Progressive-Collapse Robustness Assessment of Existing Reinforced Concrete Buildings in Diverse Geographical Regions Using the Tie-Force Method

This study evaluates the resistance of existing reinforced concrete buildings to progressive collapse using the Tie-Force Method specified in UFC 4-023-03. Five multi-storey residential reinforced concrete buildings in different regions are analysed. In situ rebar scanning and Schmidt hammer tests revealed existing reinforcement layouts and concrete strengths (14–26 MPa). From the measured geometries, material properties and design loads, the required peripheral, longitudinal–transverse and vertical tie forces are calculated and converted into equivalent reinforcement areas. The results show that none of the investigated buildings satisfies all tie-force requirements with its current detailing. In particular, approximately 40% of the total Ø12 reinforcement required for the most critical peripheral ties in the other functional areas is concentrated in a single building. For longitudinal and transverse ties within the slab plane, additional Ø12 bars are required, especially along the most unfavourable grid lines in large-span panels. Vertical tie demands are modest and can generally be met with about 1–7 Ø16 bars in the selected columns. The findings indicate that, in the investigated sample, tie-force deficiencies appear to be governed more by design era and structural layout than by geographic location, and that strengthening slab-plane ties is critical for improving progressive-collapse resistance in the investigated buildings. In typical existing RC frame buildings, tie-force inadequacy is governed primarily by slab-plane ties rather than by vertical ties and the variation in required tie reinforcement across buildings is controlled more by design era and plan-geometry/floor-load characteristics than by geographic location.