design & detail col base plate

7/27/2019 Design & Detail Col Base Plate

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STRUCTURAL STEEL EDUCATIONAL COUNCIL

TECHNICAL NFORMATION PRODUCTSERVICE

JULY 999

P r a c t i c a l D e s i g n a n d D e t a i l i n g

o f S t e e l C o l u m n a s e P l a t e s

y

W i ll i am C H o n e c k

D e r e k W e s t p h a l

F o r e l l E l s e s s e r E n g i n e e r s

Inc


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cknowledgments

T h e a u t h o r s w i s h t o t h a n k t h e f o ll o w in g p e r s o n s f o r t h e i r i np u t , r e v i e w a n d c o m m e n t s o n t h e c o n t e n t o f th i s

teel Tips

pub l i ca t i on :

M e m b e r s o f t h e S t r u c t u r a l S t e e l E d u c a t i o n a l C o u n c i l

R o g e r F e r c h , H e r r i c k C o r p o r a t i o n

B e r n i e L o r i m o r , R o c k y M o u n t a i n S t e e l

S t e v e R i c h a r d s o n , W W S t e e l C o m p a n y

R i c k W i lk e n s e n , G a y l e M a n u f a c t u r in g

D a v e M c E u e n , C a l i fo r n ia E r e c t o r s

J i m M a l le y , D e g e n k o l b E n g i n e e r s

J im P u t k e y

M a s o n W a i t e r s , F o r e l l / E l s e s s e r E n g i n e e r s .

P r o f e s s o r S u b h a s h G o e l , U n i v e rs i ty o f M i c h i ga n

Disclaimer

T h e i n fo r m a t io n p r e s e n t e d i n t h i s p u b l i c a ti o n h a s b e e n p r e p a r e d i n a c c o r d a n c e w i t h r e c o g n i z e d e n g i n e e r i n g

p r inc ip le s a nd cons t ruc t i on p rac t ices and i s fo r gen e ra l i n fo rm a t ion on ly . W hi l e i t i s be l i eved to be ac cu ra t e ,

t h i s i n fo rma t ion shou ld no t be u sed o r re li ed upon fo r any spec if ic app l i ca ti on w i thou t com pe te n t p ro fess ion a l

exam ina t ion and ve r i fi ca ti on o f i ts accu ra cy , su i tab i li ty , an d app l i cab i li t y by a l icensed p ro fess iona l eng ine e r

o r a rch it ec t. The pub l i ca t ion o f t he ma te ri al con t a ined he re in i s no t i n t ende d a s a rep re sen t a t i on o r wa r ra n ty

o n t h e p a r t o f t h e S t r u c t u r a l S t e e l E d u c a t i o n a l C o u n c i l, o r o f a n y o t h e r p e r s o n n a m e d h e r e i n , th a t t h is

i n f or m a t i o n i s s ui ta bl e f o r a n y g e n e r a l o r p a r t i c u l a r u s e o r o f f r e e d o m i n f r in g e m e n t o f a n y p a t e n t o r p a t e n t s .

A n y o n e u s i n g t h i s i n f o r m a t i o n a s s u m e s a l l li a bi li ty a r i si n g f r o m s u c h u s e .


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No

PRACTICALDESIGNAND DETAILINGOF STEEL COLUMNBASE PLATES

Table of Contents

Description Page No.

1 0

2 0

3 0

4 0

5 0

6 0

I N T R O D U C T I O N 2

1 1 Prefa ce 2

1 2 P u rp o se 2

1 3 O rg a n iz a t io n 2

D E S I G N G U I D E L I N E S F O R M A T E R I A L S A N D F A B R I C A T I O N

2 1 M a te r i a l s

2 1 1 A n c h o r B o l t s a n d N u t s

2 1 2 P l a t e s

2 2 B a se P l a t e D e s ig n fo r F a b r i c a t io n

2 2 1 Ma te r i a l v e r su s L a b o r

2 2 2 W e l d i n g

2 2 3 B a se P l a t e D im e n s io n s

D E S I G N G U I D E L IN E S R E L A T E D T O E R E C T IO N

3 1 A n c h o r B o l t s

3 1 1 A n c h o r B o l t P o s i t i o n M is lo c a t io n

3 1 2 R o t a t e d A n c h o r B o l t P a t t er n s

3 1 3 A n c h o r B o l t s S e t T o o L o w o r T o o H i g h

3 1 4 C o l u m n s N e x t to W a l l s

3 2 W a s h e r s

3 3 B a se P l a t e L e v e l in g

E N G I N E E R I N G G U I D E L I N E S F O R D E S IG N O F B A S E P L A T E S

4 1

4 2

4 3

4 4

4 5

4 6

4 7

D e s i g n f o r T e m p o r a r y C o n s t r u c ti o n L o a d s

D e s i g n f o r G r a v it y an d O t h e r D o w n w a r d L o a d s

D e s ig n fo r G ra v i ty L o a d s in C o m b in a t io n w i th U p l i f t L o a d s

D e s ig n fo r G ra v i ty L o a d s in C o m b in a t io n w i th S h e a r F o rc e s

D e s ig n fo r G ra v i ty L o a d s in C o m b in a t io n w i th S h e a r F o rc e s a n d Mo m e n t s

D e s ig n fo r Mo m e n t s d u e to S e i sm ic F o rc e s

A rc h i t e c tu ra l I s su e s

C O N C L U S I O N S

R E F E R E N C E S

3

3

3

3

4

4

4

5

8

8

9

10

10

12

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PRAC TICA L DESIGN AND DETAILING OF STEEL COL UMN BASE PLATES

1 0 INTRODUCTION

1 1 P r e f a c e

St e e l c o l umn ba s e p l a t e s a re on e o f t he mos t

~ nda me n t a l pa r t s o f a s t e e l s t r uc t u r e , ye t t he

de s i gn o f ba s e p l a t e s is c omm on l y no t g i ve n t he

a t ten tion tha t it should by engineers . This resu l t s

in base plate detai ls tha t are expensive , diff icul t to

f a b r i c a t e a nd ma y e ve n c on t r i bu t e t o t he ha z a r ds

o f t he s t ee l e r e c t ion p r oc e s s by no t p r ov i d i ng

s tab i li ty for e rec t ion load s appl ied to the co lumn.

Base p la tes se rve tw o bas ic fianct ions :

1 . They t ransfe r co lumn loads to the suppo r t ing

mem ber or found a t ion . These loads inc lude ax ia l

due to grav i ty , moments , shears and somet imes

axial due to upl if t ;

2 . The y a l low the co lum n to s tand as a

tem porary vert ical cant i lever af ter the l i f t ing l ine is

r e l ea s e d w i t hou t ha v i ng t o guy o f f t he c o l umn .

T he c o l umn a nd ba s e p l a t e mus t w i t h s t a nd

tem pora ry wind and e rec t ion loads sa fely .

