12th Congress INTERPRAEVENT 2012 Grenoble / France – Extended Abstracts www.interpraevent.at
DYNAMIC RESPONSE OF STEEL SPECIMEN UNDER DEBRIS FLOW SURGE MODEL
Nobutaka Ishikawa1,Ryuta Inoue2, Masuhiro Beppu3,Yuji Hasegawa4, Takahisa Mizuyama5
INTRODUCTION
Recently many debris flow disasters have been occurred by torrential rain fall due to the abnormal weather. Authors (2008, 2010) have examined the dynamic load of debris flow model in order to reproduce the surge formation. The objective of this study is to investigate the dynamic behaviour of a dam structure under the surge debris flow. First, the debris flow models with three different types are provided in the hydrodynamic channel test in order to examine the surge formation. Herein, the dynamic load-time relations of these debris flow models are measured by the load cell and the test loads are compared with the design load. Secondly, the dynamic response of a steel specimen (plate) under debris flow surge model is examined by measuring the strains of specimen and by increasing the thickness of plate.
DYNAMIC LOAD TEST
Figure 1 shows the hydro-dynamic channel test set-up with a slope of 18 degrees. The flow load is measured by the load cell. The debris flow models are provided as three different types, i.e.,Type A:
only water, Type B: the ratio of gravel (16-19mm) 70% and sand (less than 2mm) 30%, and Type C:
the ratio of gravel (16-19mm) 30% and sand (less than 2mm) 70%, which are flown from the distance of 5m as shown in Fig.1(b). The Sabo dam model is made by the steel specimen with length of 300mm,thickness of 2mm and width of 100mm.
Fig. 1 Dynamic load test of debris flow by hydrodynamic channel test
LOAD-TIME RELATION
Figure 2 shows the load-time relations of Types A, B and C. Type A (only water) has no clear peak load in the load-time relation and the precise surge formation was not seen in the front. Type B (gravel 7: sand 3) illustrates the very steep rise time in the load-time relation and the complete surge
1 Research Advisor, Research Association for Steel Sabo Structures, 2-7-5 Hirakawa-cho, Chiyoda-ku, Tokyo,102-0093, Japan ( e-mail:cgishikawa@m4.dion.ne.jp)
2 Civil Engineer, Kyose-Kiko,1-23-1 Shinjyuku-ku, Tokyo160-0022,Japan
3 Associate Professor, Department of Civil and Environmental Engineering, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka 239-8686, Japan
4 Senior Researcher, Civil Engineering Research Laboratory, 904-1 Tohhigashi, Ibaraki,300-2633,Japan
5 Professor, Department of Erosion and Control Engineering, Graduate School of Agriculture, Kyoto University, Kitashirakawa, Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan
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shape is formed in the front flow motion. Type C (gravel 3: sand 7) has very slow rise time in the load-time relation and the surge formation was not so clearly found in the front motion. The design load (F=ρAv2,ρ: debris flow density, A:
sectional area, v: flow velocity) is compared with the test results in which the constant load after the peak load is almost in agreement with the design load in Types A and B. Herein, ρ=1.0-1.55g/cm3, A=300cm2,v=1.61-2.70m/sec and Froude number Fr =2.1-6.6 were used.
Fig. 2 Load-time relations
DYNAMIC RESPONSE TEST OF PLATE
Fig.3 Strain-time relation of plate Fig.4 Maximum strain-time relation Fig.5 Effect of steel thickness on (d=0.5mm) of Types A,B,C (d=0.5mm) strain-time relation (Type B) The dynamic response tests under debris models were performed for steel plates (length of 300mm, width of 100mm, thickness of d=0.5mm and 2.0mm). Figure 3 shows the strain-time relation of steel plate pasted by strain gages (h=5, 10, 15cm from the bottom) under the debris flow of Type B. Figure 4 illustrates the maximum strain-time relations of Types A, B and C, where the maximum strain is measured at 5cm from the bottom. It was found that the debris flow of Type B gives remarkable influence on the maximum strain, but Types A and C don’t so much effect the maximum strain.
However, it has been confirmed that if the thickness of the plate is increased, the dynamic response may be significantly reduced as shown in Fig.5, even if the surge debris flow acts on the steel plate.
CONCLUSIONS
It was confirmed that surge shape was well formed in Type B (the ratio of gravel of 70% and sand of 30%) in which the load-time relation shows a very steep rise time and the large peak load. The maximum strain was showed at the bottom of plate in which this may be caused by the flexural action of the steel plate. The maximum strain of plate under debris flow of Type B illustrates the large dynamic response of steel plate in which this phenomenon was caused by the surge formation of debris flow. It was confirmed that the dynamic response was significantly reduced by increasing the thickness of the steel plate.
REFERENCES
Ishikawa, N. et al. (2008). Experimental approach on measurement of impulsive fluid force using debris flow model,INTERPRAEVENT 2008 Conference Proceedings, Vol.1, p.343-354.
Ishikawa, N. et al. (2010). Dynamic load characteristics of debris flow model using different gavel size distribution, INTERPRAEVENT 2010, Symposium Proceedings, April 2010,Taipei,Taiwan, p.207-216.
Keywords: dynamic response, debris flow model, surge formation, steel specimen
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