Wind Loads

 

 

 

In MIDAS/Gen, the automatic data entry of wind loads according to various standards is applicable for common buildings where each story can be defined and can reasonably act as a rigid diaphragm. The following procedure is observed :

 

  1.  Model the structure.

 

The structure must be modeled so that the gravity acts in the direction opposite to the GCS Z-direction.

 

 

  1. Use Building > Control Data to define the elevation of ground level in GCS Z-axis coordinate.

 

When the ground level is entered, the parts below this level are considered as underground stories and neglected in the wind load calculation. If the ground level is not specified, the lowest part of the modeled structure is assumed to be the ground level by default.

 

 

  1. Use Story to define stories and their floor rigid diaphragm characteristics. Enter the application position of wind loads and the width of the story subjected to pressure to generate the wind load at each story.

 

It is recommended that be used to auto-generate the data necessary for the stories and the application of wind loading. Where openings exist at a particular story, adjust the width of the wind pressure area.

 

Once the floor diaphragm is defined in Story, the X-, Y-displacement degrees-of-freedom and the rotational degree-of-freedom about the Z-axis between all the nodes on the plane (plane parallel to the GCS X-Y plane) are constrained.

 

In addition, a part or all of the constrained nodes can be separated from the rigid floor diaphragm using Floor Diaphragm Disconnect.

 

 

  1. Assign the wind load calculation standards in Wind Loads and enter the required data.

[Built-in wind load calculation standards in MIDAS/Gen]

 

IBC 2000 (ASCE7-98): International Building Code 2000

 

UBC (1997): UBC 97 standards

 

ANSI (1982): ANSI standards

 

NBC (1995): National Building Code of Canada

 

Eurocode-1 (1992): Basis of Design and Actions on Structures

 

BS6399 (1997): British Standard 6399 Loading for buildings

 

 IS875(1987): Indian Standard

 

Taiwan (2002): Taiwan Building Code

 

(available upon request)

 

Japan (Arch, 2004): Loading Specifications and Commentaries for Buildings

 

Japan (Arch, 2000): Loading Specifications and Commentaries for Buildings

 

Japan (1987): Loading Specifications and Commentaries for Buildings

 

KBC (2008): Korea Building Code 2008

 

Korean (Arch, 2000): Buildings loading criteria and commentaries

 

Korean (Arch, 1992): Regulations related to structural criteria for buildings

 

China (GS50011-2001): Code for Seismic Design of Building

 

 

Once the data required for the calculation of wind loads are defined, auto-calculate wind loads for each story in connection with the story data generated in Story. Use to verify the auto-calculated wind loads.

[Wind load generation...]

 

1) Wind load calculation

If a floor area changes at a particular story level, the area subject to wind pressure is based on the sum of (A1=B1*h1/2) and (A2=B2*h2/2) relative to the corresponding story level. [Details...]

 

 

Fig. 1 Elevation   

 

Fig. 2 Plan

 

 

2) External Pressure Coefficient

Based on L/B2 for the upper portion and L/B1 for the lower portion

 

3) Design Pressure

Actual distribution of the wind pressure is parabolic, but MIDAS/Gen expresses it in a stepped distribution because the design pressure is taken at the story levels as per Fig. 2.

 

 

 

From the Main Menu select Load > Wind Loads.

 

Select Static Loads > Wind Loads in the Menu tab of the Tree Menu.

 

 

 

Access Wind Loads to activate the dialog box defining the wind loads. Click to display the dialog box shown below.

 

Add/Modify Wind Load Code dialog box

 

Load Case Name

 

Select the load case name to be associated with the wind load. Click to the right to enter or modify new load cases.

 

 

Wind Load Code

 

Select the standards to be applied to the wind load calculation.

 

IBC 2000(ASCE7-98): International Building Code 2000

 

UBC (1997): UBC 97 standards

 

ANSI (1982): ANSI standards

 

NBC (1995): National Building Code of Canada

 

Eurocode-1 (1992): Basis of Design and Actions on Structures

 

BS6399 (1997): British Standard 6399 Loading for buildings

 

IS875 (1987): Indian Standard

 

Taiwan (2002):  Taiwan Building Code

 

 

Description

 

Enter a short description.

 

 

Wind Load Parameters

 

Enter the parameters to be applied to the wind load calculation.

