Foundation Design Philosophy for Vertical Vessel / Tower

In this page I will talk about Vertical vessel/Tower equipment foundation load calculation. Following are some pictures of Vertical vessel /Tower:

Process Column                           Vertical Vessel      

Picture-1 (Left) - A tall tower vessel resting on skirt and on Foundation.

Picture-2 (Right) - A small vertical vessel resting on legs and on fpundation.

Now you will follow the following steps to start the foundation load calculation and design:

Step-1 :  Review of vessel drawing (Vendor Equipment Drawing)

You need to review Vessel drawings from foundation design point of view and check whether you have all the following information:

  • Vessel Erection weight       (De1):  
  • Vessel Empty weight          (De2):    
  • Vessel Operating weight    (Do):    
  • Vessel Hydrotest weight     (Dt):     
  • Wind Shear and Moment
  • Seismic Shear and Moment (if the Project site is at Seismic zone)
  • Total Height of vessel
  • Vessel Center of Gravity location for Seismic load calculation and application
  • Anchor bolt location (Bolt circle Dia) with respect to center of vessel and also bolt offset with respect to Plant North line.
  • Anchor bolt supporting detail ( Anchor bolt size and  detail of anchor chair)
  • Base plate detail
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
  • Pipe supports and Nozzle loads on Equipment (Dp)
  • Location and size of Platforms around the vessel
  • 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 :  Description of Foundation Loads:

Please follow this section to understand the different loads on foundation:

Vessel Erection weight  (De1):  The erection weight is the fabricated weight of the vessel, plus internals, platforms, etc., that are actually erected with the vessel. Data from Equipment drawing.

Vessel Empty weight   (De2):  The empty weight is the in-place weight of the completed vessel, including the fabricated weight of the vessel, plus the weight of internals, piping, insulation, and platforms, but excluding the weight of fluids or products which will be contained in the vessel during operation. Data from Equipment drawings.    

Vessel Operating weight  (Do):    Vessel Empty weight  (De2)  +  Weight of Fluid inside the vessel. Data from Equipment drawings.

Vessel Hydrotest weight    (Dt):     Vessel Empty weight  (De2)  + Weight of test water

Pipe supports and Nozzle loads on Equipment (Dp): Please Coordinate with the Pipe Stress Group for determination of nozzle loads and loads due to pipe supports attached to the vessel.

Wind Shear and Moment: You will find this load data in vendor drawings. However, you have to calculate this load based on project design basis. During wind load calculation, you need to consider the pipes and platforms attached with the vessel. Compare both the data (vendor load data and your calculated data) and apply the critical one for foundation design.
Seismic Shear and Moment (if the Project site is at Seismic zone): You will find this load data in vendor drawings. However, you have to calculate this load based on project design basis. During seismic load calculation, you need to consider the pipes and platforms attached with the vessel. Compare both the data (vendor load data and your calculated data) and apply critical one for foundation design.

Step-4 :  Pedestal Sizing Criteria:
Concrete pedestals supporting vertical vessels shall be sized according to the following criteria:

Face-to-face pedestal size shall be the larger of the following:

(a)      Bolt circle + 175mm

(b)      Bolt circle + 8 x bolt diameters

(c)      Bolt circle + sleeve diameter + 150mm

(d)     Diameter of baseplate + 100mm

(e)     Bolt circle + 2 x (minimum bolt edge distance)


Pedestals having a diameter or least dimension across sides that is equal to or greater than 1.5m shall be octagonal in shape.  All other pedestals shall be square in shape.  For ease of forming, use multiples of 25mm for each octagonal side or side of square.

 It is desirable to make the pedestal deep enough to contain the anchor bolts and keep them out of the mat.

Step-5 :  Anchor Bolt Check:

Design of anchor bolts shall be based on the following considerations. Corrosion allowance  should be considered when required by the project design  criteria.

