National Academies Press: OpenBook

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

Chapter: Chapter 4 - Conclusions and Suggested Research

« Previous: Chapter 3 - Interpretation, Appraisal, and Application
Page 53
Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2004. Connection of Simple-Span Precast Concrete Girders for Continuity. Washington, DC: The National Academies Press. doi: 10.17226/13746.
×
Page 53
Page 54
Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2004. Connection of Simple-Span Precast Concrete Girders for Continuity. Washington, DC: The National Academies Press. doi: 10.17226/13746.
×
Page 54
Page 55
Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2004. Connection of Simple-Span Precast Concrete Girders for Continuity. Washington, DC: The National Academies Press. doi: 10.17226/13746.
×
Page 55
Page 56
Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2004. Connection of Simple-Span Precast Concrete Girders for Continuity. Washington, DC: The National Academies Press. doi: 10.17226/13746.
×
Page 56

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

51 CHAPTER 4 CONCLUSIONS AND SUGGESTED RESEARCH CONCLUSIONS This research detailed studies on connections for precast/ prestressed concrete bridge girders made continuous for live load. From the research, the following conclusions can be drawn. A survey was conducted of state DOTs, fabricators, and designers concerning connections for precast/prestressed con- crete bridge girders made continuous for live load. The results of the survey show the following: • Of 51 respondents, 35 had used, fabricated, or designed continuous-for-live-load bridges. • In all but one case, negative moment continuity was established by the use of a reinforced concrete deck slab. • Positive moment connections are used with the continuous-for-live-load bridges to control positive moments caused by creep and shrinkage of the precast girders. In almost all cases, the positive moment connec- tion is made by using bent-strand or bent-bar connections. Of those who used positive moment connections, 80% also embedded the ends of the girders into the diaphragm. • While the respondents reported some problems with con- structability, all indicated that the problems were minor. • Limited cost data was obtained, and it showed that the costs of providing continuous-for-live-load connections were insignificant. The highest cost was $200 per girder. Positive moment connections should be proportioned to resist the moments caused by creep and shrinkage of the gird- ers and deck slab, live load, and temperature effects. How- ever, the results of an analytical study show that providing positive moment connections with a capacity above 1.2 Mcr (where Mcr is the positive cracking moment of the composite cross section) is not efficient. It is suggested that if analysis shows the positive moment connection needs a capacity above 1.2 Mcr, steps be taken to reduce the formation of pos- itive moments. The easiest way to do this is to specify a min- imum age of the girders at the formation of continuity to allow some of the girder creep and shrinkage to occur before continuity is created. At present, there is no accepted method for designing the bent-strand connection. In this study, the number of strands and the extended length of strand needed were found from equations developed by Salmons and others (8–10, 13). Con- nections designed using these equations were found to have adequate strength. The connections fail by the strands pulling out of the concrete. In embedded connections, there is also a pull-out failure when the girder pulls out of the diaphragm. Since the equations developed by Salmons and his coauthors appear to be adequate for design of the bent-strand connec- tion, they have been placed in the commentary of the pro- posed revisions to the AASHTO LRFD Bridge Design Spec- ifications, which is found in Appendix C. Bent-bar connections designed such that the embedment of the bar into the girder and the embedment of the hooks into the diaphragm meet the provisions of the AASHTO LRFD Specifications (12) have adequate strength. There was a con- cern that congestion in the diaphragm area might reduce the capacity because of bar interactions, difficulty in consolidat- ing the concrete, or both. It was noted in the experiments that bent-bar-type connections must have the bars placed asym- metrically with respect to the cross section to allow meshing of the bars. This asymmetry causes asymmetrical responses in the connection. The connection fails by yield of the steel fol- lowed by a pull-out failure in the diaphragm. The pull-out fail- ure occurred in both embedded and nonembedded specimens. Embedding the girder ends in the diaphragm appears to reduce the stresses in the positive moment area; but, because this effect relies on the bond of a cold (or construction) joint, the effect is difficult to quantify. Additional stirrups placed in the diaphragm, just outside of the girders, do not increase the strength of the connection. However, these stirrups cross the diagonal cracks that form in the diaphragm when the final pull-out failure occurs, and the stirrups increase the ductility of the connection after the connection fails. Bars placed horizontally through the webs of the girders increase both the strength and the ductility of the connection. However, there is significant cracking in the girders at fail- ure, and this may be undesirable. To limit tensile stresses in the diaphragm, some state DOTs pour part of the diaphragm before the deck slab is cast. It is thought that the weight of the deck slab will cause the girder ends to rotate into the diaphragm and compress it. This method was found to be only marginally effective. However, it could possibly be improved if a tension tie was provided at the top of the girders.

