engineering - Girl bridging the gap

Ancient Greek technology

By girlbridgingthegap | 20 September, 2023

I had the chance to visit the Museum of Ancient Greek Technology, an unassuming facade on an Athens street. The works were beautifully presented. The cleverness and simplicity of the pieces showcase just how advanced the ancient Greeks were during their time, with many principles still in use today. A selection of interesting pieces are below.

1. Pythagorus cup

This cup has a special design which punishes the greedy – when you fill it up past a certain point, the liquid begins to empty out of the bottom, covering you in wine. Contrary to what you might expect, the flow doesn’t stop at the ‘acceptable’ level but instead will continue until the cup is empty.

As you fill the cup, the pressure at the surface of the liquid is equal to the air pressure, ie zero relative to the air. This is true both inside and outside of the centre section, since both surfaces are in contact with the air. This is why their heights are always the same during pouring.

The pressure at the bottom of the cup, relative to the air, is equal to the height of the liquid x the density of the liquid x the gravitational acceleration = hpg. You can think of it as a function of h, = f(h), since the other two values are constant. At this point the pressure at the bottom out the outer and inner sections are equal ie f(h_outer) = f(h_inner) because the heights are the same. So there is no liquid flow.

If the user keeps filling the liquid, eventually the liquid starts to flow over the inner divider, because h_inner has become fixed while h_outer has kept increasing so the unequal pressure distribution causes a flow of liquid. In other words, the liquid ‘wants’ to get to the same height as the liquid in the outer section, but is limited by the cup’s geometry.

In the last image, the liquid keeps flowing because the pressure at the bottom of the cup stem zero (relative to the air pressure) yet at the bottom of the cup is still hpg. There is an uneven pressure distribution, and therefore flow of liquid, until the cup drains completely.

This principle is exactly how modern siphons in toilets work when you flush. That’s why they have that odd curved pipe shape at the back.

2. The first crane

When the Greeks discovered mechanical advantage, they used it for hauling huge rocks and material for construction, basically inventing the crane. Mechanical advantage is the idea that you can lift a heavier load with the same force if you stand further away from the pivot point between you an the load. For example, a seesaw can be balanced with two people of different weights just by placing the lighter person further from the centre. In a crane, the ‘light person’ is the human, and the ‘heavy person’ is the load, who can be moved into the air when the ‘light person’ (human) adjusts their position further from the pivot.

the load is fixed at a chosen mechanical advantage via a series of belt gears

3. The hydraulic clock – Clepsydra

This invention is an incredibly clever piece of engineeering. The clock is able to operate continuously on its own all year, with the water provided from a local stream and flowing into the top container. The middle container has a level controlling system consisting of a conical valve, which supplies a dripper. This water drips into the bottom container, whose float (the hemisphere) rises with the water level. The float is connected to a pointer which indicates the time of day on the top chart. At the end of 24 hours, the water drains rapidly via the siphon on the right – exactly as it does in the Pythagorus cup above.

The clock is also capable of showing the day of the year. This is via a system of gears that rotates the top chart by 1/365 turns every time the bottom container is drained, ie once a day.

Wren Challenge: ongoing

By girlbridgingthegap | 12 September, 2023

This is a project I created to improve my sketching skills, analyse historical, resilient buildings and engage with the history of London architecture..

Christopher Wren may be most famous for his design of the new St Paul’s Cathedral after the old one was destroyed in London’s Great Fire of 1666, but the architect and engineer was already an experienced church designer by the time he took that challenge on. Twenty two Wren-designed buildings still stand in London today, of which fifteen are churches located mostly in the Square Mile.

I wanted to capture Wren’s genius with an ongoing project to locate, observe and draw his buildings. Having stood the test of time, I believe they really showcase what good civil engineering should look like – robust, safe, human-scale and adapted to the needs of those who use it.

Wren lived and designed in a different century, but took pride in using what resources were available in his time. Today’s civil engineers may be pioneering low-carbon concrete and solar-energy-collecting windows instead of stone and glass, but we should emulate Wren’s approaches, because city dwellers still ultimately want their buildings to be fit-for-purpose, future-proof and beautiful.

