In short: STAAD.Pro has an IS 875 (Part 3):2015 wind load generator, but it is built for vertical panels: Bentley's own documentation says sloped roof members are not loaded by the general wind generator and must be loaded separately. For portal frames and PEBs, most engineers calculate the pressure, work out Cpe and Cpi, and apply member UDLs to the columns and rafters. Wind Load does that whole job: it calculates the pressure to IS 875, tags the walls and roof in the 3D view, and writes the member loads into your STAAD.Pro load cases.

Wind is usually the governing load on a steel shed, and it is also where most of the setup time goes. This guide explains how to apply IS 875 (Part 3):2015 wind load to a STAAD.Pro model of a portal frame or PEB, what STAAD's built-in generator does and doesn't cover, and how to check the result.

If you want the code procedure itself, step by step, read our IS 875 Part 3 wind load calculation guide.

Two ways to apply wind in STAAD.Pro

  1. The built-in load generator. Define the wind (speed, factors, pressure coefficients) and let STAAD distribute it to the structure.
  2. Member loads. Calculate the design pressure yourself, multiply by the frame spacing and the net pressure coefficient, and apply a uniform load (UDL) to each column and rafter in each wind load case.

For enclosed portal frames, the second method is the one most design offices use, because it gives full control over the roof loads.

Step 1: Design wind speed

Whatever method you use, start with the design wind speed (IS 875-3, clause 6.3):

Vz = Vb × k1 × k2 × k3 × k4

  • Vb: basic wind speed for the site (33, 39, 44, 47, 50 or 55 m/s; Annex A lists cities).
  • k1: probability factor (Table 1), 1.0 for general buildings with a 50-year life.
  • k2: terrain roughness and height factor (Table 2), by terrain category 1 to 4 and height.
  • k3: topography factor, 1.0 on flat ground.
  • k4: importance factor for cyclonic regions: 1.0 general, 1.15 industrial, 1.3 for post-cyclone structures.

Step 2: Design pressure

pz = 0.6 × Vz² (N/m²)

pd = Kd × Ka × Kc × pz, but not less than 0.7 pz

  • Kd: wind directionality factor.
  • Ka: area averaging factor (Table 4): 1.0 up to 10 m², 0.9 at 25 m², 0.8 at 100 m² or more.
  • Kc: combination factor.

Example. Vb = 39 m/s, k1 = 1.0, k2 = 1.0 (terrain category 2 at 10 m), k3 = k4 = 1.0:

  • Vz = 39 m/s
  • pz = 0.6 × 39² = 912.6 N/m² = 0.913 kN/m²
  • With Kd = 0.9, Ka = 0.8 and Kc = 0.9, the product is 0.648, so Kd·Ka·Kc·pz = 0.591 kN/m²
  • That is less than 0.7 pz = 0.639 kN/m², so pd = 0.639 kN/m²

The 0.7 pz floor is easy to miss, and it governs more often than people expect.

Step 3: Cpe and Cpi

  • Wall Cpe comes from Table 5 (faces A, B, C, D), based on the building's h/w and l/w ratios.
  • Roof Cpe comes from Table 6 for pitched roofs (or Table 7 for mono-slope roofs), based on h/w and roof angle, for each wind direction.
  • Cpi depends on the openings (clause 7.3.2): ±0.2 for openings under 5% of the wall area, ±0.5 for 5 to 20%, ±0.7 for more than 20%.

Each wind direction is checked with both +Cpi and −Cpi, which is why a gable shed typically ends up with 8 wind load cases: 0°, 90°, 180° and 270°, each with +Cpi and −Cpi.

Step 4: Member loads

For each member:

UDL (kN/m) = pd × tributary width × (Cpe − Cpi)

Example. Windward wall, wind normal to the ridge, h/w ≤ 0.5: Cpe = +0.7. With Cpi = −0.2, the net coefficient is 0.9. With frames at 6 m spacing:

0.639 × 6 × 0.9 = 3.45 kN/m on the windward column, acting inward.

Repeat for the leeward column, both rafter halves and every load case, with the right sign for each surface. For a single frame with 8 cases, that is 32 member loads; for a whole building, hundreds.

What STAAD's built-in generator covers

STAAD.Pro's IS 875 (Part 3):2015 dialog (Loading → Load Generation) handles the code factors (k1, k2, k3, k4, Kd, Ka, Kc and the 0.7 pz check) for clad buildings and unclad frames. Bentley's help page for the IS 875 generator lists some limits worth knowing:

  • The wind profile is measured from Y = 0, not from the lowest point of the building.
  • It supports Y-up models only.
  • There is no change of terrain category with height, no frictional drag and no dynamic effects.
  • For unclad frames, wind is applied in one direction only.

The general wind load command has another important restriction. Bentley's technical reference for DEFINE WIND LOAD says the generator should be used for vertical panels, and that panels not parallel to the vertical axis should be loaded separately. On a portal frame, those are your rafters. That is why most engineers apply the roof wind as member loads themselves.

A faster way: Wind Load in SS Workbench

Wind Load is a tool in SS Workbench that connects to STAAD.Pro on your PC through the free STAAD Connector and does steps 1 to 4 for you:

  1. Pressure to IS 875 (Part 3):2015: choose Vb or the city, the risk class, terrain category, k3, k4, Kd, Ka and Kc. The 0.7 pz check is built in.
  2. Cpe and Cpi are worked out from your building's span, length, heights and roof angle, using Tables 5, 6 and 7, with Cpi from the opening area.
  3. Recognises your model: the frames, span and roof of the STAAD.Pro model.
  4. Auto tag the walls and roof of the frames you select (near-side and far-side walls and roof, and end walls), or tag them by hand.
  5. Tributary width is suggested from the frame spacing.
  6. 4, 6 or 8 load cases: 0°, 90°, 180° and 270° with +Cpi and −Cpi.
  7. See every load in the 3D view before anything is written.
  8. Apply to STAAD: add the loads to your existing wind load cases, or clear those cases first and write the new loads.

You also get a detailed calculation with clause references, which you can print or save as PDF for the design report. The same tool also does ASCE 7-16 and MBMA 2012 (see ASCE 7-16 vs MBMA wind loads for metal buildings).

What it doesn't do: it writes member UDLs into load cases that already exist in your model; it doesn't create load cases or combinations, and it doesn't calculate local (edge, ridge, corner) coefficients for cladding. It covers rectangular gable and mono-slope buildings. As with any design tool, check the pressures and coefficients before you use them.

Checking the result

Whichever method you use, check before you run the design:

  • Signs: windward wall loads push in, leeward wall loads pull out, roof loads are mostly suction (uplift).
  • Magnitude: one hand calculation per surface, like the 3.45 kN/m example above.
  • Load cases: all directions and both Cpi cases are present and named clearly.
  • Reactions: the total horizontal reaction for each case should roughly equal net pressure × projected area.
  • Combinations: wind cases with dead load (uplift) and with dead and live load are in your combinations.

Try it on your model

Open Wind Load in SS Workbench with your portal frame model open in STAAD.Pro, create the empty wind load cases, and apply all of them in one go.