using Gap-Element to model compression only behaviour of strip foundation 1

[jhiyoshi]

My first time post here.

I am having a problem getting non-linear gap element to work in my SAP2000 (V17.1.1).

I have a simple RC portal frame with a lateral point load pushing the structure over. I have assigned the gap elements to the bottom of column to model compression-only behaviour of the strip foundation. I have set assigned non-linear behaviour in U3 direction with stiffness of 12000KN/m and gap of 0. U1 and U2 is fixed.

How come I am still getting a tensile force on one of the column>

Please help

 

[darkjmf1]

Links are a bit tricky about local axes.
You have assigned non-linear behaviour to U3 direction, assuming that would correspond as default to Z direction... but it is not. With links, U1 represents its own longitudinal direction, while U2 & U3 are transversal directions.
You can check the orientation of your link displaying its local axes. In your case, the vertical direction corresponds to U1, so that's the value that should be assigned with the non-linear behaviour.
Or you could change the orientation of local axes to make U3 the vertical one.

 

[rn14]

I cant open your file since I'm on version 16 still but along with the local axis mention above make sure you have set up a non-linear load case.

 

[jhiyoshi]

Thank you for the reply,

In order to achieve compression-only behaviour at the link, should I be setting the gap length to 0? I don't quite know what this parameter represents when assigned as a single link (in comparison to double link).

 

[darkjmf1]

I guess you refer to the "Open" value. It represents the distance that the link shall be allowed to move until starting to develop its non-linear behaviour.
If you set it to 0, the link shall be non-linear from the very beginning.
I can't see any difference between 1-joint and 2-joints links regarding this feature.

 

[jhiyoshi]

The gap elements are working with only vertical loads applied to the structure (Load case 1: downward UDL = axial compression recorded in columns, Load case 2: upward UDL = negligible tension recorded in columns). However, when I apply a point load at the top of structure, the analysis seem to reach its maximum number of null steps after one iteration (I assume this is due to numerical instability from using gap elements rather than defined boundary condition such as pin-support). Anyway I can overcome this problem?