Wind Speed Computation Tool 2011

Journal of Engineering Research and Studies E-ISSN0976-7916

JERS/Vol. II/ Issue IV/October-December, 2011/84-87

Research Article

COMPUTATIONAL TOOL FOR WIND PRESSURE AND

FORCES ON A MULTISTORY COMMERCIAL COMPLEX 1 N G Shilu,

2Dr. H S Patel

Address for Correspondence 1 PG student, L D College of Engineering, Ahmedabad

2 Associate Professor, L D College of Engineering, Ahmedabad

ABSTRACT

Indian standard was first published in 1957 for the guidance of civil engineers, designers and architects associated with the

planning and design of buildings. It included the provisions for the basic design loads (dead loads, live loads, wind loads and

seismic loads) to be assumed in the design of the buildings. In its first revision in 1964, the wind pressure provisions were

modified on the basis of studies of wind phenomenon and its effect on structures. The latest rivision of code pertaining to

wind loads was published in 1987. To eleminate some limitation of prevailing code, GSDMA has proposed draft code for

IS:875(part-III). Here in this paper a typical example of multistoreyed building is presented for application of GSDMA

proposed draft code. The computational tool is developed for the purpose using MS Excel. The outcome obtained from tool

are validated with solved problem from explanatory of GSDMA draft code.

NOTATIONS

The following notations shall be followed unless otherwise

specified in relevant clauses. Notions have been defined in

the text at their first appearance. A few of the notations

have more than one definition, having been used for

denoting different variables:

A = Surface area of a structure or part of a structure

Ae = Effective frontal area

Az = Frontal contributory area at height z

b = Breadth of a structure or structural member normal to the wind

stream in the horizontal plane

Cf =Force coefficient

Cp = Pressure coefficient

Cpe

= External pressure coefficient

Cpi

= Internal pressure coefficient

d = Depth of a structure or structural member parallel to wind

stream in the horizontal plane

D = Diameter of cylinder or sphere; Depth of structure

F = Force on a surface

h = Height of structure above mean ground level

hx

= Height of development of a speed profile at distance x

downwind from a change in terrain category

hp =Height of parapet

Hs =Height factor for resonant response

k = Mode shape power exponent

k1 k

2 k

3 k

4 --�Wind speed multiplication factors

K = Force coefficient multiplication factor for members of finite

length

Ka = Area averaging factor

Kc = Combination factor

Kd = Wind directionality factor

Km

= Mode shape correction factor

l = Length of a member or greater horizontal dimension of a

building

pz = Wind pressure at height z

Vb = Regional basic wind speed

Vh = Design wind speed at height h

Vz = Design wind speed at height z

Vz =Hourly mean wind speed at height z

W =Lesser horizontal dimension of a building in plan, or in the

cross-section a structural member;

W′ =Bay width in a multi-bay building;

We =Equivalent cross-wind static force

X = Distance downwind from a change in terrain category; fetch

length

Z = Height above average ground level

α = Inclination of roof to the horizontal plane

β = Effective solidity ratio; Damping ratio

ε = Average height of surface roughness

φ = Solidity ratio

η=Shielding factor or eddy shedding frequency

θ = Wind direction in plan from a given axis; upwind ground / hill

Procedural steps

The general procedure for the calculation of wind

load on any structure may be as given here.

1.0 Problem Statement:

This is the description of the problem which includes

basic design data and the requirement of the solution.

Based on the data given in the problem we may find

basic wind characteristics which may be taken under

consideration while solving the problem. This

includes dimensions of the structure, situation i.e.

location of the structure, materials used,

2.0 Steps

Depending upon the description of the problem, we

may find Wind Zone, Terrain category etc. With the

help of the wind zone we may find basic wind

velocity from the table given in appendix A of the

draft. This table is based on figure No 1 given in the

draft. As per the tool developed we may select the

city directly from drop down menu. Depending upon

the selection the basic wind velocity is displayed in

the cell.

