204
CHAPTER 5. COASTAL STRUCTURE MODELS
On any particular rubble-mound structure all of the above listed loads
can be important contributors to armor unit stresses. Dynamic impact
loads are more critical for armor units placed in the upper layers because
they are less constrained by their boundary conditions and are more apt
to “rock” in response to the wave motion. Also, increased exposure in the
upper layer increases the risk of armor units being hit by smaller pieces or
other rocking armor units. Static and pulsating loads are more probable
in underlayers and in structures that are designed for a no-movement condition (Burcharth 1983). Prototype measurement of stresses in 38,000-kg
(42-short-ton) dolosse at Crescent City, California, indicated that armor
unit self-weight had created stresses that approached the limiting tensile
strength of the concrete (Melby and Howell 1989).
Complete similitude of concrete armor unit stress in a Froude-scaled
physical model requires that the model armor units be constructed from a
material in which all material properties are scaled by the Froude criterion.
Such scaling is very difficult to achieve; and when the material scaling
requirements are not met, stresses induced by static and pulsating loads
will scale differently than stresses caused by impacts. This means model
stresses due to combined pulsating and impact loads cannot be scaled unless
the relative proportions of each type of stress are known (Burcharth, et al.
1991). Fortunately, impact stresses are characterized by their sharp peak
and ultra-short duration, and thus, can be recognized in measured stress
records.
Scaling Material Properties. As mentioned, material stress similitude
under static, pulsating, and impact load conditions in a hydraulic model will
be achieved if the model armor unit material properties all scale according
to the Froude criterion used for the hydrodynamics. The relevant material
property scales for a Froude model7 are given in Table 5.1 (Timco 1981,
Timco and Mansard 1982).
7See Table 3.1 in Chapter 3.
Scaling Impact Load Stresses. Force loads that are applied relatively
slowly do not impart a shock wave through the material that makes up
the artificial armor unit, and the internal tensile, compressive, and flexural
stresses scale according to Froude similitude as
^p = NlaNL = NgNL
(5.37)
where crp represents stresses caused by static and pulsating loads, and the
material requirement that NPa = 1 gives Nya = Ng. Impacts, on the other
CHAPTER 5. COASTAL STRUCTURE MODELS
On any particular rubble-mound structure all of the above listed loads
can be important contributors to armor unit stresses. Dynamic impact
loads are more critical for armor units placed in the upper layers because
they are less constrained by their boundary conditions and are more apt
to “rock” in response to the wave motion. Also, increased exposure in the
upper layer increases the risk of armor units being hit by smaller pieces or
other rocking armor units. Static and pulsating loads are more probable
in underlayers and in structures that are designed for a no-movement condition (Burcharth 1983). Prototype measurement of stresses in 38,000-kg
(42-short-ton) dolosse at Crescent City, California, indicated that armor
unit self-weight had created stresses that approached the limiting tensile
strength of the concrete (Melby and Howell 1989).
Complete similitude of concrete armor unit stress in a Froude-scaled
physical model requires that the model armor units be constructed from a
material in which all material properties are scaled by the Froude criterion.
Such scaling is very difficult to achieve; and when the material scaling
requirements are not met, stresses induced by static and pulsating loads
will scale differently than stresses caused by impacts. This means model
stresses due to combined pulsating and impact loads cannot be scaled unless
the relative proportions of each type of stress are known (Burcharth, et al.
1991). Fortunately, impact stresses are characterized by their sharp peak
and ultra-short duration, and thus, can be recognized in measured stress
records.
Scaling Material Properties. As mentioned, material stress similitude
under static, pulsating, and impact load conditions in a hydraulic model will
be achieved if the model armor unit material properties all scale according
to the Froude criterion used for the hydrodynamics. The relevant material
property scales for a Froude model7 are given in Table 5.1 (Timco 1981,
Timco and Mansard 1982).
7See Table 3.1 in Chapter 3.
Scaling Impact Load Stresses. Force loads that are applied relatively
slowly do not impart a shock wave through the material that makes up
the artificial armor unit, and the internal tensile, compressive, and flexural
stresses scale according to Froude similitude as
^p = NlaNL = NgNL
(5.37)
where crp represents stresses caused by static and pulsating loads, and the
material requirement that NPa = 1 gives Nya = Ng. Impacts, on the other