Foundation Design Philosophy for Flare Stack In this page , I will discuss about the Guyed stack Foundation only. Self standing stack is almost similar to vertical vessel / tower. Hence, for those types, you can refer vertical vessel / tower foundation page. Guyed Stack:
From the above sketch you can see the stack, Guy sets and Deadmen. Each guy set has 3 guys connected with the stack at a particular level at one end at 120 degree apart and other end connected with Deadman. This deadman is nothing but a mass concrete foundation, which is counter acting the guy tension. So now, you can easily understand that you will design two types of foundations, one for stack and another for guy support i.e. Deadmen. To start the foundation design, you need to follow the below steps: Step-1 : Review of Stack drawing (Vendor Equipment Drawing) You need to review stack drawings from foundation design point of view and check whether you have all the following information: For Stack: • •
Stack Dead weight (De) Stack underside base plate level
•
Anchor bolt location (Bolt circle Dia) with respect to center of stack and also bolt offset with respect to Plant North line.
•
Anchor bolt supporting detail ( Anchor bolt size and detail of anchor chair) and Base plate detail
•
Wind Shear and Moment
•
Seismic Shear and Moment (if the Project site is at Seismic zone)
•
Temperature load (TL)
•
Snow load (SL)
For Deadman: • •
Location of all the deadmen with respect to Stack center line Bottom level of Guy wire fixing base plate
•
Tension at Guy wire support due to wind
•
Tension at Guy wire support due to seismic
•
Tension at Guy wire support due to Temperature
•
Tension at Guy wire support due to Snow and Ice load
•
Anchor bolt detail (size, location and projection) and Guy wire fixing (detail of fixing base plate) detail with deadman.
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, Elevation and finished grade level •
Pipe supports and Nozzle loads on stack (Dp)
•
Location and size of Platforms around the Stack
•
Location of miscellaneous skid mounted equipment around the stack
•
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 : Pedestal Sizing Criteria: For Stack: Concrete pedestals supporting Stack shall be sized according to the following criteria: Face-to-face pedestal size shall be the larger of the following:
• •
Bolt circle + 175mm Bolt circle + 8 x bolt diameters
•
Bolt circle + sleeve diameter + 150mm
•
Diameter of baseplate + 200mm
•
Bolt circle + 2 x (minimum bolt edge distance)
Pedestals shall be octagonal in shape for ease of construction. For ease of forming, use multiples of 25mm for each octagonal side. For Deadman: Concrete pedestals connected with Guy wire shall be sized according to the following criteria: Face-to-face pedestal size shall be the larger of the following: • Bolt c/c distance + 175mm • Bolt c/c distance + 8 x bolt diameters •
Bolt c/c distance + sleeve diameter + 150mm
•
Baseplate size + 200mm
•
Bolt c/c distance + 2 x (minimum bolt edge distance)
Pedestals shall be rectangular in shape for ease of construction. For ease of forming, use multiples of 25mm for each rectangular side. It is desirable to make the pedestal deep enough to contain the anchor bolts and keep them out of the mat. Step-4 : 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. For stack TensionCheck: The maximum tension force in the anchor bolts (Tmax) may be calculated according with following formula: Tmax = 4*M / (Nb x BCD) - (De) / Nb Where, M = total maximum moment on foundation BCD = Bolt circle diameter Nb = no. of anchor bolt
The above formula provides a conservative value of Tmax compared to the concrete transformed section
method. ShearCheck: 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 over sized 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. For Deadman Foundation At deadman anchor location, anchor bolts shall be checked for Tension only. If concrete around the anchor bolt is not sufficient enough to resist tension in anchor bolt from ductility point of view, provide additional reinforcement around the anchor bolt to transfer the force from bolt to foundation mat by noncontact splicing method (ACI 318 appendix-D) TensionShearInteractioncheck: When anchor bolts are subjected to combined shear and tension loads, the design shall be based on satisfying interaction formula (say ACI 318 appendix-D ). Please note that anchor bolt edge distance, spacing and load capacity shall be as per project design criteria. Step-5 : Load combinations for foundation sizing / Pile loads and Foundation design: You need to create the load combination per your project design criteria.
Please note that the weight of the foundation and the soil on top of the foundation shall be included as dead load in all the load combinations. Now from above steps, you have learnt the following: • Different types of loads on Stack foundation and on deadman anchor foundation • Different criterias for the pedestal sizing •
Maximum tension and shear force on each anchor bolt
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.
I hope this page will be very helpful to you to understand the basic foundation loads of Stack and Deadman anchor. Copyright 2009. All rights reserved. Please do not print or copy of this page or any part of this 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.
