Practical Analysis and Design of Steel Roof Trusses
The use of trusses is the most widespread alternative for roof construction in Nigeria. The primary choice of material for it is timber and steel.
The best part about truss roof is that by using the truss web the ducts and pipes that are needed to be installed can be done so by using it. Even though, it is important to pay careful attention to the design of the truss member and their connections and failure in building a proper one can lead to the loss of life and also economy.
On the 10th of December, 2016 the roof of a church building in Uyo, Akwa Ibom State, Nigeria collapsed leaving 60 worshippers dead and many injured. Its incident shows how important it is for engineers to pay careful attention to the design situations.
A system of triangulated members that are connected and designed to carry load is called a truss. Inherently, a truss is stable in its shape and can resist load by developing primary axial forces which in nature can be tensile or compressive.
It is always assumed that the connections of trusses are nominally pinned as a very stiff connection in truss can lead to secondary effects such as bending moments and shear force which are induced in the truss members.
The ratio of span to truss depth should be within a range of 10 to 15 for a good structural performance of roof trusses. Even though, it must be considered that the architectural design of the building ultimately determines the already existing geometry and that is what governs the slope that is given to the top chord of the truss.
A number of buckling models are to be considered for the designing of a compression member in a roof truss.
In most truss members only the evaluation of the flexural buckling of the compressed members in the plane of the truss structure and out of the plane of the truss structure needs to be calculated. In Eurocode3 the flexural buckling is achieved by the application of a reduction factor to the compression resistance.
Design example of simple truss roof
Measurements of the simple skeletal structure of the roof system: 18.0 m long, 7.2 m wide
Truss configuration spaced at 3m intervals
Steel grade S 275
Span of the roof truss: 7.2m
Truss spacing: 3.0m
Nodal spacing that is present in the truss: 1.2 m
Permanent dead loads
Long span aluminum roof sheet self-weight (gauge thickness- 0.55mm): 0.019 kN/m2
Weight of ceiling (10mm insulation fibre board): 0.077kN/m2
The weight of services: 0.1 kN/m2
Weight of purlin (assuming CH 150X75X18 kg/m)= (18X3m)/ (1.2X3)= 15 kg/m2= 0.147 kN/m2
Self-weight of trusses (assuming)= 0.2 kN/m2
Total dead load (gk)= 0.536kN/m2
Thereby, the total nodal permanent load (gk)= 0.536 kN/m2 X 1.2m X 3m= 1.9296 kN
Variable Imposed load
Category of the roof = Category H- Roof which is not accessible for final maintenance and repair Imposed load on roof (qk)= 0.75 kN/m2
Thereby, the nodal variable load is (Qk)= 0.75kN/m2 X 1.2m X 3m= 2.7 kN
Article Source: structville.com