Assumed Load Ratings

Assumed load ratings can be utilized for concrete or masonry structures that do not have plans or plans do not contain adequate structural information to perform calculated load ratings. These types of ratings are based on field conditions of the structure and documented engineering judgement. The following conditions must apply for an assumed load rating to be applicable:

The bridge has been carrying unrestricted traffic for at least 4 years; and

The bridge shows no signs of significant distress; and

The simple span bridge's span-to-depth ratio of the main members does not exceed approximately 20; and

Construction details such as slab thickness and reinforcement cover over any exposed reinforcing conform to specifications at the time of the estimated construction; and

Appearance of the bridge indicates that construction was performed by a competent builder.

Additional consideration for rating concrete bridges with unknown reinforcing:

Concrete decks that are a minimum of 6 inches thick and have an Item 58 (SNBI: B.C.01) condition rating greater than or equal to 5 shall not control the overall load rating of a bridge. If Item 58 (SNBI: B.C.01) <5, assume IR=HS15 [27 Tons (SNBI: RF=0.75)] and OR=HS20 [36 Tons (SNBI: RF=1.00)], or lower and post at Inventory level if the rating of the deck is controlling.

If condition ratings meet the criteria in Figure 6-1
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., then assume an Inventory Rating of HS15 [27 Tons (SNBI: RF=0.75)] and an Operating Rating of HS20 [36 Tons (SNBI: RF=1.00)]. If the condition rating criteria is not met, assume IR = HS15 [27 Tons (SNBI: RF=0.75)] and OR = HS20 [36 Tons (SNBI: RF=1.00)] or lower based on engineering judgement, and post the bridge at the Inventory level. The same ratio of SHV and EV rating factors shall be assumed. For example, if inventory rating is assumed to be HS15 and operating rating is assumed to be HS20, AASHTO Legal Vehicles, SHV and EV operating rating factors shall be 1.00.

A comparative original design rating can be used to estimate the amount of reinforcing in the main members. Normally, if the design was done prior to about 1950 and the above five considerations are met, then the amount of reinforcing can be estimated based on a percentage of the gross concrete area of the main beams (if tee-beam construction), or depth of slab (if slab construction).

Section 1: Load Ratings