I was recently reminded of a trip to Las Vegas in 2010, which included the Grand Canyon and, specifically, the Skywalk. This is a glass horseshoe walkway that cantilevers over the canyon and is, quite frankly, terrifying. On the furthest extreme of the structure I found myself next to an American woman, both of us clinging ferociously to the handrail, firmly believing that our lives depended upon this. She was terrified that the while thing would collapse and I was convinced that some supernatural force was about to pick me up and hurl me into the void. In both cases, clinging to the handrail was fundamentally futile.

Fortunately, I was able to explain to her that the building that we had just walked through was acting as a counter balance to the cantilever. For instance, if you pile enough bricks on one end of a scaffold plank you can stand confidently on the other end, even if it is over a swimming pool. The building provided the kentledge (for that is what it is called) for the pair of steel beams to stick out over the edge and form a parapet to the walkway.

By the time I had finished there was a small audience of tourists who were marginally reassured by knowing how the thing works, demonstrating that structural engineering can perform a social service for passing civilians.

Sadly, they were absolutely useless at assuring me that a mystical animal – condor, roc or dragon, maybe – was not going to toss me over the side as sport and/or food for its young. I am only here to tell the tale because I bravely closed my eyes, clung to the handrail and made my way back into the building in small jerky movements.

If anyone is in the slightest bit interested, it was my birthday last weekend. In celebration, a bunch of us went to Newcastle, mainly because I had never visited the city before, but mainly because I wanted to have a look at the Gateshead Milllennium Bridge.

The Architects for the bridge were Wilkinson Eyre who were charged with certain parameters. The footbridge had to go up to a certain height above the water, but the width of the river meant that the rise would be too steep. The solution was to extend the length of the footbridge by extending it down river. This left the problem of how to support a structure that is curved both on plan and on elevation. It is probably well known now, but the solution came to the Architects when one saw the other blink, resulting in the nickname the Winking Eye Bridge.

That’s not the end of it. The bridge also had to open to let shipping through, so the two arches rotate, also giving the impression of an eye. But the centre of gravity of the bridge moves across the axis of rotation as it goes up, meaning the pumps have to push for one section of the lift and pull for the other. This is then reversed when the bridge returns to its original position.

All in all a thing of beauty that did not disappoint when seen in the flesh, so to speak.

Still, I could have done that!

There’s a large housing estate being constructed to the south of Leighton Buzzard on what started off as a giant sand quarry, then became a giant infill site. So essentially the houses are constructed on a very deep layer of fill.

From an engineering point of view, the solution is relatively straightforward, if a little heavy handed and relatively expensive for a housing project. Reinforced concrete piles penetrate through the fill down to the virgin material, sometimes up to 20m deep. The piles work in two ways. Firstly the bottom acts like the legs of a table sitting on the floor (end bearing) but the fill material itself grips the shaft of the pile (skin friction). If there is sufficient skin friction, the end bearing is not required and the piles can be shorter.

But the design has to ignore the material immediately below the footings, and pretend that they span between the piles, usually 3m to 4m, but sometimes longer. This means reinforced concrete ground beams are needed to support the walls.

But what happens if the householder wants to add a conservatory. It’s all very well for the developer to install hundreds of piles for a large number of houses, but it is usually uneconomic to do this for a conservatory. There are such things as mini-piles for difficult ground conditions, but they are not appropriate for these sort of depths.

The solution is a reinforced concrete raft, about 300mm to 400mm deep. This foundation and conservatory act much like a cup and saucer. If the ground settles a bit, the raft tilts a bit and takes the conservatory without distorting it. This sort of solution is ideal for, say, a detached garage, but conservatories usually butt up against the house, so provision must be made for differential movement between the two different structures.

It can be that this solution is a bit more expensive than the “traditional” solution, but the oncost is relatively modest; usually a small percentage of the total cost

The government launched a campaign a couple of weeks ago to get rid of what is perceived to be unnecessary legislation. Along with many other people, there was a sigh of relief from Rawlings Towers at the thought that, at least, some of the red tape burden would be lifted. It shouldbe remembered, thought, that some of the legislation has a purpose.

A repeating Client of mine is a central London hotel, which has a ballroom, complete with stage and lighting bar. As something that holds up stage lights, this piece of kit falls under the Performance and Use of Working Equipment Regulations 1998 (PUWER98). Recently, there was an aspiration for an event that involved silk artists; scantily clad young things who interpret the complete works of Mozart in a meaningful fashion by folding themselves up in swathes of silk suspended from the ceiling. This then brings the same piece of kit under the Lift Operations and Lifting Equipment Regulations 1998 (LOLER98).

