Truss Design NS Negi

DESIGN OF STEEL TRUSS

Width of Bldg. = 8 m c/c

Length of Bldg.= 16.5 m

Height of eaves= 7.4 m

Number of bayes = 4

Angle of trust (Alfa) = 30 Deg.

Spacing of truss(assumption) = 4.05 m

Height of truss = 2.309 m

Incline length = 4.619 m

Each incline length = 1.540 m

Horizontal length = 1.333 m

Impose loads:Imposed load = 0.35 350.00

For purlin = 0.40 402.50

For truss members = 0.27 268.33

Wind loads:

Take basic wind speed(Vb) = 50 cum (Assume)

F =

Cpe & Cpi = Force coefficient for exterior & interrior of building. (Ref: Table B7 and Table B9 ;( " BOOK = Design of steel structure, second edition, LS Negi", pg:309 & 311)

A = Effective area of the structure.

Design wind pressure.= 0.6Vz^2

Where,

Design wind speed , Vz = Vb x K1 x K2 x K3

Accroading to the IS:875(part3);

Risk factor, K1 = 1.08 ref; Table B-11, Risk Coefficient K1;( " BOOK = Design of steel structure, second edition, LS Negi", pg:312)

Height and size factor, K2 = 0.91 ref; Table B-12, K2 factor;( " BOOK = Design of steel structure, second edition, LS Negi", pg:312)

Topography factor, K3 = 1 K3 = 1 for upward slope Ø < 3º; K3 = 1 to 1.36 for upwind slope Ø > 3º

Therefore, Vz = 49.14 m/sec.

1448.84 N/m2

Now,

Height of bldg., h = 9.709 m

Width of bldg., w = 8 m

Length of bldg., L = 16.5 m

h/w = 1.21

L/w = 2.06

Internal air pressure coefficient for normal permeability, Cpi = ± 0.2

Total pressure =(Cpe -Cpi) x Pd

Wind normal to ridge :Cpi = 0.2

Downward

Wind ward Cpe = -0.3 -724.422

Lee ward Cpe = -0.4 -869.306

Wind parallel to ridge

Wind ward Cpe = -0.8 -1448.844

Lee ward Cpe = -0.6 -1159.075

Max.wind load for design = 869.31 N/m2 (downward)

Max.wind load for design = -1448.84 N/m2 (Uplift)

Design of purlins

Spacing of purlins = 1.540 m c/c

Weigth of 20gauge CGI sheets = 112.7 N/m2 (Assume)

Size of CGI sheet= 1.8*0.9 m*m

Load on purlins per meter length :

Weight of sheeting = 173.513 N/m

Weight of purlin (Assuming) = 10 kgf/m

100 N/m

Total dead load = 273.513 N/m

KN/m2

KN/m2 (horizontal area)

KN/m2 (horizontal area)

(Cpe - Cpi) X A X Pz

Pz =

Design wind pressure (Pz) =

Impose load = 536.667 N/m

Wind load = -2230.641 N/m

Dead load + impose load = 810.180 N/m

Dead load + wind load = -1957.128 N/m

When wind load is consider permissible stress is increased by 33.33%,

Therefore, dead load + wind load may be considered 33.33% less effective;

Design wind load = -1471.525 N/m; combination of dead & wind load is critical

Maximum bending moment in the purlin = 2413.668 Nm

250

Permissible bending stress = 165

Therefore, section modulus (Zx) =14628.294

14.63

Required tubular purlin (as per common steel sections):Zx = 14.4

Outside Diameter = 76.1 mm

Nominal Bore = 65 mm

Class = Medium

Thickness = 3.65 mm

Weight = 6.53 kg/m

Area of X-section = 8.31

Design Load for roof trussLoads:

1) Dead loads:Length along the sloping roof = 4.619 m

Self weight of the truss = 100 N/m2 (Assume @100 N/m2 horizontally)

Gauge of CGI sheet = 20

weight of roofing material = 112.7 N/m2

No. of purling = 8

Weight of purling = 65.3 N/m2

Therefore;

Load due to :

Self weight of truss = 3240.00 N

Roofing material (CGI sheet) = 4216.37 N

Purlins = 2115.72 N

Total = 9572.086 N

No. of panels = 6

Load acting on intermediate panel point = 1595.35 N

Dead load acting on node = 1595.35 N

Dead load acting on shoe = 797.67 N

2) Imposed loads:Miscellaneouse load (Live loads) = 0.2 N/m2 (Assume)

