The customary exposed column bases of steel construction use anchor bolts. The anchor bolts may be subjected to various combinations of forces. The tensile (or pull-out) actions are one of these forces. The embedded steel sections can replace the anchor bolts in resisting pull-out forces. Depending on the shape of an embedded section, it can resist against pullout forces by three mechanisms namely: the bond resistance, the interlocking force and frictional resistance. The embedded tapered section develops the resistance against the pullout forces by the frictional resistance. The present paper, numerically and experimentally, studies the pull-out behaviour of tapered steel sections embedded in unreinforced concrete. The numerical models are generated with Abaqus 6.10-1. To support the numerical results, four tapered box and I-sections are tested under pull-out forces. The numerical models, study the effects of boundary conditions, the size of the concrete block, the tapering angle, and the coefficient of friction. The restraining boundary conditions prevent the splitting of the concrete block, which is the most common type of failure in embedded tapered sections, and could double its pull-out strength. Under proper confinement, the embedded tapered sections could have very large post-failure pull-out strength.

The large scale pullout test is used to investigate the pullout behavior of the geogrid in the anchorage zone. When pullout force is applied, tensile force gradually transmitted from the front part to end of geogrid. The active length is the part of the geogrid sample on which the activation of interaction mechanism resists against the applied force. In order to investigate more accurately the interaction between soil and geogrid, the geogrid pullout behavior should be evaluated based on the active length. In this study, a series of large scale pullout tests carried out to investigate the distribution of shear stress, pullout interaction coefficient and longitudinal strain of two types of uniaxial PET geogrid with same geometry and different stiffness embedded in two types of sandy soil and sand containing 20% clay based on active length. The results showed that, the pullout force required to move the last transverse bar of geogrid, increased with increasing of vertical effective stress in both type geogrid. Moreover, the active strain of stiffer geogrid embedded in clean sand and clayey was greater than that measured in the more extensible geogrid in both transfer load and pullout stage. At small frontal displacements, increasing the geogrid stiffness decreases the active shear stress in sandy and clayey sand soil. In all pullout tests, the lowest active pullout interaction coefficient occurred at the beginning of the pullout phase.

This paper presents the results of pullout tests on uniaxial geogrid embedded in silica sand under monotonic and cyclic pullout forces. The new testing device as a recently developed automated pullout test device for soil-geogrid strength and deformation behavior investigation is capable of applying load/displacement controlled monotonic/cyclic forces at different rates/frequencies and wave shapes, through a computer closed-loop system. Two grades of extruded HDPE uniaxial geogrids and uniform silica sand are used throughout the experiments. The effects of vertical surcharge, sand relative density, extensibility of reinforcement and cyclic pullout loads are investigated on the pullout resistance, nodal displacement distributions, post-cyclic pullout resistance and cyclic accumulated displacement of the geogrid. Tell-tale type transducers are implemented along the geogrid at several points to measure the relative displacements along the geogrid embedded length. In monotonic tests, decrease in relative displacement between soil and geogrid by increase of vertical stress and sand relative density are the main conclusions; structural stiffness of geogrid has a direct effect on pullout resistance in different surcharges. In cyclic tests it is observed that the variation of post-cyclic strength ranges from minus 10% to plus 20% of monotonic strength values and cyclic accumulated displacements are increased as normal pressure increase, but no practical specific comment can be made at this stage on the post-cyclic strength of geogrids embedded in silica sand. It is also observed that in loose sand condition, the cyclic accumulated displacements are considerably smaller as compared to dense sand condition.

The reinforcing action of soil nails is governed by its interaction with the surrounding soil generally investigated in terms of interface friction. The reported literature depicts that increase in interface friction enhances the reinforcing action of a soil nail. Thus, with the aim of utilizing additional interface friction from internal surface of a hollow pipe and bearing resistance from helical plates attached to it, the present work investigates the pullout behavior of newly developed open-ended pipe helical soil nails. The novel open-ended pipe helical soil nails are installed and subjected to pullout force in a test tank filled with frictional soil using installation/pullout machine. The present study also examined the effect of varying soil nail diameter, number of helices and phenomenon of soil plugging on pullout capacity. Additionally, torsional and axial strains developed during pullout are also observed. The installation torque and pullout capacity are correlated using a dimensionless factor called as ‘Torque factor (Kt)’ with values ranging from 27 to 55 m−1 in compression and 28.12–53.3 m−1 in tension. A theoretical torque model for open-ended helical soil nails is also developed and is verified by the laboratory test results. The test results indicate that soil plug contributes about 11.5% of the total mobilized skin friction during pullout. The pullout capacity increases with increase in the number of helices and nail shaft diameter. However, soil plug length is independent of number of helices. Besides, higher torsional and axial strains are found for helical nails with smaller diameter.

