: ESTIMATION OF BRIDGE SERVICE LIVES

CHAPTER 8 : ESTIMATION OF BRIDGE SERVICE LIVES

Bridge age has often been considered as a representation of the combined effects of load and weather effects and it is often used as a surrogate for these variables, especially in situations where data is lacking. The age of a bridge deck can be expressed as a primary age (since the bridge was constructed) or a secondary age (since the bridge was last rehabilitated). Some previous studies have found age to be the most significant variable to explain the bridge deck deterioration (Bulusu and Sinha, 1997; Chase et al., 2000). In Figure 8-1 is presented the delineation of bridge age. It should be noted that the primary age of a bridge is same as the secondary age during the period of the construction and the first rehabilitation.

Deck Condition Rating

Primary Age Secondary

Age

Figure 8-1 Delineation of Bridge Age

rehabilitation, repair, and maintenance of bridge elements. A simple approach to determining network level funding for bridges is to refer to service life as a measure of effectiveness and to determine the number of bridges that are approaching the end of their service lives. The IBMS (Indiana Bridge Management System) approximates a design life and an activity profile with reasonable accuracy (Sinha et al. 1989). Life-cycle profiles for different types of bridges, including concrete, steel, pre-stressed concrete, timber, and masonry, were developed. At the network level, IBMS established that the mean age at which replacement occurs is approximately 70 years (primary age) for most steel and concrete bridge types. Table 8-1 and Table 8-2 were developed on the assumption that bridge replacement needs occur when bridges reach the age of 70. According to profiles defined by INDOT, it is desirable that bridges received a major repair (deck rehabilitation, superstructure repair, or bridge rehabilitation) every 20 or 25 years in order to extend the design life to a service life of 70 to 80 years. For instance, for a typical concrete bridge it is suggested to have deck rehabilitation after 20 years (secondary age) of construction and a superstructure replacement after 15 years of the previous rehabilitation. Table 8-1 and Table 8-2 also show the number of bridges that would need a major rehabilitation due to the fact that 20 years have passed since their last major repair or since their construction. Current bridge replacement and rehabilitation needs are described as “backlog.” In order to avoid double counting, it was assumed that for those bridges that are close to the end of their life (older than 50 years), and have not had any repair in the previous 20 years in the subsequent 20 years, replacement, rather than rehabilitation, was a better option.

For the estimation of future bridge preservation needs, the software package IBMS was not used for the following reasons: -

The data needed for the age-based analysis is not extensive and is easy to obtain. IBMS, on the other hand, requires extensive data including age, condition, geometry, structural deficiency, etc.

The age of a bridge is closely related to its condition justifying the use of bridge age as a measure of its performance

The age based approach proved to be consistent with the historical expenditure record on bridge preservation.

Number of bridges Cumulative number of Number of bridges Cumulative number of

bridges with more than Year approaching the end

with more than 20

bridges approaching

20 years since the last of service life

years since the last

the end of service life

repair 2005 339

repair

Cumulative Number of

Cumulative

Number of

number of bridges bridges

number of bridges

bridges with more