-
90
chng V mng cc
5-1.Khi nim chung: Mng cc c s dng rng ri trong xy dng:
Khi ti trng cng trnh tng i ln, Khi lp t tt xut hin tng i su, Khi
c yu cu cao v hn ch bin dng cng trnh. Mng cc gm 2 phn: Cc: l b phn
chnh nhim v truyn ti trng CT vo t thng qua ma st bn thnh cc v phn
lc u mi cc. i cc: l cu kin dng bn c nhim v: Nhn ti trng t CT v Phn
phi ln cc cc. T mt t n y i gi l chiu su chn i h
Nu h su cho t t y i tr ln tip nhn c ti trng ngang th: mng cc gi
l MC i thp, ngc li gi l MC i cao. Cc trong MC i thp chu ti trng
ngang khng ng k m ch yu chu ti ng. Cc trong MC i cao chu ti trng
ngang ca CT kh ln nn i hi phi c cng chng un ln. Cc cng trnh giao
thng thng p dng loi mng ny gim bt kh khn cho thi cng.
Cc lm vic chia thnh 2 loi: Cc chng: Nu mi cc nm trong lp t c tnh
bin dng ln rt thp hoc thc t coi l khng bin dng. Ton b ti trng truyn
xung mi cc; ma st xung quanh thn cc khng ng k thng l nn . Tnh ton
cc chng ch yu da vo kh nng lm vic VL
-
91
Cc treo ( cc ma st): Ti trng truyn xung mt phn mi cc, mt phn
xung quanh thn cc. Tnh ton cc treo ch yu da vo kh nng lm vic t
nn.
(Phn ln cc cng trnh ca ta s dng cc ma st) 5.2 Cu to cc: Phn loi
cc: Theo vt liu: G, BTCT, thp Theo cng ngh thi cng cc:
Cc c sn: ng; p; rung, xon.. Cc ti ch: khoan nhi, Ba rt
Phn loi theo mc thay th t: Khng thay th: cc c hoc rng c bt u ng.
Thay c: khoan nhi
Thay mt phn: khoan mi + ng hoc p 5.2.1. Loi cc c sn h bng ba
thng (cc ng) 1, Cc g: Dng trong CT nh hoc tm thi.
u im: cc g nh vn chuyn d dng, ch to n gin, khng cn thit b ng ln,
nu cc g lun nm di mc nc ngm th thi gian s dng rt lu Nhc: Sc chu ti
nh, hn ch v chiu di D b xm thc & d b ph hoi khi ng - cn ngm tm
Creozot p lc cao) Khi dng cc g y/c ch nhng im sau: - /k thng chn 20
30 cm, L= 5 8m, g dng cc nn chn loi g cng thng, cong khng nn qu
1/100 so vi chiu di. - nh cc c bo v bng ai thp dy 8mm rng 710cm, mi
vt nhn, bt thp d ng, khng b dp nt. Do g him nn hin nay t dng. 2.Cc
BTCT: Dng ph bin u im: - iu kin p dng rng ri hn ( lu nc mn) tit
din, chiu di c th theo mun, cng vt liu cc tng i ln. Nhc im: - Trng
lng qu ln, gy kh khn cho vn chuyn v h cc. Cn khi lng thp ln cho vn
chuyn, cu lp. Thi cng cc hay b nt ( khc phc bng cch dng BTCT ng sut
trc) Tit din: trn, tam gic, I, T, rng.. Vi cc BTCT thng thng c cu
to nh sau: L = 425m c khi ti 100m. Cc on thng 68 m ( c khi ln hn -
ty thuc a im v thit b thi cng) ni li vi nhau trong khi h. T s gia
chiu di L ca cy cc v b rng b ca tit din cc thng khng ln qu 100. nh
cc tng cng li thp a = 50mm
-
92
Mi tng cng thm thp dc v ai thp ai hai u dy hn a= 510cm thp 68mm
Thp dc 4-8 thanh CT5; thp ai CT3, c th dng ai xon. Thng thng ngi ta
b tr ct thp nh sau: Cc 2020 dng 144 ; Cc 2525 dng
164 ; Cc 3030 dng 184 ; Mi ni cn t c kh nng chu ti t nht l tng t
nh cc tit din khc ca cc, tt nht l dng lin kt bu lng chu tt ti trng
ng khi thi cng. B tng cc M 250 # ( thng dng 300#); B tng cho cc
khoan nhi ngoi /k v cng b ng phi c st ln m bo tnh lin tc ca cc ( st
khong 7,5cm n >15 cm - ph thc /kin s dng; tham kho TCXD) Thng
thng BTcc khoan nhi c hm lng xi mng khng < 350kg/m3
C mc treo cc v vn chuyn. Lp b tng bo v: a 3cm.
