210
CHAPTER 5. COASTAL STRUCTURE MODELS
Even in cases where combined impact and nonimpact stresses cause fracture, the breakable units can provide useful information because the model
failure stress can be converted to prototype with the more conservative scale
factor. As pointed out by Mansard (1990), the main advantage to strengthsimulated armor units is better interpretation of results as compared to the
method of monitoring rocking motions. Several additional aspects related
to strength-scaled armor units are listed below:
• Breakable units can only provide information on whether
a particular design of breakwater will lead to stress levels
which may trigger breakage (Mansard 1990). No information is obtained on stress levels below the fracture stress.
• The strength-simulated armor units are fragile and usually
can only be used once. The units are more expensive than
conventional model units, so tests with the breakable units
are conducted only in the nearly final tests, and with small
sections (Mansard 1990). Zwamborn (1987) pointed out
that a 3-d breakwater stability test could require upward
of 5000 units, which would be entirely impractical.
• Initial static stresses that are developed in prototype armor
units during manufacture are not present in the strengthsimulated units (Boer 1985). This may be significant in
very large armor units where static stresses consume much
of the reserve strength.
Measurement of Armor Unit Stress
Strength-simulated model armor units provide an indication of potential
breakage that may occur on a prototype structure; however, no information
is gained about stresses below the critical level unless the model units are
directly instrumented. Developments in the area of instrumenting model
armor units for stress determination have been driven by the need to develop
design procedures that include consideration of armor unit strength as well
as hydraulic stability (Melby 1990).
Two methods used to measure stress in model armor units are (1) externally mounted strain gauges, and (2) internally placed load cells. Burcharth, et al. (1991) reported that external strain gauges had been used in
Sweden by Sandstrom in 1974; however, attempts by Burcharth to utilize
external strain gauges on 200-kg dolosse in 1987 were unsuccessful because
the strains were too small to give an accurate stress estimate. An additional problem with using surface mounted gauges was protecting the
gauge against damage during testing. It appears that Delft Hydraulics
CHAPTER 5. COASTAL STRUCTURE MODELS
Even in cases where combined impact and nonimpact stresses cause fracture, the breakable units can provide useful information because the model
failure stress can be converted to prototype with the more conservative scale
factor. As pointed out by Mansard (1990), the main advantage to strengthsimulated armor units is better interpretation of results as compared to the
method of monitoring rocking motions. Several additional aspects related
to strength-scaled armor units are listed below:
• Breakable units can only provide information on whether
a particular design of breakwater will lead to stress levels
which may trigger breakage (Mansard 1990). No information is obtained on stress levels below the fracture stress.
• The strength-simulated armor units are fragile and usually
can only be used once. The units are more expensive than
conventional model units, so tests with the breakable units
are conducted only in the nearly final tests, and with small
sections (Mansard 1990). Zwamborn (1987) pointed out
that a 3-d breakwater stability test could require upward
of 5000 units, which would be entirely impractical.
• Initial static stresses that are developed in prototype armor
units during manufacture are not present in the strengthsimulated units (Boer 1985). This may be significant in
very large armor units where static stresses consume much
of the reserve strength.
Measurement of Armor Unit Stress
Strength-simulated model armor units provide an indication of potential
breakage that may occur on a prototype structure; however, no information
is gained about stresses below the critical level unless the model units are
directly instrumented. Developments in the area of instrumenting model
armor units for stress determination have been driven by the need to develop
design procedures that include consideration of armor unit strength as well
as hydraulic stability (Melby 1990).
Two methods used to measure stress in model armor units are (1) externally mounted strain gauges, and (2) internally placed load cells. Burcharth, et al. (1991) reported that external strain gauges had been used in
Sweden by Sandstrom in 1974; however, attempts by Burcharth to utilize
external strain gauges on 200-kg dolosse in 1987 were unsuccessful because
the strains were too small to give an accurate stress estimate. An additional problem with using surface mounted gauges was protecting the
gauge against damage during testing. It appears that Delft Hydraulics