BUILT TO LAST
Jul 1, 1998 12:00 PM, Bob Schluter
There was a time when our industry needs for steel equipment enclosurescould easily be met with tooling as basic as manually operated punchpresses, hand brakes and anvils. Those days are but a faded memory fortoday's generation of rack builders who increasingly find themselves facedwith a dizzying array of complex issues involving needs for increasedstructural integrity, expansive interior geometries, quicker delivery andadaptability to varying applications. The historical force responsible forbringing about this change is the invisible hand of the marketplace. Asdemands on contractors around the globe increase, so too has the need forrack real estate to serve in a problem-solving capacity. To meet theseneeds, a new concept of rack space has emerged, and it promises to redefinethe product genre.
This notion of rack assembly borrows from design strategies serving otherindustries. Most notably, the data/networking world has been tapped forcable management techniques that bring added function to audioapplications, along with a host of other possibilities that are onlybeginning to be explored.
In its largest sense, rack assembly for data and networking applicationsdiffers from audio in that there is more cable, even to the point where 4inch (102 mm) bundles are the norm rather than the exception. Cable bundlesof this magnitude predetermine a need for roomier rack interiors, and itwas in solving this dilemma that design techniques devised for datamanagement first influenced those used for pro-audio applications.
To accommodate large cable bundles, gaining the required space became amatter of examining the interior of the standard racks and replacingconventional, protruding rackrail bracket designs with a new, low-profileconfiguration. Simple and effective, the latest generation of rackrailbrackets sits virtually flat against the sides of the rack interior. Alongwith the new bracket design, rackrail faces grew wider, sometimes gaining 2inches (51 mm) and more in width. The result is that today'srackrail/bracket hardware no longer occupies the space it once did inconventional cabinets. The badly needed room for large cable bundles wasacquired in this fashion, and when the same technique was implemented forprofessional audio racks, more space was added not only for cable, but alsofor equipment such as power strips and power sequencers. Measured ininches, this space gain is impressive. For example, racks measuring 24.5inches (622 mm) on the outside typically offered only 19 1/8 inches (486mm) of usable space on the inside. Once the new rackrail/bracket designreached professional audio and other markets, that same enclosure provided23 inches (584 mm) of usable interior space.
If anything else has been learned in the last year or so by rackmanufacturers in this area, it is that obtaining more inner rack space forcabling and equipment is one matter, but managing that space and the cableswithin is quite another. In response, rack manufacturers offered moreoptions for cleanly tying off cables and cable bundles. With the currentcatalog available of horizontal and vertical lacing bars, contractors ofall stripes-video, audio and data-can now use rack space gains in a fashionbest suited to particular styles and needs, while maintaining a clear senseof order and tidy appearance in their work.
Other new rack assembly techniques gleaned from the data industry involvethe use of surge and spike protection. Imparted just as audio contractorswere blessed with added rack space for rear-mounted power strips and powersequencers, the lesson most relevant for those seeking to safeguardvaluable equipment was that surge and spike protection should be suppliedby a large MOV (metal oxide vari-resistor) in both common and differentialmodes. In common mode, the protective circuit is provided at the powersupply's hot and neutral legs. Conversely, protection in differential moderuns from neutral to ground and hot to ground. Both modes are capable ofswallowing large power spikes and offer more than the token amounts ofsafety provided in the past.
Space gains, advanced cable management techniques and new developments inprotective circuitry are not the only trends in rack manufacturingcurrently appearing within our industry. Along with these enhancements,levels of structural integrity have arrived, including the furtherdevelopment and refinement of seismic-rated enclosures. Once believed to bea concern reserved only for those living directly atop the San Andreasfault, seismic contracting issues are as relevant in Chicago as they are insouthern California. I do not mean to imply that Chicago is in danger ofcrumbling under the stress of a major earthquake and sliding into LakeMichigan, but that preparing your rack assemblies to surviveearthquake-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 inthe back of a truck and jostled severely in a crosstown trek. They couldpossibly arrive at the job site fully loaded, only to find that theelevators are not yet operable. A rocky trip up multiple flights of stairscould ensue. There are many more hazardous situations your rack couldencounter as well-being dropped from a crane or falling down an elevatorshaft-that make a seismic situation seem like a glass of warm milk and aplate of cookies, but my point is clear. Earthquake preparedness should befor everyone, like buckling your seatbelt. This is especially true in lightof the fact that it takes very little time and expense to obtain seismiclevels of structural integrity. In most cases, the cost to contractors intime and dollars is actually significantly less than using othertraditional methods for preparing for that crosstown truck ride orstaircase journey.
Start with the basics of readying your rack for tremors and everything elsethe 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 thatwill bring the highest levels of structural integrity to your job.Obviously, the worst rack configuration would be a tall, slender unitstanding by itself with a 200 pound (90 kg) device mounted at the top andblank panels running the rest of the way down. The best configurationrequires taking the time to understand how much weight is going into therack, where the center of gravity is, how the load is dispersed and whetherthe 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. Ifthere is more heavy gear than you have space for in the bottom third of asingle rack, spread the load out over other racks. A seismically ready rackshould be filled from top to bottom. This does not exclude the use of ventpanels for cooling or blank panels for future expansion, but use theseitems judiciously.
