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2 SERVICE LIMIT STATES IN CURRENT PRACTICES AND AVAILABLE LITERATURE 2.1 State-of-the-Art Summary Two different approaches were used to collect data on the state-of-the-art of current practices with regard to service limit states. First, a questionnaire was sent to major bridge owners to collect data on current practices and to determine if there are new service limit states that bridge owners desire to be included in the design specifications. Second, a literature search was performed to review the service limit states used by AASHTO LRFD and other international bridge design specifications and to review the background of relevant design provisions. A summary of the results of these studies is presented below. The details of the state of practice and the literature search are presented in Appendix A and B, respectively. 2.1.1 Questionnaire of Bridge Owners To determine the current state of practice of design for the Service Limit State, a questionnaire was developed and sent to major bridge owners across North America including the Departments of Transportation in all 50 US states, Ministry of Transport in all Canadian Provinces, District of Columbia and many turnpike authorities, bridge authorities and commissions. The survey included twenty questions covering the following topics: ⢠Modifications to the specification loading (HL93 Loading) for Service Limit states ⢠Checking SLSs under the effects of legal loads as part of the normal design procedure ⢠Revisions to the SLS stress limits for prestressed concrete components ⢠Revisions to existing SLSs for concrete structures ⢠Method used for designing for Control of Cracking by Distribution of Reinforcement ⢠Checking concrete superstructure and substructures for any additional service load combinations beyond those in AASHTO LRFD ⢠Checking concrete structures for SLSs under overloads ⢠Cracking of pre-tensioned concrete beams immediately after prestressing force release ⢠Cracking of prestressed concrete beams in service? ⢠Damage to ends of end of prestressed beams under expansion joints ⢠Use of the deck empirical design method and the performance of these decks in service ⢠Observations of deck cracking ⢠Type of reinforcement bars used in newer decks (i.e., black bars, epoxy coated, galvanized, stainless steel, etc.) ⢠Average life span of concrete decks and the main reasons decks are replaced ⢠Types of concrete superstructures in use ⢠Problems with bearings in concrete structures ⢠Cracking of abutments and piers ⢠Average service life span of the concrete substructures 7
⢠Fatigue problems in concrete superstructures ⢠Use of coatings in concrete substructures Twenty nine responses to the questionnaire were received and analyzed. The following conclusions were drawn from the responses to the questionnaire: ⢠The majority of bridge owners use the HL-93 loading for service limit states without modifications to the load. Where changes are made, the changes appear to address issues related to state-specific weight limits and vehicle configurations. ⢠During the design process, about half the respondents indicated that they routinely check the design for strength for legal loads as an additional design case. Half of those doing so also check the service limit state design requirements under legal loads, however, it is not clear whether the same limits are used for HL-93 and legal loads. ⢠About one-third of the respondents indicated that they use a lower tensile stress limit for prestressed concrete components. ⢠Some bridge owners use modified deflection limits or load factors for some service limits. ⢠The majority of bridge owners check the control of cracking through the distribution of reinforcement using the method specified in the AASHTO LRFD albeit some bridge owners modify the requirements to be more stringent. ⢠Only one respondent indicated that they check for an additional service limit state that does not exist in AASHTO LRFD. This limit state relates to limiting the reinforcement stresses under dead load. The lack of widespread use of additional limit states indicated that bridge owners do not see a great need for adding new service limit states to the design specifications. ⢠About one-fourth of the respondents indicated that they check some service limit states when analyzing structures for permit vehicles. ⢠More than half the respondents indicating observing early age cracking near the ends of prestressed beams. Two-thirds of the cracking was inclined and the remaining was vertical cracking. ⢠About two-thirds of the respondents indicated observing cracking of prestressed concrete in service. However, the frequency cracking was observed was low (one-third reporting observing cracking indicated frequency less than 1% of bridges, one-third indicating frequency between 1 to 5% and one-third indicating frequency 5 to 10%). ⢠About one-fourth of the