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Storage Tank Foundation Design

The scope of this calculation is to design a storage tank foundation with no internal pressure. The sub-structure is designed as a ring foundation with or without footing where, ring beam is designed for hoop tension force created due to soil and surcharge pressures and the torsional moment due to the various load components. The footing is designed to resist the net moment induced due to the tank loads and base pressure.

Standards: British.

Features

  • The module gives the user ample amount of choices so as to allow them to mould the structure similar to that of actual site conditions such as,
  • Foundation type
  • Ring beam only
  • Ring beam with footing
  • Load data
  • By vendor (user input)
  • By program (load is computed by the application based on tank details)
  • Both (takes the maximum load effect from the above inputs)
  • Load combinations - Includes tank empty condition, operating load case, test load case, wind and seismic considerations for different predefined load combinations (SLS and ULS).
  • Seismic load
  • Total seismic load
  • Individual seismic components
  • In this option the module considers the effect due to individual seismic components such as sloshing contents load are taken into consideration for design purposes.
  • Wind load - Wind Load calculation using program involves an effective way of wind pressure calculation based on external pressure coefficients for circular surfaces.
  • Stability factors (sliding and overturning) and reinforcement adequacy check is carried out for the predefined load combinations.
  • The module also computes the tension and moment capacity of the ring beam to resist the hoop tension force and equivalent moment (due to torsion) respectively.
  • Additionally, the calculation checks if the anchor bolt area is sufficient to resist the net uplift force.

Design Considerations

  • The effect of buoyancy load is considered based on the water table position.
  • For by program option, the wind & seismic load calculations are done based on respective standards applicable.
  • The calculation module includes wind and seismic load considerations, giving a choice to the user to input the loads directly or to give the general site conditions such as terrain, wind speed, seismic zone, site location etc. And the program gives an accurate load output for the given conditions (subjected to the code provisions).
  • The seismic load for empty case is calculated based on the ratio of empty weight to operating weight when total seismic load is given as input, in-case of individual components input the sum of shell and roof seismic components is taken.
  • For sliding check, the loads acting interior to the footing are also considered in the resisting force.
  • The calculation is further proceeded only if the factor of safety for overturning check is > 1.
  • For anchor bolt calculations the allowable design stress is taken as follows,
  • Operating condition - 0.45 * yield stress of bolt
  • Test condition - 0.6 * yield stress of bolt
  • Wind condition - 0.8 * yield stress of bolt
  • Seismic condition - 0.8 * yield stress of bolt
  • Seismic + operating condition - 0.8 * yield stress of bolt
  • Wind + operating condition - 0.6 * yield stress of bolt
  • Wind + test condition - 0.6 * yield stress of bolt
  • Where, yield stress of bolt is 640 N/mm2 for bolt grade 8.8 and 240 N/mm2 for bolt grade 4.6
  • For hoop tension calculations the effect of seismic sloshing pressure is taken into consideration in applicable cases.
  • While calculating torsional moment due to various load components, the centroid of rectangular section (i.e., ring beam portion up to footing thickness) alone is considered as opposed to the entire tee section.

National Standards Available

British Standard

References

  • BS 8110-1:1997 -Structural Use of Concrete - Part 1: Code of practice for design and construction.
  • BS 6399 : 1997 - Loading for Buildings - Part 2 : Code of Practice for Wind Loads.
  • API 650 - Welded Steel Tanks for Oil Storage.

Revision

  • Ver 1.0 - Original version