#040 - Designing Precast Concrete Buildings - An Overview
✒Guest Article: Laurin Ernst from Structural Basics Provides an Overview of the Design of Precast Concrete Structures
Hi all, James here 👋
Today’s article is from Laurin Ernst, the creator of Structural Basics, which is dedicated to sharing structural engineering knowledge in the easiest possible way.
Firstly, I'd like to extend my appreciation to Laurin and the archive of helpful material he has developed for engineers through Structural Basics.
Structural Basics provides clear, practical insights and is a fantastic resource for those interested in engineering fundamentals and their real-world implementation, I encourage you to check out his post archive.
His article today is a high-level overview of the design process for precast concrete buildings. There are many rabbit holes in this topic, some of which i plan to explore in the near future!
The Process of Designing Precast Concrete Buildings
My name is Laurin Ernst. I am a structural engineer based in Copenhagen, Denmark. I have a bachelor’s degree in civil engineering from the University of Innsbruck, Austria, and a master’s degree in architectural engineering from the Technical University of Denmark (DTU). Originally, I am from Germany.
I mainly design precast concrete buildings and refurbishments. I also have a few years of experience in BIM modeling and coordination.
One of my passions is sharing my knowledge with others. This led me to start the structural engineering blog and weekly newsletter structural engineering blog and weekly newsletter ‘Structural Basics’, where I teach engineering mechanics, load calculation and structural designs according to Eurocode.
Since the 1950s, precast concrete has been gaining popularity as a construction method due to benefits like faster construction time, lower labor costs, improved quality control and consistency in factories, the ability to optimize designs through geometry, materials, and product settings, and sustainability as there is much less waste than in in-situ concrete.
In this article, I share my process for designing precast concrete buildings as a structural engineer. In Denmark, where I am based, roughly 80-90% of all new buildings are built with precast concrete elements.
Let’s clarify what we cover in this post because ‘process’ can mean many things. In this post, we’ll cover the design tasks the consulting structural engineer does. We assume that the geometry of the buildings is set, the loads (dead, live, snow, wind, etc.) have been calculated, and the static/structural systems are in place. Otherwise, this post would be too long. Let us know in the comments if you want an episode 2 about precast concrete buildings.
Please also note that the following process is based on my experience in Denmark. This might look different in other countries.
Here’s a summary of the design steps:
Vertical load transfer
Horizontal load transfer and stability analysis
Robustness
Foundation design
Additional structures
So let’s get started.
1. Vertical load transfer
After the basis of the design (loads, geometry, deflection and material criteria, etc.) is defined, the first thing we do is the vertical load transfer. Note that having the basis of design ready before you get started with the design is a dream scenario, which never happens. There are always aspects that lack clarity, and things change often.
Anyway, let’s assume we are all set. Vertical load transfer means that we are calculating and documenting the loads that act on all structural elements. This is usually done in Excel spreadsheets. This is possible, because 95% of all static systems of precast elements are simply supported beams (i.e., statically determinate).
1.1 The simplicity of precast concrete buildings
As mentioned above, most static systems are simply supported beams. So, let’s look at some examples - the four main structural elements used in precast concrete buildings. These are precast slabs such as hollow-core slabs, precast concrete beams or steel beams, precast concrete columns, and precast concrete walls. As always, there are exceptions.
Hollow-core slabs
Hollow-core slabs usually span one-directionally between precast beams. These beams could be precast concrete or steel profiles.
Precast concrete or steel beams
Beams usually span between precast columns. They can be supported by corbels or on top of the column. Again, in most cases, they are simply supported.
Precast concrete columns and walls
Columns mainly span between two floors with a pin at the bottom at a roller at the top.
1.2 Who Designs What in Precast Concrete Design?
Another benefit of precast concrete buildings is that the design work is distributed among specialists. The hollow-core slabs are designed by the slab manufacturer, the columns by the column manufacturer, and the beams by the beam manufacturer. Yes, these elements are often produced by different companies, each specializing in one type of element.
The manufacturers design the elements with the vertical loads calculated and documented in the vertical load transfer. But before the manufacturer joins a project, we make sure that the elements we modeled in the BIM model can resist the loads.
2. Horizontal Load Transfer and Stability Analysis
In this section, we calculate the horizontal loads on each floor diaphragm from wind, seismic loads, and earth pressure. The loads are then distributed to the shear walls or other stabilizing elements using a rigid floor approach. The loads can be distributed elastically or plastically.
Then the floor diaphragms are verified, and the joint reinforcement is designed. I usually use the Stringer method to do this. Let me know if you are interested in an article about floor diaphragms.
Finally, the shear walls are verified for the horizontal point loads that act on them. If the horizontal load is too big and the vertical load too little more shear walls or rebar anchors need to be added.
3. Robustness
Robustness refers to a structure's capacity to endure impacts without being damaged disproportionately to the initial cause. So, a column or a wall might collapse if a truck drives into a building, but because of the robustness evaluation and calculation, the building doesn’t collapse entirely.
This is a bit more challenging for precast concrete buildings because most elements are simply supported. This means that if a column collapses, the beams and decks supported by the column also collapse.
Due to the robustness requirements, joint reinforcement is added between the slab elements and anchors to columns and walls. That way, only the element exposed to the impact collapses.
Let us know if you want me to cover robustness in more detail and show some calculation examples.
4. Foundations and in-situ Concrete Foundation Slab
Foundations are not elements (except precast piles) and are, therefore, designed by consulting engineers. I have worked on projects with the following types of foundations (note that many projects have multiple types of foundations):
piles with grade/foundation beams and pile caps
strip and pad foundations
well foundations with grade/foundation beams
The in-situ concrete slab can be supported by foundations or directly by the soil (raft). This depends on the soil characteristics and the loads.
I design the in-situ concrete slab in a FE program like FEM-Design if it is supported by grade/foundation beams or other foundation types.
5. Additional Structures
Other structures, such as secondary steel, timber, concrete structures, staircases, and balconies, are usually verified and designed by the manufacturers. We document the reaction forces of these structures if they are supported by the foundations, walls, or slabs.
We verify that the structural elements we modeled in the BIM can resist the loads, but we usually don’t document this verification. For example, we dimension the elements of a secondary steel structure, verify them (usually in FE software), and add them to the BIM model. However, we do not document the verification or design the connections, as these are the responsibilities of the manufacturer.
If the manufacturer doesn’t have the time or capability, we also handle the verification and documentation.
Closing
This was a quick overview of the design process of precast concrete buildings in Denmark. If you want to read more about precast concrete structures, then let me know on Structural Basics.
In my weekly newsletter, I cover everything from basic engineering mechanics to loads and structural designs. You can also check out my blog where I published over 100 articles on structural engineering topics.
Thanks for reading and hope to see you again.
Cheers,
Laurin. ✌️✌️
Thank you to Laurin for this guest post. I look forward to other collaborations in the future. For those interested in participating, reach out to me at james@flocode.dev.
See you in the next one!
James 🌊
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