Concrete manufacture and testing

By girlbridgingthegap | 3 August, 2023

I observed the testing of concrete intended for the headhouses of London Power Tunnels 2. This took place at the batching plant Capital Concrete in east London.

There are two types of concrete batching: wet mixes and dry mixes. The former can generally produce a higher quality concrete because it can be mixed using the industrial-scale mixers at the batching plant, rather than smaller mixers available on construction sites. Today, a wet mix is being prepared for the concrete slab. The intended strength is C40/50 (meaning a failure strength of 50MPa on a 5cm x 5cm x 5cm cube).

Note on sustainability. Concrete production is one of the world’s largest sources of CO2 pollution, meaning civil engineers need to come up with alternatives which are less polluting. In normal concrete, which is based on Portland cement, about 1000kg of CO2 is produced per tonne of concrete. A new technology called Earth Friendly Concrete (EFC) is capable of reducing this to about 200kg CO2 per tonne of concrete – a significant improvement – by replacing the Portland cement with Ground Granualted Blast Furnace Slag (GGBS) and waste fly ash from industry. The concept is in its infancy so its properties can be unpredictable – previous batches have not reached the required strength because of low-quality fly ash supplied from Dunkirk in France; now that the supplier has been changed, today’s mix should pass the required tests. If the rollout of EFC on this large project is successful, it could be a significant turning point for the construction industry to reduce its emmisions from concrete production.

The batch for testing is a sacrificial batch – it will not be sent directly to the project, but if test mix passes the strength tests, the same mix will be used on the project.

Layout of the plant:

Silos storing cement (or Earth Friendly cement alternative)
Aggregate storage area
Huge industrial concrete mixer, with a ‘tap’ to transfer the mix straigh into mixing lorries for transport to site

^ The giant, industrial concrete mixer at the batching plant.

Testing the batch:

1) Slump test. This is done every half an hour for three hours. Concrete is filled into a conical mould and then the mould is removed so the mixture spreads into a heap. The change in height of the heap is the slump.

2) Bleed test. A cyclindrical container is filled with the concrete mix in five layers – between each layer a vibrating tool is used to ‘tamp’ the mixture and reduce air bubbles. In general, the way the concrete firms up is based on the water separating from the heavier particles over time until enough stiffness is achieved. The amount of water that forms at the top of the cylinder is the bleed; a lid prevents evaporated water from escaping from the test. The use of high proportions of GGBS results in a longer bleed time that conventional concrete but this can be reduced with fine aggregates (<50 micrometres). When bleed occurs on site, either the bleed water is remixed into the concrete (which may result in a weak top surface) or one can wait for the bleed water to evaporate. If the rate of evaporation is faster than the rate of bleed (e.g. on a very hot day), the concrete may experience plastic shrinkage, which is undesirable and should be avoided.

3) Cube tests. 5cm cubes are filled with the mix. They are tested, usually with a hydraulic machine, on days 1-7, day 14, 28, 56 and 96 for failure strength. Concrete gets stronger over time and the 28-day strength is usually the quoted value; the material must reach its target strength (here, 50 MPa) by that date to be acceptable.

4) Weight test. This is just a way of obtaining the concrete density, which is a vital parameter to consider for safe structural concrete design.

5) Beam test. A 3-point test (simple supports at the ends, and a point force applied at the middle) is taken on an unreinforced beam of concrete at 28 days. This tests the bending strength of the mix.

St Mary Aldermary, City of London

By girlbridgingthegap | 21 July, 2023

This church was rebuilt after the Great Fire of London when it was severely damaged. For Wren, the Gothic style makes this one of his more unique designs. I was particularly intrigued by the tower, which retains the blockiness of many of Wren’s churches yet is mellowed out by the classic Gothic arches of the windows and octagonal turrets. I’ve chosen to sketch that here.

St Mary le Bow, City of London

By girlbridgingthegap | 21 July, 2023

Exquisite stained glass is the centrepiece on entering. I appreciate the little circular windows towards the top of the facade; they make you feel like you’re on a ship. They let light in, but their height makes it impossible to see out – reminding worshippers of their smallness. In religious terms, this perhaps indicates the almighty power of God, and to non-religious observers like me, the power of the planet and the hugeness of the universe.