T l 3,12,1 DA
V d cc:
tit din 20 20 cm ( A=25cm) tit din 25 25cm ( A=32cm) tit din 30
30cm ( A=34cm) tit din 35 35cm ( A=40cm)...
Lu : Vi cc c yu cu k thut cao( a cht phc tp, ti ln, iu kin thi
cng kh khn...) cn ch to cc c l rng vi /knh khng < 30mm kim tra
thng ng ca cc sau khi thi cng.
ngng cccc CCcc tittit didinn nhnh
-
93
Mi ni cc
on ni on mi cc
L1
on ni
2L L3
Lc
b
a a
L
Mc cu Thp dc
u cc Mi cc
(thp AI)
350
30
290
30
30 290 30
350
350
A
72
31
40
D
100100100
50
50
50
50
A-a
86
70
3020 20
8 8
= D
D
B-B
50
b b
a a
1 1
-
94
3, Cc thp: Thng dng nhng CT vnh cu hoc nhng CT chu ti trng ngang
ln cng c trng hp dng lm CT tm thi nh cc vng vy thi cng mng. Trong
trng hp c bit mi dng cc thp. *Tm li: Loi cc ch to trc a n v tr ri h
cc bng phng php ng
u im: Cht lng bn thn cc c th kim sot c v dng loi b khi pht hin
thy khng m bo, tc thi cng nhanh v thun li. Nhc im: Vi cc ng l ting
n v rung ng. 5.2.2. Loi cc h bng phng php xon ( cc xon) Phm vi: cng
trnh:
Cu, cng, thu li, ct in cao th, cc neo khi th nghim nn tnh. Cu
tao: Thn: BTCT, Thp : c dng mi khoan; thp, gang, hoc trong BT ngoi
bc thp. Cch h: nh thit b to m men xon.
u im: h cc m thun. ph hp xy chen Chu nh tt do mi m rng 5.2.3.Loi
cc h bng phng php xi nc. Cu to: tng t nh cc ng
Khc: gia hoc xung quanh c gn thm cc ng dn nc. Cch h: ch xi nc
trng lng bn thn cc lm cho n h gn n chiu su thit k khi cn cch su
thit k 1 1,5m dng ba ng Va xi nc, va dng ba ng. 5.2.4.Loi cc h bng
phng php rung ( mng cc ng) Cu to: Cc ng l kt cu v mng bng thp hoc
BTCT c dng hnh ng. Cch h: Do tc dng rung ca my chn ng.
u im: C th cng nghip ho, dng hiu qu trong loi t khng dnh hoc t
dng ht Tn t vt liu Tn dng cao kh nng lm vic ca vt liu mng C th a
mng xung su m khng cn n mng ging chm (l loi mng c hi cho sc
kho)
-
95
C th thi cng quanh nm v ton b cng tc trn mt nc ci thin /kin lao
ng Vit nam: cc cu Hm rng /k = 1,55m Nc ngoi: /k n 6m. 5.2.5.Loi cc
ti ch ( Cc nhi) Cc s c ch to ngay ti ch m cc s lm vic sau khi xy
dng CT Nguyn tc: Khoan to l, c ng vch hoc khng ng vch, hnh dng c th
trn hoc ch nht (Cc Baret) BT; BT c st cao, qua ng dn. Cu M thun: cc
/k = 2,5m; L 100m C/trnh 27 Lng h: cc Bart = 1m 2,8m n 1,5m 2,8m. u
im: Sc chu ti ln Khng phi vn chuyn nn lng ct thp trong cc t hn so
vi cc ng, Thi cng m thun nn t gy chn ng cho CT ln cn nh phng php ng
Nhc im: Vic kim tra hnh dng cng nh cht lng cc rt b hn ch, c bit l
khi thi cng khng c ng vch.
Thit b thi cng
cng knh. Nu cc b hng
kh thay th.
1 1
1 1
D
Chi
u
di
to
n c
c
2000 - 3000
2000 - 3000
1-1
D
2-2
D
-
96
5.2.6.Loi cc h bng phng php p tnh:
C tn cc Mga thi k u dng sa cha CT nghing ln l chnh Nguyn l:
Cc ch to thnh on ngn. Dng kch thu lc p tng on xung t. Dng ngay
trng lng bn thn CT lm i trng
p xong on ny t on khc ln Lin kt: bng hn hoc cht
Sau khi p xung su th/k lin kt cc vi mng c. Hin nay cng ngh ny rt
pht trin Vit nam, dng trong c sa cha v xy mi. Thit b p ln n trn 300
tn Ti trng lm vic ca cc trn 100 tn Chia thnh 2 loi: Phng php p trc:
cc c p trc khi xy dng CT lc i trng neo kch l i trng hoc cc neo Phng
php p sau: l h cc c cha sn i Khi xy xong 1 phn CT th p cc Sau lin
kt cc vi i.
