e k
SIPIL MESIN ARSITEKTUR ELEKTRO
FINITE ELEMENT ANALYSIS OF REASONABLE FOUNDATION FOR SUPPORTING SILO’S TOWER
Sukima n Nurdin
Abstrac t
The limita tio n o f so il d a ta d ue to p o o r so il inve stig a tio n p ro c e ss is a c o mmo n p ro b le m in c ivil e ng ine e ring p ro je c t. The finite e le me nt me tho d wa s use d to a na lyse the c o mp a tib ility o f
fo und a tio n to sup p o rt silo s in Live rp o o l Do c ks. Bo th sha llo w fo und a tio n a nd p ile fo und a tio n we re c o nside re d. The re sults o f the a na lyse s a re p re se nte d b y c o mp a ring a na lytic a l a nd nume ric a l
so lutio n. Pa ra me tric stud y wa s c o nside re d fo r e a c h c a se . The re a re d iffe re nt re sults fo r two typ e s o f sha llo w fo und a tio n tha t ha d b e e n c o nsid e re d. Strip fo unda tio n se e me d mo re re lia b le tha n
p a d fo unda tio n, while Pile fo unda tio n c o nside re d to b e first c ho ic e due to the sa tisfa c to ry c o nd itio n fo r a ll fa c to rs.
Ke y words : Finite Ele me nt, Silo , sha llo w a nd p ile Fo und a tio n.
Abstrak
Ke te rb a ta sa n d a ta ta na h ka re na tid a k d ila kuka nnya inve stig a si ta na h se c a ra m e nye luruh se b e lum p ro se s d e sig n d a n ko nstruksi a d a la h p ro b le m ya ng um um ya ng te rja d i p a d a p ro ye k-
p ro ye k te knik sip il. Me to d e e le m e nt hing g a a d a la h d ip a ka i untuk m e ng a na lisa je nis p o nd a si ya ng d a p a t m e nd ukung d a ri silo s d i Pe la b uha n live rp o l. Po nd a si d a ng ka l m a up un p o nd a si
tia ng ke d ua nya d ip e rtim b a ng ka n untuk d ig una ka n. Ana lisa te rse b ut m e m b a nd ing ka n m e to d e num e ric m a up un a na litik. Ha sil d a ri a na lysa te rse b ut m e m p e rliha tka n ha sil ya ng b e rb e d a d a ri
d ua tip e p o nd a si d a ng ka l. Po nd a si strip m e m b e rika n nila i ya ng le b ih o p tim a l d a rip a d a fo nd a si p a d . Se m e nta ra p o nd a si tia ng m e nja d i p iliha n p e rta m a ka re na ke la ya ka n d a ri se m ua fa c to r
ya ng d itinja u.
Kata Kunci : Ele me n Hing g a , Silo s, Po nd a si tia ng d a n p o nd a si d a ng ka l
Sta f Pe ng a ja r Jurusa n Te knik Sip il Fa kulta s Te knik Unive rsita s Ta d ula ko , Pa lu
1. Background
Te n to we r silo s a re use d to sto re c o m b ine d we ig ht o f
a p p ro xim a te ly 3000 to nne s o f lo a d . To we r silo s we re e re c te d a t q ua ysid e
o f Ale xa nd ra d o c ks in Live rp o o l.
O ve r the ye a rs silo b uild e rs ha ve im p ro ve d the d e sig n a nd
c o nstruc tio n o f the a b o ve -g ro und p o rtio n o f silo s, in c o ntra st, ve ry little
ha s b e e n d o ne to imp ro ve the fo und a tio n. To we rs ha ve g e ne ra lly
b e e n e re c te d o n fo und a tio ns c o nstruc te d b y Fug ro Limite d . Who
ha ve the ne c e ssa ry te c hno lo g y fo r a d e q ua te d e sig n a nd c o nstruc te d .
The p ra c tic e wa s re a so na b ly suc c e ssful whe n silo s we re
d e ve lo p e d . As b ig g e r silo s we re e re c te d , ho we ve r, a nd the a p p lie d
fo und a tio n p re ssure s a p p ro a c he d the b e a ring c a p a c ity o f the so ils, m a ny
struc ture s se ttle d c o nsid e ra b ly, so m e tilte d , a nd so m e o ve rturne d
c o m p le te ly. This d ig e st o utline s the p ro b le m a nd ind ic a te s the ne e d fo r a
so il inve stig a tio n to d e te rmine the a llo wa b le b e a ring c a p a c ity a nd
c o m p re ssib ility o f the so il a nd thus e na b le p ro p e r fo und a tio n d e sig n.
