Posts on Berk Demir

Diaphragm Wall Intervention

Tue, 19 May 2026 00:00:00 +0000

Memories from 2007 back in Durban, building a 2010 World Cup Stadium; we hit a problem with the diaphragm wall excavation, when the grabber got stuck in the bedrock about -20m down. So we sent commercial divers down through the bentonite slurry to fix a new steel cable to the grabber so we could retrieve it and keep the project going. Those boys were hardcore.

Paper on Nordhavnstunnel

Wed, 28 Aug 2024 00:00:00 +0000

Demir et. al. 2024 — Nordhavnstunnel.pdf

Searching for an Immersed Tunnel from 1900

Thu, 07 Sep 2023 00:00:00 +0000

It started with ITA (International Tunnelling Association) WG 11’s catalogue of immersed tunnels. The third one (and also the fourth) in the list is from Copenhagen. Actually, the culvert is located in the most photographed region of the country - under Nyhavn. In this excellent catalogue prepared by Nestor Rasmussen (DK) and Walter Grantz (USA), the S.3 tunnel has been described as shown below. Then, search is started. With the help of people from Reddit, we found the location of the tunnel in the HOFOR database. The possible location is under Nyhavn’s connection to the sea - which shown with blue dashed line. In one of the documents, it says “Den dykkede ledning i Nyhavn blev nu også lagt. Den gik fra Kvæsthusgade til Havnegade.” which translates to “The submerged pipe in Nyhavn was now also laid. It ran from Kvæsthusgade to Havnegade.” Afterwards, I contacted the Københavns Museum, and Jakob Ingemann Parby, Senior Researcher and Museum Inspector, has reached out to me to provide more resources. In one of the resources online, Københavns Energi A/S’s (previous name of HOFOR) book called “Fra stinkende rendestene til computerstyrede kloakker” (in English “From smelly gutters to computerized sewers”) had more information on the topic. This beautiful image shows the lowered steel pipes: Let’s zoom in a little bit. I think they used barrel as ballast to sink the pipes. Pipe may look like very flexible, but it’s not actually, the design of the pipe is with bent corners as shown in the image. Actually, we even have a construction drawing. We can see the backfilling and longitudinal profile of the pipe. They had gate valves to periodically flush the pipes that accumulate dirt and sediments. Another construction stage photo is coming from Københavns Museum archive - Jakob Ingemann Parby. You can see the cranes and same shape:

Plaxis Output Program

Sun, 06 Aug 2023 00:00:00 +0000

The beauty of Streamlit with Plaxis is the simplicity of the UI and how minimalistic it looks. Also, the 2x2 output figure for a retaining wall is the most compact output you can imagine. Everything you need to show for all these stages is inside one figure. Moreover, this tool can deliver a really detailed excel sheet to share with other designers. Watch the video to see it on action: Also fixed-end anchors and node-to-node anchors! And lastly, how to start the Streamlit directly from Plaxis as shown in videos:

Inconsistency Between Eurocodes for Fire Analyses

Tue, 28 Mar 2023 00:00:00 +0000

Introduction

Table of Content

Last month, we had an interesting and long discussion over the email with the creator of PCTempflow, a widely used software for fire analysis, and others like PCSheetPileWall and Framework: Gerrit Wolsink. Over the course of the many emails, we have agreed that there is something wrong (or incompatible) in the Eurocodes. This brief article is our joint effort on clarifying our views on this problem. If you have any input, please feel free to reach out.

Rectangular Parametric Tunnel in FEM-Design

Sat, 21 Jan 2023 00:00:00 +0000

Another couple nights to experiment on the parametric design with FEM-Design and Grasshopper. No problem with geometry and load definitions except couple of small bumps. The load combinations require more than couple nights, so I just move to the GUI if I need.

