r1 r2 1. load on one truss - top chord, w1 7. section ... · size of bottom chord = 4x4 actual...
TRANSCRIPT
Combined StreSam Zou4/10/2018
Full span of truss, s = 18 ftHeight of truss, h = 6 ftOn Center spacing of trusses = 16inSize of bottom chord = 4x4Actual width, b = 3.5 inActual depth, d = 3.5 inSnow Load on roof, w_snow = 15 psfLive Load in attic, w_live = 65 psfFactored allowable bending stress, F'b = 1265 psiFactored allowable tension stress, F't = 776 psi
1. Load on one truss - top chord, w1 Load on one truss - bottom chord, w2
3. Total left reaction due to w1 and w2
4. Force components in truss
5. Axial stress in the truss bottom chord
6. Maximum bending moment in the bottom chord member
7. Section modulus of the bottom chord member, Sx
8. Maximum bending stress in the bottom chord member
9. Check if the combines stress passes
Top chordw1 = 15 x (16/12) = 20 plf
NDS-Supplement-Table 1B
bending stress, fb =
Case 1: + = = 1.249 > 1= Fail!
= Fail!
1473.58 psi= =
Sx = 7.146 in3
R1 = R2 = (W1/2) + (W2/2) = 960 lbs
W1 =w1s= 180 lbsW2 =w2s= 780 lbs
Draw free body diagram at 1/4 point of truss
Axial stress, ft =
Maximum Being moment, M = = =877.5 ft-lbs
= =58.78 psi
Axial Force
Sectional area
Bottom chordw2 = 65 x (16/12) = 86.7 plf
snow load unit convert
unit convert
unit convert
live load
On Center spacing of trusses
On Center spacing of trusses
R1 R2
R1
V1
H1
H29 ft
6 ft
(W1/4)
(W2/4)
∑Fv = V1 +(W1/4) + (W2/4) - R1 =0
V1/ H1 = 6 / 9 H1 = 720 lbs
(similar triangles)
V1 +90 + 390 - 960 =0V1 = 480 lbs
∑Fh = H1 - H2 H2 = 720 lbs
PA
w2l28
MSx
ftF't
fbF'b +58.78
7761473.58
1256
Case 2: = = 1.126 > 1fb - ftF'b
1473.58 -58.781256
877.5 x12 7.146
86.7 x 928
7203.5x 3.5