Page 218 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 5 LOAD AND RESISTANCE FACTOR RATING AND REDESIGN 193

length for AASHTO Type IV girders. LRFD specifications required a larger number of

strands for the Type IV girders. The differences in the required number of strands for
LRFD versus standard design increase with an increase in span length and decrease with
an increase in girder spacing and skew angle.
7. For steel bridges, the behavior of homogeneous and isotropic types of steel is well defi ned
compared to that of timber or concrete. The fabricated structural steel industry is more com-
fortable with the use of LRFD as the preferred specifi cation. The reasons are the ongoing
advancements in strength-design-specific areas such as composite systems design, design

of systems using partially restrained connections, and seismic design. The advances in steel
design have simply been applied entirely within the context of the standard specifi cations.
The LRFD specifications help to eliminate some of these limits.

The theory of plasticity can be expressed in terms of a limit state for material behavior.

The limit state specified in AASHTO LRFD was originally introduced in British standards
following the development of the plastic hinge theory for steel frames by J. F. Baker at the
University of Cambridge and for reinforced concrete frames by A. L. L. Baker at Imperial
College London.
8. Composite beam design: A simple comparison between the LRFD and ASD methods il-
lustrates the logical and rational basis of the LRFD strength model. Because concrete inher-
ently behaves inelastically, a composite section does the same. Although the ASD approach
provides a safe design for common loadings, it can be seen that the assumed elastic neutral
axis is incorrect when the failure occurs in the inelastic range. Inelastic behavior makes it
inconsistent or inefficient to use a model that is based upon elastic assumptions.

9. Wood or timber bridges are still being designed and built, especially in the nation’s federal
lands and parks.

5.2.4 Limit State Applied to Design and Rating

It may be defined as a state or condition in which a partial collapse or total collapse may

occur due to flexure or shear failure (LRFD/LRFR strength limit state). The component material
then ceases to meet the provisions for which it was designed.
For general bridge design, four limit states are defi ned:
1. Strength.
2. Serviceability.
3. Fatigue and fracture.
4. Extreme events.
An important goal of a designer is to prevent a limit state from being reached.

5.2.5 Salient Features of the New LRFD/LRFR Method
1. Changes in basic vehicular loads such as the use of HL-93 and tandem vehicles.
2. Permit vehicles shall be used for long-term live loads.
3. Use of the probability theory for equations that predict strength, workmanship, quality
control, and consequence of failure to account for uncertainties in load combination.
4. A variety of distribution factors to simplify the theoretical approach.
5. Resistance calculations for strength I to V and serviceability I to III will be based on cor-
rection factors such as f : 1.0 to account for uncertainties in material properties.
6. Fatigue and fracture: Applications of fracture mechanics theories in design are being in-
troduced. A composite reinforced concrete section may behave inelastically even at small
loads.