This report describes engineering studies that were conducted to examine thedeformation behavior of flat, welded steelsandwich panels under two quasi-static loading conditions: (1) uniaxial compression;and (2) bending with an indenter.Testingand analysisare conducted to studythe force-displacement response of sandwich structures with different core geometries: (1) pipe or tubular cores with outer diameters equal to 2, 3, and 5 in;(2) a 2-inchsquare diamond core;and (3) adouble corrugated core called an X-corewith a 5-inchcore height.Deformation and local collapse modes of sandwich panels under these loading conditions are also studied. Moreover, the work described in this report represents basic researchto investigate the concept of applying welded steel sandwich structures as a means to offer protection to railroad tankcars-especially those carrying hazardous materials-in the event of an accident