The two-story Anchorage Museum expansion provided 30,000 sqaure feet of new gallery space for the Art of the North Collection, as well as a new temporary gallery, new member and donor lounge, offices, an expansion of the Imaginarium Discovery Center, and an outdoor patio area. The original museum building, a concrete and masonry structure built in the 1960s, was not designed to support the added loads from the addition. Located in a downtown Anchorage neighborhood bordered by busy thoroughfares, there was no room for horizontal expansion. This presented a unique design challenge. The addition was added onto and above the existing building, supported on isolated footings independent of the existing structure. Portions of the new structure adjacent to existing basement walls are supported on helical piles to reduce the risk of damage to the walls. The structural engineers designed 60-foot steel columns that ran through holes in the existing concrete roof and positioned the addition off the new roof structure. The addition is suspended from steel plate girders of various sizes, topping out at 5 feet deep and 50,000 pounds. These girders extend up to 50 feet beyond the supporting columns.

Prior to conducting site explorations, Shannon & Wilson reviewed existing geotechnical site data from earlier building additions and performed preliminary engineering studies. We identified the need for additional explorations due to the high dead loads for the new structure. Following the preliminary evaluation, additional explorations were conducted, including multiple borings and lab analysis of samples in Shannon & Wilson’s Anchorage laboratory. We conducted settlement analyses due to concerns that long-term consolidation of an underlying soft clay layer could cause adverse settlements beneath the new structure as well as under existing adjacent structures. 

Our Geotechnical Report included recommendations for supporting the new structure on shallow foundations and helical piles for new foundations adjacent to existing basement walls and for underpinning existing shallow foundations during construction. We provided recommendations for lateral earth pressure/resistance, floor slab support, building settlement, surrounding sidewalk pavements, excavation slopes and utility trenches, and dewatering and drainage during construction. Because of the location of the building in downtown Anchorage, we also evaluated site stability under seismic loading.