Click for next page ( 56

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 55
53 statement that positive moment connections are not struc- fact, loss of continuity is not seen until the connection turally beneficial may not be justified in all cases. As with is near failure, and then the loss is about 30%. The cur- any design, the decision on the use of positive moment con- rent specifications state that the connection can be con- nections should be based on a rational engineering analysis sidered fully effective if ". . . the calculated stress at of the moments formed in the entire system. the bottom of the continuity diaphragm for the combi- However, even in cases in which analysis shows that the use nation of superimposed permanent loads, settlement, of positive moment connections does not reduce the midspan creep, shrinkage, 50% live load and temperature gradi- positive moments, the positive moment connection may still ent, if applicable, is compressive." The research team be useful. The results of this research show these connections, could not determine the origin of this statement, but the when properly designed, are robust. These connections will experimental evidence confirms that this is a reason- contribute to the overall structural integrity of the system, able assumption. especially in cases in which supports are damaged. 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 PROPOSED REVISIONS TO THE AASHTO moments will form. If these moments do not form, the LRFD BRIDGE DESIGN SPECIFICATIONS connection will not crack and continuity will not be affected. This point is added to the proposed specifica- Precast/prestressed concrete bridges made continuous are tions. Finally, guidance on dealing with partially effec- addressed in Section of the AASHTO LRFD Bridge tive connections is provided. Design Specifications (12). Appendix C contains proposed 5. Design limits for service and strength limit states are revisions to this section. The proposed revisions are as follows: added. This is material found elsewhere in the LRFD Specifications, but it is added here for completeness. 1. A more complete definition of precast/prestressed con- 6. The section in the current LRFD Specifications on neg- crete bridges made continuous is proposed. ative moment connections has been expanded. The pro- 2. A significant feature of precast/prestressed concrete posed changes do not add new information, but clarify bridges made continuous is the development of time- the existing section. dependent moments. The existing specifications do 7. The current LRFD Specifications do not contain a not address methods of analysis for determining these method for designing the positive moment connection. time-dependent moments or suggested values for time- The proposed changes to the specifications suggest that dependent material properties. Based on the informa- the two possible ways to make the positive moment tion gained in the literature search and the analytical connection are as follows: studies done as part of this project, suggested time- a. Extending the prestressing strand from the end of dependent material properties and analysis methods the girder, bending it at 90, and then embedding the are presented. bent strand into the continuity diaphragm. The length 3. The current specifications do not address the effect of of the strand extension can be found from the equa- tions proposed by Salmons and others (810, 13). the age of the girder(s) when continuity is established. b. Embedding mild reinforcing bar in the end of the Information obtained in this study from literature, sur- girder. The protruding end of the bar should have veys, and analytical work shows the girder age is of a standard hook, which is embedded into the dia- great importance in determining the time-dependent phragm. The connection will develop sufficient moments that form in the system. The literature search, strength as long as both ends of the bar are embed- surveys, and analytical work also shows that if the gird- ded by the required development length (given in ers are more than 90 days old when continuity is formed, Article 5.11 of the LRFD Specifications). it is unlikely that positive time-dependent moments The experimental results from this study show that pos- (called "restraint moment" in the proposed specifica- itive moment connections designed as stated above tions) will form. The proposed specifications discuss develop sufficient strength. Analytical studies done as the effect of girder age on the analysis and on the for- part of this project show that the positive moment con- mation of positive time-dependent moments. nection should be proportioned to provide a minimum 4. When cracking occurs at the connection between strength of 0.6 Mcr, where Mcr is the cracking moment precast/prestressed concrete girders made continuous, of the connection. This point is added to the body of the it is possible that the continuity between adjacent spans suggested changes. The analytical studies also show will be lost. The current specifications require that this that proportioning the connection to provide more than effect be considered, but provide no guidance. Based 1.2 Mcr is not effective. This point is added as a sug- on the experimental work in this study, the connections gested change to the commentary. between the precast/prestressed concrete girders can 8. From information obtained in the surveys conducted tolerate some cracking and still maintain continuity. In in this study and from the experience of assembling