What makes a structural section perform well under seismic loading is not just its strength, but its ductility to flex and absorb energy without failing. Hollow structural sections (HSS) can perform well in seismic applications due to their closed geometry, which provides high torsional stiffness and efficient axial behavior. Square HSS steel offers nearly equal properties about both axes, making them advantageous for multidirectional loading. However, seismic performance depends not only on member properties, but also on section compactness, connection detailing, and the ability to develop stable, ductile yielding mechanisms.

Why HSS Steel Performs Well Under Seismic Loading

Earthquakes impose lateral forces from multiple directions, often at the same time. Square HSS carry load equally in both horizontal and vertical axes, while rectangular HSS offer more balanced properties than open shapes. That makes them efficient for columns and braces in seismic frames where bidirectional loading governs.

This two-axis symmetry—notably in square sections—means a single section can resist loading from different directions with fewer structural consideration constraints than wide-flange members. HSS sections also have high torsional stiffness because their closed cross-section resists twisting along the member length. In contrast, open sections like wide-flange beams typically require additional detailing or bracing to achieve comparable torsional, buckling or compactness resistance.

Seismic design also considers providing structural capacity for its members that must undergo large inelastic deformations without fracturing or rupturing. For that reason, the American Institute of Steel Construction (AISC) 341 seismic provisions set compactness limits for HSS used in specific seismic frame systems to promote stable, ductile behavior.

Structural Engineering Designers also are aware that square and rectangular HSS braces can provide additional deformation capacity exceeding circular sections under cyclic loading. This is largely due to strain localization associated with local buckling of the flat walls and stresses near the section’s corners. Seismic design provisions and detailing practices from years of extensive research and testing account for these behaviors through section limit states, brace design requirements, and connection detailing.

How the A1085 Specification Improves Seismic Performance

Material specification matters significantly in seismic design. The American Society for Testing and Materials (ASTM) A500 Grade C sets the standard for a minimum yield strength of 50 ksi but does not place any upper limit. That variability raises the seismic overstrength factor Ry, which is the ratio of expected to specified yield strength. A higher Ry means engineers must size beams, columns, and connections for larger forces. ASTM A1085 is a much more stringent material specification which addresses this by capping yield strength at 70 ksi. That cap reduces Ry and leads to smaller required connection strengths across the frame. A1085 also requires Charpy V-notch (CVN) testing, with a minimum value of 25 ft-lb at 40 degrees Fahrenheit. That fracture toughness requirement makes A1085 more advantageous for seismic design than A500 for structures for areas in high seismic zones. Additionally, A1085 tightens wall thickness tolerance to minus 5 percent. That tighter limit eliminates the 0.93 design thickness reduction factor and simplifies section property calculations.

Specifying HSS Steel for Seismic Projects

Bull Moose Tube produces HSS steel to both ASTM A500 and ASTM A1085 across seven Domestic United States manufacturing facilities. For seismic applications where A1085 is a significant structural advantage, structural engineers must confirm size availability early, since it is produced on demand rather than stocked at service centers.

Bull Moose offers square HSS sizes running from 0.5 inch to 14 inches, with wall thicknesses from 0.065 inches to 0.750 inches. HSS Section properties for seismic design, also include radius of gyration and width-to-thickness ratios, are typically available through Bull Moose's technical documentation section.

Contact us to learn more about available HSS specifications, seismic design support, or delivery options for your project.