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. It may be prudent to framing. Choosing an appropri- beam or penetrations either side design all floor areas for equip- ate structural framing system of a member with a predeter- ment loading. requires special consideration of mined concrete compression Microelectronics facilities use many unique design parameters, flange width between openings. hazardous chemicals throughout material availability, and the A plan note on the drawings much of the wafer fabrication economics of fast track construc- indicating the design parameters process. Facilities known as tion. This article will focus on used will protect your design and Hazardous Production Material steel design, component selection alert the contractor to potential (HPM) buildings, which are and construction. deviations from the original intended for the storage of flam- Conventional wide flanged design if openings are other than mable gasses, must incorporate rolled, domestically produced, as assumed (and shown on the very specialized blast relief dam- structural shapes can be typical- contract documents) at the com- age limiting design. The design ly used for the structural frame, pletion of construction docu- of HPM buildings is a topic of its including long span truss chord ments. Composite steel deck will own and will not be discussed in and web members. Truss webs typically be 18 or 20 gage with 3- 1 detail, but the loading criteria is can be structural tubes or wide /4” light weight topping to pro- quite interesting and notewor- flange shapes depending on the duce a two hour fire rating. If a thy. The storage of volatile magnitude of loads and types of one hour rating is required nor- chemicals is usually in a sepa- connections desired. Web mem- mal weight topping can be used. rate adjacent building. Blast ber tube sizes should be limited The topping thickness will most relief vent panels are required to to TS12x12 to avoid cost premi- likely be no less than 3” to pro- relieve internal pressures and ums and potential availability duce the required live load carry- may be designed in roof struc- problems for less than mill order ing capacity. The choice of fire ture and wall areas. Design load- quantities. A microelectronics proofing systems is not only a ing can be as much as 150 psf on facility structural floor framing code issue, but also a microcont- internal walls. Roof panels are grid is similar to a commercial amination issue that needs to be typically designed to blow off at office building’s at approximately discussed early with the design- 40 psf. Design blast pressure is a thirty two feet square. As men- ers of the fab mechanical/process function of the volatility of tioned above microelectronics system. Beams and columns may stored gasses. This creates many facilities are typically high bay be wrapped with sheetrock to special design considerations not structures. Most economic col- achieve the desired fire rating. only with regard to the unbraced umn shapes are typically rolled The economics of using light length of beam member compres- wide flange shapes. Steel column weight topping is an issue that sion flanges, but more impor- supported long span trusses can also needs to be discussed early tantly the overall structural sta- produce column axial loads on in the design process, and for bility of a building system. the order of 700 - 1000 kips. design build projects can easily HPM buildings storing chemi- Columns can range in size from be handled by the construction cals and gasses subject to confla- W14x90 to W14x211 depending manager/general contractor. gration are typically single story on tributary areas supported, The support of air purification concrete masonry buildings with braced frame locations and con- high-efficiency particulate atten- steel roofs. Roof areas designed figurations. uation (HEPA) filter ceiling grid as blast relief panels have limit- Floor framing members systems can present unique ed diaphragm capacity. Blast designed as composite members design challenges not only from relief roof panels incorporate (for 32 ft. bays) will typically be a structural support and seismic specialized fasteners which limit W18s or W21s, depending on the bracing concern but also from a diaphragm capacities and allow deck span (trib. area) with W24 structural beam design stand- for venting to occur at about 40 or W27 girders. The use of com- point. Structural framing mem- psf internal pressure. Lost posite beam design should be bers supporting (above clean diaphragm capacity must be carefully considered. room/below fan deck) ceiling sys- replaced by inplane roof trusses Microelectronics facilities are tems must be designed to avoid (bracing) supporting exterior large mechanical system ware- torsional lateral buckling from (and interior) masonry walls. house type facilities. With that unbraced compression flanges. comes a tremendous volume of Designing for the full length as STRUCTURAL SYSTEM SELECTION mechanical equipment and asso- unbraced can produce uneconom- (GRAVITY FRAMING) ciated floor penetrations. It may ically large members. It has been Many different structural sys- be prudent to design composite found that that adding interme- tems have been used for micro- beams with one half the com- diate bracing members signifi- electronics facilities, and include: pression flange effective, as for a cantly reduces member sizes (as wide flange shapes. The design of beams with web openings is beyond the scope of this paper, but an introduction to the fabri- cation method and analysis is important. A repetitive pattern of hexago- nal web openings are created by cutting a member longitudinally and then off setting the two halves to create a beam