A Short Article on Structural Steel Pipe Rack Design Loads
Pipe racks are structures in petrochemical, chemical and power plants that are designed to support pipes, power cables and instrument cable trays. They may also be used to support mechanical equipment, vessels and valve access platforms. Pipe racks are non-building structures that have similarities to structural steel buildings. The design requirements found in the building codes are not clear on how they are to be applied to pipe racks. Several industry references exist to help the designer apply the intent of the code and follow expected engineering practices.
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, 2006). Similarly, most structural steel material references are prescribed in AISC 360 (AISC, 2005b). Most structural steel seismic requirements are prescribed in AISC 341 (AISC, 2005a) and AISC 358 (AISC, 2006, 2009). 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, PIPSTC01015 (PIP, 2007) and ASCE guidelines for petrochemical facilities (ASCE, 1997a, 1997b). 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 because they supplement the IBC and the referenced industry standards and specifications. However, the PIP practices and ASCE guidelines are not code referenced documents.
Dead Loads (D)
The PIP Structural Design Criteria prescribes specific dead loads for pipe racks. Pipe racks and their foundations should be designed to support these loads applied on all available rack space, unless other criteria is provided by the client.
• Structure dead load (Ds): The weight of materials forming the structure and all permanently attached appurtenances. This includes the weight of fire protection material, but does not include the weight of piping, cable trays, process equipment and vessels.
• Operating dead load (Do): The operating dead load is the weight of piping, piping insulation, cable tray, process equipment and vessels plus their contents (fluid load). The piping and cable tray loads may be based on actual loads or approximated by using uniform loads. The PIP Structural Design Criteria recommends a uniformly distributed load of 40 psf for pipe, which is equivalent to 8-in.-diameter schedule 40 pipes filled with water at 15-in. spacing. Other uniform loads may be used based on client requirements and engineering judgment. For cable tray levels, a uniform distributed load of 20 psf for a single level of cable trays and 40 psf for a double level of cable trays may be used unless actual loading is greater.
• Empty dead load (De): The empty weight of piping, piping insulation, cable tray, process equipment and vessels. When using approximate uniform loads, 60% of the operating dead load for piping levels is typically used. Engineering judgment should be used for cable tray levels.
• Test dead load (Dt): The empty weight of the pipes plus the weight of the test medium.
Live Loads (L)
Live loads are prescribed in IBC Section 1607, with no reference to ASCE 7 or any industry standard or specifi cation. The minimum live loads applied to platforms and stairs that are part of the pipe rack structure shall meet the minimum loads per IBC Table 1607.1:
• Stairs: Per item 35, “stairs and exits—all others” shall be designed for a 100-psf uniform load or a 300-lb point load over an area of 4 in.2 , whichever produces the greater load effects.
• Platforms: Per item 39, “Walkways and elevated platforms” shall be designed for 60-psf uniform load. The PIP Structural Design Criteria also prescribes specifi c live loads which may be applicable to platforms and stairs that are part of the pipe racks. These loads are higher than required by the IBC Building Code:
• Stairs: Design for separate 100-psf uniform load and 1,000-lb concentrated load.
• Platforms: Design for separate 75-psf uniform load and 1,000-lb concentrated load assumed to be uniformly distributed over an area 22 ft by 22 ft.
Thermal Loads (T)
The PIP Structural Design Criteria prescribes specific thermal loads for pipe racks:
• Thermal forces (T): The self-straining thermal forces caused by the restrained expansion of the pipe rack structural members.
• Pipe anchor and guide forces (Af): Pipe anchors and guides restrain the pipe from moving in one or more directions and cause expansion movement to occur at desired locations in a piping system. Anchor and guide loads are determined from a stress analysis of an individual pipe. Beams, struts, columns, braced anchor frames and foundations must be designed to resist actual pipe anchor and guide loads.
• Pipe friction forces (Ff): These are friction forces on the pipe rack structural members caused by the sliding of pipes in response to thermal expansion due to the design (operating) temperature of the pipe. For friction loads on individual structural members, use the larger of 10% of the total piping weight or 40% of the weight of the largest pipe undergoing thermal movement: 10% of the total piping weight assumes that the thermal movements on the individual pipes do not occur simultaneously; 40% of the largest pipe weight assumes steel-on-steel friction.
Earthquake Loads (E)
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 Guidelines for Seismic Evaluation and Design of Petrochemical Facilities (ASCE, 1997a).
• 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.
Wind Loads (W)
Wind loads are prescribed in IBC Section 1609. This section references ASCE 7 as an acceptable alternative to the IBC requirements. Most design practitioners use the ASCE 7 wind load requirements. The PIP Structural Design Criteria prescribes that wind loads for pipe racks are determined in accordance with ASCE 7 and the following:
• Wind drift with the full wind load should not exceed the pipe rack height divided by 100.
• Consider partial wind load (Wp). This is the wind load determined in accordance with ASCE 7 based on a wind speed of 68 mph. This wind load should be used in load combination with structure dead loads (Ds) and test dead loads (Dt). The ASCE Wind Guideline (ASCE, 1997b) recommends that wind loads for pipe racks are determined in accordance with ASCE 7 and the following:
• Calculate wind on the pipe rack structure, neglecting any shielding. Use a force coefficient of Cf = 1.8 on structural members, or alternatively use Cf = 2.0 below the first level and Cf = 1.6 above the first level.
• Calculate transverse wind on each pipe level. The tributary height for each pipe level should be taken as the pipe diameter (including insulation) plus 10% of the pipe rack transverse width. The tributary area is the tributary height times the tributary length of the pipes. Use a minimum force coefficient of Cf = 0.7 on pipes.
• Calculate transverse wind on each cable tray level. The tributary height for each pipe level should be taken as the largest tray height plus 10% of the pipe rack transverse width. The tributary area is the tributary height times the tributary length of the cable tray. Use a minimum force coefficient of Cf = 2.0 on cable trays.
Rain Loads (R)
Rain loads are prescribed in IBC Section 1611. The IBC requirements are intended for roofs that can accumulate rain water. Pipe rack structural members, piping and cable trays do not accumulate rain water. Unless the pipe rack supports equipment that can accumulate rain water, rain loads need not be considered.
Snow Loads (S)
Snow loads are prescribed in IBC Section 1608. This section references ASCE 7 for the determination of snow loads. The IBC provisions are intended for determining snow loads on roofs. Typically, pipe racks are much different than building roofs, and the fl at areas of a pipe rack where snow can accumulate vary. Thus, engineering judgment must be used when applying snow loads. The fl at-roof snow load could be used for determining the snow load on a pipe rack. The area to apply the snow load depends on what is in the pipe rack and how close the items are to each other. For example, if the pipe rack contains cable trays with covers, the area could be based on the solidity in the plan view. If the pipe rack only contains pipe with large spacing, the area would be small because only small amounts of snow will accumulate on pipe. By using this approach, combinations with snow load usually do not govern the design except in areas of heavy snow loading. In areas of heavy snow loading, the client may provide snow load requirements based on their experience.
Ice Loads (Di)
Atmospheric ice loading is not a requirement of the IBC code. However, atmospheric ice load provisions are provided in ASCE 7, Chapter 10. It is recommended that ice loading be investigated to determine if it may influence the design of the pipe rack.
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