Size matters, even with hidden brackets
A hidden bracket is used to fasten a concrete element beam or composite beam to the side of a concrete element column, composite column, concrete element wall or another beam. The bracket acts as a swivel joint for the bending of the head of the beam, transferring the beam’s torsional moment to the column without support during erection.
The requirements for hidden brackets have increased in high-quality construction. When I used to work as a structural designer, the concrete strengths used were C40 or even C30. The strengths have increased significantly and, today, at least the C60 strength class is used for concrete in frame structures.
As concrete strength has increased, construction has changed. Stronger concrete enables more slender concrete structures, which means an increase in the amount of steel in the structures in relation to the surface area of the concrete. Due to this, there is less space for other parts of the columns.
The mechanical reinforcement ratio, i.e. the ratio between the surface area of concrete and steel, has remained unchanged, meaning that more steel can be added to the columns. Secondly, the lever arm has decreased. The minimum concrete cover in columns used to be 25 mm, whereas the cover is now 30–35 mm. In the past, the Concrete Code specified the distance to the surface of the main reinforcement, nowadays to the surface of the stirrup reinforcement, so the lever arm has decreased, which also requires more steel and means less space.
The physical size of the hidden bracket becomes decisive
With the development of construction, hidden brackets have also had to evolve to meet the current requirements. The concrete strengths and reduced space in the columns require hidden brackets to have slimmer and slimmer design. The more compact the hidden bracket is, the better it fits into the structure.
There is a lot of talk about tolerances, i.e. permissible dimensional deviations, in prefabricated element construction. Personally, I feel that the tolerances accepted in official tolerance documents are far too large. Of course, tolerances are needed because, for example, the formwork yields a little when a column is concreted, and measurement results are always approximate.
When I was plant manager at a prefabrication factory, I halved the tolerances in order to maintain the quality of construction. Significantly smaller tolerances are possible when the hidden bracket is small enough. It requires the measurements and work to be carried out carefully.
In the manufacture of concrete elements, it is essential to measure the pre-reinforced and equipped formwork before concreting and the finished elements after disassembling the formwork.
A low beam cannot turn askew
In the longitudinal direction, the height – or lowness – of the hidden bracket becomes central. So once again, size matters. A hidden bracket with a sufficiently low longitudinal structure cannot turn askew during concreting, but remains in place, which in turn reduces the need for tolerance fluctuation.
Anstar’s SMART STEEL™ concept enables efficient, modern construction even in difficult conditions. Our hidden brackets are a good example of this. Hidden brackets where size really matters.
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Juha Pulkkanen has nearly 40 years of experience in the manufacture and design of concrete elements, the building product industry and the management of prefabrication factories. As a plant manager, Juha particularly emphasises the quality of construction and products to ensure that Anstar’s customers only get the best service on the market.