Elplanation of Excessive Long Time Deflections of Collapsed Record Span Box Girder Bridges




Explanation of Excessive Long-Time De°ections of

Collapsed Record-Span Box Girder Bridge in Palau


Abstract: Explanation of the excessive de°ections of the Koror-Babeldaob (KB) Bridge in Palau

is presented. This bridge was built in 1977 by the cantilever method and collapsed 3 months after

remedial prestressing in 1996. It was a segmental prestressed concrete girder having the worldrecord

span of 241 m (790 ft.) and maximum girder depth of 14.17 m (46.5 ft). The ¯nal mid-span

de°ection was in design expected to be 0.53 to 0.65 m (20.7 to 25.5 in), but after 18 years it

reached 1.39 m (54.6 in.) and was still growing. Presented is an accurate analysis using 5906

three-dimensional (3D) ¯nite elements and step-by-step integration in time. For the concrete creep

and shrinkage properties, the B3, GL, ACI and CEB (or CEB-FIP, ¯b) models are considered and

predictions compared. Model B3, in contrast to the others, does not give an unambiguous prediction

because, in addition to concrete design strength, it necessitates further input parameters which are

unknown. These are three mix parameters which can be set to default values but can also be varied

over a realistic range to ascertain the range of realistic predictions. The ¯ndings are as follows: 1) For

model B3, one can ¯nd plausible values of input parameters for which all the measured de°ections

(as well as Troxell et al.'s 23-year creep tests) can be closely matched, while for the other models

they cannot even be approached and the observed shape of creep curve cannot be reproduced. 2)

The 18-year de°ections calculated by 3D ¯nite elements according the ACI, CEB and GL models

are about 66%, 63% and 40% less, respectively, than those measured and calculated according to

model B3, and their de°ection curves have shapes rather di®erent from the those measured. 3)

The shear lag is important since it increases the downward de°ection due to self-weight much more

than the upward de°ection due to prestress. 4) The de°ection is highly sensitive to prestress loss

because it represents a small di®erence of two large numbers (de°ections due to self-weight, and to

prestress). According to the ACI, CEB and GL models, the prestress loss obtained by the same ¯nite

element code is, respectively, about 56%, 46% and 37% smaller, and according to the classical lump

estimate used in design about 54% smaller. 5) Model B3 is in agreement with the measurements

if 3D ¯nite elements with step-by-step time integration are used to calculate both the de°ections

and the prestress losses, and if the di®erences in shrinkage and drying creep properties caused

by di®erences in slab thickness and temperature are taken into account.



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