@_Damping of Horizontal and Uplift Forces
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DAMPING OF HORIZONTAL AND UPLIFT FORCES ON HORIZONTALLY COMPOSITE CAISSON BREAKWATERS Andreas KORTENHAUS *) ; Hocine OUMERACI *) *) Leichtweiß-Institut, Technical University of Braunschweig, Beethovenstr. 51a, 38106 Braunschweig, Germany, e-mail: [email protected] 1. INTRODUCTION For many existing vertical breakwaters where higher risks for impact loadings were observed constructional measures are needed to reduce the loading due to impacting waves (OUMERACI, 1994) and thus leading to a reduction of the dimension and costs of the structure. Horizontally composite breakwaters where a composite breakwater is covered by concrete blocks are increasingly used to fulfil this purpose. Especially in Japan, this type of breakwater is rather com- mon so that they are very often named as 'Japanese type breakwater'. A typical cross section of this type of break- waters is given in Figure 1. h h h Bb Caisson DWL B crown R H 1:m 1:m 1 1:m 2 d 0 s d B bottom B h r Sand d sand Tetrapod layer B average b 0 c b c b DWL Figure 1: Typical cross section of a horizontally composite breakwater (Japanese type) The main purposes of these structures are as follows: exclude impact forces and to reduce the wave forces acting on the structure (horizontal force F h and uplift force F u ); reduce the wave reflection from the structure; protect the structure against scour. In order to verify the existing Japanese design method for this type of structure (TANIMOTO AND TAKAHASHI, 1994) large-scale hydraulic model tests were conducted in 1994 to measure wave-induced horizontal and uplift forces on a vertical model breakwater. Comparisons were per- formed for a structure with and without a damping layer in front of the structure. 2. TEST CONDITIONS The hydraulic model tests in the Large Wave Flume of Han- nover (GWK) have been performed in the frame of a basic research programme called 'Sonderforschungsbereich 205 / TP B3' (Special Research Unit 205). This project was aim- ing at a verification of the aforementioned formulae and possibly to build a basis for further more systematic research programmes. Water levels, wave heights and wave periods have been varied throughout the tests whereas the width of the berm and of the rubble layer in front of the breakwater, the di- mensions of the structure and the slopes of both the fore- shore and the berm were kept constant. Detailed descriptions of the model setup and which tests have been selected for analysis and how comparisons be- tween tests have been made will be described in the paper. This will also include how tests were synchronised and how data are separated to be impact or non impact waves. A number of 18 data sets (only random waves) were created where the same tests (identical water level and wave pa- rameters) were performed for a vertical structure without damping layer (undamped case) and a horizontally compos- ite breakwater with a damping layer made of concrete blocks. Recorded data comprised water elevation at the structure (wave gauges), horizontal pressures at the front wall, verti- cal pressures underneath the bottom of the caisson, dis- placement of the caisson itself, and acceleration of the cais- son. More detailed information on test setup and results of the analysis are given in HOLM, 1998. 3. RESULTS Typical results of various tests conducted with random waves will be shown in the paper. It will be seen that not only the dynamic horizontal forces but also the dynamic up- lift forces are damped significantly. Furthermore, it should be noted that the damping increases with increasing magni- tude of the dynamic loading (see Figure 2 and Figure 3).
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Transcript of @_Damping of Horizontal and Uplift Forces
DAMPING OF HORIZONTAL AND UPLIFT FORCES ONHORIZONTALLY COMPOSITE CAISSON BREAKWATERS
Andreas KORTENHAUS*); Hocine OUMERACI*)
*) Leichtweiß-Institut, Technical University of Braunschweig, Beethovenstr. 51a,38106 Braunschweig, Germany, e-mail: [email protected]
1. INTRODUCTION
For many existing vertical breakwaters where higher risksfor impact loadings were observed constructional measuresare needed to reduce the loading due to impacting waves(OUMERACI, 1994) and thus leading to a reduction of thedimension and costs of the structure. Horizontally compositebreakwaters where a composite breakwater is covered byconcrete blocks are increasingly used to fulfil this purpose.Especially in Japan, this type of breakwater is rather com-mon so that they are very often named as 'Japanese typebreakwater'. A typical cross section of this type of break-waters is given in Figure 1.
h
h
h Bb
Caisson
DWL
B crown
R
H
1:m
1:m 1
1:m2
d0s
d
B bottom
B
hr
Sandd sand
Tetrapodlayer
Baverage
b0 c
b
c
b
DWL
Figure 1: Typical cross section of a horizontally compositebreakwater (Japanese type)
The main purposes of these structures are as follows: exclude impact forces and to reduce the wave forces
acting on the structure (horizontal force Fh and upliftforce Fu);
reduce the wave reflection from the structure; protect the structure against scour.
In order to verify the existing Japanese design method forthis type of structure (TANIMOTO AND TAKAHASHI,1994) large-scale hydraulic model tests were conducted in1994 to measure wave-induced horizontal and uplift forceson a vertical model breakwater. Comparisons were per-formed for a structure with and without a damping layer infront of the structure.
