Staad Pro-series 11-Wind Load Analysis

Wind Load Analysis Concept of Wind Load Analysis: Buildings are subject to horizontal loads due to wind pressure acting on the buildings. Wind load is calculated as per IS 875(Part III)-1987. The horizontal wind pressures act on vertical external walls and exposed area of the buildings. Some of the pressure acting on exposed surfaces of structural walls and columns is directly resisted by bending of these members. The infill walls act as vertical plate supported at top and bottom by floor beams, thus transferring the loads at slab level. The parapet wall is at terrace transfers the wind loads to the surface slab by cantilever action. For simplicity, the wind loads acting on exposed surfaces of a given storey are idealized to be supported by upper and lower floors. Wind load analysis as per IS 875 (Part 3)-1987: Wind forces acting on a given surface is equal to the wind pressures multiplied by the effected area. Design wind speed (Vz): Design wind speed is given by the equation Vz= Vb K1 K2 K3 where Vz =Design wind velocity (m/sec) Vb= Basic wind speed in m/sec (Based on Appendix -A of various cities in IS 875 –Part 3)

Basic wind speed Vb, depends on the location of the building. For this purpose, the country is divided in to six zones with specified wind speeds ranging from 33m/s to 55 m/s. Basic wind speed is based on gust velocity averaged over a short time interval of 3 seconds at 10m height from mean ground level in an open terrain and for 50 years return period. Appendix A (Fig.1) of the code specified for some important cities/ towns is given.

Vb has 6 values 33, 39,44,47,50 &55 m/sec.

K1=Risk Coefficient (Table 1 of IS875-part3)

The risk coefficient k1 takes in to account the degree of reliability required and the expected life of structure. 1.All general buildings (Design life 50 years) 2.Temporary sheds (Design life 5 years) 3.Less important Buildings (Design life 25 years) 4.Important Buildings (Design life 100 years) K2=Terrain, height and structure size factor (Table 2 of IS875-part3). It depends of Terrain category and building class/size of structure. Four terrain categories are specified by the code defending on the availability of obstruction to the flow of wind.

Category 1: Refers to no obstructions available to the building (e.g) sea coasts and flat treeless plains where other structures if any have heights less than 1.5m. Category 2: Refers to open terrain with scattered obstructions of 1.5m to 10m height. (e.g) industrial area. Category 3: Refers to areas of closed spaced buildings of height up to 10m (e.g) buildings at outskirts of city

Category 4: Refers to area with highly closed buildings of large heights (e.g) dense city area. K2 factor also depends on the dimensions of the building under considerations. Based on dimension of building, the structures are classified as Class A, Class B, Class C Class A: Maximum of l, b, h 50m.

K3 = Topography factor (Clause 5.3.3.1 of IS875-part3). It depends on the topography i.e hill region, cliffs and ridges. If the upward ground slope 0 ≤ 3 ◦ , value of K3 shall be taken as 1.0. For 0> 3◦, the value of K3 lies between 1.0 to 1.36. Design Wind Pressure (pz) pz= 0.6 Vz2 where pz= Design Wind speed in N/m2 at height z Vz= Design wind speed in m/s. Wind load on a building is calculated for the “ Building as a whole.”

Force coefficients are given for the building as a whole in the code for clad or unclad buildings. In this we considered clad (covered with side cladding or walls) buildings only. The wind force acting normal to the building surface is given by F= Cf Ae pd where Cf= force coefficient (Figure 4A & 4B of IS 875-part 3-1987).

Ae=effective frontal area. pd =design wind pressure.

WIND LOADS IN STAAD-PRO Built in program automatically calculates the wind loads on the structure (from the pressures given at different heights) and distributes the loads at the lateral joints. Wind loads are converted in to joint loads and exposure factors due to openings are incorporated. Exposure factor (c) is the fraction of the influence area associated with the joint(s) on which the load acts. Exposure factors depends on the openings provided in the walls of the buildings. Joint load F= Cf Ae Pd The above formula can be rewritten as, F= (Cf Pd) Ae F= pi Ae where pi = wind intensity pressure at that heights. Hence in STAAD Pro, the wind intensities at different heights are given in the input as follows: Intensity pi= p1, p2, p3 … Height h1, h2, h3 … where pi= p1, p2, p3 etc. The program automatically calculates the wind loads on the structure (from the pressure given at different heights) and distributes the load at the joints. Two stages in Wind loads command 1. Define the wind load 2. Apply the defined wind load. Note: The above “Define” command must be given immediately after the support command before giving any load command. 1. DEFINE COMMAND: SUPPORTS …………… DEFINE WIND LOAD TYPE 1 (wind intensity in X Direction) Intensity 1.8 2.0 2.1 Height 10. 15.0 20.0

