PUNCHING SHEAR STRENGTH OF COLUMN FOOTINGS IN ACTUAL DESIGN CODES AND THEIR EXPERIMENTAL...

7/26/2019 PUNCHING SHEAR STRENGTH OF COLUMN FOOTINGS IN ACTUAL DESIGN CODES AND THEIR EXPERIMENTAL INVES

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PUNCHING SHEAR STRENGTH OF COLUMNFOOTINGS IN ACTUAL DESIGN CODES AND THEIR

EXPERIMENTAL INVESTIGATION

Zoran Boni1, Vera Pro!o"i1, Ne#o$%a Da"i&o"i1, Ri$a& 'i%i(

1The Faculty of Civil Engineering and Architecture / University of Ni / Serbia

[email protected] Faculty of ining! "eology and Civil Engineering /University of Tu#la/ $i%

A#)*ra+*

The &a&er &resents a revie' of calculation of &unching shear according to actual designcodes and revie' the e(&eri)ental tests in the field of &unching shear of reinforced concrete

colu)n footings 'orld'ide* The e(&eri)ents on footings have been rare so far! and they 'ere

&erfor)ed in laboratories! 'here soil 'as si)ulated in various 'ays* The goal of this research

is to &erfor) the &unching shear e(&eri)ents in the circu)stances 'hich are as close as

&ossible to real conditions both in ter)s of the footings! and in ter)s of the soil* +n the &a&er!

the funda)ental conce&t of &re&aration of e(&eri)ental tests that 'ere carried out on the test

s&eci)ens , colu)n footing on the real soil under ulti)ate load has been &resented* The test

s&eci)ens have been described! as 'ell as the a&&lied )aterials! &re&aration of the subgrade

soil! )easuring e-ui&)ent! and &lanned testing )ethod* +nvestigation 'ould indicate the

&ara)eters 'hose influence is do)inant in the course of &unching shear of colu)n footings

and these &ara)eters should be focused on in the further research*Keywords. colu)n footing! &unching shear strength! design code! e(&eri)ent

1 INTRODUCTION

+n statical sense colu)n reinforced concrete footings are slabs loaded by dead.'eight! by

&ressure forces fro) the structure! and by reactive distributed load of the soil* $ending of the

footing due to reactive load leads to e)ergence of cracs in concrete and in case of heavy

loadings to e(trication of a body 'hose for) is so)ething bet'een the truncated &yra)id

and the truncated cone fro) the footing* +n the literature it is said that there 'as &unching of

footing* Checing of security of footings to &unching is an obligatory &art of footing design

and focuses on control of the shear stress in the control cross.section* Factori#ed shear forceshould be deter)ined by subtracting the &art of soil reaction 'ithin the control cross.section*

The subtracted value differs de&ending on the )ethods of calculation! ado&ted &osition and

sha&e of control cross.section! and varies in the regulations of individual countries* ost

codes do not distinguish bet'een &unching of the flat slabs and of the footings *

+n the further te(t! the &unching control according to our Code on technical standards for

concrete and reinforced concrete 0134! Eurocode 2! 5678 2*9:*91 :! $S 119.1;13!1.92 'ill be &resented*

E(&eri)ental investigations conducted at the Faculty of Civil Engineering and Architecture

of Nis! financed by inistry of Science and Technological ?esearch of ?e&ublic of Serbia!

'ill be &resented* The ai) of this &ro@ect is to identify behavior of the shallo' foundations
mailto:[email protected]:[email protected]


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)ade of reinforced concrete rested on defor)able subgrade and loaded by controlled

e(ternal load u& to failure! using theoretical and e(&eri)ental )eans*

( CALCULATION OF PUNCHING SHEAR ACCORDING TOACTUAL DESIGN CODES

The fact that none of calculation )odels is generally acce&ted has led to considerable

variation in the reco))endations of actual international and national codes* +n )ost of the

the) se)i.e)&irical )ethod of critical cross.section is re&resented* This )ethod is based on

the assu)&tion that the footing is &unched 'hen a vertical fracture cross.section throughout

the &eri)eter of &enetration body is for)ed near the colu)n* This cross.section is called a

critical or control section*

$ased on this! the &unching calculation 0'hich is a )andatory &art of the calculation of slabs

and foundations4 co)es do'n to the control of shear stress in the critical section! and

co)&are the shear calculating stress in the control section cal

! at so)e distance fro) the

edges of the colu)n! 'ith the the &unching shear resistance v * +f the re-uire)ent vcal


