Structures Wood

Micro-laminated wood, for structural use

It is a material made up of wood veneers (at least 5) with a maximum thickness of 6 millimeters, with the grain mostly oriented in the same direction. Its great resistance, uniformity and lightness make it an ideal material for structural use. It is used mainly when working with edge bending (in joists, pairs or loading bays), although it is also frequently used as surface panels (especially in slabs) or as part of large span trusses. Currently this material is being introduced with great success for restorations, being attached laterally to pieces of wood, to increase their resistance and rigidity or superiorly in joist floors (as a compression layer, which significantly implements rigidity). Coniferous species (especially fir) and phenolic adhesives are used for its manufacture. When it comes to dimensioning structures with this material, we find ourselves with the difficulty of not being able to go to the Technical Building Code (CTE) to see the mechanical properties of the material, which are variable according to the manufacturer and fixed resistance classes have not been established, as is the case for sawn and laminated wood. For this reason, it must be the manufacturer who provides these data, which will have been obtained in accordance with the tests established by the UNE standards (UNE-EN 14358, UNE-EN 408 and UNE-EN 789). In addition, there is a huge dispersion of values ​​in the mechanical properties, depending on the thickness and orientation of the sheets, so that the mechanical properties of apparently similar elements, but of different origin, cannot be extrapolated. Generalizing, it can be stated that the mechanical properties are remarkably superior when all the sheets are oriented in a direction parallel to the fiber (parallel composition), but, nevertheless, the dimensional stability against changes in humidity is much higher when there are sheets oriented in both directions (cross composition). This last composition is reserved for surface elements (structural boards) and sometimes for compression pieces, such as pillars. Analyzing in detail the mechanical properties, one can see a weak point of the products with parallel composition, which is the tensile strength perpendicular to the edge fiber (it barely reaches 10% of what the pieces with cross composition resist), which can be critical in variable section pieces subjected to bending. This circumstance also makes the design of the joints decisive, where various efforts are concentrated, which must be properly controlled and designed, if we do not want the structure to fail. Knots are always the points of greatest structural compromise, even more so in these “clean” systems, where each piece must do its specific job without damaging the others. As it is a product of reduced thickness, it is possible to bend it, with a limitation of the minimum radius, which depends on the thickness of the board and the orientation of the sheets. Commercially, different finishes are presented, with different degree of brushing, depending on whether the material is going to be seen or not. Its appearance is like that of plywood. The most widespread brands in our country are “Kerto” and “ilevel”, which calls this material Microllam LVL (Laminated Veneer Lumber) and also manufactures joists with double T section, with OSB (Oriented Strand Board) core and heads. made of micro-laminated wood, arranged horizontally. This product, commercially called TJI (Trus Joist, from Ilevel), has an enormous application in residential construction and due to its technical peculiarities and structural characteristics, it will be the subject of a future entry in this blog.


Mixed ladder for restricted use

Many architects will have heard (I many times) that, according to the CTE, in the turns of stairs of restricted use (interior of houses) only entire plateaus can be left or split them at 45º, that is, the regulation only allows the inclusion of 4 steps in the 180º turn and 2 steps in the 90º turns. Interestingly, although many people blindly apply such an absurd restriction, very few have bothered to read the DB-SUA, as they would have quickly realized that it is a misinterpretation: Both the round-trip stairs (180º) and the “L” stairs (90º) are nothing more than curved staircases, since their axis is so and, therefore, the conditions of design that are specified for this type of stairs, regarding the minimum size of the tread (5cm inside the curve), the minimum dimension of the tread on the axis (22cm) and the maximum useful dimension (44cm in the outside of the curve). Applying these conditions, the plateaus divided at 45º would not comply and, for this reason, point 4.1.3 of DB SUA-1 states that: “Plateaus divided with steps at 45º may be arranged …”, as an exception to the rule. Thus, the exception has been taken as a rule, possibly due to the greater ease of application of the regulations in those terms, with the damage that this entails for the quality of the design and the safety of use and accessibility of the stairs (manifest intention of this DB), which are obviously undermined by the “imposition” of an unnatural and ergonomic model, whose inadequacy from a functional point of view is easily verifiable. Nor is it true that curved staircases have to constructively formalize the curve, since the “Criteria for the interpretation and application of the DB-SUA”, published by the Ministry of Housing, clearly stated that “The useful width of a ladder must be measured according to the perpendicular at each point to the line that defines the path of the route. In the plateaus in which said trajectory undergoes a turn, it is considered that the trajectory is defined by the arc of circumference whose center is located at the breaking point of the inside edge of the staircase. ” And in case there was any doubt, he continues saying: “… on plateaus with a 90º turn, the outer limit of the useful width would be a quarter of a circumference and on plateaus with a 180º turn, this limit would be a semicircle, and the design could be adjusted to these shapes. , although the most common are straight lines “. And, since the external measurement that is limited to 44cm is the useful dimension, we should not worry about the corner. When you try to design similar sections of stairs, you will realize that you have to play with the width of the stairs, with the diameter of the eye and with the number of steps to fit all the measures restricted by the CTE. That said, it is very true that DB-SUA, on the subject of restricted-use stairs, has some contradictions that (I know) will be polished in successive updates: If you try to fit three steps into a 90º turn you will see that it is impossible, since you will never be able to meet the maximum limit of 44cm outside. There is therefore the paradox that at 90º the turn can be designed with 2 or 4 steps, but never with 3, which is absurd if we consider that it is about setting limits on safety. Everything points to the fact that, in “restricted use”, this outer limit should be abolished. Meanwhile is what there is. If to the above is added the incongruity that a plateau with a 90º turn and 2 steps, with a small interior radius, does not comply, since it has to be considered a “curved path” (due to the existence of that interior radius) and therefore Exceeding 44 cm of footprint on the outside, while another plateau exactly the same, but without an inner radius is no longer a “curved path”, therefore it does not need to meet the 44 cm of exterior footprint and therefore complies, being clearly worse and more insecure than the previous one, it is concluded that you have to review that article, because it does not work. It’s not coherent. Some advocate eliminating the second option, that is, that in restricted use, plateaus (or perhaps not all, but stairs with a width of less than “X”) that are not justified as “curved path” cannot be split at 45º, given the insecurity of its null footprint on the inside, but only at 90º those that have a 180º turn. But that would have a harsh impact on interior staircases in close-quarters single-family homes. It is not that the CTE is a perfect document and free of errors, which it is not (especially in this DB), but as technicians we must know it and apply it rigorously, without believing interpretations made by third parties or “urban legends”, however widespread they are.


