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DESIGN OF TRUSS ROOF
Chapter 7
University of Engineering & Technology, Taxila
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Properties of Trusses
Truss is a frame structure in which all the members
have axial forces due to the following facts:
1. Members are arranged in triangles for stability.
2. All the joints of a truss are semi-rigid or fully
rigid. However, theoretically these joints may
be considered pin joints and the analysis as a
pin jointed frame is valid provided that the
requirements given in No. 3 & 4 are valid. 2
3. Centroidal axes of all the members at a joint
must intersect at a single point.
4. The loads are only applied at the panel points.
Comparison Between Rigid Frames and Trusses
1. Joints are considered as having frictionless
pins in trusses with no moment at the member
ends. In case of rigid frames, the members are
rigidly connected having appreciable moments
at the member ends.
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2. The forces in case of trusses are only axial and
hence the members are equally stressed
through-out their cross-section. In rigid frames,
due to bending moment, the fibers of the crosssection away from the neutral axis have
maximum stresses and the fibers close to the
neutral axis have less stress.
3. Because of the above facts, the design of a
member in a truss is economical as compared
with the members of a rigid frame.
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Types of Trusses
•
Trusses can be divided into two categories,
Type-I & Type-II.
•
Type-I trusses are preferred in those areas
where snow fall is common and Type-II trusses
are used in hot climates.
•
The roofs of Type-I trusses are inclined at
greater angles (10° to 60°) to drain part of the
snow falling on the roof surface.
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Types of Trusses
•
The roofs of Type-II trusses are either nearly
flat or are inclined at angles less than 10°.
•
If the forces in the diagonal members are all
compressive and that in the vertical members
are all tensile, the truss is called Howe Truss.
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Types of Trusses
•
In a reverse way if the forces in all the
diagonal members are tensile while the forces
in all the vertical members are compressive,
the truss is called Pratt Truss.
•
The difference between these two trusses is
only the orientation of the diagonals in relation
to the applied loads.
•
For all the given trusses, the loads are in
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general applied at the top chord.
Type-I Trusses
Queen Post ( L ≤ 12 m)
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Type-I Trusses
Upper Chord
Slope
h = rise
ø
L= span
Lower Chord
King Post ( L ≤ 12 m)
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Type-I Trusses
Fink Truss ( L = 8 - 10 m)
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Type-I Trusses
Fan Truss ( L = 10 - 15 m)
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Type-I Trusses
Compound Fink or French Truss ( L = 10 - 15 m)
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Type-I Trusses
Subdivided Fink Truss ( L = 10 - 15 m)
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Type-I Trusses
Radius R = (4h + L)/8h
Bowstring Truss
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Type-I Trusses
Parker or Bowstring Truss
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Type-I Trusses
Radius R = (4h + L)/8h
Radius R = (4r + L)/8r
h = rise
r
Crescent Bowstring Truss
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Type-I Trusses
Compound Fan Truss ( L = 15 - 25 m)
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Type-I Trusses
Pratt Truss ( L = 10 - 30 m)
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Type-I Trusses
Howe Truss ( L = 10 - 30 m)
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Type-I Trusses
North Light Truss ( L = 5 - 8 m)
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Type-I Trusses
Saw Tooth Truss ( L = 5 - 8 m)
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Type-I Trusses
Glass
Ketchum’s Modified Saw Tooth Truss ( L = 8 - 10 22m)
Type-I Trusses
Monitor Truss ( L = 10 - 15 m)
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Type-II Trusses
Modified Howe Truss ( L ≤ 40 m)
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Type-II Trusses
L/8
L/12
Modified Pratt Truss ( L ≤ 40 m)
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Type-II Trusses
Warren Truss ( L ≤ 45 m)
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Type-II Trusses
K - Truss ( L ≤ 60 m)
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Type-II Trusses
Warren ( L ≤ 40 m)
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Type-II Trusses
Cantilever Truss ( L ≤ 60 m)
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Terms Related With Trusses
Pitch of a Roof Truss
Pitch of a roof truss is defined as the maximum
rise of top chord of the truss (h) divided by the
total span of the truss (L). For symmetrical
trusses the pitch is equal to double the
inclination of the top chord.
Pitch = h / L
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Pitch of a Roof Truss
h = rise
L= span
Pitch = h / L
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Inclination of a Roof Truss
The slope (tan θ) or angle (θ) of top chord of a
truss with respect to the horizontal is called
inclination of the truss. For un-symmetrical
trusses, inclination may be completely
independent of the pitch of the trusses.