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n , net reactive soil &ressure fro) the force in the colu)n

bF , surface area of the &unching ele)ent in the &lane of the reinforce)ent

+t should be noted that at floor slabs there is no reduction in the colu)n nor)al force given

in E-* 024*

unching shear resistance v is ado&ted de&ending on the co)&ressive strength of concrete!coefficient of reinforce)ent 'ith longitudinal reinforce)ent tased to receive the bending

tensile forces! and di)ensions of the slab! that is! foundations* The values of these

&ara)eters have not been given &recisely in the theory of reinforced concrete! so they are

given in the codes as e)&irical &ara)eters based on e(&eri)ents* The codes treat the

influence of these &ara)eters in various 'ays! and in so)e of the)! so)e of the )entioned

&ara)eters are o)itted* Further on! a revie' of the follo'ing regulations 'ill be given;

Euro&ean EN 12.1.1;299: and CE$.F+ B1D "er)an


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As the area of the base of &unching ele)ent directly de&ends on the &osition and sha&e of the

control cross.section! the reduced factori#ed nor)al force Pu,red 'ill significantly vary in

calculation of &unching shear based on the different codes*

Certain codes do not distinguish bet'een &unching of floor slabs and of the foundations!

using the sa)e e(&ressions in both cases 0$A$.34! 'hile Eurocode 2! odel code 9!


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a1cal3

2 0>4

no s&ecial calculation reinforce)ent for rece&tion of tensile forces due to the action

of transverse force Tmax is necessary*

hen the shear stress cal is 'ithin the range

b2cala13

2


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$ is reinforce)ent coefficient

2"1c#

3"2min f#&3).&v = is )ini)al shear strength of concrete

dis static height of foundations

+f c,Rdcal v> it is necessary to install reinforce)ent*

The calculation is deter)ined by the cross.section in 'hich the &unching strength value is

the lo'est* +t is &er)issible that the &ressure of the soil 'ithin the control circu)ference is

subtracted fro) the &unching force*

(-7 89:; (-


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The $ritish standards &rescribe that shear stresses are calculated for the cross.section 'hich

is at a distance of 1*=d fro) the &eri)eter of loaded surface* The critical cross.section has a

rectangular for)* +t is also &er)issible to reduce the shear force for the value of reactive

&ressure of soil calculated for the area 'ithin the critical cross.section! loaded by evenly

distributed load*

+f the shear stress along the critical cross.section is lo'er than the &unching shear resistance

vc the reinforce)ent for rece&tion of &unching shear forces is not necessary* %ere! vc is given

by the e(&ression;

31

cu*

1

3

1

v

s

mc %

2)

f'%

d

*&&'%

db

+1&&'

-.&v

=

04

here;

m is a &artial safety factor 0usually 1*2=4

=db

+

v

s is reinforce)ent coefficient

cuf is the co)&ressive strength of concrete 'hich should be assu)ed to be )a(i)u) of

:9a! 'hile for the values lo'er than 2=a this article should be o)itted*

a(i)al shear stress is also li)ited by the value 2"1cuf(.& ! i*e* vale of =*9a along the

colu)n circu)ference*

(-@ DIN 11 (


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(- ACI 713 1 , 92 assu)es the critical cross.section at d/2 fro) the

edge of the colu)n! and its for) corres&onds to the sha&e of the colu)n* +t is &er)issible

that the &unching force is reduced for the &art of soil reaction beneath the &unching ele)ent*

Calculation shear stress cal 0here designated as uv 4! should be s)aller than the

calculation shear resistance cv ;

cu vv 9 for e(ternal and 29 for cornercolu)ns

c is ratio of the longer to the shorter side of concentrated load or reactive surface

cf is co)&ressive strength of concrete for the cylinder

A se&arated foundation footing is considered rigid! 'ith even &ressure on the soil forconcentrated centric load* +t is &er)issible to reduce the shear force for the value of effective

soil &ressures inside the circu)ference of control cross.section*

+f cu vv > shear reinforce)ent )ust be installed*

7 EXPERIMENTAL ORGANISATION

According to the scientific literature data! only a fe' e(&eri)ents 'ith footings in real soil

'ere &erfor)ed until no' 0%egger et al*! 299H! 29934* ain reason for that is considerable

)aterial cost and co)&le( organi#ation of the e(&eri)ent! so in the )a@ority of other

e(&eri)ents natural subgrade soil under footing si)ulated by s&rings 0Talbot! 11>4! bys)all hydraulic @acs or by si)&le line load on the contact surface of the footing 0%allgren et