with up to 50 percent increase in depth and with the same weight. The resulting section has a much larger moment of inertia and section modulus. As an example a W18x35 beam can be (re) fabri- cated into a W24x35 or W27x35 with a large increase in stiffness and no change in weight. The lightest (conventional) W24 and Truss bottom chord drag strut connection W27 beams weigh 55 and 84 pounds per foot respectively. This equates to a weight savings would be expected). This pre- several different factors. of 36% and 58% respectively. sents additional design chal- Economics will always play a With the use of automated com- lenges in detailing intersecting major role in structural member puter controlled torches and beam connections to provide ade- selection. Roof members of welding apparatus this proce- quate bracing. Collecting bracing microelectronics facilities typi- dure has been found to be quite loads and accounting for ceiling cally need to support heavy pip- economical and can more than system seismic bracing forces ing, ceiling, and ducting loads offset the cost of the heavier sec- need special consideration since far in excess of commercial type tions. Connections to adjacent interstitial framing levels are office buildings. The uncertainty framing members are no differ- not continuous by means of a of all mechanical loads at the ent than for conventional rolled structural diaphragm. Loads outset of design produces anoth- shapes. Intersecting member must ultimately be transferred er unique design consideration. connections to beams with web to a rigid diaphragm for redistri- Rolled wide flange shapes can openings can be accommodated bution to braced frame members. more easily accommodate point with full depth shear tabs if con- Similar problems are present loads from mechanical support nections layout on a web open- at the roof. The interstitial space hangars than open web joists. ing. above the fan deck is a pressur- Joists designed to accommodate Beam analysis is similar to ized air plenum. This internal .5kip to 1kip point loads any that of a beam with a web open- pressure can produce compres- where along the joist length may ing for a mechanical pipe or sion in flanges of roof framing produce uneconomically large duct. Top and bottom chord sec- members which are typically chords sizes. However, rolled tions are analyzed a tee sections. stressed in tension. This is espe- wide flange beams designed for The beam is sized based on the cially true if the design architect 50 to 80 psf live load can better section modulus required to chooses a (relatively light) single accommodate equipment point resist the maximum design ply adhered membrane roof in loads which are usually a mov- bending moment. Secondary lieu of a ballasted mechanically ing target during design. stress are then analyzed at the applied roof or more convention- Fabrication facilities can house web openings. The beam acts al built up roof system. For open large air handling and mechani- similar to a Vierendeel truss. web joist roof systems this can cal equipment in a separate cen- Combined stresses from the tee lead to increased chord member tral utility building (CUB), or section axial load and secondary sizes and additional uplift bridg- much of this type of equipment bending from the shear stress ing. For beam framed roofs mem- can be roof top mounted. Water across the web adjacent to the ber sizes would need to be purification equipment usually is opening are checked against checked for net uplift loads on housed in a separate facility. AISC interaction equations. the unbraced length. An economical option exists in Shear stresses are checked over The selection of roof framing using beams with web openings the gross section and the net sec- members can be influenced by in lieu of conventional rolled tion at the web opening.

Modern Steel Construction / May 1997 Horizontal web shear flow is also As noted previously, long span and 4, that connections have the checked at the web post between trusses provide for column free “strength” to resist 3Rw/8 times openings. fabrication space. Truss gravity the calculated brace force. For Beams with web openings can loads are relatively heavy and braced frame structures the duc- be composite or non composite therefore a truss spacing of 32 tility factor Rw is equal to 8. and are an economical substitu- feet (matching the structural These factors must all be consid- tion for open web roof joists or building grid) will provide for ered in the schematic design rolled shape roof beams. Roof economic members sizes. Trusses phase of a project design while beams need to be braced during also serve well as lateral load choosing braced frame locations. steel erection no different than resisting (transferring) elements. Of equal importance is the over- conventional framing. Bracing is Seismic forces resisted parallel all stiffness of the structure. required based on the unbraced to the span direction of long span Ample well throughout frame length of the compression flange trusses can be collected in very locations equally resisting a pro- and expected construction loads. stiff truss elements and trans- portionate share of the structure Uplift bridging is no different ferred to (interior) braced base shear will limit building than the required bracing for frames. Truss analysis and drift and minimize stiffness conventional rolled shapes or detailing must account for seis- irregularities. open web joists. mic collector (drag) loads. Lateral forces may be resisted Interior bay braced frames are by a number of different framing LATERAL LOAD RESISTING inevitable and must be located systems. The most economical SYSTEM SELECTION accordingly. Interior braced system for high bay, heavy