Structural Engineering & Statics
where the load goes
Concept

Truss analogy — where it appears

The model in which a cracked concrete beam is treated as a truss of concrete struts, steel ties and chords. It turns a shear calculation into an equilibrium statement about members, and the angle chosen for the struts is a design decision rather than a measured property.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

the cutcompression chordtension chordstirrups in blue, struts dashedz = 495cot θ = 2.5 · 8.3 stirrups crossed · V = 563 kN from 1047 mm²/mstrut stress 8.79 N/mm² against 10.56 available

The beam that becomes a truss

Once a web has cracked in shear there is no shear stress field in it any more. There are concrete struts, two chords and whatever crosses the cracks, and the angle of those cracks is not a property of the material — it is something the designer chooses, and every quantity in the beam moves when it changes.

internal-forces · shear truss analogy load on the top chordthe same load, hung from the soffit600 kN in the hangernothing in the same memberchord forces identical to machine precision · hanger steel 1200 mm², shear steel 96 mm² over the same length

The load that has to be lifted

Every shear calculation on this site assumes the load arrives on the top of the beam and walks down a diagonal to the support. Hang the same load from the soffit and the diagonal cannot start — the load has to be carried up to the compression zone first, by a tie no sectional calculation contains.

internal-forces · indirect support

Named alongside it

The objects these essays reach for when they reach for this one.

EquilibriumFree bodyShearStrut and tieCompression strutConcrete strutCracked sectionDetailingHanger reinforcementIndirect supportLever armLoad path

All concepts