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 a solved example. So read the article till the end and comment if you got anything wrong in this article.

Table of Contents

• Steps of roof truss Wind load calculation as per is 875-2015.
  • Step-1: Angle of roof truss
  • Step-2: Determining Basic wind Speed (Vb)
  • Step-3: Wind pressure calculation
    • 1. Design Wind Speed (Vz) :
      • Basic Wind Speed as per IS 875 Part 3
      • Find K1
      • Find K2
      • Find K3
      • Find K4
    • 2. Design Wind Pressure:
      • Find Kd
      • Find Ka
      • Find Kc
  • Step-4: Wind load on individual members
    • • Find Cpi
      • Find Cpe
• Solved example of Wind load calculation as per IS 875-2015
  • Wind load calculation table

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)

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

Step-3: Wind pressure calculation

1. Design Wind Speed (Vz) :

For finding design wind speed, the formula is 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.

City/Town	Basic Wind Speed	City/Town	Basic Wind Speed
Agra	47m/s	Kanpur	47m/s
Ahemadabad	39m/s	Kohima	44m/s
Ajmer	47m/s	Kolkata	50m/s
Almora	47m/s	Kozhikode	39m/s
Amritsar	47m/s	Kurnool	39m/s
Asansol	47m/s	Lakshadeep	39m/s
Aurangabad	39m/s	Lucknow	47m/s
Bahraich	47m/s	Ludhiyana	47m/s
Bengaluru	33m/s	Madurai	39m/s
Barauni	47m/s	Mandi	39m/s
Bareilly	47m/s	Mangalore	39m/s
Bhatinda	47m/s	Moradabad	47m/s
Bhilai	39m/s	Mumbai	44m/s
Bhopal	39m/s	Mysore	33m/s
Bhubaneshwar	50m/s	Nagpur	44m/s
Bhuj	50m/s	Nainital	47m/s
Bikaner	47m/s	Nashik	39m/s
Bokaro	47m/s	Nellore	50m/s
Chandigarh	47m/s	Panjim	39m/s
Chennai	50m/s	Patiala	47m/s
Coimbatore	39m/s	Patna	47m/s
Cuttack	50m/s	Puducherry	50m/s
Darbhanga	55m/s	Port Blair	44m/s
Darjeeling	47m/s	Pune	39m/s
Dehradun	47m/s	Raipur	39m/s
Delhi	47m/s	Rajkot	39m/s
Durgapur	47m/s	Ranchi	39m/s
Gangtok	47m/s	Roorkee	39m/s
Guwahati	50m/s	Rourkela	39m/s
Gaya	39m/s	Shimla	39m/s
Gorakhpur	47m/s	Srinagar	39m/s
Hyderabad	44m/s	Surat	44m/s
Imphal	47m/s	Tiruchirappalli	47m/s
Jabalpur	47m/s	Trivandrum	39m/s
Jaipur	47m/s	Udaipur	47m/s
Jamshedpur	47m/s	Vadodara	44m/s
Jhansi	47m/s	Varanasi	47m/s
Jodhpur	47m/s	Vijayawada	50m/s
Vishakapatnam	50m/s

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.
• The terrain category is decided on the basis of the terrain of the location of the structure.
• K2 is obtained from table-2, page no 8, IS 875 part-3 2015

Find K3

K3 is obtained 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, and cyclone shelters, K4 is 1.30.
• For industrial structures, K4 is 1.15.
• For 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 obtained 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 obtained 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 the 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 a 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 obtained from clause 7.3.3, page no 11, IS 875 part-3 2015.
• Cpe is also obtained from table 6, IS 875 part-3 2015.

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

The roofing system of an industrial shed consists of trusses spaced 6 m apart. The span of the roof truss is18 m and the rise is 3 m. The level of the eaves is 7 m above the ground. Assume a suitable configuration of the truss.  The shade is situated on flat terrain with sparsely populated buildings. The shed has less than 20% permeability. Prepare the structural layout of the industrial steel shed with a 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
• The 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, and K4 from the IS 875 part-3. To find this coefficient is explained 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. To find this coefficient is explained above in steps.

• Kd  = 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

Wind load calculation table

Er. SP.ASWINPALANIAPPAN., M.E.,(Strut/.,)

Structural Engineer

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