S teel fabr ica tors and e rec tors w ho a re m embers of

t he S t r uc tu r a l S t e e l E duc a t i ona l Counc i l SSE C )

ha ve c omm e n t e d t ha t t he r e a r e a va r i e ty o f ba s e

pla te des igns and de ta i ls f rom engineers. Som e

fabr ica tors a re c r i t i ca l o f many of these des igns

beca use they a re d i f f icu l t to fabr icate , o r spec i fy

ma t e r i a l s t ha t a r e ha r d t o ob t a i n o r t ha t do no t

ex is t in the s izes spec i fied . The des igns of ten

resu l t in co lumns tha t a re hard to e rec t o r a re

uns ta b le w i t hou t guy i ng t he c o l umn . W he n a nc ho r

bol t s a re no t p roper ly se t , expens ive cor rec t ive

wo rk i s required before the co lumn can be e rec ted ,

resu l t ing in de lays in the s tee l e rec t ion process .

This publ ica t ion of teel Tips a t tempts to address

these issues.

I n o r de r t o u nde r s t a nd be t t e r a nd re s pond t o t he

fabricat ion and erect ion issues, a qu est ionna ire was

dis t r ibu ted to severa l SSEC member f i rms

r e que s t i ng t he i r c omme n t s a bou t p r ob l e ms

expe r ienced in the i r shops dur ing fabr ica t ion and

in the f ie ld durin g steel erect io n. Specif ic issues

inc luded over ly expens ive des igns and problems

with obtaining the materials specif ied. S ugg est ion s

on how t he s e de s i gns c ou l d ha ve be e n mor e

e c onom i c a l we r e s o li c it e d . T he que s t ionna i r e

asked about ~ tee l e rec t ion problems exper ienced

a nd r e que s t e d s ugge s t i ons t o m i t iga t e t hos e

p r ob l e ms . T he r e s pons e s r e c e ive d we r e ve r y

i n f o r ma t i ve a nd ma ny o f t he s ugge s t i ons i n t he

r e s pons e s ha ve be e n i nc o r po r a t e d i n t o t h i s

publ ica t ion .

1 2 P u r p o s e

T he p u r pos e o f t h is is s ue o f teel Tips i s to

provide prac t ica l gu ide l ines for engineers ,

f a b r i c a to r s and c on t r a c t o r s r e ga r d i ng t he de s ign

and de ta i l ing of s tee l co lumn base p la tes .

Gu i da nc e i s p r ov i de d t owa r d r e s o l v i ng c ommon

des ign , fabr ica t ion and e rec t ion problems. M any

of the top ics d i scussed a re s imple to implement ,

ye t a r e o f t e n ove r l ooke d .

Unf o r t una t e l y t he be ha v i o r o f ba s e p l a t e s i n

mome n t f r a me s a nd b r a c e d f l a me s s ub j e c t e d t o

ear thq uake forces i s no t f ia lly unde rs tood .

Re s e a r c h a nd c ode gu i da nc e a r e l im i t ed . T he

e ng i ne e r i s f o r c e d t o u s e j udge me n t i n o r de r t o

achieve a des i red leve l o f per formance and i t i s

hope d , tha t th i s pu bl ica t ion wi l l in i t i a te more

r e s e a r c h a nd de ve l opme n t in t he a r e a s o f ba s e

pla te behavior and des ign guide l ines for base p la te

assembl ies tha t a re subjec ted to h igh moments

wh e r e s ome s o r t o f y i e l d ing i s ne c e s s a r y t o

achieve the des i red per form ance .

1 3 O r g a n i z a t i o n

T he f oc us o f t h i s i s sue o f

teel Tips

is directed

t ow a r d t he p r a c ti c a l a spe c t s o f t he de s i gn a nd

de ta i l in g of base p la tes par t icu la r ly as they re la te

to economica l fabr ica t ion and s tee l e rec t ion .

Sec t ion 2 .0 d i scusses fabr ica t ion issues. Sec t ion

3 .0 d i scusses e rec t ion and anchor bo l t p lacement


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issues. Section 4.0 discusses the issues involved

in the design of base plates, rather than providing

how to design methods or guidelines, and lists

the names of other authoritative publications

where the reader can find design formulas and

definitive procedures for design of base plates.

Section 4.0 also discusses fixed and partially fixed

column bases, for instance, moment frames which

resist wind or earthquake forces.

2 .0 D E SI G N G U I D E L I N E S FO R

MATERIAL S AND FABRI CATION

Engineers have numerous types of steel to choose

from when designing anchor bolts and base plate

assemblies. However, materials are often specified

that are not readily available or are not suitable for

specific applications. Base pla te details often are

hard to fabricate, overly complicated, call for

expensive welds and/or specify impossible welds.

The following sections provide design guidelines

for specifying suitable materials and suggestions

for details to make fabrication easier and more

economical.

2 1 M a t e r i a l s

A c c o rd in g to t h e A IS C S p e c i f i c a t io n fo r

Structural Steel Buildings Allowable Stress Design

and P las t ic Des ign (ASD Spec i f ica t ions) , there a re

1 6 A S T M d e s ig n a t io n s sp e c i f i e d fo r s t ru c tu ra l

appl ica t ions . Fo r spec i f ic mater ia l p roper t ies ,

su i tab le appl ica t ions and comple te d imens iona l

in format ion , the reader should re fe r to the ASTM

Speci f ica t ions .

2 1 1 A n c h o r B o l ts a n d N u t s

T h e m o s t c o m m o n a n d r e a d i ly a v a i la b l e a n c h o r

bol t mater ials a re AS TM A36 and A307. Smal le r

bo l t s ge0era l ly a re suppl ied in A307 and la rger

diameter in A36. Th e material prop ert ies fo r thes e

re la t ive ly low s t reng th bo l t s a re very s imi la r.

T h e se tw o g ra d e s a r e w e ld a b le a n d sh o u ld b e

spec i f ied when poss ib le .

Wh e n h ig h - s t r e n g th b o l t s a r e r e q u i r e d , t h e

mater ia ls typ ica l ly ava i lab le a re A449, A354 and

A 1 9 3 ty p e B 7 (o f te n r e f e r re d to a s B 7 ) . B 7

b o l t s a r e th e s a m e m a te ri a l a s A IS I 4 1 4 0 a n d c a n

b e su b s t i t u te d fo r A 4 4 9 b e c a u se A 4 4 9 a n d B 7

bol ts bo th have mater ial p rop er t ies th a t a re a lmost

id e n ti c a l. A 3 2 5 b o l t s o n ly c o m e in h e a d e d

form, a re limi ted to 1 1 /2 inch d iame ter max imum

and are limi ted in the leng ths ava ilab le. The

propert ies and chemistry for A325 bo lts are s im ilar

to A449 and B7. Genera l ly , it i s be t te r to spec i fy

A 4 4 9 , A 3 5 4 o r B 7 b o l t s w h e n h ig h - s t r e n g th b o lt s

a r e n e c es sa ry . H ig h - s t r e n g th b o l ts c o m e a s p la in

bar s tock and th reads must be cu t in to bo th ends .

