In order to obtain an element with no weight you need to change, from material properties, the element density to 0. (go to Manage–>Used materials–>select the material–>Mechanical Properties and set “Density” value to 0).
You can impose smaller rigidity for an element in two ways:
1.You can change material properties to a smaller value for longitudinaltransverse rigidity (go to Manage–>Used materials–>select the material–>Mechanical Properties and insert the desired value for longitudinal rigidity).
2.Another way you can change the rigidity of an element is to select the element–> go to properties–> Cross section–> set “Concrete inertia type” to “Imposed value” and insert a value for “Cracked section inertia coefficient” as a ratio between desired rigidity and full rigidity (value smaller than 1).