For Type-I trusses,
θ ≤ 60°
(most suitable range 20° -30°)
For Type-II trusses,
θ ≤ 10°
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Inclination of a Roof Truss
Slope
ø
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Height / Rise of Truss
The maximum height of the truss (h) with
respect to the ends of the bottom chord is
called height or rise of the truss. The highest
point is called crown of the truss.
For Type-I trusses,
h = L/3 to L/5
(most suitable value L/4)
For Type-II trusses, h = L/8 to L/12
(most suitable value L/10)
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Height/Rise of Truss
h = rise
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Panel Length
In case of roof trusses, the distance between
two consecutive top chord joints is known as
the panel length.
If panel length is more, the perpendicular
beams supporting the roof (called purlins) have
to be placed within the top chord members
producing bending moment in the truss
members.
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Panel Length
The reason of doing this is that the usual roof
coverings can not have greater span lengths.
Panel lengths can be the projected horizontal or
the actual inclined lengths.
Panel lengths for type-I trusses
= 1 to 3m
(most appropriate value 1.8m)
Panel lengths for type-II trusses
= 3 to 4m
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Purlins
These are small beams which run
perpendicular to the trusses and rest at the
panel points of the trusses.
The purlins provide the lateral bracing to the
top chord and carry the load of the roof
transferring it to the panel points of the trusses.
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Purlins
The span of these beams is equal to the centreto-centre spacing of the trusses.
Usually the purlins are continuous over the
trusses but are designed as simply supported
for convenience of design and construction.
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Purlins
The purlins may consist of angle sections,
channel sections or I-sections.
Small depth trusses may be used as purlins if
the spacing of the trusses is more.
Zee section purlins are preferable for inclined
roofs if they are easily available.
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Purlins
The angle and the channel purlins are
connected to the top chord (sometimes called
rafter) by cleat angles as shown.
I-section purlins are usually bolted to the top
chord.
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Purlins
Because of the inclination of the roof, a
component of load acts along both the
centroidal axes of the member producing both
in plane and lateral bending known as double
or biaxial bending.
Sag rods may be used with channels or other
sections to reduce lateral bending.
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Purlins
The nature of the loading on the purlin requires
a beam section that is strong in bending about
both the axes.
For this reason, W-sections are preferred when
the loads are heavy and the spans are bigger.
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Clip or Cleat Angles
These angles are previously bolted, riveted, or
welded to the top chord above which the
purlins may rest while it is being fastened to
the truss.
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Sag Rods
When channels are used for purlins, it is good
design practice to use sag rods to take the
tangential component of the roof loads.
These are placed either at mid span or at the
third points, depending on the weight of the
roof, the span of the purlins, and the pitch of
the roof truss.
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Sag Rods
Max. span of purlin for one sag rod = 6 m (light roof)
= 4.5 m (heavy roof)
For roofs steeper than a pitch of 1/4 , two sag
rods should be used for a purlin span of 4.5 m.
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Roof Covering / Sheathing
Light roofing:
Corrugated Galvanized Iron (G.I) sheets
Corrugated Asbestos Cement Concrete (A.C.C.)
sheets
Heavy roofing:
Clay or cement tiles
Gypsum tiles
Slate tiles
Tar plus gravel
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J-Bolt
J-bolt, also called hook bolt, is a bolt in the
form of letter “J” used to fix roof-sheathing or
wall sheathing to purlins and other structural
members.
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Eave
The end of truss lower in level along with its
support is called eave of the truss.
Eave’s Gutter
A channel is provided at eave-level to collect
rain-water which is called eave’s gutter.
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Rafter
Sometimes beams in addition to purlins (in a
perpendicular direction) are provided to
support the roof called rafters.
Strut
Relatively short length columns without the
chances of buckling are called struts.
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Spacing of Roof Trusses
The average spacing of trusses is given in
Table 7.1.
Span of Truss (m)
15 – 18
27 – 30
> 42
Center-to-center spacing
(m)
3.5 - 6
4.5 – 7.3
15 – 18
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Truss T-2
s
Truss T-1
p
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a) Elevation of Truss
p
p
Area contributing load
at one interior panel
Point = p x s
s/2
s/2
s
p/2
p/2
b) Part-Plan of
Truss Roof
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The End
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