respondents indicated using the empirical deck design method for some bridges. In one case the respondent indicated that decks designed using this method developed more cracks than decks designed using the conventional method. ⢠Deck cracking was reported to have been observed in about 80% of the responses. More than half of the reported cracking is in the transverse direction with the remaining split between longitudinal and map cracking. About 60% of the reported deck cracking was observed immediately after curing. ⢠About two-thirds of respondents indicated using epoxy-coated rebar in almost all new decks, one-fourth of respondents indicated the use of black bars and the remaining use galvanized bars. Other types of bars, such as MMFX bars, are in few cases. ⢠The average concrete deck life span given by respondents varied from 25 years to full bridge life. Most respondents indicated the decks are replaced due to the corrosion of the reinforcement or the concrete itself with fewer cases replaced due to extensive cracking. ⢠About 60% of the respondents indicated observing cracking of concrete piers and abutments. The lack of a pattern of the observed cracking indicated that the reported 8
problems are associated more with workmanship and detailing practice more than the design provisions. ⢠All respondents indicated that no damage was observed due to fatigue of reinforcement bars, prestressing steel or concrete. ⢠The responses to the questionnaire indicated that most bridge owners apply the service limit states included in AASHTO LRFD with no, or with few, revisions. The additional limit states used by bridge owners appeared to be related either to owner-specified vehicles, or to address a specific issue that does not seem to be shared by other bridge owner as evident by the lack of use of these additional limit states by other owners. A copy of the questionnaire and a detailed breakdown of the responses are included in Appendix A. 2.1.2 Literature Review A comprehensive literature review was performed to locate information on calibrating the service limit states and to determine the background of service limit states currently existing in the design specifications. Generally, no significant information specific to the calibrating of the service limit states was found. Much of the information located was found to be too general to be useful. Many of the methods discussed for reducing serviceability issues related to non-structural aspects of the design process, which would not be useful in calibrating limit states. Some of the sources, however, provided useful methods of anticipating and determining the effects of serviceability issues such as crack width, crack spacing, and prestressed concrete fatigue. Several design specifications were reviewed to determine the service limit states included in the design. These specifications include AASHTO LRFD, the Canadian National Code and the Eurocode. The service limit states in each code are detailed in Appendix A including the background of the provisions. A complete list of the service limit states in the Eurocode is included as Appendix B. 2.1.3 Overarching Characteristics of Other Specifications 2.1.3.1 Reversible versus Irreversible Limit States The SLSs may be categorized as reversible and irreversible. Reversible SLSs are those for which no consequences of load exceeding the specified service requirement remain once the load is removed. Irreversible SLSs are those for which consequences remain. For example, for concrete structures, a crack-width limit state with limited width is a reversible limit state as the crack will close once the driving load is removed. On the other hand, a crack-width limit state defined by a high width (such as 0.02 inches or 0.5 mm) is irreversible as the crack will not fully close once the load is removed. Due to their lesser safety implications, irreversible SLSs, which do not concern the safety of the traveling public, are calibrated to a higher probability of failure and a corresponding lower reliability index than the strength limit states. Reversible SLSs would typically be calibrated to an even lower reliability index. 9
2.1.3.2 Load-Driven versus Non-Load-Driven Limit States Difference between load-driven and non-load-driven limit states is basically in the degree of involvement of externally-applied load components in the formulation of the limit state function. In the load-driven limit states, the damage occurs due to accumulated applications of external loads, usually live load (trucks). Examples of load-driven limit states include: decompression and cracking of prestressed concrete and vibrations or deflection. The damage caused by exceeding SLSs may be reversible or irreversible and, therefore, the cost of repair may vary significantly. On the other hand, in non-load-driven SLSs, the damage occurs due to deterioration or degradation as a function of time and