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performance issues relativeto extreme events

S tructural Performance

Richard M. Drake, P.E., S.E.,SECB is a Senior Fellow/Structural Engineering at Fluor in Aliso Viejo,California. He can be contacted at Rick.Drake@fuor.com .

Robert J. Walter, P.E., S.E. is a Principal Civil/Structural Engineer at CB & I in Plain eld, Illinois. He canbe contacted at walter@cbi.com.

By Richard M. Drake, P.E., S.E.,SECB and Robert J. Walter, P.E.,S.E.

Seismic Design of Structural Steel Pipe Racks

Structural steel pipe racks typically sup-port pipes, power cables and instrumentcable trays in petrochemical, chemical andpower plants. Occasionally, pipe racks

may also support mechanical equipment, vesselsand valve access platforms. Main pipe racks gen-erally transfer material between equipment andstorage or utility areas. Storage racks found in

warehouses are not pipe racks, even if they store

lengths of piping.To allow maintenance access under the pipe rack,transverse bents are typically moment-resisting frames (Figure 1). Although the bent is shown withxed-base columns, it can also be constructed withpinned bases if the supported piping can toleratethe resulting lateral displacement.

e transverse bents are typically connected with longitudinal struts. If bracing is added inthe vertical plane, then the struts and bracing acttogether to resist lateral loads longitudinal to thepipe rack (Figure 2 ).

If the transverse frames

are not connected withlongitudinal struts, thepipe rack is consid-ered unstrutted. eframe columns mustact as cantilevers to

resist lateral loads longitudinal to the pipe rack.

Design CriteriaIn most of the United States, the governing codeis the International Building Code (IBC), whichapplies to buildings and other structures. IBC

prescribes structural design criteria in Chapters16 through 23, and adopts by reference many industry standards and speci cations that havebeen created in accordance with rigorous AmericanNational Standards Institute (ANSI) procedures.For the most part, design loads are prescribed

in ASCE 7, structural steel material referencesare prescribed in AISC 360, and structural steelseismic requirements are prescribed in AISC 341and AISC 358. e edition of each standard thatis used should be based on the edition of the gov-erning building code or as otherwise approved by the authority having jurisdiction.

Design criteria for nonbuilding structures areusually provided by industry guidelines that inter-pret and supplement the building code and itsreferenced documents. In the case of pipe racks,additional design criteria are provided by ProcessIndustry Practices, PIP STC01015, and the ASCEGuidelines for Seismic Evaluation and Design of Petrochemical Facilities . In this article, the IBCrequirements govern; the PIP practices and ASCEguidelines may be used for pipe racks, as they supplement the IBC and the referenced industry standards and speci cations, but they are notcode-referenced documents themselves.

Earthquake LoadsEarthquake loads are prescribed in IBC sectio1613, which references ASCE 7. Seismic detail

of materials as prescribed in ASCE 7 Chapter 1is speci cally excluded; instead, seismic detailinof structural steel materials is prescribed in IBChapter 22.PIP STC01015 recommends that earthquake

loads for pipe racks be determined in accordanc with ASCE 7 and the following:

Evaluate drift limits in accordance with ASCE 7 Chapter 12.

Consider pipe racks to be nonbuilding structures in accordance with ASCE 7Chapter 15.

Consider the recommendations of the

ASCE guidelines. Use Occupancy Category III and animportance factor (I) of 1.25, unlessspeci ed otherwise by client criteria.

Consider an operating earthquake load (Eo)that includes the operating dead load (Do)as part of the seismic e ective weight.

Consider an empty earthquake load (Ee)that includes only the empty dead load (De)as part of the seismic e ective weight.

Seismic Design Considerations

ASCE 7 Chapter 11 de nes a nonbuilding struc-ture similar to a building as: A nonbuildingstructure that is designed and constructed in amanner similar to buildings, will respond to stroground motion in a fashion similar to buildingsand has a basic lateral and vertical seismic forcresisting system conforming to one of the typeindicated. Examples include pipe racks. As a nonbuilding structure, consideration of semic e ects on pipe racks should be in accordanc

with ASCE 7 Chapter 15, which refers to otherchapters as applicable.

Figure 1: Typical transverse bent.

is article is condensed from a paper published by the authors

in the Engineering Journal(AISC), 4th Quarter, 2011, and is

reprinted with permission.

