7 SPECIFICATIONS COMMENTARY 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. 184.108.40.206.7e Degree of Continuity at Various Limit States C220.127.116.11.7e The connection between precast girders at a continuity A fully effective joint at a continuity diaphragm is a joint diaphragm shall be considered fully effective if either of the that is capable of full moment transfer between spans, following are satisfied: resulting in the structure behaving as a continuous structure. In some cases, especially when continuity is · The calculated stress at the bottom of the continuity established at an early girder age, continuing upward diaphragm for the combination of superimposed cambering of the girders due to creep may cause cracking permanent loads, settlement, creep, shrinkage, 50% at the bottom of the continuity diaphragm (Mirmiran et al., live load and temperature gradient, if applicable, is 2001 a, b). Analysis and tests indicate that such cracking compressive. may cause the structure to act as a series of simply supported spans when resisting some portion of the · The contract documents require that the age of the permanent or live loads applied after continuity is precast girders shall be at least 90 days when established; however, this condition only occurs when the continuity is established and the positive restraint cracking is severe and the positive moment connection is moments are assumed to be 0 as allowed in Article near failure (Miller et al., 2004). Where this occurs, the 18.104.22.168.7d. connections at the continuity diaphragm are partially effective. Theoretically, the portion of the permanent or live If the connection between precast girders at a loads required to close the cracks would be applied to a continuity diaphragm does not satisfy these requirements, simply supported span, neglecting continuity. The remainder the joint shall be considered partially effective. 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 22.214.171.124.7d, 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. 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 Partially effective construction joints are designed by interior supports shall be designed as continuous structures applying the portion of the permanent and live loads for loads applied after continuity is established for strength applied after continuity is established required to close the and extreme event limit states only. assumed cracks to a simple span (neglecting continuity).
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C-8 SPECIFICATIONS COMMENTARY 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. Possible cracking of the deck due to negative moments may be neglected in the analysis, as specified in Article 126.96.36.199. 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. 188.8.131.52.7f Service Limit State C184.108.40.206.7f 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 Tensile stresses under service limit state loadings may stresses develop at the top of the girders near interior occur at the top of the girder near interior supports. This supports, the tensile stress limits specified in Table region of the girder is not a precompressed tensile zone, 220.127.116.11.2-1 for other than segmentally constructed bridges so there is not an applicable tensile stress limit in Table shall apply. The specified compressive strength of the 18.104.22.168.2-1. Furthermore, the tensile zone is close to the concrete, f' c, shall be substituted for the compressive end of the girder, so adding or debonding pretensioned strength of concrete at the time of prestressing, f' ci, in the strands has little effect in reducing the tensile stresses. stress limit equations. The Service III load combination Therefore, the limits specified for temporary stresses before shall be used to compute tensile stresses for these losses have been used to address this condition, with locations. 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. Alternatively, the top of the precast girders at interior This option allows the top of the girder at the interior supports may be designed as reinforced concrete support to be designed as a reinforced concrete element members at the strength limit state. In this case, the stress using the strength limit state rather than a prestressed limits for the service limit state shall not apply to this region concrete element using the service limit state. of the precast girder. A cast-in-place composite deck slab shall not be The deck slab is not a prestressed element. Therefore, subject to the tensile stress limits for the service limit state the tensile stress limits do not apply. It has been customary after losses specified in Table 22.214.171.124.2-1. to apply the compressive stress limits to the deck slab. 126.96.36.199.7g Strength Limit State C188.8.131.52.7g The connections between precast girders and a The continuity diaphragm is not prestressed concrete, continuity diaphragm shall be designed for the strength so the stress limits for the service limit state do not apply. limit state. Connections to it are therefore designed using provisions The reinforcement in the deck slab shall be for reinforced concrete elements. proportioned to resist negative design moments at the strength limit state. 184.108.40.206.7h Negative Moment Connections C220.127.116.11.7h The reinforcement in a cast-in-place, composite deck Research at PCA (Kaar et al., 1961) and years of slab in a multispan precast girder bridge made continuous experience show that the reinforcement in a composite
