Section 2: Appraisal RatingsAnchor: #i1005356
Definition of Appraisal Ratings
In making appraisal ratings, consider the field condition, waterway adequacy, geometric and safety configurations, structural evaluation, and safe load capacity of the bridge. Appraisal ratings should be consistent among appraisers given the same field information, project plans, materials, and geometric and waterway data.
Evaluate seven features for their effect on the safety and serviceability of the bridge and its approaches. The intent is to compare the bridge to a new structure built to current standards. Different roadway standards - such as width, grade, and alignment - exist for the various roadway systems in Texas.
Appraisal ratings are usually done in the office where access to all necessary information and specifications is available. However, an experienced bridge appraiser may make some appraisals in the field while performing the duties of a bridge inspector.Anchor: #i1005377
The detailed instructions for entering data are contained in the Coding Guide. The seven features are:
- Traffic Safety Features (Item 36)
- Structural Evaluation (Item 67)
- Deck Geometry (Item 68)
- Underclearances (Item 69)
- Bridge Posting (Item 70)
- Waterway Adequacy (Item 71)
- Approach Roadway Alignment (Item 72).
Four of the seven appraisal ratings are automatically generated from other inspection and inventory data, and include Structural Evaluation (Item 67), Deck Geometry (Item 68), Underclearances (Item 69), and Bridge Posting (Item 70). The remaining three items, Traffic Safety Features (Item 36), Waterway Adequacy (Item 71), and Approach Roadway Alignment (Item 72) are based upon observations and historical data collected during routine inspection events. The following paragraphs summarize instructions for coding the above seven features.
Traffic Safety Features (Item 36). This feature applies only to bridges carrying vehicular traffic. . It is a measure of the adequacy of traffic safety features in meeting current acceptable standards, which reflect modern design criteria. Four digits are assigned that approximately measure the adequacy the traffic safety feature. The first digit is for the bridge railings, the second digit is for the guardrail to bridge railing transitions, the third digit is for approach guardrails, and the fourth digit is for guardrail terminals. Each of these four parts to Item 36 is assigned a value of 1 if it meets currently acceptable standards, a value of 0 if it does not, or a value of N if not applicable. These values do not give a true measure of the comparative strength or crash test level for the traffic safety feature.
Collision damage or deterioration is not considered when assessing traffic safety acceptability. Assume that damage to traffic safety features will be repaired in the near future. Note rail, transition, guardrail, or guardrail termination damage or deterioration on the Bridge Inspection Record.
Bridge class culverts do not require coding of traffic safety features if the headwall of the culvert is 30 ft or more from a traveled lane. With zero to three ft of fill over a culvert and acceptable guard fence installed over the culvert and along the approaches, bridge railings and transitions are not required. Culverts with less than three ft of fill may also have guard fence instead of bridge railing if steel posts are properly attached to the culvert.
Acceptable traffic safety standards have been developed using the current AASHTO Standard Specifications for Highway Bridges1 and the AASHTO Guide for Selecting, Locating, and Designing Traffic Barriers.2
Current acceptable bridge railing details are shown in the Bridge Railing Manual.
Structural Evaluation (Item 67). This feature considers major structural deficiencies and is based on the condition ratings of the Superstructure (Item 59), the Substructure (Item 60), and the Inventory Rating (Item 66) as related to the Average Daily Traffic (Item 29). Items 66 and 29 are correlated in a table included with the detailed instructions for Item 67 in the instructions for the Coding Guide.
The Structural Evaluation Appraisal Rating should generally be no higher than the lowest of the Superstructure or Substructure condition ratings or the Inventory Rating - ADT correlation.
Deck Geometry (Item 68). This feature applies only to bridges that carry vehicle traffic. Roadway widths are measured perpendicular to traffic direction and between faces of railings, curbs, and median barriers. Mountable curbs are ignored if 4 in. or less high.
The Deck Geometry appraisal rating is determined from a four-part table included with the detailed instructions for Item 68 in the Coding Guide. This table relates the ADT (Item 29), Bridge Roadway Width (Item 51), and Number of Lanes (Item 28).
This appraisal rating is further controlled by another table in the instructions for Item 68 in the Coding Guide that relates the Minimum Vertical Clearance (Item 53) and the Functional Classification (Item 26) of the bridge.
The Deck Geometry appraisal rating is the lowest number based on width, lanes, or vertical clearance and functional classification of the highway on which the bridge is located.
Underclearances (Item 69). This feature is a measure of both vertical and lateral clearances for any roadway or railroad passing under the bridge being rated. The vertical clearance is measured down from the lowest part of the bridge to the lower traveled roadway surface (excluding paved shoulders) or top of railroad rails.
The Underclearances appraisal rating is determined from two tables included with the detailed instructions for Item 69 in the Coding Guide. These tables relate the Vertical Underclearance (Item 54) and the Functional Classification (Item 26) of the lower roadway or railroad, and the Lateral Underclearances Right and Left (Items 55 and 56) of the lower roadway or railroad.
The Underclearances appraisal rating is the lowest number based on the vertical and lateral clearances and the functional classification of the lower roadway or railroad.
Bridge Posting (Item 70). This feature compares the load capacity of the bridge to the state legal load. At this time, the term state legal load is a load equivalent to the conventional HS-20 load pattern shown in Figure 5-1. Therefore, any inventory rating less than HS-20 requires further evaluation of the bridge. Bridges are normally not load restricted unless the capacity is less than an HS-20 Operating Rating. See the section of this chapter titled Legal Loads and Load Posting for more detail on the need for load restriction.
Specific criteria for coding this appraisal rating are included with the detailed instructions for Item 70 in the file titled Coding Guide, which has five posting levels. The Bridge Posting appraisal rating is 5 if the Operating Rating (Item 64) is more than HS-20. The Bridge Posting appraisal rating has a value of 0 to 4 depending on the percentage the Operating Rating is below the state legal load, which for this item is HS-20 loading.
Figure 5-1. Standard AASHTO Design Loads
Waterway Adequacy (Item 71). This appraisal feature applies to all bridges carrying vehicle traffic over any type of waterway. It represents the capacity of the waterway opening to carry peak water flows and is based on the criteria included with the detailed instructions for Item 71 in the Coding Guide, which has eight values. The eight values range from 2, meaning the bridge is frequently overtopped by flood waters, to 9, meaning that chance of overtopping is remote.
The estimated potential for traffic delays from flood overtopping is also considered when assigning a value to Waterway Adequacy. The design flood is the maximum water flow that can pass under bridge for a given recurrence frequency, usually expressed in years.
When hydraulic information is unavailable, the design flood is assumed to be equal to the frequency of overtopping the bridge. Local officials and residents can often provide information on the frequency of overtopping.
Approach Roadway Alignment (Item 72). This feature applies to adequacy of the approach roadway to safely carry vehicle traffic considering both horizontal and vertical alignments.
Specific criteria are included with the detailed instructions for Item 72 in the instructions in the file titled Coding Guide. Approach curvature, lane and shoulder widths, surface roughness, and sight distances all enter into the evaluation of this appraisal rating. For bridges on crest or sag vertical curves, consider also headlight and stopping sight distances.
When approach alignment is questionable, drive the alignment on the approaches to the bridge in order to estimate an advisory safe speed with due consideration given to minimum sight distances. Advisory speed on approach curves is the speed above which more than usual concentration and effort on the part of a normal driver would be required to remain safely in the proper lane. Advisory speed limit should be the posted advisory speed if one exists.