I was invited on Wednesday to go and help wobble the Passerelle Simone de Beauvoir (previous posts here and here). The wobbling was being sollicted in order to conduct ongoing tests on the bridge’s dampers. The tests were being conducted by the CSTB (France’s centre for building science, where I almost ended up doing my projet de fin d’études).
Bridges such as this one, and infamously, London’s Millenium Bridge, are susceptible to wobbling caused by the excitation of one of the bridge’s natural frequencies by the pedestrians who use it. As well as forcing the bridge deck up and down with their footsteps, pedestrians also exert a sideways force as they alternatively plant their left and right feet on the deck. This sideways movement is of a similar frequency to the transverse vibrational mode of lightweight bridges such as this one and the Millenium Bridge. When a bridge does start to shake noticeably, there is a tendency to ‘lock-in’ whereby pedestrians synchronise their steps with the vibration in order to stabilise themselves, but in doing so, give more energy to the vibration. The first time that this lock-in phenomenon was observed was at the opening of the Millenium Bridge.
This sort of vibration is unlikely to cause any damage to the bridge itself but it does make the people onboard feel quite uncomfortable. It is therefore an issue of serviceability. In order to reduce its effects, such bridges are installed with tuned dampers designed specifically to damp out these effects. And in order to check if these dampers are working or not, it takes a group of fifty or so enthusiasts (usually engineers) to jump up and down to see just how much they can get the thing to wobble. I tell you, we got some funny looks from passers by…
I am correcting a document that has been translated from French into English and I have hit upon a term that keeps cropping up, and I simply don’t know what it is in English.
In French the term is ‘Paroi Berlinoise’. I always see it translated as ‘Berlin Wall’. In my (admittedly limited) experience as an engineer, I have never heard of a ‘Berlin Wall’ unless it is a type of reinforced concrete structure with graffiti on one side that typically has a design life of, say, forty years.
I don’t think this is what this document is talking about. Can anyone help? (It maybe useful to know that ‘paroi’ is a word tpyically associated with perimeter foundation walls; for example, a ‘paroi moulée’ translates as diaphragm wall)
At the beginning of the week, the architects for the project that I am working on flew into town for an intensive week of meetings. Most of yesterday was spent shuttling back and forth between our offices and La Défense for meetings about the building’s structure. For me it was a great chance to get to know the project team before the video conferencing kicks off in ernest (despite Margaret Atwood’s invention that allows her to do book signings wherever she wants in the world from the comfort of her own home, it is still not possible to shake hands over the internet). For the moment there are still a number of questions to answer about the building’s facade but once those are answered there will be a rush to design the floor which, for the moment, is where my project is going to be focused. I therefore have the sense that we are in the calm before the storm.
During a coffee break, I tried to strike up a non-engineering/architecture conversation with the architects. Struggling fora topic, I suddenly remembered that my favourite US online radio station, KCRW, is broadcast from the same town as their headquarters. They listen to my favourite show in their office on the otherside of the world, every morning. Doesn’t the internet make the world small?
In other news, old calculations that I had made on the cost of another tower have come back to haunt me. It is not that they were wrong, it is just that I was suddenly required to present my results without any notice. I was therefore glad that I had left a decent paper trail so that I could quickly see how I came to the result two weeks ago. This is basically thanks to my new strategy: to date absolutely everything, to put the date in the name and print it in the header and to include a table of modifications for each time I use a calculation sheet. This may all seem obvious now but it wasn’t when I started off. I have since been asked to carry out a cost calculation on the tower that is the focus of my project. Since cost will be an important part of the choice of floor design, I will be able to tie the overall cost calculation into my project. And now that I have the method sorted, it hopefully shouldn’t take too long to calculate. The only trouble is… none of the floors are identical…
+ one for those who moan about London Underground – spare a thought for those who ride the L in Chicago – from a new blog find: Anonymous 1%
Tube Challenge
I always dreamed of doing this when I was a lonely and boring teenager. All the tube stations in one day. Thanks to Mary for sending me this link from Jon’s blog. I will add this to the reasons to move back to London list, a list that I hope will soften the blow of leaving Paris in the autumn.
