Nov-2011

Seismic design of structural steel pipe racks

Structural steel pipe racks are structures in petrochemical, chemical and power plants that support pipes, power cables and instrument cable trays. Occasionally, they may also support mechanical equipment, vessels and valve access platforms.

Richard M Drake, Fluor
Robert J Walter, CB&I

Article Summary

They are also referred to as pipe supports or pipeways. Main pipe racks transfer material between equipment and storage or utility areas. Storage racks found in warehouse stores are not pipe racks, even if they store lengths of piping.To allow maintenance access under the pipe rack, transverse frames (bents) are typically moment-resisting frames that support gravity loads and resist lateral loads transverse to the pipe rack. See Figure 1 for a typical transverse pipe bent. Although the bent is shown with fixed base columns, it can also be constructed with pinned base columns if the supported piping can tolerate the lateral displacement.

The transverse frames are typically connected with longitudinal struts. If diagonal bracing is added in the vertical plane, the struts and bracing act together as concentrically braced frames to resist lateral loads longitudinal to the pipe rack. See Figure 2 for an isometric view of a typical pipe rack. If the transverse frames are not connected with longitudinal struts, the pipe rack is considered to be unstrutted. The frame columns act as cantilevers to resist lateral loads longitudinal to the pipe rack.

Design criteria
In most of the US, the governing building code is the nternational Building Code (IBC)(ICC, 2009). The scope of this code applies to buildings and other structures within the governing jurisdiction. The IBC prescribes structural design criteria in Chapters 16 through 23. These design criteria adopt by reference many industry standards and specifications that have been created in accordance with rigorous American National Standards Institute (ANSI) procedures.

By reference, many loads are prescribed in ASCE 7 (ASCE, 2010d). Similarly, most structural steel material references are prescribed in AISC 360 (AISC, 2010c). Most structural steel seismic requirements are prescribed in AISC 341 (AISC, 2010a) and AISC 358 (AISC, 2010b). The latest editions of the aforementioned standards have been referenced. However, the edition of the standards used should be based on the edition of the governing building code or as approved by the authority having jurisdiction.

The IBC and its referenced industry standards and specifications primarily address buildings and other structures to a lesser extent. Design criteria for non-building structures are usually provided by industry guidelines. These guidelines interpret and supplement the building code and its referenced documents. In the case of pipe racks, additional design criteria are provided by Process Industry Practices, PIP STC01015 (PIP, 2007) and ASCE guidelines for petrochemical facilities, (ASCE, 2011). In this article, the IBC requirements govern. The aforementioned industry standards and specifications apply because they are referenced by the IBC. The PIP practices and ASCE guidelines may be used for pipe racks as they supplement the IBC and the referenced industry standards and specifications. However, the PIP practices and ASCE guidelines are not code-referenced documents.

Earthquake loads
Earthquake loads are prescribed in IBC Section 1613. This section references ASCE 7 for the determination of earthquake loads and motions. Seismic detailing of materials prescribed in ASCE 7 Chapter 14 is specifically excluded from this reference. Seismic detailing of structural steel materials is prescribed in IBC Chapter 22.

The PIP Structural Design Criteria prescribes that earthquake loads for pipe racks are determined in accordance with ASCE 7 and the following:
• Evaluate drift limits in accordance with ASCE 7 Chapter 12
• Consider pipe racks to be non-building structures in accordance with ASCE 7 Chapter 15
• Consider the recommendations of ASCE Seismic Guideline (ASCE, 2011)
• Use Occupancy Category III and an importance factor (I) of 1.25, unless specified otherwise by client criteria
• Consider an Operating Earthquake Load (Eo). This is the load considering the operating dead load (Do) as part of the seismic effective weight
• Consider an Empty Earthquake Load (Ee). This is the load considering the empty dead load (De) as part of the seismic effective weight.

Seismic design considerations
ASCE 7 Chapter 11 defines a Non-building Structure Similar To Building as a “Non-building Structure that is designed and constructed in a manner similar to buildings, that will respond to strong ground motion in a manner similar to buildings, and have basic lateral and vertical seismic force resisting systems similar to buildings.” Examples of Non-building Structures Similar To Buildings include pipe racks.

As a non-building structure, consideration of seismic effects on pipe racks should be in accordance with ASCE 7 Chapter 15, which refers to other chapters, as applicable.

Seismic system selection
Select seismic-force-resisting-system (SFRS), design parameters (R, Ωo, Cd) and height limitations from either ASCE 7 Table 12.2-1 or ASCE 7 Table 15.4-1. Use of ASCE 7 Table 15.4-1 permits selected types of non-building structures that have performed well in past earthquakes to be constructed with less restrictive height limitations in Seismic Design Categories (SDC) D, E and F than if ASCE 7 Table 12.2-1 was used. Note that ASCE 7 Table 15.4-1 includes options where seismic detailing per AISC 341 is not required for SDC D, E or F. For example, steel ordinary moment frames can be designed with R = 1 without seismic detailing per AISC 341. The AISC 341 seismic detailing requirements can also be avoided in SDC B and C for structural steel systems if R = 3 or less, excluding cantilevered column systems.

The transverse bents are usually moment-resisting frame systems and the choices are Steel Special Moment Frame (SMF), Steel Intermediate Moment Frame (IMF) and Steel Ordinary Moment Frame (OMF).

In the longitudinal direction, if braced frames are present, the choices are usually Steel Special Concentrically Braced Frame (SCBF) and Steel Ordinary Concentrically Braced Frame (OCBF), although there is nothing to preclude choosing Steel Eccentrically Braced Frames (EBF) or Buckling-Restrained Braced Frame (BRBF). If braced frames are not present, the choices in the longitudinal direction are one of the Cantilevered Column Systems.