PURLIN AND ROOF DESIGN
Terminology of Truss
Rafter (Top chord)
Purlins
Nodes
Span
Bottom chord
Design of roof truss system
The loading subjected by a truss is transferred through the purlins, either directed onto the
nodes or on the top members span.
It is ideal if the loads can be transferred to the truss at the node position.
General Procedures:
1. Start
2. Loading
3. Analyse the truss members
 Tension member
 Compression member
4. Analyse load bearing member such as rafter as continuous beam supported at the nodes
and loaded by a purlins.
If the load position are uncertain, the rafter moment may be taken as wL2/6 where L is the
node to node length of rafter and w is the total load per unit length.
5. End
Loading
The loading subjected to a truss system could be dead loads, live loads and wind load.
For roof truss system, the dead load may be consisting of cladding, insulation, self-weight of
trusses and purlins, services, etc....
For live load, according to BS 6399-2 or CP3: Ch V: Part 2, 0.75 kN/m2 may used where
entranceto the roof is available for services purposes. Otherwise, use 1.5 kN/m2 may be used
if more than that.
Purlin Design
Refer to Cl. 4.12.4.3
Purlins are those members in truss system which carrying the roof sheets and transferring load
to rafters. It is normally placed perpendicular to the rafters and sag rods may be added (to
reduce purlin size of purlins) see Table 27
The purlins are not necessary to be analyzed as complicated as the other structural members.
The satisfaction of purlins is approached by the empirical rules suggested in cl 4.12.4.3 as:
(a) The slope of roof should less than 30o from horizontal
(b) The loading on the purlin should substantially uniformly distributed. Not
(c) The limitations of section modulus Z about its axis parallel to the plane of the cladding,
member dimensions D perpendicular to the plane of caldding, B parallel to the plane of
cladding are given in Table 27.
Example:
Purlins
6.324 m
2m
12 m
Figure Q3/ [Rajah Q3]
A plane truss as Figure Q3 is arranged all purlins on its nodes. With the following data
- Spacing between trusses
=5m
- Weight of roof sheet, insulation and purlins (on slope)
= 0.35 kN/m2
- Self-weight of truss (on slope)
= 0.20 kN/m2
- Imposed load (on plan)
= 0.75 kN/m2
- Section Modulus of Purlins, Zp = WPL/1800 (Refer Table 27)
(a)
Design the purlins using single angle sections
Dead load
Imposed load
Spacing of purlins
Wpurlin
Roof slope
= 0.35 kN/m2 (on slope)
= 0.75 x 6/6.324 = 0.71 kN/m2 (on slope)
= 6.324/3 = 2.11 m
= (0.35 + 0.71) x 2.11 x 5 = 11.18 kN
= tan-1 (2/6) = 18.4 < 30o
Refer Table 27:
Zp = WpL/1800
D = L/45 = 5000/45
B = (L/2)/60 = (5000/2)/60
= 11.18 x 5000/1800 = 31.06 cm3
= 111.11 mm
= 41.67 mm
From table of properties of single angle:
use: 125 x 75 x 10L (Z = 36.5 cm3)
(b) Analysis the truss.
Loading transferred to the truss
Dead Load = 0.35 +0.2 = 0.55 kN/m2 (on slope)
Imposed Load = 0.71 kN/m2 (on slope)
Total dead load = 0.55 x 2.11 x 5 = 5.80 kN
Total imposed load = 0.71 x 2.11 x 5 = 7.49 kN
Design load
= 1.4 Gk + 1.6 Qk
= 1.4 (5.8) + 1.6 (7.49)
= 20.10 kN
20.10
20.10
20.10
10.05
Please analysis the truss by yourself.
Tq
20.10
20.10
10.05
Designing a truss member
1. Tension Member
Refer Cl. 4.6.1 & 4.6.2
Pt = pyAe
a2 = Ag-Dt
Pt = py(Ag-0.3a2). If higher than Ft. OK
If less, choose new size (larger size)
Example of sizing the truss member
Force of tension members: Ft = 260 kN
py = 275 N/mm2
Area needed = 260 x 103/275 = 945.45 mm2
Refer Table of properties of angle L types:
Try 100 x 65 x 7L where Ag = 11.2 cm2
Dt= 100x7 = 700 mm2
a2 = 1120 – 700 = 420 mm2
Pt = py(Ag-0.3a2) = 275 (1120-0.3(420) = 275.4 kN > 260 kN  OK
2. Compression members
Refer Cl. 4.7.10
Example of sizing the truss member
Force of compression members, Fc = 60 kN
Assume D = 24 mm, t = 8 mm
Ah = Dt = 24 x 8 = 192 mm2
Assume pc = 0.4 py
Ag = Fc/0.4py + Ah = 60 x 103/0.4(275) + 192 = 737.5 mm2
Try 60 x 60 x 8L (Ag = 9.03 cm2)
Check like u check the column.
Use
Pc = Agpc
If Pc > Fc Ok
Otherwise redesign the section