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VESSEL IMPACT DESIGN FOR THE NEW OCEAN CITY-LONGPORT BRIDGE RUE, VESSEL IMPACT DESIGN, PAGE 1 OF 8
VESSEL IMPACT DESIGN FOR THE NEW OCEAN
CITY-LONGPORT BRIDGE

David L. Rue; Joseph Mumber; and Andrew J. Foden
Parsons Brinckerhoff, U.S.A.










Abstract
Designing for vessel impact is a complex process that requires that appropriate
consideration be given to the various design elements that are unique to a given structure,
including bridge type, span arrangement and site conditions. A logical approach was
developed using available design codes and engineering judgment for the design of the
Ocean City-Longport Bridge. The process that was used for the vessel collision design
was the Method II probabilistic approach as outlined in the AASHTO Guide
Specifications for Vessel Collision Design of Highway Bridges. After determining the
design vessel in consultation with the Federal Highway Administration (FHWA), New
Jersey Department of Transportation (NJDOT), and Cape May County Bridge
Commission (the bridge owner), certain assumptions had to be made in order to arrive at
a practical design for the extreme event of a vessel collision. The most significant of these
assumptions was the selection of an appropriate scour condition to be used in conjunction
with the various vessel impact loads. This paper will outline the procedures used,
including the critical assumptions made, to perform the vessel impact design for the
Ocean City-Longport Bridge.

1. Introduction
Replacement of the existing Ocean City-Longport Bridge is a very important project to
the community of Ocean City and to the New Jersey tourist industry. The existing bridge
was in very poor condition with an estimated remaining life of less than five years if no
major rehabilitation project was undertaken. As the last available route out of Ocean City
to the mainland in extreme high tide situations, the bridge is critically important as an
emergency evacuation route for the barrier island in the event of a severe storm.
RUE, VESSEL IMPACT DESIGN, PAGE 2 OF 8
The new structure is a 1052 m long high-level (20 m underclearance above mean high
water) fixed bridge that will replace a low-level bascule bridge spanning over the Great
Egg Harbor Inlet leading to the Atlantic Ocean on the New Jersey coast. The 26-span
structure utilizes a prestressed concrete multi-girder superstructure supporting a
reinforced concrete deck slab. The approach spans vary from 26 m to 37 m long and
consist of conventional AASHTO girders made continuous for live load. The deep-water
spans near the center of the bridge, including the new navigation span, consist of three
separate 3-span continuous units made up of 2290 mm deep spliced post-tensioned
concrete I-girders with a maximum span length of 68 m. The substructure consists of
prestressed concrete cylinder pile bents for the approach spans in the shallow water. The
deep-water main spans are supported by hammerhead piers on pile caps formed with
precast concrete cofferdams constructed near the water level and founded on concrete
cylinder piles that extend as much as 34 m below the inlet bottom due to scour
considerations.

The vessel impact load controlled the design of the deep-water hammerhead piers in the
vicinity of the navigation channel. Therefore, selection of an appropriate vessel impact
design approach was crucial to obtaining a reasonable and economical design. In
determining the design loads, some assumptions were made using engineering judgment,
because all of the design issues are not fully addressed in the available codes.

2. Design Criteria
The load combinations for extreme events specified in the 1994 AASHTO LRFD Bridge
Design Specifications differ from those contained in the 1996 AASHTO Standard
Specifications for Highway Bridges. The 1991 AASHTO Guide Specifications and
Commentary for Vessel Collision Design of Highway Bridges provides guidance on the
design process for determining the vessel impact loads and outlines a probabilistic-based
analysis method for developing a bridge design that will be able to resist these loads with
an acceptable risk against failure. However, none of the available specifications address
the issue of extreme events such as ship impact and simultaneously occurring scour
conditions. Therefore, using engineering judgment, the load combinations described in
the following sections were developed.

2.1 Scour Considerations
Scour refers to the changing conditions of the ground elevations at a waterway opening.
There are various methods prescribed in available literature for estimating the three
distinct components that must be considered for the bridge design:
1. Long-term channel degradation refers to scour across the entire waterway breadth.
Over time the water depth tends to increase as a result of natural or man-induced
operations occurring upstream or downstream of where a bridge is located, and is
independent of the absence or presence of a bridge. Long-term channel degradation
is progressive and not usually reversible. RUE, VESSEL IMPACT DESIGN, PAGE 3 OF 8
2. Contraction scour is the removal of streambed or bank material across all or most of
the width of the channel, caused by constriction of the waterway by the bridge piers
and approach embankments.
3. Local scour is the removal of streambed material at a pier or abutment by
acceleration of flow and vortices caused by the presence of the pier or abutment.
Voids from local and contraction scour are usually resedimented shortly after the
flooding event and are considered short-term or short duration events. The scoured
condition may only last a few hours or at most a few days before refilling.

2.2 Load Combinations
Since there are no well-defined procedures for combining vessel collision loads in
conjunction with scour, the following design criteria were used with respect to vessel
collision and the simultaneously occurring scour:
1. The probability of the simultaneous occurrence of a fully loaded vessel traveling at
normal speed striking a pier at the same time that short-term scour is occurring is
very small and can be neglected, since normal transportation will not occur in the
extreme weather conditions associated with such scour.
2. The probability of a simultaneous occurrence of a fully loaded barge traveling at
normal speed striking a pier and long-term scour is not negligible and should be
considered. The following load combination results:

Load Case I = DL + CV (fully loaded barge) + SC (long-term)

Where
DL = Dead Load
CV
=
Vessel
Collision
SC
=
Scour

3. The probability of a single empty barge that has broken loose from its mooring,
floating at the speed of the current, striking a pier at the same time that short-term
scour is occurring is a plausible situation. This loading is defined in Section 3.16 of
the Guide Specification. The following load combination results:

Load Case II = DL + CV (drifting empty barge) + SC (long-term + short-term)

3. Vessel Impact Design Philosophy
Using these guidelines, we performed the probability based risk analysis contained in the
Guideline Specifications and in the LRFD Design Code (Method II), wherein the vessel
size and water current velocity are determined based upon available information. After
exhaustive research in an effort to define the frequency of vessel traffic, it was
determined that relatively few vessels of significant size regularly use the inlet. However,
over the past several years there has been a periodic program sponsored by the U.S. Army
Corps of Engineers to replenish the sand that has tended to erode from Ocean Citys RUE, VESSEL IMPACT DESIGN, PAGE 4 OF 8
beaches. Sand that had been dredged from the back bays would be transported by barge
through the inlet and deposited on the ocean beaches. If one of these sand-loaded barges
were to strike the new bridge, it would result in a very large force that would have the
potential to cause failure of the impacted component. Records of the historic barge traffic
were kept in the daily logs of the operators of the existing movable bridge, since a bridge
opening was required for each passage. These records define the freq