Ultimate Pullout Load Research Articles

Mechanical behavior of screws in normal and osteoporotic bone

Fracture fixation in severe osteoporotic bone by means of implants that rely on screw anchorage is still a clinical problem. So far, a sufficiently accurate prediction of the holding capacity of screws as a function of local bone morphology has not been obtained. In this study the ultimate pullout loads of screws in the epi-, meta-, and diaphyseal regions of human tibiae were correlated to the cortical thicknesses and cancellous bone mineral densities at the screw axes determined from QCT densitometric data. Stepwise multiple linear regression showed that in regions with cortical thicknesses below 1.5 mm, cancellous density determined the ultimate pullout load (R2 = 0.85, p < 0.001), while in regions with cortices above 1.5 mm, cortical thickness alone significantly influenced the holding capacity of a screw (R2 = 0.90, p < 0.001). The findings of this study provide a basis for a bone morphology-related pre-operative estimation of the holding capacity of screws, which could help to improve their proper application in osteoporotic bone.

Response of anchors to variations in displacement-based loading

The results of an experimental investigation on the effect of variations in displacement-based loading on the pullout capacity of anchors are presented. A single-pitch, screw helical anchor was used in the testing. The tests were conducted in dry, well-graded, dense, medium, and loose sands with relative densities of 79, 47, and 19%, respectively. Anchors were installed to, and tested at, shallow and deep depths. Measurements of ultimate pullout load and displacement at failure were made. The values of time-to-failure and mean loading rate were calculated and presented. The results of this study show that variations of displacement-based loading had some effect on the pullout capacity of shallow anchors but had only a slight effect on deep anchors. For a given depth of installation and a given sand state, the ultimate pullout load of an anchor decreases with increasing loading rate. This effect was more pronounced in anchors installed to shallow depths.Key words: anchors, displacement, displacement rate, pullout capacity, strain rate, sand, foundations, uplift.

Three-Dimensional Lower Bound Solutions for Stability of Plate Anchors in Clay

Soil anchors are commonly used as foundation systems for structures that require uplift or lateral resistance. These types of structures include transmission towers, sheet pile walls, and buried pipelines. Although anchors are typically complex in shape (e.g., drag or helical anchors), many previous analyses idealize the anchor as a continuous strip under plane strain conditions. This assumption provides numerical advantages and the problem can be solved in two dimensions. In contrast to recent numerical studies, this paper applies three-dimensional numerical limit analysis to evaluate the effect of anchor shape on the pullout capacity of horizontal anchors in undrained clay. The anchor is idealized as either square, circular, or rectangular in shape. Estimates of the ultimate pullout load are obtained by using a newly developed three-dimensional numerical procedure based on a finite-element formulation of the lower bound theorem of limit analysis. This formulation assumes a perfectly plastic soil model with a Tresca yield criterion. Results are presented in the familiar form of break-out factors based on various anchor shapes and embedment depths, and are also compared with existing numerical and empirical solutions.

Performance Evaluation of Nonwoven Geotextiles in Soil-Fabric Interaction

A laboratory research program was undertaken to determine the influence of fabric mass and normal pressure on the stress-strain characteristics of a geotextile-soil inter face. Pullout tests were done in a large pullout box using three needlepunched non woven geotextiles with the same apparent opening sizes and different mass/unit areas. A dry medium sand was the cover material. Effects of mass/unit area of the fabric and normal pressure on pullout test results were analyzed in terms of ultimate pullout load and strain at different points on the geotextile specimens. Pullout tests were conducted at 7, 14, and 21 kPa normal pressures and at a constant displacement rate of 20 mm/min. Pullout resistance decreased with increased gootextile mass / unit area due to extensive necking of the geotextiles. Pullout resistance also increased with increasing normal pressure. Effective embedment length was determined at each normal pressure level based on the strain developed along the length of the geotextiles.

