The maximum stress at third-point loading (the stress at both maxima) shall increase steadily with decreasing span-depth ratio. Naschold calculated the maximum tensile stress in beams as shown in Fig. Steinhardt achieved such a stress distributio nusing the brittle lacquer test for beams of spruce. The stress distribution for centre loading was also determined in, showing at the mid-span a stress distribution with a horizontal tangent instead of the salient point. This corresponds to the stress distribution calculated by See wald evaluated by Naschold and Tucker. The mentioned methodin and results ina stress distribution with two maxima of the stress between the two load points, as can be seen in Fig. They were tested with a span length of L = 300mm under third-point loading. The specimens had a cross-section of 120 x 120mm². 1 shows as an example the frequency distribution of the location of fractureof 72 concrete specimens as a histogram. Fracture occurs mainly at or near the point of the maximum stress, and less often, where the stress is lower. The frequency distribution of the fracture origins is used to calculate the stress distribution. The stress distribution can be determined according to a method of Schneeweiß and experimentally in the following way: a great number of the same specimens is tested the locations of the fracture origins are determined. The validity of the stress distributions calculated can be proved experimentally. Seewald calculated the stress distributions for beams under centre loading and third-point loading, supposing a linear elastic, homogenous, and isotropic material. No such salient points can occur, if there are no external loads, effecting in the tension zone. Consequently, they also show up in the graphs of the stresses at the extreme fibres of the tension zone. Salient points (kinks) should appear inthe graphs of these bending stresses. The graph of the bending stresses shows according to the beam theory for centre loadinga triangular and for third-point loading a trapezoidal shape a long the beam length. The bending stresses calculated under these suppositions should be at the same time the stresses in the extreme fibres of the tension and compression zone. Stress Distribution The classical(elementary) beam theory supposes a linear distribution of the longitudinal stresses across the beam depth. Deformations can reduce the bending strength. The compression strength perpendicular to grain at the load points is important for flexural fracture. Two special cases of the annual growth ring orientations are regarded, namely annual rings parallel (vertical annual rings) and annual rings normal to the direction of the load (horizontal annual rings). The used loading configurations are centre loading and third-point loading (in some cases two-point loading). Only the range of flexural fractures is examined in this paper. The specimens show flexural fractures above a critical value of the span-depth ratio and shear fractures below. The values of the bending strength are described in dependence of the span-depth ratio (span length L /specimen depth D). Introduction Small clear specimens are considered, with fibre direction in the longitudinal direction of the beams. Bending Strength and Compression Strength Perpendicular to Grain 12. Definition of the Compression Strength Perpendicular to Grain 10.4.2. Factors Influencing the Measured Strength Values 10.4.1. CompressionStrength Perpendicular to Grain 10.1. Horizontal Rings, Pith near the Compression or Tension Side 9.3. Bending Strength and Annual Ring Orientation 9.1.
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