-
97
pp ""trtrcc" " pp ""sausau""
5.3. Cu to i cc: Vt liu: BT; BTCT i c th: ton khi hay lp ghp.
Dng: nh i ph thuc y CT
y i ph thuc b tr cc C th: n, Bng , B Chiu dy i tnh ton.
i c th: lin tc hoc thnh bc Cu to chung i cc nh sau:
h
h
o
>=100
>=100
Thp tng cng
>1,2 m
>2D
v
>=
-
98
Lin kt cc vi i ph bin hn c l lin kt ngm.
Nu u cc khng th p ly thp ch lin kt vo i (trng hp thi cng di nc
nh tr cu chng hn) chiu di cc trong i phi khng t hn 2D hoc 1,2m nu D
> 600
Trng hp p u cc neo thp vo i: 20 i vi thp gai. 40 i vi thp
trn.
on cc trong i ch yu cu n 100 l .
Trng hp c bit cc c th khng lin kt trc tip vi i m thng qua tng
gim chn c p dng trong cc vng c ng t, lp m trung gian bng vt liu ri
( dm, si, ct th v ct trung)
Khong cch t mp cc ngoi cng n mp i ph thuc vo sai s cho php khi
ng cc, tc l ph thuc vo ng knh cc, ng thi khng < 100mm
Vi MC i cao: Nn tng cng ct thp cho i bng cc li 20 25 t cch nhau
10 20 cm nh cc nn t cc li ct thp 12 cch nhau 10 20cm Cc mp phi c
tng cng bng cc thanh ct thp un mc cu.
BT yu cu M 200# 5.4.S lm vic gia cc n v nhm cc: + S lm vic ca 1
cc n v ca 1 cc trong mng cc l khc nhau, tuy nhin trong cc phng php
tnh hin nay u coi Sc chu ti (SCT) trong 2 trng hp l nh nhau cn phi
nghin cu thm hon chnh nht l vi cc ma st. + Cc cc thng c ng thnh nhm
khong cch gia cc cc thng t 3 n 4 ln ng knh hoc cnh ca cc. Vi cc
chng th c th khng thy c du hiu khc nhau nh th. L do: cc chng ti ch
yu truyn qua mi cc m mi cc li nh nn t gy nh hng n cc cc xung quanh.
Vi cc ma st din tch phn b p lc ca ti CT xung nn t mi cc ln hn nhiu
so vi cc chng nn khong cch gia cc cc b tr c nh hng ln n /k lm vic
ca cc cc xung quanh. Nu khong cch ln biu p lc do ti truyn xung mt
phng mi cc gia cc cc khng chm nhau th lc SCT ca MC c th xem nh bng
tng SCT ca mi cc n. K/q th cc cho thy: khi khong cch gia trc cc cc
a 6D khng nh hng c th coi nh cc n. + Khi cng tr s ti trng ln cc n v
ln mi cc trong nhm. p dng li gii ca Mindlin ta thy ng sut z ti im
trn trc cc do c nhm gy ra ln gp vi ln so vi ng sut di cc n, chiu
rng hiu qu ca nhm cng gp mt s ln so vi cc n. do ln ca nhm s ln hn
cc n.
-
99
Ngi ta gi l hiu ng nhm: c th vit: P u nhm = n P u n
Trong : = h s nhm. n = s lng cc trong nhm. P u n = sc chu ti ca
mt cc n. (cn nghin cu cch thi cng v cc loi t khc nhau rt ra tr s )
SCT ca cc gm hai thnh phn:
Sc khng bn, to nn bi ma st v lc dnh ca t v cc mt bn cc. Sc khng
mi.
Nhiu nghin cu thy rng: Sc khng bn t cc hn rt nhanh ( chuyn v
khong 3 5mm. Nu cc nhi c thnh bn rt nhm, th sc khng bn c th t cc hn
chuyn v ln hn, khong 10 15mm. Ngc li sc khng mi t cc hn rt chm. Di
ti trng cho php, chuyn v ca cc [ ]s kh nh, do sc khng mi mi ch huy
ng mt phn nh (trong khi , sc khng bn ca cc huy ng kh ln) 5.5. Xc nh
sc chu ti ca cc: Cc trong mng c th b ph hoi do 1 trong 2 nguyn nhn
sau: Vt liu cc b ph hoi Pvl Pmin (Pvl; Pn) [P] thit k Do t nn b ph
hoi Pn
Sc khng tng
Sc khng bn
Sc khng mi
Sc khng
Chuyn v tung igia cc v ts
Qp/Fp
QpQf/Ff
Qf
{Q}
Qu
0
-
100
Mt s nh ngha:
Sc chu ti cc hn (Pu): l ti trng m ti vt liu hoc t nn b ph hoi.
