## Robot Structural Analysis

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# Uplift

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Hi dear :

I'm modeling a grid foundation and I want to ignore uplift . I selected the uplift in +kz but after analyzing some values remain positive ? Why ? They all must negative ? What should I do now?

Could you please help me?

Thank you.

Solved! Go to Solution.

# Re: Uplift

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Unfortunately your screen capture is done is such a way that there is no information what kind of results and for which load cases they are shown. Please make sure the non linear analysis is convergent for each of the load case and combination as well as there is no instability reported. Mind also that there is no non linear seismic analysis in case you defined seismic analysis (the use of diaphragms makes me think this is the case).

*If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.*

**Artur Kosakowski**

# Re: Uplift

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I have attached a file . I think I have don every thing. But there is some thing wrong.

Could you help me?

Thank you .

# Re: Uplift

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Somehow you managed to assign elastic soil to the column ( bar 22) which results in negative values for static load cases but the main issue is as I expected seismic analysis.

In any case of non-linear properties (tension-only members, uplift supports, cables, elasto-plastic members, non-linear releases, non-linear hinges, etc.) such objects are resulting in non-linear behaviour of the structure.

In such case in general vibrations are non-linear – they are not harmonic. It is so because stiffness of the structure can be changing during vibrations.

Dynamic modal analysis is a linear analysis assuming constant stiffness of the structure.

In case of non-linear models the dynamic modal analysis is run for the linearized model, i.e. assuming constant stiffness resulting from static load case preceding given modal load case.

It means that results of modal analysis can be different depending after which static load case the modal analysis load case is defined.

In case of model with uplift supports if a/ modal analysis is defined after static load case where all supports are active (not uplift) the vibration frequencies will be different than in case b/ when modal analysis is defined after static load case where some supports are not active (uplift).

It is so because these dynamic analysis are performed for 2 different situations – respectively:

a/ situation when there is no uplift during vibrations

b/ situation when permanently the same supports are uplift during vibrations

It is different than reality – because in reality the amount of uplift supports will be changing during vibrations.

Of course it can give some estimation of non-linear vibrations – if case b/ is corresponding to maximum number of uplift supports then the precise frequencies of non-linear vibrations are between frequencies of situations a/ and b/.

Another, more precise, possibility of solving such problem is using Non-linear Time History which is possible in RSA– but it is much more sophisticated type of analysis.

Simply saing you shoud not combine non linear elements and seismic analysis together.

*If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.*

**Artur Kosakowski**

# Re: Uplift

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Thank for your response.

OK . I understood what you say.

But if I want to do modal analysis and then bring forces to design the continuous footing what should I do ? Is there any help?

# Re: Uplift

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and one more thong:

how can i make *.thf and *.thf format from *.txt format files?

thnk you.

# Re: Uplift

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There is nothing to prevent you from using the results of seismic analysis in any of the RC Design module including the RC Continuous Footing one. The point is that the results obtained for seismic analysis of a non linear model may be incorrect due to the assumptions of the analysis itself.

There are 3 approaches I can think of:

1. Delete all continuous footings from the model and replace them with supports under each of the columns (no uplift of course so there is no nonlinearity in the model). Calculate model and and get the values of reactions. Create a spread footings with loads that correspond to the calculated reactions.

2. In the current model use for the design of the continuous footings only these combinations that do not produce any uplift along the spread footings

3. As most likely you do not want to have any uplift in along the continuous footings (the intended design is such that all continuous footings are 'pressed' against the soil rather than 'pulled' out of it) change the parameters of elastic ground assigned to them and switch off uplift so that the boundary conditions are linear rather than non linear.

**Artur Kosakowski**

# Re: Uplift

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Please look at C:\Program Files\Autodesk\Autodesk Robot Structural Analysis Professional 2012\System\Cfg\vranchea.thf text file.

*If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.*

**Artur Kosakowski**

# Re: Uplift

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Thank you .

But I need your help more.

I modeled a simple 2d model with support at base and ran analysis.

After that I want to design continuous footing . When I select base node and go to the design module . I don't know what should I do . I can only design single footing in this module . I haven't any foundation design manual .

I attached picture and robot file.

Could you please help me?

Thank you in advance.

# Re: Uplift

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If you decided on the 1st approach (no bar that model a continuous footing in the model) then you have to create it manually in the RC Design module and define loads based on the values of reactions you have in the model.

**Artur Kosakowski**