Jurna l SMARTe k, Vo l. 9 No . 4. No p e m b e r 2011: 271 - 286
272 1.1 The p ro b le m
Ma ny to we r silo s c o nstruc te d o n
c la y so ils
ha ve ring -sha p e d
c o nc re te fo und a tio ns. To re d uc e c o sts, c o nc re te flo o rs a re se ld o m
p ro vid e d . Whe n the silo s a re d e ve lo p e d fo r a ny p urp o se s, p a rt o f
the lo a d is tra nsm itte d thro ug h the c ylind ric a l wa lls to the fo o ting s a nd
the re m a ind e r is c a rrie d d ire c tly b y the so il insid e the ring fo und a tio n. The
und e rlying c la ys c o m p re ss ve rtic a lly und e r the we ig ht o f the lo a d e d
struc ture in suc h a wa y tha t the a p p lie d lo a d s a re d istrib ute d
unifo rmly to the so il o ve r the who le a re a e nc lo se d b y the c irc ula r
fo und a tio n. This unifo rm p re ssure is d istrib ute d to the fo und a tio n so il in the
fo rm o f a p re ssure b ulb ; its size a nd sha p e , d e te rmine d b y e la stic the o ry,
a re re la te d d ire c tly to the d ia m e te r o f the lo a d e d a re a a s sho wn in Fig ure
1.1. He re , two fo o ting s o f d iffe re nt size c a rry the sa m e unifo rm lo a d , b ut the
p re ssure b ulb und e r the la rg e r fo und a tio n
is m uc h
la rg e r a nd
d e e p e r. In e a c h c a se the m a xim um ve rtic a l p re ssure o c c urs im m e d ia te ly
b e lo w the fo o ting a nd d im inishe s to 10 p e r c e nt o f this va lue a t a d e p th
e q ua l to twic e the d ia m e te r o f the fo und a tio n. If the a p p lie d stre sse s
within the b ulb d o no t e xc e e d the she a r stre ng th o f the so il the struc ture
will b e sta b le .
No n-unifo rm p la c e m e nt o f silo s d uring lo a d ing ha s c a use d m a ny
p ro b le m s. Whe n the lo a d fro m the we ig ht o f the silo s a nd the live lo a d is
o ff c e ntre the p re ssure b ulb will b e d isto rte d , a s sho wn in Fig ure 1.2a .
Stro ng wind s a c ting o n a ta ll silo c a n p ro d uc e the sa m e e ffe c t. The lo c a l
o ve rstre ssing o f the fo und a tio n so il m a y c a use tilting , a nd unle ss the
p ro b le m is re m e d ie d it ma y inc re a se with tim e until the silo o ve rturns.
Fig ure 1. Pre ssure b ulb s und e r la rg e a nd sm a ll ro und fo und a tio ns.
Finite Ele me nt Ana lysis o f Re a so na b le Fo und a tio n fo r Sup p o rting Silo ’ s To we r Sukima n Nurdin
273 Fig ure 2. No n-unifo rm a nd o ve rla p p ing p re ssure b ulb s.
Pre ssure b ulb s will o ve rla p , a s sho wn in Fig ure 1.2b , if two o r m o re
silo s a re c o nstruc te d to o c lo se to e a c h o the r. Be c a use p re ssure s a re
a d d itive , the re sulting p re ssure b ulb will b e m uc h la rg e r a nd will e xte nd to
g re a te r d e p ths. The so il in the o ve rla p zo ne will b e sub je c te d to hig he r
stre sse s a nd the fo und a tio ns o ve r this re g io n will se ttle m o re , c a using the
silo s to tilt to wa rd s e a c h o the r.
Mo st fo und a tio n fa ilure s in c la y so ils o c c ur whe n a silo is q uic kly
lo a d e d fo r the first tim e . As filling p ro c e e d s, the lo a d s a re a p p lie d to
the so il ske le to n a nd to the p o re wa te r c o nta ine d within the vo id s o f
the c la y. Pre ssure s g e ne ra te d in the p o re wa te r te nd to re d uc e the fric tio n
b e twe e n so il p a rtic le s a nd he nc e d e c re a se the she a r stre ng th o f the
so il. If, a t the e nd o f lo a d ing , the a va ila b le she a r stre ng th is g re a te r
tha n the a p p lie d she a r stre sse s, the struc ture will b e sta b le . With tim e the
e xc e ss p o re wa te r p re ssure s will d issip a te , the so ils will c o nso lid a te a nd
g a in stre ng th, a nd the struc ture will b e sta b le fo r sub se q ue nt lo a d ing s.
In silo s witho ut flo o rs, sila g e juic e s no rm a lly se e p into the und e rlying so il.
Po re p re ssure s a re inc re a se d e ve n m o re , c a using a furthe r d e c re a se in
the she a r stre ng th o f the so il. The fo o ting s m a y a lso b e c o m e
und e rm ine d whe n the liq uid flo w thro ug h the so il und e r hyd ra ulic
p re ssure . Eithe r o r b o th o f the se a c tio ns c a n trig g e r a b e a ring
c a p a c ity fa ilure .
