Learning Forum

[Raheel]

Hi All

I have a structure with beam bolt connection at its base. 4 beam bolt connections are employed at base. Connection type is body-ground. One edge of beam connection(body side) is on top edge of bolt hole in structure while other edge(ground side) is taken with geometry reference of bottom edge of bolt hole in the similar structure. I have put an offset of 2mm in Z direction(gravity direction) at ground side of this connection to bring gound side out of the structure . Both sides(reference and mobile) are set deformable  All four beam bolt connection are modeled in similar way. A pretension of 8000N is applied on each beam bolt. 3 steps are used in analysis. 1st for pretension, 2nd for lock and 3rd for structural load.

In solution,  whole structure went 700mm into the direction of gravity and also whole structure stretched. Beam bolt are still physically shown connected to the edges but stretched from ground to the body. I cannot figure out what is wrong with the system. Either beam bolt connection not worked well or huge pretension did this large deformation

[peteroznewman]

Hi Raheel,

Inserting an image would help me understand what you have.

I assume you want the beam that gets pretensioned to connect to ground at one end and to connect to the top of a flange of the structure at the other end.

1) The ground end of the beam should not be scoped to any part of the flange. It should have a fixed support at that vertex. Only the other vertex of the beam is scoped to the top of the flange.

2) The underside of the flange is supposed to press against a ground that is not modeled. Therefore that face needs a compression only support.

3) You can do this in 2 steps, since Lock can be immediately present at the beginning of the step, while the other load is ramped up.

[Raheel]

Hi Peteroznewman

You are right.

• I have created co-ordinate system for ground side of bolt and bolt is connected at ground with that co-oridnate system


• For top end of beam connection, I have connected hole edge.

Now how should I generate compression only support at the underside of flange. Do I generate a body-ground joint connection at the underside of flange?

[peteroznewman]

To create a compression only support, click on Static Structural, click the Face filter, click a face and ctrl-click if there are several faces making up a single plane, the click Support, Compression Only Support.

If your model starts working, you can close this Discussion by clicking  Is Solution below the post that best answered your question.

[Raheel]

An internal solution magnitude limit was exceeded. Please check your Environment for inappropriate load values or insufficient supports.  Please see the Troubleshooting section of the Help System for more information.

One or more bodies may be underconstrained and experiencing rigid body motion. Weak springs have been added to attain a solution.  Refer to Troubleshooting in the Help System for more details.

 

These msgs are displayed. I think the compression support did not work.

Also there is warning in solution details "update contact stiffness at each iteration) in order to achieve better convergence''

[Raheel]

 Dear Peteroznewman

I have also tried a ground-body joint connection at the lower edge of the each bolt hole.Ground body beam bolts are also there having a pretension load with three step analysis. It comes with a quick solution but I m not sure with the solution results. What is your opinion about it?

[peteroznewman]

Dear Raheel,

Compression only support automatically creates a frictionless contact which needs the same level of care that any manually created nonlinear contact requires. Using a large number of Initial Substeps will usually allow the contact to become established.

A joint to ground at the lower edge of each hole is a good way to get a faster solution, but it has the limitation that the joint caot open the way a frictionless contact can open. You can monitor the joint force to make sure the force is putting the flange into "compression" against the non-existent mating face and doesn't go into "tension" that represents a non-physical result. The joint will prevent sliding of the hole along the surface as well which a fricitonless contact may allow. If you actually model the bottom surface and use a frictional contact, then maybe the hole would have enough friction to not slide.

Regards, Peter

[Raheel]

Auto-step Controls didn't show a converging behavior but a diverging behavior with huge differences in every iteration. Checked even for 5000 Initial steps. 

Also set stiffness update aggressive but no solution come so far

[peteroznewman]

Please insert an image of your geometry and optionally attach a Workbench Project Archive if you want to pursue this BC. The frictionless contact can be a problem if the body is free to slide laterally off the surface. That is where actually modeling the part the flange is being bolted to is often required to get this next level of fidelity beyond the joint from hole edge to ground method that is working for you now. 

