## General Mechanical

#### How to get compression reaction force only

• thanhttdt
Subscriber

Hi everyone,

I'm working on a structure like a flap which is supported by hinges (1) at aft side and the steel pads (2) resting on the ground (3) at the forward side. The compression load is P. There are 3 hinges and 9 steel pads

• peteroznewman
Subscriber
When you say there are some minor value of reaction forces, do you mean negative values? I expect this is what you meant. I also expect that the only DOF on the 9 remote displacements representing the pads is Z=0 and all other DOF are Free.
So if you have 3 remote displacements at the 3 hinges and 9 remote displacements at the steel pads, the pressure on the top causes some bending along the length between the pads and as you have discovered, the pads with negative reaction forces do in fact want to lift off the ground.
There are two approaches to resolve this condition.
The simplest one is to suppress the remote displacement with the largest negative reaction force and rerun the solution. Repeat this until none of the pads have a negative reaction force.
The other approach requires more work to setup nonlinear contact at each of the 9 pads so they are free to lift off the ground during the solution. This approach has the advantage of allowing different load distributions on the top that might change which pad touches and which lifts off to be correctly solved in one go.
• thanhttdt
Subscriber
Hi peteroznewman
Sure, your thinking is correct what I mean. The first approach is ok, however I try to find the way to set up the support boundary condition like compression direction only. Somehow like if the steel pad lift off the ground, the reaction forces should be zero not negative value.
I'm interested in the second approach, but I still not understand how to perform. Could you please give me a bit information to clear this point
• peteroznewman
Subscriber
The there are at least two ways to implement the second approach, which changes the problem from linear to nonlinear.
One way is to use a nonlinear spring, the COMBIN40 element. See this discussion:
https://forum.ansys.com/discussion/1110/how-do-i-implement-combin40-for-springs-in-wb
Use just the Gap functionality, and set the value of K to a high number.
Another way is to use a Contact element. That is easy to do in Workbench. Create a surface to represent the ground plane. Set that surface to be a Rigid surface. Hold that surface with a Remote Displacement. Add Frictional contact between the pad and the surface.
• thanhttdt
Subscriber
Hi peteroznewman
I did the analysis with 3 ways: using contact, spring Combin14 and spring Combin40.
Case 1: Using contact between the steel pads and plate (as a ground). The reaction forces from the contact region are so strange. It seems that they did not carry out the load
Case 2: Using spring Combin14, is the same reaction forces in comparison with using remote displacement
Case 3: Using spring Combin40, there is an error, I found in the Solution information is that:
and there is an error related to rigid body motion

I still not sure about the gap functionality so could you please take a look the attached file then give me any support.
Many thanks