FEA Software

[routercnc]

This thread is a follow on to the discussion in http://www.mycncuk.com/forums/general-discussion/5129-sideways-open-bearings.html



I've answered previous posts on sideways mounted open bearings by pointing out that they will open up a bit with load in that direction. This time however I decided to carry out some stress analysis on a simplified part. The results were quite interesting.

For a 16mm open bearing:

1N load was applied vertically to inner bearing whilst restraining the outer face (the area you would clamp your gantry to).
Displacement is 1.85e-5 mm, therefore 1.85e-5 mm/N.
So for a 30kg gantry spread over 4 bearings thats 73 N per bearing, which is about 0.00136 mm or 1.36 microns.

Initially that seemed pretty small so either there was an error in the calcs or I was worrying over nothing - but then realised that I should really model the supported rail and include that since I think it would be quite significant. Will have to wait for another night . . .

Here are the pretty pictures for the open bearing under 1N vertical load. Stress on the left in N/m2 and deflection on the right in mm.

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[Tenson]

Thanks for that routercnc!

BTW, can you do modal analysis of parts?

[irving2008]

Hi RouterCNC, what package are you using for that?

[routercnc]

Hi Tenson, Irving2008

The software is called Catia V5 made by Dassault. It is a very powerful piece of software designed for high end use, and very expensive. FEA is just one small part of the software.

I'm sure there are cheaper packages out there which will do the FEA part of the job - on simple parts at least.

It will also do modal analysis although I've not used it for that.

[irving2008]

Been looking for a freeware FEA package. Been playing with LISA (http://www.lisa-fet.com/)but the 300 node limit of the free package is... limiting ! And I'm too tight fisted to spend $80 on the full one until I'm sure I can use it properly!

[routercnc]

Hi Irving2008,

I don't want to hijack Tenson's thread so if you want to ask around the forum then maybe with your access permissions post #8 onwards (that would be yours and mine then!) could be edited out into a new thread, asking about experiences of FEA software?

I did a search of this site for 'FEA' and there were a few mentions of it but not a specific discussion or list of more affordable software. Maybe only a few people become interested enough to go that far. I know Jonathan has used some software which looks a bit like something from the SolidWorks family, but could be wrong - the end results displays are often similar between packages.

[irving2008]

Ok so far I've looked at/tried...

AxisM11 Lt

Z88Aurora v2

Lisa FET

Analysis for Windows 2.04

GeoTec

All have their limitations or complexities - usually the free version is limited to a few 100 nodes. Z88 is the most flexible having no limit to the model/structure size but its harder to work out how to use.... Lisa works well, but for the 300 node limit, but I've not figured out how to model what I want in it yet... my understanding of the way to get my real world model into the tool is limited...

what I'm trying to do is model a concrete slab with reinforcement. I've not found a tutorial on this online yet. I'm guessing I can model it as a set of stacked beams/fibres?

[WandrinAndy]

... what I'm trying to do is model a concrete slab with reinforcement....

Does that explain the slower than normal progress on your workshop build Irving ;-)

Lol

[irving2008]

Does that explain the slower than normal progress on your workshop build Irving ;-)

Lol

No, progress halted by the fact the hole is doing a poor impression of a swimming pool filled with porridge :(


But before I order the reinforcing mesh I want to check out that A142 mesh is sufficient and whether I should put 1 layer mid beam or 2, top and bottom, or fold the top one down the sides and back under the footings

The reason being thast the clay soil I am 'floating' the raft into can experience 'heave' of up to 150mm potentially so there's quite a bit of stress on the raft if thats all at one end...
6994

[Jonathan]

I'm not sure about what free FEA software for stress analysis you can obtain, but for magnetics I use FEMM (http://www.femm.info/wiki/HomePage) however it does also do heat flow. I remember you were calculating the heat loss from the building earlier, perhaps it could help with that?

At the very least you'll get some pretty pictures...

[Kai]

I know its not right but as long as its not for profit you could get hold of a copy solidworks and use that. I dont condone pirate software but if its just curiosity then no profit I think that would be ok but you have to uninstall when your done

[routercnc]

Hi Irving2008

I've done some work with footings, walls, and a new driveway but I've always just followed best practice. Collins complete DIY manual (Jackson & Day) is a good read but might not go into the detail you are after.

I don't know anything about building calculations with concrete but believe it is very good in compression and very poor in tension. So depending upon how your soil movement puts loads on the base would dictate how to best reinforce it.