Open Source Tools

Sun, 01 Jan 2023 00:00:00 +0000

Note that when the apps are not used for a while, you might need to “wake” them. It might take up to a min.

https://bdem-hb.streamlit.app/ https://bdem-hs.streamlit.app/

Hardening Soil Model

Fri, 16 Dec 2022 00:00:00 +0000

During 2019 Plaxis User Meeting in İstanbul, I have presented a detailed discussion on Hardening Soil Model. The presentation was really welcomed by the audience and in fact, later on, I have been invited to two other companies for the same presentation. This short post will be a summary of that presentation. Hardening Soil model (will be called HS from now on) was presented in an excellent conference for 10th year of Plaxis in a paper called The hardening soil model: Formulation and Verification by Schanz, Vermeer and Bonnier. However, HS is tightly bonded to previous studies on the literature such as Lade, Tatsuoka and Ishihara, Cam-Clay model, Kondner and Zelasko, Jardine, Duncan and Chang, Al Tabbaa, Simson et. al.

Hoek Brown Model

Fri, 16 Dec 2022 00:00:00 +0000

I have just published a new tool and this post will detail the methods that are being used in this tool. What it does: Performs Hoek-Brown analyses for rock and recommends additional parameters based on the inputs.

https://berkdemir-bd-hoek-brown-bd-hoek-brown-vsh6i6.streamlit.app/

Theory

Introduction

The Hoek-Brown material model is the most widely used rock mechanic model due to its simplicity and ease-of-use in continuum based numerical models such as finite element or finite difference models. Hoek-Brown model is published in Hoek & Brown (1980) and after that, it is constantly updated. Latest update was published in 2019.

Modulus of Subgrade Reaction

Fri, 16 Dec 2022 00:00:00 +0000

Introduction

By now, everything should have been settled about modulus of subgrade reaction. We all know some typical statements about it:

It depends on the soil properties
It depends on the foundation size
It depends on loading type, temperature, bitcoin prices and others.

However, let’s think about a weathered rock with E=500 MPa and a foundation to be built on top of this rock with 20 x 50 m dimensions. You can say it depends on many factors as much as you like, everybody has a rough idea already: 100,000 kN/m3. So, if you are a fancy engineer and dare to make some calculations, you can find much lower values. Will they believe you or will they think that you are being too conservative (if lowering the subgrade reaction means being conservative)? Let’s start with simple terms. The equation that everybody knows and nobody wants to use: $$ K=\frac{q}{s} $$ So, the subgrade reaction is equal to a spring stiffness distributed under the foundation. If you divide the pressure by the settlement, you will find the subgrade reaction, amount of deformation for unit pressure. If we think about how we calculate settlement (how it depends on many factors), we can see actually how complex this modulus is.

Hoek Brown Model in Finite Element Analysis

Thu, 13 Oct 2022 00:00:00 +0000

Update on the Post (13/10/2022)

New update of the Plaxis, partially, solves the problem mentioned in the post below. Now, if you define a manual tensile strength, this will be reduced during safety analysis. But if you rely on the tensile strength automatically calculated by Plaxis, it will not be reduced and same problem continues for that case, in my opinion. But at least, for the users that are aware of this distinction, there is an option to correctly use Hoek-Brown model. See the note on Plaxis

Recommendations for Pseudo-Static Deformation for Seismic Analyses of Tunnels

Fri, 09 Sep 2022 00:00:00 +0000

Before Introduction

This is the paper we have written with my dear wife Pinar Akdogan Demir, based on our experience from Istanbul tunnels. I was already considering uploading the paper here, but when I saw that my paper was not inside the proceedings USB due to a mistake (which also was almost preventing me from presenting - because they simply forgot us), it was a must. So, here it is. As always, get in touch if you have any comments.

Roof Slab with Grasshopper and FemDesign

Mon, 25 Jul 2022 00:00:00 +0000

I have been working on StruSoft’s FEM-Design and Grasshopper connection for a trial case of a precast roof slab for a shaft. The results were great and even more than I expected! I was OK with some post-processing, but getting the results directly with a one-click from Grasshopper was amazing. It can handle shell connections, supports, load combinations and all I needed for this case. It is still improving a lot every day.

MR Data from Tatone et. al. (2022)

Fri, 04 Feb 2022 00:00:00 +0000

A new paper from Tatone et. al. is published here. Using the data of the authors, following violin graph is drawn to estimate Modulus Ratio (MR = E / UCS) of the different rocks. Update: Aly Abdelaziz has proposed an update on the code and graph. Thanks to him, it looks better now. Code and data are in the Github Gist.

