Structures: Underslung Cable Beam Bridge
UNDERSLUNG CABLE BEAM.   Design and Fabrication: Carl Pisaturo, 2004. Stainless Steel Rods, Polycarbonate Disks, Aluminum, Cable, Epoxy.  Length 80"
A Beam is a structure which carries load between 2 supports.  Many bridges are, structurally speaking, beams.  A beam may be as simple as a solid slab or more complex like this structure.  All beams contain compressive and tensile forces, and often (as in the case of the solid slab) a lot of extra material which performs no structural function. Complex beams are an attempt to get rid of this extra material, leaving only the tensile and compressive load bearing material.  Complexity, however, has its own costs in man-made structures which must be balanced against the weight savings gained.

This beam contains a single compressive element - a 12 rod circular group with stablizing bulkheads, similar to the column described previously.  The tensile forces are contained by a series of cables on the bottom of the structure. 

A key detail for this type of structure is how the cables join into the rest of the structure.  Custom cable terminations were developed for this structure, they serve double duty as cable grippers and turnbuckles (tension adjusters).  4 of these terminations are shown here joining into the (upside down) structure.  Where the cables pass through and support the structure, there are aluminum blocks to spread the force out and set screws to lock the form.  The 12 rods are fixed to aluminum end-plates by threading the rods and tightening threaded standoffs on each side.

This structure was tested successfully with up to 160 pounds at center of the 74" span. The "V" arrangement of tension members works well for resisting structure twist.
Polycarbonate bulkheads ready to go.   Sheet stock was marked via CAD gererated templates, bandsawn, sanded, drilled.
Rods have been laced through bulkheads.
Each rod gets an inch of 6-32 thread.  In situ threading makes lacing easier.
Anchorage Fab involves epoxy potting of splayed cable.
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works of  Carl C Pisaturo