H e a d e d b o l t s f a b r i c a t e d f ro m A 3 2 5 , A 4 9 0 o r

A588 should no t be spec i f ied s ince these a re no t

read i ly ava i lab le . A l l o f these h igh s t rength

mater ia ls a re hea t t rea ted a l loy s tee ls and a re

therefore no t su i tab le fo r weld ing . Before

specifying a bolt material , c on tact local fabric ators

for in format ion reg ard ing mater ia l av a i lab i li ty and

re v ie w th e A S T M s t a n d a rd s fo r t h e g ra d e s b e in g

cons idered to de te rmine th e i r su i tab i l ity .

I t i s im p o r t a n t t o sp e c ify t h e c o r r e c t g r a d e o f n u t

th a t c o r r e sp o n d s to t h e sp e c i f i e d a n c h o r b o l t

m a te r i a l. A S T M A 5 6 3 sp e c if i e s t h e v a r io u s n u t

g ra d e s t h a t a r e t y p i c a l ly u se d in b u ild in g

c o n s t ru c t io n a n d n u t s su i t a b l e fo r u se w i th t h e

v a r io u s g ra d e s o f b o l t s ( s e e R e fe re n c e 4 ). T h e

H e a v y H e x n u t s ty le sh o u ld b e sp e ci fi e d

regard les s o f the nu t g rade tha t i s se lec ted .

F o o tn o te A b e lo w t a b l e X 1 . 1 m a k e s r e f e r e n ce to

A S T M A 1 9 4 g ra d e 2 H a s a su b s t i t u t e fo r A 5 6 3

w h e n c e r t a in s i z e s c o n fo rm in g to A 5 6 3 a r e n o t

ava i lab le . A19 4 is a spec i f ica t ion for p ressure

vesse l and non-bui ld ing uses , bu t the grades

re f e r e n c e d in fo o tn o te A a re su it a b le fo r u se fo r

anch or bo l t s in bu i ld ings .

2 1 2 P l a t e s

T h e m o s t c o m m o n b a se p l a t e m a te r i a l s a r e A 3 6 ,

A 5 7 2 a n d A 5 8 8 . F a b r i c a to r s r e sp o n d in g to t h e

q u e s t io n n a i re r e c o m m e n d e d th a t A 3 6 m a te r ia l b e

spec i fied i f poss ib le becau se i t is the mo st read i ly

available material. Th e table on the follow ing pa ge


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contains material availabilityguidelines based on

plate thickness

T a b l e 1 - A v a i l a b i l i t y o f P l a t e M a t e r i a l

T hi c kne s s ( t )

t _< 4

4 < t _< 6

Pla te Avai lab i l ity

A 3 6

A572 G r 42 o r 50

A588 G r 42 o r 50

A 3 6

A 5 7 2 G r 4 2

A588 G r 42

t > 6 A36

2 .2 B a s e P l a t e D e s i g n fo r F a b r i c a t i o n

Typica lly , except for very la rge c o lum ns w i th very

heavy base p la tes , such as for h igh r ise bu i ld ings ,

base p lates a re shop welded to the co lumn. Unless

the weld i s a co mp le te pene t ra t ion , weld , the

bo t t om e nd o f the c o l um n ne e ds to be c u t s qua r e

so tha t there wil l be fu ll bear ing w here the co lum n

is in contac t wi th the base p la te . Som e years ago ,

th i s was accompl i shed us ing mi l l ing machines in

t he s hop . T od a y t he c o l d s a wi ng e qu ipme n t u s e d

in mos t shops provides a co lu mn f in i shed end w i th

a ma x i mum AN SI r oughn e s s he i gh t va l ue o f 500

wh i c h i s s a t i s f a c t o r y f o r c on t a c t be a r i ng

c ompr e s s i on j o i n t s.

For very la rge co lumns , the base p la te i s e rec ted

f i r s t , us ing three leve l ing bol t s a round the

per im eter of the base p la te to leve l i t, then the

c o l umn i s e r e c t e d on t o t he ba s e p l a t e a nd

c onne c t e d u s i ng a ng l e s o r o t he r c onne c t i on

me t hods . T he ba s e p la t e i s g r ou t e d be f o r e t he

column i s e rec ted . Th e ma t ing sur faces should be

p r e pa r e d by mi l li ng o r o t he r me a ns s o t ha t t he

colum n i s in fu ll con tac t wi th the base p la te . Use

o f t h i c k ba s e p l a t e s c a n i n t r oduc e we l d i ng

p r ob l e m s . due t o d i ff i cu l ty o f me e t i ng p r e he a t

requi rements .

2 . 2 . 1 M a t e r i a l v e r s u s L a b o r

A c om mo n s ugge s t i on f r om s te e l fa b r i c a to r s fo r

engin eers to remem ber is tha t mater ia l i s cheap

re la t ive to labor . I f spec i fy ing th icke r base p la tes

wi l l resu l t in no t hav ing to add s t i ffener p la tes to

the base plate , this wil l resul t in less labor to

fabr ica te and wi l l resu l t in a more economic

des ign . Add ing s ti f feners and o the r p la tes to a

base p la te assembly i s l abor in tens ive com pared to

us ing a th icker base p la te tha t cou ld e l imina te the

need for these addi t iona l s t i f fener p lates .

2 . 2 . 2 W e l d i n g

T he e ng i ne e r s hou ld a t t e m p t t o a t l e a s t ma t c h t he

th ickness of the base p la te wi th the co lum n f lange

t h i c kne s s i n o r de r t o p r e ve n t wa r p i ng du r i ng

weld ing , par t icu la r ly i f heavy weld ing , such as

par t ia l o r com ple te pene t ra t ion welds , i s requi red

t o c onnec t t he c o l umn t o t he ba se p la t e. T h i c ke r

base p la tes wi thout s t i f feners a re of ten more

e c onom i c a l tha n u s i ng a t h i nne r ba s e p l a t e w i t h

st i ffeners . St i ffeners , i f used, w il l hav e an imp act

on co lum n f in i sh d imens ions . See Sec t ion 4 .7

Arc hi tec tura l I ssues for fur ther d i scuss ion .

Ano ther com mo n sugges t ion from fab r ica tors i s to

reduce we ld s izes as much as possible (but acc ou nt

f o r m i n i mum AW S we l d s iz e s ba s e d on m a t e ri a l

th icknesses ) and spec i fy f i l l e t welds in l i eu of

c om pl e t e pe ne t r a t i on we l ds w he r e pos si b le .

Compl e t e pe ne t r a t i on we l ds r e qu i r e mor e l a bo r

due to the need to beve l the end o f the co lum n and

fi t up, and require extens ive inspect ion . I t is m ore

eco nom ica l to d e ta i l l a rger f il l e t welds , ev en i f

more w eld meta l i s requi red for the f i ll e t welds , as

a subs t i tu te for par t ia l pene t ra t ion welds .

Fabr ica tors have a lso po in ted out th a t a l l a roun d

welds should be avoided . F i l l e t welds tha t wrap

a r ound t he f l a nge t oe s ( e nds o f c o l umn f l a nge s)

a nd t he c o l um n we b- t o - f l a nge f il le t s ( t he k

reg ion) can cause c racks due to h igh res idua l

s t resses in the welds . Such welds of ten requi re

weld ing repa i r . S top f i l l e t welds 1 /2 inch f rom

these loca t ions . See F igure 1 for c la r if ica t ion .