aggressive environment or as inherent behavior due to certain material properties cause the damage. Examples of non-load-driven SLS include penetration of chlorides leading to corrosion of reinforcement, leaking joints leading to corrosion under the joints and shrinkage cracking of concrete components. In these examples, the external load occurrence plays a secondary role. 2.1.4 Lessons Learned from Review of Existing Design Specifications Review of existing design specifications revealed that the service limit states covered by different specifications are somewhat similar. It was concluded that the information reviewed suggests that other specifications do not include what can be termed as ânewâ service limit states that need to be added to AASHTO LRFD. However, the review resulted in some concepts that may be of interest. These concepts include: ⢠The target reliability index for service limit states may have different value for different limit states. Furthermore, the target reliability for a certain limit state may vary depending on the consequences of exceeding the limit state. ⢠To differentiate between different limit states based on the consequences of exceeding the limit state, the following factors may be considered: o Whether the limit state is reversible or irreversible: Irreversible limit states may have higher target reliability than reversible limit states. o Relative cost of repairs: Limit states that have the potential to cause damage that will be costly to repair may have higher target reliability than limit states that have the potential of causing only minor damage. ⢠Generally, the calibration of the SLS in other specifications is lagging behind the calibration for ULS. Many of the requirements in other specifications relate to general concepts and expert opinion rather than to actual calibration 2.1.5 Search for Concrete SLSs Not Yet Implemented Several reports were reviewed in an effort to determine whether any additional concrete SLSs should be considered when designing bridges. The additional information was meant to supplement the literature review and the bridge ownersâ survey. Reports were gathered from sources such as the NCHRP, the Federal Highway Administration (FHWA), American Concrete Institute (ACI) Structural Journal, ACI committee documents, and conference proceedings of the Structures Congress and the American Society of Civil Engineers (ASCE). The investigated reports pertained to establishing concrete cracking of beams and bridge decks, concrete shrinkage, fatigue of prestressed concrete members. Each report was reviewed to determine the usefulness of the information. Any information that could potentially be used in creating new SLSs were noted and investigated further. The search did not lead to any totally new limit states. 10
2.1.6 SLSs to be Considered in this Report Potential limit states and possible calibration approaches for service limit states related to concrete structures and some general limit states that are not material-dependent have been reviewed. Some of the potential limit states have since been determined to be uncalibrateable. For example, some are deterministic or are based on judgment and experience. The SLSs still thought to be calibrateable are listed in Table 2-1 along with whether the phenomena being addressed are reversible or irreversible and whether the live load will involve single lane loading or multiple lane loading (see Chapter 4 for live load models). A complete list of all service limit states in AASHTO LRFD is included in Appendix A. Note that SLS references to partial prestressing have been removed. AASHTO no longer accepts partial prestressing as a design strategy. Table 2-1 SLSs Identified for Development LRFD Article Reversible Lanes Multiple Presence Factor (MPF) 3.4.1 - Load Factors and Load Combinations for Fatigue No Single - 5.5.3.1 - General - Compressive Stress Limit for Concrete - A Fatigue Criterion No Single No 5.5.3.2 - Fatigue of Reinforcing Bars No Single - 5.5.3.4 - Fatigue of Welded or Mechanical Splices of Reinforcement No Single - 5.6.3.6 - Crack Control Reinforcement - Deemed to Satisfy * No - - 5.7.3.4 - Control of Cracking by Distribution of Reinforcement No N/A** No 5.9.3 - Stress Limitations for Prestressing Tendons (no revisions required)*** No Multiple Yes 5.9.4.2.2 - Tension Stresses in Precompressed Prestressed Concrete Yes Single No * The available information on this limit state does not provide a quantifiable way of assessing the provided margin of adequacy such as safety or reliability. Based on past performance, it was considered âdeemed to satisfyâ. See Section 3.3 for the application of âdeemed to satisfyâ ** Decks are affected by axle loads not full trucks or lanes loaded *** This limit state is irreversible and as such the case of multiple lanes loaded is appropriate to minimize the possibility of strand yielding under service loads 11