2. TEST CONDITIONS
The hydraulic model tests in the Large Wave Flume of Han-nover (GWK) have been performed in the frame of a basicresearch programme called 'Sonderforschungsbereich 205 /TP B3' (Special Research Unit 205). This project was aim-ing at a verification of the aforementioned formulae andpossibly to build a basis for further more systematic researchprogrammes.
Water levels, wave heights and wave periods have beenvaried throughout the tests whereas the width of the bermand of the rubble layer in front of the breakwater, the di-mensions of the structure and the slopes of both the fore-shore and the berm were kept constant.
Detailed descriptions of the model setup and which testshave been selected for analysis and how comparisons be-tween tests have been made will be described in the paper.This will also include how tests were synchronised and howdata are separated to be impact or non impact waves.
A number of 18 data sets (only random waves) were createdwhere the same tests (identical water level and wave pa-rameters) were performed for a vertical structure withoutdamping layer (undamped case) and a horizontally compos-ite breakwater with a damping layer made of concreteblocks.
Recorded data comprised water elevation at the structure(wave gauges), horizontal pressures at the front wall, verti-cal pressures underneath the bottom of the caisson, dis-placement of the caisson itself, and acceleration of the cais-son. More detailed information on test setup and results ofthe analysis are given in HOLM, 1998.
3. RESULTS
Typical results of various tests conducted with randomwaves will be shown in the paper. It will be seen that notonly the dynamic horizontal forces but also the dynamic up-lift forces are damped significantly. Furthermore, it shouldbe noted that the damping increases with increasing magni-tude of the dynamic loading (see Figure 2 and Figure 3).
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Time t [s]
F h = 0.6
F h = 1.2
Hor
izon
tal
forc
eF
h[k
N/m
]
without damping layer
with damping layer
µ D,h≈ 50 %
Figure 2: Damping for horizontal forces induced by quasi-standing waves
-10
-5
0
5
10
15
20
25
30
35
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Time t [s]
with damping layer
without damping layer
F h = 4.1
F h = 0.9
µ D,h ≈
Hor
izon
tal
forc
eF
h[k
N/m
]
80 %
Figure 3: Damping for horizontal forces induced by impactwaves
It was therefore necessary to distinguish between non impactand impact cases where a simple method was developed inthe time domain to filter the highly dynamic impacts.
The analysis of horizontal and uplift forces as well as theirrelated damping ratio (defined as 1 – Fh,damped / Fh,undamped)generally followed three different ways (KORTENHAUS &OUMERACI, 1999):
a) Comparison of obtained damping ratios to themethod of Tanimoto for non breaking waves;
b) Derivation of semi-empirical formulae for thedamping ratios for horizontal and uplift forces;
c) Statistical comparison of force distributions fordamped and undamped cases and derivation of sta-tistical transfer function between both cases.
The analysis cases A-H which have been performed with themeasured data using these methods are shown in Table 1.
Details of the analyses and formulae developed on the basisof these tests will be presented during the conference. Oneexample graph for case C in Table 1 is shown in Figure 4:
Table 1: Overview of data analysis cases A-HNon impact Impact
Horiz. Uplift Horiz. UpliftComp. Tanimoto A BDamping ratio C D E FStatist. Approach G H
0.00 0.05 0.10 0.15 0.20 0.25
0.25
0.50
0.75
1.00
b0/Lhs [-]
µd,
h[-
]
GWK, Random wavesno Fhq1.0 < Fhmax /Fhq <= 1.51.5 < Fhmax /Fhq <= 2.02.0 < Fhmax /Fhq <= 2.5Fhmax /Fhq > 2.5
Proposed damp. ratio
Tanimoto test results
Figure 4: Damping ratio for horizontal forces vs. effectivelength of damping layer for non-impact waves(case C)
ACKNOWLEDGEMENTS
This study has been funded by the German Research Coun-cil, Bonn (DFG) under project no. OU 1/1-1. Additionalsupport for further analysis of the data was given by theMAST III PROVERBS project (MAS3-CT95-0041) whichis also gratefully acknowledged.
REFERENCES
HOLM, A. (1998): Design of vertical and composite break-water foundation subject to dynamic loading. Part A:Wave Loading and Damping of Wave Loads, ResearchReport, MAST III, PROVERBS-Project: ProbabilisticDesign Tools for Vertical Breakwaters, Braunschweig,56 pp., 6 Annexes.
KORTENHAUS, A.; OUMERACI, H. (1999): Horizontallycomposite caisson breakwaters. Final Proceedings,MAST III, PROVERBS project: Probabilistic DesignTools for Vertical Breakwaters, Vol. IIa: Hydraulic As-pects, Chapter 8.2, 29 pp.
TANIMOTO, K.; TAKAHASHI, S. (1994): Design andconstruction of caisson breakwaters - the Japanese expe-rience. Coastal Engineering, Special Issue on 'VerticalBreakwaters', Editors: Oumeraci, H. et al., Amsterdam,Holland: Elsevier Science Publishers B.V., Bd. 22,Nr. 1/2, S. 57-77.
