Page 92 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 2 DIAGNOSTIC DESIGN AND SELECTIVE RECONSTRUCTION 67

• Documentation
• Move to other location

2.6.2 LRFD Code Compliance

1. Inspection reports form the basis of interpretating of fi eld deficiencies, structural evalu-
ation, rating, selection of rehabilitation method, analysis and computer aided design, and
application of AASHTO and state codes of practice. Generally, only a few components of
a bridge need replacement, while many others are retained. Sometimes this may result in a
mismatch due to differences in old and new materials.
2. Inspection of 50- to 100-year-old structures has shown that they were designed using old
AASHO code, which was in effect at that time. Both design criteria and construction tech-
niques have changed since those days. We are dealing with many thousands of older bridges.
Design practices in the olden days were based on intuition, thumb rules, case studies, and
limited practical experience. They were later followed by hand calculations using simple

formulae. Modern day design requires code compliance and proficiency in computer analysis
and software use. Stiffness matrix and finite elements methods are currently used for analysis

and ultimate load methods for design.
3. A mixed technology scenario is sometimes necessary for widening and rehabilitation. Table
2.2 summarizes the variations in old and new materials and methods. Notable differences
compiled by the author show applicable new live loads and new design criteria.
4. At the time of their design, there were no criteria for heavy permit loads and live loads were
much lighter. Also, there was no ultimate load design, fatigue resistant details, or security
considerations. Not all long span bridges for example would meet new security criteria. There
are practical diffi culties in upgrading and bringing up to date an old bridge to conform to
newer technology.
5. Multi-girder construction has taken over non-redundant through girder systems. Working
stress design has been replaced by load resistance factor design. Use of materials having steel
yield strength of 30,000 psi is to be replaced by 50,000 or 70,000 psi of HPS 70W grade.
Similarly, concrete crushing strength has increased from 2500 psi to 5000 psi of HPC.

6. The history of failures has shown that major failures are due to floods, earthquakes, or
fatigue. It is important to check the safety for both scour and seismic criteria given in AAS-

HTO LRFD Code 2007 and recommend adequate retrofits. An important part of seismic
response is the substructure stiffness and foundation capacity. Also, the effect of scour on
foundation capacity may reduce seismic response since existing pier footings, which have
become exposed, may cause settlement during a seismic event.
Hence, old bridges require added attention and special procedures need to be developed
for their rehabilitation. Repairing old structures to meet new AASHTO criteria requires
planning, use of modern materials, and innovative techniques.

2.6.3 Restoration Methods for Historic Bridges
1. Bridges which are listed in federal and state historic registers have slightly different criteria
than routine type repairs. All new modifications need to conform to the original features and

to shapes and sizes of members and connections, which are now likely to be extinct. Such
repairs and retrofits tend to be more expensive.

2. Each historic bridge needs to be treated for its uniqueness on an individual basis. Repairs
of suspension cable bridges, truss bridges, and arch bridges present special problems. For
example, the deck stiffening of the Golden Gate Bridge is an interesting case in point.
3. Relocation factors:
• Due to deficiencies in original site selection, foundation settlement may have taken place.

It may be desirable to move the bridge to a new location. In such cases, new foundations