We all need a roof over our head. Nowadays, architects frequently use roof pattern and design to enhance the aesthetics and functionality of a building, and it is very important that engineers follow up and ensure that the structural integrity of such roof systems are guaranteed. On the 10th of December 2016, the roof a church building collapsed at Uyo in Akwa Ibom state Nigeria, leaving about 60 people dead, and many other injured. This is why such designs are to be taken seriously, especially when the span is large.
Large span construction of roof systems is usually found in public or commercial buildings. This comes in handy especially when there is need to have a large uninterrupted floor area. However, the efficiency of trusses in dealing with long span structures has been widely recognised in the structural engineering community, but there is no doubt that the longer the span, the more complex the design.
Some basic considerations in the design of trusses
(1)The connection design is as important as the member design. The connections could be welded or bolted or both.
(2) 2D idealisation of the structure is usually very sufficient for analysis and design.
(3) Joints are hardly pinned in reality, but the use of pinned joint is usually encouraged for the purpose of analysis and design.
(4) Trusses can be analysed as having continuous top and bottom chords, with the internal members pinned. In this case, loads can be applied at locations other than the joints with the resulting bending moment appropriately accounted for in the design.
(5) It is usually important to consider out of plane buckling for compression members in trusses. However, purlins usually take care of the top chord, while bracing can be used to take care of the bottom chord.
Design of Roof Purlins
(6) For efficient structural performance, it is recommended that the truss span to depth ratio be kept between 10 to 15.
(7) In the layout of truss systems, it is more preferable (in terms of economy and efficiency) to have the shorter members in compression and the longer members in tension.
(8) Welded connections offer more advantages in terms of deflection behaviour of trusses. Slack displacements are possible in bolted connections especially when non-preloaded bolts are used.
(9) In the design of compression members, buckling is the most critical.
(10) When member lines do not intersect at a node, it is important that the moment that arises due to the eccentricity be included in the design.
The structural layout of a roof system is given below;
We are required to carry out the member design of the trusses for this 23m span open hall. The design data is given below;
Design code: BS EN 1993-1-1:2005
Design wind pressure: 1.5 kN/m2
Variable load = 0.75 kN/m2
To make this post simpler and shorter, a 29 page fully referenced design paper for all the trusses in the roof system has been attached.
Click HERE to download the calculation sheet in PDF format (premium).
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Practical Analysis and Design of Roof Trusses