Page 159 - Bridge and Highway Structure Rehabilitation and Repair

134 SECTION 2 STRENGTHENING AND REPAIR WORK

No reliable design is possible without accurately knowing the deformations, bending moments,
shear forces, and foundation reactions under real and projected environmental conditions. Prac-
tical considerations will not be overlooked when analyzing or redesigning for rehabilitation.
Diagnostic analysis will address a variety of issues, including:
1. Meeting any security needs against bomb blasts for maintaining important bridges.
2. Creating minimum environmental impacts.
3. Meeting constructability requirements.
4. Achieving the expected serviceability.
5. Low initial construction or life cycle costs.
6. Easy inspectability.
7. Preserving aesthetics.
8. Resisting extreme conditions of design for earthquakes and fl ood scour.
4.1.3 Diagnostic and Preservation Methods

For new or replacement design, the theoretical approach is based on conventional theories

laid down in Chapters 3 and 4 of AASHTO LRFD specifications. The application of theoretical
tools used for rehabilitation is also similar; for example, the stiffness matrix and fi nite elements
method are still applicable. However, for bridge rehabilitation the unknowns are greater. Hence,

the objectives or requirements and the physical procedures in the field are site specific and need
to be carried out on a case-by-case basis.
Examples of diagnostic and preservation methods are:
1. Analysis for staged construction: Parapet or girder replacement would require lane closure

and partial bridge shut down. Since structural behavior is modified, a new analysis of the
superstructure would be required.
2. Jacking the beam ends at abutments and piers: For bearing repairs and replacement, it is
customary to apply the load upwards by using hydraulic jacks. Jacks are placed directly
under the beams or a jacking beam is installed. Since the direction of load causes tension
in the slab, grid beam, analysis is required to limit the bending and shear stresses and to
control vertical defl ection. Jacking load is applied in successive increments of 1/16 to 1/8
inch.
3. Deck replacement: An analysis of girders is required for concrete pour sequence and for new
loads from a deck slab. Both non-composite and composite cases need to be considered.
4. Seismic retrofi t: Placing of isolation bearings would change the structural behavior of the
members during a seismic event. Seismic analysis is required to design bearings.
5. Providing scour countermeasures: Scour analysis based on HEC-18 and HEC-23 methods
would be required.
6. Rehabilitation for movable bridges, curved bridges, segmental and cable-stayed bridges, erec-
tion loads, painting loads, etc. present additional challenges. For analysis, the mathematical

model should match field conditions rather than be based on unrealistic assumptions.
7. Underpinning of foundations: To prevent the settlement of foundations, a dead load analysis
of the bridge substructure and superstructure is required.
8. Post widening behavior: It is important that the remodeled bridge behave as a single structure
rather than two or more separate structures.
• New concrete materials for widening have different shrinkage and creep strains than the
old concrete.
• A new foundation is likely to settle more than the existing foundation.
• Old fascia girders need to be analyzed as interior girders with increased live loads.
• Longitudinal joints need to be checked for seismic response in the transverse direction.