Page 223 - Bridge and Highway Structure Rehabilitation and Repair

198 SECTION 2 STRENGTHENING AND REPAIR WORK

designer to select safe and economical girder sizes in the future. Travel comfort, durability,
and vibration-free bridges will result.
6. Use of shallow girders case study for Magnolia Avenue Bridge in City of Elizabeth, New
Jersey:
Preliminary design by the author shows that the AASHTO/NJDOT vertical underclearance
requirement on the heavily used bridge over Route 1 and 9 was 16 ft 6 in while a higher
girder depth using 50 ksi yield strength would only provide 14 ft 6 in. By using hybrid 50W
and 70W girders it was possible to restrict girder depth to 3 ft 6 in over a span of 129.5 feet.
Hence, the vertical underclearance increased by 1 ft 3 in to 15 ft 9 in. This was accepted as
a design exception by NJDOT considering that other existing bridges in the corridor had
lower than a 15-foot vertical underclearance.
The bridge had two 15-foot lanes with two sidewalks. Both AASHTO LRFD and NJDOT
Bridge Manual required L/1000 live load deflection criteria (i.e. 1.5 inch maximum under

HL-93 live load). With only 3 ft 6 in girder depth, maximum deflection under live load plus
impact exceeded well over 1.5 in. However, even with reduced girder spacing of 6 ft 6 in,

girder deflection exceeded well over 1.5 in.
The AASHTO requirement of L/1000 is conservative, especially for HPS 70W steel, and
can only be met at the expense of reducing vertical underclearance. The L/D for the girders
is on the higher side amounting to 37. It was a source of concern for the QA/QC Depart-
ment, which finally understood the need for a design exception for hybrid closely spaced

girders.
7. On other HPS 70W projects in New Jersey, where vertical underclearance is not an issue,
current practice is to use a deeper 50W web with flanges using HPS 70W steel. Also, when

adjacent bridges or those on local roads have a vertical underclearance of 14 ft 6 in, a deeper
girder web can be used and no design is needed.
Composite sections with HPS 70W and HPC decks with 5 or higher strengths: Since de-
flection is inversely proportional to the EI value of the composite section, high performance

concrete is also playing an indirect role in the sizing of girder depths. Defl ection control
needs to be investigated for a variety of HPC strengths and deck slab thicknesses, with the
view to enhance performance of such bridges.

5.3.4 Mathematical Approach
The following parameters need to be considered in analysis:
1. Plan aspect ratio.
2. Skew angles and curved slabs.
3. Number of girders.
4. Spacing of girders.
5. Boundary conditions:
• Simply supported
• Continuous
• Fixed
• Girders framing into integral abutments
• Girders framing into semi-integral abutments.

Fascia girder deflections are also affected by the thickness of the sidewalk on the cantilever
side. Hence, variations in the deck slab thickness need to be considered for fascia girders
for bridge decks with sidewalks.

For serviceability and durability, the maximum live load deflection calculated from any
of the above methods must be less than the AASHTO criteria.