Load Generators

Load generators turn a pressure you already know into ordinary member loads, so you don't have to work out and type each member load by hand. Open them from the Loads dialog with the Generate Loads menu. Each generator writes member distributed loads into the load case you choose; you can edit or delete them afterwards like any other load.

• Area Load Generator: floor and roof pressures onto beams.
• Lateral Load Generator: wall pressures (such as wind) onto columns.

Neither generator calculates code pressures. You enter the pressure from your design standard; the generator shares it out between the members. Pressures and lengths use your unit settings.

Area Load Generator

The Area Load Generator takes a floor or roof pressure (for example a 2.5 kPa or 50 psf live load) and turns it into line loads on the beams that support that floor. You don't need to model the slab or deck.

Opening it

• Whole floors: Loads dialog → Generate Loads → Area Loads. Choose which floor levels to load.
• Part of a floor: select the beams around the area (one bay, a corridor, an equipment footprint), right-click and choose Add Area Load. Only the selected beams are loaded.

Add Area Load is greyed out if the selection has no level beams running along X or Z (for example only columns or braces).

One-way or two-way

• One-way: the floor spans between parallel beams, and each beam carries the strip halfway to its neighbour on each side (line load = pressure × strip width). Under Supporting beams choose whether the X-running or Z-running beams take the load, as the deck or slab span direction can't be told from the geometry.
• Two-way: each rectangular bay is split along 45° lines, the usual hand method. Beams on the long sides get trapezoidal loads and beams on the short sides get triangular loads (all four sides are triangular for a square bay). Each bay must be closed by four beams meeting at its corners.

Settings

• Pressure: the floor pressure in your unit setting.
• Direction: Y (gravity) is the normal choice; a positive pressure acts downward. X and Z apply the pressure horizontally to the floor beams (positive along the positive axis, negative to reverse). For wind on walls, use the Lateral Load Generator instead.
• Tributary width (one-way): Auto works it out from the spacing to the neighbouring beams. Manual uses the width you type for every beam. Use Manual for a 2D frame, where the next frame isn't in the model, and enter the frame spacing.
• Replace loads this tool added earlier in this case (on by default): clears loads this generator added to the chosen load case before adding the new ones. Loads you entered by hand, and loads from other generators, are kept. Turn it off to add on top.
• Excluded members (whole floors, one-way only): leave chosen beams out. Strip widths are worked out again without them.

What it loads, and checks

• A floor level is a set of nodes at the same height (within 50 mm). Only level beams running along global X or Z are loaded; sloping beams, columns, and braces are skipped.
• One-way Auto needs at least two parallel beams that overlap along their length. A beam with no neighbour beside it gets no load from Auto; use Manual for it.
• After a two-way run, the total applied load is compared with pressure × loaded area, and you get a warning if they differ by more than 1%.
• If some bays in a two-way floor could not be closed, the generator warns and lists them. Check that those bays really should be unloaded.
• You also get a warning if beams at the same level cross each other in plan, since their loaded areas may overlap.

What it does not do

• It does not load cantilevers or overhangs beyond the outermost beams.
• It does not handle skewed, curved, or non-rectangular bays for two-way loading.
• It does not subtract openings. A bay framed around an opening is loaded as a full bay.
• It does not calculate code floor loads. You enter the pressure.

Lateral Load Generator

The Lateral Load Generator takes a pressure you already know, such as a wind pressure, and turns it into line loads on the columns along one side of your building. The result is ordinary member distributed loads in the load case you choose.

The generator does not calculate code wind pressures, pressure coefficients, or height variation. You enter the pressure; the generator only shares it out between the columns.

Why you need it

In a real building the wind hits the cladding (sheeting, glazing, blockwork). The cladding spans sideways and passes that wind to the columns behind it. You usually do not model the cladding, so your model is just columns and beams: there is nothing with an area to put a pressure on.

The generator works out how much wind each column would collect from the cladding if it were there, and puts that straight onto the column as a line load. Nothing is added to the model apart from those loads, and loads do not travel between columns. Each column gets its own share, exactly as if you had typed the member loads in by hand.

How each column's share is worked out

Each column takes a vertical strip of the (unmodelled) cladding. The strip runs from halfway to the next column on one side to halfway to the next column on the other side. This is the usual tributary width method engineers use by hand.

Line load on the column = pressure × strip width

Worked example: three columns 6 m apart, pressure 0.8 kPa.

If the cladding continues past the end columns, enter that length as the overhang. It is added to the end columns' strips: with a 0.5 m overhang the end columns above carry 3.5 m and 2.8 kN/m.

The dialog lists the strip width and line load for every picked column before you apply.

Using the generator

Open Loads → Generate Loads → Lateral Loads.

1. Columns that take the load. Click Pick and select the columns along one side of the building, for example the windward side. Only vertical members can be picked. If you select columns before opening the generator, those are used.
2. Pressure. Enter the pressure and the global direction it acts (+X, −X, +Z or −Z). A negative pressure means suction and acts the opposite way. The total row in the dialog shows the direction the load will really act, so check it when you enter suction. If the picked row runs along the direction you chose (for example a row along X loaded in X), the generator stops and asks for the other axis.
3. Width each column carries. Choose how the strip width is found (see below).
4. Add to load case. Choose the load case, or click + to add a new one. There is no default, so wind cannot land in your dead load case by mistake.

From column spacing

The generator measures the gaps between the columns you picked, along the side of the building (along Z for loads in X, along X for loads in Z), and works out each strip itself. It needs two or more columns in a line on one face. If the picked columns are not all on one face (for example a column from another side, or an interior line), the generator stops and asks you to fix the selection or switch to Fixed width.

Every height needs at least two picked columns side by side. On a gable wall where only the middle post rises above the eaves, From column spacing can't work out that top part: use Fixed width, or add the gable loads by hand.

If the column layout changes up the height (for example a column that stops at a lower storey), each column's strip is worked out storey by storey, so its load changes up its height. The dialog shows the average for those columns, marked “avg”.

Fixed width

Every picked column gets the strip width you type. Use this when the neighbouring columns are not in the model: a single column, or a 2D frame where you enter the spacing between frames. For a 2D frame loaded in its own plane, From column spacing is not available because the next frame is not in the model.

Loading several sides into one case

Each side of a building normally has a different pressure (windward push, leeward suction, side walls). Run the generator once per side, into the same load case if you want them to act together.

Replace loads this tool added earlier to these columns (on by default) removes only earlier generator loads on the columns you just picked, in the chosen load case, acting along the same axis. Loads you entered by hand, and generator loads on other sides, are kept. A corner column shared by two sides keeps the load from each side. Turn it off to add on top of what is already there.

The generated loads

Each column gets a uniform line load in global axes, in the chosen direction, over its full length. The total force on a column is pressure × strip width × column height.

What it does not do

• It does not calculate wind pressures or coefficients from a design code. You enter the pressure.
• It applies one pressure over the full height. If your design pressure increases with height, use the governing value or apply the variation yourself.
• It does not model the cladding, girts, or floor diaphragms that may actually carry part of the load to the floors. Putting the full share on the columns is the usual simplification for frame design.
• It does not create load combinations. Add the case to your combinations as usual.

Load cases are described under Load Cases, and individual member loads under Member Loads.