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Seismic System Selection

Either ASCE 7 Table 12.2-1 or ASCE 7Table 15.4-1 can be used to choose a seismicforce-resisting system, which will provide theprescribed seismic detailing requirements,design parameters (R, o, Cd), and heightlimitations. Table 15.4-1 permits selecttypes of nonbuilding structures that haveperformed well in past earthquakes to beconstructed with less restrictive height limi-tations in Seismic Design Categories (SDC)D, E, and F than those speci ed in Table12.2-1. Note that Table 15.4-1 includesoptions where seismic detailing per AISC341 is not required for SDC D, E, or F.For example, steel ordinary moment framescan be designed with R = 1 without seismicdetailing. Seismic detailing requirementscan also be avoided in SDC B and C forany structural steel system if R = 3 or less,excluding cantilevered column systems.

e transverse bents are usually moment-resisting frame systems. e choices arespecial moment frame (SMF), intermediatemoment frame (IMF), or ordinary momentframe (OMF).In the longitudinal direction, when brac-

ing is present, the choices are usually specialconcentrically braced frame or ordinary concentrically braced frame. Less commonchoices are eccentrically braced frame orbuckling-restrained braced frame. If bracing is not present, the choices in the longitudinaldirection are the cantilevered column systems.In both directions, the seismic system

selected must be permitted for both theSDC and the pipe rack height. ASCE 7Table 15.4-1 footnotes permit speci cheight limits for pipe racks detailed withspeci c seismic systems:

With R = 3.25, Steel ordinary bracedframes are permitted in pipe racks upto 65 feet (20 m).

With R = 3.5, Steel ordinary momentframes are permitted in pipe racks upto a height of 65 feet (20 m) where themoment joints of eld connections areconstructed of bolted end plates. Steelordinary moment frames are permittedin pipe racks up to a height of 35 feet(11 m).

With R = 4.5, Steel intermediatemoment frames are permitted in piperacks up to a height of 65 feet (20m) where the moment joints of eldconnections are constructed of boltedend plates. Steel intermediate momentframes are permitted in pipe racks upto a height of 35 feet (11 m).

Period Calculations

e fundamental period deter-mined from ASCE 7 Chapter12 equations is not applicable fornonbuilding structures, including pipe racks, because they do nothave the same mass and sti nessdistributions assumed for build-ings. It is acceptable to use any analysis approach that accurately includes the mass and sti nessof the structure, including niteelement models and the Rayleighmethod. e determination of thepipe rack period can be a ected by the sti ness of the piping leaving the pipe rack.

When this sti ness is not accounted for in theperiod calculation, it is recommended that thecalculated period be reduced by 10%.

Analysis Procedure Selection

Static or dynamic analysis methods can beused. Static procedures are allowed only undercertain conditions of regularity, occupancy, andheight. ASCE 7 Chapter 12 speci es when a dynamic analysis is required. e philosophy underlying this section is that dynamic analysisis always acceptable for design. A dynamicanalysis procedure is required for a pipe rack if it is assigned to SDC D, E, or F and it either:

has T 3.5T s; exhibits horizontal irregularity type 1a

or 1b; or exhibits vertical irregularity type 1a, 1b,

2, or 3.e most common dynamic procedure used

for pipe racks is modal response spectrumanalysis. e equivalent lateral force (ELF)procedure is allowed for a pipe rack struc-ture if a dynamic analysis procedure is notrequired. e simpli ed alternative struc-tural design criteria for simple bearing wallor building frame systems are not appropriateand should not be used for pipe racks.

Equivalent Lateral Force Procedure

e ELF procedure involves calculating thee ective earthquake loads in terms of a staticbase shear that is dependent on the imposedground acceleration and the structures mass(e ective seismic weight), dynamic character-istics, ductility and importance. e base shearis then applied to the structure as an equivalentlateral load vertically distributed to the variouselevations using code-prescribed equations thatare applicable to building structures. Using thisvertical distribution of forces, seismic designloads in individual members and connectionscan be determined.

ASCE 7 determines design earthquake forcon a strength basis, allowing direct comparson with the design strength of individualstructural members.

Modal Response Spectrum Analysis

MRSA is acceptable for the analysis of pipracks, and may be required if certain plan anor vertical irregularities are identi ed. e basisof MRSA is that the pipe racks mass (e ectivseismic weight) and sti ness are carefully moeled, allowing the dynamic analysis of multivibration modes that result in an accurate distribution of the base shear forces throughouthe structure. e MRSA must include a suf-cient number of modes in order to achieve aminimum of 90% mass participation.Two MRSA runs are typically required fo

pipe racks. e rst run includes the operating dead load (Do) as the seismic e ective weighto determine the operating earthquake load(Eo). e second run includes the empty deadload (De) as the seismic e ective weight todetermine the empty earthquake load (Ee).

e MRSA input ground motion param-eters (SDS, SD1) are used to de ne the ASCE7 elastic design response spectrum. To obtastatic force levels, the MRSA force resumust be divided by the quantity (R/I). ASC7 does not allow an engineer to scale dowMRSA force levels to ELF force levels becathe ELF procedure may result in an underprediction of response for structures withsigni cant higher-mode participation. Onthe other hand, when the MRSA base sheais less than 85% of the ELF base shear, thMRSA results must be scaled up to no lesthan 85% of the ELF values. is lower limiton the design base shear is imposed to accoufor higher mode e ects, and to ensure that thdesign forces are not underestimated througthe use of a structural model that does noaccurately represent the mass and sti nesscharacteristics of the pipe rack.