W elds should be de ta i led to a ccoun t for c learances

a nd a c c e s s o f we l d i ng e qu i pme n t . Obv i ous l y t he

e ng i ne e r s hou l d no t s how we l ds t ha t a r e


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impossible to access. For example, a common

mistake is to specify all around welds at plate

washers that are backed up against the column

flange or web.

High strength bolts fabricated from high strength,

heat treated steel (such as A354, A449 or B7)

cannot be welded - not even tack welded - without

adversely affecting the properties of these steels.

2 2 3 B a s e P l a te D i m e n s i o n s

Wh e re p o s s ib l e , t h e p l a t e d im e n s io n s a n d b o l t

pa t te rn of base p la tes should be symm etr ica l about

b o th a x e s. T h i s w i l l p r e c lu d e w e ld in g th e b a se

p la t e ro t a t e d 9 0 d e g re e s f ro m th e c o r r e c t

o r i e n ta t i o n . H a v in g a d o u b ly sy m m e t r i c al b o l t

pa t te rn wi l l a l so he lp avoid po ten t ia l f ie ld

problems (See Sec t ion 3 .1 .2) .

T h e e n g in e e r sh o u ld t ry t o sp e c i fy t h e s a m e b o l t

h o le d i a m e te r w h e n e v e r p o s s ib l e to e l im in a t e t h e

need fo r mul t ip le d r il l b i t s izes . This a lso appl ies

to a n y v e n t h o le s re q u i r e d to v e n t o u t a i r f ro m

u n d e r t h e l a rg e r b a se p l a t e s d u r in g th e g ro u t in g

opera t ion .

O b v io u s ly t h e b a se p l a t e d im e n s io n s sh o u ld b e

suff ic ien t to accommodate the co lumn d imens ions

p lus anchor bo l t ho les wi th suf f ic ien t d imens ions

to t h e c o lu m n f l a n g e s a n d to t h e e d g e o f t h e b a se

p la te . A lso acco unt fo r any square p la te wash ers ,

i f used . Severa l fabr ica tors have s ta ted tha t

e n g in e e r s so m e t im e s e r ro n e o u s ly a s su m e th e i r

typical base plate detai l wil l cov er al l cond it ions.

C o lu m n s th a t a r e i n d i ff e r e n t s iz e g ro u p s r e q u i re

d i f fe ren t base p la te s izes . I t is genera l ly more

economica l to des ign a typ ica l la rger base p la te

to c o v e r m o re th a n o n e c o lu m n s i z e i n a c o lu m n

g ro u p ( su c h a s Wl0 , W1 2 , W1 4 g ro u p in g s ) , t h a n

to design specific base plates for each colum n size.

T h e f e w e r v a r i a t io n s o f b a se p l a t e s r e q u i r e d w i l l

g e n e ra l ly r e su lt i n e c o n o m y in f a b r ic a t io n e v e n i f

more mater ia l i s requ i red . This is t rue because o f

the labor sav ings in shop draw ing prep ara t ion and

the d i f fe ren t shop se tups requi red for each

var ia t ion in base p la te conf igura t ion . I t i s a l so

t ru e th a t h a v in g f e w e r d i f f e r e n t a n c h o r b o l t

pa t te rns wi l l lead to less conf iasion dur ing anchor

b o l t p la c e m e n t . S e e F ig u re 1 o n th e fo l lo w in g

page for sugges ted de ta i l s .

3 .0 D E S I G N G U I D E L I N E S R E L A T E D T O

E R E C T I O N

A n c h o r b o l t s a n d b a se p l a t e s sh o u ld b e d e s ig n e d

and de ta i led to a ccom mo date s tee l e rec t ion loads .

Som e simple, yet effecctive, at ten tion to detai ls a nd

d im e n s io n s c a n g o a l o n g w a y in h e lp in g to

p re v e n t so m e c o m m o n p ro b le m s e n c o u n te re d

dur ing s tee l co lum n erec t ion . A prev ious ed i t ion

o f teel Tips (Reference 7) con ta ins usef ia l

s t ra teg ies fo r dea l ing wi th common f ie ld e rec t ion

errors .

3 1 A n c h o r B o l ts

Anch or bo l t p lacement i s obvio us ly a d i f f icu l t task

but too of ten e r rors resu l t due to poor qua l i ty

cont ro l and qua l i ty assurance or lack of

prepa redne ss in the des ign . The re a re severa l

ways to mis loca te anchor bo lt s and typ ica l ly one of

the fo l lowing wi l l occur .

3 1 1 A n c h o r B o l t P o s i t i o n M i s l o c a t io n

P o s i t i o n m is lo c a t io n i s u n fo r tu n a t e ly a c o m m o n

p ro b le m . T h e h o r i z o n ta l l o c a t io n o f t h e a n c h o r

bolts is often incorrect by as muc h as 1 to 2 inche s.

In some cases one o f the anch or bo l t s i s no t in the

correc t loca t ion wi th respec t to the rem ain ing bo l t s

and in o ther cases the en t i re layou t i s in the w ron g

loca t ion . There a re severa l wa ys to avoid th is

prob lem dur ing the des ign phase .

1. T h e b e s t m e th o d fo r p r e v e n t in g a n c h o r b o l t

m i s lo c a t io n i s f o r t h e c o n t r a c to r t o p ro p e r ly s e t

a n d h o ld a n c h o r b o l t s i n t h e c o r r e c t p o s i t io n fo r

p lan loca t ion and e leva t ion . I t i s the cont rac tors

respons ib i l i ty to se t anc hor bo l t s cor re c t ly wi th in

th e to l e ra n c e g iv e n in t h e A IS C C o d e o f S t a n d a rd

P ra c t i c e (R e fe r t o R e fe re n c e 3 ). A c h e c k b y a n

in d e p e n d e n t su rv e y o r w i ll h e lp l o c a t e m i sp l ac e d

bolts before steel is erected so that corrections can

b e m a d e b y th e c o n t r a c to r b e fo re s t e e l e r e c tio n


8/19

r ~ V ~

WHEN REQUIRED@

GROUT P i N

I

L ~ J ~

~Tqq ~ - , ~ ~ ~ ~ ~L~ :

~ ~ . . . . . . ~

~

f~C] ~

II ~ ~

~

L = V J

~

SQUARE PLATE ~

_1

O W E L D T Y P . ~

2

GROUTHOLE

IF REQUIRED ~

NOWELDTYP ~

T O P

W S H E R

P L A T E r Y P .

OVERSIZE HOLE ~

~ LEVELINGNUT

AND WASHER~

ANCHOR BOLT~

~ Use square p la te and ho le pa ttem d imens ions where poss ib le to avo id p rob lems assodated w i th m is-p laced anchor bo lts , ro ta ted anchor

bo l t pa t te rns o r p la tes tha t a re a cc identa l ly ro ta ted 90 degrees du r ing fabr ica t ion .

