Intersecting beam structure - with & without stiffener plates

Abbreviations used, and explanations:


I actually welded the structure as-designed without the stiffener plates. Therefore, I observed its real characteristics.
My observations caused me to think of putting in the stiffener-plates; which are familiar in structural steelwork using I-beams and H-columns.

Purpose of this Finite Element Analysis structural modelling project

A properly implemented FEA model of a structure, a representation in a parallel world of numbers and physical laws, should show how the real structure would behave.
Therefore if I "Finite Element" the beam-structure with stiffener-plates added in the locations proposed, that should show what this alternative structure would behave like if I welded in those stiffener-plates.

How would the new structure perform? Would it be good? Would it be a worthwhile improvement over the current design?

The FEA models

The FEA models for without and with stiffener-plates are identical apart from the with stiffener-plates model having these added (so that's like what you'd do to the real structure - you'd add (weld-in) the stiffener-plates).

I've shown each model both transparent with only the element edges shown, and opaque showing element surfaces, so all details can be picked-out.
It's not so easy actually see that they have / have-not got stiffener-plates - as there's no colour variation here to make them obvious!

without stiffener-plates...

with stiffener-plates...

Load condition - for both with and without stiffener-plates

I've put 200kg-force sideways ("x-direction") on top of the column - seen as the green arrows. So this produces a moment ("twisting-force", "torque") of 2000Nm in the horizontal beam.
A sideways 200kg-force at the top of the column is not large compared to the weights of things being put on this structure!

Restraint conditions - for both with and without stiffener-plates

The red clutter at the beam ends are: the beam-ends cannot move sideways ("x") or up/down ("y"), nor can they rotate (twist) about the beam-length ("z") axis.

However, the beam-ends can "pull-in" or "push-out" in the beam-length ("z") direction - which has a significant effect on the resulting stresses. Fully fixed is not realistic compared to what the structure can do in the real world. The conditions described match what can really happen.

The red markings in the middle, under the intersecting beam / column are two restraints.
No movement in the up/down ("y") direction - as the structure is sitting on the ground and weight upon it keeps there, regardless of beam stiffness.
No movement in the beam-length ("z") direction approximates the friction of the structure sitting on the ground keeping the structure where you put it.

Rotation under load

Humans are more more likely to think of "twist" in distance, like millimetres. However, that doesn't well reveal "the story" with these structures.
Think of twist as an angle, and display as such, then the structural behavior jumps into view...

without stiffener-plates...

The twist about the long axis is quite localised to the junction.

A significant part of the rotation of the column comes from a local event at the column's junction with the beam and along the top flange of the beam.

with stiffener-plates...

The "moment" in the column is produced by uniform elastic twist along the intersected beam.
Which is as good as this overall design can give.

The moment ("torque") applied to the column passes into the beam without any local event at the junction - which is good.

Sideways displacement / distance deflected

The Deflections (the distance it's twisted under load) show the amount of deflection 1:1. What the deflection would literally look like. ie these components twist a lot! The original "no-load" shape is shown by the light-coloured outline.

without stiffener-plates...

with stiffener-plates...


So the with-stiffener-plates structure doesn't deflect under load much less than the current without-stiffener-plates version - 96mm vs 120mm, respectively.
Is that tolerable?


It will be seen that this is where the big difference is between the "without" and "with" cases, regarding a structural performance worth further consideration.

without stiffener-plates...

The stresses - are very high in the web of the intersected beam, just under the intersection / junction. The simulation suggests that for S275 structural steel, only 200kg of side-force at the top of the 1m tall column (2000Nm "moment") would permanently deform the structure.
This prediction is compatible with what is seen with the real structures.

Nothing much to see in the two top-views of the beam...
(though the stress scale hints there's something lurking out of view!).

because the problem is all here - the high-stress region just under the junction...

The web of the intersected beam, the long horizontal beam, is grossly overloaded locally under the junction with the column.

The condition would cause plastic deformation.
Controlling the angle of the column, the plastic yielding here would produce a big permanent lean on the column above.

Intuition based on other knowledge not provided by this FEA model suggests this mode of undesirable behaviour is quite calamitous. As there is nothing to make the load needed to increase the deformation increase much, above and beyond the small side-load which set the deformation going. That is; the resistance to continued yielding is not likely to increase much (?).
(The author is thinking of the behaviour of "fully plastic" steel sections which will not buckle with deformation in beam loading and present increasing resistance to bending - as applied in "plastic design" for buildings).

with stiffener-plates...


There is a stress "hotspot" in the four corners where the column meets the beam.
That said, they are at half the peak stress appearing anywhere in the "without" design.

The "hotspots" have appeared as a side-effect of the structure now being suitably rigid.
We cannot really "go there" for several reasons

Overall though, this structure, which has the small additional stiffener-plates which could be quickly and cheaply welded in during initial fabrication, performs vastly better than the original design without the stiffener-plates.

Nothing nasty hidden under here. All is well. Stress hotspot already seen on the top surface is also seen here...

The highest general stresses of up to 170MPa, seen along the length of the beam, are due to the moment ("torque") upon it. Produced by only 200kg-force upon the top of the 1m-tall column (2000Nm of moment "twisting" the beam).
This stress is completely independent of the junction design.
This observation causes thought whether a more rigid section than an I-beam, like a square box-section, is justified for the horizontal member.

Further comments

Large fillet weld sizes, in relation to the 8mm steel thickness, in vicinity of the "stress hotspots" in the "four corners" region of the column-to-beam junction, might dilute the forces across more load-path section and lower the four "hot-spot" stresses (?).

Refining a structure is a mix of what the customer wants and what is evaluated in engineering terms as good enough for purpose. So I'll back-out at this juncture, leaving you with this already much-improved structure.
Assuming the Finite Element Analysis simulated design performs in real life like the simulation. Which it has been established over a few decades is generally the case...

(R. Smith, 27Sep2016, 28Sep2016)