Exposure 1.0 YRANGE 15.0 17.0 TYPE 2 (wind intensity in Z Direction) Intensity 1.8 2.0 2.1 Height 10. 15.0 20.0 2. APPLYING THE ABOVE DEFINED WIND LOAD: LOAD 1 (DL) ………. LOAD 2 (LL) ……… LOAD 3 (WL in X Direction) WIND LOAD X 1.0 TYPE 1 LOAD 4 (WL in -X Direction) WIND LOAD X -1.0 TYPE 1 LOAD 5 (WL in Z Direction) WIND LOAD Z 1.0 TYPE 2 LOAD 4 (WL in -X Direction) WIND LOAD Z -1.0 TYPE 2 WIND ANALYSIS GRAPHICAL METHOD

1.Defining Wind Load Click Loads & Definitions Click definitions  Wind Definitions Add. Click Type 1 Comments: Wind 1 Intensity(Kn/m2) 10.0

Height(m)

Close.

15.0

2.0

20.0 Click Exposures

2.1

 Add..

Factor 1.0  Add Close.

Highlight Exposure and select node cursor and entire structure   Assign to selected nodes Assign Yes Close. Applying the Wind Load:

1.Click Load case detail Add Number : 3, Loading type: Wind Title : Wind in X +ve dir Add. 2. Click Load case detail Add. Number :4, Loading type: Wind Title : Wind in (-)X dir  Add 3.Click Load case detail Add Number :5 Loading type: wind Title : Wind in Z +ve dir Add. 4.Click Load case detail Add Number :6, Loading type: Wind Title : Wind in (-)Z dir Add Close. 2. Assigning Wind load: Click Wind in X +ve  Add. Wind Load  X Direction Factor 1 Define Y range Mini:

Max:

 Add.

Click Wind in X -ve  Add. Wind Load  X Direction Factor -1 Define Y range Mini:

Max:

 Add.

Click Wind in Z +ve  Add. Wind Load  Z Direction Factor 1 Define Y range Mini:

Max:

 Add.

Click Wind in Z -ve  Add. Wind Load  X Direction Factor -1 Define Y range Mini:

Max:

 Add.

Load combination to be considered in Wind Load : Load combination for limit state of collapse as per IS 456-2000. 1. 1.5(D+L)

1.5(D+L)

2. 1.5(D+W)

1.5(D+W in X +ve) 1.5(D+W in Z +ve)

3. 1.5(D-W)

1.5(D+ W in X -ve) 1.5(D+ W in Z –ve)

4. 1.2(D+L+W) 1.2(D+L+W in X +ve) 1.2(D+L+W in Z+ve) 5. 1.2(D+L-W)

1.2(D+L-W in X –ve) 1.2(D+L+W in Z –ve)

6. 0.9D+1.5W

0.9D+1.5W in X +ve 0.9D+1.5W in Z +ve)

7. 0.9D-1.5W

0.9D-1.5W in X –ve 0.9D-1.5W in Z-ve

Total load cases =13. Procedure to calculate Wind Intensity of Pressure at Particular height to use in Staad Pro Data: Size of building : Length(b)=36.0m Width (a)=29.0m Height of building (h)=27.0m Vz= Vb K1 K2 K3 Vb= 50m/sec K1= 1.08 m/sec K2= For Terrain category 2 and class B for 10.m height=0.98 ; 15m height=1.02 For 20m height=1.05 and 30m height=1.10 K3 =1.0 for plain category. Vz= 50 x 1.08 x 0.98 x 1.0=52.9 The coefficient Cf is found from Figure 4A and 4B of IS 875-Part 3 a/b= Width of Building/Length of Building=28/36=0.8 h/a=Height of Building/ Width of building= 27/28=0.96 h/b= Height of Building/length of Building=27/36=0.75 Referring Fig. 4A and 4B of IS 875- part 3 For h/a =0.96 Cf=1.24 in X direction in STAAD For h/b=0.75 Cf =1.215 in Z direction in STAAD Wind force in a Building = Cf x Pd x area exposed.