~ T ry to reduce numerous base p la te var ia tions by s iz ing typ ica l p la te based on the la rgest co lumn in a s ize g roup (e .g . W l0 ' s , W 12 's o r

W 14 's ) . Reduc ing the n umbe r o f var ia t ions wi l l reduce the chance fo r e r ro r dur ing e rec t ion and fabr ica tion , and a l low fo r s im p ler

ver i fi ca t ion in the f ie ld . P rov ide max imum edge d is tance to bo l t to a l low b ase p la te s lo tt ing i f bo l ts a re m is loca ted .

~ W he n add i t iona l bo l ts a re requ i red , add add i t iona l ho les to make doub le symm et r ic bo lt pa t te rns . Th is is use fu l even i f no t a l l ho les and

bo l ts a re n eeded. Four bo l ts is the suggested m in imum fo r any b ase p la te .

~ :~ Anchor bo l ts shou ld be a t leas t 1 d iameter . Th is is benef ic ia l fo r e rec t ion sa fe ty and the ancho r bo l ts a re harder to acc identa l ly bend

in the f ie ld . Spec i fy A307 or A36 m ater ia l when poss ib le . Bo th a re eas ie r to ob ta in and we ldab le .

~ Overs ize ho les in base p la tes shou ld be used when ever poss ib le to accom mod ate anchor bo l t p lacemen t to le rances.

(~ ) P la te washe rs wi th f ie ld we lds sh ou ld be used in con junct ion wi th overs ize ho les to res is t nu t pu l l- th rough and to t rans fe r shear f rom the

base p la te to the anc hor bo l ts . Spec ia l a t ten t ion shou ld be d i rec ted toward we ld a ccess. P la te wash er shou ld have ho le 1 /16 la rger than

bo l t d iameter . W elds ma y no t be needed i f the co lumn is fo r g rav i ty on ly and there a re no shear fo rces a t the ba se o f the co lumn.

~ Lev e l ing nu ts a re recommen ded in l ieu o f leve l ing p la tes o r sh ims fo r ease o f const ruc t ion , sa fe ty and e f f ic iency .

~ The th ickness o f g rou t spec if ied shou ld accommodate the leve l ing nu ts and be in p ropor t ion to the d imens ions o f the base p la te

( fo r exam ple do no t spec i fy 3 inches o f g rou t under a W6 co lumn) .

~ Spec i fy an add i t iona l bo l t ex tens ion above the top o f the base p la te to accomm odate bo l ts tha t a re se t too low. A lso spec i fy ex t ra

th readed leng th to accomm odate bo l ts se t too h igh .

~ Spec i fy f i lle t we lds whenever poss ib le . P ar t ia l penet ra t ion we lds and comple te penet ra t ion we lds shou ld on ly be spec i fied when requ i red .

~ Avo id spec i fy ing a l l-a round we lds . There shou ld be no we ld a t the ends o f the flanges and in the fi lle t ( k reg ion) o f the co lumn.

~ I f a g rou t ho le is needed, spec i fy the same d iameter as the anchor bo l t ho les to reduce the num ber o f d ri ll b i t s izes requ i red dur ing

fabr icat ion.

F I G U R E 1 S U G G E S T E D B A S E P L A T E D E T A I L S


9/19

b e g in s . T h i s r e q u i r e m e n t sh o u ld b e in c lu d e d in

the job sp ec i f ica t ions . In addi t ion , the engine er

should spec i fy 1 /8 shee t meta l templa tes fo r every

b a se p l a te . T y p ic a l ly c o n t r a c to r s m a k e o n e m e ta l

t e m p l a t e a n d c o n s t r u c t p l y w o o d c o p ie s o f t h e

t e m p la te . T h i s m e th o d o f c o n s t ru c t in g t e m p la t e s

a n d p l a c in g a n c h o r b o l t s i n t ro d u c e s s e v e ra l

obvious oppor tun i t ies fo r e r ror .

A n c h o r b o l t s n e e d to b e r i g id ly h e ld i n p o s i t i o n

b o th to p a n d b o t to m to p re v e n t m o v e m e n t d u r in g

c o n c re t e p l a c e m e n t a n d to p r e v e n t t h e a n c h o r

b o l t s f ro m t i lt i n g. P l a t e s t h a t c o n n e c t t h e a n c h o r

b o l t s a t t h e b o t to m sh o u ld b e c o n s id e re d ,

par t icu la r ly for la rge anch or bo l t s .

2 . Spec i fy overs ize bo l t ho les in the base p la te

w i th w a sh e r p la t e s ( w e ld w a sh e r s ) t h a t a r e f i eld

welded to the base p la te (See F igure 1). The w eld

wash er should have a s tandard ho le (bo l t d iam eter

p lu s 1 /16 in c h) . T h e A IS C C o d e o f S t a n d a rd

P ra c t i c e a l l o w s th e fo l lo w in g o v e r s i z e d h o le

d iameters ,

B o l t D ia m e te r i O v e r s i z e H o le D ia .

3 /4 to 1 5 /16

1 to

2 1/2

over 2 1

F o r l a rg e r b o lt s t h i s m a y n o t b e e n o u g h o v e r s i z e

a l lo w a n c e ; a l a rg e r o v e r s i z e o f u p to 2 i n c h e s

w o u ld b e b e t t er . W e ld w a sh e r s a re n e c e s sa ry

w h e n u s in g o v e r s i z e h o le s t o p r e v e n t n u t p u l l -

t h ro u g h a n d fo r sh e a r t r a n s f e r t o t h e a n c h o r b o l t.

T h e e x t r a c o s t o f t h e w e ld w a sh e r s i s sm a l l

c o m p a re d to t h e c o s t o f m a k in g f i e ld c o r re c t io n s

and e rec t ion de lays due to misp laced anch or bo l t s .

3 1 2 R o t a t e d A n c h o r B o l t P a t te r n s

A n c h o r b o l t s w i th a n o n sy m m e t r ic a l p a t t e rn a r e

so m e t im e s tu rn e d 9 0 d e g re e s f ro m c o r r e c t

o r i e n ta t i o n . D e ta i l i n g a n c h o r b o l t p a t t e rn s w i th

d o u b ly sy m m e t r i c p a t t e rn s w i l l p r e v e n t t h i s

prob lem. See Sec t ion 2 .2 .3 .

3 1 3 A n c h o r B o lt s S e t T o o L o w o r T o o

H i g h

Spec i fy ing ancho r bo l t s wi th ex t ra bo l t p ro jec t ion

wi l l he lp for anchor bo l t s se t too low. The ex t ra

p ro j e c t io n w i l l a l so p re v e n t t h e p ro b le m o f n u t s

tha t d o n o t ha ve f ia ll th read engag eme nt . I f a f i~ ll

n u t c a n n o t b e o b ta in e d , t h e re a r e m e th o d s to

e x te n d th e b o l t l e n g th . S p e c i fy in g A 3 6 b o l t

mater ia l a l lows weld ing a s tub on to the low bol t .

Somet imes , the nu t cav i ty above a low anch or bo l t

can be f il led ou t wi th weld meta l i f weldab le nu t

and bo l t m ate r ia ls were spec i f ied .

Engineers should spec i fy more of the b o l t shank to

be th readed than i s ac tua l ly needed . I f the bo l t i s

se t h igh , the ex t ra th read s w i l l a l low the nu t to be

ru n d o w n th e b o l t w i th o u t r e q u i r in g a d d i t i o n a l

washers .

