distance it can span for a given load, all other factors being equal. For example, the model

of unit cell c) should be able to span twice the distance or carry twice the load of unit cell

a), because c) has twice the depth as a) but the same module size. Consequently the depth/

module ratio of c) is twice that of a).  As a result the diagonal struts of cell c) are aligned

more parallel to a perpendicularly applied load.  This reduces the internal stresses its web

struts are subjected to. However, as with truss bridges, there is a limit to how deep a double

layer spaceframe lattice can be before other factors such as the slenderness ratio of the

struts, the dead weight of the structure, and its cost must be taken into account.  Note also

that when the depth/module ratio of a Tri-1 unit cell is equal to 0.816, all of the struts of the

spaceframe are the same length.  That is, it is a standard octet truss.

.

Another factor affecting the load bearing capacity of a spaceframe is how its perimeter is

supported.  In general, the more support that is given to the spaceframe's perimeter, the

longer it can span, and the greater load it can carry, all other factors being equal.  For

example, for a given load, the required depth to span ratio for a spaceframe entirely

supported around its perimeter is about one-half of that for one that is just supported at its

corners.

.

tri-1_spaceframe.jpg

Fig. 235 - Tri-1 spaceframes

tri-1_spaceframe.jpg

with equal spans but

different depths

  ◄  depth = 5.9 cm = .183

        span     32 cm   (all LT)

 

      depth = 2.9 cm = .092 ►

 span     32 cm   (all RT)

.

This is due to the fact that the maximum bending moment exerted by a load located mid-

span of the structure is a function of its distance from the nearest support, as you saw before

with beams and truss bridges.  Supporting the entire perimeter of the spaceframe reduces

this distance overall compared to just corner supports.

.

The Tri-1 spaceframe is a very efficient design due to its omni-triangulated geometry that

minimizes material usage while maximizing its surface area.  It is particularly appropriate

for maximum load bearing situations.

.

offshore_oil_platform.gif

Fig. 236 - Tri-1 spaceframe

bandshell.gif

structures

 

◄  offshore oil rig

band shell  ►

(scale visualization models)

click image to enlarge

.

Variations of the Tri-1 spaceframe design are generated by selectively removing struts from

the structure in a regular pattern.  Two common examples are Tri-3 and Tri-5.

.

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Page 137 - Building stability - Tri-1 spaceframe

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