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 longitudinal\transverse 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).