Cost-effectiveness, fast performance, and relatively long service life have made geosynthetics widely used in geotechnical structures such as road pavement, retaining walls, slope stabilization and foundations, bridge abutments, embankments or at the boundary between embankments and subgrade to be used. The soil geocomposite used in this research, which is made in Iran under the brand name of GC Soil 40/40, is a combination of geogrid and geotextile and is used in various projects such as embankment bed reinforcement and separation to implement embankment on fine and loose subgrade. In general, in the design of geosynthetics, including soil geocomposites, interaction mechanisms including slip and pullout as well as interaction coefficient between soil and geosynthetic should be considered. Accordingly, to evaluate the soil-geocomposite interaction, direct shear and pullout tests were performed in one-layer soil (sand-geocomposite and gravel-geocomposite) and two-layers soil (sand-geocomposite-gravel) under different vertical stresses. The results of direct shear tests showed that reinforcing the soil with geocomposite reduced the angle of friction and increased soil adhesion. Despite the increase in shear stress and pullout resistance with increasing vertical stress, the interaction coefficients decrease with an increase in the vertical stress. This issue can be related to the nonlinear behavior of pullout force and soil-geocomposite shear stress with vertical stress. Vertical stress is an effective factor to increase the pullout resistance and type of mode geocomposite rupture and also has a significant effect on the displacement at the maximum pullout resistance. The results showed that under the same loading conditions, the placement of the geocomposite on the common surface of the two layers soil changes the pullout behavior compared to the one-layer soil. In general, it can be stated that the soil-geocomposite interaction, in addition to vertical stress, is sensitive to the type and size of soil particles, one- and two-layers of soil. For sand-geocomposite and gravel-geocomposite interfaces, the average coefficients of interaction(𝑐𝑖 ) for direct shear are 0.8 and 0.91, respectively. Also, the average pullout interaction coefficients(𝑓𝑏) for sandy and gravel soils reinforced with geocomposite (one-layer) are 0.35 and 0.47, respectively, and this coefficient is obtained as the average of 0.51 for sand-geocomposite-gravel (two-layer).

The interaction between soil and geosynthetics has great importance in engineering work, especially in design and stability analysis of geosynthetic-reinforced geotechnical structures. In recent decades, several laboratory methods have been performed to properly understand the interaction between soil and geogrids, including pullout test, large-scale direct shear test. Although factors such as the geometry of the reinforced soil system and its construction process may affect the interaction properties between the soil and the geosynthetic, these properties are strongly influenced by the physical and mechanical properties of the soil and the geometrical and mechanical properties of the geosynthetic. Pullout test determines the geosynthetic pullout resistance, which is an important design parameter in relation to the internal stability of geosynthetic-reinforced geotechnical structures, and allows the measurement of displacements throughout the specimen during the pullout testing. Pullout force refers to the tensile force required to create an external sliding of geogrid embedded in soil mass. The tensile strength of the reinforcement consists of the frictional resistance on the surface of the longitudinal and transverse members of the geogrid and the passive resistance that is mobilized against the transverse members. Although fine-grained soil is recommended in the design of geosynthetic-reinforced soil structures, many geosynthetic-reinforced soil structures are constructed using soil containing a fine percentage. Therefore it is important to investigate the effect of fine grains on the stability and performance of such soil structures under different loading conditions. Geosynthetic-reinforced soil structures are sometimes affected by cyclic loads due to traffic and train crossings, vibration of industrial machinery, wave and earthquake. In this study, by performing static and multistage pullout tests, the static and post-cyclic pullout behavior of a uniaxial geogrid manufactured in Iran under the brand GPGRID80/30 is presented. The tests were carried out on a large scale pullout box with a dimension of 90 × 50 × 50 cm and with a constant rate and multi-stage procedures on three different soil types including clean sand, sand containing 10 and 20% fine silt and three effective vertical stresses of 20, 40 and 60 kPa. Results show that geogrid static pullout resistance increases with increasing effective vertical stress in all three different soil types. Also, the increase of silt in the sandy soil resulted in an increase in the monotonic maximum pullout resistance at effective stress of 20 kPa. The geogrid behavior in all three soils for 20 kPa vertical effective stress was strain softening and for the 40 and 60 kPa vertical effective stress ​​the geogrid pullout behavior was strain hardening. However, 10% increase in silt content leads to a slight decrease in monotonic pullout resistance and a 20% increase resulted the slight increase in monotonic pullout resistance of geogrid at vertical stress of 40 and 60 kPa. As the amount of silt content increased, the effect of cyclic loading on post-cyclic resistance increased, especially in vertical effective stresses of 40 and 60 kPa. Also, at effective stress of 20 kPa, the geogrid post-cyclic resistance decreased in all three sands, sand containing 10% silt and sand containing 20% silt relative to its corresponding monotonic pullout resistance.</pre>