Project update
This is just the briefest of project updates. Things have been super busy recently in the office. Most of last week was spent researching how different types of floor structure vibrate when people walk across them. Unfortunately, the classical mechanical methods that we have been taught are not very useful for the design of office buildings as the calculations quickly become unwieldy and unreliable. Instead, the literature in this area gives empirically derived formulas for checking for excessive vibrations. The problem with these quite specific methods is that it is difficult to see how applicable they are across the board to other types of flooring. In the next few days I will be talking to the manufacturers of various different flooring systems to find out which is best suited to the building that I am working on. They will provide their own methods for checking for vibrations, but it would be nice if I didn’t have to rely on them to provide the method with which I will be testing their theory!
I have also been a little more involved with preparing material for meetings with the architects for whom, being American, it is useful to prepare stuff in English. All of a sudden, from languishing on the sidelines, I have been thrust into the middle of it all. That’s fine with me!
Filed under: Engineering
Interesting comment on Geek Buffet about a make-over for engineers in the States. I have tried to add a British and French take on things.
Livic, the civil engineering newspaper of Imperial College, is now three years old. The fourteenth edition has just been published and I have to say it is the best one yet. With this edition, current editor Andrew Kosinski’s last one of the year, it is clear that it is really starting to achieve the things that I always hoped it would.
Inspired by the student newspapers that I had seen in the States, I stood in 2004 for the CivSoc post of Livic editor. At the time, the paper was but a biannual sheet of A3 paper stuck on the department wall, nothing more. (The name Livic comes from Civil spelled backwards – a previous incarnation had apparently been called ‘Concrete’ – catchy huh?) My hope was to turn Livic into a regular student newspaper much like those that I had seen abroad. Kosinski has been onboard since the beginning, realising on paper what had previously only been an idea.
I had several goals in mind when starting out. The first was to encourage student writing. It had struck me that there were precious few creative outlets at Imperial and so I hoped to add at least one to that impovrished list. The second was to encourage staff contributions, and in doing so, improve communication within the department. I had the impression at the time that there was little awareness of the research that went on in the department, and I thought that Livic could help. Finally, a slick looking paper with a broad readership, it was hoped, would attract advertising from industry which might at the very least have paid for printing, and more ambitiously, go a little way towards boosting CivSoc’s budget.
In that first year, we made some progress towards reaching those goals. For starters, some forty students contributed articles on a wide range of subjects. What was difficult was trying to get reporters to write articles that went into any depth. I seem to remember there being some staff writing, but calls for articles often went unheard, or weren’t followed up. We did manage to break a couple of important departmental news stories (the Creative Resign article being one memorable example), but these were by no means exclusives.
By comparison, the Livic of today has come a long way. The articles are much more in depth and they cover a wide range of topics. An important story about the future of the course is on the front page and inside there are staff contributions as well a revealing interview with a lecturer. All in all it is cracking read! It is also interesting to see how the layout has changed with time. It keeps getting slicker. I am certain the Arup were more than happy to place an advert I such a classy publication.
When I was editor, sure I had ideas, but I didn’t have the first clue about how to realise them on paper. Luckily there was Kosinski who did. Both the subsequent editors, Alex Morris last year and Kosinski this year, have not only had the ideas but have also had the skills necessary to assemble the paper on the screen. And I think it is these two skills combined with a desire to say something and knowing how to say it, that are now pre-requisites of a Livic editor, a role which three years ago was somewhat of a joke position on the committee.
And so what of the future? As of next year, the first three editors will all have left the department. There is always the fear that one day Livic will fizzle out for lack of enthusiasm, and it does take enthusiasm to get something like this out of the door. But with fourteen issues in the bag, Livic has now got momentum. Elections have just been held for the post of next year’s editor. I wish him or her luck and I look forward to seeing Livic’s continuing evolution.
Access Livic online here.
Today I started getting into the nitty gritty of how to stop a floor from vibrating. When dimensioning the floor slab of a building, one of the considerations is to check whether the natural frequency of the floor is in the same frequency range as that for footsteps. If the two frequencies do coincide the latter could resonate with the first causing the floor to shake.
Today I have been looking at an American document that brings together the different ways of estimation this interaction. Most of the results are based on empirical evidence of what seems to work. This lack of rigour is fine with me, and is common in engineering. The thing which has really held me up is the units: all the calculations are in pounds, feet and inches! Could there be a more unhelpful system of measurement?