3 1 4 C o l u m n s N e x t t o W a l l s

A n o th e r p ro b le m th a t f r e q u e n t ly o c c u r s i s

inaccess ib le anch or bo l t s due to a co lumn loca ted

next to a wall . Th is occu rs wh en the anc hor bo l t s

a r e l o c a t e d b e tw e e n th e c o lu m n f l a n g e s o r a t a

c o m e r w h e re tw o w a l l s i n t e r s ec t (S e e F ig u re 2 ) .

Fo r these condi t ions, spec ia l base p la te /anch or bo l t

pa t te rns a re necessary so tha t a l l anchor bo l t s a re

accessible. Refer to the AS D M anual , C onn ec t ion

S e c t io n , f o r a s se m b ly c l e a ra n c e r e q u i r e m e n t s a t

nuts .

WALL

FIGURE 2 INACCESSIBLE ANCHOR BOLT LOCATIONS

7


10/19

3 2 Washers

If high strength anchor bolts are tensioned ,

hardened cut washers should be used in addition

to any weld washer plates used. This will prevent

the nuts from galling the weld washer or base

plate. Normally, anchor bolts are not tensioned;

nuts are usually tightened with a wrench using a

cheater .

3 3 Base Plate Leveling

Some erectors favor the use of leveling nuts

instead of shim packs or leveling plates (See

Figure 1), other erectors favor shim packs.

Leveling nuts are easier to level and provide a

more stable base for resisting erection loads than

shim packs. Generally, levelingplates are reserved

for special cases and should not be specified for

typical use.

4 .0 ENGINE ERING GUIDELINES FOR

DESIGN OF BASE PLATES

This section covers the engineeringdesign of base

plates. The focus of this section is not so much

how to calculate base plates, but what the

engineer needs to consider when engineeringand

detailing base plates. The reader is referred to

other publicationswith formulas, design aids and

procedures. See References 1, 2, 3, 5, and 6.

The base plate assembly must be designed to

transfer all forces from the column to the

supporting member whether it is a girder or a

foundation. These forces include axial forces,

shears and moments from the column. The

magnitude and combinations of these loads will

determine he design and details of the base plate.

The simplest and most common condition

encountered in practice is a column supporting

gravity loads only. When there are high shear

forces and moments, such as in a moment frame,

the design becomes more complicatedand the base

plate and anchor bolts become heavier. Thd

followingsubsectionsdiscuss the issues related to

the various loads and combinationsof loads.

4.1 Design for Temporary Construction

Loads

The first fianctionof a base plate is to temporarily

support the column from overturning due to

temporary wind, earthquake, and erection loads,

and from the column getting bumped during

erectionuntil the,beamsare attached to tie in the

column. Therefore he base plates and anchor bolts

need to be at least sufficient to resist the

overturningmoment and shear from these forces.

Although erectors often check the column by

assuming a one kip load applied horizontally t the

top, this does not relieve the engineer from

providingan adequate design.

If the anchorbolts and base plate are too small, fo r

example, with only two anchor bolts or anchor

bolts that are too close together, the base plate

assembly may not be capable of resisting erection

loading (See Figure 3).

1~ h

4

FIGURE 3 UNSAFE ANCHOR BOLT CONFIGURATIONS

This can resu l t in a dangerous condi t ion dur ing

e r ec t i on i f t he f ab r i ca t o r and e r ec t o r h ave no t

checked the base p la te assembly for e rec t ion loads

and have not recognized that this con dit io n exists .

E i ther the base p la te assembly mus t b e en la rged by

the fabr icator dur ing the shop draw ing s tage or the

co l um n wi l l need t o be gu yed o f f o r he l d w i th

ano ther li f t ing l ine and t ied in before re leas ing

the column. This process is expen sive and i t s low s

the e rec t ion progress . Engine ers should recogn ize

t h i s and no t unde r s i ze ancho r bo l t s , make ba s e

p l a t e s t oo t h i n , o r have ancho r bo l t s t oo c l o s e

t oge t he r i n the anch o r bo l t pa t t er n . A t l e a s t f ou r


11/19

a n c h o r b o l ts s h o u l d b e d e t a i l ed a n d s p r e a d a p a r t a s

m u c h a s p o s s i b le . S e e F i g u r e 1 f o r s u g g e s t e d

d e t a i l s .

4 2 D e s ig n f o r G r a v i ty a n d O t h e r D o w n w a r d

L o a d s

T h e m o s t c o m m o n b a s e p l a t e c o n d i t i o n i s a b as e

p l a t e t h a t tr a n s f er s g r a v i t y l o a d s t o t h e s u p p o r t i n g

e m b e r o r f o u n d a t i o n w i t h r el a ti v e ly l o w s h e a r

f o r c e s a n d m o m e n t s a t th e b a s e o f t h e c o l u m n .

T h e s e a r e g r a v i t y o n l y c o l u m n s t h a t a re n o t p a r t

o f m o m e n t f r a m e s o r b r a c e d f ra m e s . T h e b a s e

p l a t e m u s t b e l a r g e e n o u g h s o t h a t t h e a r e a o f t h e

c o n c r e t e b e n e a t h i t i s s u f f ic i e n t t o s u p p o r t t h e

l o a d s . U s u a l l y t h e s e c o l u m n s w i l l t r a n s f e r n o m i n a l

s h e a r a n d m o m e n t s t o t h e s u p p o r t i n g m e m b e r o r

f o u n d a t i o n . S u c h f o r c e s a r e n o r m a l l y c a u s e d b y

s t o r y d ri f t d u e t o w i n d o r e a r t h q u a k e l o a d s .

T h e A I S C M a n u a l o f S te e l C o n s t r u c ti o n

( R e f e r e n c e 3 ) p r o v i d e s a tw o s t ep p r o c e d u r e f o r

t h e d e s ig n o f a x i a l l y l o a d e d b a s e p l a t e s . F i r s t , t h e

a r e a o f t h e p l a t e i s c a l c u l a t e d b a s e d o n t h e

a l l o w a b l e b e a t i n g s t r e s s d e f i n e d b y t h e f o l l o w i n g

e q u a t i o n s .

ASD : Fp ; 0.35 f/c A'-~ ~ 0.70 f/c

2

L RFD: %

P p

0 8 5 t p c f' c A) ~'~ -< qL 1.7f'c A~

w h e r e ,

F p

A l l o w a b l e b e a r i n g s t r e s s ( k s i )

f o = C o n c r e t e c o m p r e s s i v e s t r e n g t h ( k s i )

A ~ = B a s e p l a t e a r e a ( i nz)

A 2 = A r e a s u p p o r t i n g b a s e p la t e t h a t i s

g e o m e t r i c a l l y s i m i l a r (i nz)

q~o = 0 . 8 5 f o r c o m p r e s s i o n

Pp = U l t i m a t e c a p a c i t y o f t h e c o n c r e t e i n

b e a r i n g

B a s e d o n t h i s e q u a t i o n , t h e m o s t e f f i c ie n t b a s e

p l a t e a r e a ( A ~) is a t m o s t o n e - f o u r t h o f th e

c o n c r e t e s u p p o r t a r e a ( A 2) ; o r t h e c o n c r e t e

s u p p o r t i n g a r e a ( A 2 ) i s i d e a l l y f o u r t i m e s t h e b a s e

p l a t e a r e a ( A 0 .

II ]

E l

0 80bf

d

0 80D

~ 0 95b ~

F I G U R E 4 C R I T I C A L B E N D I N G

D I M E N S I O N S


12/19

I m I r

REBAR

~

DOW E L S~

STUDS

OR LU S

a ANCHORBOLTS

WITH PLATE

WASHERS

b SHEARKEY c

EMBEDDED SHEAR

PLATE WITH

WELDED SlOE PLATES

d EMBEDDEDSHEAR

STRUT

F IG U R E 5 - M E T H O D S T O T R A N S F E R S H E A R F O R C E F R O M C O L U M N T O F O U N D A T I O N

The f ina l s tep in de te rmining the requi red base

pla te th ickness i s def ined by ca lcu la t ing the

f lexura l dem and fo r a c r i t ica l sec t ion o f p la te

ac t ing as a can t i lever . Fo r Al low able S t ress

De s i g n ( ASD ) t he e l a st i c s e c t ion modu l us ( S ) i s

u s e d ; whe r e a s f o r L oa d a nd Re s i s t a nc e Fa c t o r

Des ign (LR FD ) the p las t ic sec t ion modulus (Z) i s

used . Th e d imen s ions o f the c r i t i ca l sec t ion are

based on 0 .95d and 0 .8bf for wide f lange sec t ions ,

0 . 80 t i me s t he ou t e r d i a m e t e r o f p i pe s a nd 0 .95

t i me s t he o u t - t o - ou t d i me ns i on o f t ube s ( Se e

F i gu r e 4 ) . Ac c o r d i ng t o ASD, t he a l l owa b l e

bending c apaci ty is equal

t o 0 7 5F y

w h e r e

Fy

is the

a l l owa b l e y i e ld s t r e ng th o f t he s t e el . I f L RF D i s

prefe r red , the des ign s t rength i s equa l t o 0 9 0 F y

See Refe rence s 1 , 2 , 3 , 5 and 6 for usefu l des ign

equa t ion s and des ign a ids .

4 .3 Des ign

f o r G r a vi ty L o a d s i n C o m b i n a t i o n

w i t h U p l if t L o a d s

W hen there a re ne t up l i ft loads , such as can oc cur

i n t he e nd c o l umns o f c onc e n t r i c o r e c c e n t r i c

b r a c e d f r ame s ( CBF o r E BF) , t he a nc ho r bo l t s

and base p la tes need to be checked and increased

in s ize i f necessary . W hen upl if t loads a re very

high , i t may be necessary to add s t i f fener p la tes

we l de d t o t he c o l umn f l a nge s a nd de s i gn l onge r

a nc ho r bo l ts a bove t he ba se p l a t e t o a c c om mo da t e

the s t i f feners . Ho we ver , i t s t il l ma y be more

e c onomi ca l t o u s e a t h i c ke r ba s e p l a t e t ha n t o a dd

t he s t if f e ner s be c a us e o f t he h i gh l a bo r c os t

involved wi th the s t i ffeners. An cho r bo l t s need to

be we l l e mbe dde d i n t he s uppo r t i ng f ounda t i on

c onc r e t e t o de ve l op t he t e n s il e c a pa c i ty o f t he

a nc ho r bo l t s, a nd t o p r e c l ude a nc h o r bo l t pu l lou t

due to shear cone fa i lu re in the concre te . This

de t a i l be c ome s e ve n mor e c r i t i c a l f o r b r a c e d

f r a me s o r mome n t f r a me s s ub j e c t e d t o w i nd o r

e a r t hq ua ke f o r c e s whe r e f a i l u r e o f t he ba s e p l a t e

a s s e mb l y wou l d c a us e ove r t u r n i ng due t o up l i f t

resu l t ing in loss o f l a te ra l res i s tance in the braced

f r a me o r mo me n t f ra me . Fo r e a r t hqua k e l oa ds ,

s i nc e a c t ua l l oa ds a r e muc h h i ghe r t ha n c ode

des ign forces (which have been reduced to acco unt

for duct i l i ty in the braced frame), yielding shou ld

oc c u r i n t he b r a c e f o r a CBF o r i n t he l i nk be a m

f o r a n E BF . T he ba s e p l a t e a s s e mb l y ne e ds to be

s t rong enough to ensure tha t y ie ld ing wi ll occu r in

these o ther e lements .

4 4 D e s i gn fo r G r a v i ty L o a d s i n C o m b i n a t i o n

w i t h S h e a r F o r c e s

Taking sec tion 4 .3 a s tep fur ther , i f a brace occurs

a t the base of a co lum n, a h igh shear force i s

10


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from the horizontal force component in

This force must be resisted by the base

assembly. There are various proposed

transfer this shear force:

1. Anchor Bolts See Figure 5a);

.

Shear Key - A steel shear key is welded to

the bottom of the base plate to inte rlock with

the concrete

See Figure 5b);

.

Embedded Shear Plate - Shear plates are

field welded to the sides of the base plate

and to an embed plate that has welded shear

studs or shear lugs to transfer shear forces to

the concrete foundation

See Figure 5c);

.

Embedded Shear Strut - A strut member

with welded shear studs or shear lugs is

connected to the base plate or a column

gusset plate. The shear studs or shear lugs

transfer the shear force into the slab concrete

and then to the founda tion through rebar

dowel s See Figure 5d).The following is a

discussion of the design issues pertaining o

these methods of transferring shear at the

base of a column.

1. Anchor bolts: When column shear forces are

resisted by the anchor bolts, they must be checked

for a combination of column shear, bending and

tension. If oversize holes are used in the base

plates for anchor bolt placement tolerance, welded

washer plates must be added so that the base plate

ill not slip before engaging he anchor bolts. The

washer plates are added to the top of the base

plate and the additional bending in the anchor bolts

must be accounted for due to the increased

distance from the concrete to the washe r plate.

There is a practical limit to the amount of shear the

anchor bolt/concrete nterface can resist before the

anchor bolts become very large. When shear

fcrces are high, methods 2, 3 or 4 should be

considered.

2. Shear Keys: Steel shear keys can be welded

to the underside of the base plate to provide a

shear interlock with the concrete foundation

below. The bending and shear forces that the steel

key imparts to the base plate must be accounted

for. The use of such keys requires block-out voids

to be formed in the top of the foundation to allow

space for the keys and surrounding grout. Any

rebars in the foundation under the base plate must

be positioned vertically and/or horizontally to

allow for the depression n the foundation concrete

to accommodate the steel key. Shear keys are

effective in transferring shear forces from a brace

into the foundation, so that the anchor bolts only

have to resist tension forces.

3. Side Plates; Another strategy would be to

cast an embed plate into the top of the foundation.