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In the current study, using a physical model, the behaviour of anchors with horizontal end plates at shallow depths has been investigated. In this research, the effect of various factors such as the buried depth of the end plate, the diameter of the end plate, the type of soil, and the geocell and geotextile have been investigated. Based on the load-displacement curve obtained in the experiments, the results have been investigated. The results show that the rupture mechanism and the obtained load-displacement curve are a function of the conditions in the tests. For example, the shape of the load-displacement curve when there are geocells and geotextiles is completely different from the normal state (without geocells and geotextiles). On the other hand, the results show that the maximum value of the upward force obtained for the normal state is a function of the diameter of the plate and the buried depth so that its value increases with the increase of the diameter and depth of the plate. The effect of depth is much more important in the presence of geocell and geotextile on the results. The results obtained on different tested soils show that the pull-out force of the anchor is a function of the shear resistance parameters of the soil. As the sand friction angle increases, the force has increased. The results indicate that with the increase in the depth and diameter of the plate, the effect of sand on the maximum pullout force has increased. The results show that with the addition of a layer of geocell or geotextile, the tensile force of anchors increases significantly, and the combination of two layers of geocell and geotextile causes a significant increase in the pull-out Capacity.

The performance of steel rebars in reinforced concrete is always complicated and important. Each reinforced concrete element contains two parts, including concrete and steel rebars. Under severe forces, the behavior of reinforced concrete structures and their elements is dependent on the interaction between steel and concrete.  Due to the composite nature of these structures, their performance is complex and might be studied in different aspects. Incorrect evaluation of this item will lead to the wrong design. Although this performance affects all parts of a concrete structure, development length and lap splice are the most significant parts. One of the common tests, in evaluating the steel reinforcing bars' performance, is the pullout test, however, there are deficiencies in different investigations. In this research, five pullout specimens were tested, containing four specimens with confined and one with unconfined concrete, which had three different bar sizes including 20, 22, and 25 mm. Tests were conducted on 300 mm cube specimens and 250 mm development length of steel rebars which was insufficient. Rebars with sufficient development length have a different performance. The testing method was monotonic and the compressive strength of concrete was 23 MPa. Normally two cracking and damaging mechanism is observed. It is pullout of rebar or splitting of concrete. In confined concrete pullout occurs, while in an unconfined specimen, splitting of the concrete takes place so the bonding strength and axial force tend to zero much faster. Using these results and results from other researches, some important parameters in the field of bonding and slippage of rebars, including compressive strength of concrete, rebar size, force direction (pulling or pushing), and confinement of concrete were investigated and some equations with high accuracy were proposed to generalize available results to desired results. For verifying and increasing the range of results, three valid kinds of research are used, which confirm the accuracy of results and equations. This study showed that the bonding strength would increase 20 percent by 50 percent increase in compressive strength of concrete. Moreover, by changing the bar diameter from 20 to 22 and 25 mm, for a constant compressive strength of concrete, bonding strength decreases 4.3 and 10.6 percent respectively. By using the proposed equation, maximum bond strength for different bar sizes may be evaluated. Besides, maximum bond strength is reduced up to 40 percent of a confined specimen, and the slippage corresponding to maximum bond strength is reduced 15 to 30 percent as the result of using unconfined concrete. In the same way, for the same bar diameter, pullout force reduced up to 40 percent. The slippage corresponded to maximum bonding strength is about 1.4 to 2 mm for confined concrete and 0.25 to 0.45 mm for unconfined concrete. Bond strength and slippage of rebars vary in compressive and pulling forces. This is due to the axial behavior of the rebar's head in compression. The compressive axial force of the bar may increase the bonding strength up to 10 percent, comparing to pulling axial force.

