Page 213 - Bridge and Highway Structure Rehabilitation and Repair

188 SECTION 2 STRENGTHENING AND REPAIR WORK

2. This value is typically used when evaluating overweight permit vehicle moves. Operating
rating: Operating rating will result in the absolute maximum permissible load level to which
the structure may be subjected for the vehicle type used in the rating. This rating determines
the capacity of the bridge for occasional use. Allowing unlimited numbers of vehicles to
subject the bridge to the operating level will compromise the bridge life.
3. Posting rating: The posting rating is the capacity rating for the vehicle type used in the rat-
ing. It is a load level which may safely utilize an existing structure on a routine basis for
a limited period of time. The posting rating for a bridge is based on inventory level plus a
fraction of the difference between inventory and operating.
This may lead to load restrictions of the bridge or identification of components that require

rehabilitation or other modification to avoid posting of the bridge.

4. Preparing a load rating report: A load rating report shall include the following:
• Material properties
• Loading assumptions
• Plans or sketches showing all properties and assumptions

• Printout of computer data file(s) where appropriate
• Documentation of structural model used in analysis
• Inventory, operating, and posting summary for HS-20 and all legal loads
• Electronic copies of data fi le(s).

5.1.4 Reduced Rating—Allowable Strengths of Materials
For old bridges, where possible coupons obtained from the bridges may be tested in a
laboratory:

1. In the absence of coupon tests, for wrought iron girders, strength in tension and bending for
inventory rating is 14,600 psi and operating rating is 20,000 psi.
2. For concrete, maximum allowable bending stress may be reduced as per Section D.6.6.2.4.
and D.6.6.3.2 of the Manual of Condition Evaluation and LRFR.

Upon completion of replacement or rehabilitation of a posted structure, the need for load
restriction no longer exists. The Bridge Management Section would then prepare a “Removal
of Load Restriction Resolution.”
5.1.5 Rating Live Loads

Bridge capacity depends upon bridge geometry, material strength, condition, structure type,
etc. As related to trucks, a bridge’s capacity depends not only upon the gross weight, but also
upon the number and spacing of axles and distribution of load between the axles.

Since it is not practical to rate a bridge for the countless axle configurations, bridges are
rated for six standard vehicles (including HS-20) which are representative of actual vehicles on
the highways.

5.1.6 Analytical Steps in Load Rating
The following analytical steps are required to determine load rating:

1. Determine section properties.
2. Determine allowable and/or yield stresses.
3. Calculate section capacities.
4. Determine dead load effects.
5. Calculate dead load portion for section capacity and live load effect.
6. Calculate live load impact and distribution, and allowable factor.