Structures Design Guidelines | July 2007
2 - Loads and Load Factors |

## 2.11 Vessel Collision [3.14]

### 2.11.1 General [3.14.1] (01/07) Jan 2007, Inserted wording about automatic, user changeable traffic escalation factors and recommendation that the user compare the total vessel count used in the analysis with the latest data from the Army Corps.

The design of all bridges over navigable waters must include consideration for possible Vessel Collision (usually from barges or ocean going ships). Conduct a vessel risk analysis to determine the most economical method for protecting the bridge. The marine vessel traffic characteristics are available for bridges located across inland waterways and rivers carrying predominately barges. The number of vessel passages and the vessel sizes are embedded as an integral part of the Department’s Vessel Collision Risk Analysis Software. The vessel traffic provided is based on the year 2000 and an automatic traffic escalation factor is provided by the software for the various past points which one selects. It is recommended that the engineer compare the total vessel trip count being used in the risk analysis with the latest total vessel trip count provided for the appropriate section of waterway as published by the Army Corps. The escalation factor provided by the software can be modified by the engineer. The importance classification is provided for existing bridge sites and will be provided by the Department for any new bridge location. Port facilities and small terminals handling ships are not covered by the catalog of vessel traffic characteristics. In these cases, on-site investigation is required to establish the vessel traffic characteristics. Utilize the LRFD specification and comply with the procedure described hereinafter.

### 2.11.2 Research and Information Assembly (07/05) July 2005 added hyperlink to USACE Nav Data Center.

(When not provided by the Department)

A. Data Sources:

1.) U.S. Army Corps of Engineers, Waterborne Commerce Statistics Center, P.O. Box 61280, New Orleans, LA 70161. Telephone: (504) 862-1472.

2.) U.S. Army Corps of Engineers, Navigation Data Center (http://www.iwr.usace.army.mil/ndc/publications.htm)

2.) U.S. Army Corps of Engineers, "Waterborne Commerce of the United States (WCUS), Parts 1 & 2," Water Resources Support Center (WRSC), Fort Belvoir, VA.

3.) U.S. Army Corps of Engineers, "Waterborne Transportation Lines of the United States," WRSC, Fort Belvoir, VA.

4.) U.S. Army Corps of Engineers (COE), District Offices.

5.) U.S. Coast Guard, Marine Safety Office (MSO).

6.) Port Authorities and Water Dependent Industries.

7.) Pilot Associations and Merchant Marine Organizations.

8.) National Oceanic and Atmospheric Administration (NOAA), "Tidal Current Tables; Tidal Current Charts and Nautical Charts," National Ocean Service, Rockville, Maryland.

9.) Bridge tender record for bascule bridge at the District Maintenance Office.

10.) Local tug and barge companies.

B. Assembly of Information:

The EOR must assemble the following information:

1.) Characteristics of the waterway including:

a.) Nautical chart of the waterway.

b.) Type and geometry of bridge.

c.) Preliminary plan and elevation drawings depicting the number, size and location of the proposed piers, navigation channel, width, depth and geometry.

d.) Average current velocity across the waterway.

2.) Characteristics of the vessels and traffic including:

a.) Ship, tug and barge sizes (length, width and height)

b.) Number of passages for ships, tugs and barges per year (last five years and prediction to end of 25 years in the future).

c.) Vessel displacements.

d.) Cargo displacements (deadweight tonnage).

e.) Draft (depth below the waterline) of ships, tugs and barges.

f.) The overall length and speed of tow.

3.) Accident reports.

4.) Bridge Importance Classification.

### 2.11.3 Design Vessel [3.14.4][3.14.5.3]

When utilizing the FDOT’s Mathcad software for conducting the Vessel Collision risk analysis, a “Design Vessel,” which represents all the vessels, is not required. The software computes the risk of collision for several vessel groups with every pier. When calculating the geometric probability, the overall length of each vessel group (LOA) is used instead of the LOA of a single “Design Vessel.”

### 2.11.4 Design Methodology - Damage Permitted [3.14.13] (
07/05)
July 2005 Per HTB, added para G. requiring even AF distribution over all components.

In addition to utilizing the general design recommendations presented in LRFD (except as noted herein), the EOR must also use the following design methodology:

A. At least one iteration of secondary effects in columns must be included; i.e., axial load times the initial lateral deflection.

