Page 211 - Bridge and Highway Structure Rehabilitation and Repair

186 SECTION 2 STRENGTHENING AND REPAIR WORK

There are several methods of structural evaluation of a bridge (rating). Both theoretical and
physical methods are being used:
1. Live load rating based on LRFR methods.
2. Non-destructive static tests (such as placing sand bags with known loads).

3. Moving load tests and field observations (using calibrated vehicles): Structural capacities and
loadings are used to analyze the critical members and determine the appropriate load rating.
Load rating by load testing may be feasible in special cases such as:
• When analytical results provide a posting or operating rating factor :1, but the bridge is
otherwise showing no visual signs of distress
• When construction plan records for the bridge are not available
• When the bridge is of a special type that cannot be analytically rated.
Load testing is performed by driving a truck of known axle weights over a bridge. Stresses
are then measured in the load-carrying members with strain gauges and specially designed
data analysis equipment. These axle weights and actual measured stresses are used to cali-
brate the computer input data.
4. Live load vehicles vary in intensity (Table 5.3), such as:
• First level standard vehicles (HL-93).

• Second level legal loads: AASHTO or state specified legal trucks are used. Weigh stations are
common on important routes in the U.S. to ensure that no posted weight is exceeded.
• Third level permit loads: Permit vehicles are two to three times heavier than HL-93. Per-
mits are issued for a limited number of crossings or for the whole year. Heavier vehicles
need to be escorted.
• For complex bridges or those with non-structural members, evaluation of combined
performance may not be accurate. The performance of existing components such as
girders needs to be checked against different levels of live loads and rated using a com-
mon rating factor.
5. Using engineering judgment: For bridges with no engineering plans available, it is not easy
to field measure and evaluate every structural detail. Stone masonry and concrete bridges

provide problems since reinforcing, prestressing, or encased steel details are not obvious.
A balance needs to be maintained so that load estimates are not rated too high (for safety)
or too low (to disallow use by permit vehicles). A criteria needs to be developed based on
engineering judgment and experience. Alternatively, expensive NDT techniques can be ap-
plied. Computation methods using theoretical vehicles are the least expensive.

5.1.2 Rating Factor
In the past, a rating factor was based on the LFD method as follows:
RF 3 (C 6 DA ) / A (L 4 I)
1 2
C 3 Capacity of member
D 3 Dead weight effects
L 3 Live load effects
I 3 Impact factor
A 3 Factor for dead load
1
A 3 Factor for live load.
2
RF for LFD is expanded as the general LRFR equation given below (refer to AASHTO
Manual for Condition Evaluation and Load and Resistance Factor Rating of Highway Bridges,
October 2003):
Rating factor RF 3 [C 6 ( ) DC 6 ( ) DW 4 ( ) P] / ( )/ (LL 4 IM) 9 1
P
L
DC
DW