FOREWORDAn extensive Round Robin test programme on compressive softening was carried out by the RILEM Technical Committee 148-SSC "Test methods for the Strain Softening response of Concrete". The goal was to develop a reliable standard test method for measuring strain softening of concrete under uniaxiat compression. The main variables in the test programme were the specimen slenderness hid and the boundary restraint caused by the loading platen used in the experiments. Both high friction and low friction loading systems were applied. Besides these main variables, which are both related to the experimental environment under which softening is measured, two different concretes were tested: a normal strength concrete of approximately 45 MPa and a higher strength concrete of approximately 75MPa. In addition to the prescribed test variables, due to individual initiatives, the Round Robin also provided information on the effect of specimen shape and size. The experiments revealed that under low boundary friction a constant compressive strength is measured irrespective of the specimen slenderness. For high friction loading systems (plain steel loading platen), an increase of specimen strength is found with decreasing slenderness. However, for slenderness greater than 2 (and up to 4), a constant strength was measured. The shape of the stress-strain curves was very consistent, in spite of the fact that each labora-tory cast its own specimens following a prescribed recipe. The pre-peak behaviour was found to be independent of specimen slenderness when low friction loading platens were used. However, for all loading systems a strong increase of (post-peak) ductility was found with decreasing specimen slenderness. Analysis of the results, and comparison with data from literature, showed that irrespective of the loading system used, a perfeet localization of deformations occured in the post-peak regime, which was first recognised by Van Mier in a series of uniaxial compression tests on concrete between brushes in 1984.Based on the results of the Round Robin, a draft recommendation will be made for a test procedure to measure strain softening of concrete under uniaxial compression. Although the post-peak stress-strain behaviour seems to be a mixture of material and structural behaviour, it appears that a test on either prismatic or cylindrical specimens of slenderness hid = 2, loaded between low friction boundaries (for example by inserting sheets of teflon between the steel loading platen and the specimen), yield.; reproducible results with relatively low scatter. For normal strength concrete, the closed-loop test can be controlled by using I the axial platen-to-platen deformation as a feed-back signal, ] whereas for high-strength concrete either a combination of axial] and lateral deformation should be used, or a combination of] axial deformation and axial load.
Geosynthetic reinforced soil (GRS) for bridge support was developed through the cooperative efforts of the University of Colorado at Denver, the Colorado Department of Transportation, and the Federal Highway Administration (FHWA), and was supported in recent years by a nationally funded research program. Most recently, the FHWA Turner-Fairbank Highway Research Center developed the Integrated Bridge System (IBS), a relatively fast, cost-effective method of bridge support that blends the roadway into the superstructure using GRS technology. Through the FHWA's Every Day Counts (EDC) initiative, the IBS is being rapidly deployed across the country, with about 100 in design or construction to date. This paper provides the instrumentation plan and the initial performance observations of the first wall of this type constructed in Minnesota. A robust monitoring program was developed to capture performance related to settlement, wall distortion, and pressure and deformation associated with the concrete box beams under thermal cycling. Automated monitoring will continue for a three-year period following construction.
The loading response of a single vertical pile was calibrated against strain gage load-history data acquired at a highway bridge abutment located on Steele County Highway 7 in Owatonna, MN. At this site, H-type piles were driven to a weathered bedrock layer and soil surcharging was used to reduce the anticipated settlement of a 15 m (50 ft) thick layer of clayey sand overburden. Evaluating the load in the bridge piling for this case study and subsequent sensitivity modelling provides a comprehensive case study for use as background to the Minnesota Department of Transportation's revised dragload design guidance. The simulation was performed with FLAC3D and the goal of the model was to investigate pile behavior subjected to negative skin friction (downdrag) from approximately 9m (30ft) of abutment backfilling being placed around the stickup length of the pile. The simulation approximated the construction sequence of backfilling by applying a layer-by-layer backfilling approach and approximated structural loading by applying a direct axial force to the top of the pile. A sensitivity analysis was conducted from the calibrated case by changing the stiffness of the strata along the frictional portion of the pile and in the endbearing strata. The variation in maximum force along the pile as well as the position of the neutral plane was observed by varying soil stiffness. The relative stiffness was defined as the ratio between the average Young's modulus along the shaft of the pile and the end-bearing stratum's modulus. The relative stiffness influences the amount of dragload, axial force distribution along the pile and the location of the neutral plane. From the simulations it was observed that for a relative stiffness below 0.1 (very stiff base layer), the neutral plane is at the bottom of the pile and maximum possible dragload forces are realized. At a relative stiffness above 10 (very soft base layer), the neutral plane is near the top of the pile and the drag load force is minimal. This research suggests that for many Minnesota state transportation projects the dragload is centered between the extreme cases. Findings indicate for piles driven to stiff rock, dragload must be evaluated to ensure pile structural capacity is sufficient; historically this check was often ignored.Keywords: Negative Skin Friction, Downdrag, Dragload, FLAC3D, Driven Piles Lucarelli, Blanksma, Dasenbrock, Peterson 3 INTRODUCTION Downdrag forces, dragload, on driven piles may be caused by a variety of site conditions; often dragload is a result of placing fill material on top of a consolidating soil layer near the pile which induces downdrag (a downward deformation of the piling). Design for piles or pile groups should consider the effect of downdrag in order to properly evaluate criteria for strength and service limit states in LRFD design. Current practice is still evolving and at present design guides may non-conservatively underestimate, or over-estimate, the maximum load on the pile if downdrag effects are not treated appropria...
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