AERIAL PICTURE OF THE ORIGINAL 1953 BURNABY TERMINAL DURING CONSTRUCTION
Structural Design Code for the 1953 Tanks
The engineers likely used the American Petroleum Institute API Standard 12C – 1951 Edition for the design of both external floating roof (EFR) and closed roof (IFR) oil storage tanks. API 12C had only one design load case to size tank wall plate thicknesses and no lateral loading by wind or earthquake:
“Stresses shall be computed on the assumption that the tank is filled level full of water at 60 deg. F”.
Hydraulic water pressures create the tension ring forces on the vertical welded butt joints. A second formula sized the horizontal stiffeners around the tank to keep it circular when the tank was empty. As a result, tank wall plate thicknesses would be identical for tanks of the same height but any diameter, whether they were EFR or IFR tanks, anywhere the tanks were located. Standard tables at the back gave the designer all the plate thickness required for each tank height.
However, EFR and IFR tanks have very different failure modes under lateral earthquake loading:
– IFR (Closed-Roof) tanks are restrained laterally by the tank roof diaphragm that limits horizontal deflection but increases compression on the tank wall and eventually buckle under compression just above the base – an elephant-foot failure. Additional buckling can occur at the roof/wall connection. With lower lateral deflection there is lower sloshing forces on the tank. Failure usually won’t release tank contents but welded connection failures release the contents under pressure.
– EFR (external floating roof) tanks deflect horizontally under lateral earthquake forces as they’re not restrained by a roof diaphragm. The higher deflection creates sloshing forces that amplify both lateral displacements with the contents rising up the tank walls. Amplifications are more severe if the frequency of the earthquake waves matches the tanks frequency and for higher volumes stored in the tanks. Floating-Roof Tank failure occurs when vertical displacement of the sloshing oil over-tops the tank wall or onto the top of the floating roof. The spillage will ignite if sparks are generated when the floating roof impacts against tank walls.
The existing tank foundations aren’t designed to resist uplift or sliding forces generated by an earthquake. The foundation likely is a small concrete tension ring beam that would be less than useless to resist seismic uplift even when the tank is bolted to the foundation.
One clause in the API Code Appendix F recommended flexible piping connections to the tank to allow for settlement and earthquake shaking but didn’t define the amount of movement. This was addressed in Kinder Morgan’s list of seismic upgrades but was a suggested detail in API Design Code (51 Ed).
AERIAL PHOTO OF THE EXISTING BURNABY TERMINAL, 2019