NBCC 2015 Wind

Load Case

States the Label of the load case being defined.

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.

Building Dimensions

For dynamic procedure, building dimensions are needed in calculation of Gust effect factor (C_g) and external pressure coefficient (C_p).

Select to the Use Maximum Exposure Dimensions to use the widest dimensions along the direction perpendicular to the loading direction. Otherwise you may select the Use option and type the X and Y dimensions to use for the building.

External Pressure Coefficient, Cp

External pressure coefficients C_p for Windward and Leeward surfaces can be either directly provided by the user, or they can be calculated as follows (in the following equations H is the mean roof height and D is the depth of the building in the direction of the wind. See Figure A-4.1.7.5. (2) and (3)):

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.

Topographic Factor

The topographic factor is calculated according to a method given in 4.1.7.4. This method is not implemented. Instead, the user is expected to provide Ct values for each direction.

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: 4.1.7.3. Sentence 8
- C_g = 2.0 for the building as a whole and main structural members
- C_g = 2.5 for external pressures and suctions on secondary structural members, including cladding

Dynamic procedure: 4.1.7.8 (Sentence 4)

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 (Figure A-4.1.7.8. (4)-A) $\sqrt{2 \ln_{e} v T} + \frac{0.577}{\sqrt{2 \ln_{e} v T}}$
v	=	average fluctuation rate $f_{n D} \sqrt{\frac{s F}{s F + β B}}$
T	=	3,600 s
K	=	factor related to surface roughness = 0.08 for Exposure A = 0.10 for Exposure B
C_eH	=	exposure factor at the top of the top of building (mean roof level). It is calculated according to equations given in 4.1.7.8 Sentences 2 and 3.
B	=	background turbulence factor obtained from Figure 4.1.7-8 as a function of w/H, in which w is building effective width and H is the mean roof height. Both w and H 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 A-4.1.7.8. (4)-B as a function of w/H and reduced frequency, $f_{n} \frac{H}{V_{H}}$ . It can be also calculated from the following equation: $= \frac{π}{3} [\frac{1}{1 + \frac{8 f_{n} H}{3 V_{H}}}] [\frac{1}{1 + \frac{10 f_{n} w}{V_{H}}}]$
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 $= \bar{V} \sqrt{C_{e H}}$
$\bar{V}$	=	$\sqrt{\frac{2 I_{W} q}{ρ} C_{e H}}$
q	=	reference velocity pressure (kPa = kN/m²), which is provided by the user
ρ	=	air density value used by the program = 1.2929 kg/m³
F	=	gust energy ratio at the natural frequency of the structure obtained from Figure A-4.1.7.8. (4)-C. It can be also calculated from the following equation: $= \frac{x_{o}^{2}}{{(1 + x_{o}^{2})}^{4 / 3}}$
x₀	=	1,220 f_n/V_H
β	=	critical damping ratio in the along-wind direction

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 4.1.7.3 (5)
- Open Terrain: $C_{e} = {(\frac{h}{10})}^{0.2} > 0.9$
- 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). This method is not implemented (in this case, the user needs to calculate it manually and enter it)
Dynamic procedure: the exposure factor is calculated as follows (4.1.7.8):
- 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
Referring to Figure A-4.1.7.8 (2) and (3), Exposure A is regarded as open terrain and Exposure B as rough terrain. These definitions are used in the current implementation for dynamic procedure.

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.

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.

RAM Structural System Help

NBCC 2015 Wind

Conceptual example of additional load cases for tension-only members