Page 78 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 2 DIAGNOSTIC DESIGN AND SELECTIVE RECONSTRUCTION 53

3. Selective type of reconstruction: Selective reconstruction is tailored to suit existing fi eld
conditions. Based on an alternatives study, value engineering is usually carried out. Since

structural solutions and type of foundation to be used are site specific and bridge oriented,
unique applicable solutions need to be considered.
Funding for the diagnostic design and repair would be less than the cost of replacement.
However, life cycle costs cannot be controlled due to limitations in the original design as
any deficiency is likely to be recurring.

Repair, retrofit, and rehabilitation requirements would require deficient members to be

repaired or replaced. Selected primary ailing members need to be checked for new conditions
and load combinations. If the old member is retained, selective reconstruction or a “fi x” is
required. Load posting may be required during diagnostic design.
Consider physical constraints such as matching new and old components: A newly con-
structed width in an existing bridge may have a different response to shrinkage and creep
strain of concrete.
4. Preservation design: Certain important bridges which are on the Historic Register have spe-

cial requirements for repairs and retrofit. It is a special case of diagnostic design and many
features are common. While funding based priorities are generally used in diagnostic design
and the rehabilitation process is different, funding is less of an issue for the preservation of
historically signifi cant bridges.
In diagnostic design, in place of an expensive solution it is preferred to replace the bridge.
However, in a preservation design an expensive solution is not a deterrent, and restoration is
carried out for aesthetic, historical, and sentimental reasons. Hence, the methods of struc-
tural solutions and the type of repair technology in each case tends to be different. More
sophisticated methods may be adopted for the repairs of a historic bridge since members or
their appearance need to be preserved.
Similarly, inspection frequency would be higher for preservation design than for diagnostic
design. In preservation design, load posting may be frequently required.
According to AASHTO LRFD specifications the minimum life of a bridge is assumed to

be 75 years, which incidentally coincides with the life span of the generation of engineers
who may be regarded as its worldly creators. It appears that historic bridges and most of the
older bridges have survived longer than originally estimated as a result of regular inspec-
tions.

5. Innovative repair, strengthening, and retrofit techniques listed in Chapters 7 through 12 cover
both diagnostic and preservation approaches to designing for maintenance.
6. Steps in a reconstruction process: Steps required for reconstruction include the 5 R’s and/or
widening (W) as shown in Figure 2.9.
• Bridge Management System (BMS) is used for National Bridge Inspection System (NBIS)

based on which sufficiency rating (SR) is computed.
• Bridge or highway structure is evaluated for deficiency or is declared as functionally

obsolete.
• The type of maintenance based on the 5 R’s or widening is selected.
• Funding approval from FHWA or highway funding program is obtained.
• Project scoping is developed based on which feasibility study for preferred and other
alternates is carried out. The FHWA method for accelerated bridge construction and life
cycle cost analysis is applied. Bridge security and environmental issues are considered.
For large or complex projects, Value Engineering is performed. If necessary, the prelimi-

nary design is modified. Final design and contract documents are prepared for award of the
reconstruction contract. Constructability and MPT are considered.