The design wind speed is,

Vz = V

b k

1 k

2 k

3 k

4,

Where

Vz = design wind speed at any height z in

m/s ,

k1 = probability factor (risk coefficient) (see 5.3.1),

k2 = terrain roughness and height factor (See 5.3.2),

k3 = topography factor (see 5.3.3), and

k4 = importance factor for the cyclonic region

The factors k1,

k3,

k4, are found with the help of the

tables generated in spreadsheet and are not depending

on height, whereas k2 depends on terrain category as

well as height under consideration.

This is calculated in each case separately in the sheet

with the help of the formula developed using excel

formula.



While the wind pressure at any height above mean

ground level shall be obtained by the following

relationship between wind pressure and wind speed:

pz =0.6 Vz

2

where ,

pz = wind pressure in N/m

2

at height z, and

Vz = design wind speed in m/s at height z.

The design wind pressure pd can be obtained as,

pd = K

d. K

a. K

c. p

z

where

Kd = Wind directionality factor

Ka = Area averaging factor

Kc = Combination factor (See 6.2.3.13)

While Kd, K

a,, K

c, are calculated with the help of the tables created

in the excel sheets.

The following are the general notation for data entry

and interpretation.

1.0 In the excel sheets the yellow cells with blue font

are input cells.

2.0 The cells with green fonts (or some time with

black fonts) in saffron cells are the cells which

contains formula ,so no data entry is allowed in these

cells. If data is entered in these cells it may lead to

failure as the formula may be erased by entering into

these cells.

3.0 The tables completely filled with blue colour are

tables created based on Draft code. Hence they are

database for the calculations.

4.0 Some cells have comments guiding data entry.

5.0 Though the calculations of the quantities may be

calculated for each table separately, the same is

utilized in calculations of different examples in

slightly different way. Because in particular table the

input value may be unique for the purpose of

calculation, but it may be variable while performing

the whole example calculations. For example if we

want to calculate k2 in table 2 for a particular value h,

where as the same can be calculated in say MS

building by considering h variable.

6.0 An attempt is made to develop curve fitting

equation in many tables. However the values based

on interpolation is used in the calculations. Hence

when any difference in values found by two methods

the values base on interpolation is used as

recommended by code.

7.0 For the interpolation I have used two techniques.

One is based on inbuilt excel functions and the other

is based on VBA code user defined functions. Both

the techniques give same results ,hence any of them

may be used as per convenience. However I preferred

VBA code. And used mostly in calculations.

8.0 Here values as calculated in the Ref. No. 5 kept

as they were ,so the reader may compare the results

with those calculated by using MS excel.

9.0 Caution: Please don’t enter any values manually

in saffron cells. As the formula inside may deleted.

Application

Wind Pressure and Forces on a Multistory

Commercial Complex by Force Coefficient Method:

Calculate design equivalent static wind forces on a

RCC Multistory commercial complex

12mx18mx51m tall situated in Mumbai.

It is proposed to be constructed about 200m inside

the sea front. Take average story height as 3.0m and

frames spaced 6m c/c in both directions. The building

is oriented with its smaller dimension facing the sea,

i.e. in long-after body orientation.

Figure 1: Shematic diagram of MS building

Wind and structure Data:

1. Wind Zone: Zone IV (Vb= 47m/s) (IS:875-pt.3,

Sec 5.2, Fig. 1) by selecting city Jaipur.

2. Terrain category: (IS:875-pt.3, Sec 5.3.2.1) This

building shares special location characteristics.

On one face, i.e. sea face, it is exposed to terrain

category 1 transiting into terrain category 3 from

200m distance. On the other hand, other faces

are exposed to terrain category 4, being located

in a commercially developed area with tall

structures of height exceeding 35m. Therefore ,

we have to calculate a combined wind profile as

per Appendix—B (IS:875-pt.3, Sec 5.3.2.4),

transition from terrain category 1 to terrain

category 3, for one wind direction and consider

terrain category 4 directions.Calculating

combined wind profile for TC 1 to TC3 This

may be determined using IS:875-pt.3, Sec.

5.3.2.4(b). There are two options but option (ii)

will give more rational values and therefore,

should be used.Fetch Length x3 = 200m,

developed height inTC 3, h3 = 35m (IS:875-pt.3,

Table 3)Therefore, up-to 35m height, k2 factor

shall be as per TC 3 and above 35m it will be as

per TC 1.

