I took the original post to refer to the force of the vessel on the supporting structure due to the jet effect of the contents escaping- not to the pressure of the contents when released in an enclosed building.
Suggested approach:
Assume steady force applied over short duration.
Get spring constant of supporting steel.
Using that steady force and spring constant and mass of vessel & part of supporting structure, calculate acceleration, velocity, deflection over the time period that the force is acting- should be able to integrate that numerically via spreadsheet or whatever if the solution isn't easy-
Using velocity and zero jet force, calculate additional deflection until the vessel starts bouncing back.
If that deflection is a reasonable number for the structure, then you have your solution.
If the deflection is just a little high, you could consider the reduction in pressure relative to time, with velocity decreasing as a function of time.
If that deflection is an unreasonable number (say, 4 feet), then you know you need to do something else.
But what this approach would do is give you a reasonable value IF the duration is short enough.
Also, I'm awfully hazy on compressible flow, but if there is sonic choking or something like that, it may limit the flow rate to a much lower value than Bernouli's equation would indicate, which would lower the force.
