Wind Zones - ASCE 7 All Heights Method

Tekla Structural Designer
2021
Tekla Structural Designer

Wind Zones - ASCE 7 All Heights Method

At the end of the wind wizard, the system creates default zones for all the walls and roof panels for each of the defined wind directions.

If any errors have occurred in this process, a red cross appears next to Pressure Zones in the Project Workspace.

Wind Directions

Eight zone directions evenly spaced at 45 degree intervals starting from the longitudinal direction are defined as follows:

  • +X - (longitudinal direction, uses qz for 1st principal axis)

  • +X+Y - (longitudinal direction + 45°, uses qz for 1st and 2nd principal axes)

  • +Y - (longitudinal direction + 90°, uses qz for 2nd principal axis)

  • -X+Y - (longitudinal direction + 135°, uses qz for 2nd and 3rd principal axes)

  • -X - (longitudinal direction + 180°, uses qz for 3rd principal axis)

  • -X-Y - (longitudinal direction + 225°, uses qz for 3rd and 4th principal axes)

  • -Y - (longitudinal direction + 270°, uses qz for 4th principal axis)

  • +X-Y - (longitudinal direction + 315°, uses qz for 1st principal axis)

The 4 directions +X,+Y,-X,-Y enable the modeling of design wind loadcases 1 and 2 from Figure 27.4-8. The 4 intermediate directions +X+Y etc. enable the modeling of loadcases 3 and 4.

Wall Zones

The wind wizard automatically generates wall zones, where possible, in accordance with Figure 27.4-1.

Windward Walls are split horizontally over intermediate heights. Zones are labelled with a 'W' followed by the height at top of zone rounded to nearest foot (or meter if using metric units). Each zone uses the same Cp, (0.8 from Wall Cp table in Figure 27.4-1), but a different velocity pressure, qz, z being determined at the top of the zone.

Complex wall shapes are dealt with by splitting zones intelligently as illustrated below:

Leeward walls have a single zone, 'L', using a Wall Cp from the table in Figure 27.4-1, with interpolation for L/B. The velocity pressure, qh is used for all such zones.

Side walls have a single zone, 'S', using the same Cp, (-0.7 from the Wall Cp table in Figure 27.4-1). A single velocity pressure, qh is used for all such zones.

Skew Walls

There is no guidance in ASCE 7-10 for walls not orthogonal to the principal axes. However, the wind wizard will apply zones in those cases and it is your responsibility to check the wind loads adopted.

Roof Zones

The wind wizard automatically generates Windward and Leeward roof zones ('W' and 'L' respectively, where possible, in accordance with Figure 27.4-1. This is possible for Windward and Leeward roof panels with θ >= 10°, because there is only one zone. However, for θ < 10° and Side roof panels, automatic zoning will not be carried out for all cases.

Negative and positive values of Cp are determined for each zone from the Roof Cp table in Figure 27.4-1.

For non-principal axis directions, it is assumed that roof loads are not required and so special zones are created, named "Zero". Such zones will result in zero loading on the relevant roof panels. However, if the you choose to do so you can change these to non-standard zones and enter coefficients to force loads to be applied.

When calculating the area reduction factor we use the slope area not the plan area.

Mansard roofs are not automatically detected, i.e. it is your responsibility to set the Roof Type manually. Generally, Mansard roofs are handled exactly the same as if they are:

  • Flat (for slope < 0.1°)

  • Pitched (for slope >= 0.1°)

There is no guidance on what to do with other multipitch roofs, so they are treated as pitched roofs, whether they have been flagged as Mansard or not.

Side roofs are split into to 4 zones depending on the size of the roof. See the Roof Cp table in Figure 27.4-1.

  • Zone 1 - 0 to h/2, interpolating between 2 values for h/L if necessary.

  • Note: Upper value for interpolation can be reduced linearly with the sloped area of this zone, (see ** in Figure 27.4-1).
  • Zone 2 - h/2 to h, interpolate between -0.9 and -0.7 for h/L if necessary
  • Zone 3 - h to 2h, interpolate between -0.5 and -0.7 for h/L if necessary
  • Zone 4 - > 2h, interpolate between -0.3 and -0.7 for h/L if necessary

Automatic Zoning

Automatic zoning will apply to all Windward and Leeward roof panels with θ > 10°, because there is only one zone. However, for θ < 10° and Side roof panels, automatic zoning will not be carried out for all cases.

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