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Framing Systems for Microelectronic Facilities

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Framing Systems for Microelectronic Facilities FRAMING SYSTEMS FOR MICROELECTRONIC FACILITIES ICROELECTRONICS lars a day. Most microelectronics Fast track FACILITY STRUCTURAL fabrication (fab) facilities are MSTEEL FRAMING SYSTEM designed as fast track design- construction and SELECTION can be driven by an build projects and redefine the ambitious fast track design term fast track to hyper track. It mechanical schedule for material fabrication, is not uncommon for mill order delivery and erection, longspan drawings to be produced within system design framing requirements for column the first couple of weeks of free manufacturing space and design for advanced mill order of play a crucial vibration sensitive design crite- rolled shapes. ria. Further, mechanical loads Microelectronics facilities are role in selecting may not be well defined at the classified by the Uniform outset of design precluding the Building Code as hazardous a suitable use of steel joist roof framing occupancy facilities. This haz- systems. Microelectronics ardous occupancy classification framing system Facility Design is not driven by can lead to explosion venting architectural considerations as design and damage limiting con- By Nathan T. Charlton, P.E. much as by mechanical system struction for areas used for stor- support requirements. ing volatile gasses subject to con- Microelectronics facilities are flagration. Further, the code very specialized, single-use, requires an importance factor of high-bay facilities which typical- 1.25 be used in the seismic ly incorporate long span trusses, analysis of the structure. extraordinarily sensitive vibra- Dynamic analysis are not imme- tion design criteria, and very diately required due to the H heavy permanent equipment occupancy classification but may loads. Fab buildings, as they are be required due to building stiff- referred to in the industry, are ness irregularities. typically three-story structures ranging in size up to 300,000 sq. CONFIGURATION OVERVIEW ft. Typical project costs are on Fab buildings are typically the order of one billion dollars, three-story structures. The which includes very costly ground floor (or basement level) process equipment or “tools”. is referred to as the “sub fab” Construction cost may be 30% to and houses a tremendous volume 40% of the total. The microelec- of support mechanical equip- tronics industry is very dynamic ment. Above the “sub fab” is the and product life cycle can be very “process level” or clean room short. Technological advances area and adjacent personnel and require owners to design, build, equipment support areas. The staff, qualify tools and begin clean room space can be on the operation in increasingly shorter order of 60,000 to 80,000 sq. ft. periods of time. Lost days in pro- and is typically isolated from the duction can cost millions of dol- adjacent structure with a physi- Modern Steel Construction / May 1997 cal isolation joint to preserve the provide for the support of a myri- Long-span trusses provide a mechani- extreme vibration sensitivity of ad of air purifying mechanical cal equipment floor level, lateral load the space. Clean rooms are clas- equipment, and serve as lateral resistance and column-free manufac- sified based on various classes of load resisting elements transfer- turing space. Photo by Steve Miller, air purity, class 1,10,100, and ring roof seismic and wind loads KPFF 1000. This classification repre- through their depth. sents the number of .5 micron The Process area floor is usu- particles per cubic foot of air. ally designed as a concrete waf- composite action and unexpected The process level fab environ- fle slab (or waffle grid) supported loading. ment is extremely sensitive to by concrete columns and shear Roof framing members (span- micro-contamination and is usu- walls. Composite floor framing ning between trusses) can be ally class 1 or 10 space. The may be used for the bulk of the rolled shapes or open web joists. third floor or “fan deck” level floor plate (outside the process Rolled shapes provide much houses mechanical equipment area) design. The economy pro- more flexibility for hanging loads for: air purification, space condi- vided with this common struc- due to the fact that mechanical tioning, uninterrupted power tural system is very similar to system piping and duct runs supply and electrical transform- that of commercial office build- may not be determined until the ers and switch gear. The fan ing design but can be even more design is nearly complete. The deck is typically the bottom equitable due to the relative high use of open web joists could chord level of full story depth design live loads. However, potentially require costly field long span trusses spanning numerous penetrations through modifications for the support of across the process clean room or floors are required for ducting concentrated mechanical loads. “ballroom” as it is sometimes which can compromise a compos- Design-build mechanical sys- referred. The roof level is framed ite floor design. Mechanical