Fire safety based on sound calculations
Author: Susanne Jacob-Freitag
Photos: Susanne Jacob-Freitag

BauBuche posts

BauBuche posts can be dimensioned so that they do not lose their load-bearing capacity in the event of a fire. The dimensions of construction elements are defined by the load they need to carry, and also by the fire resistance required for a specific construction. When it comes to fire safety, BauBuche posts are at par with reinforced concrete posts. This short article tells you why this is the case.

Fire safety is primarily concerned with ensuring that a building maintains its structural integrity until all persons have rescued it – by themselves or with the help of the fire rescue services. The evacuation time of a building depends of course on its type and nature: a single-storey factory hall for instance can be evacuated much more quickly than a multi-story residential block. Therefore, the time for which a building and its load-bearing elements must withstand a fire varies greatly.

In order to ensure that there is an adequate time window for the evacuation of a building, fire safety engineers determine the required fire resistance rating, which is the time in minutes that the construction and its structural elements must be able to withstand a fire. A fire resistance rating of F60 indicates for example that, in the event of a fire, the building remains structurally sound for minimum 60 minutes before it is likely to collapse.

Protective charcoal layer ensures high fire safety

The load-bearing capacity of timber construction elements exposed to fire can be calculated. Such calculations take into account that the surfaces of the elements become charcoaled, whereby this layer then acts as an insulation, protecting the rest of the element against going up in flames. The charcoal layer thus protects the core of the posts, which therefore retains its strength for as long as the surrounding layers are smouldering.

The fire resistance rating of constructions is based on hot design calculations: the core cross-section needed to meet the structural strength requirements is first determined purely on the basis of the load, and then increased by the layer that is allowed to burn away and be turned into charcoal in the event of a fire. The thickness of this layer depends of course on the required fire resistance rating, which in turn is determined by the burning rate of timber. For BauBuche, the burning rate is around 0.65 mm per minute.

Let us consider a timber skeleton construction with square, load-bearing BauBuche posts that need to withstand a fire for 90 minutes, i.e. meet a fire resistance rating of F90. Before taking into account fire safety, cross-sections of 30 cm would be sufficient to carry the load (cold design). To achieve the F90 rating, an additional surrounding layer of around 59 mm (0.65 mm x 90 minutes) is needed. For the hot design, 59 mm must thus be added to the 30 cm cross-sections of the square posts, resulting in dimensions of 42 cm x 42 cm (rounded up). Compared with the cold design cross-sections, these dimensions appear of course excessive. However, posts with such large cross-sections do not require costly fire cladding with plasterboard so that the timber can be left exposed.

BauBuche posts enable proprietors, architects and other decision-makers to opt for attractive exposed load-bearing timber constructions without in any way compromising the fire safety of the building.

Comparison of fire resistance of BauBuche and reinforced concrete posts

A comparison of the fire resistance of these two materials reveals some surprising facts. 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 of 40 cm x 40 cm or more corresponds to that of a reinforced concrete post of the same dimensions. In other words: From hot design cross-sections of 40 cm x 40 cm, the F90 fire resistance rating of BauBuche posts is at par with that of equally sized C50/60 reinforced concrete posts.

BauBuche posts with hot design dimensions are also 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 posts. Where BauBuche posts are to be clad with firecheck boards, they can of course be dimensioned according to cold design calculations. When it comes to BauBuche posts and fire safety, decision-makers have thus many options, all of them are based on sound calculations made during the planning stage.

For more information on BauBuche posts, see: Pollmeier BauBuche – Chapter 16 – Posts.

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