Apr 9, 2017

[2.70] Seek and Geek #7: Truss Geometries

2.70 coursework is pivoting towards specific components of the desk, so this seek&geek is about possible truss geometries for the desktop. I expect a constant distributed load (from desk weight and stuff placed on top) as well as a worst case point-load on the end (elbows and bodyweight). So how do other real-life objects achieve lightweight rigid planar structures?

Trusses are statically determinate structures solely consisting of two-force members, so basically assemblies of pinned beams. Trusses are most commonly seen in bridges, where the top beams "top chords" are typically under compression and the bottom chords are under tension.

Warren truss bridge

In my case, a desktop with a truss frame would experience tension on top and compression on the bottom. This design problem is similar to overhead cantilever road signs seen on the highway! Therefore, my desktop design can draw from a wealth of experience and calculations.

Highway signs also have to deal with significantly higher lateral loads than my desk does
http://s-steel.com/overhead-sign-structures/
Highway signs typically use a Pratt truss geometry to minimize sensitivity to buckling

Drawings and calculations for a highway overhead sign in I-85 Atlanta, GA

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