Hollow structural sections (HSS) have long been a popular structural steel section choice for engineers. For many applications, HSS are cost-effective, reliable and allow for diverse design innovations. HSS have historically been popular for use in seismic load resisting systems and dynamically loaded structures. However, research has shown that the material properties of HSS are important in the design of structures that resist dynamic and seismic loads. The ASTM A500 specification, which has been the most common HSS specification used in North America, presents some challenges to engineers because its outdated requirements no longer meet engineers’ needs. With the recent development of the ASTM A1085 HSS spec, that has changed.
Realizing the need to improve the state of current specifications as well as HSS, the American Institute of Steel Construction (AISC), in partnership with members of the HSS industry, recently helmed the initiative to create the brand new HSS specification, ASTM A1085-13 Standard Specification for Cold-Formed Welded Carbon Steel Hollow Structural Sections. Their efforts paid off in April 2013, when the new spec was officially adopted with the ultimate goal of producing improved HSS that offer enhanced performance in seismic and dynamic applications.
A change for the better
The 1994 Northridge, Calif., and 1995 Kobe, Japan, earthquakes were watershed events that changed the way we approached seismic design and also changed engineers’ understanding of the importance of material properties. We learned from those events that steel can indeed be too strong. It was at this time that steel mills were producing more and more steel through electric arc furnaces (EAF) and using recycled steel instead of the traditional basic oxygen furnaces (BOF). One of the results of the EAF method was higher yield strengths in steel sections. While “stronger is better” usually makes sense, it was learned to not be so when dealing with seismic loads.
Seismic design evolved to focus more on capacity design, or designing for the expected capacity of a member and not just the expected loads. Also, engineers needed to have better understanding and control of material properties. This need for better control of material properties led the steel industry in the late 1990s to develop ASTM A992, which has gone on to become the standard wide flange specification used in North America for all building construction. ASTM A992 has a cap on the minimum yield stress, as well as a yield-to-tensile strength ratio. This leads to better predictability of the material properties and prevents a steel section from becoming too strong.
Unfortunately, HSS were not affected by the advent of A992 and therefore did not inherit the benefits that this new specification brought to engineers working on structures that had to resist seismic and dynamic loads. Now, with the advent of ASTM A1085, HSS are able to offer better performance when subjected to seismic and dynamic loading.
Benefits of A1085 HSS
Minimum value for yield stress and tighter material tolerances
The wall thickness tolerances in ASTM A1085 are half of those in ASTM A500. These more stringent wall tolerances and the addition of a mass tolerance will allow for the use of the full nominal wall thickness and cross sectional properties when designing with A1085 HSS. This will likely lead to the removal of the reduction factors of 0.93 in AISC 360 Specification for Structural Steel Buildings in the U.S. and 0.90 in the CISC Handbook of Steel Construction in Canada. Using section properties based on the full nominal thickness will increase HSS column capacity.
The minimum specified yield stress in ASTM A1085 is 50 ksi (kips/square-inch) for all shapes and sizes. This is greater than A500, Grade B (42 ksi for round and 46 ksi for square), which is the most commonly specified grade. A single minimum yield stress for all shapes and sizes is much simpler and easier to remember. With more available area for design because of the tighter tolerances and a single, higher minimum yield, the A1085 spec makes HSS a more efficient design option, and the increased strength to weight ratio allows for more economical designs.
Maximum value for minimum yield stress
HSS produced to the A1085 spec has a minimum yield stress range of 50 to 70 ksi. This range puts a cap on how high the yield stress can exceed the minimum specified value. This cap does not permit the actual yield stress to exceed 70 ksi. This is similar to the minimum yield stress range that is given in ASTM A992 specification for wide flange shapes. This maximum yield stress will result in lower expected yield strength and potentially reduce the overstrength factor required for capacity design in seismic designs. A1085 is the only HSS specification used in North America or Europe that limits the maximum yield stress. Engineers expect this of structural steel utilized in seismic load resisting systems, as it makes the expected strength of steel shapes more predictable.
Standard requirement for Charpy V-Notch (CVN)
The new ASTM A1085 has a standard requirement for a Charpy V-Notch test for all HSS produced to this standard. CVN tests measure the amount of absorbed energy at a certain temperature during fracture and determine the level of toughness for a piece of steel. This measure gives insight into how steel will perform under dynamic loads and in extreme service conditions, such as low temperatures. All A1085 HSS will be required to meet the minimum CVN value of 25 foot-pounds at 40 degrees Fahrenheit, the same CVN required by the American Association of State Highway and Transportation Officials (AASHTO) for steel used in bridges located within Zone 2. This base CVN requirement in the A1085 spec makes certain that HSS produced to this spec will provide a level of performance suitable for dynamically loaded structures, such as bridges or buildings that are subjected to seismic loads. There is a supplemental requirement within A1085 that allows for a user-specified CVN value that is more stringent than the base CVN.
The new ASTM A1085 HSS specification is a dynamic step forward for HSS. This is an innovative step forward not only for the HSS industry, but for the steel industry as well. The A1085 specification brings HSS to the forefront of steel design by providing updated and innovative requirements that raise the bar on performance. The new specification provides tighter dimensional tolerances, a single minimum yield stress, a cap on the minimum yield stress and a standard CVN test requirement. All of these make HSS easier and more efficient to design with. I encourage you to continue learning about the new A1085 spec. There are resources available to help engineers understand the new spec. Helpful information can be found on Atlas Tube’s online forum (atlasconnection.com), the AISC website (aisc.org/hss) and the Steel Tube Institute website (steeltubeinstitute.org).
Brad Fletcher is a structural engineer at Atlas Tube. In this role, Fletcher leverages his 20 years of experience in the steel industry to provide technical expertise on the use of hollow structural sections (HSS) and pipe piling products to design engineers, detailers, fabricators and architects.