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al*! 14* Considering this! it 'as decided to &erfor) the e(&eri)ent on the natural

subgrade soil! in real conditions in si$u! 'ith footings , test s&eci)ens and corres&onding

load* Co)&le(ity of the 'hole &roble) re-uired serious &re&aration and detailed &lanning*

$asic conce&t of the &re&aration &lan should enco)&ass;

selection of the a&&ro&riate site for the &erfor)ing of the e(&eri)ent

selection of the a&&ro&riate site for the &erfor)ing of the e(&eri)ent

nu)ber of the test s&eci)ens , footings! their si#e! ty&e! and -uality of the )aterial

0concrete and reinforce)ent4! 'ay of )anufacturing

&erceiving of all &ara)eters of the subgrade soil and of the footing that should be

identified in advance! or )easured during the e(&eri)ent

analysis related to the 'ay of the )ode of footing loading! selection of the loading

e-ui&)ent

selection of the e-ui&)ent for )easuring and tracing do'n all scheduled

&ara)eters in the subgrade soil and in the footings

identification of the financing &lan and &roviding the necessary financing resources!

)an&o'er! and necessary accessory and trans&ort devices

a&&ro(i)ate ti)e needed for all &hases during the e(&eri)ent


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NOlting0%egger 299H!29934 ar

999 399 to 3:9

?ivin0?ivin 1H34 1H3 surface/clayand sand

H -uadratic H=9 and1999

129

?ichart0%egger 299H!29934 1: s&ring 1:

-uadratic

and

circular

H19 to >999 299 to 3:9

Talbot0Talbot 11>4 11> s&ring29

0in &unching4-uadratic 1=29 2=9

E(&eri)ental research conducted in 299 0$oniG 2919! 29114 envisaged &roduction of a

)odel in si$u'ith &revious &re&aration of the subgrade 'ith deter)ined geo)echanical

characteristics and 'ith &roduction of test s&eci)ens . colu)n footings of s&ecifieddi)ensions and defined characteristics of concrete and reinforce)ent* Fig* : &rovides a

sche)e of the structural asse)bly used for the e(&eri)ent* Asse)bly consisted of the

test fra)e! test s&eci)ens! hydraulic @ac! and &re&ared soil*

Fig* :* Sche)e of the e(&eri)ental setu& Fig* =* Fra)e lo'ered in the &it

7-1 Te)* .ra0e

The fra)e 'ith steel &late botto) is laid into the &re&ared footing &it! :*9(=*9 ) in base! and

>*9 ) dee&* Soil )aterial is &laced over the botto) of the fra)e! 'ith di)ensions of>*=(>*=*9 ) in baseD the soil has layers of s&ecified thicness! co)&acted to the re-uired

value of co)&ressibility )odulus*

Truss structure of the fra)e! as 'ell as its di)ensions should &rovide undisturbed for)ation

of sliding surfaces in the soil beneath the foundations! should! in the course of loading! the

soil failure &recedes the &unching shear*

This allo'ed foundations testing 'ith co)&letely realistic boundary conditions in ter)s of

soil and also co)&arison and verification of earlier testing results in laboratories 'ith testing

in si$u*

7-( Te)* )e+i0en)


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The selected footing di)ensions are =(= c) at the layout and corres&ond to the

e(&eri)ents of innunen and %egger 0for the &ur&ose of result co)&arison4! and also

corres&ond to the ca&acity of available )easuring e-ui&)ent 01999 N4*


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Fig* H*


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Fig ** Arrange)ent of &ressure gauges

7-= Te)*in ro+e&6reFooting is &laced on the soil surface and loaded by vertical centric force 'hich is a&&lied by

a hydraulic @ac &ositioned bet'een the cross.bea) and the footings 0Fig* *4* For footingloading! a hydraulic @ac 'ith the ca&acity of 1999 N 'as used! a&&lying the load in load

ste&s of =9 N* The load 'as e&t constant at every load ste& until the total consolidation of

the ground at that load 'as achieved* The consolidation 'as registered by observing the

&rocess of vertical dis&lace)ents of &oints at the footing corners and on the colu)n of the

footings*

The e(&eri)ent should deter)ine the influence of ey &ara)eters on the foundation

&unching )echanis)! such as; ty&e and characteristics of subgrade! irregularity of contact