sys- tems can be slow to develop at the top of the “fan deck” truss- tems may not be well developed given their extreme complexity es. at the outset of structural design and sensitivity to ever changing and it is not uncommon to cover equipment specifications. Beams FRAMING OVERVIEW plate beams and modify connec- with fabricated web openings Long span trusses provide for tions after steel erection is com- provide an economical substitute column free space in the primary plete to accommodate unexpect- for roof joists (and floor beams) fabrication (clean room) areas, ed floor penetrations, the loss of an will be discussed in greater detail later. Modern Steel Construction / May 1997 creates large vertical plenum spaces throughout the structure and through (fan deck) structur- al diaphragms. Fan deck diaphragm shears are trans- ferred through heavily reinforced “shear piers”, which provide diaphragm continuity. Heavy composite slab reinforcing may be required to strengthen struc- tural diaphragms. Diaphragm continuity can also be accom- plished in narrow piers with the use of flat plate steel elements analogous to horizontal steel plate shear walls. SERVICE LOAD DESIGN CRITERIA The design live load criteria varies throughout microelectron- ics facilities and can be quite dif- Three levels of structural framing showing “Fan Deck” and roof framing with open-web expanded beam framing. ferent. The process floor (clean room) structure is typically designed for a minimum 250 psf LATERAL LOAD RESISTING Building Code (UBC) building floor load for the support of fab SYSTEMS OVERVIEW drift limitation requirements. tools and an additional 50 -100 Lateral loads generated from; Design lateral loads are speci- psf for hanging loads incorporat- seismic loads, wind loads and fied in the UBC for building ing; ducting, piping, and ceiling blast loads, may be resisted by a design in the Western United systems. This load may also be number of different types of well States. Code prescribed seismic in the form of interstitial fram- known bracing systems, but are forces must include an impor- ing supporting mechanical most economically resisted by tance factor of 1.25 as specified equipment between the Sub Fab braced frames located on the for hazardous facilities. The and Fab levels. Fan deck areas building perimeter (unlike devel- UBC specifies that for ware- supporting air purifying equip- oper office building projects) and house facilities a percentage of ment are typically designed for throughout the interior. The permanent equipment load be 100 to 150 psf which also number of braced frames accounted for in the lateral includes; ceiling filter systems, required is proportionately larg- analysis. The calculated equip- ducting, and mechanical piping er for microelectronics facilities ment load weight can be signifi- loads. Areas designed to receive than for commercial office build- cantly higher than the UBC pre- electrical transformer and unin- ings due to the inertial mass cre- scribed mass. It is prudent to terrupted backup power equip- ated by the large volume of per- calculate the actual equipment ment should be designed for a manent mechanical equipment weight, within reason, and minimum 200 psf live load. and heavy composite slabs include this mass in the lateral Roof areas should be designed designed to support equipment analysis. for a minimum 50 psf live load loads. Numerous braces will also The analysis of structural for hanging duct work and pip- reduce the load to “long” braces diaphragms can be quite com- ing but can also support heavy at tall floor to floor areas thus plex. At the roof level mechanical equipment which reducing the brace sizes and diaphragms span between long would need to be accounted for foundation loads. Code pre- span trusses which minimize accordingly. Roofs can be sub- scribed buckling criteria can play shear loads and provide a very jected to internal plenum pres- a significant role in determining good load path to the fan deck sure loads resulting in net uplift brace sizes. Braced frames often level. Roof diaphragm shear loads (especially when added to extend through occupied spaces. loads produced from loads per- wind uplift loads) requiring spe- This can dictate the brace geom- pendicular to the truss span cial attention to the bracing of etry; “Chevron”, “V”, or single direction are resisted by full beam flanges that are typically diagonal. Moment frames height perimeter braced frames. stressed in tension. Corridors require excessively large column Fab facility tools require a and personnel support areas are sections due to tall floor to floor tremendous volume of air flow to typically designed per code mini- dimensions and Uniform provide an ultra pure clean room mum live load requirements. environment. This requirement Support areas may become part Modern Steel Construction / May 1997 l 8 of a future expansion within the conventional reinforced concrete, spandrel beam (Be = / or beam fab and this should be taken into precast concrete and spacing/2). This will accommo- account when configuring floor composite/non composite steel date a penetration adjacent to a loads.
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