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 51
49 The 2nd edition of the AASHTO LRFD Specifications (12) Skewed Connections states: Skewed connections present a particular difficulty. The 18.104.22.168.7c: If the calculated stress at the bottom of the joint skewed connection, by nature, has asymmetries, and the exper- for the combination of superimposed permanent loads, set- imental evidence suggests that asymmetry affects connec- tlement, creep, shrinkage, 50% live load and temperature tion performance. Unfortunately, no skewed connections were gradient, if applicable, is compressive, the joint may be con- tested in this program, so it is difficult to make recommenda- sidered fully effective. tions. More research is required on skewed connections. Given the experimental evidence, this specification seems both reasonable and justified. In cases in which the calculated DISCUSSION ON IMPLICATIONS stress at the bottom of the joint for the combination of super- OF SEISMIC EVENTS ON CONTINUITY imposed permanent loads, settlement, creep, shrinkage, 50% CONNECTIONS live load, and temperature gradient is tensile, it might be jus- As outlined in the research problem statement of this proj- tifiable to treat the joint as partially effective--that is, some ect, the ability of these types of connections to maintain portion of the live load would be carried as a continuous load continuity during and after seismic events has been ques- and some portion as a simple-span load. However, the data tioned and, therefore, needs to be evaluated. The authors were from the experiments reported herein are insufficient to make charged to prepare a discussion on the subject and to identify a strong recommendation concerning degree of continuity as future research needs in this area. only a single structural system is considered. Connection failures are the most common type of damage in bridge structures. Seismic events tend to overstress the entire bridge and, particularly, the connections. For the sake EFFECT OF DIFFERENT CONFIGURATIONS of this discussion on the seismic implications, three types of ON THE CONNECTION continuity connections are identified: This experimental program tested six types of connec- · Type I: Continuity diaphragms with positive moment tions, but the number of possible variations on these types of connections made of bent bars or bent strands and connections is large, as was seen in the survey. From the negative moment reinforcement in the composite experimental results, it is possible to comment on some of deck or even in the continuity diaphragm. The gird- these other configurations. ers and/or continuity diaphragms are placed on bearing pads, which act as pin or roller supports. Therefore, the superstructure is basically isolated from the substructure Connecting Different Depth Girders in that no horizontal force is transferred from ground motions in the longitudinal or transverse direction. Vari- One of the concerns with the positive moment connections ations of this type of connection has been used in many is congestion in the diaphragm area. The bars or strands used states, particularly in Tennessee and Missouri. to create the positive moment connections tend to be meshed, · Type II: Integral continuity connections, in which leaving little or no clearance between adjacent bars. The inter- the continuity diaphragm, the composite deck, and action between the bars might limit the connection strength, the columns are cast monolithically. This type of con- but this was not found to be the case. The tests showed that nection provides fully rigid continuity and, therefore, the connections had adequate strength even though the dia- the superstructure must be designed for a portion of the phragm area was congested. Anything that relieves the con- plastic hinge moment of the columns. This type of con- gestion will improve the connection. When girders of differ- nection is primarily used in California. ent depths are connected into opposite sides of the diaphragm, · Type III: Similar to Type I, but the diaphragm is the positive moment connection steel from each girder will connected to the pier cap and, therefore, limited shear not lie in the same plane, leaving more clearance between the and moment would be transferred during ground bars and reducing interaction effects. motion. This type of connection is primarily used in the However, when the girders are the same depth, the forces state of Washington, where the bottom 2 ft of the dia- in the positive moment connections are at the same level and phragm is cast before the composite deck and the rest of will act in opposition to each other. When the girders are of the diaphragm. different depths, the forces in the positive moment connec- tions are now offset by the difference in the depths of the gird- Previous studies on the subject are very few and not directly ers. This may lead to additional cracking in the diaphragm. relevant. Ductility of continuity diaphragm-deck connections The forces in the diaphragm area should be analyzed using an is an important issue. Failure at the interior supports could be appropriate model, such as the strut-and-tie model, and addi- brittle and catastrophic because of the smaller compression tional reinforcing should be provided as needed. area at the bottom of the girder-diaphragm. NCHRP Report
OCR for page 52
