Apr 20, 2013
Cast Connex is back from NASCC 2013! St. Louis is a great city and the conference was a huge success. The exhibition hall was bursting and we got to showcase our new Scorpion “Critter” (pictured below). The technical sessions were excellent, and I was very fortunate to learn all about the new additions and changes to the AISC 341-10 Seismic Design Manual. The networking events were amazing, especially the Nucor sponsored event at City Museum which had a 10-storey slide! There were so many new and exciting things happening at NASCC ‘13, but one in particular that I found very interesting was that the American Institute of Steel Construction and the Steel Tube Institute (AISC and STI) revealed that there would be a new ASTM HSS material grade specification: ASTM A1085.
Coming from a HSS background and learning about the new specification, I have to say the implications of this new grade mean exciting things for our industry. So why is Cast Connex excited about this new specification? Why should you be excited about this new steel grade?
If you have studied or worked in Canada designing with HSS, you will be familiar with using CSA G40.20/G40.21 grade steel. The G40.20 tolerances on wall thickness and mass allow an engineer to use a design wall thickness (tdesign) that is equal to the nominal wall thickness (tnominal). But if you have studied or worked in the US designing with HSS, you will be more familiar with either ASTM A53 or A500. If you’re designing using the ASTM A500 grade steel, tdesign is only 0.93 x tnominal. The design thickness using A500 grade steel is less than the nominal thickness because the specification allows wall thickness to be up to 10% less than what is reported, and there are no mass tolerances. (A53 allows the wall thickness to be up to 12.5% less than the nominal wall thickness, and up to 10% less than the reported mass).
ASTM A1085 grade steel will have tighter tolerances on wall thickness and mass. A1085 will only allow wall thickness to be 5% less than what is reported, and mass to be 3.5% less than what is reported. With these tighter tolerances, no reduction of wall thickness will be required for design calculations, that is, tdesign = tnominal. What does this imply? It implies that for a given design load, an engineer may be able to specify a lighter member size by using A1085 instead of A500 or A53. Also, the minimum yield strength of ASTM A1085 HSS is 50 ksi, regardless of the HSS shape. This means engineers will be able to design more economical structures!
But you might be thinking, if CSA G40.20/G40.21 grade already allows the design thickness to be equal to the nominal thickness, then why bother developing ASTM A1085? The A1085 specification will do something that no other North American or European tube specification has done: it will have an upper bound limit on yield strength.
If you look at any steel grade in North America and Europe, the Fy reported and that used in design calculations is the minimum specified yield strength of the material (there is no maximum specified yield strength). An engineer can reasonably expect that the actual yield strength of the steel will be higher than Fy. You may be asking, isn’t that a good thing? Generally speaking, yes, it is a good thing; however, we don’t know by how much the actual yield strength exceeds Fy and that may not necessarily be a good thing when it comes to seismic design.
If you are familiar with seismic design, you will be familiar with capacity design, that is, establishing a fuse element and designing all non-fuse members and connections to be stronger. To do this, engineers would like to know what the actual yield strength of the material will be. While it is very unlikely to know this during design, Ry values are determined to help engineers establish a probable or expected yield strength (RyFy), where Ry is the ratio of the probable yield strength to the minimum specified yield strength. Ry values are statistically determined based on numerous material property tests and reported in our codes. According to CSA S16-09, the probable yield stress of HSS shall not be less than 460 MPa, which for circular tubes of ASTM A500 grade C material implies an Ry=1.45. According to AISC 341-10, for HSS produced to ASTM A500, Ry = 1.4; and for Pipe produced to ASTM A53, Ry = 1.6.
While Ry helps engineers determine a probable or expected yield strength, with no upper bound limits on yield strength, there can be some uncertainty with the accuracy at which Ry helps predict the actual yield strength. Statistically speaking, it is possible that the actual yield strength can be more than twice than the minimum specified yield strength, which could potentially mean the fuse element would not activate when it should during a seismic event.
The new ASTM A1085 steel grade will have a minimum specified yield strength of 345 MPa (50 ksi), which makes it suitable for seismic design, but it will also have a maximum yield strength of 485 MPa (70 ksi). So why is this significant? It is significant because Ry can be determined with a lower standard deviation meaning engineers will be more certain about the actual yield strength. It could mean engineers can be more confident that their fuse elements and structures perform as they were designed to perform. An extension on this thought – by setting an upper bound limit on yield strength, it is possible that the average yield strength from the numerous material property tests will be lower, which is to say, Ry could be lower. By reducing Ry, all non-fuse elements could be designed for lower forces. Again, this means more economical buildings!
Along the same train of thought as seismic design, any dynamically loaded structure, whether under low-cycle fatigue (like seismically loaded buildings) or under high-cycle fatigue (like a bridge), requires that the material have some toughness – the ability to absorb energy. It is typical that toughness is measured using the Charpy V-Notch (CVN) test. ASTM A500 has no CVN requirements meaning it may be difficult to specify when designing for dynamically loaded structures; on the contrary, the new ASTM A1085 specification requires a minimum CVN value of 25 ft-lb @ 40°F (or 34 J @ 4°C), which makes it a better option for use in dynamically loaded structures. What’s more is that additional CVN requirements can be specified such that the engineer can tailor the material requirements for a particular application.
Cast Connex is very excited about the potential benefits of this new ASTM A1085 steel grade and wanted to send a thank you to AISC and STI for distributing the flyers at NASCC ‘13. Please check out www.aisc.org/hss for more information.