Hotel La Canela in Piedralaves (Ávila)

From time to time, and where you least expect it, you have the pleasure of enjoying a first-rate architectural experience, completely removed from the strict line marked by the architects who occupy the “Star System” and the publications that surrender to their whims and whims. This is what happened to me in a recent stay at Hotel La Canela, located in the Sierra de Gredos. Its owners (he is Dutch and she is Spanish) decided to settle in this small town, on land overlooking the Tiétar valley, and bring with them all their cultural and gastronomic experiences, accumulated in more than a decade living in Asia. They say that the project starts with some small square tablecloths that they brought from China, with the conviction that they would be the epicenter of their new adventure. Then came the tables and the inevitable next step in this Matryoshka game was the building, as a necessary container. And so they raised it. From very simple ideas and an even simpler budget, an architecture of great purity arises in which nothing is superfluous and there is not a single concession to the superfluous. Its beauty, as happened with those industrial pieces of the Bauhaus, resides in that it soberly fulfills its mission. What you see is what is available. The structure, with reinforced concrete pillars and hollow core slabs, is fully visible. The continuous floor, made of polished concrete, presents the irregularities typical of those who do it for the first time (they say that doing it was 3 times cheaper) and the retraction joints arise where the material demands it. There is no attempt to disguise these flaws. The building tells the story of its construction, showing everything without complexes. And the result is as surprising as it is welcoming. The furniture, exquisitely chosen, is the minimum necessary, almost like small oriental display pieces. The desire to save energy leads to taking special care of climate control, with huge windows facing south in all rooms and rooms and with eaves and awnings that regulate the entry of the sun in the different seasons, making very limited use of heating in winter, it is underfloor heating. A small wood stove in the dining room and a very dim lighting, put the icing on this thermal (and mental) comfort. A magnificent example of the benefits of passive architecture and bioclimatic design. The ingenuity and ingenuity of someone who is not a construction professional, but is completely involved in the project, gives the best results in this architecture without an architect. If the color palette they offer me for the render does not convince me, why not add coffee to the mix? And, if corten steel is very expensive, how will normal steel be if I oxidize it, immersing it in salted water? I can imagine the faces of contractors and masons at these suggestions. Well, they weren’t so crazy. In short, I believe that the technicians and other “experts” who participate in the construction process should learn from this passionate attitude and have an open mind, because the best path is never the one we have been taught, but the one we are capable of opening ourselves. PS: If this blog were about gastronomy, the Hotel La Canela would also deserve an entry. Those little square tablecloths are filled with Asian flavors, cooked and presented like only someone who loves their job can do.


Wood in architecture

After almost two decades working as an architect specialized in wood, I have detected a series of concerns common to most of the clients, as well as a multitude of fears and “legends” that can only be justified in the lack of knowledge of the material.

Wood, as a building material, is the oldest of those used today and its durability has not been surpassed by any other material to date. Despite these objective assessments, wood is the construction material that continues to generate the most distrust, and not only among potential users, but also among technicians themselves.

The rejection of wood architecture or the classification of constructions made with this material as “provisional” or “worse quality” is frequent, without going to assess their true architectural, constructive, spatial, structural or sustainability qualities, which should be criteria of greater weight when judging the quality of buildings. One of the purposes of this blog is to share my experiences as an architect specialized in wood, which have led me to be a stalwart of such a noble material.

I intend to seriously analyze and from technical and objective criteria, the use of wood in architecture, especially with structural functions. In recent decades, enormous progress has been made in research on wood derivatives (glued, micro-laminated, CLT, etc.) and the industry offers more and more materials at the service of architecture. Regulations and computer programs also continue to evolve in the direction of promoting the use of wood and facilitating its use and prescription to technicians.

In less than 20 years, the evolution has been spectacular, although there are still many myths to demolish. As an example of the durability of wooden architecture, this post illustrates an image of the “Temple of the Teaching of the Flourishing Law” (Horyu Gakumonji), a Buddhist temple located in Ikaruga, Nara prefecture, Japan. Inside are the oldest wooden buildings in the world, dating from 706. More than 1,400 years old! And there are still those who distrust wood …