50 322 (11) carried out a parametric study of the negative moment designed for carrying, nor should it be expected to carry, such strength of composite sections and found the typical sections moments for the two adjacent spans. Previous experiences to have adequate rotational ductility to allow formation of a with continuity connections have demonstrated the benefits of failure mechanism. The static analysis indicated that an upper such redundancy. For example, in the late 1970s, in a bridge limit on deck reinforcement equal to 50% of its balanced rein- on State Route 1 over the Wolf River in Memphis, Tennessee, forcement ratio would ensure sufficient ductility (of about a bent supporting two spans failed completely due to scour, 3.4) to develop the full-failure mechanism as well as to pro- but the simple spans--widened with continuous-for-live-and- vide enough deformation to give adequate warning of failure. composite-dead-load box beams--only sagged. Failure was Most recently, Holombo et al. (30) studied the continuity averted by the alternate load path created by the continuous of precast/prestressed spliced-girder bridges under seismic widened portion and the New Jersey parapet. loads for a typical construction in California. Since the gird- To avoid catastrophic failures due to unseating of the gird- ers were spliced, continuity was present for both self-weight ers during seismic events, appropriate detailing must be devel- and live load. Two 40% scale-model continuous bridge struc- oped based on the expected longitudinal and transverse tures, one with bulb-T and the other with bath-tub girders, movements of the support columns and the pier cap. In Type were tested under longitudinal seismic loads with up to eight II connections, the superstructure is actually designed for the and six times the first-yield displacement in the bulb-T and proper transfer of moments. In Type III connections, as dis- bathtub girder systems, respectively. Continuity was estab- cussed earlier, the dowel bars transfer some horizontal forces lished by post-tensioning the girders together with strand. to the superstructure. However, rigidity of the girders in the This is a common detail in spliced girders and a possible, but longitudinal direction and that of the continuity diaphragm in not common, detail for continuous-for-live-load systems. the transverse direction should be more than adequate to Although this system is not necessarily representative of con- resist such loads. tinuous-for-live-load systems, it is presented here to give Another important implication of seismic events is that some indication of the performance of a continuous system vertical ground accelerations may induce a significant level under seismic load. of inertia force for near-fault bridges. However, such forces In the tests, significant ductility with only minor strength have to first overcome the gravity accelerations before devel- degradation was observed. An essentially elastic performance oping an uplift force at the continuity connections. of the bent cap and superstructure was observed during the Based on the above discussion, future research on the sub- simulated seismic displacement cycles. Holombo et al. sug- ject should be focused on the analysis and testing of Type III gested approximating the superstructure seismic moments by connections, where some limited force transfer occurs, but giving two-thirds of the column plastic hinge moment to the perhaps is not designed for. Proper detailing must be devel- girders adjacent to the column and the remaining one-third to oped to avoid unseating of the girders. Use of shear keys or the nonadjacent girders (30). dowel bars may be considered. Effect of vertical ground Since seismic events predominantly develop horizontal in- motion on the vibration of the superstructure and the conti- plane loads, the continuity connection may be subjected to nuity diaphragm also need to be studied. longitudinal or transverse movements. If the superstructure Finally, in this study, no experimental work was done that is isolated from the substructure through pin or roller sup- was specifically related to seismic behavior, but the experi- ports, the only implication of seismic event may be the unseat- mental work that was done does provide some relevant infor- ing of the girders in the longitudinal or transverse direction. mation that may be applicable to seismic design. The results Piers may experience a significant level of damage or be of the connection capacity tests showed that the bent-strand "lost" after a major earthquake. At this stage, the connection connections tend to slip under cyclic loads. This type of behav- is also expected to have cracked and damaged. From a life ior might not be desirable in a seismic area. Tests on the fifth safety performance point of view, it would be important that stub specimen demonstrated that if the girder ends are embed- the girders still be able to resist their self weight plus other ded in the diaphragm and the additional stirrups are placed in superimposed gravity loads. Continuity connections do pro- the diaphragm just outside of the girders, the connection will vide enhanced structural integrity and redundancy for the be more ductile after the pull-out failure occurs. This addi- entire system. However, the positive moment connection is not tional ductility would be useful in seismic designs.