BUILT TO LAST
Jul 1, 1998 12:00 PM, Bob Schluter
There was a time when our industry needs for steel equipment enclosures could easily be met with tooling as basic as manually operated punch presses, hand brakes and anvils. Those days are but a faded memory for today's generation of rack builders who increasingly find themselves faced with a dizzying array of complex issues involving needs for increased structural integrity, expansive interior geometries, quicker delivery and adaptability to varying applications. The historical force responsible for bringing about this change is the invisible hand of the marketplace. As demands on contractors around the globe increase, so too has the need for rack real estate to serve in a problem-solving capacity. To meet these needs, a new concept of rack space has emerged, and it promises to redefine the product genre.
This notion of rack assembly borrows from design strategies serving other industries. Most notably, the data/networking world has been tapped for cable management techniques that bring added function to audio applications, along with a host of other possibilities that are only beginning to be explored.
In its largest sense, rack assembly for data and networking applications differs from audio in that there is more cable, even to the point where 4 inch (102 mm) bundles are the norm rather than the exception. Cable bundles of this magnitude predetermine a need for roomier rack interiors, and it was in solving this dilemma that design techniques devised for data management first influenced those used for pro-audio applications.
To accommodate large cable bundles, gaining the required space became a matter of examining the interior of the standard racks and replacing conventional, protruding rackrail bracket designs with a new, low-profile configuration. Simple and effective, the latest generation of rackrail brackets sits virtually flat against the sides of the rack interior. Along with the new bracket design, rackrail faces grew wider, sometimes gaining 2 inches (51 mm) and more in width. The result is that today's rackrail/bracket hardware no longer occupies the space it once did in conventional cabinets. The badly needed room for large cable bundles was acquired in this fashion, and when the same technique was implemented for professional audio racks, more space was added not only for cable, but also for equipment such as power strips and power sequencers. Measured in inches, this space gain is impressive. For example, racks measuring 24.5 inches (622 mm) on the outside typically offered only 19 1/8 inches (486 mm) of usable space on the inside. Once the new rackrail/bracket design reached professional audio and other markets, that same enclosure provided 23 inches (584 mm) of usable interior space.
If anything else has been learned in the last year or so by rack manufacturers in this area, it is that obtaining more inner rack space for cabling and equipment is one matter, but managing that space and the cables within is quite another. In response, rack manufacturers offered more options for cleanly tying off cables and cable bundles. With the current catalog available of horizontal and vertical lacing bars, contractors of all stripes-video, audio and data-can now use rack space gains in a fashion best suited to particular styles and needs, while maintaining a clear sense of order and tidy appearance in their work.
Other new rack assembly techniques gleaned from the data industry involve the use of surge and spike protection. Imparted just as audio contractors were blessed with added rack space for rear-mounted power strips and power sequencers, the lesson most relevant for those seeking to safeguard valuable equipment was that surge and spike protection should be supplied by a large MOV (metal oxide vari-resistor) in both common and differential modes. In common mode, the protective circuit is provided at the power supply's hot and neutral legs. Conversely, protection in differential mode runs from neutral to ground and hot to ground. Both modes are capable of swallowing large power spikes and offer more than the token amounts of safety provided in the past.
Space gains, advanced cable management techniques and new developments in protective circuitry are not the only trends in rack manufacturing currently appearing within our industry. Along with these enhancements, levels of structural integrity have arrived, including the further development and refinement of seismic-rated enclosures. Once believed to be a concern reserved only for those living directly atop the San Andreas fault, seismic contracting issues are as relevant in Chicago as they are in southern California. I do not mean to imply that Chicago is in danger of crumbling under the stress of a major earthquake and sliding into Lake Michigan, but that preparing your rack assemblies to survive earthquake-level shaking is a good idea no matter where you live.
Consider what your racks may encounter in the course of their lives. Normally shipped to the job site fully loaded, they could be strapped in the back of a truck and jostled severely in a crosstown trek. They could possibly arrive at the job site fully loaded, only to find that the elevators are not yet operable. A rocky trip up multiple flights of stairs could ensue. There are many more hazardous situations your rack could encounter as well-being dropped from a crane or falling down an elevator shaft-that make a seismic situation seem like a glass of warm milk and a plate of cookies, but my point is clear. Earthquake preparedness should be for everyone, like buckling your seatbelt. This is especially true in light of the fact that it takes very little time and expense to obtain seismic levels of structural integrity. In most cases, the cost to contractors in time and dollars is actually significantly less than using other traditional methods for preparing for that crosstown truck ride or staircase journey.
Start with the basics of readying your rack for tremors and everything else the world can dish out. Remember, how the rack is assembled is critical. You do not have to be Einstein to figure out the basic rack physics that will bring the highest levels of structural integrity to your job. Obviously, the worst rack configuration would be a tall, slender unit standing by itself with a 200 pound (90 kg) device mounted at the top and blank panels running the rest of the way down. The best configuration requires taking the time to understand how much weight is going into the rack, where the center of gravity is, how the load is dispersed and whether the rack in question will be standing alone or ganged together.
Here are some guidelines: Keep the center of gravity as low as possible. Heavy power amps located at the top of the rack are out of the question. The heaviest gear should be confined to the lowest third of the rack. If there is more heavy gear than you have space for in the bottom third of a single rack, spread the load out over other racks. A seismically ready rack should be filled from top to bottom. This does not exclude the use of vent panels for cooling or blank panels for future expansion, but use these items judiciously.
