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Robot Structural Analysis

Posts: 190
Registered: ‎12-06-2011

Hydrostatic Pressure

989 Views, 2 Replies
05-29-2012 06:35 PM

1).  In defining hydrostatic pressure, what value is required for the unit weight of liquid when defining variable part of the load.  The dialog box asks for the unit weight which for water would be 1000 kg/cu.m, whilst the help topic states that it is the specific gravity of the liquid which should be input, which for water would be 1.  see attached images.  Since we can't read the value of the hydrostatic force from the load table, it is difficult to check whether load has been correctly defined. 


2.) Also looking at the formula in the help topic p(hydro) = p + grav(z-h) does not quite add up since p is input in kpa and grav is input in kg/cu.m - acceleration due to gravity seems to be missing to make right side of equation consistent.  


3).  (silly question) Why is there a constant pressure component for liquids.  I thought liquids only exerted variable pressure on vertical elements?


4). Does the dialog box only apply to non-horizontal surfaces or does it also apply the weight of the liquid on say a tank base?

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Valued Mentor
Posts: 653
Registered: ‎09-07-2011

Re: Hydrostatic Pressure

05-29-2012 10:55 PM in reply to: stroxy

1.         Input 1000kg/m3 for liquid weight


2.        p is a uniform pressure on the surface so kPa is correct.  kg/m3 will be converted to kN/m2 per metre which is the same as kN/m3


3.       I use hydrostatic load definition to apply soil loads to walls.  Use weight of soil x passive pressure coefficient for weight of liquid, use kPa surface load for weight of applied load at surface


4.  as far as I remember the load will also apply to horizontal panels. 


This load definition type does not work on claddings however. 




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Active Contributor
Posts: 39
Registered: ‎05-23-2012

Re: Hydrostatic Pressure

05-30-2012 01:33 AM in reply to: stroxy

You can change the input units in job preferences. If you change the weight units from kg to kN, the input for hydrostatic pressure will be in kN/m3. See the attached pictures.


tony.ridley is right, this load applies also for horizontal panels (load of constant value).



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