Project #tweetprop: There is still life in "dead" slabs

The third proposition of my dissertation is the following:

The very large redistribution capacity of slabs is demonstrated, amongst others, by carrying out experiments on severely damaged and locally failed specimens that led, on average, to a capacity of about 80% of a virgin specimen.

or in Dutch:

Het zeer grote vermogen tot herverdeling van platen is onder andere aangetoond door proeven uit te voeren op zwaar beschadigde en lokaal bezweken laten, waarbij gemiddeld een capaciteit van ongeveer 80% van een onbeschadigd proefstuk gehaald werd.

This proposition is based on a story.

Initially, we were planning to do two or four tests per slab. For testing with the concentrated load in the middle of the width, we were planning two tests per slab: one at each support. For testing with the concentrated load near the edge, we were planning four tests: two close to every support (one east, and one west).

As we were testing our first slab, with loads in the middle of the width, we suddenly grew curious. What if we would test this completely damaged slab with the concentrated load near the edge? So, we wouldn't be loading exactly at the location of were the slab already failed completely, but at less than 1m away from the failed part.

And if I say "failed", I mean huge cracks. Like this case here, our cracks were 2cm wide, I could almost stick my fingers into it.



We didn't expect much from this experiment. We didn't plan to mark cracks, we didn't plan for load steps. We assumed we'd reach failure before 200kN (which would be the first load step in an experiment on a virgin specimen).

We were so wrong.

The load kept on increasing, and I started slowly moving towards the exit of the lab, as I thought we'd be seeing some extremely explosive failure, with all that energy held within the specimen.

We reached a large failure load, and the failure mode was very similar to an experiment on a virgin specimen - much to our surprise.

For that reason, we started to do 6 experiments per slab: either 2 "virgin" (or "uncracked") ones in the middle, and then 4 "cracked" ones at the edges, or the other way around.



This testing sequence meant of course more experiments, but also more data... and we used the comparison between the "uncracked" and "cracked" tests to see the effect of redistribution in slabs: we saw experimentally that the "cracked" tests had on average 80% of the capacity of an "uncracked" test.

In beams, the least bit of cracking might have a negative influence on the shear capacity, as we learned from experiments on beams that were also done in Delft [1].

Slabs, on the other hand, have the ability to redistribute forces. Even a local failure can't stop them from doing so. And that -again- is a reason why slabs behave structurally very different in shear than beams.

[1] Yang, Y., 2011, "Report of Experimental Research on Shear Capacity of Beams Close to Intermediate Supports," V. Stevinrapport 25.5-11-10, Delft University of Technology, the Netherlands, 58 pp.

PhD Talk for AcademicTransfer: Finding Housing in the Netherlands when Coming from Abroad

This post is part of the series PhD Talk for AcademicTransfer: posts written for the Dutch academic career network AcademicTransfer, your go-to resource for all research positions in the Netherlands.

These posts are sponsored by AcademicTransfer, and tailored to those of you interested in pursuing a research position in the Netherlands.

If these posts raise your interest in working as a researcher in the Netherlands, even better - and feel free to fire away any questions you might have on this topic!

For many students and employees who move to the Netherlands, finding housing can be a serious challenge. Housing was something that kept me awake at night during my first months in Delft. The key to finding a suitable place to live, is just giving yourself enough time to look around.

If you are coming to the Netherlands by yourself, I advise you to look for a temporary place where you can stay a few weeks first. From there, you can visit other places and look for a room or house that suits your fancy. Don't book housing for an entire year from abroad - you always get disappointed. The following websites can help you finding temporary housing:
Air BnB
Housing Anywhere
Otherwise, you can stay in a hostel, bed and breakfast, hotel or camping site for a few days while trying to secure your spot.

If you are coming to the Netherlands with your family, it might be better to apply for university housing. University housing typically gives you a place to live for the first year, while you accommodate your family in the country. Many universities cooperate with Duwo - and there are good and bad stories about university housing. The major advantage of renting through a centralized housing agency is the security that you will have a place when you arrive - something that I consider a big *must* for families. The disadvantage, however, is that they tend to charge a lot of extra application and administration fees.

When it comes to finding your place for the next few years, you might get scared off at first. Some housing is very expensive in the Netherlands - there is a shortage of student rooms. There is public housing, provided by for example Woonnet Haaglanden, but the waiting lists are very long: at least 3 years of waiting time. If you are coming for a PhD program of 4 years, then your chances of finally getting public housing are rather small. Private housing companies, such as RotsVast and Van der Vorm (from whom I rented - but their administration is a massive mess), fill the void in the housing market and offer apartments for rent - at prices higher than the public sector.

