TR.31.3 Definition of Wind Load
The general wind load generator can be used to generate lateral loads in the horizontal –X and Z (or Y if Z up)– directions only.
General Format
DEFINE WIND LOAD
TYPE j ( optional_comment )
{ intensity-definition | code-parameters }
EXPOSURE e1 { JOINT joint-list | YRANGE f1 f2 | ZRANGE f1 f2 }
Repeat EXPOSURE command up to 98 times.
Where:
| Parameter | Description |
|---|---|
| TYPE j optional_comment | wind load system type number
(integer)
The optional comment is a text string comment or description used to help identify the wind load type. |
| intensity-definition or code-parameters | data is entered based on either custom or Russian code wind definitions. See Wind Intensity Definition or See Russian Wind Loads |
| EXPOSURE e1, e2 … em |
exposure factors. A value of 1.0 means that the wind force may be applied on the full influence area associated with the joint(s) if they are also exposed to the wind load direction. Limit: 99 factors. If the command EXPOSURE is not specified or if a joint is not listed in an Exposure, the exposure factor for those joints is chosen as 1.0. |
| JOINT joint-list | Joint list associated with Exposure Factor (joint numbers or TO or BY) or enter only a group name. |
| YRANGE | ZRANGE f1 f2 | global coordinate values to specify Y (or Z if Z UP) vertical range for Exposure Factor. Use YRANGE when Y is Up and ZRANGE when Z is Up (See the SET Z UP command in TR.5 Set Command Specification). |
Wind Intensity Definition
For custom (including for ASCE 7) wind load definitions, the wind intensity at heights above ground are defined as follows:
intensity-definition = INTENSITY p1 p2 p3 … pn HEIGHT h1 h2 h3 … hn
Where:
All intensities and heights are in current unit system. The heights specified are in terms of actual Y coordinate ( or Z coordinates for Z UP ) and not measured relative to the base of the structure. The first value of intensity (p1) will be applied to any part of the structure for which the Y coordinate ( or Z coordinate for Z UP ) is equal to or less than h1. The second intensity ( p2) will be applied to any part of the structure that has vertical coordinates between the first two heights (h1 and h2) and so on. Any part of the structure that has vertical coordinates greater than hn will be loaded with intensity pn.
Only exposed surfaces bounded by members (not by plates or solids) will be used. The joint influence areas are computed based on surface member selection data entered in TR.32.12.3 Generation of Wind Loads and based on the wind direction for a load case. Only joints actually exposed to the wind and connected to members will be loaded. The individual bounded areas must be planar surfaces, to a close tolerance, or they will not be loaded.
Exposure factor (e) is the fraction of the influence area associated with the joint(s) on which the load may act if it is also exposed to the wind load. Total load on a particular joint is calculated as follows.
Joint load = (Exposure Factor) × (Influence Area) × (Wind Intensity)
The exposure factor may be specified by a joint-list or by giving a vertical range within which all joints will have the same exposure. If an exposure factor is not entered or not specified for a joint, then it defaults to 1.0 for those joints; in which case the entire influence area associated with the joint(s) will be considered.
For load generation on a closed type structure defined as a PLANE FRAME, influence area for each joint is calculated considering unit width perpendicular to the plane of the structure. You can accommodate the actual width by incorporating it in the Exposure Factor as follows.
Exposure Factor (User Specified) = (Fraction of influence area) × (influence width for joint)
Example
UNIT FEET DEFINE WIND LOAD TYPE 1 INTENSITY 0.1 0.15 HEIGHT 12 24 EXPOSURE 0.90 YRANGE 11 13 EXPOSURE 0.85 JOITN 17 20 22 LOAD 1 WIND LOAD IN X-DIRECTION WIND LOAD X 1.2 TYPE 1
The Intensity line can be continued in up to 12 lines.
So the following
INT 0.008 0.009 0.009 0.009 0.01 0.01 0.01 0.011 0.011 0.012 0.012 0.012 HEIG 15 20 25 30 40 50 60 70 80 90 100 120
could be split as
INT 0.008 0.009 0.009 0.009 0.01 0.01 0.01 0.011 0.011 0.012 0.012 0.012 – HEIG 15 20 25 30 40 50 60 70 80 90 100 120
or
INT 0.008 0.009 0.009 0.009 0.01 0.01- 0.01 0.011 0.011 0.012 0.012 0.012 HEIG 15 20 25 - 30 40 50 60 70 80 90 100 120
etc.
Russian Wind Loads
This specifies the definition of a wind load to the Russian wind code which will need to be referenced in a wind load command included in a primary load case. For wind loads per Russian codes SNiP 85 or SP20 2016, the code parameters are defined as follows:
code-parameters = SNIP 1985 PRESSURE f3 TERRAIN { A | B | C }
or
code-parameters = { SP20 } PRESSURE f3 TERRAIN { A | B | C } REGION f4 LOG f5
Where:
- SNIP 1985 — design according to previous design Code SNiP 2.01.07-85
- SP20 2016 — design according to renewed design Code SP 20.13330.2016
- A structured modeled as a set of vertical members –such as is typically used to define a cylindrical stack or chimney structure, and thus doe snot define a closed panel such as defined by a frame formed from a number of columns and beams– is considered to be a "Stick Structure." Refer to TR.32.12.3 Generation of Wind Loads for additional details.