Tension Check:

The maximum tension force in the anchor bolts (Tmax) may be calculated according with following formula:

Tmax  =  4*M / (Nb x BCD)  -  (De1 or De2) / Nb

Where,  M = total maximum moment on foundation
              BCD = Bolt circle diameter
               Nb = no. of anchor bolt

Use De1 or De2 whichever is critical.

The above formula provides a conservative value of Tmax compared to the concrete transformed section method.

Shear Check:

When anchor bolts are utilized to resist shear, the unit shear per bolt shall be calculated as follows:


Vmax = V / Nb    where, V = total shear force on anchor bolt.

When oversized anchor bolt holes are provided in the vessel base plates or when anchor bolt sleeves that are not grout-filled are used, anchor bolts should be designed to resist tension only.


Frictional resistance to shear between the vessel base plate and the concrete or grouted bearing surface shall be utilized to resist shears induced by wind or by other static loads.  Frictional resistance shall not be employed to resist shear induced by seismic loads.  For seismic-induced shear, adequate mechanical means shall be provided to resist horizontal shear, either by means of properly detailed anchor bolt / bolt hole arrangements or through a combination of anchor bolts, shear lugs, or other anchorage devices.  The static coefficient of friction between steel and concrete or between steel and cementitious grout shall be considered as 0.4 or specified in project design criteria.

Tension Shear Interaction check:

When anchor bolts are subjected to combined shear and tension loads, the design shall be based on satisfying  interaction formula (say Appendix-d of ACI 318).

Please note that anchor bolt edge distance, spacing and load capacity shall be as per project design criteria.

Step-6 :  Load combinations for foundation sizing / Pile loads and  Foundation design:
You need to create the load combination per your project design criteria. However, I have created this load combination based on ACI 318:

Load combination for Foundation sizing and Pile load calculation (un-factored load calculation): 
  • LC1:  Do + Dp
  • LC2:  (De1 or De2) + Wind
  • LC3:  De2 + Seismic
  • LC4:  Do + Dp + Wind
  • LC5:  Do + Dp + Seismic
  • LC6:  Dt + 025*Wind
Load combination for Pedestal and Foundation design (factored load calculation): 
  • LC7:  1.4*(Do + Dp)
  • LC8:  0.75 [1.4 De2 (or 1.4 De1)] +1.6 Wind
  • LC9:  1.2 De2 +1.0 E
  • LC10:  0.75 (1.4 Do + 1.4 Dp) + 1.6 Wind
  • LC11:  1.2 (Do + Dp)  + 1.0 E
  • LC12:  0.75 (1.4 Dt)  + 1.6 (0.25 W)

The weight of the foundation and of the soil on top of the foundation shall be included as dead load in all of these load combinations.
 

Now from above steps, you have learnt the following:
  • Different types of loads on foundation
  • Different criterias  for the pedestal sizing
  • Maximum tension and shear force on each anchor bolt
  • A sample load combinations.
To complete the foundation design, your work will be to create following calculation sheets:
    • A calculation sheet for anchor bolt embedment length check (ex: ACI 318 appendix-D).
    • A calculation sheet for foundation sizing (considering soil bearing pressure, Sliding, Buoyancy and overturning) or pile load (tension, compression and shear on each pile) calculation and check with soil consultant for acceptable values.
    • A calculation sheet for foundation and pedestal reinforcement calculation per your project design criteria.
 For typical Platform and foundation for a Vertical Vessel / Tower click here  

Discussions:

Question from visitor: What is the reference of anchor bolt tension calculation formula?
Answer from Subhro:  Refer "ASCE task committee on anchor bolt design" clause #3.5.1.1 - vessel anchor bolt design.

Question from visitor: How do I calculate the wind load on vertical vessel?
Answer from Subhro:  Refer "Wind load and anchor bolts design for petrochemical facility by ASCE"


I hope this page will be very helpful to you to understand the basic foundation loads of a Vertical Vessel / Tower.

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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.