Loads

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Loads

Loads acting on truss:

:Dead load)1

Weight of corrugated and weight of steel structure

W c = (58) kg / m2 for single layer (default)

= (1218) kg / m2 for double layer (if given)

W s = (2035) kg / m2/hz proj. We take W s= B

i.e. If the span of truss = 28 m take W s= 28 kg / m2

aSsWSa

cWDP += acos

PDLP /2 PDLDLP DL

DLPPDLDL PPDL DLPPDL P /2DL

2) Live load:

W LL = 60 66.66 tan a inaccessible roof (default)

= 200 300 tan a accessible roof

In kg / m2 /hz proj.

i.e. If the slope 1 : 10

\W LL = 60 66.66 * 101 = 60 6.7 = 53.3 kg / m2 /hz proj.

PLL = W LL a S

Loads

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PP /2 PPPP PP PP P /2

LL LL LL LLLL LL LL LL LL LL LL

3) Wind load:

W w = C e k q kg / m2

a - K height factor k = 1.0 h ? 10 m

k = 1.1 10 < h ? 20

b q = wind intensity in kg / m2

= 70 kg / m2 Cairo 80 kg / m2 Alexandria

c For C e: divide truss into 2 parts

Pressure side Suction side

For wind left (WL) For wind right (WR)

Suction side Pressure side Winddirection

Winddirection

For suction side: C e = - 0.5 ( - ve means suction outside truss)

For pressure side: C e is calculated using the following curve

Loads

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0.8

- 0.8

0.4

C e

tan0.80.

2

0.05

ie: For all trusses of slope 1:5 (0.2) to 1:20 (0.05), Ce will be = -0.8 (suction)

although it lies in the pressure side.

Only fink truss can have slope more than 0.4

ShP *2

*1*70*8.01 = SaP *

cos*1*70*8.02 a

-=

SaP *cos

*1*70*5.03 a-= ShP *

2*1*70*5.04 -=

P

1P

1P

22P /2 P2P2 P2 P /23

P3 3P 3P 3P2P/2 P/2 3

Note loads at the joint at mid span

Loads

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main types of loads2There are

Primary loads: Case I (A): dead and live load

Secondary loads: Case II (B): wind load, braking force & lateral shock

For case II (B) increase the allowable stresses by 20 %

IMP: To determine the design case:

If 2.1+

++LD

WLD Case I If 2.1>+

++LD

WLD Case II

Example:

Determine the design value & case for the following members:

DL LL WL WR5 7 1 -25 7 3 -25 7 3 55 7 -3 -15 7 1 -75 7 3 -7-5 -7 3 -3-5 -7 3 -1-5 -7 7 3

Solution:

Case A Case B Comparison Design caseD+L D+L+WL D+L+WR D+WL D+WR and value12 13 10 6 3 13/12=1.081.2 15 T (B)12 15 17 8 10 17/12=1.4>1.2 17 T (B)12 9 11 2 4 12 T (A)12 13 5 6 -2 13/12=1.081.2 15T (B), 2C (B)-12 -9 -15 -2 -8 15/12=1.25>1.2 15 C (B)-12 -9 -13 -2 -6 13/12=1.08