Summary： Triangles are the most stable structure in the world, why don’t all buildings look like pyramids?

A few days ago, a roommate of a classmate in our laboratory came to help us tie the steel bars. She studied psychology, and she asked me when the steel bars were being tied. "Since triangles are the most stable, why do we rarely see triangles in our buildings?"

"Look at the steel cages we are piercing, they look like rectangles, right?"

"Yes, yes."

"Do you think these two sides are two right-angled sides of a right-angled triangle?"

"Yes. Where is the hypotenuse of this triangle?"

"We said this is a reinforced concrete structure. Now that we have steel bars, where is the concrete?"

"It seems to be finishing the steel cage, then pour the concrete, and pour the steel bars in the concrete?"

"That's right, so after the concrete inside has solidified, this beveled edge."

"Is every rectangle?"

"Yes. It looks like we are piercing rectangles one by one. In fact, after the concrete has solidified, these are all triangles one by one."

Perhaps in your opinion, the house is like this, square, without any triangles.

Later, you learned by accident that it turned out that there were steel bars inside. But there are still no triangles?

But in the eyes of structural engineers like me, the house is actually like this.

All densely packed are triangles.

A concrete beam, longitudinal bars and stirrups form a rectangular grid; there is a person standing on it. How does this person's weight transfer to the supports at both ends?

The concrete beam itself is a truss model composed of tension and compression rods. Assuming that the weight of the person standing on it is 100, how much does the longitudinal reinforcement at the bottom of the beam need? It needs to be able to withstand a pulling force of at least 200. How much do you need for stirrups? It needs to be able to withstand a pull of at least 50. What is the thickness and strength of the concrete? The concrete at the top of the middle needs to withstand a pressure of 150, and the concrete at an oblique angle of 45 degrees needs to withstand a pressure of 70.71.

The same is true not only for reinforced concrete beams, but also for steel structure beams.

If the beam is a steel structure, it is also a truss model composed of triangles. In the calculation of the shear resistance of steel structure beams, if the spacing of the vertical stiffeners is not too large, the contribution of TFA (Tension field action) in the diagonal direction can be fully considered.