3. harmonic ((, , y gp, s, rayleigh & love)...
TRANSCRIPT
3. HARMONIC
(P, S, Rayleigh & LOVE)( , , y g )
WAVESWAVES
George BouckovalasP f f N T U AProfessor of N.T.U.A.
October, 2016
The Seismic motion is due to …….
Pressure (P), Shear (S), Rayleigh (R) and other wave typespropagating through the ground (soil & bedrock) ...GEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.1
3.1 ELASTIC WAVESELASTIC WAVES ((P & S)P & S)in UNIFORM MEDIAin UNIFORM MEDIA
P WAVE S (SH SV) WAVE
in UNIFORM MEDIAin UNIFORM MEDIA
P- WAVEwith planar front
S (SH, SV) - WAVEwith planar front
ww
SV
SH
v v
u u
Compression - extension
P-waves
Wave length
S-wavesshearing
S waves
Wave lengthGEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.2
Wave equation dx
Wave equation . . .A
x dxx
2
x dxxx
u¶2
2
tu
x ux x
uΌμως σ Dε D
x
¶= =-
¶x, u
2 2x
2 2
σ u uκαι D ρ
x x t
¶ ¶ ¶=- =-¶ ¶ ¶x x t¶ ¶ ¶
DC
General solutionGeneral solution...
( )( )u f Ct x=
why;
Y Ct x=
( ) ( ) ( )22
2 2f Yu ¶¶
( ) ( ) ( )2 2
2 2
f YuC C f ''
t Y
¶¶ = =¶ ¶
¶ ¶2 22u u
άρα πράγματι
2 2
2 2
u f (Y )ενώ f ''
¶ ¶= =
¶ ¶=¶ ¶
22 2
u uC
t x
2 2f
x Y¶ ¶GEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.3
Physical meaning...Physical meaning...
The problem variables are NOT two independent The problem variables are NOT two independent ones (‘x’ and ‘t’) but a combined one: Y= x±Ct
If Y=Ct-x If Y=Ct+x
then, t=0 Y= -xo
t≠0 Y=Ct-xt=-xo
then, t=0 Y= xo
t≠0 Y=Ct+xt=xot 0 Y Ct xt xo& xt = xo + Ct
f(Ct-x) f(Ct+x)
t 0 Y Ct xt xo& xt = xo - Ct
tt=0
f(Ct x)
o)
t t=0
( )
f(-x
o
xo+Ctxo x xo-Ct xo x
P & S – wave propagation velocities& p p gin soil & rock formations
EGS- waves: 12
EG,GVs
P- waves:
21
1G2
211
1ED,DVp
21211
( / )VS (m/s) VP (m/s)dry saturated
loose-recent SOIL deposits <400 <1000 ≈1500
stiff SOILS – soft ROCKS 400-800 800-1600 1500-2000
Rocks >800 >1600 >2000GEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.4
Vibration velocity (at a point) VVibration velocity (at a point) V
f ( Ct )u f ( x Ct )
* *u u X u x XV u
* *V u
t t x tX X
xV C x
xά V C CD
D
Vibration velocity (at a point) V
f ( Ct )
Vibration velocity (at a point) V
u f ( x Ct ) ATTENTION:
* *u u X u x XV u
The velocity of VIBRATION is different
(2 to 3 orders of magnitude lower!)* *
V ut t x tX X
than the velocity of PROPAGATION
xV C x
xά V C CD
DGEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.5
Special Case: Harmonic wavesp m
( )( ) ( )
ωi Ct x i ωt kxCf Ct x Ae Ae
+ ++ = =( )af Ct x Ae Ae+ = =
( ) ( ) ( )ω
i Ct x i ωt kxCbf Ct x Ae Ae
- -- = =( )bf Ct x Ae Ae
where
k /C 2 /(Τ C) 2 /λ bk =ω/C = 2π/(Τ•C) = 2π/λ = wave number
Ground VIBRATION at a given POINT (i e x=x )
titiikxf * Harmonic vibration
(i.e. x=xο)
tio
tiikxba exueAef o *, with frequency
ω=2π/Τ
fa,b
T=2π/ω
tku*(xo) =Ae+ikxo
fa,b u*(xo) =Ae+ikxo
xxo
GEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.6
Ground DISPLACEMENT at a given INSTANT OF TIME (i e t=t )at a given INSTANT OF TIME (i.e. t=tο)
ikxikxti etueAef o * oba etueAef o ,
Όπως προηγουμένως αλλά τώρα τον ρόλο As before, but now the role of t ς ρ γ μ ς ρ ρτης συχνότητας τον παίζει ο κυματικός
αριθμός k
,frequency is played by
the wave number kΤ=2π/ωx=xkλ=2π => λ=2π/k
Τ=2π/ω
to
x=xo
[k= ω/C λ = 2π/ωC = T·C]u(xo,t)
xo
u(x) t=to
λx
EXAMPLE: Two rods in contact
x k kxti
oeuu 1
xkti2 eu