Under this second lot of regulations there is a requirement to publish a Safe Working Load (SWL) and this has to be determined by testing rather than calculation. The testing is relatively straightforward; simply attach a bracket to the threaded rod and jack against the roof and measure using strain gauges. This requires the removal of the ceiling, as to jack against the ceiling would cause the finish to compress and register a false reading. Except that the room in question is Listed, and so there was an investigation into whether or not Listed Building Consent was required for such testing work. Apparently not, we found out some time later.

And so the great day arrives when the ceiling is to be removed. It was found that the plaster is attached to expanded metal lathing, which in turn spans between small steel channels that are fixed to the soffit of the slab, creating a small gap between the structure and the ceiling. We also discovered that the bar was screwed to a piece of plywood that was simply resting on the expanded metal lathing; not attached to anything.

No doubt the safe working load test would have revealed a fault in the construction anyway, but if it had not been for the requirements of the regulations there is a distinct possibility that the silk artist would have ended up as someone’s table decoration. I am no great fan of over regulation, but on this occasion they did exactly what they were designed to do.

I was hobnobbing with the toffs a couple of weekends ago.

Out in the pound note area of the Buckinghamshire countryside is a rambling complex consisting of barns, houses, fields and forest over about 60 acres. The buildings range from 1930’s back many hundreds of years, featuring just about every example of barn construction that you can imagine. This is the home a Music Camp. Musicians arrive; pitch tents; eat, sleep and generally socialise in a sort of communal way; and play music. Once a year they have a summer ball and I squeezed into the dinner suit and attended the ball and was royally entertained by a swiftly put together big band playing all sorts of jazz numbers.

After what the organisers called an “interval for sleep”, we reconvened for brunch and were let loose to wander the grounds at will. In almost every barn, outhouse or awning there were a small collection of musicians playing chamber music of one kind or another. Even a walk in the woods was accompanied by a solo flautist producing magical noise. It would not have been at all surprising if it had turned out he had goats legs, or a Fairy Godmother appeared in answer to his summons.

Part of the afternoon’s entertainment included a brief tour of the grounds in the company of the owner, where I had the opportunity to resolve a couple of building issues that had been foxing him for some while.

I’m not above doing this for anybody, really. It often requires an informal assessment to allow the building owner to determine the best course of action, and I am usually content to perform this signposting exercise without necessarily charging a fee from the outset.

Of course, laying on an event over two days, involving top notch musicians in sumptuous surroundings to receive this informal advice is not absolutely necessary, but I won’t turn it down.

They say that it’s good to scare yourself every now and then, and I managed this spectacularly on Tuesday last week. A regular commercial client has a 40 year old building with concrete cladding, and it could be seen that some of them were deteriorating. A survey from close quarters was needed and, as the building is four storeys high and surrounded by plant, landscaping and car parking, the only solution was a cherry picker.

It’s all very well being told that the machine is perfectly safe, as well as wearing a five point harness that attached me to the basket, but it’s quite a different thing to actually get in one and fly over low lying things and up four storeys.

Frankly, the first half an hour was somewhat scary, but it is surprising how quickly one gets used to the conditions. By the time we got to the second elevation I was getting quite cocky…..until we got to the corner. As we slid sideways and I could see straight down the next elevation, I was reminded how high up we were and had a small wobble.

Soon recovered though, and managed to exceed the work programmed for the day, cutting the number of required visits down from four to two, with the resultant saving.

The BBC have published an article on the number of ghost estates in Ireland. These are housing developments where the houses are empty. http://news.bbc.co.uk/1/hi/world/europe/8653949.stm 

It seems that there are 621 of these estates; approximately one in every five house in Ireland is unoccupied. By way of comparison, they say that the number of unoccupied houses in the UK is 1 in 32. According to the 2001 census there were 24.14 million households in the UK and the Office for National Statistics projected that this number would rise to 25.3 million in 2006 and 26.2 million in 2011. Given the current economic climate, let’s go with the 2006 figure. At a rate of 1 in 32, that means the UK has over 800,000 empty houses.

So why were told a few years ago that tens of thousands of houses had to be built to house our ever increasing population? I recall saying at the time that I haven’t seen too many families wandering the streets looking for somewhere to live. I was told it was because many children couldn’t afford starter homes; that there were a large number of immigrants from the new European states; and that the number of break ups and divorces meant that there was pressure for a lot more houses.

It is understandable that the banking crisis has meant that funding for developers has dried up and that construction on some of the large estates that surround Leighton Buzzard, Aylesbury and Milton Keynes has come to a halt. But where have all these people gone? I’m sure many economic migrants have gone home, or elsewhere, but where are the rest of them?

The BBC report states that if all of the people in Ireland’s social housing list were housed, there would still be hundreds of thousands of houses left empty. They ask, who was supposed to live in these ghost estates, even if there was not an economic downturn. You would therefore think that over 800,000 empty houses would put a fair dent in the UK’s socal housing list.