From a construction perspective, the main advantage of such a window placement is that the glass does not have to hold any weight – this structure would hold up just fine even if the glass were damaged – the stone walls and arched ceiling form the essential structural elements. Modern glass-facaded buildings use a ‘curtain wall technique’, where again the glass doesn’t hold any weight, but instead of a massive stone outer structure, the building is supported by internal columns, beams and braces made of steel and reinforced concrete. The disadvantage being, of course, the loss of an unbroken internal space, a key component in many of Wren’s churches.

*^ the tower from the outside; on the right, the facade with the little circular windows*

St Lawrence Jewry, City of London

By girlbridgingthegap | 21 July, 2023

Missable from the street – you’d easily wander past this one. But tucked away behind the buzz of the City, the people and their pints and their suits, stands a huge expanse of courtyard. Empty and flat, the pale stone transports us to southern Europe; it feels almost Milanese. Yet the roughness of the church bricks brings in that earthy British tone. There the church stands, spire up into the sky on this strange island of peace in inner London.

St Lawrence Jewry has a very familiar style – very much a main block with a sloping roof, with a tower about double the height, also rectangular, on one end. Unfortunately I was unable to go inside this church to examine the structure in more detail.

The church was actually rebuilt after extensive damage during the Blitz, but the architect Cecil Brown stuck to Wren’s initial design.

^ the view into the courtyard; the facade of St Lawrence Jewry is the building on the very right and in the centre is Guildhall

On the other side of the courtyard stands another church-like building, which is actually Guildhall, the HQ for the City of London Corporation.

Chelsea Royal Hospital, Chelsea

By girlbridgingthegap | 10 July, 2023

Today, this complex acts as a retirement and care home, but it has housed a range of other groups including ware veterans in the past. It is generally regarded as a luxury home for the mostly upper classes, and only admitted women in 2009.

The courtyard reminded me most of an Oxford college – in particular the Queen’s college – with its distinctive symetrical windows and neatly kept grass.

I decided to use two-point perspective as a sketching technique for this because it felt appropriate to the building scale and shape.

*^ Chelsea Royal Hospital in two-point perspective*

^ *Queen’s College, Oxford, for reference*

In more detail, I did a close-up sketch of the entrance in the middle of the courtyard. The neoclassical influence is very distinctive here.

Creation of a simple beam bridge for road traffic

By girlbridgingthegap | 5 September, 2022

A beam bridge is the simplest type of bridge, consisting of a deck resting on vertical columns. Despite their simplistic physics and appearance, the construction of such a structure in the modern world takes more than just resting a plank on a column. I’ve made a short video showing an example of the construction process.

The main materials of interest are:

Reinforced soil. This is a specialist technique whereby the nearby soil is made stronger by the addition of a grid of a different material – usually metal or composite layers. This is a vital process in weak soils and is usually carried out by a specialist subcontractor on a big project.

U beams. The deck consists of these concrete beams, whose cross section is a U shape. This has a large second moment of area (see below for calculation), making the deck very resistant to torsional and bending stresses. The beams tend to be large enough that construction workers are able to stand inside them to attach the next layers of material.

Temporary works scaffolding. Usually consisting of metal bars which can be assembled and disassembled quickly, temporary works structures are needed to prop the structure up before all structural elements are completely secure.

Rebar. Rebar is a type of steel shaped into rods of about 2cm diameter. When worked into a cage shape and inserted during the concrete pour, the resulting ‘reinforced’ concrete, is much stronger in tension that its raw counterpart. This is because concrete tends to be strong in compression but not tension; using pure steel would be way too heavy, not to mention way too expensive. Combining these properties creates an excellent new material which is used in all large modern infrastructure projects.

Permadec plastic panelling. This is a very strong overlay material that is manufactured by a specialist company. It contains a shell of fibreglass with steel strips inside and is placed on top of the U-beams.

Composite top layer, consisting of some of the materials described above. This diagram is an expansion of the diagram of the U-beams above. It is a zoomed in version of the green box.

Calculation of the U-beam second moment of area:

We have to split the cross section up, calculate the value of I about the centroid of each section, then use the parallel axis theorem to find the total I about the centre of the section.

