• Used UPVC roof sheet • Designed for Sylhet • Used BNBC code 2006 • Computer Analysis of truss (Using GRASP)

1. Roof Truss Design
STRUCTURAL ANALYSIS & DESIGN SESSIONAL
Department of
Civil Engineering.
Leading University, Sylhet

2. Presented by
FOYSOL MAHMUD
1512060014
Batch: 10th ; Section: B
Department of
Civil Engineering.
Leading University, Sylhet

3. Key Facts
• Used UPVC roof sheet
• Designed for Sylhet
• Used BNBC code 2006
• Computer Analysis of truss (Using GRASP)

4. Design Data:
Span = 40’
Rise = 10’
Span to rise ratio = 4:1
Slope = tan-1(6/20) = 26.5651 ⁰ (Degree)
Location: Sylhe, Basic Wind Speed 56.7 m/s = 204.12 km/hr
Exposure Category: A
Truss is supported on brick wall of height = 12 feet

5. Design Loads:
Dead load:
Self-weight of truss = 60 lb per ft. horizontal span of truss.
Sag rod + bracing = 1 psf. (approximately known)
UPVC sheet roofing = 340kg per 100 square meter = 1psf
Purlin (self-weight) = 1.5 psf. (assumed)
Wind load = according to BNBC 2014 (Bangladesh National Building Code 2014).
Snow load = not applicable for our country.
Design Method:
Design method followed here is AISC/ASD
Steel to be used: A36 (Yield stress (Fy = 36 ksi))
Electrode to be used: E60XX (electrode material tensile strength (FEXX) = 60 ksi)

6. Analysis and Design of Purlin for Dead Load:
Calculation of total dead load on purlin:
UPVC sheet roofing = 1 psf (known)
Self-weight of purlins = 1.5 psf (assumed but will be checked later)
Sagrod weight = negligible
Total dead load = 2.50 psf
Uniformly distributed load (UDL) on purlin, WDL = 2.50 psf × purlin spacing
= 2.50 psf × 7.453 ft. = 18.6325
Component of WDL in X direction, WDLx = WDL ×sinθ = 18.6325× sin(26.565) = 8.33 lb/ft
Component of WDL in Y direction, WDLy = WDL ×cosθ = 18.6325× cos(26.565) = 16.66 lb/ft
Purlin span = 25 feet for loading in Y direction (loading perpendicular to the plane of roof surface)
Purlin span = 12.5 feet + 12.5 feet for loading X direction (loading in the plane of roof surface)

7. Analysis and Design of Purlin for Dead Load:
Mxx = 1.3 Kip-ft, Myy = 0.163 Kip-ft
Allowable bending stress, Fb= 0.66Fy = 0.66 X 36 = 23.76 ksi
Section Sxx (inch3) Syy (inch3) Actual bending
stress (f) in ksi
Allowable
bending stress
(Fb) in ksi
Comments
C 3×4.1 1.10 0.202 23.86 23.76 Not OK
C 3×5 1.24 0.233 20.97 23.76 OK
Check self-weight of purlin:
For C 3×5 channel, self-weight is 5 lb/ft which is
equivalent to = 0.6708 psf

8. Wind Load Calculation (according to BNBC 2014)
Truss location: Sylhet
Vb= Basic wind speed in km/h = 204.12 km/h
B = Horizontal dimension of the building,
in meters measured normal to wind direction = 25 feet = 7.6219 meter.
L = Horizontal dimension of the building, in meters measured parallel to wind
direction = span of truss = 40 feet = 12.1951 meter.
H = average/mean height of the roof in meters = 17 feet = 5.1829 meter.
z = Height above the ground in meters
θ = Angle of the plane of roof from horizontal, degrees = 26.56510 (degree)
Cc= Velocity-to-pressure conversion co-efficient = 47.21 x 10-6
CI = Structure importance co-efficient = 1.00 for standard occupancy structures
CZ = Combined height and exposure co-efficient = 0.3897 for exposure A
Cz = 0.1879(z)0.4435 = 0.1879(5.1829)0.4435 =0.3897(>0.368)
CG = = Gust response co-efficient = 1.6257 exposure A
Cpe = External pressure co-efficient = -0.14345 for windward side
Cpe = External pressure co-efficient = -0.70 for leeward side
qz = Sustained wind pressure in kN/m2
Pz = Design wind pressure in kN/m2
qz = Cc CI CZ Vb
2 = (47.2x10-6 ) × (1) × (0.3897) × (204.12)2 = 0.766
kN/m2
Pz = CG Cpe qz
Design wind pressure for windward side:
Pz = -0.1786 kN/m2 = -3.72 psf
Design wind pressure for leeward side:
Pz = -0.872 kN/m2 = -18.20 psf

