Monday, November 18, 2013


For convenience, support types are described  for beams, but apply to other horizontal  elements, like trusses, as well.  The type of support affects analysis and design, as well  as performance.  Given the three equations of statics defined above, ΣH=0, ΣV=0, and  ΣM=0, beams with three unknown reactions are considered determinate  (as described  below) and can be analyzed by the three static equations.  Beams with more than three  unknown reactions are considered  indeterminate and cannot be analyzed by the three  static equations alone.  A beam with two pin supports (1 has four unknown reactions, one  horizontal and one vertical reaction at each support.  Under load, in addition to bending,  this beam would deform like a suspended cable in tension, making the analysis more  complex and not possible with static equations.

By contrast, a beam with one pin and one roller support (2) has only three unknown  reactions, one horizontal and two vertical.  In bridge structures such supports are quite  common.  To simplify analysis, in building structures this type of support may be  assumed, since supporting walls or columns usually are    flexible enough to simulate the  same behavior as one pin and one roller support.  The diagrams at left show for each  support on top the physical conditions and below the symbolic abstraction.

Beam with fixed supports at both ends subject to bending and tension
2  Simple beam with one pin and one roller support subject to bending only
3  Beam with flexible supports, behaves like a simple beam

Simple beams, supported by one pin and one roller, are very common and easy to  analyze.  Designations of roller- and pin supports are used to describe the structural  behavior assumed for analysis, but do not always reflect the actual physical support.  For  example, a pin support is not an actual pin but a support that resists horizontal and  vertical movement but allows rotation.  Roller supports may consist of Teflon or similar  material of low friction that allows horizontal movement like a roller.

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