Research Project. concrete footings taking into account the soil structure interaction

Size: px

Start display at page:

Download "Research Project. concrete footings taking into account the soil structure interaction"

Louisa Bennett
6 years ago
Views:

1 Research Project Development of an advanced design model for the punching behaviour of reinforced concrete footings taking into account the soil structure interaction Author: Dipl.-Ing. Dipl.-Wirt.Ing. (FH) Bernd Ulke supported by: Arbeitsgemeinschaft industrieller Forschungsvereinigungen Otto von Guericke e.v. (AiF) Joint Project between Institut für Massivbau (IMB) and Geotechnik im Bauwesen (GiB)

2 1,5 d Geotechnik im Bauwesen Abstract The 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, and the shear slenderness seems to affect the punching shear capacity significantly. Introduction 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 aim of the present investigation is to derive an advanced design model for footings taking into account the soil-structure-interaction. The bearing 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. The design codes allow a part of the soil reaction to be subtracted from the punching load. However, the amount to be deducted differs from one code to the other (Fig.1). BS ,0 d 0,5 d DIN 1045 (7/88) ACI DIN (50% von A crit ) column Coextensive ring Fig.1 Control perimeter for different codes

3 Most design codes with the exception of Eurocode 2, and Model Code 90 do not differentiate between the treatment of the punching shear strength of flat slabs, and footings using the same equations. 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 (Fehler! Verweisquelle konnte nicht gefunden werden.). 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 (Fehler! Verweisquelle konnte nicht gefunden werden.) 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. The common approximation procedures for the calculation of the soil pressure distribution in soil mechanics, for example the classical Winkler solution or the coefficient of subgrade reaction method, simulate the ground as an elastic, isotropic half-space with a system of linear springs. The redistribution of the soil pressure towards the foundation centre at an increased load is not sufficiently described by these methods. Experimental program 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. The footing DF3 included heavy shear reinforcement consisting of vertical bars with a diameter of 12 mm and a yield strength of 548 MPa. At the top and the bottom of the slab the bar ends were anchored by welded plates because the height of the slab was only 200 mm. The footings DF4 and DF5 were more compact and were designed to investigate the effect of a small shear slenderness = a/d on the punching behaviour. Ready-mixed concrete with a maximum coarse aggregate size of 16 mm was used in all footings. Fig. 2 shows the experimental setup. 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.

Fig.1 Test setup Measured soil pressure distribution Pressure gauges were used

For test DF1 17 pressure gauges were used, for test DF2 20, and 21 for the

4 Fig.1 Test setup Measured soil pressure distribution Pressure gauges were used to measure the soil pressure distribution beneath the footing. For test DF1 17 pressure gauges were used, for test DF2 20, and 21 for the remaining tests. The arrangement of the load cells is revealed in Fig.3. Fig.2 Arrangement of the load cells The soil pressure distribution beneath the footing DF2 is represented in Fig.3 for specimen DF3 (top), DF4 (middle) and DF5 (bottom) near the collapse load.

used to eliminate the measurement deviation.

5 Fig.3: Soil pressure distribution near collapse load for specimen DF3 (top), DF4 and DF5 (bottom) 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

6 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 ( m = V/A) is about 1.06 close to limit state. Experimental investigations dealing with the soil pressure distribution under footings 35 years ago an experimental investigation was carried out to determine the soil pressure distribution at the University of Karlsruhe. The results of these tests were again evaluated in a database to verify the results of the present experimental investigations. The measuring technique, the screening characteristic of the soil, and the foundation measurements used in Karlsruhe are comparable with the own experiments. The footings were designed that no punching failure occurred. Therefore, the tests were incrementally loaded until the bearing capacity was reached. Provided that the load is equal to the calculated bearing capacity according to BS EN the measured stresses beneath the punching cone were about 8 percent higher than the average value of the soil pressure (V/A). These results confirmed the own experimental investigations. 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 BS EN , 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 15.7 percent greater than the average value of the soil pressure (V/A). Publications Hegger et al., 2005: Hegger, J.; Ricker, M.; Ulke, B.; Ziegler, M.: Entwicklung eines Bemessungskonzeptes zum Durchstanzen von Fundamentplatten unter Berücksichtigung der Boden-Bauwerk-Interaktion. Abschlussbericht zum gleichnamigen AiF-Forschungsvorhaben, unveröffentlicht, Aachen, Hegger et al., 2006: Hegger, J.; Ricker, M.; Ulke, B.; Ziegler, M.: Untersuchungen zum Durchstanzverhalten von Stahlbetonfundamenten. Beton- und Stahlbetonbau (101) 2006, S Hegger et al., 2007: Hegger, J.; Ricker, M.; Ulke, B.; Ziegler, M.: Investigations on the punching behaviour of reinforced concrete footing. Engineering Structures 29 (2007), S

CHAPTER 4 PUNCHING SHEAR BEHAVIOUR OF RECYCLED AGGREGATE CONCRETE TWO WAY SLABS

CHAPTER 4 PUNCHING SHEAR BEHAVIOUR OF RECYCLED AGGREGATE CONCRETE TWO WAY SLABS 103 CHAPTER 4 4.1 GENERAL PUNCHING SHEAR BEHAVIOUR OF RECYCLED AGGREGATE CONCRETE TWO WAY SLABS The problem of punching shear in reinforced concrete slabs under a concentrated load or around a column has