Page 216 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 5 LOAD AND RESISTANCE FACTOR RATING AND REDESIGN 191

methodologies with load and resistance factors based on the known variability of applied
loads and material properties.
3. The plastic theory is a direct approach since the failure of a beam or column will occur by
the yielding of steel or crack formations in concrete. Compared to the yield point limit for
applied load used in the elastic theory, most materials are capable of resisting even higher
loads beyond yield and prior to failure. To a great extent, the true material behavior can be
represented by applying the plastic theory. Redistribution of stresses in the member and
formation of plastic hinges prior to failure lead to an accurate determination of reserve
capacity of the member and ultimate loads.

5.2.2 Applicable AASHTO Codes
All construction activities associated with rehabilitation require planning, analysis, and
compliance with design codes. In the U.S., the entire highway inspection, monitoring, repair,
and rehabilitation of bridges and highway structures is governed by the provisions laid down in
the following codes:

1. Manual for Condition Evaluation of Bridges, 2nd Edition, AASHTO, 2000 using load factor
design (LFD) (for rating analysis).
2. Manual for Condition Evaluation and Load and Resistance Factor Rating (LRFR) of Highway
Bridges, 1st Edition, AASHTO 2003, Washington D.C., ISBN: 1-56051-283-0 using load
resistance factor design (for rating analysis; supersedes LFD).
3. AASHTO LRFD Bridge Design Specifi cations, 2007 (for redesign).
nd
4. AASHTO LRFD Bridge Construction Specifi cations, 2 Edition, 2006 Interim Revisions
(for reconstruction).
5. The state or agency technical specifications covering the type of materials, the method of

measurements and payments, etc.

5.2.3 History of Design and Rating Specifi cations
In the U.S., there has been a progressive evolution of structural design codes. Existing bridges
have been constructed based on one of the following codes:

• Original AASHO code.
• AASHTO specifi cations.
• AASHTO Manual for Condition Evaluation.
• AASHTO English standard specifi cations.
• AASHTO metric standard specifi cations.
• AASHTO LRFD specifi cations.
• AASHTO LRFR Condition Evaluation Manual.
1. Additional rules are usually supplemented by a state bridge design manual.

AASHTO first introduced the Standard Specifications for Highway Bridges (Standard)

in 1931, and since then these specifications have been updated through 19 editions. The last

edition was published in 2007. The standard specifications were based on the allowable stress
design (ASD) philosophy until 1970, after which the load factor design (LFD) philosophy
was incorporated into the specifi cations.
Earlier methodologies provide a desirable level of safety for bridge designs, but do not

ensure a uniform level of safety. AASHTO introduced the LRFD specifications in 1994 to
bring consistency to the design safety level. AASHTO also adopted the Guide Manual for
Condition Evaluation and Load and Resistance Factor Rating (LRFR) of Highway Bridges
in 2002, which incorporates the same philosophies into the rating of bridge conditions.