Publikationsserver der RWTH Aachen University
Einfluss der Bodenreaktionskraft beim Durchstanzen von Einzelfundamenten
Abstract
dc:descriptionThe punching shear capacity of footings varies significantly for different codes. The amount of the soil reaction to be deducted from the punching load differs from one code to another. The aim of the present investigation is to derive an advanced design model for the punching behaviour of footings taking into account the soil-structure-interaction. The results of five punching tests on reinforced concrete footings supported on soil are presented. The experimental results indicate that the angle of the shear failure plane is steeper than observed in punching tests on flat slabs. Since the beginning of the 20th century only few punching tests on footings have been performed. Because the experimental study of a footing under realistic boundary conditions is associated with considerable expenditure, most researchers avoid using real soil in their experiments for bedding. The punching shear capacity of foundations is influenced by the stress distribution beneath the footing. For the calculation of the punching shear capacity the design codes suggest a reduction of the shear force by the soil pressure within the control perimeter. The assumption of a uniform, or linear stress distribution is commonly accepted but does not match the real stress distribution, which itself depends on the magnitude of the effective load. In 1885 Boussinesq showed that for rigid, vertically loaded, and rotationally symmetric foundations a concave stress distribution beneath the footing arises. This theory is correct as long as the soil behaviour is assumed to be elastic isotropic With increasing load the soil plasticizes under the foundation edges and the stresses redistribute towards the centre of the footing. This redistribution is completed when the bearing capacity is achieved. At limit state the stress distribution is convex according to the theory of Prandtl-Buisman. For a convex stress distribution the soil reaction beneath the punching cone is greater than for a uniform one. This results in a more economic design. Five reinforced footings were tested under realistic boundary conditions. The test parameters included the compactness and stiffness of the sand, the shear span ratio, and the shear reinforcement. The dimensions of the test specimens were chosen to model an ordinary footing at a scale of 1/2 to 1/3 and to fit into the experimental sandbox. All tested footings had a footprint of 900 mm x 900 mm and were designed to fail in punching. Thus, all footings can be regarded as rigid compared to the soil. A hydraulic jack (maximum capacity 2000 kN) applied the load incrementally. During testing the vertical displacements at the slab centre, and the slab corners as well as the steel and concrete strains were measured. Furthermore, pressure gauges were used to monitor the soil pressure distribution. The equilibrium of the vertical forces was used to eliminate the measurement deviation. For loads below 300 kN/m² the soil stress distribution according to Boussinesq can vaguely be recognized. A local stress maximum beneath the column can already be seen in serviceability state. The concentration progresses with increasing load level. The ratio of the average measured stress beneath the punching cone to the average soil pressure (sigma m = V/A) is about 1.06 close to limit state. However, the measured concentration of the soil pressure beneath the column was not as distinctive as expected according to the theory of Prandtl-Buisman. The experimental bearing capacity was significantly above the calculated one according to DIN 4017, or other codes. The redistribution of the soil pressure towards the centre of the footing was not completed when the calculated bearing capacity was achieved. An evaluation of the soil stress distribution close to ultimate load showed that the measured stresses beneath the punching cone were about 15.7 percent greater than the average value of the soil pressure (V/A).
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2008
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Ulke, Bernd Andreas
- Contributors dc:contributor
-
- Ziegler, Martin
Subjects
dc:subject × 13Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- ger