Page 270 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 6 APPLICATIONS OF BRIDGE DESIGN AND RATING METHODS 245

• Structure’s serviceability and functional obsolescence—accounting for 30% and based

on ability of bridge to meet current traffic conditions, and
• How essential the bridge is for public use - accounting for 15%.
3. Vulnerability rating
If rating is based on extreme conditions to which the bridge may be subjected in its life time
and is vulnerable to fail as a result it is called vulnerability rating.
Examples are seismic vulnerability or scour vulnerability etc. The behavior of the substruc-
ture or superstructure components under such cases is studied so that a failure can be avoided
by a retrofi t etc.

4. Rating of historic bridges
A record of bridges of historic importance is maintained by National Register of Historic
Places.
Survey information is available on a database kept by the state. General requirements are:
1. Over 50 years age of bridge
2. Stone arch, metal truss and bridges with technological importance.
3. Bridges located in on historic routes or districts.
Instead of replacement of original components, repair is preferred. Strengthening is carried
out at a higher cost than nonhistoric bridges. Nondestructive testing methods to verify component
performance are applied. State guidelines or Secretary of Interior’s Standards for the treatment
of Historic Properties are a good guide for maintenance criteria.

6.3.9 Posting Analysis
As described in AASHTO Rating Manual:
1. When RF 9 1.0 Posting is not required.
2. When RF 9 0.3 but : 1.0 the safe posting load 3 W (RF – 0.3)/0.7 AASHTO LRFR Eq.
(6-7).
3. When lane load governs, W 3 40 Tons.
4. For any vehicle when RF : 0.3 the vehicle type should not be allowed on the span.
5. When RF for all three legal loads (Type 3; Type 3-S2 and Type 3-3 units) : 0.3 the bridge
shall be closed for vehicular traffi c.
6. Speed limit may be lowered to reduce Impact live load.
7. All Regulatory or warning signs shall conform to AASHTO Manual on Uniform Traffi c
Control Devices (MUTCD).

6.3.10 Diagnostic Load Testing as an Alternative or a Supplement to Load Rating
A nondestructive load test is useful for important or complex bridges. It provides in-depth
structural data. In most cases a theoretical analysis or rating is carried out. In some cases as built

drawings may not be available to perform theoretical analysis. After analysis of field test results
the live load rating may be revised. In Section 3, methods of nondestructive testing, repair and

retrofit are discussed. The test vehicle weight is selected.
Cost of load test and condition of bridge determines the need for load test.
Load test is performed by installing instrumentation without closing the bridge.
1. Both linear and nonlinear behavior is studied.
2. Strains, deformations and rotations are measured by field observation. Strain sensors, electrical

resistance gages, acoustic strain gages or strain transducers are attached to critical locations.
Measurements are taken by remote sensors both at the start of test and at selected increments.
Displacements are monitored by electrical, mechanical and optical measurements.