Load cases

Actions, self-weight, imposed loads and load arrangements — how to build cases that stay separable.

Loads & combinations 7 min read

A load case is one action, unfactored, applied to the model. Combinations do the factoring. The rest of the process depends on keeping to that.

One action per case

Good case names	Why they work
`Self-weight`	Generated by the model, factored as permanent
`Superimposed dead — finishes`	Permanent, but separable from structure weight
`Imposed — offices`	Variable, its own ψ factors
`Imposed — plant`	Different ψ factors from offices
`Snow`	Its own combination rules
`Wind + X`	One direction, one internal pressure assumption

Bad case names	Why they hurt
`Dead + finishes + services`	Cannot be factored separately later
`All imposed`	Cannot produce adjacent-span arrangements
`Wind`	Which direction?
`Worst case`	The combination generator exists precisely to find this

Self-weight is not a load you type in. A load case whose type is Self-weight carries a generator: a scope (all elements, so that new members are included automatically, or a selection), a factor, and a direction. The value is never stored. At solve time each element's line load is worked out from its section area, its material density and the project's gravity, so changing a section changes the self-weight with it and there is nothing to keep in sync. The properties panel shows the value a member will receive, in N/m and as a total over its length.

The direction defaults to the opposite of the project's up axis, set in project settings, and a case can override it, which is how a horizontal mass effect or an inclined structure is handled.

The Loads dock audits the generators: it reports how many elements self-weight covers, how many are weightless because no case reaches them, and how many are counted twice because two cases do. Either count above zero is a button that selects the elements in question. An element with no section or a material with no density contributes nothing and is listed by id.

Non-structural weight (screeds, finishes, services, ceilings) is not self-weight. It belongs in its own permanent case, as an ordinary load.

Load types

Type	Applied to	Typical use
Point force or moment	Node or position on a member	Column reactions, plant, secondary beams
Uniform distributed	Member, full length	Slab loads on beams, cladding
Linear	Member, varying between the ends	Wind on a rafter, varying soil pressure
Surface	Plate, wall, shell, uniform or graded	Floor loading, wind pressure
Edge load	Edge of a surface, varying between the corners	Walls on slabs, parapets
Temperature	Member or surface	Uniform change or through-thickness gradient
Load panel	An area over bare members	A roof or floor pressure carried to the beams under it, with no slab to mesh

The docked Loads table with 107 rows, showing loads EL1 to EL4, their load cases Roof cladding and Snow and Wind transverse, type Uniform distributed, target E25 and E26, and qx, qy, qz values in kN/m — The loads table of the steel hall: one row per applied load with its load case, type, target member and components — EL1 is a uniform distributed load of −0.525 kN/m in z on member E25 in the roof cladding case, EL2 the snow on the same member.107 rows

Load types. Line and surface loads can act in global or local directions; temperature splits into a uniform change and a gradient through the depth.

Load panels

A roof or a floor on a bare frame carries a pressure, but there is no slab to mesh and the load has to reach the purlins or the beams as line loads. A load panel is an outline drawn over the members that does that arithmetic: put a surface load on the panel and it is delivered to the members in its plane.

Setting	Options
Spans	Two way, every point goes to its nearest beam (the 45° rule), or one way along a global axis, so beams along that axis carry nothing
Delivers to	Beams, every member in the panel's plane including interior joists; Edges, only the members along the outline and hole rims; Corners, the corner nodes as point loads
Method	Tributary, the 45° construction or one-way strips that a hand calculation assumes; Standard, shared by length and weight and laid on uniformly; Accurate, the panel solved as a plate with its reactions used as the loads, truest where spans are unequal

The panel's properties list every receiving member with its share, the total delivered, and the area that reaches no member at all, which is the number to look at when a reaction is missing. The Display options of a viewport show or hide the outlines.

Local versus global direction

A load in the global Z direction is gravity. A load in the member's local direction acts perpendicular to the member. On a horizontal member they coincide, which is why the distinction is only ever discovered on a sloping one.

On a sloping rafter, snow acts vertically over the plan area, wind acts perpendicular to the roof plane, and self-weight acts vertically along the member length. That is three conventions in one load path, which is why a roof is where units and directions go wrong.

Global or local axes

Uniform, linear, surface, graded and edge loads each carry a Global / Local switch in the load editor. Local values turn with the member: x′ runs from its start node to its end node, y′ and z′ are its section axes.

Load on	Local directions
A member	Along the member (local x′), Local y′ (section axis), Local z′ (section axis)
A plate	Local x′ and Local y′ (in the plate plane), Local z′ (normal to the plate) — the axes its results are reported in, with z′ pointing down on a slab

Wind on a sloping roof plate is one local pressure, not a pair of global components you worked out by hand; a surface pressure or a graded (hydrostatic, earth) pressure starts in the plate's local axes for that reason. The local axes travel with the load into the solver, the result tables, the report, IFC and SAF, and through Mirror and Divide beam.

How the load is drawn

The load editor draws the load on the member's real geometry — its supports, its dimensions and a legend of its local axes — beside the target, the case and the values. Linear, edge and graded loads take a Start / End table, with buttons to swap the ends or make them equal. How forces are drawn shows a load as its Components or as one Resultant arrow.

In the viewports a member load is drawn as a band along the member, a surface load as a volume over the plate, and in a 2D view a global load on a sloping member points the way it really acts.

Load arrangements

For continuous structures the governing arrangement of variable load is rarely "all spans loaded":

Maximum sagging in a span: that span loaded, alternate spans loaded.
Maximum hogging at a support: the two adjacent spans loaded.
Maximum shear at a support: adjacent span loaded.

Each arrangement is a separate load case, related to the others in a group so that the combination generator treats them as alternatives, not simultaneous actions. Model them as separate cases from the start: retrofitting arrangements into a model built with one blanket imposed case means redoing the load application.

Grouping

Every case belongs to a load group, and the group answers the two questions the combination generator cannot answer from a case alone.

Action, permanent or variable. Permanent cases act in every combination, in full or with the favourable factor; variable ones act or not. The two never share a group.

Relation, how the group's cases may appear together:

Relation	Behaviour	Use it for
Standard	No constraint; each case is considered on its own	Independent actions
Exclusive	At most one case of the group acts in any combination	Wind directions, arrangements of the same imposed load
Together	All of the group's cases act, or none of them	A load that is split across cases for bookkeeping but is physically one action

Category, for variable groups: an entry from a shared catalogue (imposed A to H, snow by altitude, wind, temperature, seismic and so on) that supplies the $ψ_{0}$ , $ψ_{1}$ and $ψ_{2}$ the generator reads. Custom ψ lets you type the three values for a National Annex the catalogue does not carry. The catalogue is shared between EN 1990 and ASCE 7, so a group tagged wind stays wind if the project changes standard.

There is no accidental group: EN 1990 6.11 and the EN 1998 seismic combinations are not generated, and have to be written by hand. ASCE 7 seismic combinations (§2.3.6, §2.4.5) are generated, with a single seismic effect $E$ .

Correct groups are what make automatic combinations produce a sensible set instead of a very long one.

Before combining

Every case contains one action, unfactored.
Self-weight is counted exactly once, which the Loads dock audit confirms.
Wind and imposed load have the arrangements the code requires.
Cases are assigned to the right group.
Each case has been solved and looked at on its own. The deflected shape under a single case is the fastest way to spot a load applied in the wrong direction.

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