Page 316 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 7 BRIDGE WIDENING AND DECK REPLACEMENT STRATEGY 291

steel girder or type of abutment or pier. Fixing (repairing or rehabilitating) old bridges versus
building new ones is an engineering decision guided by the condition of the old structure and

available funding. A cost benefit analysis needs to be carried out prior to funding in millions of
dollars. The engineer needs to determine the remaining useful life of the existing structure, its
maintenance cost per lane per year, and compare that to the corresponding cost for a new one.

7.3.8 FHWA Deterministic Approach to LCCA (Refer to Chapter 2 for LCCA Application)
Structural engineering projects can cost hundreds of millions of dollars. LCCA is an asset
management and economic analysis tool useful in selecting the preferred alternative by assigning
values to relative merits. LCCA considers both short-term and long-term costs.
FHWA lists the following steps for a deterministic approach:
1. Establish design alternatives.
2. Determine activity timing.
3. Estimate costs (agency and user).
4. Compute life cycle costs.
5. Analyze the results.

6. Effects of inflation: Expenditures occur in the past or future and are therefore measured in
different value units because of infl ation.
7. Effects of discounting:
Adjusting for the opportunity value of time is known as discounting.
(Dollar) 3 (Dollar) 8 (Price index) /(Price index)
base year dats year base year dats year
The consumer price index for each year is available.
Real discount rate ranges from 3 to 5 percent.
Discount future constant value costs to present value by using the formula:
Present value 3 Future value 81/Discount factor
n
(14 r) 3 Discount factor
Where r 3 Real discount rate
n 3 Number of years in the future when cost will be incurred
Computer software is available to compute life cycle costs. (Reference FHWA, Lifecycle
Cost Analysis Primer)

7.3.9 Evaluating the Relative Cost of Rehabilitation versus Replacement

A significant portion of the nation’s inventory of bridges is rapidly approaching the end of its
intended design life. Therefore, it is helpful to understand the processes which cause reduction
in service life. Introducing innovative methods for extending the life of these structures through
quick construction needs to be encouraged. The following factors need to be evaluated:
• Condition of deck, superstructure, and substructure
• Cost of rehabilitation of substructure and superstructure
• Cost of replacement of substructure and superstructure
• User cost of delays and detour during rehabilitation or replacement
• Life cycle cost analysis of rehabilitation and replacement.

All conclusions drawn in the replacement versus rehabilitation discussion process must be
fully documented to discuss the differing viewpoints and gain the knowledge and experience of
the team before making the fi nal decision.