Gravity

Gravity loads are a special case of body loads i.e. loads that apply internally throughout the body rather than being applied externally to the body. Unlike the other load types, gravity loads are an acceleration applied to mass in the structure.

Gravity loads are one way to account for the dead weight of the structural elements, but excluding any non-structural loads. However gravity loads can be applied in any global direction so it is possible to use gravity loads to model transverse effects as well as vertical effects. The gravity load considers all the mass of the structure that is modelled. Non-structural elements, floor slabs etc which are not modelled explicitly are excluded. The gravity loads are defined in terms of 'g', thus a normal gravity loading on the structure is modelled with an acceleration of −1g (1g in the negative Z direction).

Normally gravity is thought of applied to the whole structure. In some cases it is convenient to look at gravity applied to a subset of the structure. In this case gravity loads are applied to elements specified by a list.

This is useful where design checks which require different gravity factors on different spans. So for example if elements 1, 3, 5, … are assigned gravity loading in load case 1 and elements 2, 4, 6, … are assigned gravity loading in load case 2. Then it is possible to examine the effects of adverse and beneficial loading using L1 + 1.4L2 and 1.4L1 + L2.

Definition

Name

The name is only used as a convenient way of identifying a load. Optional.

Element List

This specifies a list of elements to which the gravity loads apply using any one of the forms detailed in “Lists”. The list may be a single item, but is typically “all”.

In sculpt: The element list is set to the elements in the current selection set. The element list cannot be edited in the dialog.

Load Case

The load case in which the gravity load applies. The load case gives a way of grouping load effects together.

Gravity Factors

These are the factors that apply on “gravity”. They will nearly always be used with gravity factors of (0, 0, −1) for normal static structures with the global Z-axis vertically upwards. However to model seismic effects it may be useful to apply a static horizontal loading of, for example, 0.25g in the X direction in which case the factors may be (0.25, 0, 0).

Application

Gravity loads apply to the following element types:

• Beams, bars, rods, ties and struts – through the area, density and length.
• Masses – through the element mass
• Cables – through the mass per unit length and mass
• 2D Elements – through the thickness, density and area. plus the additional mass per unit area and area
• 3D Elements – through the volume and density

Spring, damper, link and spacer elements are considered massless, so attract no gravity load.

Load cases and titles

The loading is stored by case number. However it is good practice to give load cases names. The load title gives access to the Load Case Titles wizard to simplify this procedure. The wizard is opened for the currently displayed load case.