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Thread: Support Types and Their Reaction Forces

  1. #1

    Support Types and Their Reaction Forces

    Hello all

    Please dont shoot me down for asking this question as I know it maybe simple for most people.

    I am trying to remove doubt from my mind regarding the 3 x types of supports and their reactions.

    To me the types of support and their reaction forces are:-

    Fixed Support = Vertical Reaction, Horizontal Reaction, Bending Moment Reaction.
    Hinged Support = Vertical Reaction, Horizontal Reaction.
    Roller Support = Vertical Reaction, Bending Moment Reaction.

    To me the above supports can ONLY have those reaction forces - would this be correct?

    The reason i ask is because someone told me that a roller support does NOT have a bending moment reaction and i was immediately thrown into confusion.

    Can anyone confirm?

    Thank you.

    Supports.PNG

  2. #2
    Administrator Kelly Bramble's Avatar
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    We assume an idealized behavior of a support in order to simplify the analysis. Generally, one would assume a x-y analysis separate from an x-z.

    I think your instinct is correct in that a roller support can realize a bending load/moment perpendicular to the translation direction allowed by the support though this can be minimized by design.

    Assumptions in analysis:

    Roller supports are free to rotate and translate along the surface upon which the roller rests. The surface can be horizontal, vertical, or sloped at any angle. The resulting reaction force is always a single force that is perpendicular to, and away from, the surface. Roller supports are commonly located at one end of long bridges. This allows the bridge structure to expand and contract with temperature changes. The expansion forces could fracture the supports at the banks if the bridge structure was "locked" in place. Roller supports can also take the form of rubber bearings, rockers, or a set of gears which are designed to allow a limited amount of lateral movement.

    A roller support cannot provide resistance to a lateral force. Imagine a structure (perhaps a person) on roller skates. It would remain in place as long as the structure must only support itself and perhaps a perfectly vertical load. As soon as a lateral load of any kind pushes on the structure it will roll away in response to the force. The lateral load could be a shove, a gust of wind or an earthquake. Since most structures are subjected to lateral loads it follows that a building must have other types of support in addition to roller supports.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
    Hello Kelly

    Thank you for your responses, I really appreciate it.

    Just a follow on question.

    The attached drawing (Picture 1) shows a member that support a UDL of 200kN.

    The member sits directly on 3 x supports, to me I would describe these supports as roller supports - would you agree?

    If this is correct then they have no horizontal reaction forces which I am happy with.

    However since it is a roller support it has no moment reaction (which you have kindly informed of) but I struggle to visualize this, the flat underside of the member sits directly on a flat horizontal supports - would it be correct to say that the the member rotates about the edge of the supports as shown in picture 2 (forgive my poor sketch, I have highlighted in red circles the rotation points).

    Picture 1 = Picture 1.PNG

    Picture 2 = Picture 2.PNG

    Thank you.

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