Complex requirements turn conventional bending on its head
December 13, 2005
Today's architects develop designs and concepts that push past the boundaries of yesterday. Fabricators are faced with a sometimes daunting challenge to make unusual components to assist architects in completing unusual buildings, to the extent that they sometimes have to rely on themselves to develop new equipment and processes.
As architectural designs grow more sophisticated, fabricators that serve this market must develop new skills and techniques to keep up. Albina Pipe Bending Co. is one such fabricator. Although the word pipe appears in the company name and implies that this is the only material the company works with, Albina actually bends and rolls an entire spectrum of structural steel profiles, including hollow structural tubes (square and rectangular), wide-flange and standard sections, angles, channels, T's, flat bars, all solids, and pipe and round tubing. Yet in today's market, even this variety of capabilities cannot satisfy the challenges created by evolving trends in design and architecture.
In recent years many fabricators have been pressed to respond to the greater demands in bending and rolling. Architects' and designers' continuously expanding visions drive these demands. In response, rolling and bending companies are compelled to devise new processes to produce bent and rolled structural steel into forms that are far more complex than simple geometries. Two main driving factors for these innovations are customers' increased use of exposed structural steel and requests for bending outside standard configurations and profiles.
Examples of this increasing level of sophistication include rectangular parts bent in two planes (both hard way and easy way), off-axis bends, and S curves with zero tangent transitions. Through more difficult bending requirements and a heightened emphasis on aesthetic qualities, designers have created a market for bent structural steel forms that cannot be processed using off-the-shelf equipment and tooling. After receiving a request for quote, fabricators sometimes ask themselves, "Is this possible, and if so, how?"
Requests for spiral bending of certain types of structural steel shapes and pipe are nothing new to the bending trade. For years benders have been able to roll plate, flat bar, channel, and pipe in spiral configurations for use in many applications, such as circular stringers and handrails for staircases and piping coils for heat exchangers. Until recently fabricators bent circular stringers from flat bar and channel for industrial applications, such as those that encircle large tanks. Fabricators can bend these two materials into spiral shapes with ease, which puts this process within reach of most benders with the proper standard equipment.
Spiraled pipe is a typical material for making handrail and piping coils. Handrails usually are made from small-diameter pipe or round tubing that is easily rolled into a spiral with a little persuasion. Like round tubing, piping coils usually are not difficult to bend or roll because of the shallow pitches or minimal coil spacing required. Pitches required for staircase spirals are far steeper, which results in a more complex bending scenario.
One of the recent trends is the use of rectangular steel tube for making circular staircases. On occasion large-diameter pipe serves as the stringers. Historically, grand circular staircases made from wood served as high-profile centerpieces. These staircases are beautiful in their own right, but wood has limited use in today's larger steel buildings. The desire to replicate this vision of circular sweeping ascension in exposed steel has placed one of the greatest demands for innovation on bending specialists. Because these staircases have functional and aesthetic purposes, bending processes must factor in appearance as much as dimensional accuracy. Bending large hollow sections into spirals without crushing or distorting the material puts greater emphasis on the care with which these materials are processed.
To meet the design requirements that call for square and rectangular steel tube to be spiral-bent, Albina, like many other bending companies, had for years employed numerous variations of bending and fabrication techniques. These techniques depend on labor-intensive fabrication of built-up box tube sections composed of bent plates and/or channels to duplicate the tube dimensions specified in the plans. Striving to move beyond the limitations of simple channel and flat bar spirals, Albina went through intense brainstorming, research and development, and several failed attempts before finally developing several innovative processes. The company also designed and fabricated its own bending equipment and tooling.
Three unique processes evolved that enable Albina to spiral-bend hollow structural tubing and structural steel materials such as wide-flange sections and large-diameter pipe.
First is a cold bending process for large-radius requirements on structural shapes such as steel tube and wide-flange sections. Second is a hot bending process that bends large square and rectangular steel tube to very tight radii and steep pitches with minimal bend distortion. The third is a modified roll bending process for smaller diameters or a cold bending process for larger diameters.
Two of the more difficult jobs Albina has processed illustrate the lengths to which a fabricator must go to meet the requirements of today's architectural designs.
This curved staircase was installed at Pacific University in Forest Grove, Ore.