Curious about this archaic standard, I started hunting around on Wikipdedia and found a wealth of information on the origins of both systems. Apparently, the only countries still to use imperial units of measurement are the USA, Liberia and Myanmar, although, I might add that here in the office my colleagues were surprised to here that in the UK we also use the metric system.
I urge anyone who is similarly disposed towards the imperial system, to use this site to help them out.
Last night Lorenzo (a fellow engineer from work) and I blagged our way into the opening of the permanent exhibition at the newly refurbished Cité National de l’architecture et du patrimoine, France’s national architecture museum. Neither of us had thought to check if this was and invite only event and indeed, it was. Luckily, a few charming smiles and a couple invitations made themselves available from the large group loitering outside the front door and then we were in.
The Cité is housed in one of the wings of the Palais de Chaillot, that splendidly curving art deco building opposite the Eiffel Tower on the other side of the Seine. Unlike any other architecture exhibit that I have visited, this one had not one model of a building. Instead, the main exhibition space, itself a long and wide curving corridor, was filled with fifty odd floor-to-ceiling screens onto which a series of short films were being projected. Every film was about a project, but every film had been shot differently: some showed in speeded-up time a building going up, others showed people flowing in and out, the building through the different seasons. There were images taken from satellites that showed whole areas being redeveloped. My favourite was a series of photos taken from a balcony, of an American city skyline. The photos start in the 60s and go on, lets say one a month until the present. As the images tick pass, the downtown skyscrapers grow like mushrooms after a rainstorm. One by one the pop up out of a hole in the ground, until eventually, one pops up right in front of the balcony and the view is completely obscured.
It is not just buildings that are showcased. One video was taken from a car driving across the Milau Viaduct. Another, from a helicopter flying over an offshore wind farm.
I think that the exhibition rather successfully shows the dynamic side to buildings. How they change, during their lifecycle, fro, construction, to use, to decay, to demolition and also how people interact with them. None of these aspects are static and so the moving image is an ideal medium for communicating them. My one criticism of the exhibit is that in the dark room where the videos are projected, it is difficult to read the programme that tells you what the projects are. Maybe you are just supposed to know already. I wonder?
The free lemony champagne was worth the effort it was to get through crowds of people in order to see the exhibit. It is hardly surprising that on the opening night I saw a lot of architects and not a lot of architecture. I shall therefore definitely be making another visit before too long.
Filed under: Final Year Porject
After much rudderlessness, it seems I now have something fairly conretely (haha!) defined subject for my Projet de Fin d’Etudes. Up until yesterday, I had been increasingly frustrated at my lack of progress. I just seemed to be doing calculations while newer ’stagiaires’ were wading deep into reading material for their projects. So this morning, with a little more determination I brought the subject up with the powers that be, and this time it was with success.
It now looks like the bulk of my project will be a study of the floor design for an innovative new tower. As well as dynamics calculations, it will involve plenty of discussion with the architect and the other engineers involved, which suits me. I am happy because, though complicated, it is a well defined subject. I too now have piles of documents to wade knee-deep into.
Despite this sudden change of course, I do not regard the work that I have done so far to have been a waste. If anything it has been a good intorduction to working in the company. The time spent thinking about how best to decompose the problems I have solved will not have been wasted as, and I said this in a recent blog post, that my method should be applicable to a wider range of problems and, if not, at the very least, next time in my career when I have to work out the cost of a building.
The other morning I found myself in northwestern central Paris around the Gare St.Lazare. This is not my normal stomping ground, so I took the time to go and have a look at the striking curved-glass metro entrance that was built as for Paris’ newest metro line, the driverless 14.
The architects and the engineers on the project were Arte Charpentier and RFR respectively.
The glass has a double curvature: that is to say, like a dome or the saddle for a horse, the glass curves in two different directions. The lateral stability of the structure is assured by the fine metal cross-bracing that can be spanning diagonally across the frames. The frames are in stainless steel, a material that, thanks to its many different crystal faces, reflects light from many different aspects.
Unfortunately I didn’t have the time to have a closer look at how the various elements are joined, which is always the devil in projects such as this where transparency is the goal.
You can see a slideshow of photos of the Gare St.Lazare station entrance from my engineering photo site by clicking here.