52 When the deck slab is cast, it heats up while still plastic because of the heat of hydration. After the deck slab sets, it cools and contracts. This contraction causes the formation of negative moments at the connection. At present, analytical models do not account for this negative moment formation. Because this negative moment causes compression of the con- nection, it helps to mitigate later positive moment formation. Temperature effects on the system are significant. Daily temperature changes caused end reactions to vary ±20% per day. Seasonal temperature variations also affect the behavior of the bridge. At present, few design methods account for temperature effects, but they can be as significant as the live- load effects. Current analytical models predict that differential shrink- age between the deck slab and the girders should cause neg- ative moment formation at the connection in some systems. For a particular bridge, the predicted development of this negative moment depends on a number of factors, such as the creep and shrinkage properties of the girder and slab, the amount of prestressing force, the dead load, and the span length. In this project, the analytical models predicted that a negative moment should have formed for full-size Specimen 1 and that there should have been a gradual downward deflection of the girders and a decrease in the end reactions. This was not observed. The data showed that the end reac- tions gradually increased, which indicates positive moment formation. An examination of data from other projects (26) confirms that differential shrinkage of the deck does not seem to cause a negative moment formation. In the other projects, the girders were anywhere from 60 to 300 days old when continuity was established. With girders at these ages when continuity is established, the analytical models predict that differential shrinkage should cause negative moments, which would be accompanied by a downward deflection of the gird- ers. However, since the observed deflections remained almost constant (except for daily temperature-induced changes), neg- ative moments do not appear to have developed. This finding has a potentially significant impact on the girder and connec- tion design. The negative moment predicted by the models reduces the predicted maximum positive moment. If the neg- ative moment does not form, the models may underpredict the positive moment formation. However, if the negative moment formation is ignored (by choosing a very low value of deck shrinkage), the models may predict unrealistically high val- ues for the positive moment. The presence of positive moment cracking at the diaphragm does not necessarily reduce continuity as predicted by ana- lytical models. The analytical models treat the joint between the girder and the diaphragm as monolithic when it is really a cold joint. As a result, the analytical models tend to over- predict the load at which the cracks form. However, the mod- els also predict that when the joint cracks, the crack will immediately propagate into the slab and continuity will be reduced. This does not happen. The crack propagates slowly along the joint, and as long as the connection remains elas- tic, continuity is maintained. When the positive moment con- nection approaches failure—which is signified by large crack openings at the diaphragm, cracking on the bottom of the diaphragm, diagonal (pull-out) cracking in the diaphragm, and propagation of the joint crack into the slab—there is an observed loss of continuity of 20% to 30%. This is consistent with models because, at this point, the cracking in the speci- men matched that predicted by the models. The presence of positive moment cracking does not affect negative moment capacity of the connection. However, if the positive moment cracking extends into the slab, the negative cracking moment is reduced. COMPARISONS WITH PREVIOUS RESEARCH Previous research done on the continuous moment connec- tions was published as NCHRP Report 322 (11). This report concluded that positive moment connections were of no struc- tural benefit. If a positive moment connection is not used, the formation of time-dependent positive moments at the dia- phragm due to creep and shrinkage will crack the joint. This cracking will release the positive moments, but will cause the joint to behave as a hinge. Continuity will be lost, and the gird- ers will behave as simple spans for both dead and live loads. If a positive moment connection is used, the connection will resist cracking at the diaphragm and preserve continuity, and the midspan positive live-load moment in the girders will be lower than in the simple-span case. However, because the con- nection resists rather than releases the time-dependent positive moments, it causes time-dependent positive moments to form throughout the girder. The total midspan positive moment is then the sum of the dead-load moment (assumed simple span), the live-load moment (assumed continuous), and the time- dependent positive moments. NCHRP Report 322 concluded that this total midspan positive moment was virtually identi- cal to the midspan positive moment obtained by assuming simple spans for all loads. Thus, the conclusion is that posi- tive moment connections provided no benefit. NCHRP Report 322 was an analytical study and, as with any research study, considered only a limited set of parame- ters, mostly related to the creep and shrinkage characteristics of the concrete girders and deck (11). The conclusions of the NCHRP Report 322 study are valid, within the limits of that study; however, the actual continuous system is more com- plex. Positive moment formation at the diaphragm is influ- enced by many factors including girder size and length, the amount of prestressing force, the age of the girders at the for- mation of continuity, the construction sequence, the use of partial diaphragms, contraction of the deck because of cool- ing after casting, deck reinforcement, and thermal effects. In some cases, it is possible to have a system in which there is little or no time-dependent positive moment formation at the diaphragm. In other cases, the time-dependent moments formed at the diaphragm are negative. Therefore, a blanket

