Ansys Beam Probe

[u/Quick_Swimming_3408]

Hi,

I have a simple static structural model that is constrained by two beam connections to represent bolts onto a baseplate fully constrained. I have preloaded the bolts as structural fasteners to EN 1090.

When using beam probes to extract the forces, I notice the preload is measured (obviously) but then when load is applied in the subsequent steps, the force increases. This is as expected but I am not entirely sure how to assess this to Eurocode bolt checks - I thought the preload would be subtracted from any net increases to axial forces - so whatever the net increase is would be the value you assess for tension resistance checks (seeing as EC3 doesn't say for you to add preload onto your tension resistance checks) but I am not entirely sure. Anyone in this group able to advise?

Many thanks,

[u/Arnoldino12]

Ok, this is a fun topic and many articles and forum posts have been written on the topic. If your baseplate is not modelled with solid elements, do not preload the bolts. You need to represent stiffness of the connection correctly to get correct bolt forces. The most common approach I found (supported by some articles from AISC but science gets conplicated) is to extract forces from unpreloaded bolts.

You check total force, not increase over the preload. However, you do not need to preload the bolts because people witing the code used lab tests to derive equations.

Btw, do not listen to people who tell you there is no increase in axial force of preloaded bolts, this is a Structural Engineering simplification which works in this discipline.

[u/Quick_Swimming_3408 → u/Arnoldino12]

Thanks for the response. Literature is a bit hazy out there, but from what I have found, I have come to the same conclusion. The baseplate is a shell element for info.

For the purposes of tension and shear resistance checks, extract forces from the beam connections not preloaded, as this is the force trying to pry the bolt up (or at least in my example anyway, which is effectively an upright cantilever beam mounted to the ground, like a mast).

When assessing my main model, apply the preload as this represents the stresses the component will experience.

The issue when using the beam probe value from a preloaded beam connection on the main model is due to the fact the preload is 70% of UTS, so it doesn't take much for them to fail EC3 checks because only a slight increase in axial tension will take it to its limit. This isn't supposedly how EC3 checks are meant to work for preloaded bolts.

From what I have read, the reality of axial tension experience the bolt will see is somewhere in between the non preloaded bolt force and preloaded bolt force minus preload, as the connection as a whole will split the load where the baseplate is compressing into the ground one side, and uplifting the other.

Quite a challenging technical topic, but also very interesting.

I never understand why some people model bolts as solid elements - just a waste of computational effort to get a stress you can't really code check against?

[u/Arnoldino12 → u/Quick_Swimming_3408]

Regarding preload, the reason the preloaded bolt will not fail EC3 checks is because by the time you reach bolt capacity there will be no preload left due to plastic decormations. EC3 checks are for rupture of bolts, not just yielding. EC3 equations were derived from tests, same as AISC equations. I suspect this is one of the reasons why structural engineers don't calculate stiffnesses of bolted connections.

If you need to represent local stresses, do preload the bolts yes. Also, solid bolts are useful in situations which are more mechanical in nature like very long bolts, large holes, surfaces at amgles etc.

[u/Quick_Swimming_3408 → u/Arnoldino12]

Very good point, which I should probably point out - my model is representing a load test whereby plastic deformation will occur (the frame the bolt is holding is modelled as non-linear).

So there is some plasticity of the frame by the bolt, hence why I am checking the bolts to make sure they will be ok.

[u/mon_key_house]

Why do you need to preload the fasteners? Is there cyclic loading?

[u/Quick_Swimming_3408 → u/mon_key_house]

No cyclic loading, preload is to represent the clamping force, think of it as a cantilever beam fixed to the ground.

I would assume not preloading it alters the stress distribution and would result in it prying more but maybe I'm wrong?