Sc chu ti cho php [ ]P : l ti trng m ti cc lm vic an ton (vi mt h s
an ton sF thng ln hn 2) A. Xc nh SCT theo phng dc trc: 5.5.1.Theo
Vt liu: 1. Cc b tng ct thp ch to sn tit din c, chu nn. '( )vl b n a
aP m R F R F= +
nR cng chu nn tnh ton ca b tng. F din tch tit din ngang ca
cc.
'
aR cng chu nn tnh ton ca ct thp.
aF din tch ct thp dc.
bm h s /k lm vic ca BT. i vi cc c tit din nh hn 30cm;
mb=0,85
h s un dc. Khi cc xuyn qua lp bn, st yu v mng cc i cao, s un dc
c k n trong phm vi chiu di t do ca cc. Chiu di t do ca cc c tnh t y
i n y lp t yu ( tra bng). Cn li ly =1 2. Cc ng BTCT c lp y phn rng
bng b tng: ' ' '( 2,5 )vl b n a a ax axP m R F R F R F= + +
lF din tch tit din ngang ca li BT
'
ctF din tch tit din ngang tnh i ca ct thp xon ' l x
ax
x
D FFB
pi=
lD ng knh li BT
xF din tch tit din ngang ca thanh thp xon
xB bc ca ct thp xon.
axR cng chu nn tnh ton ca ct thp xon. 5.5.2. Theo t nn: 1. Phng
php th nghim: 1. Phng php nn tnh:
-
101
-
102
Vit nam Ta c cc quy phm nh sau: 88-82; 189-190; 269:2002 TCVN:
269: 2002 Cc Phng php th nghim bng ti trng tnh p dc trc: Thi gian
ngh ca cc: Thi gian ti thiu: 21 ngy cc khoan nhi 7 ngy vi cc khc
Quy trnh gia ti tiu chun: (C th gia ti theo chu k) Gia ti tng cp n
ti trng th nghim ln nht theo d kin. Mi cp 25% ti thit k. Cp mi ch c
tng khi tc ln u cc t n nh quy c nhng khng qu 2h. Gi cp ti ln nht
cho n khi ln u cc t n nh quy c hoc 24 gi, ly thi gian no ln hn. Sau
khi kt thc gia ti, nu cc khng b ph hoi th gim ti v 0, mi cp gim ti
bng 2 ln cp gia ti, thi gian mi cp 30 pht, ring cp 0 c th lu hn
nhng khng qu 6 gi.
-
103
n nh ln quy c: Khng qu 0,25mm/h i vi cc chng vo lp t hn ln, t
ct, t st t do cng n cng
Khng qu 0,1mm/h i vi cc ma st trong t st do mm n do chy. i vi cc
thm d: th nghim n 250% 300% ti thit k i vi cc kim tra: th nghim n
150% n 200% ti thit k. Trnh by kt qu: V Biu quan h ti trng chuyn v(
P-S) V Biu quan h chuyn v thi gian( S t) Da vo xc nh Sc chu ti gii
hn Da vo th P S
Trng hp ng cong bin i nhanh, th hin r im c dc thay i t ngt ( im
un), sc chu ti gii hn bng ti trng tng ng vi im ng cong bt u thay i
dc.
Nu ng cong bin i chm, kh hoc khng th xc nh chnh xc im un th cn c
vo cch gia ti v quy trnh th nghim chn phng php xc nh sc chu ti gii
hn ( xem ph lc sau)
Vi quy trnh 88-82: Sc chu ti ca cc c ly nh sau:
Pgh c xc nh da trn trn biu ti trng - ln ng vi tr s ln bng: S* =
.Sgh
Trong : Sgh = ln gii hn cho php ca cng trnh = h s chuyn i k n s
khc nhau gia thi gian tc dng ca ti trng khi
th nghim v ca ti trng thc t. Quy phm Vit nam cho php ly =
0,2
25
25
40
45
35
30
20151050
S*
10
15
20
5
45403530
Pgh
S(mm)
P(t)
Ti trng cho php c ly: [ ]s
ghFP
P = ; (Fs = 1,25)
-
104
Ghi ch: Gi tr sc chu ti gii hn xc nh theo phng php khc nhau c th
khc nhau * Sc chu ti cho php:
[ ]s
gh
FP
P = ; Fs thng thng ly = 2. Tuy nhin h s an ton c th cao hn hoc
thp
hn tu thuc mc quan trng ca CT, iu kin t nn, c im cc v phng php
th nghim. (Tham kho Ph lc E TCXDVN 269: 2002) Theo quy phm ASTM
D-1443 th c 7 cch gia ti khc nhau. Tuy nhin quy trnh
gia ti nhanh ( QL Quick Load) thng c s dng nht. Quy trnh ny nh
sau: D tnh ti nn ti a Pmax. Thng thng Pmax= 2 3 [P]; [P] l sc chu
ti thit k.
Mi cp s l 1/20 Pmax ( nh vy s c 20 cp ti) Mi cp ch gi 2,5 n 5
pht. Sau gim ti.
