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 (V_b)

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 (V_z) :

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

V_z = V_b × K₁ ×K₂ × K₃ × K₄

Where,

• V_b = 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	Punducherry	50m/s
Darbhanga	55m/s	Port Blair	44m/s
Darjeeling	47m/s	Pune	39m/s
Dheradun	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.
• 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

• K_{4 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:

P_d = K_d ×K_a × K_c × P_z

Where,

P_z = wind pressure

P_z = 0.6 × V_z²

• K_d = wind directionality factor
• K_a = Area averaging factor.
• K_c = Combination factor.

Find K_d

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

Find K_a

• K_{a 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 K_c

K_c 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 = ( C_pe – C_pi ) A × P_d

Where,

• C_pe  = external pressure coefficient
• C_pi = internal pressure coefficient
• A = surface area of structural element or cladding unit
• P_d = design wind pressure

Find C_pi

• C_{pi is obtained} from clause 7.3.2, page no 11, IS 875 part 3 2015.
• C_pi depends on the opening area in a structure.

Find C_pe

• C_pe  is obtain from clause 7.3.3, page no 11, IS 875 part-3 2015.
• C_pe 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 (V_b)

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, K_3,  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

V_z = V_b × K₁ ×K₂ × K₃ × K₄

= 50 × 1 × 1 × 1 × 1.15

= 57.5m/s

Design Wind Pressure:

P_d = K_d ×K_a × K_c × P_z

Direct finding K_d, K_a, K_c from the IS 875 part-3. For find this coefficient is explain above in steps.

• K_d  = 0.9
• K_a = 0.92 (getting this by interpolation between 10 and 25)
• K_c = 0.9

P_z = 0.6 × V_z²

= 0.6 × 57.5²

=1983.75 N/m2

P_d = K_d ×K_a × K_c × P_z

= 0.9 × 0.92 × 0.9 × 1983.75

= 1478.29 N/m2

= 1.478 KN/m2

Design wind pressure is less than 0.7 × P_z

= 0.7 × 1983.75

= 1388.62 (N/m2)   Hence OK

Step-4: Wind load on individual members

F = ( C_pe – C_pi ) A × P_d

Find C_pi

The shed has less than 20% permeability

Therefore, C_pi = +/-(0.5)

Find C_pe

For truss angle 18.43 and,

h/w = 7/18

= 0.38

the C_pe 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 × P_d  = 7.115 × 1.478

= 10.52 KN

Wind load calculation table

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