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Put wind on a building ​

Load the walls of a building with a wind pressure, and its frame with a wind load per metre, as a load case of their own.

A box of shell walls under wind: 0.8 kN/m2 pushing on the windward wall and 0.5 kN/m2 of suction on the leeward one

Pressure on the windward wall, suction on the leeward one, both along the wind.

You need ​

  • The walls as ASD Shell elements, each wall's mesh with an ElementId of its own: Wall_West, Wall_East.
  • For a frame, its columns as beam elements with an ElementId: Col_West.
  • The wind pressure on each face, in kN/m², worked out to your code of practice: Alpaca4d does not compute it.

Steps ​

  1. Place a Mesh Load for each face that takes a different pressure. Type the wall's name into Shell|Id: every face of its mesh is loaded.
  2. Give Force as a vector in global axes, in kN/m². Wind blowing along +X pushes the windward wall with q, 0, 0 and pulls the leeward one, in suction, also along +X.
  3. For columns or beams in the wind, place a Linear Load with their name in Beam|Id and Force in kN/m, also in global axes: the pressure times the width each one catches.
  4. Wire all of them into one Load Pattern of its own, the wind case, and the pattern into Assemble's LoadPatterns.
  5. Set the pattern's Factor for a combination: 1.5 for wind as the leading action at the ultimate limit state, say, under the factors your code gives.

Check ​

  • In Model View, tick Show Loads in its Loads menu, and give LoadPattern the wind pattern's Id to draw it alone: the arrows on each wall should point the way the wind pushes or pulls it, and Show Values writes what each is worth.
  • Reaction Forces: with the wind case alone, the reactions along X add up to the pressure times the area of the walls, plus the line loads times their lengths.

Good to know ​

  • The pressure is uniform over each Mesh Load and not along each face's normal: it acts in the global direction you give. A sloping roof or a curved wall needs its faces grouped by direction, with a Mesh Load each.
  • A name that matches no shell or beam loads nothing, and Assemble warns about it. Two names that match the same element load it twice.
  • Every pattern wired into Assemble acts in the same analysis. To see the wind alone, assemble a second model with only the wind pattern.
  • The pressure becomes loads on the nodes of each face, its area shared among them, so a coarse mesh carries the same total load as a fine one.

See also ​