Nhn xt:
- c coi l phng php tin cy nht. - lu, cng knh, tn km.
Bi 1
Kt qu th nghim nn tnh cc BTCT tit din (30 30) cm2 cho trong bng
sau. Hy xc nh ti trng gii hn ln cc v ti trng cho php ca cc theo
TCVN. Bit rng ln cho php ca cng trnh [ S] = 6cm P(tn) 5 10 15 20 25
30 35 40 42 44 S(mm) 2 3,5 5,1 6,9 8,8 12,6 17,2 24,3 30,4 35,2
Bi lm:
-
105
S(mm)
0
P(t)5
10
15
20
25
30
35
40
45
5 10 15 20 25 30 35 40 45
12mm
28T
Trc ht da vo kt qu th nghim ta xy dng th quan h S=f(P) nh hnh
trn, ng vi ln quy c S* = 0,2[S] = 12mm ta xc nh ti trng tng ng. T
th ta c: Pgh = 28T
v [ ] TFP
Ps
gh 4,2225,1
28===
chn [P] = 22 T 2.Phng php ng: * Nguyn l: Sau khi h cc n su thit
k, cc ngh, tin hnh ng th vi ba trng lng Q, cao ri ba H, o ln 1 nht
ba gy ra l e - chi ca cc - xuyn su bnh qun ca cc do 1 nht ba. (ng
cc th phi tin hnh t 3 n 5 nht ba. cao ri ba phi ng u cho tt c cc
nht, v ly gi tr trung bnh.)
Pgh 1/e Thit b: Ba ng cc: Q; H Dng c o ln.
Din dch kt qu:
Theo Gerxvanov:
QHe
nFqQqkQnFnFP
TNgh
+
++
+=
21
2
22
F = din tch tit din ngang ca cc. Q = trng lng phn ba ri q = trng
lng cc bao gm c m cc, m cc v cc dn nu c.
HL
-
106
H= chiu cao ri ba tnh ton. k1= h s phc hi khi va chm; khi thp vi
gang va chm vi g th ly k1 = 0,2 n = h s kinh nghim, ph thuc vt liu
lm cc v cch ng cc.
theo bng. H s n
Loi cc v m cc n(kG/cm2) Cc g: - c m - khng m Cc b tng ct thp c
cc dn Cc thp: - c m g - c m thp v cc dn - c cc dn, khng m
8 10 15 20 30 50
Ti trng tnh ton c xc nh nh sau:
tc
gh
kP
P = ktc = ly theo ph lc A TCXD 205:1998 ( =1,25 hoc 1,4)
TCXD 205: 1998 ( Quy phm mi ca VN) c kin ngh 2 cng thc tnh Pgh
ng vi trng hp e 0,002m v e< 0,002m.
Khi chi o c meTN 002,0
( )
++
+++= 141
2 11
2
www
www
nFeEnFMP
cn
cn
gh
Trong : n = h s ly bng 150t/m2 i vi cc BTCT c m cc. F= din tch
cc. M = h s kiu ba, ly bng 1 khi ng cc bng ba tc dng va p cn khi
h
cc bng ba rung th ly theo bng. E = nng lng tnh ton ca 1 va p ba
theo bng. ef = chi thc t, bng ln ca cc do mt va p ca ba cn khi
dng
my rung l ln ca cc do cng ca my trong thi gian 1 pht,m. e = chi
n hi ca cc xc nh bng my o chi. w = trng lng ca phn va p ca ba, T.
wc = trng lng ca cc v m cc, T. w1 = trng lng ca cc dn ( khi h cc
bng rung w1=0),T. wn = trng lng ca ba hoc ca my rung,T. = h s phc
hi va p, khi ng cc ng b tng ct thp bng ba tc ng va
p c dng m m g ly 2 = 0,2 h bng rung ly 2 =0
Khi chi o c meTN 002,0< nn chn ba sao cho tho mn /kin trn nu
khng c th i c thit b ng th tham kho cng thc trong TCXD
205:1998.
-
107
Hin tng chi gi khi ng cc: chi o c ngay sau khi va ng cc xong khc
vi chi thc. Vi t ct chi o c ngay sau khi ng cc < chi thc. Vi t
st th ngc li.
L do: Khi ng cc do rung t dn cht li lm ma st tng ln chi nh i.
Sau
mt thi gian cc tnh cht ca t c phc hi chi o c ln hn.
Vi t st khi ng t b p cht nn nc b thot ra, nhng mi ch thot nhiu
ra pha xung quanh cc lm ma st gim i nn chi tng ln, mt thi gian t hi
phc chi gim i.
Vi: t ct sau: 23 ngy th nghim t st 1530 ngy hoc lu hn.