Rear support is another essential consideration for structural integrity.If, after rack-mounting a component, you can push down or lift up on therear of the unit and it flexes easily, you need rear support. In morespecific terms, the following formula is an aid to determining the need forrear 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 theoverall depth of the device is greater than 2.5X its racking height, thatpiece of equipment must be rear supported. Rear-support mechanisms can takemany shapes and forms. There are rear-hanging brackets available thatattach to equipment outfitted with rear-hanging ears secured to arear-mounted set of rackrails. For transporting a loaded rack to a jobsite, contractors have long relied upon the rear-supporting technique ofcutting wood chocks and inserting them between the equipment. Whileeffective, this method calls for the removal of the chocks once the rack isinstalled because the presence of the wood in an electronics enclosureviolates fire codes. Given the amount of time it takes to create theblocks, install them and remove them once the rack is secured in itspermanent resting place, it makes more sense to choose something morepermanent and secure that will also serve in helping to complete a rackthat meets the seismic Uniform Building Code (UBC).
Drawing once again upon lessons learned from the data industry, thisbecomes a simple and inexpensive task with the addition of a pair ofmid-mounted rackrails and a rectangular horizontal lacing bar, which isemployed here as a support directly beneath your equipment, secured at eachside along the mid-mounted rackrail. Simple, sturdy and value-oriented,this configuration is also ideal for rear-supporting components that arenot equipped with rear-hanging mounting ears. This method is quicker,cheaper, stronger and more permanent than wood chocks were ever intended tobe, and it is color coordinated to match your rack.
In a true seismic event, the cornerstone of rack integrity resides at floorlevel. All other physical and structural pragmatics considered, if youdon't anchor the unit to the floor properly, all other preparations willnot matter. Although I do not know of any rack manufacturer that sellsseismic fasteners (simply because they can't tell what kind of floor willbe present in each application), I recommend that any anchor you choose bepurchased along the same guidelines one would use in buying a parachute. Gofor the best quality. Fasteners with a high shear rating and high tensilestrength are the best choice.
Seismic-rated clamp kits for your rack should be installed in the rack'scorners. Along with the fastener holding the rack to the floor, flangedwashers 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 andare designed to resist the violent twisting and wrenching they wouldreceive in an earthquake, while transferring all the heaving motion of atrembler directly to the rack's vertical surfaces via the corners where itis strongest.
There are some other items not to overlook in your quest for seismicrating. Tighten all rack screws that secure your components to therackrail. This ensures that your rack remains square, and that all of theside-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 inmeeting budgets, choose a rack seismically rated right out of the box at noextra expense. They are readily available; you should not pay more to meetseismic codes. In choosing this type of unit, look for a rack with cornerbraces composed of structural steel measuring at least 1/8 inch (3.2 mm)thick designed for seamless side construction. Some racks are built usingcorner butt or lap joints that are held together by welds at the top andbottom. For aesthetic reasons, these welds are ground down, sometimesremoving approximately 80% of their strength. Racks ideal for seismicapplications are seamless in that the sides are created by taking a fullsheet of metal and cutting the center out, thereby alleviating the need toemploy stress fracture-prone welds at the corners.
Switching gears, consider Northridge, CA, January 1994. We are at theepicenter of this decade's worst earthquake thus far, and major sections ofthis town have been reduced to rubble. On Reseda Boulevard, where abuilding collapsed is a rack enclosure standing unscathed. An entire floorof a building which collapsed on top of it lies in ruin at its base. Nowthat is seismic rating. Too bad they would not let photographers into thearea. Take my word for it---there were witnesses. It is now your turn tocreate the next generation of rack-mounting survivors using the availablematerials and these guidelines.
Based in San Francisco, EQE International is a consulting firm with officesaround the globe. Founded as an entity devoted almost entirely toearthquake engineering, today the firm's more than 500 associates alsoprovide expert counsel on other hazards and disasters ranging from fire,floods, tornadoes and explosions to risky situations of a more subtlenature in the worlds of security and finance.
Before EQE was contacted a few years back, there were no establishedguidelines for providing seismic ratings for steel rack assemblies.Determined to obtain seismic ratings for a series of products found in hiscompany's catalog of racks and rack accessories, Bob Schluter, president ofMiddle Atlantic Products, enlisted the aid of EQE's Leo Bragagnolo indeveloping the necessary testing procedures. Using the Uniform BuildingCode (UBC) as a roadmap, Bragagnolo devised a series of lateral stresstests.
"Primarily, I was interested in determining the amount of lateralforce-like an earthquake would impart-that the cabinets could take,"Bragagnolo recalled. "I wanted to know at what point the cabinets wouldbreak or fail in some manner where their structural integrity wascompromised."
Bragagnolo's test racks were outfitted with dummy loads approximatingreal-life situations. Based upon the results of his tests, he determinedwhat forces the cabinets could safely withstand using the 1994 UBC's mostsevere Seismic Zone 4 Upper Floor rating requirements
"Because we were the first agency of record I know of engaged in this typeof rating process, what it really came down to was developing testingprocedures that would provide me with the level of confidence I needed tosay they met the requirements of the UBC," Bragagnolo added. "As a licensedengineer 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 inestablishing seismic ratings for steel electronics enclosures."