class Element {
    public:
        int width;
        double depth;
        int bearingLength;
        double secMmtArea;

    Element() {
        width=0; depth=0.0; bearingLength=0; secMmtArea=0.0;
    }

    // constructor that sets members to passed-in variables
    // using the initialiser list
    Element(int w, double d, int bl, double sma)
    : width(w)
    , depth(d)
    , bearingLength(bl)
    , secMmtArea(sma)
    {
        // i could equally have done this within the function body
        // but the initialiser list is preferred when possible
        // width = w;
        // depth = d;
        // bearingLength = bl;
        // secMmtArea = sma;
    }

    double SecondMomentofArea() {
        secMmtArea=width*depth*depth*depth/12;
        return secMmtArea;
    }
};

class Beam : public Element {
    public:
        int span;
        int spanDiff;

    // constructor that will call the base 'Element' class constructor with our
    // parameters and then set the members of this class as well
    Beam(int s, int sd, int w, double d, int bl, double sma)
    : Element(w,d,bl,sma)
    , span(s)
    , spanDiff(sd)
    {
        // again, could have done:
        // span = s;
        // spanDiff = sd;
        // but must use the initialiser list to call the base-class constructor...

    }

    Beam() {
        span=0;spanDiff=0;
    }
};

and then later on in your main function:

Beam beamOne(10,2,50,2,49); // replace numbers with what you actually want to match up with the order they're passed into the beam constructor