MEP-Technical Manual CDN 0408.qxd - Hambro

8
DESIGN PRINCIPLES AND CALCULATIONS
4.2 NON-COMPOSITE DESIGN
2
D
Y
Fig. 8
N.A. of top chord
The top chord must be verified for the loads applied at the
non-composite stage. From the previous example, we
have the following results:
1- Factored Loading
• Dead load:
70 mm slab: 1.65 kN/m2 Formwork and joist: 0.24 kN/m2 1.89 kN/m2 x 1.25 = 2.36 kPa
• Live load:
Construction live load: 0.95* kN/m2 x 1.5 = 1.43 kPa
Total factored load = 3.79 kPa
* Reduces beyond 7 620 mm span at a rate of
0.05 kN/m2 each 760 mm of span.
2- Factored moment resistance
Mr nc =Crd or Trd i.e. Wnc L2 =Crd or Trd, whichever is the lesser
Wnc = 3.79 x joist spacing = kN/m
L = clear span + 100 mm
C = area of top chord (mm2 ) x factored
compressive resistance (MPa)
T = area of bottom chord (mm2 8
) x factored
tensile resistance (MPa)
d = effective lever arm (m)
= (D + 2 mm - y) /1 000
From the above formula, the maximum “limiting span”
may be computed for the non-composite (construction
stage) condition. For spans beyond this value, the top
chord must be strengthened or joist propped.
Strengthening of the top chord, when required, is usually
accomplished by installing one or two rods in the curvatures
of the “S” part of the top chord.
The bottom chord is sized for the total factored load which
is more critical than the construction load.
Hambro top chord properties are provided to assist you in
computing the non-composite joist capacities.
d
C r
T r
4.2.1 TOP CHORD PROPERTIES - D500 TM
2 mm
(0.08”)
17 mm
(0.68”)
X X
Fig. 9
METRIC
t = 2.3 mm
Anet * = 361 mm2 Ix = 2.74 x 105 mm4 Top Chord Fy = 350 MPa
Bottom Chord Fy = 380 MPa
IMPERIAL
t = 0.090 in.
Anet * = 0.560 in. 2
Ix = 0.658 in. 4
Top Chord Fy = 50 ksi
Bottom Chord Fy = 55 ksi
A net *= Effective area according to CAN3-S136-01
Y
Y
t