B. The analysis must include the effects of force transfer to the superstructure. Bearings, including neoprene pads, transfer lateral forces to the superstructure. Analysis of force transfer through the mechanisms at the superstructure/ substructure interface must be evaluated by use of generally accepted theory and practice.

C. The ultimate bearing capacity (UBC) of axially loaded piles must be limited to the compressive and/or tensile loads determined in accordance with the requirements of SDG Chapter 3
. Load redistribution must not be permitted when the axial pile capacity is reached; rather, axial capacity must be limited to the ultimate limit as established by analysis.

D. Lateral soil-pile response must be determined by concepts utilizing a coefficient of sub-grade modulus provided or approved by the Geotechnical Engineer. Group effects must be considered.

E. For the designer’s Vessel Collision risk analysis, the FDOT will determine whether a bridge is critical or non-critical. A list is provided with the Department’s software.

F. Use Load Combination “Extreme Event II” as follows:

(Permanent Loads) + WA+FR+CV

With all load factors equal to 1.0. Nonlinear structural effects must be included and can be significant. It is anticipated that the entire substructure (including piles) may have to be replaced and the superstructure repaired if a bridge is subjected to this design impact load; however, the superstructure must not collapse.

Commentary: Further refinement or complication of this load case is unwarranted.

G. Unless directed otherwise by the Department, distribute the total AF acceptance criterion equally (as practical construction considerations allow) among the bridge components. Ignore any benefit provided to the channel piers if a fender system is provided.

### 2.11.5 Widenings

Major widening
of bridges spanning navigable waterways must be designed for Vessel Collision. Minor widenings
of bridges spanning navigable waterways will be considered on an individual basis for Vessel Collision design requirements.

### 2.11.6 Movable Bridges

For movable bridges, comply with the requirements of this chapter.

### 2.11.7 Main Span Length (01/06) Revised per TDB C05-11, August 19, 2005.

A. The length of the main span between centerlines of piers at the navigable channel must be based upon the Coast Guard requirements, the Vessel Collision risk analysis (in conjunction with a least-cost analysis), and aesthetic considerations.

B. Where the vessel traffic volume, at high level fixed bridges, is such that the risk analysis results in channel pier strength requirements in excess of 1,500 kips, a continuous girder superstructure is required. The channel span will be set at a minimum of 200 feet or as specified by the Department.

Commentary:

Additional safety and structural redundancy is provided at bridge locations where large volumes of commercial traffic exist. Safety considerations and unknowns surrounding the probability of vessel collision justify the relatively small additional construction expense.

### 2.11.8 Scour with Vessel Collision [3.14.1]

A. Substructures must be designed for an extreme Vessel Collision load by a ship or barge simultaneous with scour. Design the substructure to withstand the following two Load/Scour (LS) combinations:

1.) Load/Scour Combination 1: |
LS(1)= Vessel Collision @ ½ Long-Term Scour | [Eq. 3-4]
Where: |
Vessel Collision: Assumed to occur at normal operating speed. |
Long-Term Scour: Defined in Chapter 4 of the FDOT Drainage Manual |
2.) Load/Scour Combination 2: |
LS(2) = Minimum Impact Vessel @ ½ 100-Year Scour | Eq. 3-5]
Where: |
Min. Impact Vessel as defined in LRFD [3.14.1] with related collision speed. |
100-Year Scour as defined in Chapter 4 of the FDOT Drainage Manual |

B. When preparing the soil models for computing the substructure strengths, and when otherwise modeling stiffness, analyze and assign soil strength parameters to the soil depth that is subject to Local and Contraction Scour that may have filled back in. The soil model must utilize strength characteristics over this depth that are compatible with the type soil that would be present after having been hydraulically redeposited.

Commentary: In many cases, there may be little difference between the soil strength of the natural streambed and that of the soil that is redeposited subsequent to a scour event.

### 2.11.9 Application of Impact Forces [3.14.14]

When the length to width ratio (L/W) is 2.0 or greater for long narrow footings in the waterway, apply the longitudinal force within the limits of the distance that is equal to the length minus twice the width, (L-2W), in accordance with Figure 2-1.

### 2.11.10 Impact Forces on Superstructure [3.14.14.2]

Apply Vessel Impact Forces (superstructure) in accordance with LRFD [3.14.14.2].