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OCR for page 54
52 When the deck slab is cast, it heats up while still plastic tic, continuity is maintained. When the positive moment con- because of the heat of hydration. After the deck slab sets, it nection approaches failure--which is signified by large crack cools and contracts. This contraction causes the formation of openings at the diaphragm, cracking on the bottom of the negative moments at the connection. At present, analytical diaphragm, diagonal (pull-out) cracking in the diaphragm, models do not account for this negative moment formation. and propagation of the joint crack into the slab--there is an Because this negative moment causes compression of the con- observed loss of continuity of 20% to 30%. This is consistent nection, it helps to mitigate later positive moment formation. with models because, at this point, the cracking in the speci- Temperature effects on the system are significant. Daily men matched that predicted by the models. temperature changes caused end reactions to vary ±20% per The presence of positive moment cracking does not affect day. Seasonal temperature variations also affect the behavior negative moment capacity of the connection. However, if the of the bridge. At present, few design methods account for positive moment cracking extends into the slab, the negative temperature effects, but they can be as significant as the live- cracking moment is reduced. load effects. Current analytical models predict that differential shrink- age between the deck slab and the girders should cause neg- COMPARISONS WITH PREVIOUS RESEARCH ative moment formation at the connection in some systems. For a particular bridge, the predicted development of this Previous research done on the continuous moment connec- negative moment depends on a number of factors, such as the tions was published as NCHRP Report 322 (11). This report creep and shrinkage properties of the girder and slab, the concluded that positive moment connections were of no struc- amount of prestressing force, the dead load, and the span tural benefit. If a positive moment connection is not used, the length. In this project, the analytical models predicted that a formation of time-dependent positive moments at the dia- negative moment should have formed for full-size Specimen phragm due to creep and shrinkage will crack the joint. This 1 and that there should have been a gradual downward cracking will release the positive moments, but will cause the deflection of the girders and a decrease in the end reactions. joint to behave as a hinge. Continuity will be lost, and the gird- This was not observed. The data showed that the end reac- ers will behave as simple spans for both dead and live loads. If tions gradually increased, which indicates positive moment a positive moment connection is used, the connection will formation. An examination of data from other projects (26) resist cracking at the diaphragm and preserve continuity, and confirms that differential shrinkage of the deck does not seem the midspan positive live-load moment in the girders will be to cause a negative moment formation. In the other projects, lower than in the simple-span case. However, because the con- the girders were anywhere from 60 to 300 days old when nection resists rather than releases the time-dependent positive continuity was established. With girders at these ages when moments, it causes time-dependent positive moments to form continuity is established, the analytical models predict that throughout the girder. The total midspan positive moment is differential shrinkage should cause negative moments, which then the sum of the dead-load moment (assumed simple span), would be accompanied by a downward deflection of the gird- the live-load moment (assumed continuous), and the time- ers. However, since the observed deflections remained almost dependent positive moments. NCHRP Report 322 concluded constant (except for daily temperature-induced changes), neg- that this total midspan positive moment was virtually identi- ative moments do not appear to have developed. This finding cal to the midspan positive moment obtained by assuming has a potentially significant impact on the girder and connec- simple spans for all loads. Thus, the conclusion is that posi- tion design. The negative moment predicted by the models tive moment connections provided no benefit. reduces the predicted maximum positive moment. If the neg- NCHRP Report 322 was an analytical study and, as with ative moment does not form, the models may underpredict the any research study, considered only a limited set of parame- positive moment formation. However, if the negative moment ters, mostly related to the creep and shrinkage characteristics formation is ignored (by choosing a very low value of deck of the concrete girders and deck (11). The conclusions of the shrinkage), the models may predict unrealistically high val- NCHRP Report 322 study are valid, within the limits of that ues for the positive moment. study; however, the actual continuous system is more com- The presence of positive moment cracking at the diaphragm plex. Positive moment formation at the diaphragm is influ- does not necessarily reduce continuity as predicted by ana- enced by many factors including girder size and length, the lytical models. The analytical models treat the joint between amount of prestressing force, the age of the girders at the for- the girder and the diaphragm as monolithic when it is really mation of continuity, the construction sequence, the use of a cold joint. As a result, the analytical models tend to over- partial diaphragms, contraction of the deck because of cool- predict the load at which the cracks form. However, the mod- ing after casting, deck reinforcement, and thermal effects. els also predict that when the joint cracks, the crack will In some cases, it is possible to have a system in which there immediately propagate into the slab and continuity will be is little or no time-dependent positive moment formation at reduced. This does not happen. The crack propagates slowly the diaphragm. In other cases, the time-dependent moments along the joint, and as long as the connection remains elas- formed at the diaphragm are negative. Therefore, a blanket