Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
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| 8.4.3 Vessel Collision Loads Collision of a ship or barge with abridge structure can be a severe loading condition that may result in collapse of the bridge. AASHTO (2014) requires that all bridge components in navigable waterways, located in water depths greater than 2 feet, be designed for vessel impact. The requirements in AASHTO (2014) have been adapted from the AASHTO Guide Specifications and Commentary for Design for Vessel Collision Design of Highway Bridges (1991) using the Method II risk acceptance alternative, and modified for the second edition (2009). Vessel collision loads are determined based on the selection of a design vessel and considerations of the vessel and waterway relative to the bridge, such as the waterway geometry, the waterway depth, the size, type, loading, and frequency of vessels in the waterway, the vessel speed and direction, and the structural response of the bridge to the collision. The determination of the design vessel involves probabilistic analyses and risk assessments, and generally requires a multidisciplinary team. These analyses are beyond the scope of this manual refer to AASHTO (2014) for detailed information on the determination of the design vessel. Once a design vessel is selected, the design weight tonnage (DWT) is determined and the head-on ship collision impact force on a pier can be determined as follows ππ π π = 8.15ππβπ·π·ππππ Equation 8-7) 124 Where P s = Equivalent static vessel impact force (kips. DWT = Deadweight tonnage of vessel (tonne. V = Vessel impact velocity (ft/sec). Since the design load is treated as an equivalent static force, the lateral loading analysis for the foundation is performed using static loads rather than dynamic or cyclical loads. Two design cases are assessed for substructure design once the equivalent static force is determined (AASHTO 2014): β’ 100 percent of the design impact force is applied in a direction parallel to the alignment of the centerline of the navigable channel β’ 50 percent of the design impact force in the direction normal to the direction of the centerline of the channel. All components of the substructure exposed to physical contact by any portion of the vessel are to be designed to resist the applied loads. The assessment should consider the geometry of the vessel in determining the portions of the substructure that maybe in contact with the vessel, as well as crushing of the bow of the vessel. The impact force is applied as follows (AASHTO 2014) for each of the two design cases described above For overall stability, the design impact force is applied as a concentrated force on the substructure at the mean high water level (MHL) of the waterway as shown in Figure 8-2. For local collision forces, the design impact force is applied as a vertical line load equally distributed along the vesselβs bow depth as shown in Figure 8-3. The vesselβs bow is considered to be raked forward in determining the potential contact area of the impact force on the substructure. Fora barge impact, the local collision force is taken as a vertical line load equally distributed on the depth of the head block as shown in Figure 8-4. The foundations are designed for the force effects calculated for all the above conditions (parallel and normal to centerline, for concentrated and distributed force applications. The most critical case governs the design. Additional vessel impact collision loads may include collision with the bow, deckhouse, or mast with the superstructure. As these loads are directly applied to the superstructure, a structural analysis is required to develop the force effects at the head of the deep foundation elements. Once the force effects on the foundations are determined, a lateral loading analysis can be performed according to the procedures previously outlined in this manual. Protection against vessel impact loads can be provided by inclusion of physical protection systems such as fenders, pile clusters, dolphins, islands, or other measures. Such measures may reduce or eliminate the vessel collision forces applied to the bridge structure and its foundation. The design of such protective systems usually involves an iterative process to evaluate the energy absorption capacity of the system including flexure, torsion, shear, and displacement of the system components) versus the kinetic energy of the vessel (AASHTO 2014). This type of analysis is beyond the scope of this manual. |