 

Revision of Gen 2012

 

IBC 2000(ASCE7-98)

Simplied Procedure

 

Basic Wind Speed

 

Importance Factor

 

Exposure Category

 

Analytical Procedure

 

Basic Wind Speed

 

Wind Directionality Factor

 

Important Factor

 

Exposure Category

 

Mean Roof Height

 

Gust Effect Factor

 

Gust Factor Parameter

 

X-Breadth (Bx)

 

Y-Breadth (By)

 

X-Natural Frequency

 

Y-Natural Frequency

 

Damping Ratio

 

Calculate : Calculate Gust Factors

 

Gust Factor X

 

Gust Factor Y

 

Load Evaluation Using Force Coefficient : Determine whether to calculate load using wind force coefficient.

 

Revision of Gen 2012

 

Auto. Calculation

 

Chimneys, Tanks, and similar structures

 

Horizontal Cross-Section Type

Square(wind normal to face)

Square(wind along diagonal)

Hexagonal or octagonal

Round

 

Diameter of circular cross-section and Least horizontal dimension of square, hexagonal or octagonal corss-sections(d)

X-Dir

Y-Dir

 

Depth of protruding elements such as ribs and spoilers(d’)

X-Dir

Y-Dir

 

Solid signs

Calculate the force coefficient for solid outdoor signs. It can be applied when the effective wind area is larger than 70% of gross area.

 

Installation Type

At Ground Level : Ground supported signs

Height of Solid Signs(hs)

Breadth of Solid Signs(b)

X-Dir

Y-Dir

 

Above Ground Level : Above ground signs

Larger dimension of Solid Signs(m)

Smaller dimension of Solid Signs(n)

X-Dir

Y-Dir

 

Open signs and lattice Frameworks

Calculate the force coefficient for open outdoor signs and lattice frameworks. It can be applied when the effective wind area is larger than 30%.

 

Section Member Type

Flat-Sided Members

Ratio of Solid area to Gross area

 

Rounded Members

Ratio of Solid area to Gross area

Diameter of a typical round member(d)

 

Trussed Towers

 

Plan Type

Square Plan

Triangle Plan

 

Member Type

Steel Rounded members

Square cross-sections members

Other Type: Other member type such as ladders, ducts, mechanical facilities, elevators, etc.

 

Ratio of Solid area to Gross area

 

Assign Force Coefficients

Target Story: Specify the story for which wind coefficient is applied.

 

UBC (1997)

Projected Area Method

 

Exposure Category

 

Basic Wind Speed

 

Importance Factor

 

Pressure Coefficient

 

Normal Force Method

 

Exposure Category

 

Basic Wind Speed

 

Importance Factor

 

Mean Roof Height

 

ANSI (1982)

Exposure Category

 

Basic Wind Speed

 

Importance Factor

 

Windward Coefficient: Windward wind pressure coefficient

 

Leeward Coefficient: Leeward wind pressure coefficient

 

NBC (1995)

Simple Procedure

 

Reference Wind Speed

 

Gust Effect Factor

 

Detailed Procedure

 

Reference Wind Speed

 

Gust Effect Factor

 

X-Breadth (Wx)

 

Y-Breadth (Wy)

 

X-Natural Frequency(Nox)

 

Y-Natural Frequency(Noy)

 

Damping Ratio

 

Building Height

 

Exposure Category

 

Include Topography Effect

 

Hill Shape

 

2-D Ridge or Valley

 

2-D Escarpment

 

3-D Axisym. Hill: 3-dimention axisymmetrical hills

 

Building Location: Building Location in case 2-D Escarpment

 

Upwind

 

Downwind

 

Hill Height: Height of the hill or the difference in elevation between the crest of the hill and that of the terrain surrounding the

upstream

 

Hill Length: Distance upwind of the crest to where the ground elevation is half the height of the hill

 

Crest-Building Distance (x): Distance from crest to the building site

 

Load Evaluation Using Force Coefficient: Determine whether to calculate load using wind force coefficient.

 

Eurocode-1 (2005)

Terrain Category: Exposure category

 

Friction Coefficient (Cfr)

 

Fund. Basic Wind Velocity (Vb,o)

 

Directional Factor (Cdir)

 

Seasonal Factor (Cseason)

 

Turbulence Factor (KI)

 

Building Height (h): Automatically inputted by the program

 

External Pressure Coefficients

 

Windward(A=10): Windward wind pressure coefficient for the area of 10m2

 

Windward(A=1): Windward wind pressure coefficient for the area of 1m2

 

Leeward Coef.: Leeward wind pressure coefficient

 

Lack of Correlation Factor: Lack of correlation of wind pressures between the windward and leeward sides

 

Parameters for Mean Wind Velocity

 

Consider Orographic Effects: Consider the increase of wind velocities over orography.