Theo tiu chun ca H lan:
( )eqQHQPgh
+=
2
75
=ghPP ; H s an ton thng ly bng = 6 ( B tc k s)
Bi 2:
Kt qu ng th cc bng ba dizen kiu ng c: Trng lng qu ba: Q= 12,5kN.
Trng lng ton phn ca ba: Qn = 26kN Chiu cao ri ti a ca qu ba: H= 3m
Cc b tng ct thp c tit din (30 30) cm2, trng lng cc: q= 20,5kN Cc c
m lt bng g. Trng lng m g v tht thp ca my trn u cc: q1= 2KN Kt qu th
cho chi ca cc l: e = 0,008m
Hy xc nh sc chu ti ca cc.
Bi lm:
Khi th ng cc ng, nu chi thc t o c ef 0,002m; Qu xc nh theo cng
thc:
]1)(.41[2 1
12
++
+++=
WWWWWW
nFenFMQ
cn
cn
f
pu
Nu chi o c < 0,002m th nn i thit b c nng lng ln hn h cc. Nu
khng i c th dng cng thc trong 205: 1998 n = h s kinh nghim ph thuc
vt liu lm cc v cch ng cc tra bng:
Cc BTCT c m lt bng g: n =1500kpa
-
108
F = din tch cc M = h s ly bng 1 khi ng, cn khi rung tra bng loi
t di mi cc. p = nng lng tnh ton ca mt va p ca ba tra bng loi ba. ef
= chi thc t bng ln ca cc do mt va p ca ba. W = trng lng phn va p ca
ba. Wc = trng lng ca cc v m cc. W1 = trng lng ca cc dn. Wn = trng
lng ca ba. = h s phc hi va p khi ng cc v cc ng BTCT bng ba c dng m
m g ly 2 = 0,2 Thay s:
=
++
+++= ]1
25,2026)25,20.(2,026
.
008.0.09,0.15003.5,12.9,0.41[
21.09,0.1500
uQ
KNQu 7744,534)19226,8.(5,67]16289,0.1251.[5,67]15,485,30
.
08,11351.[5,67 ==+=+=
Sc chu ti cho php ca cc:
tc
tca k
QQ = ; ktc = 1,4 h s an ton; du
tc kQQ = ; k = 1 h s an ton theo t
Vy TKNKNQa 383824,17744,534
===
3, Phng php th ng bin dng ln: ( PDA- Pile Driving Analyzer) L
phng php ca gio s G.C. Goble, Hc vin cng trnh Case ca M o ng xc nh
gn ng sc chu ti ca cc n v phn on cht lng thn cc, gi l phng php
Case. Phng php Case l phng php o ng v phn tch dng l thuyt truyn
sng
lm c s. ng thi nghin cu ch to thit b phn tch ng cc PDA c th ngay
hin trng thu c cc tham s nh sc chu ti ca cc, cht lng thn cc, ng sut
ng cc, nng lng ng ba v tnh nng lp m.
Thit b th nghim gm 2 b phn: + Thit b ng: Phi lm cho gia cc v t c
chuyn v tng i, nng lng tc ng ln u cc phi tng i ln, v vy phi dng ba
nng ng ln u cc. Trng lng ba thng ly bng 1% sc chu ti ca cc. + My o:
C nhiu: M, H lan, Thu in, Trung quc. Hin nay c phn mm CAPWAP vn s
dng kt qu o c trong th nghim PDA qu trnh phn tch ca CAPWAP cht ch
hn, chnh xc hn v a li nhiu kt qu hn so vi phng php Case trn.
-
109
4.Xc nh SCT theo phng php thng k: Da vo k/qu chnh l nhiu s liu
thc t v th nghim th tnh cc h trong cc loi t khc nhau tm tng quan
gia ma st v phn lc t mi cc vi mt s ch tiu c l.