3. Life span 50 years by selection

4. Main Building Height = 51 m, Width =12

m, Length =18 m

5. Importance All other Structures (selection from

respective table)

6. Type of Building as buildings (selection from

respective table)

7. Column Spacing =3 m

8. Floor Height = 3 m



Calculation of Design Factors: As per solved example As per calculation tool

Risk Coefficient Factor k1= 1.00

(IS:875-pt.3, Sec 5.3.1, Table-1)

:1

Terrain & Height Factor k2 Varies with height and terrain category, as given in Table 1

(IS:875-pt.3, Sec 5.3.2, Table-2)

:1.1714

Topography Factor k3 = 1.00

(IS:875-pt.3, Sec 5.3.3.1)

:1

Importance Factor for Cyclonic Region k4 = 1.00

(IS:875-pt.3, Sec 5.3.4)

:1

Wind Directionality Factor Kd= 0.90 :0.09

Area Averaging Factor Ka = 1.00*, for glazing/cladding :1

= 0.8**, for 12m face

(IS:875-pt.3, Sec 6.1.2, Table-4)

:0.08

= 0.8**, for 18m face :0.8

* Tributary area for glazing/cladding shall be less than 1 0m2, depends on the supporting system. (IS:875-pt.3, Sec 6.1.1)

Design Wind Pressure:

Design Wind Speed = VZ= Vb×k1×k2×k3×k4 = :47 xk2

= 47x 1.0x k2x 1.0x 1.0 = (47x k2) m/s

(IS:875-pt.3, Sec 5.3)

pz= 0.6 (Vz)2& pd=pz *Kd*Ka

(IS:875-pt.3, Sec 5.4 & Sec 6.1)

Table 1 : Calculations of Variation in Design Wind Speed with Height As per solved example Using Computational tool

TC1 TC4 k2* VZ(m/s) Height

from

ground

k2*

VZ(m/s) Height

from

ground,

m For sea

face•

For other

faces •

For sea

face

For other

faces

m For sea

face•

For other

faces •

For sea

face

For other

faces

Up to

9m

0.91 0.8 42.77 37.6 9 0.9100 0.8000 42.770 37.600

12m 0.934 0.8 43.9 37.6 12 0.9340 0.8000 43.898 37.600

15m 0.97 0.8 45.59 37.6 15 0.9700 0.8000 45.590 37.600

18m 0.994 0.8 46.72 37.6 18 0.9940 0.8000 46.718 37.600

21m 1.015 0.817 47.7 38.4 21 1.0150 0.8170 47.705 38.399

24m 1.03 0.87 48.41 40.8 24 1.0300 0.8680 48.410 40.796

27m 1.045 0.92 49.115 43.24 27 1.0450 0.9190 49.115 43.193

30m 1.06 0.97 49.82 45.59 30 1.0600 0.9700 49.820 45.590

33m 1.07 0.99 50.29 46.53 33 1.0690 0.9895 50.243 46.507

36m 1.165+ 1.009 54.755 47.423 36 1.1650 1.0090 54.755 47.423

39m 1.1725 1.0285 55.107 48.34 39 1.1725 1.0285 55.108 48.340

42m 1.18 1.048 55.46 49.256 42 1.1800 1.0480 55.460 49.256

45m 1.1875 1.0675 55.81 50.17 45 1.1875 1.0675 55.813 50.173

48m 1.195 1.087 56.165 51.09 48 1.1950 1.0870 56.165 51.089

51m 1.2012 1.102 56.456 51.8 51 1.2012 1.1020 56.456 51.794

* : k2 values are linearly interpolated. Fetch Length =0.2km

+ : Effect of terrain category change from TC3 to TC1 above this height

More Distant Category =TC1

6 : For terrain category 1 transiting to category 3 Near Category =TC3

Development Height = 35 Table 2: Calculations of Variation in Design Pressure with Height