 

Orography Type

 

Building Location

 

Height of Topographic Feature

 

Length of Upwind Slope

 

Length of Downwind Slope

 

Crest-Building Distance: Distance from crest to the building site

 

Consider Effects of Neighbouring High-rise Structures: Consider the influence of higher neighboring structures on the wind

velocities.

 

Building Height: Automatically inputted by the program

 

Average Height of Nearby Structures

 

Distance to the High-rise Structure: Distance from the building site to the neighboring high-rise building

 

Larger Horizontal Dimension: Larger horizontal dimension of the neighboring high-rise building

 

Height: Height of the neighboring high-rise building

 

Consider Raising of Displacement Height: For buildings in Terrain Category IV, closely spaced buildings and other

obstructions causes the reduction in wind velocities.

 

Obstruction Height: Height of the neighboring structure or the obstruction

 

Upwind Spacing: Distance to the neighboring structure or the obstruction

 

Structural Factor: Gust effect factor

 

Along Wind Breadth

 

Along Wind Depth

 

Along Wind Natural Frequency

 

Logarithmic Decrement of Damping

 

Load Evaluation Using Force Coefficient: Calculate load using wind force coefficient

 

Force Coefficient

 

Eurocode-1 (1992)

Simplified Procedure

 

Roughness Category

 

Ref. Wind Speed (Vref)

 

Windward Pressure Coef.

 

Leeward Pressure Coef.

 

Friction Coef.(Cfr)

 

Topography Coef. at Building Ground Level Ct

 

Vertical Range for Ct

 

Force Coefficient (Cf)

 

Detailed Procedure

 

Roughness Category

 

Ref. Wind Speed (Vref)

 

Windward Pressure Coef.

 

Leeward Pressure Coef.

 

Friction Coef.(Cfr)

 

Topography Coef. at Building Ground Level Ct

 

Vertical Range for Ct

 

Gust Response Factor

: Gust Response Factor Calculator

 

Building Height (H)

 

Along Wind Breadth (B)

 

Along Wind Depth (D)

 

Along Wind Natural Freq (NI)

 

Fund. Flex. Damping (delta)

 

: Calculate Gust Response Factors

 

Force Coefficient (Cf)

 

BS6399 (1997)

Standard Method

 

Site Category

 

Building Type Factor (Kb)

 

Basic Wind Speed (Vb)

 

Mean Roof Height (Ho)

 

Seperation of Building (X)

 

Friction Drag Coef. (Cf)

 

Closet Diatance to Sea

 

Directional Method

 

Distance to Town Edge (Sa)

 

 

Altitude Factor (Sa)

 

Directional Factor (Sd)

 

Seasonal Factor (Ss)

 

Probability Factor (Sp)

 

Topographic Increment (Sh)

 

IS875 (1987)

Standard Method

 

Basic Wind Speed

 

Terrain Category

 

Building Class

 

Friction Drag Coef.(Cf')

 

Risk Coefficient

 

Class of Structures

 

Risk Coefficient (k1)

 

Include Topographic Effects

 

Topographic Factor (k3)

 

Vertical Range for k3

 

Taiwan (2002)

Site Category

 

Shape Factor

 

 

Wind Load Direction Factor

 

Enter the loading direction and the magnitude of wind load to be applied.

 

X-Dir.: Scale factor to be applied in GCS X-direction

 

Y-Dir.: Scale factor to be applied in GCS Y-direction

 

Z-Rot.: Scale factor to be applied in torsion about GCS Z-direction

 

Note

It is activated only when Japan (Arch, 2004) is selected.

 

 

Additional Wind Loads

 

Enter additional wind loads that the auto-calculation does not take into account.

 

Press to enter the stories to apply additional wind loads and the magnitudes for each direction.

 

: Display Tables and Graphs in a spreadsheet form for each loading direction and component of the auto-calculated wind loads.

 

Component: Assign the wind loading direction for a graphic display

 

Select Profile: Select the items to be displayed

 

Story Force

 

Story Shear

 

Overturning Moment

 

: Display a spreadsheet Text Output file showing the wind load calculation process.

 

Text Editor is automatically executed.

 

: Apply the auto-calculated wind loads to the model.

 

Note
Refer to the relevant code for details regarding the wind load calculation.