RAM Structural System Help

NBC of Canada 2010

Equivalent Static Force Procedure is implemented according to Section 4.1.7, Division B of NBC 2010. The specified external pressure or suction (i.e., windward and leeward pressures) is calculated according to the following equation:

p = IwqCeCgCp

where
p
=
external wind pressure, calculated in kN/m2 (kPa)

Importance Factor for Wind Load (Iw)

It is given in table 4.1.7.1 and this parameter is expected to be provided by the user.

Reference Velocity Pressure ( q )

The reference velocity pressure is referenced in Sentence (4) of Section 4.1.7 and this parameter is expected to be provided by the user. Acceptable units are kN/m2 (kPa) psf, or kg/m2.

Exposure Factor (Ce)

Exposure Factor can be determined based on one of the following three options (in calculations below, h is height above ground level, and it is defined in meters):
  • Use provided value
  • Static procedure: It is given in Sentence 4.1.7.1.(5)
    • Open Terrain: C e = ( h 10 ) 0.9 > 0.9
    • Rough Terrain: C e = 0.7 ( h 12 ) 0.3 > 0.7
    • Intermediate value between these two options (see Commentary I, p. I-7, Paragraph 12). This is not implemented. Instead, the user is allowed to enter Ce
  • Dynamic procedure: If dynamic approach to the action of wind gust is used, the exposure factor is refereed to Commentary I, p. I-24, Paragraph 41 and it is calculated as follows:
    • Exposure A: C e = ( h 10 ) 0.28 and 1.0 ≤ Ce ≤ 2.5
    • Exposure B: C e = 0.5 ( h 12.7 ) 0.50 and 0.5 ≤ Ce ≤ 2.5

Gust Effect Factor (Gg)

Gust effect factor can be determined based on one of the following two options (in calculations below, h is mean roof height in meters):
  • Static procedure: Cg = 2.0
  • Dynamic procedure: see Commentary I, p. I-26, Paragraph 47
    C g = 1 + g p ( σ μ )
    where
    ( σ μ )
    		=
    K C e H ( B + s F β )
    gp
    		=
    statistical peak factor for the loading effect (as shown in Figure I-21 (Commentary I, p. I-28)
    2 log e v T ⁡ + 0.577 2 log e v T ⁡
    T
    		=
    3,600 s
    v
    		=
    average fluctuation rate, =
    f n s F s F + β B
    K
    		=
    factor related to surface roughness
    • = 0.08 for Exposure A
    • = 0.10 for Exposure B
    CeH
    		=
    exposure factor at the top of the top of building (mean roof level). It is calculated according to dynamic procedure (Commentary I, p. I-24, Paragraph 43)
    B
    		=
    background turbulence factor obtained from Figure I-18 as a function of w/H (Commentary I, p. I-27), in which w is building width at a given height and H is mean roof height. Both are in meters.
    = 4 3 ∫ 0 914 H [ 1 1 + x H 457 ] [ 1 1 + x w 122 ] [ x ( 1 + x 2 ) 4 / 3 ] ⅆ x
    s
    		=
    size reduction factor obtained from Figure I-19 as a function of w/H (Commentary I, p. I-27) and reduced frequency, f n H V H ,
    = π 3 [ 1 1 + 8 f n H 3 V H ] [ 1 1 + 10 f n w V H ]
    fn
    		=
    natural frequency of vibration for given direction (in Hz). It is either given by the user or computed by the program
    VH
    		=
    mean wind speed (m/s) at the top of the structure
    = V ¯ C e H
    V ¯
    		=
    39.2 q
    q
    		=
    reference velocity pressure (kPa = kN/m2), which is provided by the user
    F
    		=
    gust energy ratio at the natural frequency of the structure obtained from Figure I-20 (Commentary I, p. I-28) as a function of fn/vn
    = x o 2 ( 1 + x o 2 ) 4 / 3
    x0
    		=
    1,220 fn/VH
    β
    		=
    critical damping ratio in the along-wind direction

External Pressure Coefficient (Cp)

External pressure coefficients are calculated based on I-7 to I-14 and I-15. In the program implementation, the following is applied: for each direction, you are given two options:
  • Either Windward and Leeward Cp values are entered by the user
  • Or it is calculated from Figure I-15 as follows (where H is the mean roof height and D is the depth of the building in the direction of the wind):
    • For Windward case:
      C p = { 0.6 H D < 0.25 0.27 ( H D + 2 ) 0.25 ≤ H D < 1 0.8 H D ≥ 1
    • For Leeward case:
      C p = { − 0.3 H D < 0.25 − 0.27 ( H D + 0.88 ) 0.25 ≤ H D < 1 − 0.5 H D ≥ 1

Loading Directions

Loading directions Cases A-D as given in Figure I-16 are implemented. Regarding Case B, it is assumed that only half of the building surface is loaded with indicated wind pressures (see the following figure and note that h is the height of the surface.).

Generated partial wind load cases NBC of Canada 2010

Total Force = (PW + PL) (w/2) h

ex = w/4

Similarly, it is assumed that partial loads are applied to half surface for Case D:

Generated wind load Case D of NBC of Canada 2010

ΣFx = 0.565(PW + PL)wh

ΣFy = 0.565(PW + PL)dh

ex = ±0.082w

ey = ±0.082d

Based on Figure I-16, the following load cases are generated by the program (the load cases indicated with orange color are only generated if Additional Load Cases for Analysis with Tension Only Members option is checked).

Wind Load Case A (NBC of Canada 2010)

Wind Load Case B (NBC of Canada 2010)

Wind Load Case C (NBC of Canada 2010)

Wind Load Case D (NBC of Canada 2010)

Parent topic: Story Forces: Wind