The values are tabulated as follows: Height in metres 10 15 20 30

Vz= Vb K1 K2 K3

K2 0.98 1.02 1.05 1.10

52.92 55.08 56.70 59.4

Pd=0.6 Vz2

Pi = Cf x Pd KN/m2

N/m

X direction Z direction

1680.32 1820.28 1928.93 2117.02

2.08 2.26 2.39 2.63

2.04 2.21 2.34 2.57

1.Defining Wind Load Click Loads & Definitions Click definitions  Wind Definitions Add. Click Type 1 Comments: Wind 1 (W in X dir) Intensity(Kn/m2)

Height(m)

2.04

10

2.21

15

2.34

20

2.57

30

 Add.

Click Loads & Definitions Click definitions  Wind Definitions Add. Click Type 2 Comments: Wind 2 (W in Z dir) Intensity(Kn/m2) 2.08

Height(m) 10

2.26

15

2.39

20

2.63 Click Exposures

30

 Add.

Factor 1.0  Add Close.

Highlight Exposure and select node cursor and entire structure   Assign to selected nodes Assign Yes Close. Example: 1

Five storey building with wind Load

DATA: Plan of Building

: 12m x 10m

Height of Building :18.5m ; Typical Floor height=3.0m; Ground Floor height=4.0m Depth of Foundation: 2.50m Size of RCC members: COLUMNS C1,C2, C3

:300 x 600mm

BEAMS

:230 x 300 mm (End beam) : 230 x 450mm (Main Beam)

SLAB

Thickness

:120mm

1.

Geometry Creation :

Open STAAD.Pro V8i. New Project 

Select  Space  Length =meters;

Force=KN; File Name= Five storied Building 

Next

Select  Open Structure Wizard  Finish. Change to Frame Models from Truss models  Select Bay Frame and double click on it. Length (x)= 12.0m No. of bays =4

Click Bay No.1=3.0

Bay No.2=3.0; Bay No.3=3.0; Bay No.4=3.0 Height (y)= 18.5m No. of bays =6

Click Bay No.1 =2.50

Bay No.2 =4.00; Bay No.3 =3.00; Bay No.4 =3.00; Bay No.5 =3.00; Bay No.6 =3.00

Width (z)= 10.0m No. of bays =2

Click

Bay No.1= 5.0 Bay No.2 =5.0

Apply Transfer model click yes o.k. Go to Front view icon (first view) 3. Member Properties : Front view  Select beam along X axis for top 5 levels by windowing  Go to Isometric view and confirm the selection of beams in X direction of top 5 levels of beams  From main menu  Commands  Member property Prismatic Rectangular  YD=0.30 ZD=0.23 Assign De select the member. Go to side view  Select beam along Z direction for top 5 levels by windowing  Go to Isometric view and confirm the selection of beams in z direction of top 5 levels of beams  From main menu  Commands  Member property Prismatic Rectangular  YD=0.45 ZD=0.23 Assign De select the member and select the beam at lower level (Plinth beam along x and z direction)  From Main menu Commands member property PrismaticRectangular YD=0.4

ZD=0.23 Assign Close De select the member and select column member (Go to top view). Window the Corner column on four ends (type 1 column 0.6m x 0.3m)  From Main menu  CommandsMember PropertyPrismatic YD=0.60 ZD=0.30  Assign Confirm by viewing the column in Isometric view. Go to top view Window the intermediate column of end rows (type 2 column 0.6m x 0.3m)  From Main menuCommandsMember PropertyPrismaticRectangular YD=0.6m ZD=0.30 Assign Confirm by viewing the column in Isometric view. Window the internal column (type 3 column YD=0.60 ZD=0.3)  From Main menu  CommandsMember PropertyPrismatic YD=0.60 ZD=0.30  Assign Confirm by viewing the column in Isometric view. To confirm whether any missing property Main menu SelectMissing attributes Missing Property No entity with missing property is foundo.k. 4. Supports : Change to Node cursor (joint) and select the nodes (joints). (If more than one node use CTRL key and select the nodes). Go to front view  Select the bottom most nodes for supports  From Main menu  Commands  Support specification  Fixed  Assign  Close  De select the nodes and change to beam cursor. 4. Loading : *DEFINE WIND LOAD: (1)

X DIRECTION

From Main menu Commands Loading  Definitions  Wind Load Type 1 Comments: Wind  Add. Click Type 1 Add. Wind Definition screen will appear. Intensity  Select type1. Intensity (KN/m2) 1.8 2.0 2.10

Height(m) 10 15 20

 Add. Exposure Factor = 1.0  Add  Close.

