Click for next page ( 44

The National Academies of Sciences, Engineering, and Medicine
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 43
43 CHAPTER 4 Conclusions and Suggested Research This report has reviewed recent advancements in ABC tech- protected connections between components and comprise the niques that are either being used currently or show promise integral connection type that has been considered here. for use in regions of the United States that are subject to mod- In some cases, emerging nontraditional concepts seek not to erate-to-high seismic hazards. ABC techniques have been dissipate energy so much as act compliantly and accommodate applied on many projects, primarily in regions of low seismic seismically induced displacements with minimal damage. Such activity. However, their use in moderate-to-high seismic connections are called deformable, and are used in lieu of regions of the country has been limited, because the conven- energy-dissipating connections at locations where inelastic tional, linear, precast elements used with ABC cause the con- deformations are expected. They may allow the bridge designer nections to be located at the intersections of framing elements, to improve the seismic response of the system by selecting an and those locations are typically the regions expected to expe- optimal distribution of moments within it. rience the highest demands under earthquake loading. Accord- Significant knowledge gaps remain to be closed for energy- ingly, significant work is under way and more is needed to dissipating connections, so a focus of additional research ensure that ABC connections can meet the required seismic should be on energy-dissipating and deformable connections. performance, in addition to having the necessary non-seismic The reason for this is that capacity-protected connections may properties of constructability, cost effectiveness, durability, be designed largely with data that supports the use of these and inspectability. same elements in non-seismic areas. Typically, such design data exists. Gaps in the experience and knowledge base for these capacity-protected components must eventually be Conclusions closed, but the energy-dissipating work is a more pressing The use of precast or prefabricated elements in bridges impediment to implementation. located in seismic areas can be characterized into two cate- gories, energy-dissipating and capacity-protected, and this sep- Long-Range Needs aration is useful in focusing the development of SABC. Bridge systems are designed such that the inelastic response that is The status of the existing state-of-knowledge and practice unavoidably induced by the ground motion is concentrated in for SABC, coupled with the wide range of construction prefer- a few predetermined components. These components are typ- ences by owners and engineers around the country, suggests ically the columns, which act like fuses. Other components are that a broad and extensive testing program will ultimately be thereby protected from the heavy loading demands and do not necessary to fully support the use of SABC in the United States. need to be designed for the more rigorous conditions experi- Such a program should eventually include large-scale sub- enced by the columns. Many of the connections that were assemblage (full pier) tests, as well as field demonstration proj- reviewed participate in the dissipation of earthquake-induced ects to build confidence in the use of SABC. Experience energy. These elements are termed energy dissipating and they suggests that a single technology will not fit the needs of all the significantly influence the overall seismic performance of the states with moderate-to-high seismic areas, especially in view bridge. The components and connections of a bridge that are of the fact that SABC technologies are a significant departure protected by the fuse-like behavior of the columns are designed from conventional CIP systems. A realistic estimate of the total with capacities that are large enough to prevent damage from time to develop the required knowledge could be in the range occurring in them. Such elements are denoted as capacity- of 20 years. Therefore, this section summarizes the broader,

OCR for page 43
44 long-term needs and proposes the first, but significant, step of provide seismic performance that is superior to that available a journey that will likely last a number of years. This assessment from present designs, but the effort required to get those tech- of duration is supported by recalling the time taken for the les- nologies both to a level of maturity that will instill high confi- sons learned in the 1971 San Fernando (California) earthquake dence in owners and designers and to imbue them with to be formally adopted into bridge design practice in 1990. characteristics of rapid constructability will take longer than Compared with conventional bridge technology (e.g., CIP), that needed for CIP-emulative types. Thus, the recommended SABC elements are somewhat more complex and include a strategy is to give higher priority to development of the tech- wide range of possible behaviors, which suggests that time will nologies that align with current preferences and to take them be required to fully develop the technology. Nevertheless, such to a deployable level. a research undertaking is crucial for the implementation of The review of existing technologies undertaken in this proj- ABC in seismic areas and is an important step for the states, the ect led to the generalization of connections into seven types, traveling public, and work-zone safety. which are listed in Table 34. The use of connection types in a Many ABC systems have been proposed for seismic use and bridge follows a building-block approach, where the overall limited testing has vastly improved the knowledge base and bridge system is built of SABC connections, along with more reduced the gaps in it. However, for owners and designers to conventional connections in non-seismic critical locations. have the confidence to deploy SABC technology, it is necessary For each of the seven connection types, the available informa- to develop definitive design and construction specifications, tion is insufficient to justify implementation as an SABC sys- design examples, demonstration projects and field experience. tem in the field. Future efforts should fill the remaining gaps in this knowledge The concept of TRL has been adapted for the use of SABC, in a systematic way. The objective is to provide the user with a providing rankings within nine categories that range from palette of connections that can be constructed easily and that initial concept development to the system having successfully will accommodate inelastic deformations expected at inter- performed in the intended environment (in this case, a design mediate piers--an essential "toolbox" for bridge designers in earthquake). In addition to the ranking of TRLs, a judgment moderate-to-high seismic regions. of the level of completeness at each TRL has been used and An ancillary benefit of development work on SABC systems this helps identify three ways in which connection or system is that such systems may address other extreme events, such as development may be deficient. The deficiencies fall into one vessel impact, blast, or other loadings that may load a bridge of the three broad classifications defined above by the degree beyond its elastic limits in ways not addressed by design for of completion of the steps in the TRL. The generalizations of gravity load alone. The design principles used for seismic load- the deficiencies are meant to represent a composite status for ing require continuity of load path, reserve inelastic strength, the type of connection under consideration. and a high level of structural integrity, and such attributes directly benefit the structure for other extreme events with The catch-up classification indicates that a step along the strong lateral effects. connection's development is missing altogether and must Previous NCHRP and state DOT-funded research to develop be provided to justify the connection's use at the highest workable solutions to meet seismic performance requirements level at which other information is available. In some cases, for ABC applications has produced a good start, and the sug- the connection may be in use today, despite the lack of gested additional effort represents the next logical step on a information in one previous step. longer journey. The infill classification indicates that in one or more steps, Leveraging existing knowledge and experience is necessary the needed information has been partially, but not fully, to prevent redundant research. An example of such leverage developed. In some cases, the partial knowledge may jus- is the use of existing data for the design of capacity-protected tify use of the technology in moderate seismic areas where connections. Deferring consideration of capacity-protected connections focuses the next stage of SABC work on a smaller universe of connections that are affected by the seis- Table 34. Work remaining by SABC connection type. mic demands placed on energy-dissipating and deformable- element connections. Connection Type Catch-up Infill Advancement Bar couplers The overall strategy for the implementation of SABC should Grouted ducts also account for owner preference in deployment of technol- Pocket ogy. There is a distinct preference apparent in the survey results Socket for technologies that emulate CIP construction performance, Hybrid and this is a manifestation of comfort level with the known. It Integral is also apparent that there are emerging technologies that may Emerging