pZ (kN/m2) pd, for building pd, for cladding All faces* Height from ground, m

Sea face Other face Sea face Other faces

Up to 9m 1.097 0.848 0.79 0.61 0.987

12m 1.156 0.848 0.832 0.61 1.04

15m 1.247 0.848 0.9 0.61 1.122

18m 1.31 0.848 0.943 0.61 1.178

21m 1.365 0.885 0.983 0.637 1.228

24m 1.406 1 1.012 0.72 1.265

27m 1.447 1.122 1.042 0.808 1.302

30m 1.489 1.247 1.072 0.898 1.34

33m 1.517 1.3 1.092 0.936 1.365

36m 1.799 1.349 1.295 0.971 1.619

39m 1.822 1.402 1.312 1.01 1.64

42m 1.845 1.456 1.328 1.048 1.66

45m 1.87 1.51 1.346 1.087 1.683

48m 1.893 1.566 1.363 1.127 1.704

51m 1.912 1.61 1.377 1.159 1.721



Notes: 1. For building faces Ka = 0.8 is used vary it from face to face.

Below is the result calculated using computational tool.

WIND LOAD CALCULATIONS

Wind Induced Lateral Force on Structure: This will

be calculated at every story level and separately for

each wind direction, three cases in this problem.

F=CfxAexPd

(IS:875-pt.3, Sec 6.3)

FORCE COEFFICIENT CALCULATIONS

Long-afterbody orientation

a/b = 18/12 = 1.5, h/b = 51/12 = 4.25

Cf= 1.222375

Cf 1.2 (IS.•875 -pt.3, Fig. 6)

a/b =1.5

h/b =4.25

Short-afterbody orientation

a/b = 12/18 = 0.667, h/b = 51/18 = 2.833

Cf= 1.35 ( IS.•875 -pt.3, Fig. 6)

Cf=1.3421527

a/b =0.66666

h/b =2.83333

Effective area (Ae) calculations:

6.0 x3.0 = 18m2, for intermediate frames

3.0 x 3.0 = 9m2, for end frames

For Cladding: depending on the spacing of

supporting structure, but the effect of enhanced force

at the corners and edges should be considered for

fasteners by taking local coefficients from IS:875-

pt.3, Table 5.

Tributary area for calculating wind forces on building

frames = 51 x 6 = 306m2 in either direction, being the

product of height of building & frame spacing in

either direction. As brought out in the commentary

also, the area averaging factor has been introduced in

this proposed draft, in order to account for loss of

correlation between peaks of wind generated force

over an area. Since all peaks do not occur

simultaneously, the net effect of wind force exerted

on the exposed surface is less than the case when

whole face is considered to be acted upon by design

wind force at a time. Net wind force goes on reducing

with increase in the net effective area for the element

being analyzed.

CONCLUSION:

As per above mentioned example the calculated

values using the tool is matching with corresponding

values of solved example. Hence the tool may be

considered as validated. So it can be used for similar

structure data and wind data. The developed tool is

very user friendly and one can use the tool without

help of relevant code.

REFERENCES 1. Indian standard 875 (part 3) -1987 “code of

practice for design loads (other than earthquake)

for buildings and structures part 3 wind loads(

second revision)”

2. DR.PREM KRISHNA ,DR. KRISHEN KUMAR ,DR. N.M.

BHANDARI ” is:875(part3):wind loads on

buildings and structures -proposed draft &

commentary”

3. DR.PREM KRISHNA ,DR. KRISHAN KUMAR,DR.

N.M.BHANDARI “is: 875 (part 3) – 1987 a

commentary on indian standard code of practice

for design loads (other than earthquake) for

buildings and structures part 3 wind loads

(second revision)”

4. DR. N.M. BHANDARI DR. PREM KRISHNA

DR.KRISHEN KUMAR, DR. ABHAY GUPTA “an

explanatory handbook on is 875 (part3):1987

wind loads on buildings and structure”

5. DR. N.M. BHANDARI , DR. PREM KRISHNA ,

DR.KRISHEN KUMAR, DR. ABHAY GUPTA “an

explanatory handbook on proposed is 875 (part3)

wind loads on buildings and structures”

6. Explanatory handbook on indian standard code of practice for design loads (other than earthquake)

for buildings and structures part 3 wind loads [is

875 (part 3): 1987] bureau of indian standards 7. Prof. H.s. Patel ,prof. G.n. PATEL “a

compuational tool for wind loads”journal of

structures and bridges,1997

Documents

Wind Speed Computation Tool 2011