Yes, I know it might not be practical to do this without errors and there’s reasons like running utilities through and quality control with inexperienced workers, but technically speaking, could you make stronger structures with less materials using stronger structural shapes? Maybe it’s just more pleasing to our eyes everything being straight and square and such, but what about utility canopies and tents where you want to have your structural members as light and portable as possible? Why do we not have tetrahedron shaped tents that have 3 small, collapsible, yet rigid poles and a firing they go in at the top instead of having long flexible ones?
In: Engineering
If you run joists diagonally to the beams, it increases the span of the joists, which would make the floor or ceiling weaker. To combat this, you have to put the beams much closer together, which would require more beams, which would cost you in material.
In this scenario any extra strength you gain would be from the addition of the beams. The joists running on diagonals wouldn’t make it any stronger. The reason is because a floor joists receive sheathing, typically plywood in residential construction. The plywood ties it all together and provides the lateral strength, the resistance to “rocking,” that a triangular shape has.
Joists are there to provide specific kinds of strength or strength in specific directions relative to the forces acting on the structure. In residential construction ceiling joists, for example, do a few things:
-They provide a nailing surface and support for whatever sheaths the ceiling.
-They’re nailed to the tops of walls to keep them the correct distance apart and prevent them from moving.
-They keep the tops of exterior walls from being pushed outward by the roof.
All of these functions are made worse if the joists are running diagonally to the walls and or roof. If you want to stop something from moving away from you, it’s much better if the force you apply to it is perpendicular to the direction it is trying to move. If you’re at an angle, it’s much less effective. There’s also the fact that the roof’s rafters in most cases lie alongside the ceiling joists. They overlap, and the rafters are frequently nailed to the side of the joist as well as the top of the wall they’re sitting on. Not only would this be impossible if the joists were diagonal to the walls, but it might not even be possible to set the rafters in place on top of the walls on a proper layout. In fact it would certainly be impossible. It’s hard to explain why, but it’s the nature of the geometry of it all. You’d run into situations where the bottom side of the rafter hits the top of a ceiling joist.
The triangle shape is strong because it resists “rocking.” You can’t move any of the vertexes relative to the other parts without breaking something. The lengths and angles are locked in. In a rectangle, the angles aren’t locked. You can shift the top to one side and make it into an increasingly exaggerated diamond shape all the way until it is basically flat, but the lengths of all the sides are all still the same. The angles changed, but nothing broke apart. Triangles are “stronger” because they can’t do that.
The floor of a house isn’t really vulnerable to that kind of force to start with, especially once the plywood subfloor is on. Those kinds of shear forces are things that walls have to deal with, and they get that triangular strength from the sheathing put on them.
We do see that triangle shape in one place in residential construction, though. The roof. Once we get to a point where something has to hold itself up, like a roof, we make a triangle. If you go inside an attic above a certain size, you will probably see horizontal braces connecting every third rafter (or so) to the one directly across from it, creating a full triangle.
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