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C-9 SPECIFICATIONS COMMENTARY shall be proportioned to resist negative design moments deck slab can be proportioned to resist negative design at the strength limit state. Requirements of Article 5.7.3 moments in a continuous bridge. applicable to the strength limit state shall be satisfied. Longitudinal reinforcement used for the negative Limited tests on continuous model and full-size structural moment connection over an interior pier shall be anchored components indicate that, unless the reinforcement is in regions of the slab that are in compression at strength anchored in a compressive zone, its effectiveness becomes limit states and shall satisfy the requirements of Article questionable at the strength limit state (Priestly and Tao, 18.104.22.168.3. The termination of this reinforcement shall be 1993). The termination of the longitudinal deck slab staggered. All longitudinal reinforcement in the deck slab reinforcement is staggered to minimize potential deck may be used for the negative moment connection. cracking by distributing local force effects. Negative moment connections between precast girders A negative moment connection between precast into or across the continuity diaphragm shall satisfy the girders and the continuity diaphragm is not typically requirements of Article 5.11.5. These connections shall be provided because the deck slab reinforcement is usually permitted when the bridge is designed with a composite proportioned to resist the negative design moments. deck slab and shall be required when the bridge is designed However, research (Ma et al., 1998) suggests that without a composite deck slab. Additional connection mechanical connections between the tops of girders may details shall be permitted if the strength and performance also be used for negative moment connections, especially of these connections is verified by analysis or testing. 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. The requirements of Article 22.214.171.124 shall apply to the reinforcement in the deck slab and at negative moment connections at continuity diaphragms. 126.96.36.199.7i Positive Moment Connections C188.8.131.52.7i Positive moment connections at continuity diaphragms Positive moment connections improve the structural shall be made with reinforcement developed into both the integrity of a bridge, increasing its ability to resist extreme girder and continuity diaphragm. Two types of connections events and unanticipated loadings. Therefore, it is shall be permitted: recommended that positive moment connections be provided in all bridges detailed as continuous for live load. · Mild reinforcement embedded in the precast girders Both embedded bar and extended strand connections and developed into the continuity diaphragm. have been used successfully to provide positive moment resistance. Test results (Miller et al., 2004) indicate that · Pretensioning strands extended beyond the end of the connections using the two types of reinforcement perform girder and anchored into the continuity diaphragm. similarly under both static and fatigue loads and both have These strands shall not be debonded at the end of the adequate strength to resist the applied moments. 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 Analytical studies (Mirmiran et al., 2001 a, b) suggest shall be proportioned to resist the calculated positive that a minimum amount of reinforcement, corresponding moment, except that the positive moment connection shall to a capacity of 0.6 Mcr, is needed to develop adequate be proportioned to provide a factored capacity (fMn) of at resistance to positive restraint moments. These same least 0.6 Mcr. 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
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C-10 SPECIFICATIONS COMMENTARY 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 computed using Equation The cracking moment, Mcr, is the moment that causes 184.108.40.206.2-2 with the gross composite section properties for cracking in the continuity diaphragm. Since the continuity the girder and the effective width of composite deck slab, diaphragm is not a prestressed concrete section, the if any, and the material properties of the concrete in the equation for computing the cracking moment for a continuity diaphragm. 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 220.127.116.11.2 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 18.104.22.168.2 does not apply. Allowing a positive moment connection with lower quantities of reinforcement will relieve congestion in continuity diaphragms. 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 22.214.171.124 shall apply to the reinforcement at positive moment connections at continuity diaphragms. · Positive Moment Connection Using Mild Reinforcement The positive moment connection is designed to utilize The anchorage of mild reinforcement used for positive the yield strength of the reinforcement. Therefore, the moment connections shall satisfy the requirements of connection must be detailed to provide full development of Article 5.11 and the additional requirements of this article. the reinforcement. If the reinforcement cannot be detailed Where positive moment reinforcement is added between for full development, the connection may be designed pretensioned strands, consolidation of concrete and bond using a reduced stress in the reinforcement. of reinforcement shall be considered. Potential cracks are more likely to form in the precast The critical section for the development of positive girder at the inside edge of the bearing area and at moment reinforcement into the precast girder shall consider locations of termination of debonding. Since cracking within conditions in the girder. The reinforcement shall be the development length reduces the effectiveness of the developed beyond the inside edge of the bearing area. The development, the reinforcement should be detailed to avoid reinforcement shall also be detailed so that debonding of this condition. It is recommended that reinforcement be strands does not terminate within the development length. 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 Where multiple bars are used for a positive moment is staggered to reduce the potential for cracking at the ends connection, the termination of the reinforcement shall be of the bars. staggered.
