Punch shear

The shear modulus of a material can be determined by a static torsion test or by a dynamic test employing a torsional pendulum or an oscillatory rheometer. The maximum short-term shear stress (strength) of a material can also be determined from a punch shear test. 

Depth of Slab. The thickness of the foundation slab (bottom course) required for punching shear must also be sufficient to withstand the tendency to shear along line (Z-Z2), (Figure ll-4c) at the edge of the pedestal. This shearing load may be determined as follows ... 

Example 5. To illustrate the procedure, the punching shear will be calculated for the foundation considered in Example 1. 

In actual practice, the depth of the base in the examples given could be reduced, if desired. All of the stresses for diagonal tension, punching shear, bending (upward and downward), and bond in the reinforcement are well below the allowable values. 

There are a number of loading systems that give rise to shear stresses. In this section we shall consider variations on the lap shear, punch shear, torsion, and four-point loading. 

Punch shear geometry is relatively popular for plastics to measure shear strength, although it can be used for any material. The approximation to pure shear conditions are achieved by a punch bearing on a sheet of material supported by a die. The smaller the difference between the internal diameter of the die and the external diameter of the punch, the nearer the approximation. 

Punching shear parameter. (After Hanna, A.M. and Meyerhof, G.G., Canadian Geotech. /., 17, 300-303, 1980. Reprinted with permission of Canadian Geotechnical Journal.)... 

The three principal modes of soil failure, established by the patterns of the shearing soil zones, are defined as general shear failure, local shear failure, and punching shear failure. Figure 4.2 illustrates the differences in the three modes and the foundation load settlement curve typical to each mode. 

The local shear failure condition (Figure 4.2b) involves some of the characteristics of both the general shear and the punching shear failure modes. As for general shear, the well-defined wedge and slip surfaces are formed beneath the foundation, but the slip surfaces fade into the soil mass... 

For application to shallow foundation design, it is commonly considered that the general shear case applies to dense granular soil and to firmer saturated cohesive soils subject to undrained loading. The punching shear case is considered appropriate for compressible soil, such as sands having a low to medium relative density, and for cohesive soils subject to slow loading. 

Top reinforcement over the internal supports is concentrated in a strip half the width of the span this arrangement provides better performance with regard to punching shear. Top reinforcement over the external supports is concentrated in a strip one-fifth the width of the span. The reinforcement is designed by using an appropriate multiplier in the calculation of the support moment. 

Carter, J.M., Sorensen, S.E., Johnson, R.R., Teitelbaum, R.L. and Levine, M.S. (1983) Punch Shear Testing of Extracted Vital and Endodontically Treated Teeth. J. Biomechanics 16(10), 841-848. 

Utilized a miniature punch shear apparatus to determine shear strength and toughness perpendicular to the direction of dentinal tubules. Dentin harvested from the cemento-enamel junction to one-third the distance to the root apex. Strengths novel measurements, precise measurements, defined specimen location, defined orientation of testing. Limitations tooth type not defined for constrained tests, teeth stored in mineral oil prior to testing. 

There are five types of machine motion (see Figure 13-2) that create machine hazards rotation, reciprocating or transverse motion, in-running nip points or pinch points, cutting actions, and punching, shearing, and bending. 

Shock-resisting tool steels S Shock-resistant tool steels, with Cr-W, Sl-Mo, and Sl-Mn as major alloys, are strong and tough, but they are not as wear resistant as many other tool steels. These steels resist sudden and repeated loadings. Apphcatlons include pneumatic tooling parts, chisels, punches, shear blades, bolts, and springs... 

FIGURE 8.4 Bearing capacity failure modes for shallow foundations, (a) General shear (b) local shear and (c) punching shear. (Adapted from Vesic, A. S., Foundation Engineering Handbook, Edited by Winterkorn, H. F. and Fang, H. Y., Van Nostrand Reinhold Company, 1975.)... 

The most common practice for soil nail wall construction includes applying two layers of shotcrete for the face of the wall. Each shotcrete layer is usually about 4-6 in (100-150 mm) thick. The first layer of shotcrete is placed directly against the excavated soil face and then the steel anchoring plate and beveled washer with hex nuts are installed to lock the soil nail in place. The second layer of shotcrete is then subsequently installed. For these soil nail walls, it is important for the designer to check the flexure and punching-shear demand generated by the soil nail in each slab layer. 

The temporary face slab system needs to be checked with flexure and punching shear strength. Then, the final face slab system needs to be checked in a sirnilar manner but with different strength factors. 

Nail force converted from punching shear capacity... 

The end anchor assembly for the soil nails should be covered by the finished layer of shotcrete. It is a common practice to install steel studs on the end bearing plate for typical application. These studs can help ensure a sufficient connection between the temporary layer of shotcrete and the final shotcrete layer, while also increasing the punching shear capacity. 

Tests performed by Wight et al (2007) confirmed enhanced strength when a high volume of fibres was applied, and dispersion of cracks into microcracks were observed. Similar results related to punching shear resistance have been obtained by Cheng and Parra-Montesinos (2007) after tests of slab-column systems that were subjected to monotonic loading that simulated the effects of a seismic event. 

Cheng, M.-Y., Parra-Montesinos, G. J. (2007) Punching shear resistance and deformation capacity of fiber reinforced concrete slab-column connections subjected to monotonic and reversed cyclic displacements, in International Workshop HPPRCC 5 in Mainz, RILEM, Proc. S3, W. H. Reinhardt and A. E. Naaman eds pp. 489-96. 

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