Braunschweig reinforcement

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The term Braunschweig reinforcement or Braunschweig protective reinforcement refers to the reinforcement of shelters in connection with concrete, which is characterized by high strength and stability, as is required for the construction of the walls and ceilings of air raid shelters . It was developed in Braunschweig and has been the standard construction method for air raid shelters in Germany since 1941.

background

During the Second World War , cities were bombed more and more from the air. Therefore, the construction of reinforced concrete bunkers became an urgent task in order to provide adequate protection for the civilian population. Special attention was paid to a material-saving yet effective construction method. The protective reinforcement that was introduced played an important role here. The sole purpose of this is to minimize damage to the building caused by bomb impacts or detonations. This is to ensure that those seeking protection are accommodated as safely as possible and cannot be injured by falling parts of the building.

The former test site of the Institute for Structural Air Protection

In order to develop the most efficient construction method possible for the bunker systems, a number of large-scale and model tests have been carried out at the Technical University of Braunschweig since 1939 (other sources say 1938, i.e. before the start of the war) at the Institute for Structural Air Protection , which was founded in 1937 . The aim was to find out which reinforcement was best suited for the construction of new air defense systems. The requirements were, among other things, a low consumption of steel, cost-effective production, high protection and high strength of the concrete. From these tests a new type of reinforcement, later called “Braunschweig reinforcement”, was developed. Under the direction of Theodor Kristen, the “Institute for Structural Air Protection” became a leading institution in the field of bunker development. For the tests on the outdoor area in the Querumer Forest, bunker models in a size of 1: 5 were produced.

Conventional reinforcement

Since concrete ceilings without any reinforcement can cause damage from explosive funnels on the outside as well as larger blasting off (explosive lenses) in the interior, these were excluded as a construction measure. Even with reinforced concrete ceilings with an even distribution of the steel layers, an external explosive funnel is created, but instead of the explosive lens, if there is sufficient steel reinforcement, a bulge or arching forms on the inside, which reduces the ceiling height. The different arrangements of the steel inserts were therefore tested.

Benzinger reinforcement
Saardrahtwerke GmbH offered reinforcement with a steel weight of 60 kg / m³ for the construction of the bunker. A wire mesh (diameter 2.5 mm) was crosslinked with one another using steels with a diameter of 8 mm. Each layer of reinforcement had a height of 15 cm, the individually concreted layers were held together with steel clips. The reinforcement was well suited for road construction, as it withstood dynamic loads, but explosive tests showed that the individual layers flaked off and the wire mats tore easily.
Cubic reinforcement
Here steel inserts (diameter 10 to 12 mm) are spatially evenly distributed and connected to one another. Due to the close networking, the use of low-water concrete was only possible to a limited extent and did not allow large building thicknesses, as it had to harden for a long time and was not very strong. In addition, a considerable amount of steel (around 150 kg / m³) had to be built in to achieve a protective effect.
Spiral reinforcement
The company Dykerhoff & Widmann KG offered a variant similar to the cubic reinforcement, in which mats made of spirally wound steel were used. This design also showed no real advantage over this design in the test. Therefore the spiral reinforcement was increased to a steel content of 70 kg / m³ in 1940 due to an instruction for the construction of bomb-proof air raid shelters. Although the protective effect could be increased, the consumption of steel was more than twice as high as the desired amount of around 30 kg / m³. In this area, however, the reinforcement no longer offered adequate protection.
Lattice reinforcement
The company Luz-Bau GmbH offered a reinforcement in which the steels were attached in a grid shape at an angle of inclination of 60 degrees. This arrangement also showed disadvantages in the experiment, as cracks could form in the building structure. It achieved a protective effect similar to that of the spiral reinforcement.
Standard protective reinforcement
According to a proposal by the Reich Aviation Ministry (RLM), the spiral and lattice reinforcement were combined with one another, which led to a more effective protective effect than if they were used individually.

Construction of the Braunschweig protective reinforcement

Based on the test results, a new form of reinforcement was designed. This was called the "Braunschweiger Schutzwehr" and in July 1941 it was laid down in the "Regulations for the construction of air raid shelters" as the only uniform type of reinforcement for the Reich.

The principles for the construction were that around 60% of the steel had to be installed on the underside or inside of the building and that the steel mats should have a large mesh size. The exception was the lower (inner), which was made more closely meshed. The steel weight was around 30 kg / m³ and still offered protection equivalent to that of similar buildings with a steel content of 55 to 80 kg / m³. The protective effect of the Braunschweig protective reinforcement against other bunker types has been proven in tests. Even after four attempts to blast it, it still held up, while the cubic reinforcement, for example, had already been penetrated.

