Foundation Design for Elevated, Cryogenic, Field-Erected, Flat-Bottom Storage Tanks

1.         PURPOSE

 

1.1       This global engineering standard defines the philosophy and characteristics that are to be followed in the engineering and design of elevated, cryogenic, field-erected, flat-bottom, storage-tank foundations.

 

 

2.         scope

 

2.1       This standard applies to the design of piling and foundations for elevated, cryogenic, field-erected, flat-bottom storage tanks. Elevated tanks are defined as those that are arranged to locate the inner-tank withdrawal-nozzle penetration through the bottom of the tank. In most applications, this distance will be at least 4 m (13 ft) above the centerline of the pump‑suction flange.

 

2.2       Foundations for low-pressure, flat-bottom tanks that do not follow the elevated criteria discussed in this document shall be designed in accordance with the requirements specified in 3CS02004.

 

 

3.         related documents

 

3.1       Air Products Engineering Documents

 

3CS02004        Frost Heaving Prevention of Cryogenic Equipment Foundations

3EQ15001        Flat-Bottom, Cryogenic Storage Tanks

4WEQ-1516      Field-Erected, Flat-Bottom, LOX and LIN Storage Tanks

 

3.2       International Code Council (ICC)

 

International Building Code – Latest Edition (unless project dictates otherwise)

 

 

4.         PHILOSOPHY

 

4.1       Typically, the foundation design for elevated, low-pressure, flat-bottom tanks for cryogenic-liquid storage will consist of one of the following systems. The first method is to have a tank-foundation slab that is supported directly on augured or driven piles that project above finished-grade elevations as required. The second method is to have a soil-supported base mat at grade with formed-concrete columns that in turn support an elevated-foundation slab. If the soil profile and subsurface material properties permit the use of either augured or driven piles or drilled piers, this method will typically be the most cost effective.

 

4.2       In areas of high wind and/or low to moderate seismic activity, the cost impact and effect to the foundation design for this arrangement is usually not significant. For pile-supported foundations the cost difference is primarily from the additional pile length and the forming required for the elevated slab. Industry-standard foundation design practices and details shall be followed. In areas of high-seismic activity, the foundation cost is increased in a number of areas. Special consideration to the items listed in paragraph 5.2 shall be followed.

 

4.3       It is critical to perform a site-specific geotechnical investigation. The report that is prepared from the site investigation and subsequent laboratory testing of the material must provide foundation and piling recommendations specific to the properties, configuration, and arrangement of the tank(s). Vertical and lateral load capacities for piles must be established. The load capacity of the pile is affected by the fixity of the pile head to the foundation. The recommendations from the geotechnical engineer must provide information for a variety of possible end-fixity conditions. Lateral capacities shall also be based on a maximum allowable lateral displacement of 6 mm (1/4 in) at the pile head, which will be approximately 4 m (13 ft) above finished grade.

 

5.         Foundation concerns

 

5.1       General

 

5.1.1   For many soil profiles the appropriate foundation support will require the use of some type of augured or driven pile. The design of the pile shall account not only for the vertical loading condition but the lateral loads imposed from wind and earthquakes. Displacements at the top of the pile under design-load conditions shall be limited to 6 mm (1/4 in).

 

5.1.2   Piping penetrations from the tank through the supporting foundation, as well as valve boxes and interconnecting piping that might be located beneath the tank foundation mat, must be considered in the placement of any piles or piers that are required for the support of the tank-foundation mat. Typically, piping entering or exiting the tank through the foundation is grouped in one or two areas and passes through thermal sleeves that are provided with the tank and cast in the foundation. Significant information on typical details and requirements of these storage tanks is available in 4WEQ-1516.

 

5.1.3   Embedded plates, cast directly in the foundation, shall be used to the greatest extent possible for the support of any auxiliary equipment and piping that is to be supported from the foundation. Field-drilled anchors shall be avoided, particularly on the top and bottom of the foundation, because of concern that the installation drilling might cause damage to the primary structural reinforcing steel.

 

5.1.4   If a finite element analysis is performed on the pile cap, care shall be used when selecting the design elements from the analysis program element library. The dimensional grid established for the model and the mat thickness might require the use of a plane-strain-type element instead of a plate-bending-type element. The engineer must understand the formulation and limitations of the finite element selected such that the element(s) selected for the analysis provide the most accurate results.

 

5.1.5   Additional reinforcing steel shall be provided in the foundation around all piping penetrations to ensure the continuity of the primary reinforcing steel.

 

5.1.6   Typically, sloshing effects of the tank contents are accounted for in the codes used for the design of the tank. Foundation loads provided by the tank supplier should include these effects. Specific information concerning the design of the tank can be found in 3EQ15001 and 4WEQ-1516.

 

5.2       High-Seismic Areas

 

5.2.1   When the foundation type is the style where the elevated mat is supported directly by concrete piles that extend above finished grade without a supplemental pile cap at grade, care shall be taken to utilize a fairly symmetrical piling arrangement. In most applications, a static analysis is typically adequate. There may be applicable special provisions of the design codes, in these high seismic areas, such as special concrete reinforcing detailing requirements that may be required.

 

5.2.2   In high-seismic regions it is important to consider the liquefaction potential of the soil when designing the tank foundation, piling, auxiliary equipment, and piping. Liquefaction is a phenomenon whereby saturated granular materials lose their inherent shear strength as a result of increased pore-water pressures that might be induced by cyclic loading such as that caused by an earthquake. Granular materials of a low relative density, shallow ground water, and the long duration/high acceleration of seismic shaking are some of the factors that can cause soils to liquefy. When the potential for soil liquefaction is a concern, the piping and close-coupled equipment shall be supported from the tank-foundation piling or the pile cap.

 

5.2.3   When the tank is supported on an elevated slab which is in turn supported directly on drilled piers that extend from grade to the bottom of the elevated slab without a supplemental pile cap at grade, the circular cross section of the pier shall be maintained for the entire length of the shaft. A change in cross section from round below grade to square above grade for ease of construction is not acceptable. Structural failures in high‑seismic events have shown that changes in column stiffness can create problems and shall be avoided.

 

5.2.4   Spiral-reinforcing ties shall be used in all columns. Spacing of the ties shall be based of the appropriate design codes. Spiral ties are used to improve the confinement of the primary column reinforcing in a seismic event.

 

5.2.5   If a fixed-end condition is assumed in the drilled-pier design at the pier/foundation mat interface, care shall be taken to ensure the reinforcing steel bars in the pier have sufficient length to be fully developed in the foundation mat.

 

 


 

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