1.2 Site Inve stig a tio ns Re lia b le a nd e ffe c tive site
inve stig a tio n is wid e ly a p p re c ia te d in c ivil e ng ine e ring a nd b uild ing p ro je c t,
the g ro und c o nd itio n is usua lly g ive n the la rg e st risk e le m e nt o f fina nc ia l
a nd te c hnic a l a sp e c t. Alm o st e xc lusive ly, the sc o p e o f
g e o te c hnic a l inve stig a tio n is g o ve rne d b y wha t is ne e d e d to
c ha ra c te ristic the sub surfa c e c o nd itio n a p p ro p ria te ly b y ho w m uc h
the c lie nt a nd p ro je c t m a na g e r a re willing to sp e nd M.B. Ja ksa e t
a ll,2005
Jurna l SMARTe k, Vo l. 9 No . 4. No p e m b e r 2011: 271 - 286
274 Be a ring c a p a c ity a nd ve rtic a l
se ttle m e nt a re d ire c tly re la te d to the e ng ine e ring p ro p e rtie s o f the so il a t a
site , so tha t a so il inve stig a tio n is re q uire d to p ro vid e info rm a tio n o n so il
p ro file , lo c a tio n o f the g ro und wa te r ta b le , ind e x p ro p e rtie s, she a r stre ng th
a nd c o m p re ssib ility.
The site inve stig a tio n wa s c a rrie d o ut a lo ng Ale xa nd ra d o c ks b y
Fug ro Limite d . Ho we ve r, the site inve stig a tio n la c ke d d a ta to e nsure a
d e sig n o f p ro p e r fo und a tio n to sup p o rt the silo s. So m e b o re ho le s
we re d rille d a ro und the site .
1.3 Te c hnic a l Asp e c ts o f Silo
Fo und a tio n Eve ry o wne r sho uld insist o n
a n a d e q ua te fo und a tio n fo r his silo a s p ro p o se d b y C a na d ia n Build ing
Dig e st C BD 80, C BD 81. The fo llo wing p o ints sho uld b e c o nsid e re d to e nsure
a sta b le struc ture :
a . Re info rc e d Fo o ting s A ring fo und a tio n is sub je c t to
b e nd ing stre sse s fro m ve rtic a l wa ll lo a d s, so il p re ssure s, a nd the
c irc um fe re ntia l lo a d s tha t e xist in the wa lls a t the b a se o f the silo . As
c o nc re te ha s a lo w te nsile stre ng th, the fo und a tio n sho uld b e
re info rc e d with ste e l to re sist suc h b e nd ing m o m e nts a nd te nsile
stre sse s. If the fo und a tio n sho uld c ra c k b e c a use o f insuffic ie nt
re info rc e m e nt, the m o no lithic b e ha vio ur o f the ring fo und a tio n
will b e d e stro ye d a nd its a b ility to sup p o rt the sup e rstruc ture
a d e q ua te ly will b e re d uc e d .
b . C o nc re te Fo und a tio n It is m o st imp o rta nt to use g o o d
q ua lity c o nc re te in a we ll p re p a re d e xc a va tio n in whic h the sid e s a re
ne a tly trimm e d a nd the flo o r c le a ne d o f a ll d isturb e d so il. In
g ra nula r so ils o r so ils c o nta ining b o uld e rs, fo rm wo rk m a y b e
ne c e ssa ry. The c o nc re te m ust b e p la c e d with the sa m e c a re a s
wo uld b e fo llo we d in c o nstruc ting the silo wa lls. To e m p lo y no q ua lity
c o ntro l o n e ithe r c o nc re te o r wo rkm a nship is una c c e p ta b le
c . C e ntring Silo o n Fo o ting s Lo a d -b e a ring wa lls sho uld b e
c e ntre d o n fo o ting s whe ne ve r p o ssib le in o rd e r to a p p ly unifo rm
lo a d s to the fo und a tio n so il a nd thus p e rmit full use o f its a llo wa b le
b e a ring c a p a c ity.
