# Roof truss Wind load calculation as per is 875-2015

In this article we explain wind load calculation on roof truss as per revised is code 875 -2015. Explained all steps of wind load calculation with solved example. So read the article till the end and comment if you got anything wrong in this article.

## Steps of  roof truss Wind load calculation as per is 875-2015.

### Step-1 : Angle of roof truss

Angle of roof truss = tan-1( Rise/(Span/2))

### Step-2 : Determining Basic wind Speed (Vb)

Finding basic wind speed from page no 6 or 51 of  IS 875 part-3 -2015 as per location.

### Step-3: Wind pressure calculation

#### 1. Design Wind Speed (Vz) :

For finding design wind speed, formula given on page no 5 of IS 875 part-3 2015.

Vz = Vb × K1 ×K2 × K3 × K4

Where,

• Vb = Basic wind speed
• K1 = Risk Coefficient
• K2 = Terrain roughness and height factor
• K3 = Topography factor
• K4 = Importance factor for cyclonic region
##### Basic Wind Speed as per IS 875 Part 3

Basic wind speed at 10m height for some important cities of India as per is 875 part 3 2015 is tabulated below.

##### Find K1
• K1 is obtained from page no 7, table-1, IS 875 part-3 2015
• K1 depends on the class and life of the structure.
##### Find K2
•  K2 depends on the terrain category and height of the structure.
• Terrain category decided on the basis of terrain of location of structure.
• K2 is obtain from table-2, page no 8, IS 875 part-3 2015
##### Find K3

K3 is obtain from clause 6.3.3, page no 8, IS 875 part-3 2015.

##### Find K4
• K4 is obtained from clause no 6.3.4, page no 8, IS 875 part-3 2015.
• For hospitals, schools, communication towers, cyclone shelters, K4 is 1.30.
• For industrial structures, K4 is 1.15.
• All other structures, K4 is 1.00.

#### 2. Design Wind Pressure:

Pd = Kd ×Ka × Kc × Pz

Where,

Pz = wind pressure

Pz = 0.6 × Vz2

• Kd = wind directionality factor
• Ka = Area averaging factor.
• Kc = Combination factor.
##### Find Kd

Kd is obtain from clause no 7.2.1, page no 9, IS 875 part-3 2015.

##### Find Ka
• Ka is obtained from clause no 7.2.2, page no 10, IS 875 part-3 2015.
• Ka is dependent on the tributary area.
• Tributary area = spacing or pitch × rise
##### Find Kc

Kc is obtain from clause no 7.3.3.13, page no16, IS 875 part-3 2015.

### Step-4: Wind load on individual members

Wind load on individual members is determined by formula which is given in IS 875 part 3, clause 7.3.1, page no 10.

F = ( Cpe – Cpi ) A × Pd

Where,

• Cpe  = external pressure coefficient
• Cpi = internal pressure coefficient
• A = surface area of structural element or cladding unit
• Pd = design wind pressure
##### Find Cpi
• Cpi is obtained from clause 7.3.2, page no 11, IS 875 part 3 2015.
• Cpi depends on the opening area in a structure.
##### Find Cpe
• Cpe  is obtain from clause 7.3.3, page no 11, IS 875 part-3 2015.
• Cpe is also obtain from table 6, IS 875 part-3 2015.

## Solved example of Wind load calculation as per IS 875-2015

Roofing system of an industrial shed consists of trusses spaced at 6 m apart. The span of roof truss is18 m and rise is 3 m. The level of eaves is 7 m above the ground. Assume suitable configuration of truss.  The shade is situated on flat terrain with sparsely populated buildings. The shed has less than 20% permeability. Prepare structural layout of industrial steel shed with suitable configuration. Determine the wind forces on the truss. Location Chennai.

Given Data:

• Spacing : 6m
• Span = 18m
• Rise = 3m
• Height = 7m
• Terrain: flat terrain with sparsely populated buildings
• Shed has less than 20% permeability

Solution:

Assume howe type truss for 18m span.

Step-1: Angle of roof truss

Angle of roof truss = tan-1( Rise/(Span/2))

= tan-1(3/(18/2))

= 18.43

Step-2: Determining Basic wind Speed (Vb)

For Chennai, the basic wind speed is 50m/s from page no51, IS 875 part-3 2015.

Step-3: Wind pressure calculation

Direct finding K1, K2, K3,  K4 from the IS 875 part-3. For find this coefficient is explain above in steps.

• K1 = 1
• K2 = 1
• K3 = 1
• K4 = 1.15

Vz = Vb × K1 ×K2 × K3 × K4

= 50 × 1 × 1 × 1 × 1.15

= 57.5m/s

Design Wind Pressure:

Pd = Kd ×Ka × Kc × Pz

Direct finding Kd, Ka, Kc from the IS 875 part-3. For find this coefficient is explain above in steps.

• K= 0.9
• Ka = 0.92 (getting this by interpolation between 10 and 25)
• Kc = 0.9

Pz = 0.6 × Vz2

= 0.6 × 57.52

=1983.75 N/m2

Pd = Kd ×Ka × Kc × Pz

= 0.9 × 0.92 × 0.9 × 1983.75

= 1478.29 N/m2

= 1.478 KN/m2

Design wind pressure is less than 0.7 × Pz

= 0.7 × 1983.75

= 1388.62 (N/m2)   Hence OK

Step-4: Wind load on individual members

F = ( Cpe – Cpi ) A × Pd

Find Cpi

The shed has less than 20% permeability

Therefore, Cpi = +/-(0.5)

Find Cpe

For truss angle 18.43 and,

h/w = 7/18

= 0.38

the Cpe is given below.

Finding area:

A = Spacing × ( ((span/2)2 × (rise)2)^(0.5))/number of panels )

=  6 × (((9)2 × (3)2) ^ (0.5)) / 8 )

= 7.115 m2

Therefore,

A × Pd  = 7.115 × 1.478

= 10.52 KN

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### 6 thoughts on “Roof truss Wind load calculation as per is 875-2015”

1. Hi,
Was working through this example – now have following questions:
A = Spacing × ( ((span/2)2 × (rise)2)^(0.5))/number of panels )
should be
A = Spacing × ( ((span/2)2 + (rise)2)^(0.5))/number of panels )?

As we appear to be calculating roof slope area on one side only, the number of panels should be 4 rather than 8? ie. where the purlins would match the truss panels on one side of the roof?

A minor typo?
The shade is situated on flat terrain with sparsely populated buildings
should read The shed is situated…

Other than those points, a great example well explained and sequenced.

Regards

As per is 875/part 3 /2015
We want wind speed,
Vb=50m/s,
Length=16m,