Cement based material such as mortar and concrete are brittle in nature and crack under low tensile stress and strain levels. Adding discontinuous fibers as reinforced concrete remedy some concern related to cement based material brittleness and poor resistance to crack growth. After cracking the fibers arrest between two crack faces and provide mechanisms that abate their unstable propagation []. Fibers bridging force is achieved by transmission of the bond interfacial stress between the fiber and surrounding matrix. The resistance of the section to further crack opening depends largely on the fiber pullout mechanisms and related possibilities including complete fiber pullout or fiber fracture []. The high levels of interfacial shear strength may prevent fibers from complete debonding and result in fiber fracture. Although the strength of composite may increase, its toughness reduces significantly and failure is brittle. On contrary the low interfacial shear strength causes complete fiber debonding from matrix and fiber pullout. The effectiveness of fiber is often assessed by using single fiber pullout test. The experiments have shown that in improving the pullout resistance, hook-end fiber is more effective than straight fiber [,, ].The pullout process of hooked-end fibers is more complex than that of straight steel fibers and there is one additional deformation mechanism because of mechanical anchorage. So the analytical models for straight fiber are not valid for the fibers having mechanical anchorage. The main objective of this paper is to develop an analytical model for hook-end steel fiber pullout behavior. In this model the concept of bond shear stress versus slip relation between fiber and matrix has been used to develop fiber force and bond stress. Also the interfacial stress has been supposed that to be distributed uniformly. Based on two mentioned assumption a theoretical relation have been developed for aligned straight fiber at first. Then this relation is promoted for hook ended steel fiber pullout response. In order to do this, the effect of hooks on force and stress distribution has been analyzed along the fiber length and utilized for developing the pullout response of hook ended steel fiber. Based on obtained relation, the hooks change the fiber along the fiber length at the hooks and this force will be decreased with constant coefficient which is the function of fiber geometry. Despite that a normal force and its frictional force will be occurred at the hook bent. Decreasing the fiber force and creating a normal force at the hook bend are the factors that create an extra resistance force against the pullout in hook-end fiber. This study investigates these factors and develops the relations in order to calculating the maximum load required for pulling out the hook-end fiber. Finally the model has been validated by experimental results on the hook-end steel fiber. Proposed model is able to estimate the main pullout mechanism due to mechanical anchorage of hooks.

Back pain is a common medical problem. There is no clear cause for the back pain problem so far, but in most cases, spinal instability can be noted. Lumbar spine fixation is performed to treat the problems of low back pain. Spinal fixation can be done with or without surgery. One of the surgical methods is the use of spinal screws in which the strength and stability of the screw are of great importance. The strength and stability of the screw in the bone reduces the time and cost of treatment, reduces the amount of bleeding and accelerates the patient's treatment. In this study, screws were inserted using a digital torque meter. An impact was applied using an impact hammer and resonated sound was recorded using a microphone. The vibration mode of the screw was obtained by processing the signal generated by MATLAB R2017 software and plotting the fast Fourier transform. Finally, tensile test was performed to obtain the ultimate pull-out force. The innovation of this study was to use modal analysis method and to correlate its results with that of the ultimate pull-out force and peak insertion torque. In this study, five screws with different screw depth, and screw thread crest thickness were examined. Also, the effect of self-tapping was investigated. The peak insertion torque, ultimate pull-out strength and natural frequency occurred at 182 Nm, 992 N and 1916 Hz, respectively, for the cylindrical pedicle screw. By comparing the obtained data, results showed a linear relationship between insertion torque and pull-out force of the screws. Due to the lack of significant difference between natural frequency and pull-out force of the self-drilling and non-self-drilling tip screws (comparing between screws number 3 and 4 and between 1 and 5), the use of self-tapping screws can be advantageous. The trend of the dependent parameters in all three methods i.e. insertion torque, pull-out force and natural frequency are the same, indicating the non-destructive advantage of modal analysis in in-vivo surgical application.