Page 269 - Bridge and Highway Structure Rehabilitation and Repair

244 SECTION 2 STRENGTHENING AND REPAIR WORK

The following formula is recommended:
Live load capacity rating 3

(Allowable load – Dead load)
8 Vehicle Weight (tons)
(Rating vehicle live load + Impact)
Rating vehicles include:

• HS vehicles
• H vehicles
• Legal load vehicles
• Military loads
• Permit loads
• Type 3 unit; Type 3-S2 unit; Type 3-3 unit.
LRFR Factor 3 (C 6 DC 6 DW 4 – P) / (LL 4 I)
C 3)c )s ) Rn
3 LRFD Resistance Factor. )s 3 1 for redundancy
)c )s 9 0.85
C 3 Allowable stress in the Service Limit State.
Live load rating is for two load levels

• Inventory rating: A load level which can safely utilize a bridge for an indefinite period of
time. Inventory level reflects the existing bridge and material conditions with regard to

corrosion, loss of section and other deficiencies. It does not exceed the design stresses.
For steel girders the allowable inventory stress is 55% of yield stress:
Grade A 36 – 19.8 ksi
Grade 50 or 50W – 27.5 ksi
Grade 70W – 38.5 ksi
• Operating rating: The absolute maximum permissible load to which the bridge can be
subjected. The use of bridge by unlimited number of heavier vehicles would exceed the
capacity and is not permitted.
For example the load produced by a permit vehicle must be lower than the structural capac-
ity determined by operating rating.

2. Sufficiency Rating: It is defined as a calculated rating indicating the bridge’s sufficiency or
capability.
Ratings range from 100 (entirely sufficient) to 0 (entirely insufficient or defi cient).

Sufficiency Rating is considered by the federal government when a state requests federal
bridge dollars to improve the condition of the bridge. Bridges with low suffi ciency ratings are
eligible for more funds.
Sufficiency Rating vs. Funding Eligibility

0–49 Eligible for costs to replace bridge
50–79 Eligible for costs to rehabilitate or refurbish bridge
80–100 not available
Factors included in the calculation are:
• Structure’s adequacy and safety - accounting for 55% and based on inspection data,