OCR for page 43
45 demands are somewhat lower, but not in higher seismic beam, which is subsequently filled with concrete. The primary regions. information was developed in NCHRP Project 12-74. The sys- The advancement classification indicates that all the infor- tem provides considerable promise, but mechanics-based mation up to and including a given step is available and the design procedures are needed for the joint region and more remaining developments are those needed to push the extensive testing is needed to advance their development. The technology to a higher level of readiness. joint region includes not only the confined pocket itself, but also the surrounding region in the cap beam. The dimensions In an ideal world, all TRLs would be completed before of that region are limited by the width of the cap beam and the moving on to the next level. In the real world of bridge engi- size of the pocket, so it may be quite small and, therefore, neering, this has not been done. highly stressed. In particular, the required quantity of tie and The reasons for the foregoing classifications are as follows. other confining reinforcement and procedures for computing Bar Couplers refer to devices that connect reinforcing bars it need to be established. The connection is placed in the for tension or compression using grouted sleeves or various "infill" category. types of mechanical connections. A wide range of tension In a Socket Connection, the footing or cap beam is CIP capacities are available, but only a limited set of such couplers around a precast column, from which no reinforcement proj- are potentially suitable for seismic applications. Of these, the ects. These are simple to fabricate and transport and offer grouted sleeves have been used in a number of applications, excellent onsite constructability characteristics. Some cyclic and several versions are commercially available. A primary testing has been conducted on both precast concrete and steel shortcoming is that a comprehensive test series on grouted- columns embedded in footings that are typical of bridge con- types of bar couplers is lacking. Partial information is avail- struction. Other studies have investigated footings suitable for able (tests on couplers in air under high strain rates; isolated buildings, but those results appear to translate poorly to bridge tests on members connected using particular couplers, [e.g., construction. A more extensive study is needed to define the Splice Sleeve Japan Ltd., undated, Riva 2006]), but tests relationships between the embedded length of the column, the covering the full range of behaviors under seismic loading column diameter, surface roughness, and confining reinforce- have not been conducted. Open questions include not only ment. Clear, mechanics-based design guidelines are needed for the cyclic response of the couplers themselves, but also sys- the design of the critical connection region. The connection is tem effects, such as the influence of the coupler stiffness on placed in the "infill" category. the strain distribution in the plastic hinge zone and its effects Hybrid Systems typically contain unbonded prestressing on the strain penetration in the opposing connected element tendons that remain elastic at all times during an earth- and on the overall deformation capacity of a coupled-bar quake and re-center the bridge system when the lateral load system. Because grouted sleeve bar couplers have already been is removed--a highly advantageous characteristic for post- deployed in seismic regions, the paucity of cyclic performance earthquake use. Hybrid systems differ from many of the others data for that type of bar coupler represents a serious short- discussed here in that their primary purpose is to provide supe- coming, so they are placed in the "catch-up" category. rior seismic performance, with rapid erection seen as a desir- Grouted Ducts refer to the anchorage of bars from one ele- able, but not essential, additional feature. This ranking of ment into another by means of grouting the projecting bars priorities is the opposite of most of the other systems presented into ducts. The load is then transferred from the duct into the here. The principles of hybrid structures have now been well surrounding concrete by bond. A number of test programs established in the vertical building industry and a number of have demonstrated the high anchorage capacity of grouted such structures have been built. This level of development in a ducts under monotonic loading, but only a few tests have been parallel industry, coupled with their demonstrated potential conducted using inelastic cyclic loading. Other areas where for improved performance, justifies their being treated differ- more information is needed include the effects of the size of ently from other emerging technologies. However, details suit- duct, the type of duct (particularly the nature and roughness able for bridge construction have not yet been fully worked of the corrugations), the location of the bar in the duct out. This is the case partly because many variants on the con- (eccentric or otherwise), group pullout failure, and transfer figuration are possible. The primary questions include the of the load from the duct wall through the concrete to neigh- choice between pre- and post-tensioning, use of bars or strand, boring reinforcement. Grouted ducts have been deployed in corrosion protection, anchorage details in the footing and pier non-seismic applications, on the basis of the available static cap, confinement needed at the rocking interface, and so forth. strength data. The connection is therefore placed in the "infill" Hybrid systems are, thus, classified as "advancement" in terms category. of needed work to advance to deployment. In a Pocket Connection, bars projecting from the top of a Integral Connections are taken here to mean the connec- column are fitted into a single void, or pocket, in the cap tion between girders, cap beam, and columns that resists