7/29/2019 Seismic Design of Structural Steel Pipe Racks - Structure Magazine Feb 2012

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performance issues relativeto extreme events

StructuralPerformance

Richard M. Drake, P.E., S.E.,

SECB is a Senior Fellow/StructuralEngineering at Fluor in Aliso Viejo,California. He can be contacted atRick.Drake@fuor.com.

Robert J. Walter, P.E., S.E. is aPrincipal Civil/Structural Engineer atCB&I in Plaineld, Illinois. He canbe contacted atwalter@cbi.com.

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

Seismic Design of StructuralSteel 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-resistingframes (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. Teframe columns mustact as cantilevers to

resist lateral loads longitudinal to the pipe rack.

Design Criteria

In 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 manyindustry standards and specications that havebeen created in accordance with rigorousAmericanNational 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. Te edition of each standard thatis used should be based on the edition of the gov-erning building code or as otherwise approved bythe 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 ofPetrochemical Facilities. In this article, the IBCrequirements govern; the PIP practices and ASCEguidelines may be used for pipe racks, as theysupplement the IBC and the referenced industrystandards and specications, but they are notcode-referenced documents themselves.

Earthquake Loads

Earthquake loads are prescribed in IBC section1613, which references ASCE 7. Seismic detailing

of materials as prescribed in ASCE 7 Chapter 14is specically excluded; instead, seismic detailingof structural steel materials is prescribed in IBCChapter 22.PIP STC01015 recommends that earthquake

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

Evaluate drift limits in accordance withASCE 7 Chapter 12.

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

Consider the recommendations of the

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

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

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

Seismic Design Considerations

ASCE 7 Chapter 11 denes a nonbuilding struc-ture similar to a building as: A nonbuildingstructure that is designed and constructed in amanner similar to buildings, will respond to strongground motion in a fashion similar to buildings,and has a basic lateral and vertical seismic force-resisting system conforming to one of the typesindicated. Examples include pipe racks.As a nonbuilding structure, consideration of seis-mic eects on pipe racks should be in accordance

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

Figure 1: Typical transverse bent.

Tis article is condensed from apaper published by the authors

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

reprinted with permission.

Te online version of thisarticle contains detailedreferences. Please visit

www.STRUCTUREmag.org.

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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 be

constructed with less restrictive height limi-tations in Seismic Design Categories (SDC)D, E, and F than those specied 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 ordinaryconcentrically braced frame. Less commonchoices are eccentrically braced frame orbuckling-restrained braced frame. If bracingis 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 specicheight limits for pipe racks detailed withspecic 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. Steel

ordinary 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 determined from

ASCE 7 Chapter 12 equations is not applicablefor nonbuilding structures, including pipe racks,because they do not have the same mass andstiness distributions assumed for buildings. Itis acceptable to use any analysis approach thataccurately includes the mass and stiness ofthe structure, including nite element modelsand the Rayleigh method. e determinationof the pipe rack period can be aected by thestiness of the piping leaving the pipe rack.

When this stiness 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 species when adynamic analysis is required. e philosophyunderlying this section is that dynamicanalysis is always acceptable for design.

A dynamic analysis procedure is requiredfor a pipe rack if it is assigned to SDC D,E, or F and it either:

has T 3.5Ts;

exhibits horizontal irregularitytype 1a or 1b; or

exhibits vertical irregularity type1a, 1b, 2, or 3.

e most common dynamic procedureused for pipe racks is modal responsespectrum analysis. e equivalent lateralforce (ELF) procedure is allowed for apipe rack structure if a dynamic analysisprocedure is not required. e simpliedalternative structural design criteria for

simple bearing wall or building frame systemsare not appropriate and should not be used

for pipe racks.

Equivalent Lateral Force Procedure

e ELF procedure involves calculating theeective earthquake loads in terms of a staticbase shear that is dependent on the imposedground acceleration and the structures mass(eective 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 this

vertical distribution of forces, seismic designloads in individual members and connectionscan be determined.ASCE 7 determines design earthquake forceson a strength basis, allowing direct compari-son with the design strength of individualstructural members.

The easiest to use software for calculatingwind, seismic, snow and other loadings forIBC, ASCE7, and all state codes based onthese codes ($195.00).

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Concrete beams with torsion ($45.00).