# Data from Tatone, B. S., Abdelaziz, A., & Grasselli, G. (2022). Novel Mechanical Classification Method of Rock Based on the Uniaxial Compressive Strength and Brazilian Disc Strength. Rock Mechanics and Rock Engineering, 1-5.
import pandas as pd
import seaborn as sns

# Rock Category
rock_type = {"Sedimentary": ["SL", "SSh", "SS", "SC"],
 "Metamorphic": ["MG", "MS", "MQ", "MM"],
 "Igneous": ["IG", "IF", "ID"]}

ordered_box_list = []
for i, v in rock_type.items():
 ordered_box_list += v

df = pd.read_csv("data.csv")

df = df[df.E != "-"]
df = df.astype({"E": float, "UCS": float})

df["MR"] = df["E"] / df["UCS"] * 1000

sns.set(rc={"figure.figsize": (20, 8.27), "figure.dpi": 300})

ax = sns.violinplot(x="Type", y="MR", data=df, order=ordered_box_list)
ax.set_ylim(0)

The data.csv file can be saved from here or from Gist. [[Notion/Quick Note/BDEM (1)/Blog Posts/_assets/data.csv]]

Topology Optimization in Plaxis

Thu, 23 Dec 2021 00:00:00 +0000

Topology optimization is very similar to sculpting. You give a bulk volume to the algorithm and some constraints, it gives you the best shape that it can find. It is possible to use this in any software, as long as you can calculate the stresses - or any other criteria that you can use to eliminate the unnecessary volumes. I have been planning to try this in Plaxis for a long time. Using a couple of hours of coding and trials, it turns out Plaxis - Python connection works pretty well for topology optimization. This code lacks too much such as unstable volume check (stiffness matrix errors) or stop-criteria. But it was fun.

Plaxis and Steel Fibre Reinforced Concrete

Tue, 06 Jul 2021 00:00:00 +0000

Crack propogation during three point bending beam test using Concrete model in Plaxis. In this case, we have tried to capture the behavior of steel fibre reinforced concrete and results are really promising. The actual photo of the 3 point bending beam test is shown below from Buttignol et. al. (2008).

Plaxis-Python MN Interaction

Sun, 02 May 2021 00:00:00 +0000

Recently, I have prepared a Moment-Axial Force Interaction Diagram that fetches the structural forces from Plaxis automatically. Using the amazing Streamlit module, I have created a simple GUI for MN diagram and published its video in Linkedin. The response was amazing and I got a lot of questions regarding the procedure, Python-Plaxis connection and Streamlit. You can see the video here. I will not publish the code since it will require me to check every aspect of the code, do extensive tests and prepare a documentation. Instead, I want to give some insights on the methods that I have used in the code. I had to try and fail too many times and contacted Plaxis support several times. Since this is a gray area still with lot to develop, it is not easy to find discussions on the internet, so even a brain-storming with Plaxis support is really valuable. (Thanks to Stefanos) So, to create a record of these functions, I will share small gists (a little code snippets sharing tool by Github.)

Hoek-Brown Parameters Database

Fri, 12 Mar 2021 00:00:00 +0000

If you will use Hoek-Brown in your Python code, you may want to recommend some constants based on rock type. There is a widely used table in literature by Hoek and others that we use to select Modulus Ratio and material constant (mi) in the absence of high quality laboratory tests. I have done the manual labour, don’t write it all again. A dictionary called RockDict is given in the following Gist. Rock types are given as keys of dict and a sub-dictionary with:

Plaxis and Plate Properties - A Look into the FE Adaptation of Long Term Stiffness Changes

Thu, 25 Feb 2021 00:00:00 +0000

There was a question regarding Plaxis in Bentley forum: A user asked the following question -paraphrased-:

When we changed the plate parameters to simulate long term stiffness, there is no change in deformations. Is there something wrong?

