International Journal of Science and Engineering Research (IJ0SER),

Vol 3 Issue 10 October-2015

3221 5687, (P) 3221 568X

Analysis Of Long Span Roof Truss

Nalladurai V* Shankar P* *
* P.G Student, Structural Engineering, Department of Civil Engineering,
** Assistant Professor, Department of Civil Engineering,
Nandha Engineering College, Erode, Tamil Nadu,
India

Abstract- The usage of steel roof truss for long span area is more economical than RCC structure. The planned long span roof truss was
analysed by both manually and using software package. In manual design, there are more different methods available to determine
member forces in the roof truss. The methods except force coefficient may not be feasible to determine the member forces, so force
coefficient method is taken for the designing of long span roof trusses by the researchers and design companies. In this project, the
member forces in the roof truss of 36m long have been determined using force coefficient method and also by using STAAD.Pro and the
results were compared.

Keywords: Long span roof truss, Force co-efficient method

INTRODUCTION Dead load = 4.563 kN at each node

A standard truss is a series of triangles - a stable Live load:

geometric shape that is difficult to distort under loads.
Regardless of its overall size and shape, all the chords and Live load on plan area = Load x Purlin spacing x Bay
webs of a truss form triangles. These triangles combine to spacing
distribute the load across each of the other members,
resulting in a light structure that is stronger than the sum of = 0.75 x 1.825 x 10
the strength of its individual components. Trusses are
provided to support roof covering. The weight of roof = 13.69 kN at each node
covering through purlins is transferred at joints along the
Wind load:
rafters. These joint loads cause axial forces – tensile or
compressive – in all the members of a truss since all the Basic wind speed at Coimbatore (Vb) = 39 m/s
joints of a truss are assumed to be hinged. Finally all loads
including self-weight are transferred to the supports through Risk coefficient (K1) = 1
the joints at supports. There are two types of Truss
configurations- Terrain height and structure size factor (K2) = 0.9816
(1) Pitched roof Trusses
(2) Parallel Chord Trusses Topography factor (K3) = 1

This study is confined to Pitched roof Truss (N- Design of wind speed (Vz) = Vb x K1 x K2 x K3
type)
= 39 x 1 x 0.9816 x 1
LOAD CALCULATIONS
= 38.2824 m/s
Dead load:
Design wind pressure (Pz) or Pd = 0.6 Vz2
(a) Roofing material – GI sheeting with unit weight of
150 N/m2 = 0.6 x (38.2824)2

(b) Purlins – Assuming unit weight of purlin is 100 (Pz) or Pd = 879.33 N/m2
N/m2
F = (Cpe – Cpi) x A x Pd
2
(c) Total dead load – 150 + 100 = 250 N/m = 0.250
kN/m2 Wall opening between 5% - 20% of wall area (IS 875 part-3
clause 6.2.3.2)
Dead load on plan area = Load x Purlin spacing x Bay
spacing = 0.250 x 1.825 x 10 Cpi = ± 0.5

Nalladurai,Shankar…. (IJ0SER) October - 2015

International Journal of Science and Engineering Research (IJ0SER),
Vol 3 Issue 10 October-2015
3221 5687, (P) 3221 568X
Value of Cpe: Height of the building to eaves (h) = 12m

 3 Short dimension of the building in plan (w) = 36m

Roof angle θ = tan -1   = 9⁰27′
 18 
h 12
  0.333 < 0.5
w 36

Fig.1 Truss Profile

Fig.2 Shear force and Bending moment diagram

Wind load force (kN)

Pressure Co-efficient

Nalladurai,Shankar…. (IJ0SER) October - 2015

International Journal of Science and Engineering Research (IJ0SER),
Vol 3 Issue 10 October-2015
3221 5687, (P) 3221 568X
Wind Cpe Cpe ± Cpi AxPd Wind load F (kN)
Cpi
angle WW LW WW LW (kN) WW LW

-0.5 -1.67 -0.9 16.04 -26.79 -14.44

0⁰ -1.168 -0.4 0.5 -0.67 0.1 16.04 -10.75 1.60

-0.8 -0.8 -0.5 -1.3 -1.3 16.04 -20.85 -20.85

90⁰
-0.6 -0.6 0.5 -0.1 -0.1 16.04 -4.81 -4.81

Bottom chord co-efficient

Mi MB 9.5
Fi = ; fBC = =  10.56 ; FBC = 10.56 x 1.8 = 19
di d BB ' 0.9

Bending 0 9.5 18 25.5 32 37.5 42 45.5 48 49.5 50

Moment(kN.m)
A′ B′ C′ D′ E′ F′ G′ H′ I′ J′ K′

Depth(vertical) 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3 3.3 3.6

F0 0 10.56 15 17 17.78 17.86 17.5 16.85 16 15 13.89

Bottom chord 0 19 27 30.6 32 32.15 31.5 30.33 28.8 27 25

Force (kN)
AB BC CD DE EF FG GH HI IJ JK

(-ve indicates member in compression, +ve indicates member in tension)

Top chord co-efficient

Member A′B′ B′C′ C′D′ D′E′ E′F′ F′G′ G′H′ H′I′ I′J′ J′K′

Fo 10.67 15.15 17.17 17.96 18.04 17.68 17.02 16.16 15.15 14.03

cos(9  27 ' )

Top chord -19.21 -27.27 -30.91 -32.33 -32.47 -31.82 -30.64 -29.09 -27.27 -25.25
Force (kN)

(-ve indicates member in compression, +ve indicates member in tension)

