Bolted connections (with initial slip)

Husesset

I frequently model 2-D frames and trusses with bolted connections.

These connections have a lateral stiffness in the frame's plane, as well as a rotational stiffness if there are more than one bolt per connection.

Typically, I model the frame using beam elements in the XY plane. To simulate the stiffness of each connection, I offset the different beam components in the z-plane and connect the ends with a circular beam element with properties (diameter, E-modulus, Poissons ratio, and lenght) such that it represents the torsional and lateral stiffness of the actual connection. Then constrain end beam nodes rotation so that they are only allowed to move in XY plane.

This works very well.

However, bolted connections may have an initial slip. That is, there is usually some clearance in the bolt holes (for example 12 mm bolts may have 13 mm holes). This means, the connection will not absorb force until the bolts surface meets the edge of the hole.

I have not been able to work around this. It is obviously some kind of non-linear problem.

Using contact constraints wouldnt work, too tedious.

Does anybody have any ideas on how to approach this?

Best Regards

Ola, Sweden

Sergio

I suffer that problem designing and testing (FEA and physical tests) an engine mount aluminium bracket twenty years ago (or more). For low loads the initial bolt preload was enough to keep the part in place with the bolts centered in the holes (slots, even worst), so fatigue was in that condition, but for extreme loads (crash and curb impact tests) the part slips and rotate a little until it get in contact with the ends of the slots, so the boundary condition was completly different from the low loading condition.

In order to simulate such loading condition we just move in CAD the part until that position and then we make plastic analysis. For testing in house... we build an impact test machine during a weekend, we need to replicate, solve and convince our customer that the new part would not fail again.

FERNANDO

A non-linear calculation with a gap is necessary

Husesset

I have found one way that could work so far. The initial slip of the connection is known. Lets say it´s 0,5 mm. I can add a temperature load to each beam member equal to this amount of displacement. However, this demands I first establish which members are in tension and compression, and I would prefer a neater way to do it.

disla

If you are using ccx, beams can also be offset.
I would test extensively cause i'm not sure if that's safe to use and how it does affect deflections, stresses, and if it will make any effect.

EDITED: Internal also has offset from ver.25.Beam offset for Static 3D.