Global Surface Load [FEA]

A Global Surface Load applies a distributed pressure over a polygonal area defined by a set of points, distributing it to the elements and nodes of the nominated group that fall within that area.

Use it for deck overlays, snow, equipment pads, soil surcharge, and any area load whose footprint is defined geometrically rather than by naming elements. Like the other Global loads, it survives re-meshing.

Global Surface Loads generate load. They add nothing to stiffness or mass.

Surface Load

Load Case: The analysis case the load belongs to. With no case assigned it applies nowhere.

Group: The FE Group whose elements can receive the load.

Engineering influence. Only elements inside this group and inside the polygon receive load. Both filters apply, so a load can be lost either by a polygon that misses the structure or by a group that excludes it — and neither produces a warning.

Points: The polygon defining the loaded area, edited in its own sub-sheet.

Engineering influence. The polygon sets the footprint, and the total load is pressure × enclosed area. Its accuracy therefore directly scales the applied load. A polygon that does not close properly, that is self-intersecting, or whose points are ordered inconsistently can enclose a different area than intended — sometimes a much smaller one. On a curved deck the polygon must have enough points to follow the edge, or it will cut across the curve and under-apply near the outside.

Fx / Fy / Fz: Pressure components, in force per unit area.

Engineering influence. Response scales linearly with pressure in a linear analysis. Sign follows the axis direction — a downward overlay pressure in a Z-up model is a negative Fz.

The pressure is applied over the polygon's area. Where the deck is sloped, an overlay quantity derived from plan area must be converted, or the applied load will differ from the intended total by the slope factor.

Convert to Node Load: When enabled, the pressure is resolved into equivalent concentrated loads at nodes rather than applied as a distributed pressure.

Engineering influence. Node conversion is more predictable but loses the correct distribution within each element, so local moments differ while the total and the global response are close. Leave it off unless nodal application is specifically needed, and if it is on, ensure the mesh is fine enough for the discretization to be fair.

Load Distribution

These settings control how the pressure varies across the footprint rather than being uniform.

Load Type: Selects the distribution pattern applied over the polygon — uniform, or varying between a base and an apex value.

Engineering influence. A uniform pattern is right for an overlay or snow. A varying pattern is what soil pressure needs, since lateral earth pressure grows with depth; applying a uniform pressure where a triangular distribution is correct puts the resultant at the wrong height and produces the wrong overturning moment on a wall or abutment even when the total force happens to match.

Direction X / Direction Y / Direction Z: Define the axis along which the distribution varies.

Engineering influence. This is the direction the "base to apex" variation is measured in — for earth pressure on a wall, the vertical. Setting it to the wrong axis makes the pressure vary horizontally across the wall instead of with depth, which produces a completely different force distribution while still summing to a plausible total.

Load Coeff 1 (Base) / Load Coeff 2 (Apex): The multipliers applied at the base and apex of the distribution.

Engineering influence. These scale the pressure at each end of the variation. Equal values give a uniform distribution; 0 at one end and 1 at the other gives a triangular one, which is the usual earth-pressure idealization. The resultant force and, more importantly, its line of action depend on the ratio between them — a triangular distribution puts the resultant at one third of the height, a uniform one at mid-height, and the overturning moment differs accordingly.

Verification

• Sum the case's reactions and compare against pressure × polygon area computed by hand. This is the definitive check.
• Display the applied loads and confirm the loaded footprint matches the intended area, with no gaps at the boundary and no spill beyond the structure.
• Confirm the polygon closes and does not self-intersect.
• For a varying distribution, check the line of action by comparing the overturning moment at the base against a hand calculation — a correct total force with a wrong distribution shows up here and nowhere else.
• On a sloped deck, confirm whether your pressure was derived from plan or surface area.
• Check the sign by inspecting the deflected shape.

Common mistakes

• A polygon that misses the group's elements, so the load is silently lost.
• Too few polygon points on a curved edge, under-applying load near the outside.
• A self-intersecting or unclosed polygon, enclosing the wrong area.
• Using a uniform distribution for earth pressure, putting the resultant at mid-height and understating overturning at the base.
• The distribution direction set to the wrong axis, varying pressure across the wall rather than with depth.
• Using plan-area quantities on a sloped deck without converting.
• Sign error, applying overlay pressure upward.
• No Load Case assigned, so the load does nothing.