FEA Software

[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:



Then imported that into Z88Aurora and meshed it:



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:



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 slab

Not 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...
Attachment 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.

[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)


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



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



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...