RAM Structural System Help

AS/NZS 1170.2:2002

This standard covers building heights with less than 200 m (666.67 ft). Wind actions are defined in two categories:
  • Wu : ultimate limit states
  • Ws: serviceability limit states

In the following sections, AS/NZS 1170.2:2002 Structural design actions - Wind actions manual is referred.

Site Wind Speed (Vsit.β)

It is defined for the 8 cardinal directions (β) at the reference height (z) above ground. The current implementation does not calculate site wind speeds.

Design Wind Speed (Vdes,θ)

The building orthogonal design wind speeds shall be taken as the maximum cardinal direction site wind speed Vsit,β linearly interpolated between cardinal points within a sector ± 45 degrees to the orthogonal direction being considered. For the ultimate limit state design, Vdes,θ shall not be less than 30 m/s. For leeward walls, side walls, and roofs, wind speed shall be taken as the value at z=h, (h: the reference height and it shall be taken as the average height of roof). For Windward wall, V might vary with height > 25 m (~83.3 ft.)

Note that windward design wind speed is not explicitly calculated by the program. Instead, the user is provided two choices: either a constant wind speed is entered (so the wind over the height of the building is assumed to be the same) or it is exclusively defined and read in from a user defined file (so that one can consider variation of windward wind speed over the height of the building). Further information is provided in coming sections.

A constant Leeward wind speed is assumed in the current implementation.

Design Wind Pressure

The design wind pressure, in pascals, is calculated according to the following equation:

p = 0.5pair(Vdes,θ)2CfigCdyn 2.4(1)

where
pair
=
density of air (1.2 kg/m3)
Vdes,θ
=
building orthogonal design wind speed (usually at θ=0. 90, 180, 270 degrees)
Cfig
=
aerodynamic shape factor (depending on which part and geometry of structure
Cdyn
=
dynamic response factor ( =1 for non wind sensitive structures)
  • If f > 1 Hz (T < 1.0 s), Cdyn = 1.0
  • If f < 0.2 Hz (T > 5 s), AZ/NZS 1170.2:2002 does not cover this type of structure
  • If 0.2 < f < 1, (1 < T < 5), refer to Sections 6.2 & 6.3.
  • Note that Cdyn is calculated at a given height, z.

External Pressure

External Pressure for enclosed buildings is calculated according to the following equation:

Cfig = Cp,eKαKcKlKp 5.2(1)

where
Cp,e
=
external pressure coefficient (Section, 5.4.1 in the building code). It depends on which part of structure considered (leeward, windward, upwind slope of roof, or downwind sloped of roof, etc…)
Kα
=
area reduction factor (Section 5.4.2 in the building code)
Kc
=
combination factor (Section 5.4.3). For all structures, Kc cannot be less than 0.8/Kα (i.e., Kc× Kα≥ 0.8
Kl
=
local pressure factor (Section 5.4.4). There is also another correction applied to Kl, which is a reduction factor in the lee of the parapet. (Kl is 1.0 in all cases except when determining the wind forces applied to cladding, their fixings)
Kp
=
reduction factor for porous cladding (Section 5.4.5)

Internal Pressure

Internal Pressure for enclosed buildings is calculated according to the following equation:

Cfig = Cp,iKc 5.2(2)

where
Cp,e
=
internal pressure coefficient (Section, 5.3). Applies to inside of all structure, depending on openings and permeability of walls. The height at which the wind speed is determined shall be the average roof height (h).
Kc
=
combination factor (Section 5.4.3)

Friction Pressure

Friction Pressure for enclosed buildings is calculated according to the following equation:

Cfig = CfKc 5.2(3)

where
Cf
=
friction drag force coefficient. (it is only applied where the ration d/H or d/b is greater than 4)
Kc
=
combination factor (Section 5.4.3)

Forces Derived from Wind Pressures

F = Σ ( p g A g )
where
F
=
wind forces (in Newtons)
pz
=
design wind pressure in pascals (normal to surface) at height z, calculated according to 2.4(1) in the building code.
Az
=
reference area, in square meters, at height z

For enclosed buildings, internal pressures shall be taken to act simultaneously with external pressures. The most severe combinations of internal and external shall be selected for design.

Forces Derived from Frictional Drag

F = Σ ( f g A g )
where
fz
=
design frictional distributed forces parallel to the surface, calculated according to Section 2.4.2 (the equation given in Section 2.4.2 is similar to Eq. 2.4(1) except different values of Cfig and Cdyn must be used for frictional drag forces)

Forces and Moment on Complete Structures

For rectangular enclosed buildings where the ratio d/h or d/b is greater than 4, the total resultant force on a complete structure shall include the frictional drag calculated in accordance with Section 5.5 in the building code.

Implementation Details

The following assumptions are enforced for the current implementation:
  • It is assumed that building is effectively sealed and having non-opening windows. Hence, it is further assumed that internal pressures cancel each other.
  • It is assumed that sidewalls external pressures cancel each other (note that internal pressure also cancels out each other). See Different pressure types on buildings .
  • Frictional drag forces are ignored (assumed that d/h or d/b is less than 4)
  • Kl (local pressure factor for cladding) is taken as 1
  • Kp (reduction factor for porosity) is taken as 1
  • Ka (area reduction factor for sidewalls) is taken as 1 (since external sidewalls pressures are ignored (assumed to cancel each other)
  • Kc (combination factor) is taken as 1 (since only external pressures are applied in the current implementation. This factor is used if external pressures are applied with other types of pressures such as internal, side wall or upwind roof, etc…)
  • For buildings with height less than 25 meters, the design wind speed (Vdes,θ) can be assumed as constant over the height of the building. Otherwise, it varies with height. Thus, the following options are provided:
    • For leeward side wind speed, it is always constant and measured at reference height
      • For windward side wind speed, two options are provided: Either
      • It is assumed as constant over the height of the building
      • Or, it varies with height. In this case, the engineer needs to provide this information stored in a file (with an extension of .WND) placed in the directory "Tables." An example is given below.
  • Vdes,θ must be defined for each orthogonal directions.

    Different pressure types on buildings

Example .WND File

Also, the user has to provide values in consistent units: SI (speed: m/s, height: m); Metric (speed: Km/s, height: m); or English (speed: mph height: m); 

// wind profile (height versus wind speed, height(m) vs Speed (m/s))
 0.0	35.0
10.		45.0
20.		60.0
30.		70.0
40.		80.0
Note: The first line (beginning with "//") is a comment and is ignored by the program.

Wind Action Directions

Four orthogonal directions are considered for the load case. It is assumed that building wind characteristics are the same for +X and -X directions so that the same wind speed profile, windward\leeward exposure constants and dynamic response factors (Cdyn) are used for +X and -X. This is also true for +Y and -Y.

Orthogonal load cases

Parent topic: Story Forces: Wind