Steve Jackson Games Forums - View Single Post - [Magic] Using Shape Earth and Earth to Stone in construction

Anaraxes · 06-20-2019, 11:50 AM

If you fuse all the stones, you don't necessarily get a true arch. You get a shape that could be described as post-and-lintel with a semicircle chopped out of the center bottom of the lintel. I'm not an engineer, but it's not obvious to me that an homogeneous block with a semicircular cutout is stronger than the full post-and-lintel (itself weak compared to an arch).

The point of the arch is converting the downward stress into stress along the perimeter of the arch, and thus making it compressive around the arch span rather than tensile at the bottom edge of the lintel. Qualitatively speaking, the wedge shapes of the arch stones (or the stones + their mortar) gradually change the downward stress into more transverse compression, starting with trying to drive the keystone down into the rest of the arch, the driving of which wedge becomes a stress forcing the two blocks to either side apart horizontally, and so on around the boundary of the arch. For another comparison, there's a shape called a "jack" arch, which isn't curved at all, but rather a horizontal series of wedge-shaped blocks or bricks slanted outward at the tops, with those wedges filling a flat horizontal shape, identical in outline to a lintel. If it were only the bottom edge of the shape that mattered and not the internal connections, then a jack arch wouldn't be any stronger than a lintel, but merely decorative at best, but structurally not worth the bother.

If the crystals in solid stone could do that redirection of the downward force on their own, then it seems post-and-lintel would have been a fine solution to start with -- though cutting out the middle reduces the load the lintel itself contributes, it also weakens the lintel by making in thinner where the most stress is. Maybe removing part the stone makes the forces go sideways just because they no longer have a choice (other than to go down "into the air" and break the stone). But maybe it's the more macroscopic geometry of those "voussior" wedges that make the real difference, rather than the crystalline structure of the minerals in the stone or molecular bonds, or just the curved outline of the span over the doorway.

I don't suppose anyone out there just happens to be a civil engineer or architect?

06-20-2019, 11:50 AM	#18
Anaraxes Join Date: Sep 2007	Re: [Magic] Using Shape Earth and Earth to Stone in construction If you fuse all the stones, you don't necessarily get a true arch. You get a shape that could be described as post-and-lintel with a semicircle chopped out of the center bottom of the lintel. I'm not an engineer, but it's not obvious to me that an homogeneous block with a semicircular cutout is stronger than the full post-and-lintel (itself weak compared to an arch). The point of the arch is converting the downward stress into stress along the perimeter of the arch, and thus making it compressive around the arch span rather than tensile at the bottom edge of the lintel. Qualitatively speaking, the wedge shapes of the arch stones (or the stones + their mortar) gradually change the downward stress into more transverse compression, starting with trying to drive the keystone down into the rest of the arch, the driving of which wedge becomes a stress forcing the two blocks to either side apart horizontally, and so on around the boundary of the arch. For another comparison, there's a shape called a "jack" arch, which isn't curved at all, but rather a horizontal series of wedge-shaped blocks or bricks slanted outward at the tops, with those wedges filling a flat horizontal shape, identical in outline to a lintel. If it were only the bottom edge of the shape that mattered and not the internal connections, then a jack arch wouldn't be any stronger than a lintel, but merely decorative at best, but structurally not worth the bother. If the crystals in solid stone could do that redirection of the downward force on their own, then it seems post-and-lintel would have been a fine solution to start with -- though cutting out the middle reduces the load the lintel itself contributes, it also weakens the lintel by making in thinner where the most stress is. Maybe removing part the stone makes the forces go sideways just because they no longer have a choice (other than to go down "into the air" and break the stone). But maybe it's the more macroscopic geometry of those "voussior" wedges that make the real difference, rather than the crystalline structure of the minerals in the stone or molecular bonds, or just the curved outline of the span over the doorway. I don't suppose anyone out there just happens to be a civil engineer or architect?