Page 150 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 3 BRIDGE FAILURE STUDIES AND SAFETY ENGINEERING 125

2. Fire resistant design: In 2008, the National Institute of Standards and Technology (NIST)
in Gaithersburg, Maryland recommended the use of a load combination with extreme fi re
conditions in their Report on the (2001) Collapse of World Trade Center Building 7. This

collapse was caused primarily due to fires. Fire damage to several bridges has shown:
• Longer span bridges with thinner web plates are more vulnerable to local buckling and need

to be designed to resist fire. Currently, temperature forces are calculated for an increase

between 68° and 112° F. Thermal stress is added to flexural stress under a dead load and
the cumulative effect may exceed the allowable.

• The failure mechanism under fires needs to be studied. At high temperatures allowable
shear and bending stress decrease. Also, the load path is likely to shift and loads may
become eccentric to the center line of web causing additional moments not allowed for in
design.
• High temperatures cause heat corrosion. Hence, corrosion resistance needs to be improved.

Another method of increasing fire resistance is to use a non-flammable spray paint.
• The author developed Section 45 (Scour at Bridges) and Section 46 (Seismic Design and

Retrofit) for the NJDOT LRFD Bridge Design Manual. Based on research on New Jersey
scour critical bridges, a “Handbook of Scour Countermeasures” was developed by the
author jointly with City University of New York and was approved by FHWA. Based on
studies of material response to fire, a fire resistant design criteria and design method for

steel and prestressed girders needs to be developed.
3. Additional topics which require attention are:
• Applying a more realistic LRFR method for rating
• Applying load and resistance factors based on the LRFD method
• Studying failure mechanisms of different types of structural systems
• Maintaining quality control and personnel safety during construction
• Ensuring seismic retrofit against minor and recurring earthquakes

• Providing scour countermeasures
• Developing and making available codes for rehabilitation of mixed structural systems
• Developing codes for new materials such as FRP decks
• Considering new techniques of repairs similar to those discussed in this book need to be
considered for inclusion in the codes.
4. Provide redundancy in design with more than one load path.
5. Risk management.

3.21.3 Construction Related Activities
Preventing accidents during construction: The construction industry has the highest rate of
accidental deaths compared to say mining or mountain climbing. OSHA (Occupational Safety
and Hazards Administration) maintains records of the recurring events. While original construc-
tion is done only once, maintenance is continuous. Hence, construction issues and diffi culties
are always present. Examples are:
1. Scaffolding failures.
2. Crane failures.
3. Subsidence during deep excavations or land slides.
4. Fire, combustion, and respiratory problems.
5. Underwater diving hazards.
6. Unprotected welding operations.
7. Wind gusts.
8. Explosions.