Total live load = 8724.48 N

Load acting on intermediat panel point = 1454.08 N

Live load acting 0n node = 1454.08 N

Live load acting on shoe = 727.04 N

3) Wind loads:Total wind load = 1448.84

Wind load acting on one intermediat panel = 9034.10 N

Wind load acting on shoe = 4517.05 N

Yield stress(fy) = N/mm2

N/mm2

mm3

Cm3

Cm3

Cm2

Force coefficient for exterior & interrior of building. (Ref: Table B7 and Table B9 ;( " BOOK = Design of steel structure, second edition, LS Negi", pg:309 & 311)

ref; Table B-11, Risk Coefficient K1;( " BOOK = Design of steel structure, second edition, LS Negi", pg:312)

ref; Table B-12, K2 factor;( " BOOK = Design of steel structure, second edition, LS Negi", pg:312)

K3 = 1 for upward slope Ø < 3º; K3 = 1 to 1.36 for upwind slope Ø > 3º

Total pressure =(Cpe -Cpi) x Pd

Cpi = -0.2

Uplift

-144.884

-289.769

-869.306

-579.538

N/m2

N/m2

N/m2

N/m; combination of dead & wind load is critical

Load Combinations:

30 Deg.

49.11 Deg.

60.00 Deg.

1) Dead loads:Dead load acting on node = 1.595 kN

Dead load acting on shoe = 0.798 kN

2) Imposed loads: 1.45 kN

3) Wind loads:Wind load acting on one intermediat panel = 9.03 kN

Wind load acting on shoe = 4.52 kN

Sign convention (Compression -ve, tension +ve)

Member

Forece (KN) due to Load combination

Design load

Dead laod Imposed load Wind load Maximum Minimum

(a) (b) (c ) (a+b) (a+c) / 1.33 KN KN

Principal LoU1 -7.98 -7.27 39.12 -15.25 23.42

27.34 -15.25Rafter U1U2 -7.98 -7.27 44.33 -15.25 27.34

Rafter U2U3 -6.38 -5.82 39.12 -12.20 24.61

Main tie L0L1 6.91 6.30 -31.62 13.20 -18.58

13.20 -18.58Main tie L1L2 5.53 5.04 -22.59 10.56 -12.83

Main tie L2L3 4.14 3.78 -13.55 7.92 -7.07

Main sling U3L2 2.76 2.52 -18.07 5.28 -11.51 5.28 -11.51

Main strut U2L2 -2.39 -2.18 15.65 -4.57 9.97 9.97 -4.57

Minor sling U2L1 2.11 1.92 -13.80 4.03 -8.79 4.03 -8.79

Minor strut U1L1 -1.60 -1.45 10.43 -3.05 6.64 6.64 -3.05

Angle of trust, ө =

Angle of trust, α =

Angle of trust, β =

Design of truss member1) Rafter (L0-U3 & U3-L5):

Design load =27.34 KN; Tension

15.25 KN; Compression

Effective length, l = 1077.72 mm; since rafter is continuous over panel points

Maximum bending moment = 3175.11 Nm


19.24

Section adopted for rolled steel beam Section type = Medium

Nominal bore, d = 160 mm

Outer diameter, D = 165.1 mm

Weight per meter, w1 = 19.2 kgf/m

Radius of gyration, r = 56.7 mm

Yield stress of steel, fy = 250 N/mm2

19.01

148.92 N/mm2

150 N/mm2

11.71 N/mm2; Safe in compression

20.99 N/mm2; Safe in tension

2) Main ties (Lo-L5):



Effective length, l = 933.33 mm



9.81






27.05

146.61 N/mm2

150 N/mm2



3) Main sling (U3L2 & U3L3):



Effective length, l = 2266.67 mm; since ends of copression section partially restrained for lateral bending



35.83






65.70

116.48 N/mm2

150 N/mm2

mm3

Cm3

Ratio, λ (l/r) =

Permissible stress in axial compression, σac =

Permissible direct stress in axial tension, σat =

Actual compression stress, σac,cal =


mm3

Cm3

Ratio, λ (l/r) =





mm3

Cm3

Ratio, λ (l/r) =



mm; since ends of copression section partially restrained for lateral bending

U3

Lo L1 L2 L3 L4 L5

fig: Truss components

ө= 30 α = 49 β = 60

1.333 m 1.333 m 1.333 m 1.333 m 1.333 m 1.333 m

fig: Truss

U1 U4

U1 U5

1.54 m 1.54 m

1.54 m 1.54 m

1.54 m 1.54 m

1.595 KN

1.595 KN 1.595 KN

1.595 KN 1.595 KN

0.798 KN 0.798 KN

4.786 KN 4.786 KNfig: Dead load

23.471 KN 23.471 KNfig: Wind load

4.517 KN 4.517 KN

9.034 KN 9.034 KN

9.034 KN 9.034 KN

4.517 KN 4.517 KN

7.824 KN

7.824 KN 7.824 KN

4.517 KN7.824 KN 7.824 KN

4.517 KN 4.517 KN3.912 KN 3.912 KN

4.517 KN 4.517 KN

2.259 KN 2.259 KN

fig: Wind load components

Truss Design NS Negi

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