For a cantilever, where only one of the footings was taking the load (and the floor was not supported by the clay if it moved away) then the tension would be on the top surface. This would suggest reinforcement at the top.

For a simply supported beam where both footings are taking the load but there is also alot of weight on the floor and the clay has moved from under it then the tension would be on the lower surface. This would suggest reinforcement at the bottom.

I don't know what reinforcement in the footings would give you, if anything.

I remember seeing a programme about applying and holding a pre-load onto the reinforcement whilst the concrete set. This would then apply a permanent compression on the concrete enabling it to be used in a situation where it would experience some tensile loads (the limit being where the tensile load equalled the built in compression load). In effect a DC offset towards the compression side.

These are just my thoughts on it - best get proper building advice!

[irving2008]

So by way of experiment here is the concrete raft sans reinforcement (I haven't worked out how to make Sketchup successfully export the stl file for that without breaking it). I drew it in Sketchup as a 6m x 4m x 300mm slab with a 5.4m x 3.4m x 200mm void underneath, exported it as an STL:

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Then imported that into Z88Aurora and meshed it:

6999

Then applied material info and fixed constraints at 3 corners and a 100N upward load at the 4th corner (nearest to camera), run the solver and get this:

7000

which suggests a 7mm upward displacement, which intuitively feels very large for 100N load....

more experimentation to come :)

fun this learning curve tho...

[irving2008]

Hmmm...further experiments with just a 300mm square 6m long beam as a test case...

modelled as a beam between two nodes, deflection is .356mm as calculated by hand

modelled using an STL file, deflection is 353mm... so somewhere a factor of 10^3 is creeping in....

onwards into the mire lol...

[mike mcdermid]

what did you hand calculations come up with?? for the 6m slab
secondly what did you use for a mesh /element type

[irving2008]

what did you hand calculations come up with?? for the 6m slabNot done them by hand for the slab, its too difficult. But fixed the issue... as i suspected its in the STL export from Sketchup... you have to export in millimetres and not metres as it gets the sizing wrong in some way. Its clearly not obvious as if all the measurements were out by 1000 it would have been a 6mm beam of .3mm sq cross section and that clearly couldnt have a 353mm deflection... anyway it's working so now I'm back to my original model...

[Peter.]

No, progress halted by the fact the hole is doing a poor impression of a swimming pool filled with porridge :(


But before I order the reinforcing mesh I want to check out that A142 mesh is sufficient and whether I should put 1 layer mid beam or 2, top and bottom, or fold the top one down the sides and back under the footings

The reason being thast the clay soil I am 'floating' the raft into can experience 'heave' of up to 150mm potentially so there's quite a bit of stress on the raft if thats all at one end...
6994

I've never seen weldmesh folded - don't even know if it can be!

From a dismantling point of view, I would prefer that you only put one layer of mesh in the top, that would guarantee an easy job of demolition. One layer in the mid would be next-easiest with the top layer and bottom of footing third easiest. Most robust would be top & bottom layer of steel in the slab. U-bars on the egdes fully tied into both meshes even better again, you have a strong slab then. Best of all would be a full cage in the footings with top/bottom layer in the slab with u-bar links but you're looking at a few bob for that.

[routercnc]

Hi Irving2008

I modelled your 6 x 4 meter voided slab and applied the same load case (restraint at 3 corners and vertical load 100N in the fourth corner). Displacement was around 0.01mm, with stress around 23,300 N/m2. Hopefully this is similar to your answer. Yield stress for concrete under pure tensile load is quoted as 3,000,000 N/m2 which suggests it could take about 1280 kg loaded like this.

7011

7012

[m.marino]

Irving you might want to look at the Ferrocement Education Network. They might have some ideas for you on construction.

[irving2008]

Finally got the sofware doing its thing and got the answers I need...

Worst case scenarios, fully loaded with 4.5tonnes of wall/roof, live load and mill etc... and including 8.8tonnes of self weight!

Clay shrinks under hard core in centre and theres only support at corners (centre sags 3mm)

7030
Clay heaves under hard core and lifts slab off footings (corners drop about 2mm)

7028

Clay shrinks at one end and leaves end floating in space.... (it drops 7mm or so)

7029

Using BS8110 part 3 tables I now know that i need some substantial mesh/reinforcement...

In slab I need B1131 (100mm/12mm dia x 200mm/8mm dia)
In beams I need 10 x T12mm bars, 5 top and 5 bottom on the long side and 8 x T8 (4 top, 4 bottom) on the short sides.

Its cheaper to use the more expensive mesh than to make the concrete thicker...