r1 r2 1. load on one truss - top chord, w1 7. section ... · size of bottom chord = 4x4 actual...

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