Analysis Of Long Span Roof Truss

Transcription

1 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 A standard truss is a series of triangles - a stable geometric shape that is difficult to distort under loads. Regardless of its overall size and shape, all the chords and webs of a truss form triangles. These triangles combine to distribute the load across each of the other members, resulting in a light structure that is stronger than the sum of the strength of its individual components. Trusses are provided to support roof covering. The weight of roof covering through purlins is transferred at joints along the rafters. These joint loads cause axial forces tensile or compressive in all the members of a truss since all the joints of a truss are assumed to be hinged. Finally all loads including self-weight are transferred to the supports through the joints at supports. There are two types of Truss configurations- (1) Pitched roof Trusses (2) Parallel Chord Trusses This study is confined to Pitched roof Truss (Ntype) Dead load: LOAD CALCULATIONS (a) Roofing material GI sheeting with unit weight of 150 N/m 2 (b) Purlins Assuming unit weight of purlin is 100 N/m 2 (c) Total dead load = 250 N/m 2 = kn/m 2 Dead load on plan area = Load x Purlin spacing x Bay spacing = x x 10 Dead load = kn at each node Live load: Live load on plan area = Load x Purlin spacing x Bay spacing Wind load: = 0.75 x x 10 = kn at each node Basic wind speed at Coimbatore (V b ) = 39 m/s Risk coefficient (K 1 ) = 1 Terrain height and structure size factor (K 2 ) = Topography factor (K 3 ) = 1 Design of wind speed (V z ) = V b x K 1 x K 2 x K 3 = 39 x 1 x x 1 = m/s Design wind pressure (P z ) or P d = 0.6 V z 2 = 0.6 x ( ) 2 (P z ) or P d = N/m 2 F = (C pe C pi ) x A x P d Wall opening between 5% - 20% of wall area (IS 875 part-3 clause ) C pi = ± 0.5

2 Value of C pe : Roof angle θ = tan = 9⁰27 Height of the building to eaves (h) = 12m Short dimension of the building in plan (w) = 36m h < 0.5 w 36 Fig.1 Truss Profile Fig.2 Shear force and Bending moment diagram Wind load force (kn) Pressure Co-efficient

3 Wind angle C pe C pe ± C pi AxP d Wind load F (kn) C pi WW LW WW LW (kn) WW LW ⁰ ⁰ Bottom chord co-efficient F i = M i ; fbc = d i M B 9.5 = d BB ' ; F BC = x 1.8 = Bending Moment(kN.m) 0 A 9.5 B 18 C 25.5 D 32 E 37.5 F 42 G 45.5 H 48 I 49.5 J 50 K Depth(vertical) F Bottom chord Force (kn) 0 AB 19 BC 27 CD 30.6 DE 32 EF FG 31.5 GH HI 28.8 IJ 27 JK 25 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 F o ' cos(9 27 ) Top chord Force (kn) Vertical chord co-efficient

4 Shear (kn) (Q) AB BC CD DE EF FG GH HI IJ JK Top chord force(f c ) Q = F c Sinθ Force in vertical F = Q - Q 6.35 B B 4.03 C C 2.43 D D 1.2 E E 0.17 F F G G H H I I J J K K Diagonal chord co-efficient Force in vertical B B C C D D E E F F G G H H I I J J K K Diagonal length Vertical length Force in diagonal (kn) A B B C C D D E E F F G G H H I I J J K FORCES IN TRUSS MEMBERS (BY MANUAL CALCULATION) LOCATION BOTTOM TOP VERTICAL DIAGONAL MEMBER FG E'F' K'K J'K LENGTH FORCE CO-EFFICIENT D.L (4.563) L.L (13.69) W.L X1 (-26.79) W.L X2 (-10.75) W.L Z1 (-20.85) W.L Z2 (-4.81) D.L + L.L (D.L + L.L) (D.L + L.L) W.L X (D.L + L.L) W.L X (D.L + L.L) W.L Z (D.L + L.L) W.L Z

5 1.2 (D.L + L.L + W.L X1) (D.L + L.L + W.L X2) (D.L + L.L + W.L Z1) (D.L + L.L + W.L Z2) (D.L + W.L X1) (D.L + W.L X2) (D.L + W.L Z1) (D.L + W.L Z2) COMPARISON OF MANUAL RESULTS WITH STAAD.PRO RESULTS

6 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 , Code of practice for general construction in steel. [2] IS (part-1) Code of practice for design loads (other than earthquake) for buildings and structures. [3] IS (part-2) Code of practice for design loads (other than earthquake) for buildings and structures. [4] IS (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.