Page 147 - Bridge and Highway Structure Rehabilitation and Repair

122 SECTION 1 ADMINISTRATIVE ISSUES

Both fascia and interior girders need to be designed. If the fascia is lighter than the in-
terior, good practice will be to adopt the heavier interior girder design throughout. This is
necessary for future widening, otherwise the fascia girder and part of the deck will need
replacement.
7. Longitudinal joints shall be avoided. In place of transverse deck joints, integral abutments
may be preferred. Deck joints usually have maintenance issues due to higher impact from
vehicles.
8. Walls of the abutment and pier should be placed at 90 degrees to the direction of traffi c fl ow
to avoid any skew. Minimum vertical clearance for new bridges shall be 16 feet 6 inches.
The horizontal distance to the face of the abutment and pier needs to be a minimum 30 feet
from the edge of the travel lane. This distance may be reduced if guard rails are provided in
front of the abutment or pier.
9. Geotechnical borehole results must be applied in selecting shallow or deep foundations.
Weaker soils require deep foundations, such as piles, drilled piers, or caissons. For fi rm soils
with a bearing capacity of 2 ksf and above, shallow footings should be considered.
10. Traffic count is required to determine the number of lanes, with traffic projections for 20

years ADTT. A provision for deck widening will be included.
3.20.5 Steps to Avoid Superstructure Failures
1. Durability requirements need to be addressed on a scientifi c basis.
2. The engineer needs to become familiar with the principles of related engineering disciplines,
such as dynamic analysis, hydraulics, and materials science.
3. AASHTO LRFD code provisions and other relevant FHWA guidelines to be followed:
• Strength and serviceability load combinations for constructability that are not covered in
AASHTO LRFD code are presented in Chapter 5. Application of these loads will minimize
the number of construction accidents.
• Strength and serviceability load combinations for ship collisions that are not covered in
AASHTO LRFD code are presented in Chapter 5.
• Collapse can be reduced by using standard details and ductile moment resistance connec-
tions.
• The expected life of 75 years or more for modern bridges and their components may not
always be achieved without regular inspection, structural evaluation, and timely rehabilita-
tion.

4. Deficient modeling: The following are some of the unknowns a bridge designer will typically
encounter:

• Inability to define loads accurately, such as magnitude and unpredictable level of stress
distribution from settlement
• Limited redundancy in structural system
• Inability to fully include plastic behavior of composite action between the concrete deck
slab and repeated beams, arching action, creep, and shrinkage strain distribution in the
deck slab
• Lack of information on fracture mechanics in general and lack of understanding of fracture
of new materials in particular
• Inelastic behavior of connections and joints, splices, gusset plates, bolts, and welds
• Complex behavior as a unifi ed assembly of uncombined multiple components of mixed
(old and new) materials and structural systems, resulting from rehabilitation or widening
methods
• Delamination and reduction in strength of concrete deck due to deicing salts (as observed
from chain drag test)