Moment Diagram



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    Raphael Arsenian

    If you are talking curves, that means the load is linear (vs. point). All gravity loads on simple beams create concave moment diagrams. The only "convex" diagrams I can think of are on cantilevers (that is why we put the tension steel in the upper half of a cantilever concrete beam), retaining walls, or on a portal frame (where lateral forces result in an upward component).

    The exam will test you on very simple loading conditions. I had a question straight out of the Structural References Diagrams tab.

    Hope this helps.

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    Timothy Martin

    Yes, the shape of the moment diagram depends on the shear diagram, which depends on the load diagram, which depends on the beam's applied forces (concentrated and uniform) it supports and it's boundary conditions (fixed, pinned, etc.)... all held in equilibrium, on a good day. If you draw a load, shear and moment diagram for a simple case then you'll recall how they are related to each other. One rule is the area under the first curve (i.e. shear) equals the magnitude of the second curve (i.e. moment) at any point along the beam. Another rule is the magnitude of the first curve (i.e. shear) at any point along the beam, is equal to the slope of the second curve (i.e. moment). For example, in the middle of a simply-supported, uniformly loaded beam is where the shear goes to zero and the slope of the moment curve is zero, at the peak of the concave shape. The key to drawing good diagrams is being consistent in the sign convention, defining positive and negative direction. After working through a simple example, you might start to gain an intuitive sense for other more complicated cases, whose diagrams can be reviewed in standard beam formulas (i.e. google search, or chapter 3 of AISC Steel Construction Manual) while studying for the exam. 


    Good news, after the exam you'll never have to mess with this again, unless you happen to be discussing the results of a computer structural analysis with a structural engineer, which could happen if they are kind enough to step you through the basis of their design, most likely for a complicated and challenging problem you've given them. :)

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    Caroline James


    Thanks for these tips--I found the structural resources from AISC listed in the NCARB ARE Handbook online. Super helpful!

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