Rural alpine natural water system

By girlbridgingthegap | 15 July, 2022

I visited a low-tech mountain hut during a walking tour in the Alps in France. At 1500m elevation, it can only be accessed via cable car and then a mountain walk, and it is isolated from both electricity and water networks. This means the only power comes from gas, used for cooking, and wood, burned for heating. Water must be obtained from a nearby stream, yet I found the system to be surprisingly sophisticated.

To check it out, I walked about a hundred metres from where the hut stands, to locate the stream. Basically, the system consists of two plastic tanks, each holding about 100 litres of water, sitting in the stream. Water enters the first tank directly from the stream – it is fresh as it has come straight from the glacier higher up. This tank contains a filter to catch any sediment; at present, this is improvised with an old stocking.

The water that passes through the filter is led via a plastic hose to the second tank, which is a couple of metres down into the valley and is a storage tank: when supply exceeds demand, some water can be held here until it is needed on drier days. An overspill pipe attached to the top allows water to run safely back into the stream if the tank is full.

The main pipe leads from the storage tank, downhill to the hut, where it can be used cold or heated with gas.

Some calculations for quantifying the system

We can examine two situations; the first, where the second is not in overspill so nothing flows through location 6; the second where it does. Bernoulli’s equation is useful here. It is an energy balance equation and states that [pressure + kinetic energy + gravitational potential energy] = constant in a flow system, as long as the mass flow rate is constant (there are a couple more technicalities, but I won’t dwell on them here). Bernoull’s equation allows us to consider only start and end point, ignoring what happens in between.

Where p is pressure, rho is density, v is velocity, g is gravitational acceleration and h is height from a fixed point.

Also, with a constant mass flow rate (and incompressible fluid) we can assume:

where A is pipe area.

Situation 1: At both 5 and 1, the density of the water is the same and the area of the pipe is the same. Therefore the velocity of the water entering the house is the same as the velocity of the water from the stream. To calculate the pressure change, we can cancel the second term in Bernoulli’s equation (since velocity and density don’t change), giving p1 + (rho)(g)(h1) = p2 + (rho)(g)(h2). Rearranging gives p2 = p1 – (rho)(g)(h1-h2).

Sticking into this that p1 = air pressure = 1 bar = 100kPa and h1-h2 = 30, p2 is 3.94 bar.

French alpine viaducts – the old and the new

By girlbridgingthegap | 11 July, 2022

The A40 motorway in southern France is a busy highway through the mountains that provides the most direct route from Geneva to the mountain resort of Chamonix and surrounding towns. Together with a railways along a similar route, they are used all year round by both French and international tourists and local people; the area is famous for skiing, mountain hiking, climbing and local French culture – so the roads and railways along this route are essential.

Two prominent and impressive viaducts caught my eye as I drove down the motorway. For each, I sketched a front-on, fine-lined shape and a more visual sketch of what the bridges actually look like when you’re driving on the road.

First, a modern, slender concrete structure – the Viaduc des Egratz de Passy. This one is part of the westbound A40 motorway.

^ see the column cross sections at the bottom of the drawing

Viaduc des Egratz de Passy, 1981 (road)

The (presumably reinforced) concrete posts are generally rectangular, with their short side aligned with the length of the motorway, with one exception. The column on the right of the drawing above is hexagonal instead – the reason is not clear, but it may be required because of the harsh bend at that point on the structure.
The deck does not appear to be simply fixed straight on to the columns – at a glance, it looks like it is levitating slightly. This is probably because of the damping system between the deck and the columns. Allowing some small damped rocking, rather than rigidly fixing the two together, helps the structure deal with the vibrations of the road without sudden plastic collapse or fast fracture of the joints.

Second, a more traditional, heavier-weight arch design – clearly from a much older era – the Viaduc de Saint-Marie.

Viaduc de Saint-Marie, opening date unclear (rail)

Straight, sturdy columns form the bottom section. Semi-circular arches have been utilised for structural stability in the top section.
The main material is masonry – probably stone masonry by observation.
Arch bridges are excellent at dealing with the continuous vibrations of railway traffic without the need for external damping systems (which were likley not developed at the time of construction).
The project would have been advanced for its time, fitting in with such an uncertain landscape – not to mention massively expensive as it would have been build by human power alone. Impressive!