Demos at: www.struware.com

Figure 2: Typical 4-level pipe rack consisting of nine transverseframes connected by longitudinal struts.

continued on next page

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Modal Response Spectrum Analysis

MRSA is acceptable or the analysis o piperacks, and may be required i certain plan and/or vertical irregularities are identifed. Te basiso MRSA is that the pipe racks mass (eective

seismic weight) and stiness are careully mod-eled, allowing the dynamic analysis o multiplevibration modes that result in an accurate dis-tribution o the base shear orces throughoutthe structure. Te MRSA must include a su-fcient number o modes in order to achieve aminimum o 90% mass participation.wo MRSA runs are typically required or

pipe racks. Te frst run includes the operatingdead load (Do) as the seismic eective weightto determine the operating earthquake load(Eo). Te second run includes the empty deadload (De) as the seismic eective weight to

determine the empty earthquake load (Ee).Te MRSA input ground motion param-eters (SDS, SD1) are used to defne the ASCE7 elastic design response spectrum. o obtainstatic orce levels, the MRSA orce resultsmust be divided by the quantity (R/I). ASCE7 does not allow an engineer to scale downMRSA orce levels to ELF orce levels becausethe ELF procedure may result in an under-prediction o response or structures withsignifcant higher-mode participation. Onthe other hand, when the MRSA base shearis less than 85% o the ELF base shear, theMRSA results must be scaled up to no lessthan 85% o the ELF values. Tis lower limiton the design base shear is imposed to accountor higher mode eects, and to ensure that thedesign orces are not underestimated throughthe use o a structural model that does notaccurately represent the mass and stinesscharacteristics o the pipe rack.

Drift

o obtain amplifed seismic displacements,the displacement results calculated rom the

elastic analysis must be multiplied by thequantity (Cd/Ie) to account or the expectedinelastic deormations when checkingagainst the drit limits o ASCE 7 able12.12-1. Te displacement results mustbe multiplied by Cd or checking pipe ex-

ibility and structure separation.It is important that the drit o pipe racks

be compared to other adjacent structureswhere piping and cable trays run. Te pipingand cable trays must be exible enough toaccommodate the movements o the piperack relative to these structures.

Seismic Detailing Requirements

Te selection o a seismic orce-resistingsystem rom ASCE 7 able 12.2-1 dictatesdetailing requirements prescribed in ASCE7 Chapter 14. Because this chapter is specif-

cally excluded by the IBC, seismic detailingrequirements or structural steel systems mustbe taken instead rom IBC Chapter 22 and

AISC 341. Te selection o a seismic orce-resisting system rom ASCE 7 able 15.4-1directly dictates seismic detailing require-ments prescribed in AISC 341.AISC 341 includes such requirements or 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 designorces are accompanied by minimal seismic

detailing requirements. Higher R values andlower earthquake design orces 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 AISC 360.However, beams in IMF and SMF systemshave progressively more restrictive require-ments or lateral bracing o beams that canonly be met by the addition o a horizontalbracing system at each pipe level. For this

reason, it may be more economical to selectan OMF system or the transverse bents.AISC 341 prescribes that beam-to-column connections or IMF and SMFsystems be based on laboratory testing.OMF beam-to-column connections maybe either calculated to match the expectedplastic moment strength o the beam orbased on laboratory testing. AISC 358prescribes specifc requirements or labo-

ratory-tested systems appropriate or use inseismic moment rames. One o the systemsincluded in AISC 358 is the bolted end-plate moment connection, commonly usedin pipe rack construction (Figure 3). Teseconnections are popular in industrial plantsbecause they involve no feld welding.

Redundancy in SDC A, B, or C

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

Redundancy in SDC D, E, or F

Te typical pipe rack has no horizontal brac-ing system that serves as a diaphragm. I oneindividual bent ails, there is no load path orlateral orce transer to the adjacent rame.

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

For a transverse bent to qualiy or =1.0, it must have our or more columnsand three or more bays at each level.Tis ensures that the loss o momentresistance at both ends o a single beamdoes not result in more than a 33% losso story strength. Otherwise, = 1.3.

For an individual longitudinal bracedrame to qualiy or = 1.0, it musthave two or more bays o chevron or Xbracing (or our individual braces) at eachlevel on each rame line. Tis ensuresthat the loss o an individual brace orconnection does not result in more thana 33% loss o story strength nor cause anextreme torsional irregularity (ype 1b).Otherwise, = 1.3.

I the pipe rack is provided with a horizontalbracing system that serves as a diaphragm andprovides a load path or lateral transer, it can

be treated as a redundant structure. For a pipe rack to qualiy in the

transverse direction or = 1.0, it needsto have horizontal bracing between alltransverse bents and a minimum o ourtransverse bents. Otherwise, = 1.3.

For a pipe rack to qualiy in thelongitudinal direction or = 1.0, thereneeds to be a minimum o our transversebents, and each longitudinal rame lineneeds to have two or more individualbraces at each level. Otherwise, = 1.3.

Figure 3: AISC 358 extended end plate connections.