Don’t get me wrong. If someone wants houses built, I’ll design them. But I’m still puzzled. On what basis was it decided that all of these houses were needed? Not just put up as a speculative developments for profit, needed . And if they were needed before the construction industry slowed down, where are all the people they were being built for?

I think we should be told.

Spent an enjoyable hour or so taking a look around the MK Dons stadium and the Doubletree hotel attached to it. Not only is the structure interesting (large cantilevered roof and large beams supporting precast concrete units as terracing) the design and ambition of the whole campus is really impressive.

The pitch is 8m below the surrounding ground level, which means that the public enter the stadium at the top of the lowest tier (and only tier, at the moment) and can circulate the entire stadium at that level, where there are concessions and bars. It currently holds 22,000, but the middle tier will bring it up to 44,000 and if the FIFA World Cup comes to England in 2018, then the top tier can be installed bringing it up to 66,000 or so.

The ambition to host some matches in the World Cup did cause a bit of sniggering, but this is a UEFA Elite class stadium, with excellent infrastructure, so it is sure to attract some high profile matches in the future.

The website of MK Dons is www.mkdons.com

I don’t recall signing a non-disclosure agreement, but it’s probably inappropriate for me to say too much about the nuts and bolts of my involvement, if for no other reason than there are dozens of companies and government agencies involved and I don’t know what is politically sensitive. Suffice to say that I was called in at short notice to offer advice and design for some temporary works that are being used to install electrical equipment in one of the arenas.

Rawlings Structural Design were called in by a repeat Client at very short notice because of contractual problems elsewhere. As a Sole Practitioner it is important that the office programme has built-in gaps to accommodate such issues. Whether it be an issue at the Olympic Park, a local refurbishment or a domestic extension, unexpected issues always raise their heads and, when there is work proceeding on site, this has to take priority. If time is not allowed for these events in the programme, it would soon be a seven day a week 24 hour a day office. It only feels like that, at the moment.

Security at the Olympic Park is airport quality, and the development itself is fascinating. To see one stadium go up is fun, but to see four or five being developed at the same time and within sight of each other is really exciting. It is possible to see an entire community taken shape as accommodation, hotels and support buildings are going up at the same time as the main arenas. The infrastructure is pretty much in place (all though awaiting final tittivation). Roads, walkways, bus services and sign posts are already there is a small town emerges out of the backlands of east London.

So what was a Sole Practitioner doing on such a large site? In truth, it doesn’t matter how large the project is, when it comes to putting pen to paper, turning a spanner or offering advice, personal relationships are of utmost importance.  And it was those personal relationships that allowed a difficult situation to be resolved rapidly and amicably.

Keith

The landslides in Brazil over the last day or so are an indication of the consequence of a build up of water within the soil.

The friction between the granules in sand, or the platelets in clay, create the “angle of repose”. If you emptied a bag of sugar onto the table it would form a cone, the angle of which would be the angle of repose, typically 30 to 40 degs. Amongst the factors that change the angle of repose is the water content of the soil as it lubricates the elements within the soil.

If you were feeling particularly messy, you could get several bags of sugar and add water to them in increasing amounts and pour them onto the table, noting the angle of repose getting more and more shallow. Eventually the sugar will become saturated, flow across the table and drip onto the floor where it will be licked up by the dog.

A similar mechanism occurred in Rio de Janeiro, and periodically occurs in this country. When the rainfall becomes so heavy that the ground water has no chance to disperse, the soil becomes increasingly saturated until all internal friction is gone. When this soil is the side of a hill a mudslide occurs. The forces generated under such conditions are immense, which is why structures originally designed to retain the slope have no chance of doing so.

It is rare for Structural Engineers to have to design for such large forces, but due cogniscance is often taken of the possibility. In commercial situation, a site investigation will reveal the necessary parameters to design appropriate structures to resist most conditions, whether this is bored piles beneath a building or a horribly expensive retaining wall.

On a much smaller scale, the same mechanism can cause problems in residential garden retaining walls. These animals are relatively cheap to build, so a site investigation (or a Structural Engineer’s fees – bah!) is often considered uneconomic. Such walls often survive for decades (many Victorian examples still stand) and if they do decide to start giving up, there is usually plenty of warning in the shape of horizontal displacement or cracking.

Many builders overlook the need to allow the water behind the retaining wall to escape. In normal conditions, the water disperses naturally, but in unusually heavy rain the water pressure builds up behind the wall.

There are proprietary products that can be used when it is known the water table is a problem, but usually simply leaving the vertical mortar joints (perpends) open or installing small plastic tubes within the wall is enough to allow the water to percolate the wall and disperse.

The photo was taken from the BBC website at http://news.bbc.co.uk/1/hi/world/americas/8605386.stm

Keith