For structures designed in the frames may terminate at the loaded structures is a braced western part of the United truss bottom chord floor level. frame “Chevron” or “V” brace States, lateral loads are derived Braced frames resisting seis- configuration. Economy can be as prescribed in the Uniform mic loads perpendicular to long gained, with these configura- Building Code. West coast facili- span truss bays are typically tions, from the brace length, ties are designed for seismic located on the exterior building frame geometry, and the fact Zone 3 or 4 loading, while plants grids, but may locate on interior that braces work in tension and in the south west are designed grids as required. The designer compression. The UBC dictates for seismic Zone 1, 2, or 2b load- will not have the benefit of truss- that “Chevron” braces be ing. The design differences are es as collector elements and designed for 1.5 times the calcu- considerable and are beyond the braced bays would need to lated force, in part, to lessen the scope of this paper. This paper extend full height to the roof chance of a tension member fail- will discuss the design of facili- level. Diaphragm shears for ure resulting in compression ties designed in seismic Zones 3 loads perpendicular to truss bays buckling of the remaining brace. and 4. will be considerably higher for “V” braces, or inverted “Chevron” Microelectronics facilities are brace bays located only at the braces can be used in lieu of classified by the UBC as H building perimeter. “Chevron” braces to help accom- (Hazardous) occupancy build- Microelectronics facilities are modate corridor door locations ings. The Uniform Building Code quite rectangular in plan. The close to grid line columns or dictates that hazardous facilities layout of process equipment is mechanical duct work centered be designed with an importance linear and building plan aspect in a corridor. This type of brace factor of 1.25 times the pre- ratios can be between 1.8:1 to configuration requires special scribed earthquake loading and 3:1. The choice of braced frame foundation considerations where 1.15 times the prescribed wind locations is not as sensitive as in braces meet the slab on grade loading. Design loading used for speculative office building design between columns. The use of sin- the analysis of equipment brac- work. Frame locations, in heavy gle diagonal braces is limited by ing is to incorporate an impor- fabrication facilities can be gov- the code. Design forces for single tance factor of 1.5. Further, facil- erned by limiting diaphragm diagonals must be increased by ities with permanent equipment shear capacity, foundation/frame 3R w/8 to account for the non should be designed to account for overturning (column) loads, and redundant nature of the frame. 25% of the permanent equip- the magnitude of brace loads. In The overall brace length and ment mass in the seismic analy- high bay structures brace mem- increased design load makes this sis as required by the UBC. It is ber lengths can dictate the use of type of frame system uneconomi- prudent to calculate the actual large tube or wide flange mem- cal and left to be used only when expected mass of permanent bers. The UBC limits the architectural or mechanical duct equipment and compare that to “Chevron” brace slenderness routing requirements dictate. 1 the UBC criteria. The actual ratio, Kl/r, to 720 / F y 2 . For Moment frames are not partic- equipment weight may be con- long braces this quickly elimi- ularly economical for high bay siderably more and should be nates many tube sizes. The UBC heavy loaded structures. This accounted for in the seismic also requires, in seismic Zones 3 framing system yields heavy col- analysis of the structural frame. bers. “Standoff” connections, or extended shear plates may be required to physically enable the erector to install beams and make bolted connections. Many factors affect the design and analysis of long span truss- es. Truss depths, to optimize effi- ciency, should be limited to approximately one tenth of the truss span. There is a diminish- ing return in optimizing chord sizes by increasing the depth to reduce axial loads. Compression members increase in size with increasing length governed by slenderness Kl/r. Chords are usually not parallel in order to provide a minimum roof slope. It may be desirable from an owners “Fan Deck” long span truss bay with open-web expanded beam roof framing standpoint to slope the bottom chord. This bottom chord slope allows for accidental spills to umn and beam sections. Heavy a mid span truss column pre- “roll” off the truss into adjacent structure loads would increase sents a vibration isolation prob- plenum space instead of possibly the number of frames required to lem which precludes the use of down into clean room fabrication limit member sizes and control the process level to reduce truss space. Midspan camber may be building drift. The complexity of column unbraced lengths. Truss desirable to offset dead load post Northridge moment connec- depths are typically a full floor to deflections. tion field welding has recently floor story depth, a minimum of Truss geometry can take any created schedule problems on 15 feet. The bottom chord level number of forms based on the fast track projects not only due or fan deck provides support for designers preference, but may to increased field welding but air purification equipment. Total also be governed by mechanical also as a result of more stringent truss load including fan deck and duct size penetration require- inspection criteria. Further, con- roof can be in excess of 300 psf. ments. Large mechanical ducts siderable ongoing review and Truss chord and web members and a myriad of pipes extend