Vi cc chu nn:
+=
=
n
iiitt
tc
FRlumk
P i1
211
Hnh 2. B tr th nghim PDA
Hnh 3. S thit b th nghim PDA
-
110
Vi cc chu ko: in
iitt
tc
lumk
P =
=
11
1
P = sc chu ti tnh ton. m = h s /k lm vic ca cc trong t theo TCXD
205: Cc ng tit din
-
111
Bng: Cng tnh ton ca t di mi cc, nR (kPa)
t loi ri cht va si Ct to Ct va Ct nh Ct bt
t st c ch s st Il bng
Chiu su h cc (m)
0,0 0,1 0,2 0,3 0,4 0,5 0,6 7000 3400 2200 5 8800 6200
4000 2800 2000
1300 800
7300 3700 2400 7 9700 6900
4300 3300 2200
1400 850
7700 4000 2600 10 10500 7300
5000 3500 2400
1500 900
8200 4400 15 11700 7500
5600 4000
2900 1650 1000
4800 20 12600 8500 6200 4500
3200 1800 1100
23 13400 9000 6800 5200 3500 1950 1200 35 15000 10000 8000 6000
4100 2250 1400
S trn cho t ct, s di cho t st. Bng: Cc h s 1 v 2 dng trong cng
thc thng k
H s iu kin lm vic ca t c k n mt cch c lp vi nhau khi tnh
ton sc chu ti ca cc
Phng php h cc
Di mi cc, 2 mt bn cc, 1 1.H cc c, cc rng bt mi bng ba 1 1 2.Cc
ng vo l khoan mi, vo trong t cha khoan ti thiu 1m, vi ng knh l
khoan Bng cnh cc vung Nh hn cnh cc vung 5cm Nh hn cnh cc vung hoc
ng knh cc
trn 15cm
1 1 1
0,5 0,6 1
3.H cc trong t ct c xi nc nhng ng tip m cui cng khng xi nc
1
0,9
Bi 3
Hy d bo sc chu ti ca cc BTCT tit din vung (25 25) cm2, di 12m c
ng vo nn t c a tng gm 3 lp (theo th t trn xung) nh sau:
Ct pha do dy 6m c st Il = 0,6 Ct bt cht va dy 4m St do cng Il =
0,3
(cha kho st ht chiu dy trong su h khoan 18m) Bit rng nh cc cch
mt t 0,5m; y i cch mt t 1,0m
-
112
Bi lm:
1, i vi cc chu nn: Chu vi tit din cc u = 0,25m 4 = 1,0m Tit din
ngang cc Fc = 0,25 0,25 = 0,0625m
2 Chiu di lm vic ca cc lc = 11,5m; Mi cc su 12,5m k t mt t. S lm
vic ca cc nh trn hnh v. Da vo s lm vic ta thy:
Cc phn lp 1;2;3 thuc lp t ct pha do; Cc phn lp 4;5 thuc lp t ct
bt cht va; Phn lp 6 thuc lp t st do cng.
Sc chu ti ca cc d bo theo cng thc sau:
66
1RFluP ci
iigh +=
=
Ct pha do
Ct bt cht va
6000
4000
2500
0.000
500 800
200
1000
300
800
-1.500
-5.000
-7.000
-9.000
-11.250
-12.500
-3.000
St do cng
-1.000
-6.000
-10.000
-12.500
0.000
-
113
Tra bng: Trong : hi su trung bnh ca mi lp t chia.
Lp t Loi t li (m)
hi (m)
i Kpa
1 Ct pha do Il=0,6
1 2 2
1,5 3,0 5,0
10 14 17
2 Ct bt cht va 2 2
7,0 9,0
32 33,5
3 St do cng Il=0,3
2,5 11,25 47
R6=f(Il=0,3; L6=12,5m) tra bng = 3750 Kpa
Sc chu ti gii hn ca cc:
1,0.{[1.10 2.14 2.17 2.32 2.33,5 2,5.47]
0.0625.3750} 320,5 234, 4 554,9 55,5ghP
Kpa T
= + + + + + +
+ = + = =
[ ] 55,5 401, 4
gh
tc
PP T
k= = ; i n v: 1 kPa = 0,1T/m2
2, i vi cc chu ko:
cii
igh GluP +==
6
1
TKpalu ii 325,320]47.5,25,33.232.217.214.210.1.{[0,1..6
1=+++++=
TmTmGc 88,1/5,2.12.25,0.25,0 3 == ; [ ] TTP
1568,1488,18,1288,15,232
=+=+=
3.Mt s cng thc thc nghim xc nh SCT ca cc: Cng thc tnh SCT ca cc
gm 2 thnh phn + Tng lc khng bn quanh cc Qs + Lc khng ti mi cc Qc
1.Da vo kt qu xuyn tnh: CPT
cc
n
i
ciigh qFkqluP +=
1
ciq = sc khng xuyn trung bnh ca lp t th i cq = sc khng xuyn
trung bnh ca t trong phm vi 3d pha trn chn cc v 3d pha di chn cc.(d
l ng knh hay cnh ca cc)
kc = h s tra bng.
hd
l1l2
-
114
s
ghd F
pP = theo TCVN 205:1998: Fs = 23
Bng: H s chuyn i ki v i t kt qu CPT i vi cc BTCT
H s ki H s i Gi tr cc i ca i Loi t qc (kPa) Cc
khoan Cc ng Cc
khoan Cc ng Cc
khoan Cc ng
t loi st chy,bn
0-2000 0,4 0,5 30 30 15 15
t loi st cng va
2000-5000
0,35 0,45 40 40 (80) 35
(80) 35
t loi st, cng n rt cng
>5000 0,45 0,55 60 60 (80) 35
(80) 35
Ct chy 0-2500 0,4 0,5 960) 120
(60) 80
35 35
Ct cht va
2500-10000
0,4 0,5 (100) 180
100 (120) 80
(120) 80
Ct cht n
rt cht
>10000 0,3 0,4 150 150 (150) 120
(150) 120
Cc ga tr trong ngoc c th s dng khi:
i vi cc nhi, thnh h c gi tt, khi thi cng khng gy ph hoi thnh h v
b tng cc t cht lng cao. i vi cc ng c tc dng lm cht t khi ng.
