problem about the elastic support with uplift

[SOTECO-YAN]

Hello,

I created a model of a slab placed and anchored in some places on a rigid support (look at the picture below).

 

With regard to the supports, I used two types of supports at the bottom of this slab.

1)      Punctual pinned supports (nodes: 6 to 16)

2)      Planar elastic supports (look at the definition below). These supports make it possible to block the deformation towards UZ- but to authorize uplift towards UZ+.

The loads applied are as follows:

1)      Dead load

2)      Heat gradients ±80°C/m

the combinations are made as follows:

1)      ULS : 1.35*G+1.5*T

2)      SLS : 1*G+1*T

But after calculation, I find that the bending moments in ULS and SLS are almost similars, it is not logical because normally these two bending moments have a ratio of 1.5. I do not know why

 

However, after having replaced the planar elastic supports by planar simple supports or simply removed the planar elastic supports, I find the normal results as follow

 

Can you bring an answer to me? I add my small model in the enclosure

 Thank you in advance.

Yan

[Artur.Kosakowski · Accepted solution]

Running your example with the current version of Robot (2013) I can see that this situation has been corrected.

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.

[SOTECO-YAN]

Thank you for your answer! But I do not have a version 2013, I have only the version 2011 and 2012. I already update the software by downloading the new service packs in your site. The problem of the bending moments exists always. So this is a insufficient for the version 2011 et 2012? Then the version 2013 is absolutely needed  in order to resolve this problem? I must pay more to have this version?

Sincerely

Yan

[Artur.Kosakowski · Accepted solution]

The solution for the older versions is to create a new simple load cases with loads of magnitudes as in the combination it is going to replace. See the attached picture.

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.

[SOTECO-YAN]

Excuse me to disturb you again. I still have a question for this model. When I want to calculate the section of reinforcements of this slab, the software presents a message "Required types are not ascribed to SLS combinations. Calculations may be incorrect". What it means?

In fact, I have created the combinations for SLS and ULS pour this model as follows. The definition of my combination SLS is not correct?

in fact, I have met the same type of message in other projects. I don't know why.

[Artur.Kosakowski]

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/SLS-combination/m-p/3316923/ 

[SOTECO-YAN]

Hello, I have a question concerning the sign convention of the heat gradient. As the figure presents below, I have a slab on which the thermal gradient applies, I would like to know which side is hot, which side is cold.

[Rafal.Gaweda]

Check these two threads

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/Thermal-Load/m-p/3806569/highlight/true#M115...

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/Application-of-thermal-loads-to-a-slab/m-p/3...

[Artur.Kosakowski]

Doubt? Check the deformation of a simply supported slab

[SOTECO-YAN]

So, in my case, the top side is hot, the bottom side is cold, right?  The direction of heat transfer (from up to down) is contrary to the direction of the red arrows (towards up) (red triangle in the figure)?

[Artur.Kosakowski · Accepted solution]

Correct.

[SOTECO-YAN]

Hello, I still have a question. for this time, my slab bears two loads: a heat gradient load, a linear travelling railway load.

The client asks us to make a following calculation:

<< Usually, the design of a railway slab which undergoes a heat gradient is done in 2 steps:
- Application of the heat gradient on the model - >calcul of a deformation;
- Application of the other loads on the “deformed model”.>>

To achieve this goal of the client, how to make in the ROBOT a calculation in two steps? In other words, we apply a thermal load on the slab, after its deformation, we apply un another variable load to the deformed slab. Lastly, Robot gives the final combined results . We can carry out this type of calculation in ROBOT?  

I can creat the combinations below, but in my opinion, a simple combination as below cannot satisfy my request, because these combinations mean that the two loads are applied at the same time. There is not a sequence between both.

[Artur.Kosakowski]

For linear static analysis the result of combination is the superposition of the results of simple load cases. The sequence of applying load is irrelevant. In most cases this is all you need but check with the client what kind of effects he or she actually wants to take into account.

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.

[SOTECO-YAN]

In fact, it is a nonlinear calculation because I put elastic supports  with upward uplifit in the bottom of my slab. I created  linear combinations as what you see above, but after calculation, the analysis type of combinations become automatically nonlinear (see below).

The client wants just the results of a calculation in two steps:
1. to apply the heat gradient to the slab

 2. to apply the live load to the slab whic have deformed because of the previous heat gradient.

By the way, I join my model in the attachment for your analysis. Thank you!

[Artur.Kosakowski · Accepted solution]

1. Set the temperature load as the first load case (with the lowest number)

2. Set it as the staring case for other cases

Mind that you can do this for one temperature load at the time so you need to have two separate models for + and - gradients.

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.

[SOTECO-YAN]

Hello, I still have a  question concerning the application of the heat gradients in the model.

I created a model of a railway slab with two walls anti-derailment of the train (see figure below)

I applied heat gradients to these three panels (1 slab + 2 walls). (see figure below).

But the client says that the application of the heat gradients on the two walls is not correct. Because my model take into account of the application of the heat gradients according to the diagram n°1, but in fact, the way that the heat gradients are applied in the structure must be like the diagram n°2.

I know that the ROBOT applies the heat gradient according to the thickness of the panel. So, is there a solution where the heat gradient can be applied to the wall in the vertical direction like the diagram n°2 (local axis Y in the model)?

In fact, I found a solution where I can create three horizontal slabs with various thicknesses, but in addition to the heat gradients, there is still a linear shock applied to the top of the wall qui will create the bending moments at the bottom of walls (see figure below). If I do like this, the bending moments will not be produced. This solution is not thus good, I think.