What is a simple support?
A simple support (Dutch: vrije oplegging) is a support on which a beam, girder or floor rests freely and can rotate, without being fixed. It transfers a vertical (and possibly horizontal) force, but no bending moment. A beam on two such supports is called a simply supported beam.
How it works
In structural analysis a simply supported beam is usually modelled with a pinned support at one end and a roller support at the other. The beam can then deflect freely, rotate at the ends and expand or shrink with temperature. The beam is statically determinate: the reactions follow directly from equilibrium.
Rules of thumb for a simply supported beam with span L and uniformly distributed load q:
- Maximum moment at midspan: M = q·L²/8.
- Reactions: q·L/2 at each end.
- Midspan deflection: 5·q·L⁴/(384·E·I).
For comparison: a beam fixed at both ends deflects five times less at midspan (q·L⁴/(384·E·I)), but develops moments at the supports that the supports must resist.
In practice
Simple supports are found with timber joists bearing on masonry, precast concrete floor units, lintels over openings and many steel beams. Tips:
- Provide enough bearing length, so the load is introduced properly and the masonry below does not fail. The required length follows from the design or the manufacturer’s instructions.
- Let the beam move. A beam designed as simply supported but then cast in solid behaves differently from the design and can cause cracking.
- Check deflection: for a simply supported beam it often governs, especially in timber. If in doubt, have a structural engineer do the calculation.
Related terms
- Pinned support
- Roller bearing support
- Fixed support
- Statically determinate
- Moment
- Bending moment diagram
- Beam
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