Page 212 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 5 LOAD AND RESISTANCE FACTOR RATING AND REDESIGN 187

Capacity C 3 7 7 7 R where 7 7 9 0.85
s
c
c
n
s
7 3 Condition factor
c
7 3 System factor
s
7 3 Resistance factor.
For service limit state C 3 f (allowable stress specifi ed in LRFD specifi cations):
R
DC 3 Dead loads
DW 3 Dead load of wearing surface and utilities
and 3 LRFD load factors, replacing A factor for dead load in LFD rating
DC DW 1
equation
3 Evaluation live load factor, replacing A factor for live load in LFD
L 1
rating equation
P 3 Permanent load other than dead load
LL 3 Live load effect
IM 3 Dynamic load allowance.

Rating is calculated in tons and standard truck weight is multiplied by rating factor to evalu-
ate capacity.
RT 3 W 8 RF

If the rating factor is : 1 the following measures are required:
1. Load posting using AASHTO Manual on Uniform Traffic Control Devices

(MUTCD): When RF is between 0.3 and 1, safe posting load 3 (RF 6 0.3) W/0.7.
RF : 0.3, a lower legal load vehicle is posted.

2. Strengthening: When RF : 0.3 for the three AASHTO specified legal trucks, the bridge is
due for strengthening or partial replacement.
• Girder replacement: If it is not feasible to strengthen, girders need to be replaced with
deck slab and parapets.
• Bridge replacement: If substructure requires major repairs, the entire bridge needs to be
replaced.
• Strengthening and replacement of members requires detailed design for all possible envi-
ronmental loads and hence many more design load combinations are applicable than for
the load rating.
• Rating is followed by design. LRFD computer software computes and prints out ratings
in addition to design results. Hence, rating and design usually go hand in hand.
• Live load deflection control such as L/800 is not be applicable for rating.

• Wind loads, temperature effects, and earthquakes are not considered for load ratings.
• Loads on substructures are both vertical and horizontal. Any redesign of substructures
may require applying all AASHTO LRFD load combinations.

5.1.3 Types of Ratings
Load rating analysis of bridges is performed to determine the live load that structures can
safely carry. Bridges are rated at three different stress levels, referred to as:

1. Inventory rating: Inventory rating is the capacity rating for the vehicle type used in the rating
that will result in a load level which can safely utilize an existing structure for an indefi nite
period of time. Inventory load level approximates the design load level for normal service
conditions.