NBCC 2005 Wind

Load Case

States the Label of the load case being defined.

Exposure

Select the appropriate exposure category from the drop-down list.

The exposure category reflects the characteristics of ground surface irregularities arising from natural topography and vegetation and of the constructed features at the site in which the building will be constructed. See building code for more information.

Direction

Use the Direction box to indicate the direction of the force (X, Y, or both) by clicking on the corresponding box so that the box is marked. One load case will be generated for each direction at which the force acts.

Mean Roof Height/Structure Height

Building height used in the calculation of Gust Response and Pressure Coefficients (It is not used in the calculation of the actual exposed area). Building Height refers to the height of the building above ground level as defined in the Ground Level dialog. If parapet height is to be included, it is defined in the Exposure dialog.

You have the option to override the program’s selection of Building Height by selecting the third option, Use, and entering a value in the corresponding edit box.

See building code for more information.

External Pressure Coefficient, Cp

External pressure coefficients are calculated based on I-7 to I-14 and I-15. In the current implementation, the following is applied: for each direction, the user is given two options:

Either Windward and Leeward C_p values are entered by the user
Or it is calculated from Figure I-15 as follows (below H is the mean roof height and D is the depth of the building in the direction of the wind):
- For Windward case:
  $C_{p} = {\begin{matrix} 0.6 & \frac{H}{D} < 0.25 \\ 0.27 (\frac{H}{D} + 2) & 0.25 \leq \frac{H}{D} < 1 \\ 0.8 & \frac{H}{D} \geq 1 \end{matrix}$
- For Leeward case:
  $C_{p} = {\begin{matrix} - 0.3 & \frac{H}{D} < 0.25 \\ - 0.27 (\frac{H}{D} + 0.88) & 0.25 \leq \frac{H}{D} < 1 \\ - 0.5 & \frac{H}{D} \geq 1 \end{matrix}$

Importance Factor

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

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: $C_{e} = {(\frac{h}{10})}^{0.9} > 0.9$
- Rough Terrain: $C_{e} = 0.7 {(\frac{h}{12})}^{0.3} > 0.7$
- Intermediate value between these two options (see Commentary I, pg. I-7, Paragraph 11). This is not implemented. Instead, the user is allowed to enter C_e
Dynamic procedure: If dynamic approach to the action of wind gust is used, the exposure factor is refereed to Commentary I, pg. I-24, Paragraph 41 and it is calculated as follows:
- Exposure A: $C_{e} = {(\frac{h}{10})}^{0.28}$ and 1.0 ≤ C_e ≤ 2.5
- Exposure B: $C_{e} = 0.5 {(\frac{h}{12.7})}^{0.50}$ and 0.5 ≤ C_e ≤ 2.5
- Exposure C: $C_{e} = 0.4 {(\frac{h}{30})}^{0.72}$ and 0.4 ≤ C_e ≤ 2.5

Gust Effect Factor, Cg

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: C_g = 2.0

Dynamic procedure: see Commentary I, pg. I-25, Paragraph 46-47

C_{g} = 1 + g_{p} (\frac{σ}{μ})

where

$(\frac{σ}{μ})$	=	$\sqrt{\frac{K}{C_{e H}} (B + \frac{s F}{β})}$
g_p	=	statistical peak factor for the loading effect $\sqrt{2 \log_{e} v T} + \frac{0.577}{\sqrt{2 \log_{e} v T}}$
T	=	3,600 s
v	=	average fluctuation rate $f_{n} \sqrt{\frac{s F}{s F + β B}}$
K	=	factor related to surface roughness = 0.08 for Exposure A = 0.10 for Exposure B = 0.14 for Exposure C
C_eH	=	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 41)
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. $= \frac{4}{3} \int_{0}^{\frac{914}{H}} [\frac{1}{1 + \frac{x H}{457}}] [\frac{1}{1 + \frac{x w}{122}}] [\frac{x}{{(1 + x^{2})}^{4 / 3}}] ⅆ x$
s	=	size reduction factor obtained from Figure I-19 as a function of w/H (Commentary I, p. I-28) and reduced frequency, $f_{n} \frac{H}{V_{H}}$ $= \frac{π}{3} [\frac{1}{1 + \frac{8 f_{n} H}{3 V_{H}}}] [\frac{1}{1 + \frac{10 f_{n} w}{V_{H}}}]$
f_n	=	natural frequency of vibration for given direction (in Hz). It is either given by the user or computed by the program
V_H	=	mean wind speed (m/s) at the top of the structure $= \bar{V} \sqrt{C_{e H}}$
$\bar{V}$	=	$39.2 \sqrt{q}$
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 as a function of f_n/v_n $= \frac{x_{o}^{2}}{{(1 + x_{o}^{2})}^{4 / 3}}$
x₀	=	1,220 f_n/V_H
β	=	critical damping ratio in the along-wind direction

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.

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 2005

Total Force = (P_W + P_L) (w/2) h

e_x = 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 2005

ΣF_x = 0.565(P_W + P_L)wh

ΣF_y = 0.565(P_W + P_L)dh

e_x = ±0.082w

e_y = ±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 2005)

Wind Load Case B (NBC of Canada 2005)

Wind Load Case C (NBC of Canada 2005)

Wind Load Case D (NBC of Canada 2005)

Generate Additional Load Cases for Analysis with Tension-Only Members

RAM Frame utilizes a nonlinear analysis algorithm to keep track of tension-only members during solution of the structural model under applied loads. Since the process has a nonlinear (iterative) nature, analysis results can not be simply superposed with other results as in load combination option in RAM Frame. Therefore, each lateral load case (wind or seismic) are solved separately.

Since the direction of the lateral loads become important in an iterative analysis, RAM Frame provides an option such that additional load cases can be created upon user request. When Generate Additional Load Cases for Analysis with Tension-Only Members in generating lateral load cases (wind and seismic load cases only) is invoked, additional load cases are created. For instance, there are 12 load cases generated for a regular IBC 2000 (ASCE 7-98) Wind load case. If this box is checked, the number of the load cases becomes 22. Additional load cases are created to account for directional effects of applied loads so that the most severe loading case is captured.

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