Simulate Dead Load due to self-weight

[gyom]

Dear Forum Members:

I would appreciate if you can give me some hints about:
(a) How to simulate the 'dead load' due to self-weight for a structure.
(b) Do we need to apply 'dead load' and 'live load' in two different STEPs.

Thank you in advance.

gyom

 

[corus]

Self weight is input via a body load with 'grav'. I put the value of -g (-9.81m/s^2) as the component in the relevant direction.
You can impose dead and live loads together in the same step, but as you appear to be referring to a design standard then I would follow their rules for assessing each load case. You can combine multiple loads later in Viewer if you wish.

corus

 

[barrind]

Just to make you aware, we have found in Abaqus 6.5 that you have to use a value of -9.81 in the gravity dialog, even if you are working in units of mm. We found that if you converted 9.81m/s^2 into mm/s^2 you get an incorrect result. It would be a good idea to model a 1m cube block resting on a rigid surface and extract the vertical reaction forces to be sure the thing weighs as it should. I don't kno if this is intentional or a glitch.

 

[corus]

9.81m/s^2 is the correct unit if you wish the self wt load to be in Newtons when your density is in kg/mm^3, geometry in mm. It's always useful to check your total reaction force anyway in a static analysis to make sure you haven't made any mistakes in your input.

corus

 

[barrind]

I should also add you need to input the density of the material in the property module, though you probably guessed that already. So in my models I have all the geometry in mm, the density in kg/mm^3 and gravity as the oddball -9.81 (m/s^2)

 

[Yoman228]

ABAQUS is unitless so it is always worth to check the Unit by using the following;
F = ma
F = density * volume * a
and make sure the young modules and the true stress in material use the same order of unit too.

 

[barrind]

I understand what corus is saying now. To me it appears the -9.81 is not an acceleration per se, but a conversion factor to convert whatever unit of mass you are using in your density to whatever unit of force you want in the output and is therefore independant of whether you are working in m, mm or whatever. The unwary could however assume that if it was an acceleration it should be converted into the appropriate distance units being used.

 

[corus]

It's not a conversion factor, it's an acceleration. It's 9.81 m/sec^2 which is the acceleration due to gravity. If you use that and your density and distances are in consistent units then you'll get Newtons as the force because Newtons are in units of Kg.m/sec^2. Young's modulus and stress/pressure are of course in N/mm^2 for these consistent units. The only oddball is Newton, so rumour has it.

corus

 

[Cansand]

Now.. what if one has another time-dependent parameter ,say permeability/conductivity (Distance/Time), should one also have flexibilty with the dimension of this parameter ; e.g, can one enter it as m/h while keeping the accerlation in as 9.81 m/s^2. I think that unit of accerlation should be consistent with other units

 

[corus]

For a dynamic analysis use 9810 mm/s^2 with mm as the unit of distance. Density has to be tonnes/mm^3. It's equivalent to the static case.

corus