Bi 4:
Hy d bo sc chu ti theo t nn ca cc BTCT tit din (25 25) cm2, di
12m c thi cng theo phng php p trc vo nn t c a tng gm 3 lp nh sau (k
t mt t xung)
ct pha do dy 6m: sc khng xuyn qc = 15 kG/cm2
bn st dy 4m: sc khng xuyn qc = 4 kG/cm2
ct ht trung cht va: sc khng xuyn qc = 45 kG/cm2
(cha kho st ht chiu dy trong su h khoan 18m) Bit rng nh cc cch
mt t 0,5m; y i cch mt t 1,0m
-
115
Bi lm S lm vic ca cc v a tng nh hnh v:
Theo kt qu xuyn tnh, ta xc nh c chiu di cc qua cc lp t v cc gi
tr i
v Rn theo cc bng sau: ccn
i
ciigh qFK
qluP += 1
; Kc ph thuc loi cc v phng php h cc: tra bng l1 = 5m; 1 = 30
l2 = 4m; 2 = 30 l3 = 2,5m; 3 = 100 k3 = 0,5 Sc chu ti gii hn ca
cc:
=+++= 45.5,0.625]10045
.250304
.4003015
.500.[100ghP
32 667,0+14 062,0 = 46729kg = 46,7T
[ ] TTpP gh 197,185,27,46
5,2===
2.Da vo kt qu xuyn tiu chun ( SPT) Theo Mayerhof:
s
tbp
tbgh NulKFNKP += 21 s
tbp
tb NN ; = tr s SPT trung bnh trong khong 1d di mi cc v 4d trn mi
cc, v ca cc lp t dc theo thn cc.
K1 (kN/m2) = 400 cc ng; =120 cc nhi
K2(KN/m2) = 2 cc ng ; = 1 cc nhi
[ ]s
gh
FP
P = 35,2 =sF ( TCVN 205:1998)
5000
4000
0.000
-1.000
-6.000
-12.500
1000
2500
-
116
Theo cng thc ca Nht bn:
[ ]
++= ucLLNFNP
n n
cisisia1 1
2,031
aN - ch s SPT trung bnh trong khong 1d di mi cc v 4 d trn mi cc
(d l ng knh hay cnh ca cc)
siN - ch s SPT trung bnh dc thn cc ca lp ct th i m cc i qua.
siL - chiu di on cc nm trong lp t ct th i m cc i qua, m;
siL - chiu di on cc nm trong lp t st th i m cc i qua, m; - H s,
ph thuc vo phng php thi cng cc; cc BTCT thi cng bng phng php ng hoc
p: =30; cc khoan nhi =15. Bi 5: Cng nn t nh trn, chiu di cc nh trn.
Kt qu th nghim SPT cho kt qu sau: su th nghim (m)
2 4 6 8 10 12 14 16
Gi tr N 3 3 1 1 20 21 22 22
Lc dnh ca lp ct pha c=1,1T/m2 ; lp bn st c=0. Hy d bo sc chu ti
ca cc.
Bi lm:
* Theo cng thc ca Nht bn: [ ]
++= ucLLNFNP
n n
cisisia1 1
2,031
; 21aN
[ ] ( ){ }( )[ ] ( ) T
P
183,1815,154,3931155,104,39
31
25,0451,15,2212,00625,0213031
=+=++=
++=
Lu : Theo TCXD205: 1998: * Cn k n trng lng cc nh ti trng tc dng
trong tr/ hp cc chu ma st m.
Sc chu ti cho php ca cc l: [ ] wFP
wwFP
Ps
ghdatcoc
s
gh== )(
w = l hiu s gia khi lng bn thn cc v khi lng bn thn t do n chim
ch c xt n lc y acsimet ca phn cc di mc nc ngm. Sc chu nh cc hn ca
cc theo t nn ly bng tng ma st bn cc c k thm trng lng cc.
* SCT trng ngang cc hn ca cc c tnh ton khi cc chu tc dng ng thi
ca m men un, lc ngang, lc dc trc v phn lc ca t nn - Tham kho ti
liu.
SCT theo phng ngang trc xc nh theo th nghim hin trng hoc theo
kinh nghim nh bng trong gio trnh NM.
-
117
5.6.Tnh ton mng cc i thp: Ni dung: 1.Chn loi, kch thc i v cc.
2.Xc nh SCT ca cc. 3.S b xc nh s lng cc. 4.B tr cc trn mt bng v ng.
5.Kim tra theo cc trng thi gii hn. nu khng tho mn thay i li.