- The loading has two
components: static and dynamic. The dynamic effect is determined by the number
of modes included in the dynamic load case and changing the number of modes
considered using the
CUT OFF MODE SHAPE command may result in a change
in the resulting wind force. Refer to
TR.30.1 Cut-Off Frequency, Mode Shapes, or Time
for details.
As the analysis will requires extraction of eigen solutions to determine the dynamic effects of the wind loading, the number of modes to be used will also affect the results. Thus setting the command CUT OFF FREQUENCY or CUT OFF MODE should be considered and specified as required prior to the definition of the load cases.
If the cut-off command is omitted, six mode shapes are computed by default.
- If the PRINT STATICS CHECK option is included in the analysis command, then the output file will include a section on the SNiP wind load. This output will indicate both the static and dynamic contributions to the wind load as well as the total applied at each node.
- This command cannot be used with models that have been defined with the SET Z UP command.
- The building classification for wind load definitions per SNiP 2.01.07-85 and SP 20.13330.2016 is limited to prismatic building structures. General type concrete and steel framed structures are not supported.
The Russian wind load generation commands must also be used in conjunction with the SNiP wind load definition.
STAAD.Pro can generate both static and dynamic wind loads per the SP 20.13330.2016 code.
Example
An example with various static wind loads from different directions that use a SNiP 1985 definition:
DEFINE WIND LOAD
TYPE 1
SNIP 1985 PRESSURE 0.38 TERRAIN A
EXP 0.5 JOINT 1 3 5 7 9 11
*
LOAD 1 LOADTYPE WIND TITLE Wind load in the +ve X direction
WIND LOAD X 1 CONFIG 0 NU 1 TYPE 1
* Mass model required in first wind load case
JOINT LOAD
3 TO 6 FZ 62.223
9 TO 12 FZ 62.223
*
LOAD 2 LOADTYPE WIND TITLE Wind load in the –ve X direction
WIND LOAD X -1 CONFIG 0 NU 1 TYPE 1
* No mass model or additional loads in this load case
*
LOAD 3 LOADTYPE WIND TITLE Wind load in the +ve Z direction
WIND LOAD Z 1 CONFIG 0 NU 1 TYPE 1
* No mass model or additional loads in this load case
*
LOAD 4 LOADTYPE WIND TITLE Wind load in the –ve Z direction
WIND LOAD Z -1 CONFIG 0 NU 1 TYPE 1
* No mass model or additional loads in this load case
*
LOAD 10 LOADTYPE DEAD TITLE Selfweight load case
SELF Y -1 ALL
*
LOAD COMBINATION 100 Wind plus selfweight
1 1.0 10 1.0The following example uses SP 20.13330.2016 with both static and dynamic load cases:
DEFINE WIND LOAD TYPE 1 SP20 2016 PRESSURE 0.38 TERRAIN A REGION 0 LOG 0.3 * * Reference Mass Definition for Modal Analysis DEFINE REFERENCE LOADS LOAD R1 LOADTYPE Mass TITLE REF LOAD CASE 1 JOINT LOAD 3 TO 6 FX 62.223 3 TO 6 FZ 62.223 9 TO 12 FX 62.223 9 TO 12 FZ 62.223 END DEFINE REFERENCE LOADS * * Request mode shapes CUT OFF MODE SHAPE 3 * * Static Joint Load Required for Dynamic Wind Load LOAD 1 LOADTYPE None TITLE LOAD CASE 1 JOINT LOAD 3 5 9 11 FX -100 * * Modal Analysis LOAD 2 LOADTYPE None TITLE LOAD CASE 2 MODAL CALCULATION REQUESTED * * Dynamic Wind Load Command LOAD 3 LOADTYPE None TITLE SNiP Dynamic WIND LOAD X 1.0 TYPE 1 DYN 1 PERFORM ANALYSIS PRINT LOAD DATA
Persistence of Parameters used to Generate ASCE Wind Loads
In the Analytical Modeling workflow, in the Add New : Wind Definition dialog, you can click the Calculate as per ASCE-7 button to generate the pressure versus height table per the ASCE 7 wind load specifications per the 1995, 2002, or 2010 editions. The parameters which go into the derivation of this table are not retained by the graphical environment but rather added into the STAAD input file so they may be edited as needed. These values are not read by the STAAD engine directly and, therefore, are not directly processed as a load but are rather used to generate the wind intensity values which are used by the engine. An example of it is shown below.
Example
DEFINE WIND LOAD
TYPE 1
This entire section of the input file must not be edited.<! STAAD PRO GENERATED DATA DO NOT MODIFY !!!
ASCE-7-2002:PARAMS 85.000 MPH 0 1 0 0 0.000 FT 0.000 FT 0.000 FT 1 -
1 40.000 FT 30.000 FT 25.000 FT 2.000 0.010 0 -
0 0 0 0 0.761 1.000 0.870 0.850 0 -
0 0 0 0.866 0.800 0.550
!> END GENERATED DATA BLOCK
INT 0.0111667 0.0111667 0.0113576 0.0115336 0.0116972 0.0118503 0.0119944 -
0.0121307 0.0122601 0.0123834 0.0125012 0.0126141 0.0127226 0.012827 0.0129277 -
HEIG 0 15 16.9231 18.8461 20.7692 22.6923 24.6154 26.5385 28.4615 -
30.3846 32.3077 34.2308 36.1538 38.0769 40