testing of moment frame connec- are typically wide flange and through trusses. This requires tions continually produce new tube shapes depending on the that truss web member gusset information regarding design, load magnitudes and member plates be drawn to within as lit- 1 detailing, and material specifica- lengths to achieve the best econ- tle as /2” of actual size for multi- tion requirements. omy. Economy in truss design, as ple computer aided design (CAD) It is not uncommon for braced it relates to the overall steel overlay for mechanical coordina- frame bays to be assembled on erection sequence, is not only a tion. the ground and lifted into place. function of the truss weight, but The move in, and replace- This gives the steel erector a in the ease (or complexity) of: ment, of large mechanical equip- means of controlling the building web and chord connections, ment in the fan deck truss bays erection tolerances and econo- attention to detail of intersecting is a very important design con- mizes the fit up eliminating awk- secondary floor and roof framing sideration and not usually a con- ward position welding of brace members, zero tolerance truss to cern in commercial buildings. members. column connection details, and Several options have been tested top chord bracing requirements to accommodate equipment move ONG SPAN TRUSS DESIGN L to control “tl” buckling. in: roll up doors between truss Trusses spanning across clean Truss chords at web intersec- bays at the building exterior, room areas provide for column tion node points “picking” up cantilever trusses with beam in- free manufacturing space. A floor and roof secondary framing fill framing or Vierendeel trusses steel weight penalty is paid for members may be congested with to provide openings transverse to clear span trusses in lieu of stiffener plates which control their span. Design requirements shorter multi-span continuous truss chord web buckling prob- for microelectronics facilities trusses. Clean room process lems. Special attention must be typically require access to con- equipment is extremely vibration paid to the ease of connection of cealed truss bay spaces after the sensitive and the introduction of intersecting beam framing mem- design is complete. Access to con-

Modern Steel Construction / May 1997 cealed spaces can be accommo- overall (axial) load will typically ends rather than parallel to the dated with removable wall pan- not control the design. Chord truss chord which should be els. Removable roof panels are members should align at mid accounted for in the gusset plate not an especially good option for depth to reduce excessive transi- design. obvious reasons. Exterior bays tion welding at adjacent (offset) Special attention must be paid with braced frames can be flanges of chords with different to the unbraced length of truss designed incorporating remov- thickness. Aligning chord depths top chord compression members. able braces with bolted connec- will produce steps at the top of Intersecting beam connections tions. the top chord which may require may be detailed to provide ade- There are many variables the addition of shim plates to quate compression member brac- involved in the erection of long provide for a uniform bearing ing. Open web joists do not pro- span trusses. Field splices of surface for roof decking. All of vide adequate top chord bracing shop fabricated trusses are these suggestions will help pro- because the bottom chord flange almost inevitable due to shipping duce well engineered, efficient, is not restrained in a typical limitations. This requires field cost effective designs. bearing connection. Specially welding of at least one truss Connection design and detail- detailed top chord, bottom flange diagonal web member to com- ing is of particular importance bracing is required to economize plete the assembly. High and worth some further discus- top chord member sizes and pro- strength bolted chord splices are sion. The design of truss web vide structural stability of the typical. The addition of plates connection gusset plates can pro- system. Contract documents either side of the chord member vide interesting challenges. should explicitly instruct the web, above the bottom flange Aside from the shear and contractor on the appropriate and below the top flange will, moment checks that must be erection sequence of truss top create a double shear connection. made to preliminarily size gusset chord bracing as roof construc- Locating the splice as near to the plates and welds, plates must be tion proceeds. It is not unusual center of the truss or the calcu- designed as column compression for long span trusses with light lated point of zero shear will elements. Web members may be single ply roofing systems or reduce the web connection held up off of the truss bottom standing seam metal decks to requirements. Analysis and chord so as to not interfere with experience load reversals with detailing of trusses to bear on metal deck installation. Gusset bottom chords in compression top of supporting columns will plates must be sized for axial from wind uplift loading. This is aid the steel erector with stabili- compression loading. This short not the case in microelectronics ty problems during erection. Fit plate column section can facilities with fan decks at the up tolerances of truss (or braced increase in size very quickly as bottom chord level, but more frame) web connection plates to web member end distance prevalent in large single story support columns is always of increases from the chord. The high bay manufacturing build- concern to the steel erector. column being designed has a ings. For trusses with net uplift The analysis of long span radius of gyration of d 12 , or “problems”, bottom chord diago- trusses can be made using well 28.8 percent of the gusset plate nal bracing may be required up understood design principles. thickness. This grossly limits the to an adjacent truss top chord to Special attention must be paid to axial load carrying capacity of a provide structural stability. the design of web and chord con- plate column even with a very nections. It is recommended to short length. VIBRATION CRITERIA minimize web and chord sections The choice of an appropriate Many papers have been writ- as internal forces dictate. But, “K” value to determine the ten on vibration analysis and the member lengths should be maxi- design column length must be derivation of applicable imperi- mized to reduce the number of determined. A “K” of one is clear- cal formulas to assist engineers costly full penetration splices. ly unconservative and a “K” of in the design of floor systems, Sophisticated design programs two (as for a cantilever column) avoiding perceptible vibrations. are available to simplify design. is clearly way too conservative. It is not the intent of this article If trusses are modeled the This author recommends using a to re-introduce vibration design author recommends fixing the “K” of 1.2. Stiffeners can be criteria for buildings, but to dis- web members and designing con- added to the each side of a gus- cuss the uniqueness of microelec- nections with the appropriate set plate to change the “column” tronics facilities’ vibration design fixed end moment. Truss ele- geometry and increase the “col- parameters and investigate a ments have continually been umn” radius of gyration. This structural steel framing system designed as two force members, will help force plate shear, not suitable to replace a convention- but some fixety exists at the web compression buckling to govern al concrete waffle slab floor sys- to gusset plate connection point the design. An economical tem. which should not be ignored. The advantage exists in detailing Microelectronics facilities percentage of web moment to web members with square cut

Modern Steel Construction / May 1997 ics facilities air flow is required through the process floor. Large openings between beams will reduce the structure mass and damping characteristics. This needs to be carefully considered when designing composite beams to support heavy design live loads and assigning a trans- formed moment of inertia for beam stiffness calculations.

This article is adapted from a paper to be presented at the National Steel Construction Conference. Nathan T. Charlton, P.E. is an associate and senior project manager with KPFF require extremely vibration sen- perform well with limited per- Consulting Engineers in sitive equipment for use in the ceptible vibration. This is easily Portland, Oregon. KPFF is a manufacturing process of silicon obtainable with a 30x30 typical structural/civil engineering firm wafers. Plants require a “vibra- office building grid using com- with offices in Portland, Seattle, tion free” environment. posite framing with normal San Francisco, Los Angeles, San Suspended, structured floors can weight concrete topping. Diego and Phoenix. Charlton has be steel framed or cast in place Vibration perception is limited a bachelor of science degree in concrete. Process (clean room) by: damping, structure mass, civil engineering and a masters structured floor areas may be and beam span. of science degree in structural designed as much as 10 times Microelectronics facilities can engineering from Portland State stiffer than commercial office require kf products as high as University in Portland, Oregon. floors. Commercial, office struc- 64,000. This equates to a struc- He is a registered professional tures are designed with natural ture velocity as low as 125 engineer in Oregon, Washington, frequencies in the 5-8 hertz (hz) microinches per second (dB). Idaho, and California. range (depending on assumed This extremely stiff structural damping). Wafer fab facility frame is typically designed as a process floors can require natur- concrete waffle slab or waffle al frequencies on the order of 50 grid (without a slab). To create hz. The design goal is to elimi- this same type of system using nate ambient vibrations or structural steel framing would “noise” from foot traffic and sur- require a heavy composite slab, rounding rotary machinery. on the order of 3 inch deck with 1 The process floor structure, in a minimum 3 /2” normal weight some cases, may be physically topping, and stiff beams with isolated from the balance of the short spans. Using well known building superstructure. This equations for beam stiffness and requires a completely separate frequency a beam’s natural fre- structural system resisting grav- quency and deflection can be ity and lateral loads. determined. It has been suggest- A floor systems’ vibration ed that an appropriate ampli- characteristics can be measured tude (deflection) be calculated by comparing the calculated using a point load midspan of a beam stiffness frequency prod- simply supported beam, account- uct, kf (or f/(), to a known con- ing for partial fixity from end stant. The results of instrumen- connections. A structural fram- tation of existing office floors to ing grid of W21 or W24 beams at measure vibration levels can pro- 8 feet on center (or less) span- vide a basis for comparison. ning sixteen feet would be Instrumented commercial office required to come close to match- floor systems with kf products of ing the stiffness of a concrete 350-500, velocities of up to waffle grid, depending on the 16,000 microinches per second, cracked moment of inertia used and moderate damping appear to in design. In some microelectron-

Modern Steel Construction / May 1997