July 21, 2014
The construction industry’s use of hollow structural sections (HSS) has been on the rise since the recession. Because of their high strength, HSS allow engineers to use less material, extending the maximum unbraced length between structural members. It’s in this environment that the tube cutting laser is beginning to open up new opportunities.
Editor’s Note: This article is based on “The Growth of HSS Applications,” presented by Al Bohlen, vice president and general manager, Mazak Optonics Corp., Elgin, Ill., at The FABRICATOR’s Leadership Summit, Feb. 27-March 1, 2013, in Palm Harbor, Fla.
Although the automotive industry shapes people’s perception of U.S. manufacturing, and metal manufacturing in particular, it isn’t the nation’s largest consumer of metal—not by a long shot. It’s commercial construction.
To lower cost, builders have relied on design techniques to reduce the amount of steel a structure needs, as well as to reduce on-site erection time. One design avenue that has become significantly more popular within the past two decades has been to use alternatives to the standard wide-flange beam.
These beams have evolved into an efficient building material of choice. But when it comes to strength, the shape of the wide-flange beam pre-sents a challenge. It can span only a certain distance (or “unbraced length”) before requiring support. From a purely strength perspective, it would be much more efficient for beams to take on a circular, square, or rectangular shape, which would extend the maximum unbraced length. The longer distance these structural members can span, the fewer braces and supports a building needs. Ultimately, this means builders can use less of what’s often their biggest expense: the structural metal itself.
Enter hollow structural sections, or HSS (see Figure 1). These round, rectangular, or square tubes have shapes that provide inherently higher strength and can span greater lengths between braces. A square steel tube with a 3⁄16-in.-thick wall thickness has an allowable load of 79 kips over a column length of 32 ft., while a similar wide flange (ASTM designation of W12 x 40) has an allowable load of 64 kips over the same column length (see Figure 2).
For decades HSS have been used for their dramatic effect. Builders and architects have used them to make an artistic statement, not to save money, and this remains true in many cases today. But because HSS are so strong, architects can design buildings with less material. HSS also save on finishing costs, because compared to wide-flange beams, tubular sections have less surface area to paint or fireproof. Combine this with the fact that tube production costs have fallen in recent years, and building with HSS begins to make real economic sense. This is one main reason that demand for HSS has been on the rise since the recession, and it’s in this environment that the tube cutting laser is beginning to open up new opportunities.
HSS represent a departure from many tube laser cutting applications that tend to work with relatively thin-walled workpieces. Shops providing HSS often must deal with workpiece weights (called “stick weights”) up to 2,000 pounds. These workpieces are not just long, but also large; 14-, 16-, and 20-in. diagonal cross sections aren’t uncommon.
To produce such large workpieces cost-effectively on a tube laser calls for careful planning. It’s far more complicated than using a cutoff saw, but it also adds a lot more value to the workpiece. Modern tube lasers have load/unload functions that can handle mill-length pipe and structural material.
This capability gives designers significant flexibility when it comes to designing for mated sections. Mating a round tube to another round tube seems simple, but the bevel required to create a tight fit-up between the two sections can be extremely complicated, especially if tubes are of different diameters or shapes, or if they intersect at unusual angles.
From an architectural engineering perspective, such angles may produce the best transfer of loads and most efficient use of HSS. But to the welder and fabricator, such a complicated joint can be a nightmare.
When designing a building, engineers perform a lot of give-and-take when it comes to costs. They may choose a thinner wall thickness for HSS, saving on material, but then deal with the added expense of additional through-plates or other connectors to ensure the structure has sufficient strength. Or they may choose thicker-walled HSS to ensure connections between structural members meet requirements. Architectural engineers may call for connections between HSS, or connect HSS to wide-flange beams. It’s a continual balancing act.
This balancing act can be easier—and this is where a tube cutting laser can really shine. The machine effectively makes complex geometries at HSS end sections cost-effective to fabricate. Six-axis laser cutting heads create complex bevels as well as tab-and-slot arrangements to simplify fit-up. This includes tilting from side to side (moving along the A/B axis), which can be extremely beneficial not just for HSS, but for cutting various structural geometries, including wide-flange beams. By tilting, the head can cut geometries in corners, eliminating secondary operations.
This tilting allows for cutting angles for bevels as well as precise fit-up between two HSS of different diameters. What if you need one tube to slide in at an angle with another tube? A 3-D cutting head can cut the necessary angles to ensure complete surface contact; that is, no gap between the two workpieces. Systems also have secondary tapping units to tap holes within the laser cutting work envelope (see Figures 3-7).
This done-in-one concept reduces handling and total production time, at least that’s the ideal. But this is far more complicated than a tube cutoff operation with a saw, and quite different from typical light-gauge laser cutting; again, stick weight can be thousands of pounds. This makes proper planning and inspection even more important.
It starts with the 3-D CAD model, which in the architectural world is often integrated into BIM, or building information modeling. The architectural industry also transfers data via files formatted as Industry Foundation Classes (IFC), an object-based building model format developed by the International Alliance for Interoperability (IAI). Such files now can be imported directly into machine tool software.
The software shows how the laser cutting machine will process the HSS workpiece, simulating the entire work cycle. This includes the loading automation, when a series of V arms position a new piece of material, be it round, rectangular, or square. The simulation then shows the master chuck grabbing onto the material and pushing it through another chuck (the slave chuck) and into the laser work envelope.
As the material moves into position, the software reveals exactly where the probe will contact the workpiece. Touch sensing can be critical with heavy HSS. The probe compares the actual workpiece geometry to the one programmed in the machine. For instance, the longitudinal weld in a tube production process can create distortion in extremely long HSS, and the touch probe can account for that distortion.
The software simulates the laser cutting and (if needed) tapping work cycle, ensuring there are no interferences between the processing heads and workpiece. It simulates chuck movement throughout the cycle and then shows how the machine will unload the finished workpiece and remnant.
All this is planned before anything moves to the shop floor. This kind of simulation can benefit a variety of fabrication processes, of course, but it becomes even more important when dealing with large sections. Moving and fabricating bad components from a 2,000-lb. tube represents a lot of wasted time and money.
When you consider how long wide-flange beams have been in use, HSS are still newcomers, but now more builders are calling for them. Look at various building designs today, and you’ll see HSS becoming more prevalent, either dominant in a building’s design or providing efficient support between wide-flange beams.
On the fabrication side, most beams being shipped to construction sites are processed through beam lines, and some of the latest technologies in that arena include aspects of the done-in-one concept: cutting, beveling, tapping, drilling, and more, all in one machine. This concept has carried over to the laser cutting arena, in which both the workpiece and multiaxis cutting heads move in concert to produce extremely complex geometries, many thought to be too expensive or simply impossible not too long ago.
Now the laser has made these possible and cost-effective, because process simulation, touch probing, and the done-in-one concept reduce overall fabrication time. And once these heavy sections reach the work site, erectors can assemble them quickly, shortening overall construction time—which, in the scheme of things, has the most dramatic effect on construction costs. This has been core to the success of many architectural and structural fabricators in recent years: Do more in the controllable environment of the fabrication shop to make things easier in the relatively uncontrollable environment of the construction site.
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