Marcelo Villalva

Videos and images of roofs that collapse in the event of rain and wind frequently circulate on the internet. Everything would be normal if there were no photovoltaic systems installed on these roofs. After all, do solar panels encourage roofs to collapse? Should we be concerned about the safety of roofs before installing the systems? Whenever a roof with a photovoltaic system collapses, before blaming it on the wind, a question should be asked: "Was a structural analysis of the building and roof carried out before installing the system photovoltaic?". Adding the photovoltaic panels and the fixing structures, there is typically an overload of up to 15 kgf/m² on the roof. This is no small feat. Structural analysis is necessary in any photovoltaic installation. Naturally, in small installations, with half a dozen panels on residential roofs, structural analysis is a precious thing that ends up being dispensed with by common sense. Even so, common sense sometimes fails, as there have been reports of residential roofs collapsing during installation. Wind and rain must be anticipated in designs. We cannot simply install photovoltaic systems without any concern for the safety of the roof or even the building. Did the roof collapse after a gale of rain? We cannot blame the wind. Installing photovoltaic systems on roofs without structural analysis of the roof and the building is a crime of responsibility. A falling roof can cause material damage and loss of life. This matter is serious and must be treated with all the rigor that engineering allows. Roof structure analysis The photovoltaic designer does not need to be a roof expert. To resolve this issue, there are two engineering specialties: civil and mechanical. The company responsible for the design and installation of the photovoltaic system must seek the help of an engineer specialized in structures to carry out an analysis of the roof or the building. Creating a good project and sleeping peacefully does not cost much more. The value of the structural project must be part of the cost composition of the photovoltaic project. Assessment of the structure It is recommended that the structural engineer visit the site to survey the characteristics of the structure and roof. In many cases, however, due to the distance, the assessment can be carried out by another professional, through sketches and photos, as occurs in some cases. The dimensions and locations of the pillars, purlins, and trusses must be surveyed. and roof shears. It is necessary to identify the characteristics of the material that was used in the manufacture of the structures (type of profile, thickness, type of steel) and the type of tile used in the roof. A tip: you can seek the help of a locksmith to carry out this assessment . Photos and films of the structures can also help the structural engineer in carrying out the project. Structural project Does this mean that we are going to carry out the structural design of the building and the roof just to be able to install the photovoltaic system? Yes, let's do it. But it’s not just about installing the photovoltaic system. It's so you can sleep peacefully. Obviously, the project should have been done before the construction of the roof, but it was not done (or the project even existed, but was lost). A structural project for a medium-sized warehouse, say, something around 2 thousand m², It's a week or less job. It is a service provided by civil or mechanical engineers, who have the professional responsibility for calculating structures. The structural design does not have a significant impact on the schedule or cost of the photovoltaic system. It is a service that must be considered from the beginning of the photovoltaic project design. How many roofs will need to fall for the market to become aware of this? After surveying all the characteristics of the construction and the roof, the structures are modeled. This work is carried out by the structural engineer with the help of specialized computer programs, such as STRAP, ANSYS, CYPECAD and others. What will the structural engineer analyze? Structural analysis is a work of relative complexity, which considers several technical standards, among which we can highlight: ABNT NBR 8800/2008: Design of steel structures and mixed steel and concrete structures for buildings; ABNT NBR 14762/2010: Design of steel structures made up of cold-formed profiles;ABNT NBR 6123/1988: Forces due to wind in buildings;ABNT NBR 14323/2013: Design of steel structures and composite steel and concrete structures of buildings in fire situations ;ABNT NBR 8681/2003: Actions and safety in structures. In the structural analysis, checks are carried out regarding the actions of gravitational loads, accidental overload and wind. Gravitational loadsSelf weight (must be calculated, as it depends on the materials used and the geometries of the structures); Weight of tiles (from 7 kgf/m2 to 12 kgf/m2, for simple metal tiles, for example); Photovoltaic panels, including the fixing system (15 kgf/m2); Utilities (10 kgf/m2 typically) – accessories that are not part of the structure but may be supported or suspended from it, such as electrical and hydraulic installations, linings, etc. Accidental overloadAccording to standard NBR 8800/2008, a minimum accidental overload of 25 kgf/m2 is considered, corresponding to actions or efforts that were not initially foreseen. This consideration is mandatory by law.Wind actionsFinally, wind action must be added to the analysis. The basic wind speed at the construction site must be considered, in addition to topography, roughness and statistical factors - which change the type of analysis carried out by the structural software. All this knowledge is part of the curriculum of civil and mechanical engineers. With these considerations, after carrying out all the analyses, we cannot blame the wind for the collapse of a roof. Known data, statistical factors, safety factors, accidental overloads apply. In an engineering project there should be no chance for bad luck.Placing the photovoltaic system within the existing safety marginIn some cases, at the discretion of the structural engineer, aiming to reduce the cost of structural reinforcements, a more aggressive analysis can be carried out, but still safe, according to item B.5.1 of NBR 8800/2010. The value of utility loads can be included in the overload up to the upper limit of 0,05 kgf/m², which reduces the total weight considered on the structure In addition, the load from the solar panels (15 kgf/m²) can be included within the accidental load limit (originally 25 kgf/m²), leaving a safety margin for accidental load of 5 kgf/m². In summary, the paragraph above says that it is not necessary to have an oversized structure to accept the installation of the photovoltaic system. This means that, if the structure was designed in the right way, within the standards, it should support the photovoltaic system without any changes. , as the weight of the solar panels will be included in the safety margin that already existed in the project. This would be perfect in an ideal world. Unfortunately, most of the structures we find are built outside of standards or without any type of engineering design - in this case, reinforcements are necessary not only because of the presence of the photovoltaic system. But because the structure was already deficient anyway. The presence of the photovoltaic system will only precipitate disaster - that's why any "wind" brings everything down. Structural reinforcement: to do it or not? In many cases it is really necessary to reinforce the structures. The result of the structural project will highlight this need. The structural engineer will suggest a solution and provide a list of materials. A locksmith can then be hired to carry out the reinforcements in accordance with the engineer's instructions. After the structural analysis of the roof and the application of the reinforcements suggested by the responsible engineer, the result is a photovoltaic project carried out in compliance with all standards and correct engineering practices. The installing company and the customer can sleep peacefully. The cost of structural reinforcement, whatever it may be, must be passed on to the customer. Fortunately, the cost of the structural design and reinforcements does not make the photovoltaic system unfeasible, which is good news. Conclusion There is no such thing as a miracle. Do you want to install a solar system safely? So you will have to carry out a structural design and, perhaps, reinforce the structures. It's part of the game. A photovoltaic project is an engineering project and the market needs to see this. We have seen all types of blunders in photovoltaic installations. The lack of safety in the use of roofs is just one of the aberrations found. Responsibility, professionalism and safety in photovoltaic installations must be part of the rules. There is no room for amateurism in the photovoltaic market.

Marcelo Villalva

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