This effect was observed as far back as the mid-1800s, when it was found that trains passing in opposite directions tipped precariously toward one another. The reason is the same: The high velocity of the air between the car and the truck creates a region of lower pressure between the vehicles, and they are pushed together by greater pressure on the outside ( (Figure)). Similarly, when a car passes a truck on the highway, the two vehicles seem to pull toward each other. This pressure difference results in a net force, pushing the curtain inward. Calculate Arch Bricks, Bricks Wall, Circle Wall,Wall Volume by Bricks. The reason is that the high-velocity stream of water and air creates a region of lower pressure inside the shower, whereas the pressure on the other side remains at the standard atmospheric pressure. The thickness of mortar in brickwork is generally assumed to be 10mm or inch. For instance, shower curtains have a disagreeable habit of bulging into the shower stall when the shower is on. There are many common examples of pressure dropping in rapidly moving fluids. As a result, the pressure drops in a rapidly moving fluid whether or not the fluid is confined to a tube. The net work done increases the fluid’s kinetic energy. In general a large diameter circular tube is the most efficient shape under axial loading. This is probably offset by the fact that a circle would deflect projectiles better. The hexagon may be more resistant to localised impact, particularly if that impact occurs at a corner. localised buckling of tubes under bending (not a straightforward analysis - especially for the hexagon). A 9-kip force is applied at a bar (not shown) that is welded to the end of the tube. Some factors which I have not considered: Science Engineering Question The structural tube shown has a uniform wall thickness of 0.3 in. So this basic analysis shows that a hexagon of equal area and wall thickness would be weaker under bending and have a lower buckling load. Again, the circle is more resistant to buckling since its $I$ is larger. The axial stiffness is $EA$, where $E$ is Young's modulus which depends on the material (assumed to be the same in this case) and $A$ which is the cross sectional area.Ī hexagon with an equal cross-sectional area to a circle of radius $r$ and (thin) wall thickness $t$ must have side lengths of: $a=\frac $, where $L$ is the length of the tube. For them to be the same strength (and weight) they would have to have the same cross-sectional area. If it is assumed that both the round and hexagonal tubes are extruded (which is likely for a bike frame) we can ignore any cold-forming or welding issues. Hexagonal tube is likely to be much more expensive as it is a non-standard shape. I can't think of much advantage to using a hexagonal tube, except perhaps cosmetic - but this is subjective. It seems likely to me that hex tube would be a fairly poor compromise between square and round/elliptical section tube. Of course there are other pragmatic considerations to be taken into account for example square or rectangular tube is much easier to join than round as square tube can just be mitred to the required angle with a saw whereas round tube needs to be notched with a specialist machine or painstakingly hand fitted. Although this is, in most cases, impractical for fabricated frame structures. An ideal structure will have a smoothly varying section with section size proportional to the stresses on it, in fact bones are an excellent example of this. It is a fairly good rule of thumb in structures that any sort of discontinuity represents at best an inefficiency and at worst a potential failure point. If the section has a uniform wall thickness then it certainly means that some of the material is effectively wasted as the corners will encounter yield before the flats and at worst it can lead to crack propagation points and fatigue. With tubing this can be a double effect that you potentially have work hardening from the manufacturing process concentrated at the corners as well. An additional factor is that any section which has defined corners will tend to concentrate stress at the corners rather than it being evenly distributed throughout the section.
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