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correspond to the bottom chords of the bridge, experience mainly tensile stresses. The |
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diagonal struts making up the web can experience either type of stress depending on their |
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position in the framework relative to the load. Such spaceframes are statically determinate. |
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That is, their internal forces can be predicted using static analysis techniques. It is common |
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practice, however, to construct real life spaceframes using rigid joints for the hubs to reduce |
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the forces experienced by the members and the amount of deflection the spaceframes |
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exhibit. Like the rigid gusset plate joints of truss bridges, and the fixed ends of columns and |
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beams, rigid hubs impart additional bending stresses in the struts which have to be figured |
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into the load calculations. Accounting for these perpendicular acting forces in addition to |
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the axial forces increases the required cross sectional area of the struts, and the amount of |
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material used for them, compared to spaceframes with flexible joints. In general the small |
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benefit achieved by rigid hubs is offset by the increased cost of materials. Although both |
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types of structures are referred to here as spaceframes, technically, lattices with flexible |
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hubs should be referred to as space trusses, and those with rigid hubs as spaceframes. |
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. |
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Just as you saw that there is a relationship between the H/S ratio of a truss bridge and the |
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maximum load it can carry, so too the ratio of the depth of a spaceframe to its span affects |
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its load bearing capacity significantly. In the following example, the depth of the unit cell |
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of the Tri-1 spaceframe is varied. This involves varying the length of the struts in the inner |
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and outer layers, as well as the web diagonals. You can model this with Polymorf by simply |
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building the unit cells out of either small triangles (ST), or right triangles (RT), or large |
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triangles (LT), or isosceles triangles (IT) as shown by the following. |
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. |
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side view |
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. |
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As shown by the forgoing models, The length of each strut comprising the grid layers is |
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referred to as the spaceframe's module size [e.g. cell a) module = 7.18 cm]. And the |
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distance between the layers is its depth [e.g. cell a) depth = 2.95 cm]. As with A-frame and |
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truss bridge structures, increasing the depth, or height, of a spaceframe increases the |
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Back to Knowhere |
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