Challenge

Wolf Point East is a 60+story residential tower under construction and located on the Chicago River with an estimated cost of $350 million dollars. The tower includes 6 levels of underground parking and a core structure which extends approximately 35ft and 53ft below grade respectively.  The design of an Earth Retention System (ERS) presented challenges due to the small size and irregular shape of the lot; presence of adjacent parking structures which would remain operational during construction; Wolf Point Plaza; North Orleans Street; bracing layout and conflicts with column/shear wall lines; and the Chicago River on two sides. In addition, City Authorities required hand calculations for the design of the main members of the ERS system in preference to computer output.


Solution

GZA engineers performed the design using DeepXcav software and validated it through hand calculations. Hand calculations were performed in MathCAD with a special function tool that allows to report all the “numbers” that go into every equation. After submission of the hand calculations to the City, the Engineer requested a 2D structural model of the internal bracing scheme to show the load distribution around the bracing. This analysis was performed using RISA3D structural software.

The ERS was composed of a steel sheet pile perimeter system down to elevation -50 (City of Chicago Datum (CCD)) and then a Core ERS composed soldier piles and lagging that extended from -29 to -59 (CCD). The two-level internal bracing system was composed of skewed 24 to 36-inch diameter Grade 3 pipe sections with varying wall thicknesses and heavy single & double W36 wales. The internal bracing system also supported the neighboring parking garage which extended to similar elevation below grade. Vertical transfer beams were used to span between the neighboring parking garage slabs to avoid unfavorable loading of its exterior walls with high corner brace loads. The design load on the braces reached up to 2,100kips and jacking loads were between 180 to 1050kips. The excavation included the removal of approximately 40,100 yd3 of material.  

The tower structure is supported on large diameter drilled shaft foundations. The requirement for an internal braced ERS and the space for construction of large diameter shafts severely limited the remaining workspace.  The core excavation required another 19-ft of excavation using internally braced soldier piles and lagging. GZA performed overall global stability analysis to ensure the main ERS, including the core ERS was stable. As the internal brace system and corner struts had to be supported on the neighboring parking garage, GZA designed a system to spread the loads to floor elevations and checked the parking garage slab against the applied thrust.


Benefit

The underground portion of the proposed building was constructed on schedule. The use of hand calculations in conjunction with geotechnical and structural software provided accurate estimates of the loads imposed on the ERS, which ultimately substantiated the installation of a more cost effective ERS resulting in savings in materials and labor during construction. The designed system was effective in providing the needed lateral support and allowed the below grade construction to be completed efficiently meeting the contractor’s requirements and satisfied the OUC concerns. The project received an SEAOI Excellence Award.  

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