Crashworthiness of Bridge Fenders Impacted by a Barge

Rafal Wuttrich, Jerry W. Wekezer, ASCE fellow, Nur Yazdani, ASCE fellow, and Jack Toth ASCE member

Civil Engineering Department, FAMU-FSU College of Engineering, 2525 Pottsdamer St. Tallahassee FL, 32310 wuttrich@eng.fsu.edu,

ABSTRACT

Bridge fender systems are used to guide vessels through navigational channels under bridges. They appear to be an attractive and inexpensive solution for protecting integrity of bridge piers. However, the fenders are not designed to withstand any lateral forces, thus their existing impact capacity is ignored in the design. The paper presents results of research efforts initiated by the Florida Department of Transportation, aimed at evaluating the impact capacity of bridge fenders, and developing possible retrofit recommendations. Project efforts were focused on computational analysis of a Jumbo Hopper barge impacting the fender system. The non-linear finite element computer code LS-DYNA was adopted for analysis. Different initial velocities and impact angles were used to represent possible collision scenarios. Specific retrofit recommendations, allowing for more efficient kinetic energy absorption, were developed and compared with the original design. INTRODUCTION

After thirty-five deaths and excessive damage resulting from the ship impact to the Sunshine Skyway Bridge in Tampa, Florida in 1980, the Federal Highway Administration (FHWA) and eleven state agencies funded a research project to develop design guidelines for bridge protection against such impacts. These guidelines call for design of stronger bridge components, capable of withstanding impacts, or for using protective structures (Knott and Larsen, 1990). Protective structures, built from less expensive materials than structural components of bridges, can efficiently dissipate kinetic energy during impacts. The research was motivated by an idea of utilizing existing bridge fender systems, currently used due to US Coast Guard requirements to navigate vessels through shipping channels, as protective structures (see Figure 1a). Their design didn’ t consider crashworthiness due to lateral forces, thus the impact capacity of the fenders and their retrofit or redesign became the main concern. The major goal of the research was, to investigate ways to absorb as much kinetic energy of the barge, as possible using existing or redesigned structures. Moderate cost of the proposed retrofit was also an important concern in this investigation. Several impact scenarios of a vessel-structure collision were considered. A Jumbo Hopper barge was selected as the impacting vessel, being one of the most popular barge types in the US waterways system. In 1975, the market share of

Jumbo Hoppers was estimated of 27% of all barges (Knott and Larsen, 1990). The collision speed was set to 1.955 m/sec (3.8 knots), as the bound value of current velocities that can be found in midland waterways in Florida. These conservative assumptions should represent most of the actual collisions that can take place.

9.6

m

1.62

5 m

3.38 m

4.87 m14.63 m

a) b)

Figure 1. An example of currently used bridge fenders: a) a picture of an actual bridge fender system b) finite element model of three sections of

fender

The collision scenarios analyzed included initial impact at 90 degrees, as well as 45, 30, and 15 degrees impacts. Different impact angles produce different demands for protective structure design. However, 30 degrees initial angle was considered as the most interesting case. FINITE ELEMENT MODEL

A complete dynamic finite element model of the bridge fender consisted of square prestressed concrete piles embedded in soil. The structure consisted of 355mm-by-355mm vertical and battered piles connected by cables with horizontal square timber walers. The section width taken as the distance from both ends of a waler is 4.87 m. A fender system may consist of several sections, depending on the bridge width. For this study, a response of three sections, with overall width of 14.63 m, was analyzed (see figure 1b). The prestressed concrete piles were modeled using solid, tetrahedral finite elements. The cross-section mesh reflected the strand pattern used in the piles. The strands were modeled using explicit truss elements having common nodes with piles. Initial stress was applied to elements to truss elements to represent the prestressing force. LS-DYNA material Pseudo-Tensor (Material 16) was used as the most suitable for modeling the non-linear behavior of concrete subject to impulsive load (LS-DYNA, 1999). The piles were connected by 15.8-mm (5/8 in) steel cables, according to the original fender design.

The original fender model consisted of 19,924 solid elements along with 8,008 beam elements, from which 7,327 were subjected to the initial stress. The model also included 1,320 non-linear springs attached to the piles to model the soil response. The spring model followed the approach used by Habibagahi and Langner, 1984. The major concern of the project was an assessment of the lateral impact capacity of the fender system. Thus, the impacting vessel model should be only as accurate as it is necessary to provide sufficient and reliable information about general dynamic behavior of the vessel and its influence on the fender. Two areas of the barge have been identified for meshing. The rear of the barge (representing the cargo area) was modeled with coarse mesh, while the front of the barge was modeled with considerable accuracy. For the most reliable results, LS-DYNA contact algorithm requires similar size of meshes of both contacting bodies. Thus, the front of the vessel was modeled by shell elements of size matching those of the fender (See figure 2).

