Page 77 - Bridge and Highway Structure Rehabilitation and Repair

52 SECTION 1 ADMINISTRATIVE ISSUES

2.3 PROGRESSIVE DESIGN PHASES FOR CONTINUITY
2.3.1 Routine, Diagnostic, and Preservation Design
1. Original or routine design (shown in Figure 2.10): New bridge design is based on the latest
codes of practice from AASHTO and the state DOT. The latest technology is utilized. There
are many options available for superstructure planning and design. Structural solutions and
the type of foundation to be used are broad based.
Major funding is required. However, life cycle costs can be controlled through judicious
planning and design. In original design, no load posting is required.
AASHTO LRFD specifications deal primarily with routine design of new members and

not with the redesign of deficient or “rogue” members. At present there are no separate

specifications for diagnostic design. Although there are many types of defects, the practical
issues related to all types of defects need to be addressed on their own merits, perhaps on a
case-by-case basis. A comprehensive maintenance code would therefore be desirable.
2. Diagnostic design: Older bridges were designed to different criteria, using different mate-
rials and for different loads, a long time ago. Fatigue and environmental constraints such
as corrosion have given rise to structural defi ciencies. To identify the defi ciencies and the
degree of damage, a diagnosis based on inspection or remote health monitoring is carried
out on a regular basis. Some major and some minor repairs may be required, based on the
diagnosis.
Smart solutions need to be based on diagnostic type design. Diagnostic design steps
required for a typical pre-reconstruction planning process are shown in Figure 2.10.
An example requiring diagnostic design is when a lower strength of concrete exists. This
can be known by taking core samples from abutment or wingwall and testing the samples
in a laboratory. Older bridges may have used lower strengths (below 3000 psi) while new
bridges the require Class A concrete (4000 psi or higher) in some states. Similarly, current
requirements of 60,000 psi for yield of reinforcing bars may not be met, since in olden days
yield strength was not even 40,000 psi. Such bridges may be strengthened if they are not
beyond repair.
Another example requiring diagnostic design is at zones of high fatigue in a steel beam
subject to reversal stress from moving loads. This would require flange plates to be added. If

new plates are not added, allowable stresses in bending and shear may be lowered in keeping
with the fatigue deficiency. Load posting to a lower live load may be required.

Figure 2.10 Progressive design phases that may occur in the life of a bridge.