kxtiB kxtiBeu 1
12ρ C
ρ1,C1
ρ2,C2
i t kx i t kx i t kxou e u u e Be 2 1
x xu uM M M Mx x
2 12 1
i t kx i t kx i t kxoM ke Mu ke MBke GEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.7
EXAMPLE: Two rods in contact
Boundary conditions:
x=0 u1=u2 d l & x=0 u1=u2
σ1=σ2
displacement & stress compatibility
tititioo MBkekeMuekM o21 MBkekeMuekM
22
o C
C
M
M
k
kBu
h k /C M C2 d kΜ C
11CMk
where k=ω/C, M=C2ρ and kΜ=ωCρ
EXAMPLE: Two rods in contact
Buouou 22C1
o21
C
Buouou o
22
11 u
C
C1
C1
B
11
22o C
CBu 11C
S i l
o22
u
C
C1
2
Special cases:
F d C 0 B & Γ 2
11C
Free end: ρ2C2=0 B=uo & Γ=2uo
Fixed end: ρ2C2=∞ B=-u & Γ=0Fixed end: ρ2C2 B uo & Γ 0
GEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.8
3.2 ELASTIC WAVES IN nonELASTIC WAVES IN non--UNIFORM UNIFORM MEDIA ( ith i t f ) MEDIA ( ith i t f ) MEDIA (with interfaces) MEDIA (with interfaces)
SNELL’s LAW:SNELL’s LAW: 2121
C
sin
C
sin
C
sin
C
sin
2121 sspp CCCC
C1 > C2
VIBRATION AMPLITUDE of reflected & transmitted refracted wavesof reflected & transmitted- refracted waves
I th l d k im l f 1 D ti :
medium 1: ρ1 C1 medium 2: ρ2 C2
In the already known, simple case of 1-D wave propagation:
transmitted incident wave
medium 1: ρ1, C1 medium 2: ρ2, C2
wave
reflected wave Cwave 2 2
1 1. .
2 2
1
1refl inc
CC
u uC
Vibration amplitude u:
2 2
1 1
.
1
1 refl
C
uu u
. .
.
1
2
ftransm inc
inc
u uu
.2 2
1 11
incuCC
GEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.9
VIBRATION AMPLITUDE of reflected & transmitted refracted waves
The amplitude of vibration of the reflected and refracted of reflected & transmitted- refracted waves
pwaves in 2-D problems, is computed based on stress equilibrium and strain compatibility (continuity) at the interface. In this case, the amplitude is also a function of the angle of wave incidence.
EXAMPLE: SH waves (Richter 1958)
2 2
. 1 1
cos1
cosrefl
Cu C
θδ
2 2.
1 1
cos1
cosinc
CuC
2
1 1
. . 21refr reflu u
C
θπρ θαν
1
2 2. .
1 1
cos1
cosinc inc
Cu uC
C1>C2SHinc
Criticalrefraction angle: 121 C
sinsinsinsin
refraction angle:1
221
21
1C
i1i
Csinsin
CC
2
2
121
Cii
1C
sin1sin
21
1
2cr2
C
Csinsin
1
21cr2 C
Csin
C1>C2What happens for θ2 > θcr ?
Application:Application:
Seismic refraction method for geophysical exploration geophysical exploration ……….
GEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.10
3.3 SURFACE WAVESSURFACE WAVESWave length
αδιατάραχτο μέσοαδιατάραχτο μέσο
P-wave+
SV wave RayleighRayleighSV-wave+
FREE C/CS
Rayleighwave
SURFACECR=0.94CS
Ground displacements due to R-waves
( ) ( )RR Rz z2 5.8 i ωt k xλ λu u z x t iB 0 55 e e e
- - -ì üï ïï ï= = -í ý( )
( ) ( )RR Rz z2 5.8 i ωt k xλ λ
u u z ,x ,t iB 0.55 e e e
w w z x t B 1 66e 0 92e e- - -
= = í ýï ïï ïî þì üï ïï ïí ý( ) ( )RR Rw w z ,x ,t B 1.66e 0.92e e= = -í ýï ïï ïî þ
k / b w/wo & u/uo
/
kR=ω/CR wave numberλR=CRT wave lengthΒ = constant u/uoΒ = constant
λR z/λR
w/wo
R
u
w|w(0)||w(0)|
|u(0)|=1.6
GEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.11
LOVE WAVES (in layered soil profile)( y p )
They are SH waves “trapped” within the upper soft layer
of a non-uniform soil profile.of a non uniform soil profile.
CS11 S11
CS22
LOVE WAVES (in layered soil profile)( y p )
They are SH waves “trapped” within the upper soft layer
of a non-uniform soil profile.of a non uniform soil profile.
