This project involved the renovation and restoration of a National Historic Landmark constructed in 1917 at 1785 Massachusetts Avenue, NW, Washington, DC. The renovated structure would serve as the new headquarters building for the American Enterprise Institute (Fig. 1). The existing structure consisted of five levels above grade and one level below grade. The scope of new construction involved extensive renovations and structural upgrades to the superstructure along with construction of an additional basement level. A new deep foundation system and perimeter shoring system was required to allow excavation beneath the existing spread footing foundations. To accomplish this, existing interior steel columns were supported by one hundred thirty-eight micropiles that provided temporary support during excavation to new subgrade and later served as the permanent foundation system for the renovated structure. Load transfer from interior columns to micropiles was accomplished using transfer beams and a trussed-tower system consisting of the micropiles and lacing (Figs. 2 and 3). To support the perimeter walls and columns, sixty-seven battered micropiles were installed first along the perimeter followed by hand dug concrete pit underpinning. The piles were strategically positioned to provide support for the completed underpinning. During excavation to new subgrade, the underpinning was laterally supported by two levels of temporary tiebacks drilled through the pits. Once excavation reached subgrade, new pile caps were constructed that were supported by the interior micropiles. Concrete columns were then constructed connecting the caps to the base of the steel columns. A load transfer program was performed as the final step in transferring the building loads to the new foundation system. The project required a total of two hundred and thirty-six micropiles installed in low headroom conditions.
Subjects: Bridge, approach slab, abutment, embankment, settlement, downdrag, negative skin friction
Author: SungJe Chi, P.E.
Publisher: DFI