Page 214 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 5 LOAD AND RESISTANCE FACTOR RATING AND REDESIGN 189

The stress levels used to analyze critical members and determine an appropriate inventory
and operating rating are outlined in AASHTO’s Manual for Condition Evaluation of Bridges.
5.1.7 Selection of Members for Rating
1. It is transportation policy to rate only the primary load-carrying members in a bridge. For
superstructure, this is normally the slabs, girders, trusses, or arch ring.
2. Concrete box culverts are rated as rigid frames.
3. Not included in the load rating are the piers, abutments, and foundations. The condition
of these elements shall be considered, and they shall be assumed to safely carry the loads
transmitted to them unless there is evidence of serious deterioration.

5.1.8 Restricted Live Load Posting
The Bridge Management Section implements load restrictions by preparing a “Load Restric-

tion Resolution.” The Bridge Management Section then informs public officials about the intended
load restrictions. Those receiving information include local fire companies, school transportation

directors, the local authority for regional transport, senators, and representatives.
When a bridge is not able to safely carry the loads allowed by state statute, it is posted for
its reduced capacity. The policy of most agencies is to restrict loads on bridges when the post-
ing-rating factor drops below 1.0 for any of the state legal truck loads. The minimum posting
is 3 tons.

5.1.9 Analytical Differences between Design and Rating
1. Rating is based mainly on live loads. Available live load capacity of a bridge can be compared
with various levels of truck weights such as:

• AASHTO defined HL-93 and legal loads and state defined permit loads or

• County defined reduced truck loads and maximum legal loads.

For selecting live load trucks for rating, several lighter truck loads such as H-20 and ML-80
are also considered. A port authority may use special heavy intensity truck loads on its local
bridges for high density loading on freight trains.
2. Rehabilitation covers both rating and redesign aspects. In fact, redesign of an existing
member is dependent upon the rating. Dead load analysis is common to both design and
rating. In both cases basic consideration is the effective use and application of construction
materials, namely steel, reinforced concrete, prestressed concrete, or timber. Replacement
or rehabilitation of bridges is based on detailed analysis and design which includes checking
the strengths of members and connections for a variety of primary and secondary effects
including extreme events.

3. Deflection control is not an essential part of the rating but may be required for rehabilitation
and redesign.
4. For important bridges, seismic vulnerability and scour sufficiency may be evaluated on a

case-by-case basis, and bearings, piers, or foundation retrofit may be implemented. Service

load combinations are for deflection control and crack control.
5. Due to the small probability of occurrence, bridge girders are not rated for wind, temperature,
or extreme events, although they are retrofi tted in detail for seismic forces, accidents, and
scour.
6. Only steel girders are rated for fatigue.
7. Timber girders are only rated for strength I and II but not for service load.
8. Ultimate loads and service load combinations will be based on the following:
Strength I: Relating to normal use without wind
Strength II: Permit vehicles without wind