10 July 202110 July 2021 by Omotoriogun Victor

[Opinion] Choice of Structural form for a Monopitch Roof

You’re faced with the design of a steel truss for a monopitch roof spanning 18.35m. The roof is invariably enclosed hence is considered predominantly subjected to gravity loads, which scheme would you choose or consider more appropriate and why?

Howe Truss
Pratt Truss

Warren Girder

***Figure 1: Structural forms of steel truss***

This is a simple and very straightforward question in my opinion, for engineers who are very farmiliar with design of structural steel-work. But the answer to this question indeed lies in your understanding of the behaviour of the structural forms listed above.

Firstly, we must note that, the howe and pratt truss are essentially opposite of each other. When subjected to gravitational point actions, in a howe truss, the vertical members are in tension while its diagonal members are in compression, the reverse becomes the case with a pratt truss – the verticals are in compression while its diagonals are in tension. A warren girder is very different, different in the sense that, it doesn’t have vertical members, only diagonals which are alternately carrying tension and compression respectively.

Secondly from the purview of steel-work design, steel elements in compression are highly susceptible to buckling failure. Buckling reduces the effectiveness and member resistance of steel sections. Lest we forget, buckling becomes more critical with increase in the design length of a compression member. Hence it is generally preferable and accepted to keep the shorter elements of steel trusses in compression with the longer members in tension. Giving this facts, for a monopitch steel roof as shown in figure 1 the pratt truss is easily favoured over the howe, since it is clear that the verticals are typically shorter than the diagonals.

However, there is a caveat here. For a pratt truss to be practiceable, the steel truss must be predominantly subjected to gravitational actions. For this to happen the roof must be fully enclosed, or the influence of wind actions on the roof must be light and not critical. In a situation were the structure is experiencing high wind uplift because the building is completely open or a large opening exist within the building, there would be stress reversal, as a result a howe truss now behaves like a pratt while the pratt behaves like the howe. In other words the members that were initially in compression are now in tension while those that were in tension are now in compression.

The most appropriate scheme is unarguably a “pratt truss” giving that the building is predominantly under the influence of gravitational forces. A warren girder is also plausible but this engineer thinks they’re particularly more economical and suitable for longer spans and where there is no dominating force. And also where a truss is subjected to uniformly distributed loading.

My Encounter with Steel Trusses

Two years ago, one of my mentors called me – ” you’ve to design a light roof truss of 12m at a pitch of 33°. Please mail the calculations sheets and detail drawing to me tommorow morning”. With no prior design experience in steel trusses, i responded affirmatively and set out to carry out the design. I would almost later regret this because it was quite a task, a very tough grind and a bitter sweet experience.

After a lot of back and forth and after one full week, I finally spent the entire night preparing a design which ended up very quickly in the trash bin the following morning. Why? Even though calculations were correctly done, detail drawings were classy and top of the shelf, the scheme was very wrong. In fact an experienced steel fabricator could easily spot the error because this was clearly evident in the massive steel sections required. How on earth do you propose UB sections for a light truss of just 12m?. Long story short, I received an extensive lesson on trusses that day after which I had to repeat the design.

Let’s get one thing straight; most Nigerian engineers are not so good with steel designs like they are with concrete. Most of us just do detailing using simple rules of thumb which might not be applicable in every case while some basically use softwares that do steel designs and would easily claim to be expert at it. We’ve talked about the dangers of softwares in engineering and one of the points raised was that the use of softwares often result in engineers who do not necessarily have the technical capacity and expertise to do complex or even simple designs, doing them simply because a software program enables them to do so. For example, you’ve never designed a corbel before, you dont have any prior knowledge about corbels, as matter of fact you’re unwilling to even do a little research about Corbels, but you’re faced with the challenge of designing corbels and you’re going to design it anyway because there is a tab on your favourite software that reads “Corbel Design”. What is seriously lacking ofcourse is the competence to design Corbels. Same thing that happened to me when tasked with the design of that 12m roof truss.

This is not to vilify anybody, trust me i’m not even trying to make this up. My mentor did raised questions of wether Nigeria trained engineers were not been taught steel design in schools, because he had noticed the same thing. Also how ironic that a design which i was suppose to complete before the following morning took a full week. Worst of all it was wrong in it entirety. We can all trust his judgement, giving his wealth of experience as a chief structural engineer and a director at the FCDA with over 30years of quality experience.

We can as well argue that, because steel is not a popular choice for structural works in this clime, most of us have either forgotten our steel designs or don’t pay enough attention to it. But this excuse is not reasonable enough because structural steel still find many applications in this clime; in portal frames for single story industrial buildings and roof trusses for multi storey buildings which is even the concentration of this post. While these can be considered by some as very simple designs in structural steel-work, they must’nt be treated with carelessness.

In Dec, 2016 the roof over a church auditorium in Uyo, Nigeria suddenly collapsed killing over 160 people with several hundreds of casualties (Figure 2). This is how interesting it gets and goes further to proove the importance of taking what we might perceive as simple designs extremely serious.

roof truss collapse in uyo, nigeria — ***Figure 2: Roof Collapse in Uyo, Nigeria (Source: Democrat Gazette)***

See: Displacement of Trusses using Principle of Virtual Work

On a final note, as engineers we’re sometimes conflicted between our ego and super ego. You should chose between being dynamic and be seen to continuously strive to acquire and grow your knowledge or chose to remain static and stay out of things that are outside your competence. The choices are yours to make. However, whichever decision you make, it’s not a crime to say – I don’t have the competence, knowledge and experience and then get a more competent hand to get the job done, it doesn’t make you less an engineer but saves stress and ensures that the right thing is done always.

Thank you for reading, let’s have your opinions in the comment section!!!

Published by Omotoriogun Victor

A dedicated, passion-driven and highly skilled engineer with extensive knowledge in research, construction and structural design of civil engineering structures to several codes of practices View all posts by Omotoriogun Victor