Foundation Design Philosophy for Equipment on Skid
In this page I will talk about the foundation design philosophy for Equipment on skid. These equipment are static in nature and are resting on Channel section or Wide beam section. A very simple analysis and design is required to produce a Foundation for equipment on skid. You need to follow the following steps to complete the foundation design:Step-1 : Review of Equipment Drawing (Vendor Equipment Drawing)
- Plan dimension of Equipment base frame
- Height
of Equipment
- Anchor bolt location, size and embedment depth
- Empty weight of Equipment (De)
- Operating weight of equipment (Do)
- Location of center of gravity both vertically and horizontally
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 Equipment, if any
- 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.
Please follow this section to understand the different loads on foundation:
Equipment Empty weight : The empty weight is the in-place weight of the Equipment, including the fabricated weight of the equipment, plus the weight of internals, piping and insulation, but excluding the weight of fluids or products which will be contained in the equipment during operation.
Equipment Operating weight : Equipment Empty weight (De2) + Weight of Fluid inside the Equipment
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 Equipment.
Wind Shear and Moment: Most of the time you will not find this load data in vendor drawings. You need to calculate this load based on project design basis. During wind load calculation, you need to consider the pipes and platforms attached with the equipment.
Seismic Shear and Moment (if the Project
site is at Seismic zone): Most of the time
you will not find this load data in vendor drawings. You need to
calculate this load based on project design basis.
During seismic load calculation, you need to consider the pipes and
platforms attached with the equipment.
Step-4 : Block Sizing Criteria:
Concrete foundation block supporting equipment,
shall be
sized according to the following criteria:
Face-to-face Block size shall be the larger of the following:
(a) Bolt center line distance + 200mm
(b) Bolt center line distance+ 8 x bolt diameters
(c) Bolt center line distance + sleeve diameter + 150mm
(d) Out to out dimension of skid + 100mm each side
(e) Bolt center line distance + 2 x (minimum bolt edge distance)
Face-to-face Block size shall be the larger of the following:
(a) Bolt center line distance + 200mm
(b) Bolt center line distance+ 8 x bolt diameters
(c) Bolt center line distance + sleeve diameter + 150mm
(d) Out to out dimension of skid + 100mm each side
(e) Bolt center line distance + 2 x (minimum bolt edge distance)
It is desirable to make the pedestal deep enough to contain the anchor bolts.
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 = M / (Nb x BCD) (Equipment weight) / Nb
Where, M = total maximum moment on foundation due to wind or seismic
BCD = Bolt center line distance
Nb = no. of anchor bolt
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 equipment skid 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.
The maximum tension force in the anchor bolts (Tmax) may be calculated according with following formula:
Tmax = M / (Nb x BCD) (Equipment weight) / Nb
Where, M = total maximum moment on foundation due to wind or seismic
BCD = Bolt center line distance
Nb = no. of anchor bolt
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 equipment skid 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):
Load combination for Foundation sizing and Pile load calculation (un-factored load calculation):
- LC1: Do + Dp
- LC2: (De) + Wind
- LC3: Do + Seismic
- LC4: Do + Dp + Wind
- LC5: Do + Dp + Seismic
Load
combination for Pedestal and Foundation design (factored load
calculation):
- LC6: 1.4*(Do + Dp)
- LC7: 0.75 [1.4 De] +1.6 Wind
- LC8: 1.2 Do +1.0 E
- LC9: 0.75 (1.4 Do + 1.4 Dp) + 1.6 Wind
- LC10: 1.2 (Do + Dp) + 1.0 E
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 concrete block 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 Concrete block 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 a Skid Mounted Equipment.
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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.