53 statement that positive moment connections are not struc- turally beneficial may not be justified in all cases. As with any design, the decision on the use of positive moment con- nections should be based on a rational engineering analysis of the moments formed in the entire system. However, even in cases in which analysis shows that the use of positive moment connections does not reduce the midspan positive moments, the positive moment connection may still be useful. The results of this research show these connections, when properly designed, are robust. These connections will contribute to the overall structural integrity of the system, especially in cases in which supports are damaged. PROPOSED REVISIONS TO THE AASHTO LRFD BRIDGE DESIGN SPECIFICATIONS Precast/prestressed concrete bridges made continuous are addressed in Section 5.14.1.2.7 of the AASHTO LRFD Bridge Design Specifications (12). Appendix C contains proposed revisions to this section. The proposed revisions are as follows: 1. A more complete definition of precast/prestressed con- crete bridges made continuous is proposed. 2. A significant feature of precast/prestressed concrete bridges made continuous is the development of time- dependent moments. The existing specifications do not address methods of analysis for determining these time-dependent moments or suggested values for time- dependent material properties. Based on the informa- tion gained in the literature search and the analytical studies done as part of this project, suggested time- dependent material properties and analysis methods are presented. 3. The current specifications do not address the effect of the age of the girder(s) when continuity is established. Information obtained in this study from literature, sur- veys, and analytical work shows the girder age is of great importance in determining the time-dependent moments that form in the system. The literature search, surveys, and analytical work also shows that if the gird- ers are more than 90 days old when continuity is formed, it is unlikely that positive time-dependent moments (called “restraint moment” in the proposed specifica- tions) will form. The proposed specifications discuss the effect of girder age on the analysis and on the for- mation of positive time-dependent moments. 4. When cracking occurs at the connection between precast/prestressed concrete girders made continuous, it is possible that the continuity between adjacent spans will be lost. The current specifications require that this effect be considered, but provide no guidance. Based on the experimental work in this study, the connections between the precast/prestressed concrete girders can tolerate some cracking and still maintain continuity. In fact, loss of continuity is not seen until the connection is near failure, and then the loss is about 30%. The cur- rent specifications state that the connection can be con- sidered fully effective if “. . . the calculated stress at the bottom of the continuity diaphragm for the combi- nation of superimposed permanent loads, settlement, creep, shrinkage, 50% live load and temperature gradi- ent, if applicable, is compressive.” The research team could not determine the origin of this statement, but the experimental evidence confirms that this is a reason- able assumption. As stated in the previous conclusion, if the girders are at least 90 days old when continuity is established, there is a low probability that time-dependent positive moments will form. If these moments do not form, the connection will not crack and continuity will not be affected. This point is added to the proposed specifica- tions. Finally, guidance on dealing with partially effec- tive connections is provided. 5. Design limits for service and strength limit states are added. This is material found elsewhere in the LRFD Specifications, but it is added here for completeness. 6. The section in the current LRFD Specifications on neg- ative moment connections has been expanded. The pro- posed changes do not add new information, but clarify the existing section. 7. The current LRFD Specifications do not contain a method for designing the positive moment connection. The proposed changes to the specifications suggest that the two possible ways to make the positive moment connection are as follows: a. Extending the prestressing strand from the end of the girder, bending it at 90°, and then embedding the bent strand into the continuity diaphragm. The length of the strand extension can be found from the equa- tions proposed by Salmons and others (8–10, 13). b. Embedding mild reinforcing bar in the end of the girder. The protruding end of the bar should have a standard hook, which is embedded into the dia- phragm. The connection will develop sufficient strength as long as both ends of the bar are embed- ded by the required development length (given in Article 5.11 of the LRFD Specifications). The experimental results from this study show that pos- itive moment connections designed as stated above develop sufficient strength. Analytical studies done as part of this project show that the positive moment con- nection should be proportioned to provide a minimum strength of 0.6 Mcr, where Mcr is the cracking moment of the connection. This point is added to the body of the suggested changes. The analytical studies also show that proportioning the connection to provide more than 1.2 Mcr is not effective. This point is added as a sug- gested change to the commentary. 8. From information obtained in the surveys conducted in this study and from the experience of assembling

54 the experimental specimens, detailing requirements are proposed. SUGGESTED FUTURE RESEARCH This experimental program tested only AASHTO I shapes. Limited testing to verify positive moment connection capac- ity for other shapes may be advisable. This experimental program tested only a single beam line as a full-size specimen. The result should be verified by mon- itoring the response of a complete bridge. The analytical models do not accurately assess the effects of differential shrinkage between the deck slab and the gird- ers. Work needs to be done to create more accurate models. The effects of skew and seismic effects were not tested in this program. Additional experimental work on seismic and skew effects is advised.

Next: References »
Connection of Simple-Span Precast Concrete Girders for Continuity Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB’s National Cooperative Highway Research Program (NCHRP) Report 519: Connection of Simple-Span Precast Concrete Girders for Continuity includes recommended details and specifications for the design of continuity connections for precast concrete girders. Also included in the report are examples illustrating the design of four precast girder types made continuous for live load.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!