Analysis of the mechanical properties of Caixeta wood in relation to moisture content variations: Implications for ABNT NBR 7190:2022

Wood is a remarkable building material, prized for its ability to carry heavy loads while remaining light enough to be lifted by hand. This high strength-to-weight ratio makes it ideal for constructing everything from simple frames to complex engineered beams. However, wood is not a static substance like steel or concrete; it is a living, breathing material that constantly exchanges water with the air around it. This ability to absorb and release moisture, known as being hygroscopic, means that the wood's internal structure changes as the weather shifts. When wood absorbs water, its fibers swell, and its ability to resist breaking or bending generally weakens. Engineers must account for this behavior to ensure that a wooden bridge or building remains safe throughout its life, adjusting their calculations based on how wet or dry the wood is expected to be.

In Brazil, a specific set of rules known as ABNT NBR 7190:2022 guides how engineers design with timber. These rules provide a standard reference point, assuming that wood will be used at a moisture content of 12 percent. If the wood is wetter or drier than this, the code offers a mathematical method to estimate how much stronger or weaker the wood will be. The code essentially acts as a translator, converting measurements taken in the lab into values that can be safely used in construction. But for this translation to work perfectly, the rules must accurately reflect how different types of wood actually behave. While the code works well for many species, there is a growing need to verify if these standard adjustments hold true for the diverse and unique tropical woods found in the Amazon and other regions of Brazil.

This is the question Nicolas Freitas and Francisco Rocco from the University of São Paulo set out to answer. They focused on a specific tropical tree called caixeta, scientifically known as Simarouba amara. This species is gaining attention for its potential use in structural applications, including the production of glued laminated timber, which is a type of engineered wood made by bonding layers together. The researchers wanted to see if the standard Brazilian code correctly predicted how caixeta wood would perform when it was wet, specifically when it was close to its fiber saturation point. This saturation point is the moment when the wood's cell walls are completely full of water, but no free water exists in the open spaces between the cells. For caixeta, the team found this point to be around 24.25 percent moisture content.

To test this, the team gathered thirteen large beams of caixeta wood from a supplier in São Paulo, which had been harvested in the state of Mato Grosso. They cut these beams into smaller test pieces, ensuring that for every single piece of wood, they created two sets of samples: one set dried down to the standard 12 percent moisture level, and another set kept wet at 25 percent, just slightly above the saturation point. This careful setup allowed them to compare the exact same wood under two different conditions, removing any guesswork about whether the differences were due to the wood itself or just natural variation between trees. They then subjected these samples to a battery of rigorous tests, measuring everything from how hard the wood was to how much force it could take before snapping in tension, compression, or bending.

The results revealed a clear pattern: as the moisture content increased, the mechanical properties of the caixeta wood generally decreased. The wetter wood was indeed weaker and less stiff than the drier wood. However, the story became more complex when the researchers compared their actual measurements against the numbers predicted by the official code. The standard rules suggested that the wood's strength would drop by a certain amount as it got wet, but the real-world tests showed a different story. In many cases, the actual wet wood was stronger than the code predicted it would be. For example, when looking at the wood's ability to resist bending, the experimental value was about 16.66 percent higher than the estimated value. Similarly, the stiffness of the wood in compression was roughly 12.75 percent higher than what the standard formula calculated.

Perhaps the most significant finding concerned how the wood should be classified for construction purposes. Based on their tests at the standard 12 percent moisture level, the researchers calculated a specific strength value that placed this batch of caixeta in a higher strength category, known as D30. This was a notable shift, as the existing Brazilian code currently lists caixeta as a D20 species, which implies it is weaker. This discrepancy highlights a crucial reality: wood from the same species can vary significantly depending on where it was grown and which specific batch is being used. The standard classification might be underestimating the potential of this particular timber, or it might simply reflect a lack of recent, specific data for this species.

The study concludes that while the standard correction factors in the Brazilian code provide a useful baseline, they do not perfectly capture the behavior of caixeta wood when it is wet. The code tends to be conservative, predicting a greater loss of strength than what actually occurs in the lab. This suggests that for this specific species, engineers might be able to use the wood more efficiently than the current rules allow, provided they have access to better data. However, the authors are careful to note that their results come from a single batch of wood from one specific location. They emphasize that these findings should not be immediately applied to all caixeta trees everywhere. Instead, their work serves as a call to action to expand the database of experimental results for native tropical species. By gathering more data from different origins and timber lots, the scientific community can refine these rules, ensuring that the use of tropical wood in construction is both safe and optimized for its true potential.