[Raheel]

Dear Peteroznewman

Here is the image of problem 

• Two Dark Blue Surfaces at the bottom of structure are used to apply compression only support.


• Image is of step 1 so equipment weight is zero in first step but at last step it will be 500N


• Body ground bolt co-ordinate system is shown at every bolt alongwith a pretension

[Raheel]

A ground joint connection(along with body-ground beam bolt) type solution showed red contours at the bolt edge with 1000s of MPa Stress. I think it is singularity( It will increase with mesh refining) and I should consider stress value just at the neighboring element of bolt edge element. This singularity occurred due to pretension.

Am right?

[peteroznewman]

Dear Raheel,

Thanks for the image which makes it much easier to talk about the BCs.

A high fidelity model of a bolted connection is to model the base with the hole and the fastener with the bolt head. The shank of the bolt is split so the shank face in the hole in the base is bonded, while the shank face in the clearance hole in the part you are modeling has the Bolt Pretension load applied. There is frictional contact of the bolt head to the top face of the flange and frictional contact between the bottom face of the flange and the base.

The problem of a high fidelity model is the extra work modeling the geometry and adding the additional contact definitions, then the extra care needed to get this model to converge while the bolt pretension is applied.

There are a couple of features you could incorporate from the high fidelity model in a simpler model that is less work to build and more likely to converge easily. For example, you can add an offset circle on the top and bottom faces of the flange around each hole to split those faces. Then the joint to ground can be scoped to the annular face on the bottom of the flange instead of the circular edge to avoid a stress singularity. The same can be done to the beam element that gets bolt pretension. Scope that to the annular face on the top of the flange.

I mentioned that using a Compression Only support could add some fidelity to the joint-based model by replacing the joint to ground. Now that I see the image, I can add that the joint was providing 6 DOF to support the model. When you remove that joint, you have to replace the 6 DOF with something else. The Compression Only support only provides 3 DOF. That is why it will not converge. If you want to use the Compression Only support, all you need to do is use a General joint instead of a Fixed Support. I can't see a Triad in your image so I will assume that "up" in the image is the Y-axis.  Since you have 4 bolts, you can have 4 General Joints that have X and Z fixed at 0 while leaving Y free since the Compression Only support provides the Y axis constraint. You can leave all the rotations free since the compression only support is in the XZ plane and therefore supports the rotation about X and Z and the fact that you have four bolts, the Y rotation is also constrained for the whole model.

Regards, Peter

[Raheel]

Thank Peteroznewman for sharing your valuable point of view

• The problem is that I have a large assembly with more than 100 Bolts so modeling for high fidelity will be laborious. After modeling, convergence in huge system will also be a headache. Although Object generator will help alot but yet managing  100 bolts is a challenge


• Annular face at both bolt ends is a good option for system simplification. Singularity can be dealt well with this.


• Compression only support with a general joint( gravity axis free) will work in this scenario according to my dealing experience in this model apart from a fixed joint

Since my system is based on multiple parts bolted together, so body body beam bolts between parts alongwith pretension also show non converging behavior. But after applying a joint connection between them, system response is converging. However pretension create singularity problems so I get an idea from CAE webinar. Idea is to get stress value on neighboring element rather than singularity location edge

[Raheel]

 Peteroznewman 

I have tried the compression only support with a general joint(bolt axis direction free) at body-ground on the structure being discussed. The results are similar to simulation done with joint(All fix) at  body-ground with around 5% variation.

We can say that the both approaches are similar.

But for body body connection, compression only support may not work. However I have developed body body joint along with beam bolt (+ pretension) and it holds the parts well in place

Sparse Direct Solver is used in all solutions

[peteroznewman]

Yes, Compression Only Support is only to ground. You have to use contact or joints to connect two bodies. Glad to read your model is working. Please mark one of my posts above with Is Solution to show that the topic is Solved.