Design of Concrete Ring Type Foundation for Storage Tank

Design of Concrete Ring Beam for Storage Tank http://www.civildesignhelp.info/tf.html In this page I will talk about the design philosophy of ring beam for storage water tank. The granular fill foundation of the tank shall be designed per project design criteria / specification. This type of foundation is mostly common to all project site. However, sometimes we design concrete ring beam around the tank foundation. Following are some reasons for design of concrete ring beam, though this is more costly and take longer to construct than granular fill ring:    

Sometimes clients ask to provide concrete ring beam around tank foundation. Prevent uplift of the tank due to wind or earthquake Prevent edge failure of the soil at the tank shell Prevent local uplift of the tank due to internal pressure.

Now you will follow the following steps to start the foundation load calculation and design: Step-1 : Review of Tank detail drawing (Vendor Drawing) You need to review tank drawings from foundation design point of view and check whether you have all the following information:           

Tank Dimension, Diameter and Height Type of Roof (Floating or fixed roof), weight of roof Detail of tank shell and weight of tank shell Detail of tank base plate, location of base sump, annular plate and total weight of base plate Detail of anchor bolt (BCD, no of bolt and dia of bolt) and anchor bolt fixing detail Location and detail of man-hole at bottom portion of tank Product density and and maximum height of product Maximum height of water inside the tank for the hydrotest* Internal pressure or suction Live load Wind Shear and moment on tank shell Seismic shear and moment on tank shell

Step-2 : Verification of foundation location, elevation and external fittings loads You need to review Plot plan, Equipment location drawings and 3 -D Models and check whether you have all the following information:  

Verify the area available for foundation. Verify Foundation location and Elevation

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Pipe supports and Nozzle loads on tank (Dp) Location and size of Platforms around the tank Locations of underground pipes Electrical and Instrument duct banks Locations and extent of adjacent foundations Verify the location and extent of new/existing foundations not shown in 3D model or plot plan.

Step-3 : Loads on concrete ring beam and on the confined compacted granular fill inside the ring: You need to place concrete ring beam in such a way that outer surface of the tank shell should be the center of ring beam. Consider the following loads on ring beam and on granular compacted fill inside the concrete ring. Geotechnical Data: Before starting the design, you need to collect the following information about soil:   

Allowable Bearing Pressure Density of Soil Co-efficient of earth pressure at rest (Ko)

Loads on Ring beam:       

Total weight of tank shell (vertical load), kN / m (DL) Total weight of roof , for fixed roof case. For floating roof, part of the roof weight will come on the ring beam, kN / m (DL) Total live load on roof , for fixed roof case. For floating roof, part of the live load will come on the ring beam, kN / m (LL) Part of annular base weight on ring beam, kN / m 2 (DL) Part of product / test water load on ring beam, kN / m2 (PL) Seismic shear and wind shear on ring beam, kN / m Part of internal pressure / suction load on ring beam, kN / m 2(IP)

Loads on compacted granular fill inside the ring beam:     

Floating roof weight on compacted granular fill, kN / m 2(DL) Annular base weight on compacted granular fill, kN / m 2 (DL) Live load on floated roof, kN / m2 (LL) Product / test water load on compacted granular fill, kN / m2(PL) Internal pressure / suction load on compacted granular fill, kN / m 2 (IP)

Following load combimations can be used for soil bearing pressure check (at bottom of ring beam level) :

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Load Combination: LC1 - Self weight of soil / Beam + Self weight of tank + Product weight + Internal pressure Load Combination: LC2 - Self weight of soil / Beam + Self weight of tank + Product weight + Internal Pressure + Live Load Load Combination: LC3 - Self weight of soil / Beam + Self weight of tank + Product weight + Internal pressure + Wind Load Load Combination: LC4 - Self weight of soil / Beam + Self weight of tank + Product weight + Internal pressure + Seismic Load Load Combination: LC5 - Self weight of soil / Beam + Self weight of tank + Product weight + Internal pressure + Live Load + Wind Load Load Combination: LC6 - Self weight of soil / Beam + Self weight of tank + Product weight + Internal pressure + Live Load + Seismic Load Load Combination: LC7 - Self weight of soil / Beam + Self weight of tank + Test water weight

Load combinations for Ring beam design for Hoop tension:  

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Load Combination: UC1 - 1.7 x (Surcharge load of confined soil) + 1.7 x Surcharge load of (Self weight of tank + Product weight + Internal pressure) Load Combination: UC2 - 1.7 x (Surcharge load of confined soil) + 1.7 x Surcharge load of (Self weight of tank + Product weight + Internal pressure)+ 1.4 x surcharge of Live Load Load Combination: UC3 - 1.7 x (Surcharge load of confined soil) + 1.7 x Surcharge load of (Self weight of tank + test water weight)

Step-4 : Determination of concrete ring beam size: The ring wall should be a minimum 300 mm thick and extend to a suitable bearing stratum, whch may be natural ground or built-up compacted granular material. It should be 500 mm below ground level and extend below frost line. The bearing capacity of the soil below the ringwall should be calculated using a strip foundation analysis loaded with vertical load as mentioned in step-3. API 650, appendix-B, clause B.4.2.2 states that it is desireable that the ringwall width be such that the average unit soil loading under the ring wall will be approximately equal to the earth pressure under the confined earth at the same depth (in maximum liquid level condition). Once, the ringwall thickness is determined from above condition, it should be reviewed to ensure that excessive quantities of concrete are not used for tanks with low liquid levels and that the permissible ground pressure for the width of the wall is not exceeded. Please note that, soil bearing pressure under the ring beam and under the confined earth at same depth should not exceed the allowable soil bearing pressure for any of the above described load and any load combinations.

Step-5 : Determination of Hoop Tension on concrete ring beam and reinforcement calculation: The concrete ring beam shall be designed for hoop tension. This hoop tension will be generated from surcharge load due to confined soil and loads on confined soil. Load calculation: Surcharge due to confined soil: Sursoil = 0.5 x (height of ringwall)2 x soil density x Coefficient of earth pressure at rest (Ko) Surcharge due to uniform load on confined soil: Surudl = (Load on confined soil) x (height of ringwall) x Co-efficient of earth pressure at rest (Ko) Total Hoop tension (T) = (Sursoil + Surudl) x (0.5 x centerline diameter of ring beam) Factored Hoop Tension load can be calculated as per step-3. Required area of Hoop reinforcement is, Ast = (Factored Hoop tension) / (0.9 x yeild stress of rebar---fy) The ringwall must also be designed to take care circumferential bending moments due to the vertical load being applied eccentrically to the ringwall center line. The ringwall should be reinforced on both faces, with vertical reinforcement (stirrups) closest to the concrete surfaces. Not more than 50% of the hoop reinforcement should be lapped at any one position. Step-6 : Anchor Bolt Design: Anchor bolt shall be checked per design criteria and Tenssion & Shear load supplied by vendor. If wind and shear forces are not supllied by vendor, you need to calculate the anchorage load from API 650. Anchor bolt shall be designed for ductility failure. If required, additional reinforcement to be provided around the anchor bolt. For a typical concrete ring beam detail click here.

I hope this page will be very helpful to you to understand the basic design of a concrete ring beam for storage tank.

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page without written permission from Subhro Roy. Disclaimer: This page is prepared based on experience on Civil Engineering Design. All definitions and most of the explanations are taken from different text books and international design codes, which are referenced in the contents. Any similarity of the content or part of with any company document is simply a coincidence. Subhro Roy is not responsible for that.