Effects of Sustained and Repeated Tensile Loads on Geogrid Embedded in Sand

Abstract Design methods for reinforced soil structures under static loading conditions are relatively well established. Little research has been conducted on the behavior of embedded geosynthetics subjected to repeated loadings. Such research is needed to improve the design of reinforced soil structures subjected to traffic and seismic loadings. The study reported here is related to the long-term performance of a geogrid embedded in Ottawa sand under several confining pressures and subjected to various tensile loads. Different magnitudes of sustained and repeated tensile loads were applied to the geogrid incrementally using a pullout device. The confining pressure increased the soil-geogrid interface friction force and thus affected strain distribution along the geogrid length. Creep developed in the geogrid as the applied tensile load increased. The geogrid creep strain rate at a given tension force was found to be independent of confining pressure at the point of this force application, that is, creep can be viewed as an intrinsic property of the geogrid. A rapid pullout failure of geogrid occurred as the applied sustained load approached the ultimate pullout capacity. Conversely, under repeated loading the pullout occurred progressively. The ultimate pullout load and interaction coefficient, Ci, obtained from repeated loading tests were about 20% less than the values obtained from sustained loading tests. This suggests that a Ci smaller than that obtained in static (conventional) tests should be used in structures subjected to dynamic load. Creep under repeated load was smaller than that under sustained load.

Model investigation of the performance of single anchors and groups of anchors

Experimental investigations on the performance of single and groups of vertical screw anchors installed in dense, medium, and loose sands are presented. An experimental setup was instrumented to allow the measurement of the total pullout load, upward displacement, sand surface deflection, and stress development in the sand layer during all phases of testing. A sand placing technique was developed and utilized over all the testing program to ensure reproducibility of the predetermined unit weight. Stresses measured within sand deposits indicated that the tested sands were overconsolidated due to the application of mechanical compaction. Special tests were conducted on colored–layered sand to define the nature of the failure mechanism. The results of these tests, together with the measurements of the deflection of the sand surface, were employed to establish the shape of the rupture surface which could be represented by a segment of a logarithmic spiral. Groups of three, four, six, and nine anchors were tested in this investigation. The effect of installation depth, spacing between anchors, and sand characteristics on the ultimate pullout load of the group was examined. The experimental setup was instrumented to allow the measurement, of the total pullout load of the group as well as that of individual anchors in the group. Load distribution among the anchors of a group is discussed in terms of anchor location and the applied load level. At failure, all anchors contribute almost equally to the uplift capacity. Group efficiencies were calculated and compared. An installation procedure was proposed to avoid differential upward displacement during the uplifting process and to provide uniform load distribution on the different anchors of the group. Key words : anchors, failure mechanism, group action, model tests, sand, uplift capacity.

Ultimate Pullout Capacity of Shallow Vertical Anchors in Clay

ABSTRACT Model test results for the ultimate pullout capacity of shallow rectangular vertical anchor plates embedded in saturated clay have been presented. The width-to-height ratio of the anchor plates used in the tests has been varied from one to five. The ultimate pullout load of an anchor can be expressed in the form of a nondimensional breakout factor. Based on the experimental results, an empirical equation for determination of the ultimate pullout load has been presented.

Anchor Behavior in Sand

This paper describes an experimental investigation of the behavior of isolated anchors and groups of anchors in sand. It is concerned with medium scale laboratory tests. It is concluded that the ultimate pull out resistance of an anchor in sand depends on the standard variables such as diameter, depth of embedment, and sand density as reported by others, but also on the stress history of sand. Carefully controlled tests show that overconsolidation of the sand increases pull out resistance several fold. Quantitative measurements of sand movements near to the anchor were made. Prestressed anchors behave in a very different manner than dead anchors and test data quantify these differences. In groups, anchors interact. Generally the ultimate pull-out loads were less than those of an equal number of isolated anchors. Load distribution amongst the anchors was initially near uniform but with load increase the outer anchors carried larger loads than the central anchors. Movements of the group generally were greater than those of an isolated anchor, and depended primarily on applied load level.