# Structural Engineering and Coffee ... Who Knew?



## Lighty (May 31, 2013)

Okay fellas, following the tamper caper last night I was looking at a work calculation today and thought to myself 'Hang On A Minute'

So, here we are.

When a Structural Engineer designs a foundation it is widely accepted that the pressure bulb beneath the foundation is as per the diagram shown.

Essentially directly beneath the foundation the pressure bulb spreads at 45 degrees.

I then did a quick check on the untamped / uncompacted volume of grounds for 2 different diameters - 58.5mm and 58.35mm.

I had to assume that the walls of the basket were vertical, and that the depth of grounds was say 15mm in the basket.

Does that sound reasonable?

Surely this is the way to look at it - volume not area?

Pressure does NOT act directly vertically NOR is it the same at all depths within the basket ....

Essentially 'Stop Worrying Fellas'

Cheers

Lighty


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## Lighty (May 31, 2013)

Attachemnts for better viewing ...


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## Lighty (May 31, 2013)

dash how do i attach them not in the post!


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## Lighty (May 31, 2013)

Might actually be 0.0047% difference ...

its getting late and I may have missed x 100 for %


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## CallumT (Aug 23, 2013)

Loving this, makes sense is the pressure bulb derived as a simplification that works in most cases? Or is it accepted mathematical proof, I only mention this because reality is often simplifications work and are accepted practice.

The pressure distribution below the UDL does make sense I just think there is more packing issues with the puck than initially meets the eye, wonder about the distribution under a curved tamp.


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## Lighty (May 31, 2013)

I'd have to grab my text books .. Can't remember the geotechnical guy who put his name to this.

We tend to use a simplified 45' line generally.

Its a similar case for 'punching shear'

lets say a 200mm square column sits on a 300mm thick slab - we check that the column won't punch through the slab by checking perimeter shear lines. So say at mid depth of the slab, assuming a 45' spread, the length of perimeter to check would be (200+150+150)x4 = 2000mm

the packing issue is a fair point too.

a specialist report is generally procured by the engineer and a geotechnical specialist takes samples and tests them in the lab. They would then provide an 'allowable pressure' for which we design to, including a factor of safety.

In simple terms if you are advised to design for 5 tonnes per square metre and you have a column carrying 10 tonnes you would allow for a base with area of 2 square metres - 1.41m x 1.41m

The report would also advise that you are likely to have say 25mm of consolidation of the subsoils over the life of the building - hence you try and equalise the pressure in all foundations to prevent differential movement.

Some columns will have 20 tonnes, sone maybe 5 tonnes to keep it simple.

So again as with coffee we will have localised soft spots in the ground / changes in strata / etc that all impact on the end result

Most of the public think we just get the builder to chuck some concrete in a hole in the ground ...

I'm not saying a larger tamper won't make any difference as it might have an impact on the basket to puck perimeter bond, or the flow of water at that point due to decreased loose grounds.

The maths behind the 'area' increase just doesn't stack up for me on its own though - just my humble opinion.

Lighty


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## Dylan (Dec 5, 2011)

This is all a bit french to me, so I have a question...

Is it correct to say the pressure applies like this because of the way the particles interact below the load? i.e. the load is even, but the particles distribute the weight creating the 'bulb' of pressure.

If so would the pressure of the water hitting the puck enact a similar bulb of pressure? As the puck infuses I imagine this pressure bulb would change depending on factors including distribution, and how the water emits from the shower screen. Due to the relation in pressures, if it were possible to achieve a totally downward pressure force with no 'bulb' would this actually negatively affect the distribution of water as it passes through the puck?

I guess some of that will be impossible to answer accurately, but what does you more mathematical mind think?


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## Lighty (May 31, 2013)

You would imagine that water would take the path of least resistance - thus we have pre infusion where water molecules under low pressure fill the voids between coffee grains until the puck is saturated.

The grains then absorb water and more water fills the voids.

The water then builds a head up on top of the puck as the pressure point is reached where there is no more room effectively and the pour begins - I think Scott Rao covered this in his book (I don't have a copy but have read it as a friend let me have it for a couple of weeks)

I suppose that's why we tamp - to try and create an even bed of particles and prevent water just flying through the least resistive path and straight through the basket.

I'm pretty new to the coffee game so there will be more betterer ( ;-) ) explanations around !


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