In the case of walls, the reinforcement has dense steel inserts on the interior side, which form increasingly larger distances to the outside. This reinforcement is arranged horizontally on the ceilings. In order to optimally utilize the desired strength and the advantages of the Braunschweig protective reinforcement, no processes may be used that use pure liquid, cast or pumped concrete.

Concrete ceilings with Braunschweig protective reinforcement
The reinforcement is put in with a soft concrete mix (grain size 0 to 30 mm) made of 400 kg cement per cubic meter, about 20 cm thick, and then compacted. This is followed immediately by the introduction of a firmer concrete layer (grain size up to 100 mm). The workers responsible for compaction stand on the reinforcement deposits and climb upwards as the concrete height rises, like on ladders.
Concrete walls with Braunschweig protective reinforcement
For the walls, pouring pipes are mostly used for concreting, which are adapted to the reinforcement and have a diameter of 20 cm (inside) to 40 cm (from the center of the wall). Here, too, the inner layer is made from the softer, finer and the outer layer from the firmer, coarser concrete mass. If possible, this should be a little higher than the more fluid inner layer so that it spreads out towards the finer reinforcement.

The original structure of the Braunschweig reinforcement was designed for ceilings with a thickness of 1.40 m and was used for the first construction wave in Braunschweig from 1940 to 1941. The second construction wave required slab thicknesses of at least 2 m, so the construction had to be adapted to the new specifications. For this purpose, wide-meshed steel mats were installed at different heights. Further additional changes increased the protective effect, whereby the weight of the reinforcement was retained.

Old design
With a thickness of 2 to 2.5 m, the reinforcement consists of stirrups with a diameter of 14 mm and for 3 m with a diameter of 16 mm and individual mats with steel with a diameter of 10 mm. There is a larger number of mats in the lower part of the bunker ceiling or the inside of the bunker walls. In the middle and on the outside there is another mat.
New design
Here the reinforcement consists of four crosswise arranged mats, which are enclosed by brackets. The lower mat is made of a network of round steel (diameter 22 mm) with a mesh size of around 13 mm. The mats are anchored with stirrups (diameter 30 mm) with a hook length of 45 cm. The close-meshed net of the lower mat is intended to prevent larger concrete blocks from breaking out. The reinforcement was only in the lower two thirds of the building thickness, the upper third remained unreinforced. After the end of the war, the Braunschweig reinforcement continued to be used for civil air protection.

literature

  • Study society for Benzinger constructions: The Benzinger reinforcement in concrete, reinforced concrete, road and air raid protection. Karlsruhe 1934, OCLC 174236373 .
  • Th. Kristen, K. Ehrenberg: The development of the protective reinforcement of defense structures, especially LS structures made of reinforced concrete. In: Messages from the Institute for Structural Air Protection at the Technical University of Braunschweig. Braunschweig 1944, OCLC 257320993 ( ibmb.tu-braunschweig.de , PDF).
  • Leutz, Kern: Structural repair of civil protection bunkers. In: civil defense. gsb.download.bva.bund.de (PDF; 21.3 MB) Verlag Gasschutz und Luftschutz Dr. Ebeling, Koblenz-Neuendorf 1962, issue 1 p. 20ff. OCLC 33914825 .
  • K. Ehrenberg, U. Finsterwalder, G. Kern: Explosive tests on protective structures. Braunschweig reinforcement. In: Bundle of seven parts. 1981, OCLC 258709119 .

Web links

Individual evidence

  1. ↑ a b c d e f g h i protective reinforcements on amaot.de
    Th Kristen, K Ehrenberg: The development of protective reinforcement in defense structures, in particular LS structures made of reinforced concrete.
  2. ↑ Braunschweig, Querumer Forst test site. ( Memento from October 15, 2013 in the Internet Archive ) on vergierter-geschichte.blogspot.de, accessed on October 16, 2013.
  3. ↑ a b c Dietrich Janßen: Concreting ceilings and walls with Braunschweig protective reinforcement. 2003 ( bunkermuseum.de PDF; 85 kB).
  4. ^ A b Leutz, Kern: Structural repair of protective bunkers for civil protection. In: civil defense. Koblenz 1962, issue 1 p. 20 ( gsb.download.bva.bund.de PDF; 21.3 MB).