Design of selected wood species for longer span trusses

The trends of researches in the built industry are exploration of the available alternatives; either as raw materials or finished products construction materials, to most construction materials that have been playing dominant roles in construction industry for decades. Aside that the cost of using them in construction keep rising, the associated health hazards in their production and usage are huge and costly to rectify. The usage of locally available construction materials is being hindered by the lack of useful engineering data of the materials for structural design. Wood is an alternative local, green and sustainable material to steel in roof truss design and construction. Its usage has been limited to short span roof trusses based on inherited knowledge especially in Africa. There is need to know the actual capacity and potential of various wood species in relation to roof design. This work studied the properties of; Gmelina arborea, Tectona grandis (Teak), Terminalia superba (Afara), Ayin (Anogeissus leiocarpus), and Acacia (Robinia pseudoacacia). The values obtained from the laboratory experiments conducted on them were used to design rafter and tie beam of king post roof truss spanned to 12 m and 8 m using timber section 0f 50 mm x 150 mm of all the species. Then section sizes of 150 mm x 250 mm and 250 mm x 350 mm of Acacia were then designed separately. It was found that the five species were not suitable for long span roofing using the chosen three section sizes because the tie beams failed in deflection, which is most critical to deliver a safe, durable and serviceable roof. However, all the species, except Afara, are satisfactory for rafter whose span is not more than 7.5 m. It was also discovered that the higher the section size of timber used in the design the lower the deflection.