A strong combination
Author: Susanne Jacob-Freitag
Photos: Susanne Jacob-Freitag

BauBuche posts in skeleton construction

Most skeleton constructions are mode of one single building material, such as steel, reinforced concrete or timber. However, systems that combine two or more materials are becoming ever more popular – at least since BauBuche has been available. Thanks to its high compressive strength, this unique material is a viable substitute for reinforced concrete in the construction of posts and pillars, opening up new design options for skeleton constructions.

Skeleton constructions consisting of posts and beams are among the most efficient and cost-effective supporting structures. That is why they are widely used for the construction of office blocks and commercial premises, as well as residential buildings. Post and beam structures allow for greater flexibility in floor plan design than load-bearing walls. This adds value to a building, particularly with regard to its long-term use.

The higher the load capacity of the material, the slimmer the posts can be made. BauBuche for instance allows for posts with particularly small cross-sections whose load capacity is however in line with that of reinforced concrete. Another advantage of BauBuche is the fact that posts made from this material can easily be combined with reinforced concrete elements. The respective connecting points at the top or bottom of a post can be designed according to established solid construction standards.

By using BauBuche and combining it with other more conventional materials, developers can save money and reduce the share of energy-intensive materials in construction while gaining extra flexibility and safety. BauBuche is thus a resource-efficient material that opens up new options in skeleton construction.

Comparison of BauBuche and reinforced concrete posts

Depending on the supporting grid, the number of storeys and the envisaged building use, posts must be able to carry very heavy loads. Posts made in BauBuche score particularly high when it comes to centrally applied loads. Studies reveal that BauBuche posts have a much higher load capacity than posts with the same cross-section made from reinforced concrete, so that, for a given load, BauBuche posts can be made slimmer than concrete posts. At the same time, the weight of BauBuche posts is only about a third of that of reinforce concrete posts with the same dimensions.

In office and residential buildings, the characteristic compressive strength of BauBuche in grain direction is between 59.4 N/mm² and 70 N/mm² and thus higher than that of C50/60 concrete. This means that BauBuche posts with a cross-section of 20 cm x 20 cm easily match the load capacity of reinforced concrete posts of the same dimensions, and – depending on the actual requirements – can be made even narrower. From cross-sections of 40 cm x 40 cm, BauBuche posts can even replace equally sized C50/60 reinforced concrete posts with a fire resistance rating of 90 minutes.

Protective charcoal layer ensures high fire safety

Timber construction elements of a certain thickness come with a high fire resistance rating. In the case of a fire, their surfaces become charcoaled. This layer acts as an insulation, protecting the rest of the element against going up in flames. The burn rate slows down and the load capacity of the core of the timber element is retained for some time.

After 90 minutes of exposure to flames from all sides, the load capacity of a BauBuche post with an average length of 3.05 m and a cross-section from 40 cm x 40 cm corresponds to that of a reinforced concrete post of the same dimensions. In many cases, BauBuche posts are a viable substitute for reinforced concrete posts with cross-sections of 30 cm x 30 cm or 35 cm x 35 cm. In such constructions, and for fire safety reasons only, the BauBuche posts must be slightly larger than the concrete ones. Alternatively, the BauBuche posts can be clad with firecheck boards, so that the cross-sections can be reduced to the required load capacity dimensions (cold-state design), doing away with overdimensioning the elements for fire safety reasons (hot-state design: extra thickness of posts to cater for the charcoal layer).

BauBuche comes in all standard post cross-sections commonly used in multi-storey constructions. At the moment, posts with cross-sections of 20 cm x 20 cm and 30 cm x 30 cm and lengths of 2.80 m to 10 m are available in stock. From 2021, Pollmeier intends to extend its product range to include cross-sections of up to 60 cm x 60 cm. The company is currently working on a system that allows it to ship all finished posts as precision-machined, hydrophobized elements fully equipped with base and head fasteners, and safely packed for temporary storage on the construction site. Until this system is in place, Pollmeier offers machining only for larger cross-sections, in cooperation with production partners. This means that, for the moment, the design of joints and the fitting of fasteners are still the responsibility of the project planners.

However, BauBuche posts for skeleton constructions are available with square cross-sections other than the standard dimensions and also with rectangular cross-sections.

For more information on this topic, please download the brochure: Pollmeier BauBuche – Chapter 16 – Posts.

Early adopters pave the way to innovations of the future

Two of the first lighthouse projects where BauBuche posts were used in skeleton constructions can be found in Switzerland, namely on the site of a former oxygen factory known as “Suurstoffi” located close to the railway station of Rotkreuz. Here, we find the ten-storey “Suurstoffi 22” and the adjacent 15-storey, 60 m high “Arbo” building. This 41.40 m long and 20.40 m wide timber-concrete composite construction consists essentially of a timber skeleton made from glulam posts and beams combined with timber-concrete composite floors. Posts exposed to particularly high loads where the dimensions could not exceed those of the glulam supports are made from BauBuche.

Another ground-breaking project was recently completed in Norway, where the architects chose BauBuche for the striking new building of SR Bank in Stavanger. Most of the office block is made in timber, and BauBuche posts and beams were used primarily at ground floor level, resolving many structural strength issues that would have been nearly impossible to overcome without the use of this exceptional timber material.

There is also a pioneering project in Germany, dating from 2015, namely the three-storey office block of Euregon in Augsburg, a timber skeleton construction of BauBuche posts and beams. What we are still looking for is a reinforced concrete skeleton structure that incorporates BauBuche posts. Early adopters might consider such a project to position themselves as innovative trend setters. Pollmeier will supports pilot projects in any way it can, to make sure that its advanced technology is combined in the best possible manner with established reinforced concrete skeleton construction designs and methods.

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Advice for architects, engineers, commercial builders and timber construction:

+49 (0)36926 945-560 baubuche@pollmeier.com

Advice on timber, BauBuche and for Pollmeier LVL:

+49 (0)36926 945-163

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