Chapter 15Trussed Rafter

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 653

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15.1 General

A roof truss is essentially a plane structure which is very stiff in the plane of the members, that is the plane in which it is expected to carry loads, but very flexible in every other direction. Thus it can virtually be seen as a deep, narrow girder liable to buckling and twisting under loads. In order, therefore, to reduce this effect, eccentricity of loading and promote prefabrication for economy, low-pitched trussed rafters are designed with bolt ply/nail ply joints. Plywood as gussets, besides being simple have inherent constructional advantage of grain over solid wood for joints, and a better balance is achievable between the joint strength and the member strength.

Trussed rafters are light weight truss units spaced at close centres for limited spans to carry different types of roof loads. They are made from timber members of uniform thickness fastened together in one plane. The plywood gussets may be nailed or glued to the timber to form the joints. Conceptually a trussed rafter is a triangular pin jointed system, traditionally meant to carry the combined roof weight, cladding services and wind loads. There is considerable scope for saving timber by minimizing the sections through proper design without affecting structural and functional requirements.

Trussed rafters are required to be supported only at their ends so that there is no need to provide load bearing internal walls, purlins, etc. In comparison with traditional methods of construction they use less timber and considerably reduce on-site labour, and mass production of reliable units can be carried out under workshop controls.

15.2 Design

15.2.1 The design may be based on the use of Group 'C' species of standard grade timber in accordance with good practice [6-3A(7)] for nail jointed timber construction. As the strength and stiffness of any framing depends upon efficiency of joints, design calls for proper size and number of fasteners, their spacings

PART 6 STRUCTURAL DESIGN — SECTION 3 TIMBER AND BAMBOO: 3A TIMBER

Table 21Strength Characteristics of Glued Finger Joints (Clause 13.1.3.1)

Sl No.	Character → Treatment ↓	Compressive Parallel to Grain Tests			Tension Parallel to Grain Tests			Static Bending Tests
		Maximum Compressive Stress (N/mm2)	Fibre Stress at EL (N/mm2)	MOE ( $\times 10^3$ N/mm2)	Maximum Tensile Stress (N/mm2)	Tensile Stress at EL (N/mm2)	MOE ( $\times 10^3$ N/mm2)	Edge-wise			Flat-wise			Edge-wise			Two Point Loading
		Maximum Compressive Stress (N/mm2)	Fibre Stress at EL (N/mm2)	MOE ( $\times 10^3$ N/mm2)	Maximum Tensile Stress (N/mm2)	Tensile Stress at EL (N/mm2)	MOE ( $\times 10^3$ N/mm2)	Fibre Stress at EL (N/mm2)	MOR (N/mm2)	MOE ( $\times 10^3$ N/mm2)	Fibre Stress at EL (N/mm2)	MOR (N/mm2)	MOE ( $\times 10^3$ N/mm2)	Fibre Stress at EL (N/mm2)	MOR (N/mm2)	MOE ( $\times 10^3$ N/mm2)	Fibre Stress at EL (N/mm2)	MOR (N/mm2)	MOE ( $\times 10^3$ N/mm2)
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)	(12)	(13)	(14)	(15)	(16)	(17)	(18)	(19)	(20)
i)	UL1	31.196	16.971	10.202	30.019	23.446	10.202	20.993	29.136	6.769	39.142	46.401	8.142	25.114	28.743	8.044	32.471	38.553	9.516
ii)	UL2	37.474	17.462	11.282	30.607	23.642	10.301	20.209	31.686	6.671	35.120	46.205	7.161	23.446	42.085	7.652	41.202	51.306	9.712
iii)	UL3	35.905	19.914	10.595	32.667	23.838	10.006	22.171	35.022	6.867	38.750	47.480	8.437	25.898	29.234	7.848	29.626	38.357	8.633
iv)	PL1	28.743	21.484	9.908	20.895	17.560	10.497	20.699	23.446	8.142	22.955	27.076	9.123	18.443	21.876	7.848	25.506	27.174	8.927
v)	PL2	28.743	19.424	10.006	18.149	17.069	9.614	16.285	18.639	5.984	24.133	26.095	8.731	16.579	17.854	7.259	26.389	28.547	8.535
vi)	PL3	27.860	20.111	9.908	24.721	19.326	10.497	15.009	17.069	7.652	21.680	26.193	8.829	10.301	11.183	6.867	26.487	30.313	11.183
vii)	C	35.512	27.959	10.301	31.490	23.152	10.104	25.506	45.518	7.063	36.788	57.192	7.063	31.883	44.537	7.946	38.161	52.582	9.221
MOE — Modulus of elasticity (values rounded off for decimals)
MOR — Modulus of rupture
EL — Elastic limit
U — Urea formaldehyde with liquid hardener
P — Cold setting phenolic glue with hardener
L1 — Finger length 38 mm
L2 — Finger length 50 mm
L3 — Finger length 62 mm
C — Control (unjointed specimen)
NOTE — The information given in the above table is based on the tests conducted on glued finger joints of varying lengths/pitch with two types of adhesives.

FIG. 15 TYPICAL CROSS-SECTION OF WEB BEAMS

FIG. 17 STRESSED SKIN PANEL CONSTRUCTION (SINGLE SKIN OR DOUBLE SKIN)

and thickness of gusset plates for transfer of stress from one member to the other.

Trussed rafter shall be designed to sustain the dead and imposed loads specified in Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code and the combinations expected to occur. Extra stresses/deflections during handling, transportation and erection shall be taken care of. Structural analysis, use of load-slip and moment, rotation characteristics of the individual joints may be used if feasible. Alternatively, the maximum direct force in a member may be assessed to be given by an idealized pin-jointed frame-work, fully loaded with maximum dead and imposed load in the combination in which they may reasonably be expected to occur.

NOTE — Some trussed rafters with plywood gusseted joint, span 6 m, designed in Pinus roxburghii (Chir) were tested in prototypes at Forest Research Institute, Dehra Dun for structural adequacy as under, in accordance with accepted standards [6-3A(14)],

a) destructive test (short-term); and

b) proof test (long-term).

Results demonstrated an apparent factor at safety of 4 and actual factor of safety of 3. Deflection was well within the allowable deflection at design load. A pre-camber of 1/240 of span at the bottom chord was recommended.

15.3 Timber

The species of timber including plantation grown species which can be used for trussed rafter construction and permissible stresses thereof shall be in accordance with Table 1. Moisture contents shall be as per zonal requirements in accordance with 4.4.

15.4 Plywood

Boiling water proof (BWP) grade preservative treated plywood shall be used in accordance with accepted standards [6-3A(13)]. Introduction of a plywood gusset simplifies the jointing and in addition provides rigidity to the joint. Preservation of plywood and other panel products shall be done in accordance with good practice [6-3A(15)].

15.5 Some design data on the lateral load bearing strength of common wire nails in plywood-to-wood joints are presented for timber species, Sal and Chir in Table 22, for guidance in design.