Climbing the Steps to the Academic World. Isley Welding Service, a welding shop in Portland, Ore., contacted Albina with a request for processing spiral-bent tubing for fabricating a curved staircase (see Figure 1). This job required bending 16- by 4- by 0.500-in. rectangular tube steel the easy way for the stringers and the landing sections. Albina used its proprietary hot bending process to bend the spiraled stringer sections because of the radii and pitch requirements. The staircase was 5 feet wide and had an intermediate landing that was also curved at the same plan radii as the stringers. The stringers were left exposed with the exception of the inboard faces where the treads were attached. The radius of the inside stringer was a 10-ft. plan radius, with a pitch of 7 in. in rise over 12 in. of run. The longer spiral sections of the staircase required spiral-bent arc lengths of 16 ft. for the inside stringer and 21 ft. for the outside stringer.
This job also required handrails curved to follow the staircase on both the inside and outside stringers. After successfully bending the large tubing with a new process, the company found that spiral rolling some small pipe for the handrails seemed like a breeze.
A Cluster of Coils. Another job was a spiral coil fabricated by Northwest Copper Works Inc. This coil was bent from 2-in.-OD INCONEL® alloy and was far more difficult than small-pipe handrails or standard piping coils. Many factors made this a challenging job. The completed coil was a cluster coil with four rows that were progressively larger in diameter. Each row at a given diameter contained 25 tubes and achieved from 1-1/2 to 2 full turns of revolution. All 100 tubes terminated at either end into circular headers with very close hole-to-hole spacing. To successfully fit into this scenario, each tube had to meet close tolerances.
The blueprint called for minimal spacing between the tubes as they fit into the header with terminal ends equally spaced around the header's circumference. All the tubes were spaced tightly, with 0.5 in to 0.75 in. of gap between rows and between each coil within the rows. Each tube had a spiral bend that was rolled into it first, followed by rotary draw bends at each end for the terminals.
Architects can specify a staircase's pitch as either the rise over the run or the degree of pitch. Likewise, other specifications can be stated in more than one way. Although architects might not provide all of the specifications explicitly, they must provide enough information for fabricators to calculate the plan radius; the number of degrees of the arc; the pitch; the overall rise; and the spiral direction.
Several modifications were required to both the equipment and the company's normal approach to fabricating piping coils. The pitches required for the coils were nearly double their respective diameters, which forced Albina to rebuild the frame of its bending machine so it could stand on end, making it possible to reroll coils as needed.
For the coils to terminate into the headers at the correct locations, the coils had to meet the exact specifications for diameter, pitch, and overall rotation. To ensure they met the specifications, Albina fabricated drums at the appropriate inside diameters along the full length of the coils to check them. These drums also allowed the bending machine operators to locate the terminal bends precisely before bending them on each coil.
Regardless of the type of spiral, the architect or designer must provide several key specifications to the bender for proper processing. Furthermore, the fabricator needs to use a variety of methods to verify the components meet the specs.
What Are the Specifications?The specifications should include the radius in plan view; the number of degrees of arc in plan view of the spiral section only; the overall rise of the spiral section only; the degree of pitch; and the direction of rotation, interpreted as clockwise up or counterclockwise up (seeFigure 2).
These two views are the clearest way to illustrate and interpret these types of bends. If the architect or designer represents these specifications in some other manner that does not provide these dimensions, the information must be comprehensive enough that the fabricator can calculate them.
Does the Product Meet the Specifications?Performing dimensional verification on spirals depends on many factors, such as the type of spiral, the size of the material, bend radius, and the degree of bend. This is just a sample — some bends may require more verification than this.
Verifying the dimensional tolerances of large spiral sections such as those discussed in the first job requires mocking up the stringers at full scale and checking all the necessary dimensions. The fabricator can verify the accuracy of the radius by laying out the plan view radius at full scale on the floor and then checking the stringer with a plumb line to this radius. The actual bend radius is verified using chord and rise measurements throughout the full length. Stringers also are checked over their entire vertical faces for plumb in relation to the correct pitch. Heights at several increments along the plan view are verified to ensure that the elevation of the spiral is accurate as it climbs from end to end.
Spiral coils for piping or other coils that typically complete more than one revolution are easier to verify dimensionally. More often than not, it is simply a matter of checking the diameter and the spacing between the turns in the coil. However, exceptions do crop up, such as the Northwest Copper job mentioned previously.
Fabricators that have refined or perfected techniques for today's architectural challenges cannot stand still. A quick look at modern building trends is proof that architects aren't satisfied with conventional designs, simple geometries, and components that are easy to fabricate. Given how far architects have taken structural steel, one thing is certain: Future designs are likely to cause fabricators even more surprise, head-scratching, and occasional bewilderment than today's designs do.
Mark King is estimating manager for Albina Pipe Bending Co., 12080 S.W. Myslony St., Tualatin, OR 97062, 503-692-6010, fax 503-692-6020, www.albinapipebending.com.
INCONEL is a registered trademark of Huntington Alloys Corp.