When you want to live in a room in a shared house, you often have to go to an "instemming" (literally translated: "voting in"). Sometimes that means that 100 students vying for one room are gathered in a house, get to introduce themselves in one sentence, and then the residents of the house randomly point at a number of faces they like, to further interview these folks. You might take your chances at such an event, but I must say that I never got to the second round.
Moreover, an "instemming" is typical for Bachelor's and Master's students. As a PhD student in the Netherlands, you are an employee of the university, and you're typically expected to show up to your office early in the morning. The "student life" thing just might not fit you anymore...

If you want to cut costs, I recommend you looking for a shared house/apartment. Many PhD students in the Netherlands live in shared accommodation. A good website to look for a room for rent or prospective roommates (when you decide to take a contract and look for peers to share costs) is Kamernet. Also, when you move to the Netherlands by yourself, it can be nice to be in a shared house, so that you can make friends and ask for advice from senior PhD students, who might have had to do all the paperwork that you are about to embark upon.

The best advice that I can give you, is to let all your colleagues know upfront that you are looking for housing.
I found my first room in Delft because a schoolmate from high school told a friend in Delft that I was going there too, and his roommate happened to be away for a semester to study abroad - that was the sublet from where I could get started on finding a place. I then found my home in Delft because my colleague knew a PhD student who was finishing and he asked her if her room would be available. I ended up taking over her room, and buying all her furniture.

Introducing PhD Talk for AcademicTransfer

Starting Thursday, you will find a new biweekly series titled "PhD Talk for AcademicTransfer" showing up here on PhD Talk.

These posts are sponsored by AcademicTransfer, the portal for academic job positions in the Netherlands - and the contents of this series is aimed at those of you who might be toying with the idea of coming to the Netherlands for an academic position, or who are thinking of starting a career in science in the Netherlands by applying to one of the PhD programs. 

Why would I do a series of sponsored posts?

In short, my 4 years in Holland were the bomb (don't tell anyone in Belgium I said that, though, especially not next summer when the soccer craze will hit). I landed a PhD program in the Netherlands by accident, and at that time it felt like a Plan B since securing funding in the USA was just not going to work out for me. 

But a few months after starting in The Netherlands I realized I was in a very good place. Social security. Awesome lab. Great colleagues. Funding for traveling to conferences. No coursework. Seriously, you'd be surprised to see how much that tiny country by the North Sea has to offer.

Now allow me to introduce AcademicTransfer to you - in their own words:

Since 1997 AcademicTransfer supplies all academic job openings at non-profit research institutes, universities and university medical centers in the Netherlands. In recent years, jobs in the corporate sector have also been added in response to intense demand for talented researchers.

Our goal is to promote PhD positions, research and post-doctoral positions and job openings for assistant, associate and full professors.
Although AcademicTransfer focuses on providing access to jobs in the Netherlands, the website is published in several languages. All job openings that specifically solicit foreign applicants are also offered in English.
AcademicTransfer originated as initiative of three Dutch organisations:

• VSNU, Association of Universities in the Netherlands
• NFU, Dutch Federation of University Medical Centers
• WVOI, Employers Association of Research Institutions (Dutch)

Using AcademicTransfer for your next step in excellence

Job-seekers can use AcademicTransfer completely free of charge. A job search field is placed prominently at the top of the home page, followed by the latest ten jobs posted on the site. Notable or unusual job openings are featured in text boxes, with a short summary of the position.
Ready for your next step in excellence? AcademicTransfer enables you to build your scientific career.

Finding relevant jobs

Start your search by browsing all jobs, entering a search string, or searching for employers in our employer listing, or refine your search results by adding extra criteria in an advanced search, such as academic field, salary or occupation.
Once your search results have been tailored to your preferences, you can set up an RSS feed or email alert for new jobs that match those parameters. You will be notified if new job openings are added that match your criteria.

Applying for jobs

You are always free to apply for a job that interests you, even if you are not a registered user. However, once you reach this point, it may be advisable to join us, since AcademicTransfer supports job applications by providing a user profile. Registered users can create a CV in their profile; you can choose to allow potential employers to access your CV, or send it along with a job application. When applying for a job, you can also attach up to five additional documents. In addition, you can use the ‘Match’ button to ask the system to display suitable job openings based on your profile.

Learn more about employers

Next to the job opening information most employers provide additional information. This information can be accessed by clicking the employer logo shown in job openings or via the employers section. The information provided by employers may include application procedures, tenure tracks, PhD programmes, frequently asked application questions and contact details.

Your guide to additional career information

AcademicTransfer also offers a number of external links to websites with career related information. These links are moderated: checked for relevancy and reliability and provided with a short description. Furthermore they are categorized to facilitate quick access to more information on topics relevant to academic career building. Topics include expat information, career counseling, post-doc programmes and job opening listings in the Netherlands and abroad.