I have tried my best to explain this in the forum, but it is not an isolated case. In fact, there are many examples of design reports that do not take this into account. For example, changing the plate parameters from shotcrete to final lining is not a correct approach to simulate long-term degradation of the temporary lining. Why are deformations not changed when we changed the stiffness of structural elements? There is a clear explanation for that: Because finite element method does not work that way. As can be seen below or in Appendix of Scientific Manual of Plaxis, the incremental deformations are caused by unbalanced load. If there is no unbalanced load, the deformations will not increase since the equation results in 0. But let’s consider a tunnel. As you can see below, the real case is in first row. If we want the ground loads to act on the permanent lining, we can’t just change the material properties and hope for the best. We have to simulate the degradation of the temporary lining. There are several methods for this such as gray rock or assuming a certain thickness of shotcrete thickness is degraded. However, some project requirements do not allow for the consideration of temporary lining for permanent lining analyses at all. This is the case for subway projects in Turkey. In the second row, you see the wrong way of using plates for tunnel design. If we use this method, structural forces will be less than actual. Why? Let’s remember few things and consider the above case of wrong use of plates:

Plaxis-Python Seismic Deformation

Sun, 21 Feb 2021 00:00:00 +0000

For underground structures, a rough but reasonable simplification is pseudo-static deformation method. In this method, we apply seismic strain which can be calculated as the ratio of effective PGV (Peak Ground Velocity) to effective shear wave velocity. $$ \gamma = \frac{PGV_e}{VS_e} $$ Effective PGV can be multiplied with depth dependent reduction factors (see FHWA-NHI-10-034) and maximum shear wave velocity obtained from geophysical tests with almost zero strain can be converted to effective shear wave velocity based on recommendations of FHWA or Eurocode 8. A simple Python code can be written to implement lateral deformation profile in Plaxis to simulate seismic loading. If you locate this Python file inside the Bentley folder (< PLAXIS installation folder >\pytools\input) this can be directly called from Plaxis Input.

Comparison of Building Rigidity Calculation Approaches to Estimate Tunnelling-induced Deformations

Sun, 24 May 2020 00:00:00 +0000

Estimating damage level on the existing buildings due to tunnelling is a tricky business which depend on many factors and cannot be over-simplified. But one of the simplifications that has been used commonly is equivalent beam method. Equivalent beam method allows us to simplify the rigidity of buildings in a simple Mindlin beam. Why do we need the rigidity of the building to come into our numerical models? We have many evidences that show us greenfield deformations (which are deformations due to tunnelling on level ground without any structure.) are much higher and narrower than building deformations caused by tunnelling. One evidence is given my Frischman et. al. (1994) and reported by Mair, Taylor and Burland (1996). So, we need the rigidity of the structure to be modelled and we need it as close as possible to the reality. How? We can model the structure every time we model a tunnel problem, this is an option of course, but an option for people with lot of time. We need a simpler solution. The simpler solution is equivalent beam method. To determine the EI and EA of the beam, we have to come up with a methodology that allows us to simplify the building. Potts and Addenbrooke (1997) have proposed calculating moment of inertia of the building with respect to neutral axis, considering only slabs using parallel axis theorem. (If you forget: Moment of inertia of a beam at point P at a distance y to its neutral axis is I_P=I+Ay². As authors state, this is an over-estimation since this assumption holds true only for rigidly framed structures. But they also summarize another method which is also described in CIRIA report 200 by Mair and Taylor (2001). Using this method, authors have predicted the deformations before the construction (Class A prediction) and results are very close to measurements. In this method, moment of inertia of each slab is summed up by neglecting Ay² term. Note that in both methods equivalent area of the beam can be calculated using sum of the area of each slab. Goh and Mair (2014) developed a third approach which adds column stiffening factor, C of Meyerhof to approach given in Mair and Taylor (2001). However, calculation of C requires detailed information on column and beam positions which may not be readily available in every project. But, to make a note, I should state that, if you will perform a detailed analysis for a significant building, you should definitely use this paper (if you will not model the building in full detail.)s Goh and Mair (2014) states that parallel axis method is over-estimation of the building rigidity while algebraic sum of moment of inertias is under-estimation. To be honest, parallel axis method was looking better for me. However, a very very simple experiment proved me wrong. I have modelled a building in almost-full detail. Of course, there are still many simplifications such as connections of slabs and columns, but it is a simple trial, so this should do. I have created three models using Plaxis 2D 2019: