Equivalent Static Load
Equivalent Static Load
(OP)
I have an application in which a vessel must be restrained against movement if gases escape suddenly. I have calculated the instantaneous force generated by the escaping gases easily enough, but the structural guys are asking me to convert that into an equivalent static load. Despite my Googling and reference book digging I am not sure of the exact number they are looking for. Can anyone suggest a reference or a series of equations that I should be using? Thanks in advance.






RE: Equivalent Static Load
if the force decays quickly thee you can say that the support structure doesn't need to react the peak load, but some other "static equivalent" load.
it might be enough for you to calculate how the force decays, then tell them "make your structure withstand these loads, invent your own static equivalent".
Quando Omni Flunkus Moritati
RE: Equivalent Static Load
Dan
www.eltronresearch.com
Dan's Blog
RE: Equivalent Static Load
justifying an "equivalent static load" is difficult ... 1/2 the peak load might be reasonable (based on load magnification for a load applied as an impact, yes?). for you (personally) it's easiest just to calc the load vs time and have the stress guys say the structure is good. But it's easier for them if they can get you (or someone else) to take that "responsibility" on !
In today's world, i think it's not unreasonable to run a time domain FEA (with something like LS-DYNA or MARC)
Quando Omni Flunkus Moritati
RE: Equivalent Static Load
1. Permanent deformation , but not rupture of the enclosure..
Pes=1.5Pred/Fu
2. Permenant deformation of the enclosure not acceptable..
Pes=1.5Pred/Fy
Pes=enclosure design press
Pred=maximum pressure developed in a vented enclosure...
get a copy of the latest NFPA standard
RE: Equivalent Static Load
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.
RE: Equivalent Static Load
According to "Design of Welded Structures" p.2.8-3: "It is common practice to apply an impact factor to a load and design as though it were a steady load." Where the equivalent static load is equal to 2 times the weight of the body impacting the structure. So, basically multiply the force by two.
According to "UFC-3-340-02": (Section 4-47) An axial load times 1.2 equals the equivalent static load. However, this is for columns loaded axially. So I don't think it necessarily applies in this situation. Section 5-22.3 states that the equivalent static load is equal to a dynamic load factor times the pressure times the length of a beam, ending up with lbs/ft units.
Obviously, these equations will all give me different results. I'm just not sure which one (if any) to apply in this instance.
Dan
www.eltronresearch.com
Dan's Blog
RE: Equivalent Static Load
you've cal'd the initial pressure (and derived a load) ... could you develop the load as a time function, and use the average pressure over a time period, 1 sec, 1 minute, ... ?
Quando Omni Flunkus Moritati
RE: Equivalent Static Load
RE: Equivalent Static Load
RE: Equivalent Static Load
Dan
www.eltronresearch.com
Dan's Blog
RE: Equivalent Static Load
Thanks for the help everyone.
Dan
www.eltronresearch.com
Dan's Blog
RE: Equivalent Static Load
You mention a very fast decay, which makes me think it is either a very large opening, or it is not a high-volume, low specific gravity discharge.
(At NASA, we always pointed discharges in a direction that made sense, if for no other reason than to not unscrew a pressure relief device as the result of a discharge.)
RE: Equivalent Static Load
RE: Equivalent Static Load
RE: Equivalent Static Load
RE: Equivalent Static Load
Dynamics of structures J. Humar Champter 8.7
http://books.google.ca/books?hl=en&lr=&id=...
or
http://www.civil.utah.edu/~bartlett/CVEEN6330/Duha...'s_integral.pdf