Coming to the Netherlands

The English section of the AcademicTransfer provides many links to information that will be specifically interesting to people from other countries who are considering coming to the Netherlands. This section includes information on visas, housing, cultural tips, taxation, social security and insurance. The information in this section is provided in part by Euraxess.nl, AcademicTransfer’s European partner in the Netherlands.

Expand your horizon

Interested in expanding your horizon beyond the Netherlands? AcademicTransfer to the rescue! In 2008, AcademicTransfer acquired Career|edu, a job board for the international academic and research community. Originally American, Career|edu now operates in 38 countries. AcademicTransfer is also the preferred supplier of job openings in Dutch research organisations for Euraxess, a European initiative. The European Euraxess website offers an overview of all the research jobs in Europe.

Stay informed when and wherever

Always and everywhere informed about the newest job openings on AcademicTransfer or Career|edu? The AcademicTransfer iPhone app might be of help: supplying all the latest job openings in Dutch and English on AcademicTransfer and Career|edu on your mobile.

Posting your job opening on AcademicTransfer

Do you have a job opening and are you considering a posting on AcademicTransfer or Career|edu?
Please let us inform you about the benefits and post your job on AcademicTransfer and/or Career|edu.

Project #tweetprop: the Modified Bond Model

The second proposition of my dissertation is the following:

By combining two-way quadrants and one-way strips, the Modified Bond Model bridges the gap between the one-way and two-way shear approaches.

or in Dutch (as the propositions are in English and Dutch:

Door in twee richtingen dragende kwadranten te combineren met stroken die in één richting dragen overbrugt het Modified Bond Model de kloof tussen de methodes voor pons en dwarskracht

Admittedly, this proposition is more difficult to explain without dwelling upon all the technical details - but bear with me for this one, the lighter propositions will be up soon.

The Modified Bond Model is the theoretical model that I propose to determine the capacity of a slab subjected to a concentrated load close to the support. As its name suggests, it is a modification of the Bond Model by Alexander and Simmonds [1], which was developed for concentric punching shear in slabs.

In other words: while the Bond Model studies a single load (or column) on an infinitely large slab, the Modified Bond Model looks at the practical case of a slab with a certain geometry subjected to a concentrated load.

The Bond Model divides the slab into 4 strips, that branch out from the load, and 4 quadrants. All loading is carried from the slab to the column, via the strips. As such, the governing cross-section is the interface between the quadrants and strips. The capacity is defined as the sum of the capacities of the 4 radial strips.



The Modified Bond Model goes one step further. As we found in the experiments that the geometry is decisive for the shear capacity of slabs subjected to concentrated loads close to supports, we defined reduction factors that reduce the capacity of the strips. One of the advantages of the Modified Bond Model is that it is easy to calculate (you can do it by hand), and that it can incorporate a variety of different geometries.

So these ideas explain you the Modified Bond Model. But how does it bridge the gap between one-way and two-way shear approaches, and what is so cool about that?

Let me start by saying that our experiments, and thus the case of a slab subjected to a concentrated load close to the support, is a type of shear failure that is in-between the typical beam shear failure and punching shear failure modes. We see inclined cracks at the bottom face, but also punching damage and shear cracks at the side faces. The codes deal with these two failure modes in a very separated way, while in reality there is a transition zone. You can understand that it is thus important to find a method that describes this transition zone.

The Modified Bond Model does exactly that. The strips work in arching action, and the quadrants work in beam shear. The strips carry load in one direction (cfr. one-way shear) and the quadrants carry load in both directions (cfr. punching shear), off to the two strips that border each quadrant.

As such, the Modified Bond Model uses elements of one-way and two-way shear approaches, and helps us to describe the transition zone between pure beam shear behavior and punching shear behavior.

[1] Alexander, S. D. B. and Simmonds, S. H., 1992, "Bond Model for Concentric Punching Shear," ACI Structural Journal, V. 89, No. 3, pp. 325-334.

Project #tweetprop: On Slabs versus Beams

The first proposition of my dissertation is the following:

The two-dimenional shear-carrying behaviour of one-way slabs under concentrated loads close to supports should be treated differently than the one-dimensional shear-caryring behaviour of beams.

or in Dutch (as the propositions are in English and Dutch:

Het tweedimensionale afschuifdraagvermogen van in één richting dragende platen onder geconcentreerde belastingen nabij de opleggingen moet anders behandeld worden dan het eendimensionale afschuifdraagvermogen van balken.

This proposition is one of the main findings of my research, and a conclusions that I have previously published in the ACI Structural Journal as well as in a number of conference papers.

Let me break the justification of this proposition down into the observations, and the explanations based on the experimental evidence as well as theoretical reasons.

What did we observe?

