3-108 AASHTO LRFD B RIDGE D ESIGN S PECIFICATIONS (SI) For waterways with widths less than 6.0 times the

length overall of the design vessel, LOA, the acceptance criterion for the annual frequency of collapse for each pier and superstructure component shall be determined by distributing the total bridge acceptance criterion, AF, over the number of pier and span components located in the waterway.

For wide waterways with widths greater than 6.0 times LOA, the acceptance criterion for the annual frequency of collapse for each pier and span component shall be determined by distributing the total bridge acceptance criterion over the number of pier and superstructure components located within the distance 3.0 times LOA on each side of the inbound and outbound vessel transit centerline paths.

Based on historical collision data, the primary area of concern for vessel impact is the central portion of the bridge near the navigation channel. The limits of this area extend to a distance of 3.0 times LOA on each side of the inbound and outbound vessel transit path centerlines. For most bridges, these vessel transit path centerlines coincide with the centerline of the navigable channel. Where two- way vessel traffic exists under the bridge, the vessel transit path centerline of the inbound and outbound vessels should be taken as the centerline of each half of the channel, respectively.

The distribution of the AF acceptance criterion among the exposed pier and span components is based on the Designer’s judgment. One method is to equally spread the acceptable risk among all the components. This method is usually not desirable because it fails to take into account the importance and higher cost of most main span components. The preferred method is to apportion the risk to each pier and span component on the basis of its percentage value to the replacement cost of the structure in the central analysis area.

3.14.5.1 Vessel Frequency Distribution

The number of vessels, N, based on size, type, and loading condition and available water depth shall be developed for each pier and span component to be evaluated. Depending on waterway conditions, a differen- tiation between the number and loading condition of vessels transiting inbound and outbound shall be considered.

C3.14.5.1

In developing the design vessel distribution, the Designer should first establish the number and characteristics of the vessels using the navigable waterway or channel under the bridge. Because the water depth limits the size of vessel that could strike a bridge component, the navigable channel vessel frequency data can be modified, as required, on the basis of the water depth at each bridge component to determine the number and characteristics of the vessels that could strike the pier or span component being analyzed. Thus, each component could have a different value of N.

Vessel characteristics necessary to conduct the analysis include:

• Type, i.e., ship or barge;

• Size based on the vessel’s deadweight tonnage, DWT;

• Inbound and outbound operating characteristics; • Loading condition, i.e., loaded, partly loaded,

ballasted, or empty; • Length overall, LOA; • Width or beam, BM;

• Draft associated with each loading condition; • Bow depth, DB;

SECTION 3(SI):LOADS AND LOAD FACTORS 3-109 • Bow shape;

• Displacement;

• Vertical clearances; and

• Number of transits under the bridge each year. Sources for the vessel data and typical ship and barge characteristics are included in the AASHTO Guide Specifications for Vessel Collision Design of Highway Bridges (1991).

The Designer should use judgment in developing a distribution of the vessel frequency data based on discrete groupings or categories of vessel size by DWT. It is recommended that the DWT intervals used in developing the vessel distribution not exceed 20 000 DWT for vessels smaller than 100 000 DWT, and not exceeding 50 000 DWT for ships larger than 100 000 DWT.

3.14.5.2 Probability of Aberrancy

3.14.5.2.1 General

The probability of vessel aberrancy, PA, may be determined by the statistical or the approximate method.

C3.14.5.2.1

The probability of aberrancy is mainly related to the navigation conditions at the bridge site. Vessel traffic regulations, vessel traffic management systems and aids to navigation can improve the navigation conditions and reduce the probability of aberrancy.

The probability of aberrancy, PA, sometimes referred to as the causation probability, is a measure of the risk that a vessel is in trouble as a result of pilot error, adverse environmental conditions, or mechanical failure.

An evaluation of accident statistics indicates that human error and adverse environmental conditions, not mechanical failures, are the primary reasons for accidents. In the United States, an estimated 60 percent to 85 percent of all vessel accidents have been attributed to human error. 3.14.5.2.2 Statistical Method

The probability of aberrancy may be computed on the basis of a statistical analysis of historical data on vessel collisions, rammings, and groundings in the waterway and on the number of vessels transiting the waterway during the period of accident reporting.

C3.14.5.2.2

The most accurate procedure for determining PA is to compute it using long-term vessel accident statistics in the waterway and data on the frequency of ship/barge traffic in the waterway during the same period of time (Larsen 1983). Data from ship simulation studies and radar analysis of vessel movements in the waterway have also been used to estimate PA. Based on historical data, it has been determined that the aberrancy rate for barges is usually two to three times that measured for ships in the same waterway.

3-110 AASHTOLRFDBRIDGE DESIGN SPECIFICATIONS (SI)