The embed plate would have shear studs or lugs

welded to the bottom to transfer shear forces into

the foundation. The embed plate would be larger

than the base plate to accommodate setting

tolerance and to accommodate side plates to

transfe r shear forces from the base plate to the

embed plate and foundation. The column would

be erected and leveled in the same manner as any

conventional column. Loose plates would be

added and field welded to the sides of the base

plate and to the embed plate. Grouting between

the base plate and embed plate would be the final

step in the process. This detail is practical because

it provides a template for the anchor bolts and

allows for confinement of the grout.

4. Struts; When shear forces are high and shear

keys or embedded plates are not practical for

detailing reasons, steel struts can be added that are

embedded into the slab concrete. The strut is

welded or bolted to the base plate or to a stiffener

or gusset plate welded to the base plate. The strut

should have shear studs or lugs welded to it to

transfer axial forces from the strut to the concrete

slab. The slab adjacent to the strut should be

doweled to the foundation to transfer forces from

the slab to the foundation. Attention to

construction details and sequencing is important

so that the rebars around the strut do not interfere

11


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with being able to positionand connect the strut to

the base plate.

4 5 Design for Gravity Loads in Combination

with Shear Forces and Moments

When a base plate assembly must transfer column

base moments to the foundation, the mechanism

for resisting the moments is typically aken by the

combinationof the tensile capacity of the anchor

bolts and the bearing capacity of the concrete or

masonry. This forms a couple consistingof the

tension force in the anchorbolts and the equivalent

force at the centroid of the bearing area under the

base plate. This is analogous o the internal forces

to resist bending in a concrete cracked section.

The other gravity, shear, and uplift forces acting in

combinationwith the bending moment must also

be added and accounted for.

Two methods are presented for consideration

when designing base plates subject to bending

moment. See References 1, 2, 3, 5 and 6 for

additional information on how to calculate and

design for this combinationof loads.

The first method is based on the assumption hat

stresses caused by the moment are linear across

the base plate length. The tensile force in the

anchor bolt is dependent on the bearing area. An

equation is provided in Reference 1 to calculate

the effective length of bearing. (See Figure 6,

Method 1). This may not be consistentwith actual

behavior since the assumption relies on linear

deformationof the base plate.

The second method is based on the assumption

that the resultant of the bearing length is directly

beneath the column flange. The basis of this

assumption s that the flange experiencesa greater

axial load compared to the web because of the

higher combinationof axial and flexural stresses,

and the relative width of the column flange to the

web (See Figure 6, Method 2). This method may

produce inaccurate results as well since the

bearing length may extend over to the anchor bolt

in tension. More testing and research is required

to confirm he validityof either method with actual

~ M

P T

M E T H O D 1

TENSIONB

FORCE

IN BOLT

AXIAl. FLEXURAL

STRESS N

COLUMN

FORCE

P I TRANSMITTED

~ ~ M TOBASE LATE

ANCHORBOLT

\

PT

M E T H O D 2

F I G U R E 6 - C O L U M N W I T H A X I A L

F L E X U R A L D E M A N D

2


15/19

behavior. See References 1 and 5 for further

explanationand useful equations.

When the column moments are known the design

is more straightforward han if the moments are

more unpredictable such as at the base of

moment frame columns which resist earthquake

forces where ductilitybecomes an important issue.

See Section 4.6 for additional discussion of base

plate assemblies hat resist seismic forces.

4 6 D e s i g n f o r M o m e n t s d u e t o S e i s m i c

F o r c e s

Unfo r tuna te ly , the behav ior of base p la tes in

mome n t f r a me s a nd b r a c e d f l a me s s ub j e c t e d t o

ear thquake forces i s no t fu l ly unders tood .

Re s e a r c h a nd c ode gu i da nc e a r e l im i t ed . T he

engineer i s forced to use judgment and the

in te rpre ta t ion of the resu l t s f rom tes t s o n

a s s e mb l i es w i t h s imi l ar c ompone n t s i n o r de r t o

achiev e a des ign tha t hopefu l ly wi l l have the

des i red leve l o f per formance .

Try ing to f ix , o r par t ia l ly f ix , the base of mo me nt

f r a me o r b r a c e d f r a me c o l umns a ga i n s t r o t a t i on

m ay be necessary to reduce the dr if t: in the s tory

above the base p la te loca t ion . Cons ider the

fo l lowing scenar ios :

1. Con t i nue t he c o l umn i n t o t he f ounda t i on o r

in to a basem ent leve l be low;

2 . De s ign a heavy base p la te assembly s t rong

enough to force a p las t ic h inge in the co lumn.

This i s d i f f icu l t to accom pl i sh even fo r re la t ive ly

smal l co lumns . The base p la te and anch or bo l t s

be c ome ve r y l a r ge a nd a nc ho r bo l t a nc ho r a ge

becom es difficul t. The found at ion must be capable

o f r e s i s t i ng t he h i gh mome n t s f r om t he c o l umn

base assembly (See F igure 7a). Curren t ly ,

research i s ong oing a t the U nivers i ty of Michigan

by P r o f e s s o r Subha s h Goe l on ba s e p l a t e

assembl ies o f th i s type ;

3 . I f the s tee l f rame i s suppo r ted on spread

f oo t i ngs w i t h mome n t r e s i s t i ng g r a de be a ms

be t we e n t he f oo t i ngs o r s uppo r t e d on a g r a de

beam gr id sys tem, par t ia l ly f ix the base of the

c o l umn by de s i gn i ng t he f oo t i ng / g r a de be a m

sys tem to form p las t ic h inges in the grade b eams

tha t b ehav e in a duc t ile manner . S ize the base

p l a t e a s se mb l y to de ve l op t he s t r e ng t h o f t he

f oo t i ng / g r a de be a m c ons i de ri ng t he ove r s t r e ng t h

o f t he c onc r e t e s e c t i ons . ' Any p l a s t ic h inge s

should occu r in the grade beam s;

4. Design a part ia l ly f ixed base plate assembly.

This wil l help l imit dr if t , and the base plate and

a nc ho r bo l t d i me ns i ons w i ll be m or e ma na ge a b l e

than wi th a f ixed base so lu t ion . Dr i f t can be

redu ced dramat ica l ly because the co lu mn wi l l be

forced to bend in double curva ture . The cha l lenge

is to des ign the base p la te assembly to b ehave in a

duc t i le manner . I f par tia l fix i ty i s los t dur ing an

e a r t hqua ke due t o t he f a i lu r e o f t he ba s e p l a t e o r

stretching or breakage o f the an chor bol ts , the drif t

o f the f i rs t s to ry wi l l increase dram at ica lly

resu l t ing in more damage and poss ib le fa i lu re of

the co lumn. A fa i lu re of the second f loo r beams

could al so occur i f they w ere no t des igned for the

ext ra bending or a re no t duc t i l e enough to

accomm odate the ex t ra ro ta t ion . (See F igures 7b ,

7c, and 7d);

5 . P in the base of the co lumn by des igning a

base plate assem bly that wil l have relat ively l i t t le

mo me nt res i s tance , bu t wi l l be du c t i le enough to

acc om m od ate the f i rs t s tory seismic drift :.

So m e d es ign i ssues re la tive to scenar ios 4 and 5 .

will be discussed further. