Mass and weight

Mass in the structure is represented by the mass of the structural elements and mass properties applied to nodes. Weight is the effect of gravity on this mass. Mass is the key aspect for dynamic analysis while weight is key for static loads.

Mass modification

There is scope to modify the basic mass of the elements, which can have an effect on both static and dynamic analysis.

For a beam, bar, tie or strut elements the mass is the area × length × density. There is an option in the section properties to modify the effective area of the section and this modification can either be applies or ignored in the mass (and weight) calculations.

For a 2D element the mass is the area × ( thickness × density + additional mass). There is an option to modify the effective thickness for mass and weight calculations

For mass properties on nodes the mass is specified directly. There is also an option to modify the mass in different directions. This is only available for dynamic analysis and is primarily intended for fluid structure interaction calculations.

Additional mass in dynamic analysis

When building a model it is convenient to ignore non-structural parts of a building and represent these as loads: usually beam loads and 2D face/edge loads. If these are ignored in a dynamic analysis the response will be incorrect. The additional mass option for dynamic is intended to allow for these to be properly included. In general these loads represent the effect of gravity of the non-structural aspects of the building, so to convert these to mass the load vector is assembled and the mass is calculated (assuming z is vertical) from

$$\begin{aligned} w &= f_z\\ m &= \frac{w}{m} \end{aligned}$$

Any gravity loads are ignored in the additional mass calculation as the mass of these loads is defined explicitly.

Weight from mass in static analysis

In a static analysis the weight of the structural elements is accounted for by gravity loading. The general case is

$$\begin{Bmatrix} f_x\\ f_y\\ f_z \end{Bmatrix} = \begin{Bmatrix} a_x\\ a_y\\ a_z \end{Bmatrix} mg$$

But the weight calculation corresponds to the factor being 0 for $x$ and $y$ and -1 for $z$ so

$$f_z = −mg$$

This is always based on a scalar mass so the mass modifiers applied to a mass properties are excluded.

The weight calculated from the mass is likely to be an underestimate as the mass of connections, stiffening plates, etc. is ignored. This can be adjusted for by modifying the gravity factor, so for a 10% allowance the gravity factor can be set to −1.1.