Open structure wind loads

Tekla Structural Designer
2021
Tekla Structural Designer

Open structure wind loads

For a wind pressure acting in a specified direction, open structure wind loads can be calculated for structural members, ancillaries or equipment as follows:

  • Structural members (beams, columns etc) - wind pressure is determined, a distributed load is then calculated from this pressure, a shape factor and the area exposed to the wind (including fire protection if relevant)
  • Ancillary pipework and cable trays - wind pressure determined, a distributed load is then calculated from this pressure, a shape factor and a calculated effective area (based on largest pipe for pipework and height for cable trays) (including insulation if relevant)
  • Ancillary ladders, guardrails and staircases - wind pressure determined, a distributed load is then calculated from this pressure, a shape factor and a user defined effective area.
  • Equipment - wind pressure determined, a point load is then calculated from this pressure, a shape factor and an effective area.

Open structure wind load workflow

The pressure can be applied by running the wind wizard to create a wind model, or, by creating a simple wind load. A workflow is provided for each.

Wind load calculation for members and ancillaries

Wind load is determined as two full length UDLs, one acting in local y (minor) and one acting in local z (major) on the member.
Full UDLMajor
 = 
z x Cf x Effective Areaxy x Cos(θ) x Sin(φ) / L
Full UDLMinor
 = 
z x Cf x Effective Areaxz x Sin(θ) / L

where

z
 = 
wind pressure calculated at the height of the member for that wind direction (for vertical member average is taken at top and bottom level)
Cf
 = 
shape factor
Effective Areaxy
 = 
effective area of member in xy
Effective Areaxz
 = 
effective area of member in xz
θ
 = 
angle of the wind to the member x axis projected vertically into the horizontal plane
φ
 = 
angle of the member to the horizontal plane
L
 = 
length of the member

Wind load calculation for equipment

Wind load is determined as a single point load and applied to the CoG of the equipment at mid height.
FWind
 = 
z x Cf x Effective Areaxz x Sin(θ) + Effective Areayz x Cos(θ))
Fx
 = 
FWind x Cos(θ)
Fy
 = 
FWind x Sin(θ)

where

z
 = 
wind pressure calculated at the height of the equipment for that wind direction
Cf
 = 
shape factor
Effective Areaxy
 = 
effective area of equipment in xy
Effective Areaxz
 = 
effective area of equipment in xz
θ
 = 
angle of the wind to the equipment x axis projected vertically into the horizontal plane
L
 = 
length of the equipment
Note: All wind loads considered are horizontal, no vertical loading is considered.

Shape factors and effective area factors

The default shape factors (Cf) come from the document ‘Wind Loads For Petrochemical And Other Industrial Facilities’ published by ASCE. These shape factors, and the effective area factors are assigned to members/ ancillaries and equipment as follows:
  • Members/ancillaries
    • To specify the defaults for new members/ancillaries:
      • Edit the factors under Model Settings > Loading > Wind Loading
    • To edit the factors for existing members/ancillaries:
      • Select the member/ancillary
      • In the Properties window, under Wind loading select Apply open structure wind load
      • Edit the factors as required
  • Equipment
    • The shape factors and effective area factors assigned to equipment are determined from Table 1 below and cannot be edited.
Table 1: Equipment shape factors and effective area factors
Equipment type Shape factor Effective Area Side Elevation Effective Area End Elevation
Sphere 0.5 π x (D/2)2 π x (D/2)2
Vertical cylinder

ASCE 7 Fig 6-21

see Table 2 below
D x L D x L
Horizontal cylinder - perpendicular wind

ASCE 7 Fig 6-21

see Table 2 below
D x L -
Horizontal cylinder - longitudinal wind rounded end 0.5 - π x (D/2)2
Horizontal cylinder - longitudinal wind flat end 1.2 - π x (D/2)2
Cuboid see Table 3 below H x L H x B
Table 2: Cf for cylinders
Cross-section Surface L/D
1 7 25

D x √qz > 5.3 (metric units)

D x √qz > 2.5 (US customary units)

Moderately smooth 0.5 0.6 0.7
Rough 0.7 0.8 0.9
Very rough 0.8 1.0 1.2

D x √qz > 5.3 (metric units)

D x √qz > 2.5 (US customary units)

All 0.7 0.8 1.2
Note:
  • in metric units D in m and qz in N/m2
  • in US customary units D in ft and qz in psf
  • interpolation can be used between h/D values
Table 3: Cf for cuboids

c = h

a = L or B whichever is in line with the wind direction

b = B or L whichever is perpendicular to the wind direction

c/b a/b Max Cf
0.00 0.25 0.5 1.00 1.50 2.00 2.50 3.00 Cf a/b
0.25 1.20 1.20 1.00 0.92 0.91 0.91 0.91 0.90 1.23 0.20
0.5 1.16 1.21 1.16 0.97 0.92 0.91 0.91 0.90 1.21 0.34
1.00 1.14 1.15 1.18 1.16 1.02 0.95 0.92 0.90 1.20 0.70
2.00 1.16 1.17 1.19 1.17 1.04 0.97 0.93 0.92 1.21 0.70
5.00 1.21 1.22 1.24 1.22 1.11 1.03 0.99 0.97 1.27 0.70
8.00 1.28 1.29 1.35 1.30 1.17 1.09 1.04 1.01 1.36 0.70
10.00 1.31 1.34 1.41 1.36 1.24 1.14 1.08 1.04 1.46 0.70
Note: This Table is generated from Figure 1 of ESDU (1978) Fluid Forces, Pressures and Moments on Rectangular Blocks, ESDU Data Item 71016, Engineering Sciences Data Unit, London.

Assumptions and limitations

Assumptions
  • All wind loads are assumed to act perpendicular to the members, equipment and ancillaries
  • Applicable for all 1D structural members irrespective of their characteristic except
    • analysis elements
    • DELTABEAM
    • FABSEC beams
Limitations
  • Only currently applicable for US, EC and IS wind codes
  • Cannot be applied on structural walls
  • Shielding effect is not considered
  • There are no frictional wind forces accounted on any surfaces

Open structure wind loads report

An Open Structure Wind Load report can be generated for the open structure loads in user defined wind load cases on members, ancillaries and equipment.

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