A new material specification for cold-formed welded steel
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Many fabricators and erectors of hollow structural sections
(HSS) are familiar with the ASTM International specification of A500. Currently, this is the most common specification for HSS producers in North America, and has been the preferred standard for engineers for quite some time. In April 2013, ASTM introduced a new material specification for cold-formed, welded steel tubes, ASTM A1085, which provides functionality and durability benefits over A500. An additional advantage of A1085 is its uniformity across categories, whereas A500 has four defined grades. To simplify matters, we will compare A1085 to the commonplace A500-10 Grade B.
A variety of HSS manufacturers that produce A500 have the capability to make A1085 tubes in approximately the same size ranges. The difference is primarily in the composition of the carbon steel used to form the product. A1085 has more stringent material tolerances than A500; specifically in tensile strength, yield stress and wall thickness.
These differences are not problematic for manufacturers, though it is likely a company that primarily focuses on A500 would need to order different steel coils to meet these requirements. Organizations like Glenview, Ill.-based Steel Tube Institute (STI) are working with producers to provide capability and availability tools to help connect end users with appropriate suppliers.
Strength
Tensile strength is the maximum amount of stress a material can withstand before breaking or failing. The new spec has a minimum tensile strength of 65,000 pounds per square inch, the Grade B equivalent only requires a 58 ksi minimum. A1085 will also hold its shape better and avoid deforming at a higher yield point since it requires a single minimum yield stress of 50 ksi. Correspondingly, A500 Grade B minimums are 42 ksi for rounds and 46 ksi for squares and rectangles.
Not only have these minimums been raised, but a cap of 70 ksi has been placed on the yield strength. Traditionally, HSS specifications used in North America and Europe have not limited the yield strength. This tighter specification will reduce capacity design requirements and column required strengths in seismic designs, while forcing the steel properties to be more predictable for designers.
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Additionally, the more stringent wall tolerances and mass tolerance mentioned before will likely please many engineering professionals, as this means the full nominal wall thickness can be used for designs. Kim Olson, PE, technical advisor for STI, mentions in a webinar about A1085 that these changes better align with other structural steel products and eliminate the need for designers to use the 0.93 factor in calculations. This results in more economical designs with improved efficiencies that are better for everyone involved.
Tolerance
Beside the tolerance differences, A1085 also requires a Charpy V-notch (CVN) test. This CVN test, or impact testing, essentially determines the toughness of the material at a controlled temperature. A1085 must meet the minimum CVN value of 25 foot-pounds at 40 degrees, which is the same as American Association of State Highway and Transportation Officials (AASHTO) requirements for steel used in bridges in Temperature Zone 2. This opens the door for A1085 to be used in more AASHTO projects like bridges and other infrastructures across the United States.
It is evident that this new specification was created specifically with seismic design properties in mind, and could become the staple specification for HSS moving forward. While A500 will likely remain the most common standard for designers to specify in the near future, the prevalence of A1085 will continue to rise as the advantages become more widely known. Today, this newest specification is touted as an ideal option for not only buildings and bridges, but cranes, amusement park rides and more.
Patrick Limbaugh is a specialist at Southland Tube, Birmingham, Ala. To learn more, visit www.southlandtube.com or reach him at patrick.limbaugh@southlandtube.com.