Click the Exposure 1.0 and using node cursor select entire structure  Assign. Note: STAAD TEXT MODE DEFINE WIND LOAD TYPE 1(WIND INTENSITY IN X DIRECTION) INTENSITY 1.8 2.0 2.1 HEIGHT 10.0 15.0 20.0 EXPOSURE 1.0 YRANGE 15.0 20.0 Z DIRECTION From Main menu Commands Loading  Definitions Wind Load Type 2 Comments: Wind  Add. Click Type 2 Add. Wind Definition screen will appear. Intensity  Select type 2. Intensity 1.8 2.0 2.10

Height 10 15 20

 Add. Exposure Factor = 1.0  Add  Close. Click the exposure and Select node cursor & entire structure  Assign. STAAD TEXT MODE TYPE 2 (WIND INTENSITY IN Z DIRECTION) INTENSITY 1.8 2.0 2.1 HEIGHT 10 15 20 EXPOSURE 1.0 YRANGE 15.0 20.0 

DEAD LOAD

From Main menu  Commands  Loading  Primary Load  Load case 1=Dead Load  Add  Close . Select Load case1Add Self weight (dead load) Factor=--1 Direction  Y (Beams and columns)  Select entire structure and self weight Assign to view  Assign.

Floor Load (Dead Load of slab) Pressure=--4.00 (Self wt =3.00 KN/m 2 + Floor finish=1.0 KN/m2 ) Define Y range Mini.=6.50(2.50+4.0). Max.=18.50 Define X range Mini.=0.

Max.=12.0

Define Z range Mini.=0

Max.=10.0  Add

Member Load (Wall Loads) Uniform Force w1=--12 KN/m (9” wall load) Add Member load 

w1=--6 KN/m(4½” wall Load)  Add Close

Assign 9” wall load to external beams and 4½” wall load to internal beams in Top view by selecting each floor separately LIVE LOAD Select load case 2 (live load)  Add Floor load (Live load on slab) Pressure=--3.00( LIVE LOAD FOR OFFICE BUILDING ) Define Y range Mini.=6.50(2.50+4.0). Max.=18.50 Define X range Mini.=0.

Max.=12.0

Define Z range Mini.=0

Max.=10.0  Add  Close

WIND LOAD: *APPLYING THE ABOVE DEFINED WIND LOAD Click the Load case details in data area  Add. Number 3 Loading type: wind Title: Wind X +ve Add. Click load case 3  Add Wind load X direction Factor=1. Y range Minimum: 2.50

maximum:18.5  Add.  Close.

Click the Load case details  Add. Number 4 loading type: wind Title: Wind X –ve  Add Click Load case 4 wind X –ve  Add. Wind load X direction Factor=-1. Y range Minimum: 2.50

maximum:18.5  Add.  Close.

Click the Load case details  Add. Number 5 Loading type: wind Tittle: Wind Z +ve  Add Click Load case 5 wind Z +ve  Add. Wind load Z direction Factor=1. Y range Minimum: 2.50

maximum:18.5  Add.  Close.

Click the Load case details  Add. Number 6 loading type: wind Tittle: Wind Z –ve  Add Click Load case 6 wind Z -ve  Add. Wind load Z direction Factor=-1. Y range Minimum: 2.50

maximum:18.5  Add.  Close.

AUTO LOAD COMBINATION Main menu  Commands  Loading Edit Auto Loads  Select Indian Code Select : New category say Wind No. of rule :4 Rule No. 1 2 3 4

Dead 1.5 1.5 1.2 0.9

Live 1.5 -1.2 ---

Wind -1.5 1.2 1.5

Click Update Table Close. Main menuCommands  Loading Auto Load Comb (OR) Click Load case details  Add  Auto Load Combination. Select

:

Category:

Indian code Wind  Generate loadsAdd Close.

Main menuCommandsLoading Load List (selecting Design loads)  Load List Available Load

Load list

1.DL

>

2.LL

>>

3.WIND X+ve

<

4.WIND X-ve



2.LL

>>

3.WIND X+ve

<

4.WIND X-ve