Figure 2. Finite Element model of the barge

ANALYSIS OF BARGE IMPACT WITH ORIGINAL FENDER

90 Degree Impact

Although rare, an impact at 90 degrees is possible when the barge is drifting towards the pier with the river current and the bridge fender is constructed transversally to the shipping channel. The barge produced severe damage to the fender during this impact. The cables connecting the piles failed in the early stage of the collision, which resulted in insufficient interaction between piles. As a result, the piles absorbed the impact energy independently. During the first 800 milliseconds, the resultant velocity of the center of mass of the barge decreased from 3.8 knots to 3.1 knots (0.7 knots drop). This represents

cargo area – solid block elements

bow area – shell elements

18% drop of speed. The collision dissipated approximately 30 percent of the kinetic energy of the barge. This value may vary with the different soil conditions for actual structures. 30 Degree Impact

The damage to the fender structure was still severe. Similarly to the impact from right angle, the pile connections were found to be the weakest structural elements, as they did not provide sufficient resistance. After approximately 2 seconds of impact, the barge was slightly redirected but it still was drifting towards the bridge pier with the residual velocity of 3.3 knots. RETROFIT ANALYSIS

Piles with Larger Cross-Sections

Piles with the larger cross-section were considered as one of the viable retrofit options. Larger piles were assumed as having 455-mm square cross sections with 20 prestressing strands. Different strand pattern required more elements within the pile cross-section. The 3-bay fender model with 455-mm piles consisted of 35,244 solid elements, 15,336 beam elements, and 2,200 springs attached to the piles. This significant increase of the model size was primarily due to the larger number of elements within cross-section of the pile. The computational time increased approximately 2.5-times as compared to 355-mm pile fender. As expected, output data revealed improved crashworthiness of the fender structure. It dissipated more kinetic energy, however, the gain was not as good as expected. Damage was still judged as excessive and the amount of absorbed kinetic energy appeared to be unsatisfactory (see Table 1). Pile Connections using Steel Plates

Weak pile connections were identified as the major contributors to the poor performance of the current fender design. Cables did not provide enough strength when subject to severe load. They failed early and did not sufficiently promote pile-to-pile interaction. A replacement of cables by plates adjacent to piles (as shown on Figure 3) was considered. The piles in the middle of the each section were connected by straight plates, while the piles at the section ends were connected using angle sections. The plates were to be connected to the piles by bolts. Bolts were modeled as rigid constraints with brittle failure criteria. This model improved the connection strength. However, possible excessive corrosion of steel elements in the humid Florida environment would offset these gains.

Figure 3. Plate connection retrofit

Concrete wedge retrofit

Concrete wedges, as shown on Figure 4, were used as another feasible retrofit. Thus, the wedge and pile were merged, using common nodes in the finite element model. This assumption can be satisfied only if the wedge-to-pile connection is constructed accurately.

Figure 4. Concrete wedge retrofit

The solution provided substantial increase of the structure stiffness resulting in significant change of the barge behavior during the collision. The strong pile connections promoted sufficient interaction of adjacent piles. More kinetic energy of the vessel was absorbed than in the original design. In addition, the new fender redirected the barge, so that the barge did not drift toward the bridge structure. This is an essential gain in crashworthiness of the structure.

CONCLUSIONS

Bridge fenders can serve as protection for bridge structures against errant vessels. Their crashworthiness can be improved by solutions suggested, providing protection for the main bridge sub-structure. They can substantially reduce the damage to the bridge structures due to collisions. Behavior of the fender system during ship impact largely depends on the pile connections. Stiffer connections provide better energy transfer between adjacent piles resulting in improved crash energy distribution throughout the system. The concrete head seems to be especially suitable for new fender systems, where some elements can be precast. Connecting of the concrete head to the pile will require a customized solution, extensive labor, and a precision technology regime to avoid damage to the pile strands. ACKNOWLEDGEMENTS AND DISCLAIMERS

This publication was prepared in cooperation with the State of Florida Department of Transportation under Project No. WPI-0510846. The authors would like to acknowledge the financial support of the Florida Department of Transportation that made this research project possible. Special thanks are extended to Mr. Ned Kawar (Project Manager) and Dr. Joe Bhuvasorakul from the Florida Department of Transportation. The opinions, findings, and conclusions expressed in this publication are those of the authors and not necessarily those of the Florida Department of Transportation or the U.S. Department of Transportation. REFERENCES

Habibagahi, K, Langner, J. A. (1984) Horizontal Subgrade Modulus of Granular Soil, in Laterally Loaded Deep Foundations: Analysis and Performance, Langner, Mosely and Thompson eds., ASTM Publication code No. 04-835000-38, American Society for Testing Materials, 1984 pp. 21-34. LS-DYNA (1999). “LS-DYNA Keyword User’s Manual. Nonlinear Dynamic Analysis of Structures in Three Dimensions” . Livermore Software Technology Corporation. Knott, M. and Larsen, O. D. (1990), “Guide Specifications and Commentary for Vessel Collision Design of Highway bridges” , Federal Highway Administration, Report No. FHWA-RD-91-006, McLean, VA.