Structural Assessment of an Existing Precast Concrete Superstructure for Change of Use
We investigated the load capacity of an existing office building constructed with precast prestressed concrete approximately 40 years ago.
The second floor framing and superstructure were to support a sanctuary; this is considered an assembly load according to the 2006 International Building Code versus the office loading of the original design.
We conducted non-destructive testing to identify the concrete strength and reinforcement with a Concrete Rebound Hammer and a Rebar Detector and Cover meter for the primary structural elements: Columns, Beams and Quad Tee floor planks supporting the second floor structure.
The bays of the structure were approximately 25 ft x 27 ft and were repetitive throughout the area that was to be converted from an office loading use to an assembly use.
We provided engineering calculations to determine the code allowable structural capacity based on the information gathered through the non-destructive testing at our site visit.
The structure was to be renovated and the existing roof of the building was to be removed and new columns installed to raise the ceiling height inside the structure to accommodate the larger assembly space. The new columns are to sit on new spread footings and thereby relieved the load on the existing foundation system. This alleviated the need to reinforce the existing foundation supporting the tributary area loaded with the new assembly load.
Scanning and concrete testing was conducted on the Tees and Flanges of the floor planks as well as the beams and columns. We visited the site and collected dimensions, conducted the reinforcement scans and performed concrete testing.
We determined that the existing building was built in stages and it is believed that parts of the structure date back to the 1960s and may have been constructed as late as the 1980’s.
Schmidt Hammer Tests were used to determine the in place concrete strength. The limiting factor for the floor plank Quad Tees was the shear capacity which increased due to the increase in strength of the concrete as it aged. Based on our calculations we found that the floor planks would carry the new occupancy live load within the stress limitation of the existing building code.
In conclusion, the code required ultimate floor load was estimated to increase by 27% when changing the use from office loading with the interior partition dead load to an assembly use with no fixed seating.
The load capacity with the increase in concrete strength and strands required at time of construction met the load requirements of the new use without modification. The existing columns had the capacity to carry the new loads and be within the code allowable load capacity for the new assembly use.
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