Civil Design Help

In this page I will talk about how to detemine the size of oil containment for transformer. Following is a typical picture of a transformer and its foundation with oil containment.

Now, you will follow the below steps to determine the foundation and size of spilled oil containment.

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

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

Transformer Erection weight (D_e)
Transformer Operating weight (D_o)
Plan dimension of Transformer base
Height of transformer and location of oil tank
Total volume of oil in the oil tank
Transformer Center of Gravity location in empty condition and operating condition for Seismic load calculation and application
Anchor bolt detail (size, location, projection, etc..) and transformer supporting details

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 and containment.
Verify transformer Foundation and containment location and Elevation
Electrical and Instrument duct banks Bus duct support and foundation detail, on and around the transformer pit
Locations of underground pipes
Location of fire hose and sprinkler around the transformer
Locations and extent of fire wall and construction type of fire wall
Verify the location and extent of new/existing foundations not shown in 3D model or plot plan.

Step-3 : Soil / Geotechnical information:

Following Geotechnical information are required to start the foundation and spilled oil containment:

Soil allowable Bearing pressure or pile capacity (Tension, compression and Lateral force capacity)
Soil density
Active soil pressure co-efficient of soil
Earthquake soil pressure co-efficient
Ground water table location
Frost depth (for winter snow)

Step-4 : Transformer Pedestal sizing criteria:

Transformer pedestal shall be sized according to the following criteria:

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

(a) Bolt c/c distance + 175mm

(b) Bolt c/c distance + 8 x bolt diameters

(c) Bolt c/c distance + sleeve diameter + 150mm

(d) Size of base frame + 200mm

(e) Bolt c/c distance + 2 x (minimum bolt edge distance)

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

Step-5 : Transformer spilled oil containment sizing criteria:

Containment size shall be calculated for worst condition. It is assumed that worst condition will be happened when total oil is in the containment + Transformer on fire + Heavy rain fall. So, total containment volume will be, addition of following items:

Volume of transformer oil (mentioned in the equipment drawing)
Transformer on fire: When transformer is on fire (refer IEEE-980 annex-B or NFPA-850 chapter-6 ) all the hose pipe (deluge system) will spray the water on all four sides and top of the transformer. So total volume of water will be: Water volume = (Total surface area of the transformer (all 4 sides) + top plan area of transformer) xrate of water flow from hose pipe per unit area x total fire rating time.
Rain water: Total volume of rain water shall be calculated for total fire time. So volume of rain water = Rain fall intensity (mm/hr) x Plan area of containment x total fire rating time.

Generally, you will find that containment area is full of stones (40 mm down). In this case, we consider that 35% void is available to accommodate the above volume of oil and water mix. So, you need to increase the capacity of the containment accordingly.

Step-6 : 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 (T_max) may be calculated according with following formula:

T_max = M / (Ny x BCD) - (D_e/ D_o) / N_b

Where, M = total maximum moment on foundation
BCD = Bolt c/c distance
Ny = No. of bolt row
N_b = no. of anchor bolt

Use D_e or D_o whichever is critical.

Shear Check:

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

V_max = V / N_b where, V = total shear force on anchor bolt.

Frictional resistance to shear between the transformer 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-7 : 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: D_o
LC2: (D_e) + Wind
LC3: D_e + Seismic
LC4: D_o + Wind
LC5: D_o + Seismic

Load combination for Pedestal and containment mat foundation design (factored load calculation):

LC6: 1.4*(D_o)
LC7: 0.75 [1.4 D_e] +1.6 Wind
LC8: 1.2 D_e +1.0 E
LC9: 0.75 (1.4 D_o ) + 1.6 Wind
LC10: 1.2 (D_o) + 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.

Step-8 : Loads on containment wall

Containment wall shall be designed for following loads and load combinations:

Active soil pressure on the wall
Surcharge load on wall due to live load on soil. You need to discuss with construction about any site crane movement around the transformer pit.
Earthquake load on wall due to soil movement. Use Monobe Okabe Equation for Earthquake load calculation.

Typical foundation and oil containment drawing for a Transformer

For requirement of firewall refer NFPA-850 chapter-5.

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, uplift of foundation due to frost 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, pedestal and containment wall reinforcement calculation per your project design criteria.

Discussions:
Question from visitor: Do I need to consider soil passive pressure on transformer wall for sliding check, when the transformer pit is in high seismic zone?

Recommendation from Subhro: It is advisable not to consider any passive pressure on wall for sliding check, when pit is in high seismic zone. If it is absolute necessary to consider the passive pressure on wall for sliding check, you must consult with geotechnical engineer for recommendations.

Recommendations courtsey: Soumyabrata Roychowdhury, Civil/Structural Engineer

I hope this page will be very helpful to you to understand the basic loads for transformer foundation and containment pit design

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.

Design Philosophy for Transformer Pit