Lutak Dock Structural Assessment (DRAFT)
The 68-page draft Lutak Dock Structural Assessment by PND Engineers is attached.
As stated in the executive summary:
The City of Haines, Alaska has commissioned this report to document results of the structural assessment of the Lutak Dock located in Haines, AK. The effort was directed to reviewing existing documentation for both maintenance and repairs, and to determine the probable remaining service life under static (non‐seismic), and seismic conditions. This structural assessment has been based on the guidance established by the United States Army Corps of Engineers (USACE) for cellular structures as identified in manual EM 1110‐2‐2503 “Design of Sheet Pile Cellular Structures, Cofferdams and Retaining Structures”.
Notwithstanding the maintenance and repairs/modifications to the structure since the original USACE design and construction in 1953, it is the opinion of PND Engineers, Inc. (PND) that the structure has reached the end of credible 60‐year service life. Further utilization is effectively on “borrowed time.” The presence of sink holes in the working surface of the structure is consistent with loss of fill arising through observable gaps between the main cells and the Z‐sheet pile sections utilized for repairs. Per Ref. #4, this work is assumed to have been done around 2003. It could be argued that these gaps and material loss can be repaired by re‐sealing/backfilling the closure arcs.
A source of significant risk to the structure arises from failure of the “tee” connection between the lower closure arc(s), and main arcs themselves. This risk is in addition to the damage at 5 of 11 closure arcs that arose after the 2002 repair, and the splitting failure at Closure Arc 7.5. Other structural weaknesses were revealed in this assessment though none as historically troublesome as the welded connection between the main cell and the closure arc. In 1965 the connection detail utilized for the original Lutak Dock was specifically prohibited by the USACE after a series of arc connection failures on temporary cellular structures utilized on the island river system. Given the USACE design background, the current corrosion loss, and the results of this assessment, it is the view of PND that failure conditions exist at all other closure arcs.
Notwithstanding the strong evidence of distress at closure arcs, the results of this assessment demonstrate that the Lutak Dock does not meet current USACE minimum factors of safety for cellular structures for the classic failure mode of vertical shear under the conditions of dead load plus operating live load, and for dead load plus phreatic water pressure. While it can be said that absent the failure of Closure Arc 7.5 the structure has remained serviceable, the calculations by PND demonstrate that the facility is “near the edges” and that in our view it is prudent to begin the process to replace the structure to meet current minimum standards under operating conditions and potential seismic loading. The seismic forces arising in Haines are modest, and the structure meets criteria USACE critera for low intensity earthquake with ground acceleration of .072g (50% likelihood of occurrence in 50 years). Above this level of ground shaking the structure cannot withstand earthquakes at the current “design event” level criterial mandated by building codes, waterfront design guides, or departments of transportation manuals.
