Horizontal Load Transfer
Episode #15 of the Structural Loads Series
Hi friends, ☀️☀️
Today is the last post of the structural loads series before the announcement about the next series next week. But before getting into the topic, I feel like sharing some personal stuff today.
The last few weeks and months have been a blast. Lots of fun things happened, and I made one of the biggest decisions of my life. I’ll share more about that in a few weeks.
As I am writing this, I just came back from vacation. I spent time in my hometown Kempten attending our local beer festival (similar to Oktoberfest) which always happens in the middle of August. I love it because I always meet people from my past that I haven’t seen in years and everyone is in a good mood. We also call it the 5th season of the year because most people take vacation for the 10 days the festival is happening.
At the Structural Basics newsletter, we have also seen an incredible growth over the last month. We have grown by more than 2000+ people in the last 30 days (compared to 600/month before). Welcome to everyone who is new to our weekly structural engineering newsletter. It’s great to see the community grow. 🔥🔥
Another thing I am also spending a lot of time on, besides my full-time job as a structural engineer and content creation for Structural Basics, is preparing for my second marathon. I am running the Palma Marathon on Mallorca on the 20th of October.
I finished my 1st marathon in 3 hours 39 mins. This time, I’ll try to finish in under 3:30. You can see my runs on Strava. Let’s connect.
Structural Basics is a community about structural engineering.
But community is also about getting to know each other and sharing personal stuff. We learn and develop much better in an environment of friends.
Besides, the healthier we are, the more successful we are as structural engineers long-term. Being active and sporty has a big influence on our health. We can handle stress better and we feel overall better.
Alright, story time over, let’s get into the topic of today…
Horizontal Load Transfer
A building needs to transfer the horizontal and vertical loads that act on it down to the foundation. The loads travel from one structural element to another until they reach the foundation/soil. And of course all structural elements need to resist these loads.
Now, we don’t use the same structural elements to resist horizontal loads as we do for vertical loads. Here are examples of structural elements that stabilize structures:
Diaphragms
(Wind) braces
Usually, a building/structure consists of at least 2 of these 4 stabilizing elements.
In today’s article, you’ll learn how horizontal loads like the wind load are resisted by stabilizing systems of buildings and how these systems transfer the lateral loads down to the foundation.
The step-by-step process I always follow when transferring horizontal loads through a building:
Step #1: Defining the static systems of the stabilizing elements (support conditions, hinge or moment stiff connections, etc.)
Step #2: Applying the horizontal loads like wind load, seismic load, earth pressure and/or imperfection load to the structural elements they act on
Step #3: Horizontal load transfer using the static systems
As said in last week’s article, mastering vertical AND horizontal load transfer takes a lot of practice. That’s why we’ll run through 2 different examples today.
Example #1: Precast concrete building
The stabilizing elements of a typical precast concrete building are shear walls and floor diaphragms.
Now, I have also designed precast concrete buildings which included steel/concrete frames and braces. But typically, at least 90% of all stabilizing elements are shear walls and floor diaphragms.
Step #1: Static systems of the stabilizing elements
Floor diaphragms are more tricky than other stabilizing elements and there are different methods to calculate these diaphragms but also to distribute the horizontal loads to the shear walls.
As mentioned above, there are a few ways to distribute the horizontal loads to the shear walls. We can use an elastic or plastic distribution. More on these methods in the future.
Step #2: Apply the horizontal loads
Let’s say the design horizontal wind load on the facade is 1.5 kN/m2 as the total of wind load area D and E. In the horizontal load transfer, we can simplify and apply the total load on one side of the building. But when you verify the diaphragms, you of course need to split them up and apply them separately.
Step #3: Horizontal load transfer using the static systems
We apply the load to the static system(s) and calculate the reaction forces. These reaction forces are then applied to the next structural element. We then again calculate the reaction forces of that element and apply it to the element that supports it.
The area load of 1.5 kN/m2 is applied to the facade. These facades distribute the area load to the floor diaphragms. Facade elements usually act as simply supported beams.
This line load we can now apply to the floor diaphragms. We apply 2 ⋅ pd.f to the 1st floor.
The reaction forces of the beams are calculated as:
Which leads to the reaction forces of Vf.2 = 18.4 kN and Vf.1 = 36.8 kN.
Now, Vf.2 and Vf.1 are applied to the static system of the shear walls.
The horizontal loads on the shear walls lead to a bending moment at the bottom of the walls and a horizontal reaction force. This article isn’t about shear walls, but I also added a vertical reaction force because there are always vertical loads acting on shear walls.
And the bigger the vertical loads the better because it reduces the bending moment.
The bending moment is then transferred into a compression zone.
nc and c depend on the magnitude of M. The bigger M the smaller c which means that the bigger the eccentricity of the load. If c become too small, we add reinforcement at the other side of the wall to take up tension.
You can check out our article about precast shear wall design to learn how you do these verifications.
Example #2: Steel warehouse
The next example is a steel warehouse that consists of frames and wind braces as the stabilizing elements.
Step #1: Static systems of the stabilizing elements
In the scenario of lateral load acting perpendicular to the facades, the stabilizing elements are the frames.
In the scenario of lateral load acting perpendicular to the gables, the stabilizing elements are wind braces, which act together with the rafters, beams and columns like a truss.
Step #2: Apply the horizontal loads
Let’s stick with the wind loading of 1.5 kN/m2.
Step #3: Horizontal load transfer using the static systems
Let’s split this up into lateral loading perpendicular to the facades and to the gables.
Lateral loading perpendicular to the facades
The area load of 1.5 kN/m2 is applied to the facade elements. These facades distribute the area load to the columns of the frame. You could also choose a design where the facade elements span vertically. Then the line loads would be applied to the roof and the foundation raft. In some warehouse designs you also see “horizontal purlins” which reduce the span of the vertical facade elements
The line load on the columns is calculated as:
This line load we can now apply to the columns.
The frame is now transferring the horizontal loads down to the foundations with its moment stiff connections.
Modelling the frame in a frame analysis tool like Ftool or any FE program leads us to the reaction forces which the foundations have to be designed for.
Lateral loading perpendicular to the gables
The area load of 1.5 kN/m2 is applied to the facade elements. These facades distribute the area load to the foundation raft and the beam of the frame.
The line load on the beam is calculated as:
This line load we can now apply to the beam/truss. How the loads travel through the compression and tension members is a topic for another article.
The reaction forces Vb are calculated like for a simply supported beam:
Vb can now be applied to the vertical wind bracing system.
With the SkyCiv truss calculator, we calculate the reaction forces, because the static system is statically indeterminate.
Now, you can design the pad foundation with these reaction forces.
Final Words
This was it! 5 example of how to do vertical load transfer last week, and now 2 examples explaining horizontal load transfer this week.
I hope these examples with 3D visualizations helped you understand load transfer better. It took me hours to write this article and model the 3D structures in Rhino. I hope the hours I put into it were worth it.
This was the last post from this series about structural loads.
I am excited!
If you missed episode #1 - #14 of this series, then you can find all previous posts → here ←.
See you next Wednesday, friends. 🙋♂️🙋♂️
Have a great rest of the week,
Laurin. 😎😎