La rg e fo und a tio ns a re re q uire d in so ft, we a k so ils to m a inta in a p p lie d
p re ssure s within the a llo wa b le b e a ring c a p a c ity. La rg e -d ia m e te r
ring fo und a tio ns p ro je c ting b e yo nd the silo wa lls a re o fte n use d to
p ro vid e m a xim um sta b ility a g a inst o ve rturning . The inte rio r flo o r is
usua lly o m itte d fo r re a so ns o f e c o no m y. The inne r d ia m e te r o f
this fo und a tio n is no rm a lly slig htly le ss tha n the inne r d ia m e te r o f the
silo wa ll. Be c a use the he a vy wa ll lo a d s a p p lie d to the inne r e d g e
te nd to d e fo rm the ring fo und a tio n it c o uld e a sily b re a k into ind ivid ua l
se c tio ns unle ss it is p ro p e rly re info rc e d . In this c a se the fo o ting
p re ssure s wo uld no t b e unifo rm a nd lo c a l o ve rstre ssing o f the und e rlying
so il c o uld o c c ur. With a d e q ua te re info rc ing , ho we ve r, a nd b y
e xte nd ing the fo o ting furthe r insid e the silo , the c o nta c t p re ssure s
wo uld b e re d istrib ute d m o re unifo rm ly a nd a ny te nd e nc y o f the
fo und a tio n to d e fo rm wo uld b e re d uc e d . The a d d itio na l we ig ht o f
the sila g e o n the e xte nd e d fo o ting wo uld b e m inim a l b e c a use m o st o f
Finite Ele me nt Ana lysis o f Re a so na b le Fo und a tio n fo r Sup p o rting Silo ’ s To we r Sukima n Nurdin
275 the sila g e lo a d is tra nsfe rre d to the
silo wa lls thro ug h fric tio n. d . Silo G ro up s
To a vo id a ny inte ra c tio n b e twe e n silo s c o nstruc te d o n c o m p re ssib le
c la ys lo a d e d to the a llo wa b le b e a ring c a p a c ity it is
re c o m m e nd e d tha t the minim um ho rizo nta l c le a ra nc e b e twe e n
the m sho uld b e no t le ss tha n the d ia m e te r o f the ring fo und a tio ns. If
a sm a lle r sp a c ing is d e sire d , the silo s sho uld b e c o nstruc te d o n p ile s
o r o n a c o m m o n m a t fo und a tio n a d e q ua te ly re info rc e d to re sist the
a p p lie d b e nd ing m o m e nts.
e . Silo Lo a d La rg e q ua ntitie s o f silo lo a d fo rm
whe n the c o nte nts o f the silo s a re sto re d we t, i.e ., whe n the ir m o isture
c o nte nt is to o g re a t. In to we r silo s c o nstruc te d witho ut flo o rs, the
liq uid c a n flo w und e r hig h hyd ro sta tic p re ssure into the
fo und a tio n so il a nd und e rmine the fo und a tio ns. The y c a n inc re a se the
p o re wa te r p re ssure s in sa tura te d c la y so ils, re d uc ing the she a r
stre ng th. In a d d itio n, c he m ic a l re a c tio n with the so il m a y furthe r
d e c re a se so il stre ng th.
An im p e rm e a b le flo o r sho uld b e insta lle d to p re ve nt a ny liq uid s fro m
p e ne tra ting the sub so il, a nd d ra ins sho uld b e p ro vid e d to c a rry the m
a wa y a nd re d uc e the hyd ro sta tic p re ssure s in the silo . It is im p o rta nt
tha t the d ra ins c o ntinue to func tio n fo r the life o f the struc ture .
1.4 Be a ring C a p a c ity The ultima te b e a ring
c a p a c ity o f so ils d e p e nd s o n the she a r stre ng th c
u
o f the so il, a nd the d e p th a nd sha p e o f fo und a tio n.
Fig ure 3. Attitud e o f a silo fo und a tio n a fte r fa ilure .
Jurna l SMARTe k, Vo l. 9 No . 4. No p e m b e r 2011: 271 - 286
276 Whe n a silo o ve rturns the fo und a tio n
so il ro ta te s a lo ng a c irc ula r a rc Fig ure 2.1. The d ire c tio n o f the a p p lie d
she a r stre ss a lo ng the slip c irc le c ha ng e s fro m 0 to 90 d e g fro m the
ve rtic a l. In ra re c a se s the she a r stre ng th o f the so il is iso tro p ic
c o nsta nt in a ll d ire c tio ns, b ut g e ne ra lly it is a niso tro p ic va rie s with
the d ire c tio n o f a p p lie d she a r stre ss. Inve stig a tio ns o f m a rine c la ys ha ve
sho wn o rie nta tio n d e p e nd e nt re d uc tio n in she a r stre ng th o f a s m uc h
a s 35 p e r c e nt whe n c o m p a re d with the stre ng th m e a sure d in the ve rtic a l
d ire c tio n. It is im p e ra tive , the re fo re , tha t a n a d e q ua te fa c to r o f sa fe ty
sho uld b e inc lud e d to a llo w fo r stre ng th a niso tro p y, no n-unifo rm
p re ssure s a p p lie d to the so il d ue to e c c e ntric lo a d s a nd o ve rturning
m o m e nts fro m hig h wind s, a nd to p re ve nt e xc e ssive ve rtic a l se ttle m e nts.
This fa c to r o f sa fe ty is a p p lie d to the she a r stre ng th o f the so il. a fte r
M. Bo zo zuk, 1976
2. Analytical Design