9. Wind Load Calculation (according to BNBC 2014)

10. Calculation of UDL on purlin:
UDL on purlin on windward side = (design wind pressure on the windward side × purlin spacing)
= -3.72 psf × 7.4535 feet = - 27.73 lb/ft
UDL on purlin on leeward side = (design wind pressure on the leeward side × purlin spacing)
= -18.20 psf × 7.4535 feet = - 135.654 lb/ft
Since wind load acts perpendicular to the roof surface, these loads will be combined with the Y
component of the dead load (WDLy) to get the resultant load
Resultant load in Y direction Wy = WDLy + Pz = 16.66 lb/ft - 135.654 lb/ft = -118.994 lb/ft
Again Checking Purlin:
Mxx = -9.296 Kip-ft,
Myy = 0.163 Kip-ft (moment about Y axis remains the same)
Section Sxx (inch3) Syy (inch3)
Actual
bending
stress (f) in ksi
Allowable
bending
stress
(Fb) in ksi
Comments
C 3×5 1.24 0.233 81.566 23.76 Not OK
C6x8.2 4.38 0.492 21.49 23.76 OK
Finally selected channel for
purlins: C6x8.2; self weight
= 8.2 lb/ft = 1.100154 psf.

11. Analysis of the Truss: (Dead load calculation &
wind load calculation)
Dead load calculation
UPVC sheet roofing = 1 psf (known)
Self-weight of purlins = 1.5 psf (assumed value is greater than the purlin)
Sagrod + bracing = 1psf
Total dead load = 3.50 psf
Total 4.50 psf dead load = (3.50 psf) × (purlin spacing) × (bay)
= 3.50 psf × 7.4535 feet × 25 feet = 652.18 lb
Self-weight of the truss (assumed) = 60 lb/ft horizontal span of truss
The self-weight of the truss will be equally divided among the top chord and bottom chord.
Total 60 lb/ft = 30 lb/ft in top chord & 30 lb/ft in bottom chord

12. Dead Load calculation
+
=

13. Wind Load Calculation (wind blows from
left to right):

14. Wind Load Calculation (wind blows from
left to right)

15. Truss Analysis (Computer Analysis)
For Dead Load:
Member Fx.i [kips] Fx.j [kips]
1 5.2608 5.2608
2 5.2608 5.2608
3 4.2086 4.2086
4 4.2086 4.2086
5 5.2608 5.2608
6 5.2608 5.2608
7 0.2000 0.2000
8 0.7260 0.7260
9 2.3040 2.3040
10 0.7260 0.7260
11 0.2000 0.2000
12 -5.8816 -5.8816
13 -4.7053 -4.7053
14 -3.5289 -3.5289
15 -3.5289 -3.5289
16 -4.7053 -4.7053
17 -5.8816 -5.8816
18 -1.1763 -1.1763
19 -1.4879 -1.4879
20 -1.4879 -1.4879
21 -1.1763 -1.1763

16. Truss Analysis (Computer Analysis)
For Wind (Left to Right)
Member Fx.i [kips] Fx.j [kips]
1 -10.4208 -10.4208
2 -10.4208 -10.4208
3 -9.6857 -9.6857
4 -10.1530 -10.1530
5 -11.3554 -11.3554
6 -11.3554 -11.3554
7 0.0000 0.0000
8 -0.3675 -0.3675
9 -1.9371 -1.9371
10 -0.6011 -0.6011
11 0.0000 0.0000
12 4.9000 4.9000
13 4.6260 4.6260
14 4.3520 4.3520
15 3.2675 3.2675
16 7.2994 7.2994
17 11.3314 11.3314
18 0.8218 0.8218
19 1.0395 1.0395
20 1.7003 1.7003
21 1.3442 1.3442

17. Truss Analysis (Computer Analysis)
Wind (Right to Left)
Member Fx.i [kips] Fx.j [kips]
1 -14.2314 -14.2314
2 -10.6310 -10.6310
3 -7.2757 -7.2757
4 -7.5208 -7.5208
5 -7.5208 -7.5208
6 0.0000 0.0000
7 -1.7999 -1.7999
8 -3.8449 -3.8449
9 -0.1225 -0.1225
10 0.0000 0.0000
11 8.1288 8.1288
12 7.3069 7.3069
13 6.4851 6.4851
14 5.4007 5.4007
15 6.7427 6.7427
16 -14.2314 -14.2314
17 8.0847 8.0847
18 5.0913 5.0913
19 4.0252 4.0252
20 0.3466 0.3466
21 0.2740 0.2740

18. Thank you