Rear support is another essential consideration for structural integrity. If, after rack-mounting a component, you can push down or lift up on the rear of the unit and it flexes easily, you need rear support. In more specific terms, the following formula is an aid to determining the need for rear support: If the unit to be mounted has a weight greater than 10 pounds (4.5 kg) per rack space, the center of gravity is toward the rear, and the overall depth of the device is greater than 2.5X its racking height, that piece of equipment must be rear supported. Rear-support mechanisms can take many shapes and forms. There are rear-hanging brackets available that attach to equipment outfitted with rear-hanging ears secured to a rear-mounted set of rackrails. For transporting a loaded rack to a job site, contractors have long relied upon the rear-supporting technique of cutting wood chocks and inserting them between the equipment. While effective, this method calls for the removal of the chocks once the rack is installed because the presence of the wood in an electronics enclosure violates fire codes. Given the amount of time it takes to create the blocks, install them and remove them once the rack is secured in its permanent resting place, it makes more sense to choose something more permanent and secure that will also serve in helping to complete a rack that meets the seismic Uniform Building Code (UBC).
Drawing once again upon lessons learned from the data industry, this becomes a simple and inexpensive task with the addition of a pair of mid-mounted rackrails and a rectangular horizontal lacing bar, which is employed here as a support directly beneath your equipment, secured at each side along the mid-mounted rackrail. Simple, sturdy and value-oriented, this configuration is also ideal for rear-supporting components that are not equipped with rear-hanging mounting ears. This method is quicker, cheaper, stronger and more permanent than wood chocks were ever intended to be, and it is color coordinated to match your rack.
In a true seismic event, the cornerstone of rack integrity resides at floor level. All other physical and structural pragmatics considered, if you don't anchor the unit to the floor properly, all other preparations will not matter. Although I do not know of any rack manufacturer that sells seismic fasteners (simply because they can't tell what kind of floor will be present in each application), I recommend that any anchor you choose be purchased along the same guidelines one would use in buying a parachute. Go for the best quality. Fasteners with a high shear rating and high tensile strength are the best choice.
Seismic-rated clamp kits for your rack should be installed in the rack's corners. Along with the fastener holding the rack to the floor, flanged washers must be used at each corner. More than simply a standard washer, these flanged units are made from 1/8 inch (3.2 mm) structural steel and are designed to resist the violent twisting and wrenching they would receive in an earthquake, while transferring all the heaving motion of a trembler directly to the rack's vertical surfaces via the corners where it is strongest.
There are some other items not to overlook in your quest for seismic rating. Tighten all rack screws that secure your components to the rackrail. This ensures that your rack remains square, and that all of the side-to-side motion experienced in a quake travels right down to the floor, where you are best equipped to handle the situation. Also, as an aid in meeting budgets, choose a rack seismically rated right out of the box at no extra expense. They are readily available; you should not pay more to meet seismic codes. In choosing this type of unit, look for a rack with corner braces composed of structural steel measuring at least 1/8 inch (3.2 mm) thick designed for seamless side construction. Some racks are built using corner butt or lap joints that are held together by welds at the top and bottom. For aesthetic reasons, these welds are ground down, sometimes removing approximately 80% of their strength. Racks ideal for seismic applications are seamless in that the sides are created by taking a full sheet of metal and cutting the center out, thereby alleviating the need to employ stress fracture-prone welds at the corners.
Switching gears, consider Northridge, CA, January 1994. We are at the epicenter of this decade's worst earthquake thus far, and major sections of this town have been reduced to rubble. On Reseda Boulevard, where a building collapsed is a rack enclosure standing unscathed. An entire floor of a building which collapsed on top of it lies in ruin at its base. Now that is seismic rating. Too bad they would not let photographers into the area. Take my word for it---there were witnesses. It is now your turn to create the next generation of rack-mounting survivors using the available materials and these guidelines.
Based in San Francisco, EQE International is a consulting firm with offices around the globe. Founded as an entity devoted almost entirely to earthquake engineering, today the firm's more than 500 associates also provide expert counsel on other hazards and disasters ranging from fire, floods, tornadoes and explosions to risky situations of a more subtle nature in the worlds of security and finance.
Before EQE was contacted a few years back, there were no established guidelines for providing seismic ratings for steel rack assemblies. Determined to obtain seismic ratings for a series of products found in his company's catalog of racks and rack accessories, Bob Schluter, president of Middle Atlantic Products, enlisted the aid of EQE's Leo Bragagnolo in developing the necessary testing procedures. Using the Uniform Building Code (UBC) as a roadmap, Bragagnolo devised a series of lateral stress tests.
"Primarily, I was interested in determining the amount of lateral force-like an earthquake would impart-that the cabinets could take," Bragagnolo recalled. "I wanted to know at what point the cabinets would break or fail in some manner where their structural integrity was compromised."
Bragagnolo's test racks were outfitted with dummy loads approximating real-life situations. Based upon the results of his tests, he determined what forces the cabinets could safely withstand using the 1994 UBC's most severe Seismic Zone 4 Upper Floor rating requirements
"Because we were the first agency of record I know of engaged in this type of rating process, what it really came down to was developing testing procedures that would provide me with the level of confidence I needed to say they met the requirements of the UBC," Bragagnolo added. "As a licensed engineer in California, I put a stamp on a document stating just that. Now, I guess it's fair to say we were pioneers within the industry in establishing seismic ratings for steel electronics enclosures."
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