Vertical chord co-efficient

Nalladurai,Shankar…. (IJ0SER) October - 2015

International Journal of Science and Engineering Research (IJ0SER),
Vol 3 Issue 10 October-2015
3221 5687, (P) 3221 568X
Shear (kN) (Q) 9.5 8.5 7.5 6.5 5.5 4.5 3.5 2.5 1.5 0.5

AB BC CD DE EF FG GH HI IJ JK

Top chord force(Fc) 19.21 27.27 30.91 32.33 32.47 31.82 30.64 29.09 27.27 25.25

Q′ = FcSinθ 3.15 4.47 5.07 5.30 5.33 5.22 5.02 4.77 4.47 4.14

Force in vertical F = 6.35 4.03 2.43 1.2 0.17 -0.72 -1.52 -2.27 -2.97 -3.64
Q - Q′
B′B C′C D′D E′E F′F G′G H′H I′I J′J K′K

(-ve indicates member in compression, +ve indicates member in tension)

Diagonal chord co-efficient

Force in vertical 6.35 4.03 2.43 1.2 0.17 -0.72 -1.52 -2.27 -2.97 -3.64

B′B C′C D′D E′E F′F G′G H′H I′I J′J K′K

Diagonal length 1.9 2 2.16 2.34 2.55 2.77 3 3.25 3.5 3.76

Vertical length 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3 3.3

Force in diagonal 20.1 8.96 4.37 1.87 0.24 -0.95 -1.9 -2.73 -3.47 -4.15
(kN)
A′B B′C C′D D′E E′F F′G G′H H′I I′J J′K

(-ve indicates member in compression, +ve indicates member in tension)

FORCES IN TRUSS MEMBERS (BY MANUAL CALCULATION)

BOTTOM TOP VERTICAL DIAGONAL

LOCATION CHORD(kN) CHORD(kN) CHORD(kN) CHORD(kN)
MEMBER FG E'F' K'K J'K
LENGTH 1.8 1.825 3.6 3.76
FORCE CO-EFFICIENT 32.15 -32.47 -3.64 -4.15
D.L (4.563) 146.7 -148.16 -16.61 -18.936
L.L (13.69) 440.134 -444.51 -49.83 -56.814
W.L X1 (-26.79) -861.3 869.871 97.516 111.179
W.L X2 (-10.75) -345.61 349.053 39.13 44.6125
W.L Z1 (-20.85) -670.33 677 75.894 86.5275
W.L Z2 (-4.81) -154.64 156.181 17.508 19.9615
D.L + L.L 586.834 -592.67 -66.44 -75.75
1.5 (D.L + L.L) 880.251 -889.01 -99.66 -113.62
1.2 (D.L + L.L) + 0.6 W.L X1 187.422 -189.29 -21.22 -24.193
1.2 (D.L + L.L) + 0.6 W.L X2 496.833 -501.78 -56.25 -64.132
1.2 (D.L + L.L) + 0.6 W.L Z1 302.004 -305.01 -34.19 -38.983
1.2 (D.L + L.L) + 0.6 W.L Z2 611.416 -617.5 -69.22 -78.923

Nalladurai,Shankar…. (IJ0SER) October - 2015

International Journal of Science and Engineering Research (IJ0SER),
Vol 3 Issue 10 October-2015
3221 5687, (P) 3221 568X

1.2 (D.L + L.L + W.L X1) -329.36 332.636 37.29 42.5143

1.2 (D.L + L.L + W.L X2) 289.466 -292.35 -32.77 -37.365
1.2 (D.L + L.L + W.L Z1) -100.19 101.19 11.344 12.9331
1.2 (D.L + L.L + W.L Z2) 518.631 -523.79 -58.72 -66.946
1.5 (D.L + W.L X1) -1071.9 1082.57 121.36 138.363
1.5 (D.L + W.L X2) -298.37 301.338 33.781 38.5141
1.5 (D.L + W.L Z1) -785.44 793.258 88.927 101.387
1.5 (D.L + W.L Z2) -11.912 12.0301 1.3486 1.53758

COMPARISON OF MANUAL RESULTS WITH

STAAD.PRO RESULTS

Nalladurai,Shankar…. (IJ0SER) October - 2015

International Journal of Science and Engineering Research (IJ0SER),
Vol 3 Issue 10 October-2015
3221 5687, (P) 3221 568X
CONCLUSION

The following points have been arrived based on the

analysis carried on N-type long span roof truss with 36m
span.

 Initially the member forces and its action are

calculated in both manual and software
(STAAD.Pro) manner.

 The member force variations between the manual

and software (STAAD.pro) calculations are differ
by 10% which are closely enough to the limit.

 The causes of those variations is due to ommission

of selfweight for the manual calculation because
the pre-determination of selfweight in members is
not possible in the manual calculations.

 For the concern of long span roof truss, software

based analysis is more efficient. The manual
calculations can be used for cross checking.
REFERENCES

[1] IS 800-2007, Code of practice for general

construction in steel.

[2] IS 875-1987(part-1) Code of practice for design loads

(other than earthquake) for buildings and structures.

[3] IS 875-1987(part-2) Code of practice for design loads (other

than earthquake) for buildings and structures.

[4] IS 875-1987(part-3) code of practice for design loads

(other than earthquake) for buildings and structures.

[5] Design of Steel Structures by L.S.Jayagopal,

D.Tensing.

[6] Design of Steel Structures N.Subramanian.

[7] Design of Steel Structures by S.K.Duggal.

Nalladurai.V.,
B.E(Civil) Degree: KPR Institute
of Engineering and Technology
Arasur, Coimbatore.
M.E(Structural Engineering)
Degree(Pursuing): Nandha
Engineering College Erode.

Nalladurai,Shankar…. (IJ0SER) October - 2015