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March 24, 2005

Piers, wharves could be the next quake victims

  • But new seismic standards are on the way
    • By FARHAD ROWSHANZAMIR
    Reid Middleton

    Remember the waterfront labor action in 2002? Its impact was felt throughout the United States economy, which is not surprising considering that $260 billion in cargo flows through the 29 major West Coast ports.

    Now imagine if a sizeable earthquake struck part of the West Coast — say Puget Sound. What if that earthquake crippled the ports of Seattle and Tacoma, and its effects rippled down to the ports of Vancouver and Portland? Besides the personal losses, what damage would be inflicted on the local economy, the national economy, and to the port districts saddled with the costly construction repair bill?

    Can port authorities risk having seismically substandard port facilities and what impact will enhancing these facilities have on construction costs?

    New standards coming

    Marine structures do not behave the same as buildings or bridges. Design codes specifically tailored for unique waterfront structures such as piers and wharves have not been well developed. That is about to change.

    The Coasts, Oceans, Ports, and Rivers Institute (COPRI) of the American Society of Civil Engineers (ASCE) is trying to help. The institute has established a committee focused on developing a new standard for seismic design of piers and wharves. The standard will provide analysis and design guidance. It will also address the unique load combinations of piers and wharves, including berthing and mooring loads as well as geotechnical issues.

    The question is what type of impact will these new standards have upon the construction of pier and wharf facilities?

    Prior standards

    The issue of inadequate seismic standards for waterfront structures has been raised before. The ASCE-sponsored Ports '92 conference in Seattle had a session called "Proposed Seismic Design Methods for Piers and Wharves." The session discussed standards and made some design recommendations.

    The Ports '95 conference in Tampa, Fla., presented "Code Recommendations for Waterfront Structures." This session focused on the lack of direction in the seismic design of waterfront structures and the disparity between various codes and methodologies.

    ASCE published a 1998 document called "Seismic Guidelines of Ports" with a comprehensive overview and assessment of current practice in that area.

    Finally, the importance of our publicly owned and operated port assets to our region's economic well-being and their potential seismic vulnerability was presented and discussed at the Seattle Fault Earthquake Scenario conference held in February in Bellevue.

    Current codes

    The International Building Code 2003, adopted by Washington state for building design, does not directly address waterfront structures, but references the U.S. Navy's Military Handbook MILHBK 1025/1 for design of piers and wharves. In turn, that handbook references the bridge code by American Association of State Highways and Transportation Officials (AASHTO) for seismic design.

    However, the Marine Facilities Division of the California State Lands Commission recently formalized the seismic design of marine structures through its Marine Oil Terminal Engineering and Maintenance Standards (MOTEMS) 2004. While this standard is explicitly tailored for waterfront structures, its use is primarily focused on marine oil terminals subject to heavy loading that may cause extensive environmental damage from fuel or oil spills due to poor earthquake performance.

    AASHTO vs. MOTEMS

    The most significant difference between AASHTO and MOTEMS is performance-based design. Like most codes, AASHTO uses a probabilistic seismic hazard model recommended by the National Earthquake Hazards Reduction Program.

    Designers use AASHTO to calibrate all ground motions and the corresponding force estimates for a major seismic event that would happen every 475 years. AASHTO then classifies structures by identifying "seismic performance zones" based on that ground motion. Structures are ranked as critical, essential, or not. Estimated force levels are adjusted accordingly. For example, plumb reinforced concrete piles in an essential structure would be detailed for double-the-force compared to a non-essential structure in the same location.

    Other levels of seismic risk or further ranking of risk aversion are not recognized by AASHTO. The design remains linear in the sense that a force is applied and components are designed to resist the resulting stresses without yielding.

    The AASHTO code approaches the seismic design problem by the less complicated and more traditional method of using smaller force levels, where inelastic material behavior is not explicitly considered in the analysis, but where each element is detailed with increased toughness to accommodate some level of expected yielding. Deflections are recognized explicitly in the design of the bridge seats and implicitly in the detailing requirements for various components.

    MOTEMS uses a performance-based seismic design approach, which allows an owner to select a prescribed level of structural performance. It is also probabilistic based, but it explicitly quantifies different risks associated with differing seismic events.

    For MOTEMS, risk is classified in three main categories of high, moderate and low. For each class, there are two levels of performance, depending on risk aversion to damage: minor or no structural damage with temporary or no interruption of operations; and prevention of collapse with some damage and temporary loss of operations "restorable within months." Each category has a corresponding probability or risk with return periods ranging from 36 years (frequent and low intensity) to 475 years (infrequent and high intensity).

    Authorities and owners can associate the cost of risk aversion — you get what you pay for — and weigh it against an estimated life-cycle cost including cost of possible loss of operations.

    MOTEMS and forces

    Force when applied to any structure causes displacements. Inside a structure, the forces are associated with stresses and displacements with strains. Traditionally, structural analysis calculated stresses and the required member size and detail to resist them.

    MOTEMS sets "target displacement" as the basis of design. A structure is pushed to that displacement and various components are sized and detailed such that their corresponding strains associated with deflections, not stresses associated with forces, are within acceptable limits. This follows the latest performance-based design standard ASCE 31, and the FEMA 356 document, for seismic rehabilitation of buildings.

    The methodology may have extra detailing requirements that affect construction time. The exhaustive engineering effort involves soil--structure interaction analysis with many inherent assumptions and "push over" analysis that requires quantification of "plastic hinges" in the components. This type of analysis considers inelastic behavior of structures when they are pushed beyond their elastic limit and form hinges. At each step, a hinge is formed somewhere in the structure, the analysis has to stop and reconfigure the structural model to accommodate the new hinge and resume. This non-linear analysis requires much iteration.

    This newer technology is now made possible by the increasingly less expensive hardware and software appropriate for the elaborate numerical analysis and, like any new technology, requires the test of time.

    On the horizon

    The Port of Seattle and the Navy recently applied MOTEMS to their waterfront structures. This code is highly detailed in its quantification of risk levels and, therefore, requires costly engineering at the onset. The extra engineering may or may not result in lighter structures, but it is certainly hoped to result in better performing structures.

    An important feature is that the designs using this new standard should perform more reliably in earthquakes. Not all waterfront structures may need this high level of engineering and earthquake performance; however those related to critical lifeline systems, regional transportation systems and port-based businesses may benefit.

    Farhad Rowshanzamir is the senior structural engineer with Reid Middleton in Everett. He specializes in bridge and wharf design.


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