Ground displacement variation with depth
CS11 S11
CS22
GEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.12
( )1 L 1 L Lir k z ir k z i( ωt k x )1u Ae Be e- -= +
C
L
( )2 L Lir k z i( ωt k x )
2u Γe e- -=CS1
C
L Lk ω / C=
CS2
2 2L L
1 22 2
C Cr 1, r 1
C C= - = -
2 2s1 s2C C
( )1 L 1 L Lir k z ir k z i( ωt k x )1u Ae Be e- -= +
C( )2 L Lir k z i( ωt k x )
2u Γe e- -=CS1
C
1( ) : 0z H ύ ά
CS2
1 2 1 20 ( ) : ,z ά u u
1 1 0
0
ir kH ir kHAe Be substituting for
A B
1 1 1 1 2 2
0 , ,
0 :
A B
A G r B G r G r gives
2LC1 «Dispersion»
relationship
L22S 2L 2
2 2S1 1
1CC GωH
tan 1C C G C
-é ùê ú- =-ê úê úë û relationshipL S1 1 L
2S1
C C G C1
Cê úë û -
GEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.13
Observe that, since tan(*) must be a real number, the following condition must be satisfied for the previous following condition must be satisfied for the previous equation to have a real solution:
C C CCS1 < CLOVE < CS2
CCs2
2LC1
C
L22S 2L 2
2 2
1CC GωH
tan 1C C G C
-é ùê ú- =-ê úê úë ûCs1
0
L S1 1 L2S1
C C G C1
Cê úë û -
0 ΩΗ/CL
In addition, LOVE waves are not possible unless the surface layer is softer than the underlying mediumy y g(i.e. CS1 < CS2).
Dispersion: Code name for LOVE (and other) waves h h h b f d d l which exhibit a frequency dependent propagation velocity
(even for a uniform medium).
Implications of dispersion:
The propagation velocity CLOVE decreases with increasing frequency ω (decreasing period Τ).q y g pAs a result….. the low-frequency components of the excitation get separated from the high-frequency excitation get separated from the high-frequency components, as they propagate faster. Thus:
Th d ti f h ki i ith di t f The duration of shaking increases with distance from the source
Th f t t f th ti ( th l ti The frequency content of the motion (e.g. the elastic response spectrum) changes with distance from the sourcesource
GEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.14
Special Case:Soft soil layer upon BEDROCK (i.e. CS2=∞)
CLOVE
CSH2122
LOVE
H
HC
bedrock
2
sC
bedrock
Fi ll LOVE
CLOVE
Finally, LOVE waves cannot exist for f i l th th frequencies lower than the fundamental vibration frequency of the soft soil
CS
frequency of the soft soil layer, i.e. when
Critical frequency
π/2 ωΗ/CS
ω < ωC= (π/2) Cs/HCritical frequencyωc= (π/2) CS/H
GEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.15
PROBLEM SOLVING FOR CHAPTER 3PROBLEM SOLVING FOR CHAPTER 3PROBLEM SOLVING FOR CHAPTER 3PROBLEM SOLVING FOR CHAPTER 3
HWK 3.1:HWK 3.1:The free end of an infinite rod is displaced according to the The free end of an infinite rod is displaced according to the following relationship:
U=0 for t<0U=t (cm) for 0<t<0.1sU=0 2 t for 0 1s<t<0 2sU=0.2-t for 0.1s<t<0.2sU=0 for 0.2s<t
Find the displacement variation along the rod at t=0.3s. Assume that the wave propagation velocity is 300m/s.
(aπό «Σημειώσεις Εδαφοδυναμικής» Γ. Γκαζέτα)
GEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.16
HWK 3 2:HWK 3 2:HWK 3.2:HWK 3.2:
To isolate a precision measurement laboratory (e.g. a geotech lab.) from traffic vibrations it is proposed to Α traffic vibrations, it is proposed to construct the trench ΑΑ’. Which of the following two fill materials is Lab
Α
gpreferable :(α) concrete, with ρ=2.5 Mgr/m3 &
Lab
C=2000 m/s, or(b) pumice, with ρ=0.8 Μgr/m3
groundρ=2.0 Mgr/m3
C=100 m/s ?ρ gC=500 m/s
Α’
HWK 3.3:HWK 3.3:D h H h h h h l f l Draw the SH wave propagation path through the soil profile shown below, and compute the horizontal acceleration applied t h il lto each soil layer.
± 0mCs=300m/s ρ=1.6Mgr/m3
- 20mCs=600m/s ρ=1.8Mgr/m3
- 40mCs=1200m/s ρ=2.2Mgr/m3
- 60m
Cs=2400m/s ρ=2.4Mgr/m3
SH50o
SH
(uo=1.6cm, T=0.40s)GEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.17
HWK 3.4:HWK 3.4:
Draw the reflected and refracted waves
(type and propagation di ti ) direction)
in the special cases h n in th fi shown in the figure .
.
HWKHWK 3 53 5HWKHWK 3.53.5::To get a feeling of the depth (from the free ground surface) which is affected by R-waves,
1) Compute the normalized depth z/λR where ground displacements are reduced to 10% of the value at the free ground surface.
2) What is the value of the above critical depth (range of variation) in the case of soft soil, stiff soil-soft rock and rock;rock;
(Aπό «Σημειώσεις Εδαφοδυναμικής» Γ. Γκαζέτα)
GEORGE BOUCKOVALAS, National Technical University of Athens, 2016 3.18