Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
NCHRP Project 12-103 210 Table 7-10 - Tolerable LD support movement occurring at the pier. Strength I Flexure Strength I Shear Service III CIP Multi-Cell 1 4.8 in. 27.4 in. 0.2 in. CIP Multi-Cell 2 10.4 in. 20.7 in. 0.05 in. CIP Multi-Cell 3 21.1 in. > 30 in. 0.00 in. Table 7-11 - Tolerable TD support movement occurring at the pier. Strength I Flexure Strength I Shear Service III CIP Multi-Cell 1 9.6 in. 16.7 in. 0.3 in. CIP Multi-Cell 2 17.8 in. 15.4 in. 0.08 in. CIP Multi-Cell 3 25.8 in. 27.8 in. 0.00 in. 7.4 Conclusions Based on this secondary study, steel and PS concrete box girder bridges display similar behavior (in terms of superstructure tolerance to support movement) that was observed for steel and PS concrete multi-girder bridges. The results for shorter span bridges may not be relevant, however, as shorter spans of these bridge types are not typically constructed. If a more complete understanding of the performance of these bridge types when subjected to support movements is desired, an addition study focused exclusively on these bridge types should be carried out. Such a study would follow the approach employed in this research. In lieu of such a study, a general framework is provided in Section 9 to allow designers to estimate the tolerable support movements for such bridges during the design process. 8 Functional Limitations on Tolerable Support Movements (Task 2.4) 8.1 Background on Rideability Limitations to Tolerable Support Movement There is little direct guidance on the nature of bridge support movement and rideability concerns outside of the relation of differential movement between bridges and approach slabs and pavements. Here, differential is referring to the relative displacement between supports or between the supports and the approach slab (essentially LD movements). The effects of differential settlement at bridge joints on cars and trucks was studied as early as 1969. It has frequently been described as âapproach faultsâ or
NCHRP Project 12-103 211 âthe bump at the end of the bridgeâ, although little primary or independent research on the mechanism behind rideability issues have been published (James et al. 1990, Briaud et al. 1997). Both absolute and relative limits on settlements have been suggested in order to improve rider comfort. This research is limited, however, in that it has focused on settlement of the approach/bridge interface or the interface between two adjacent, disconnected spans and not the interior supports of continuous bridges. The literature on approach slab settlement reaches back to 1969, where Hopkins, according to Puppala et al., noted that these approach faults may result in driver discomfort or even safety concerns (Puppala et al. 2012). Bridge surveys performed in Ohio and Kentucky from the 1960s to the 1980s were reviewed by later researchers who noted that settlement is a riding comfort concern for the traveling public and made recommendations on maintenance (fix the bump) and foundation design (avoid the bump) (James et al. 1990). These studies, however, did not specify limits for the allowable degree of settlement (James et al. 1990). Separate studies performed by Walkinshaw (1978), Grover (1978), and Moulton et al. (1985) noted the limits of vertical settlement at which rider discomfort may begin (Barker et al. 1991). Moulton et al. (1985) noted that the âtolerableâ limit of bridge support settlement was related to the length of the bridge, or angular distortion (Moulton 1985). This limit was based on structural safety, not rideability, however, and Moulton noted that rider discomfort is not likely to control the magnitude of tolerable settlement. The limits noted by Wilkinshaw (1978) and Grover (1978), meanwhile, were absolute measurements, with no relation to the length of the approach slab or bridge span. In the 1990s, four separate studies provided recommendations on tolerable limits of support settlement based on independent research. Wahls (1990) suggested a relative tolerable rotation of 1/250 for continuous span bridges and 1/200 for simply-supported bridges. Stark et al. suggested that an absolute gradient of 1/200 was appropriate for approach slabs (Stark et al. 1995). Tan and Duncan contended that the relative gradient limit of 1/200 for simply-supported spans provided by Wahls (1990) was too liberal and recommended a relative gradient of 1/250 for all bridges (Tan and Duncan 1991). A study by Long et al. noted that a relative gradient of 1/100 was preferable, although this recommendation was based solely on the measured settlement of the cracked approach slabs on either side of a single bridge in Illinois (Long et al. 1998). No further discussion or investigation was performed to develop this criterion for allowable settlement and thus this recommendation will not be used in this study.
NCHRP Project 12-103 212 More recent studies on settlement of the approach/bridge interface or adjacent bridges have reviewed the available work performed from 1969 to 1991 and made concurring recommendations on the tolerable relative slope or gradient. More studies have used absolute tolerable settlement as a criterion for limits on ride-ability, however these suggestions are so variable that it would be difficult to make a recommendation based on them. 8.2 Recommendations for Rideability Limitations to Tolerable Support Movements The following assumptions are made for the recommended ride quality criteria: (1) The bridge was constructed with negligible differences in grade between spans and between the bridge and approaches. (2) For ride quality purposes, continuous spans are assumed to develop twice the differences in grade as a simple span. Essentially this assumes that due to the elastic curve of a continuous bridge, the change in angle at the support is twice that associated with a simple span that undergoes just rigid body deformation. (3) Ride quality criteria is based only on LD support movement (4) An angular distortion of L/250 (as recommended by various sources, see Section 8.1) is assumed to be a reasonable limit for ride-ability concerns. These assumptions translate into the following recommendations related to tolerable support movements based on rideability criteria (Table 8-1). Table 8-1 - Tolerable movement limits for ride quality. Movement Case Limits for Ride Quality For movements occurring at the abutment of simply supported bridges with an approach slab ß Ü®à¯ + ß Ü®à¯¦ < 1 250â For movements occurring at the abutment of continuous bridges with an approach slab ß Ü®à¯ + 2ß Ü®à¯¦ < 1 250â For movements occurring at the pier of multiple- span simply-supported bridges 2ß Ü®à¯¦ < 1 250â
NCHRP Project 12-103 213 For movements occurring at the pier of continuous bridges 2ß Ü®à¯¦ < 1 250â Where: ß = absolute support movement ܮ௠= length of approach slab ܮ௦ = length of span These formulas may result in rideability concerns being the limiting factor for tolerable support movements. For example, a tolerable relative gradient between equal length bridge spans of 1/250 is equal to an angular distortion of 0.002L for each span. These recommendations do not include considerations for mitigating or exacerbating factors that may influence the experience of rider discomfort in the traveling public. Vehicle speed, mass, and suspension may all contribute to different sensation of support settlement or approach faults. Likewise, bridge joint damage, large skews, and surface roughness may also influence experiences of rider discomfort. A TD support movement, due to the transverse variability of the riding surface, may produce greater rider discomfort than a LD support movement of equal magnitude. Global support movements, not discussed in this report, will contribute to a change in gradient at the approach slab/bridge interface. In these cases the change in gradient is based solely upon the length of the approach slab. 8.3 Clearance Limitations on Tolerable Support Movement In addition to rideability concerns, clearance issues may also result from support settlements. The limits of settlement due to clearance are based solely upon existing clearances and associated clearance criteria. Waterways, rail, and roadways all present unique issues when dealing with settlement of spans over the feature. LD support movement will result in changes in clearance under the structure in the longitudinal direction. Special care should be taken in instances of TD support settlement: the change in clearance under the structure will be different in both the longitudinal and transverse directions.