&ressures! concrete co)&ressive and tensile strength! &osition of reinforce)ent! a&&lied

reinforce)ent &ercentage etc*


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characteristic values of a&&lied force* Bbserving these figures! one )ay conclude that

)a(i)u) strains in concrete are registered by the )easuring ta&e i))ediately near the

colu)n 0)easuring &oint 1 in Fig* H*4! 'hile )a(i)u) strain are )easured by the )easuring

&oints in the a(is of the colu)n or i))ediately ad@acent to the colu)n 0)easuring &oints 2

or > in Fig* H*4*

Fig *19* Co)&arative revie' of strains in reinforce)ent and in concrete of footing T++


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Fig *11* Co)&arative revie' of strains in reinforce)ent and in concrete of footing T+.19

Fig *12* Co)&arative revie' of strains in reinforce)ent and in concrete of footing T+V


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Fig *1>* Co)&arative revie' of strains in reinforce)ent and in concrete of footing TV

Fig *1:* Co)&arative revie' of strains in reinforce)ent and in concrete of footing TV+


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Fig *1=* Co)&arative revie' of strains in reinforce)ent and in concrete of footing TV++

The registered strains in concrete considerably differ fro) those recorded by %egger and

others* Na)ely! %egger recorded )ostly concrete co)&ression strains 'ith very s)all tensile

strains of )assive foundations! 'hile in this case! in al)ost all the foundation ty&es 0e(ce&t

T++ foundations4 a&art fro) co)&ressive strains! also the significant tensile strains 'ere

recorded*

Co)&arison of obtained strains in reinforce)ent and strains in foundation reinforce)ent

e(a)ined by %egger and other sho's a fairly 'ell congruence* +n both e(&eri)ents!

)a(i)u) strains 'ere detected in the colu)n #one! in the a(is or on the edge of the colu)n*

For three foundation ty&es 0out of five4 'hich 'ere e(a)ined by %egger! there 'as

reinforce)ent yield &rior to &unching! 'hile in this case! there 'ere reinforce)ent yield in

cases out of * %o'ever! great strains 'ere detected even in those foundations 'hose

reinforce)ent did not start to yield! so it )ay be concluded that only the eccentricity of

a&&lied force is the reason for absence of yield strains* This indicates the i)&ortance of

reinforce)ent in the footing &unching &rocess! so it is necessary to analy#e its influence

through the a&&lied &ercentage of reinforce)ent! arrange)ent of reinforce)ent and

)echanical &ro&erties of reinforce)ent 0yield li)it4*

= CONCLUSION

The conducted e(&eri)ental research )ade it &ossible to observe considerable differences in

&unching shear of colu)n footings on gravel! tested in realistic conditions in si$u, in

co)&arison to the earlier researches 'hich 'ere &erfor)ed in si)ulated conditions in

laboratories*

Bn the basis of the results of conducted e(&eri)ental research and subse-uent analysis! the

follo'ing conclusions can be dra'n;

The largest strain of reinforce)ent in all the e(a)ined footings are registered at the)easuring &oints i))ediately ne(t to the colu)n or in the colu)n a(is


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+n the footings 'hich failed by &unching shear! strains in reinforce)ent reach the

yield &oint! or they are close to it! so it can be said that footing &unching shear is

related to generation of large strains in reinforce)ent

The highest strains in concrete 0both co)&ressive and tensile4 'ere achieved at

)easuring &oints i))ediately ne(t to the colu)n! 'hile the re)aining )easuring&oints registered &rogressive decrease of strain as the distance fro) the colu)n

increases*

A+no!e&e0en*? The &a&er is result of the investigation in the &ro@ects T? >H92financed by the inistry of Education and Science of the ?e&ublic of Serbia

REFERENCESAC+ Co))ittee >1! L$uilding Code ?e-uire)ents for Structural Concrete 0AC+ >1.924

and Co))entary 0>1?.924! +merican !oncre$e /ns$i$u$e! Far)ington %ills! ich*! 2992!

::> &&*

$oniG X! Wacev T! roloviG W! i@aloviG ! =.1:>*

Eurocode 2;


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\7]^7_ 5*`; \ __j]!;?ABC?DE G=HDABIJK! k7]! 1H3 0in ?ussian4*

Talbot! AN; ?einforced Concrete all Footings and Colu)ns under Concentrated oads*

?esearch and

PUNCHING SHEAR STRENGTH OF COLUMN FOOTINGS IN ACTUAL DESIGN CODES AND THEIR EXPERIMENTAL...

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