Figure 2: Typical 4-level pipe rack consisting of nine transverse frames connected blongitudinal struts.

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Drift

To obtain ampli ed seismic displacements,the displacement results calculated from theelastic analysis must be multiplied by thequantity (Cd/I e) to account for the expected

inelastic deformations when checking against the drift limits of ASCE 7 Table12.12-1. e displacement results mustbe multiplied by Cd for checking pipe ex-ibility and structure separation.It is important that the drift of pipe racks

be compared to other adjacent structures where piping and cable trays run. e piping and cable trays must be exible enough toaccommodate the movements of the piperack relative to these structures.

Seismic Detailing Requirements

e selection of a seismic force-resisting system from ASCE 7 Table 12.2-1 dictatesdetailing requirements prescribed in ASCE7 Chapter 14. Because this chapter is speci -cally excluded by the IBC, seismic detailing requirements for structural steel systems mustbe taken instead from IBC Chapter 22 and

AISC 341. e selection of a seismic force-resisting system from ASCE 7 Table 15.4-1directly dictates seismic detailing require-ments prescribed in AISC 341. AISC 341 includes such requirements for eachstructural steel system listed in the two ASCE 7tables. In general, there is a relationship betweenR values and seismic detailing requirements.Lower R values and higher earthquake designforces are accompanied by minimal seismicdetailing requirements. Higher R values andlower earthquake design forces are accompaniedby more restrictive seismic detailing require-ments to provide greater ductility. AISC 341 prescribes that beams in OMFsystems do not require lateral bracing beyondthose requirements prescribed in AISC360. However, beams in IMF and SMFsystems have progressively more restrictive

requirements for lateral bracing of beamsthat can only be met by the addition of a horizontal bracing system at each pipe level.For this reason, it may be more economicalto select an OMF system for the transversebents. AISC 341 prescr ibes that beam-to-column connections for IMF and SMFsystems be based on laboratory testing.OMF beam-to-column connections may be either calculated to match the expectedplastic moment strength of the beam orbased on laboratory testing. AISC 358prescribes speci c requirements for labo-ratory-tested systems appropriate for use inseismic moment frames. One of the systemsincluded in AISC 358 is the bolted end-plate moment connection, commonly usedin pipe rack construction (Figure 3). eseconnections are popular in industrial plantsbecause they involve no eld welding.

Redundancy in SDC A, B, or C

In accordance with ASCE 7, for all structures, = 1.0.

Redundancy in SDC D, E, or F

e typical pipe rack has no horizontal brac-ing system that serves as a diaphragm. If onindividual bent fails, there is no load path flateral force transfer to the adjacent frame

As a result, the pipe rack must be treated aa non-redundant structure.

For a transverse bent to qualify for =1.0, it must have four or more columns

and three or more bays at each level.is ensures that the loss of momentresistance at both ends of a single beamdoes not result in more than a 33% lossof story strength. Otherwise, = 1.3.

For an individual longitudinal bracedframe to qualify for = 1.0, it musthave two or more bays of chevron or X bracing (or four individual braces) at eaclevel on each frame line. is ensuresthat the loss of an individual brace orconnection does not result in more thana 33% loss of story strength nor cause an

extreme torsional irregularity (Type 1b).Otherwise, = 1.3.If the pipe rack is provided with a horizonta

bracing system that serves as a diaphragm anprovides a load path for lateral transfer, it cabe treated as a redundant structure.

For a pipe rack to qualify in thetransverse direction for = 1.0, it needsto have horizontal bracing between alltransverse bents and a minimum of fourtransverse bents. Otherwise, = 1.3.

For a pipe rack to qualify in thelongitudinal direction for = 1.0, there

needs to be a minimum of four transversbents, and each longitudinal frame lineneeds to have two or more individualbraces at each level. Otherwise, = 1.3.

Figure 3: AISC 358 extended end plate connections.

References AISC (2010a), AISC 341-10, Seismic Provisions for Structural Steel Buildings , Including

Supplement No. 1, American Institute of Steel Construction, Inc., Chicago, IL.

AISC (2010b), AISC 358-10, Prequali ed Connections for Special and Intermediate Stee Moment Frames for Seismic Applications , American Institute of Steel Construction, Inc.,Chicago, IL.

AISC (2010c), AISC 360-10, Speci cation for Structural Steel Buildings , American Instituteof Steel Construction, Inc., Chicago, IL.

ASCE (2010d), ASCE 7-10, Minimum Design Loads for Buildings and Other Structures,Including Supplement No. 1, American Society Of Civil Engineers, Reston, VA.

ASCE (2011), Guidelines for Seismic Evaluation and Design of Petrochemical Facilities, American Society Of Civil Engineers, Reston, VA.

ICC (2009), International Building Code , International Code Council, Whittier, CA.

PIP (2007), PIP STC01015, Structural Design Criteria , Process Industry Practices, Austin, TX