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C-11 SPECIFICATIONS COMMENTARY · Positive Moment Connection Using Prestressing Strand Pretensioning strands that are not debonded at the end Strands that are debonded or shielded at the end of a of the girder may be extended into the continuity diaphragm member may not be used as reinforcement for the positive as positive moment reinforcement. The extended strands moment connection. There are no requirements for shall be anchored into the diaphragm by bending the development of the strand into the girder because the strands into a 90o hook or by providing a development strands run continuously through the precast girder. length as specified in Article 5.11.4. 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 (126.96.36.199.7i-1) fpul = (dsh 8)/0.163 (188.8.131.52.7i-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). · Details of Positive Moment Connection Positive moment reinforcement shall be placed in a Tests (Miller et al., 2004) suggest that reinforcement pattern that is symmetrical, or as nearly symmetrical as patterns containing significant asymmetry may result in possible, about the centerline of the cross section. unequal bar stresses that can be detrimental to the performance of the positive moment connection. Fabrication and erection issues shall be considered in With some girder shapes, it may not be possible to the detailing of positive moment reinforcement in the install prebent hooked bars without the hook tails continuity diaphragm. Reinforcement from opposing girders interfering with the formwork. In such cases, a straight bar shall be detailed to mesh during erection without may be embedded and then bent after the girder is significant conflicts. Reinforcement shall be detailed to fabricated. Such bending is generally accomplished without enable placement of anchor bars and other reinforcement heating, and the bend must be smooth with a minimum in the continuity diaphragm. bend diameter conforming to the requirements of Table 184.108.40.206-1. 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. 220.127.116.11.7j Continuity Diaphragms C18.104.22.168.7j The design of continuity diaphragms at interior The use of the increased concrete strength is permitted supports may be based on the strength of the concrete in because the continuity diaphragm concrete between girder the precast girders. ends is confined by the girders and by the continuity
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C-12 SPECIFICATIONS COMMENTARY 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 continuity diaphragm shall be detailed to provide The width of the continuity diaphragm must be large the required anchorage of reinforcement extending from enough to provide the required embedment for the the precast girders into the continuity diaphragm. 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. The sequence for concrete placement in the continuity Several construction sequences have been diaphragms and deck slab shall be shown in the contract successfully used for the construction of bridges with documents. 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. Precast girders may be embedded into continuity Test results (Miller et al., 2004) have shown that diaphragms. 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. If horizontal diaphragm reinforcement is passed The connection between precast girders and the through holes in the precast beam or is attached to the continuity diaphragm may be enhanced by passing precast element using mechanical connectors, the end horizontal reinforcement through holes in the precast precast element shall be designed to resist positive beam or attaching the reinforcement to the beam by moments caused by superimposed dead loads, live loads, embedded connectors. Test results (Miller et al., 2004; creep and shrinkage of the girders, shrinkage of the deck Salmons, 1974, 1980; Salmons and May, 1974; and slab, and temperature effects. Design of the end of the Salmons and McCrate, 1973) show that such reinforcement girder shall account for the reduced effect of prestress stiffens the connection. The use of such mechanical within the transfer length. 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. Where ends of girders are not directly opposite each A method such as given in Article 5.6.3 may be used other across a continuity diaphragm, the diaphragm must to design a continuity diaphragm for these conditions. be designed to transfer forces between girders. Continuity diaphragms shall also be designed for situations where an angle change occurs between opposing girders.
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C-13 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. 3858. 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. 2137. 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. 3070. 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. 4056. 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. 5178. 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. 1946. 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. 3266. 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, SeptemberOctober 2001, pp. 6080. 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, JulyAugust 2002, pp. 90103. 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, JanuaryFebruary 1993, pp. 5869. 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.
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C-14 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.