Slide 1
Triangles carry loads through pulling and pushing forces
A truss uses connected members mainly in tension, which pulls, and compression, which pushes. Triangles resist shape change without changing the members’ lengths.
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A pin-jointed rectangle can deform into a parallelogram, while adding a diagonal restrains that mechanism. Real joints are not perfect pins, but the comparison explains why triangulation is structurally useful.
Slide 2
The ideal model places loads at the joints
In an ideal truss, loads act at joints. Each member is a two-force element: it carries forces at its two ends, acting along its axis.
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A load applied between joints introduces bending into that member. The simple truss model is then incomplete unless the extra effect is included, showing why load placement matters as well as the outline.
Slide 3
Pushed members can buckle before they crush
Tension members resist being pulled apart. Compression members also need to resist buckling, which can occur before material crushing.
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A slender compression diagonal may require a larger section or bracing even when its axial force matches that of a tension member. Equal force does not imply identical design requirements.
Slide 4
Strong bars still need strong connections
Bolts, welds or other joints transfer forces between members. A truss with strong bars can still fail at a weak connection.
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Eccentric connections, where force lines do not meet at one point, create local bending. Actual joint geometry should therefore be reflected in analysis rather than replaced automatically by an ideal pin.
Slide 5
Extra depth can reduce force but needs more space
A deeper truss can reduce forces in its upper and lower members, called chords, for a given bending demand. It also occupies more space and changes bracing requirements.
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The upper and lower chords act as separated force paths forming a resisting couple. Increasing their separation can reduce the required force, but transport, clearance and construction constraints limit available depth.
Sources
- Trusses
MIT / David Roylance
- Force and Stress Analysis Issues
Federal Highway Administration