In our shear experiments, we subjected slabs to a concentrated load close to the support. We studied the influence of different parameters on the shear capacity. Most shear experiments in the literature are experiments on beams, that typically have a small cross-section, that are heavily reinforced in bending and that are tested in four-point bending. Our understanding of how different parameters effect the shear capacity is thus mostly supported by these experiments. When we compared our experiments to the knowledge from beams, we found differences. The shear capacity of slabs depends mostly on geometric parameters. In beam shear experiments, the capacity depends strongly on the concrete compressive strength. For the range of concrete strengths that we tested, we did not see such a strong dependence.

To study how the behavior changes from beam to slab, we tested a series of specimens with an increasing width. We found how the dependence of the shear capacity on parameters changes as the specimen width changes. We also saw how different the cracking pattern is for slabs as compared to beams.

In this figure, we see the bottom face after an experiment of a specimen of 0,5m wide and a specimen of 2,5m wide. All other parameters are kept identical. The "beam" specimen only has horizontal cracks, while the "slab" specimen has a grid-like pattern of horizontal and vertical cracks following the rebar, but also has inclined cracks on the bottom that indicate shear distress, and even some punching damage.

How can we explain this?

Let's talk strut-and-tie models here. If we have a beam with a load close to the support, the load is carried by a compression strut between the load and the support - that's in the Eurocode as well.
But what happens when we apply this to a slab? If we have a concentrated load, we know that the forces can "fan out" in the width direction.

This figure shows why the influence of the distance between the load and the support is different for slabs as compared to beams. We see the top view of a slab subjected to a concentrated load. In a beam, our compression strut is the line of a/dl = 1 from the figure. In a slab, we have many struts that can develop - an entire fan of them. So, we can say that in a slab, we develop a three-dimensional strut-an-tie model. Now, two dimensions play a role in our shear capacity: the distance between the load and the support (a or av), and the width of the specimen. If we then apply this idea to the effect of the distance between the load and the support to the shear capacity, we see that we need to account for a sort of "average distance" for all these struts in a slab (a/dl > 1 on average), while for a beam this is defined only by a/dl = 1.

To conclude: we saw in our experiments a difference between slabs and beams in their dependency on parameters to define the shear capacity. This difference can be explained by acknowledging that slabs carry concentrated loads in what can be represented by a three-dimensional strut-and-tie model. As such, slabs have a two-dimensional load-carrying behavior, which is different from the one-dimensional load-carrying behavior in shear that we know from beams (and that is well-documented in the literature).

Writers' Lab: Introducing Project #tweetprop

Through Wetenschapper 2.0, reblogged from Roy Meijer, I discovered Project #Tweetprop.

Project #Tweetprop was initiated by Felienne Hermans, who blogged about the 10 propositions of her PhD thesis. Subsequently, Eric Bouwers, Rolf Hut and Sander van der Burg blogged about their propositions.

Besides discussing their propositions, the researchers in these blog posts explain why in the Netherlands, a PhD defense is not only about the dissertation, but also about the 10 propositions. In short: it comes from the Old Days when only the propositions were used during the defense, and the candidate had to show his eloquence and general ability to argue upon a point.

In some defenses nowadays, the committee members stick to the contents of the dissertation only. In mine, as in many others', there can be a discussion about the propositions as well. Good news for the paranymphs - for they are asked to stand up and read the propositions out loud before the candidate replies to the committee member (prevents the paranymphs from falling asleep).

I agree with Roy Meijer and Felienne Hermans that the propositions are great material for blog posts. As Roy wrote (and I translate): "It gives some material for your blog, it should be fairly easy to write because you already did the research, and it's a good platform to prevent that those propositions that cost you blood, sweat and tears will be forever forgotten." Wise words!

Over the next few weeks, and in between other posts, you will see #tweetprop posts, in which I'll discuss the 10 propositions of my PhD thesis.

And you might wonder, why devote a Writers' Lab post to it? Well, the answer is simple: because I would like you to blog about your propositions as well! Not doing a PhD in the Netherlands and not writing propositions at all? In that case, I have a challenge for you: give me 5 statements from your research (or remotely related to this) that condense the wisdom of your dissertation, and blog about them. Yes, 3rd and 4th year PhD students as well as post-docs, I am looking at you. Are you reading to join this challenge?

The First 20 Hours - How to Learn Anything

If you find 20 minutes to spare, watch this video:



Not only does Josh Kaufman show in this video that it only takes us 20 hours to learn something new, but it also shows how research results can be misinterpreted: how we went from needing 10000 hours to become a world-class expert in a highly competitive field, to the idea that it takes 10000 hours to learn something.

So, do you need to learn a new coding language for your PhD, or learn a new hobby? Set aside 20 hours for deliberate practice (using the hacks that Josh Kaufman shows in his video), and you'll be well on your way to learn something new.