Dynamic Mechanical Behavior and DIF-Based Capacity Prediction of Steel–CA–UHPC Composite Beams Under Impact Loading

Steel–concrete composite beams are widely used in building and bridge engineering; however, the impact response of Steel–Coarse Aggregate–Ultra-High Performance Concrete (Steel–CA–UHPC) composite beams remains insufficiently quantified, and no beam-specific dynamic capacity formula is available. To address this gap, companion static testing and drop-weight impact tests were performed on full-scale simply supported steel–CA–UHPC composite beams under single and repeated impacts, followed by development of a strain-rate-dependent dynamic increase factor (DIF) model and a capacity prediction framework. The companion static specimen reached 448 kN, whereas the 5 m impact cases produced peak forces of 930.0–940.4 kN, corresponding to 2.08–2.10 times the static level, with the initial peak forming within 1.0–1.1 ms. Dynamic failure was marked by rapid mid-span cracking of the CA–UHPC slab and brittle shear fracture of studs, while repeated impacts mainly accelerated cumulative damage before the final high-energy strike. Static–dynamic displacement comparison further revealed much more abrupt deformation concentration under impact loading. A revised static capacity formula reduced the prediction error from 4.46% for the code-based method and 1.00% for the literature model to 0.74%. Combined with the fitted DIF–strain-rate relation, the proposed framework reproduced the measured dynamic capacities with errors of −4.63% to 9.75%. The study provides member-level evidence and a practical DIF-based method for evaluating the impact resistance of steel–CA–UHPC composite beams.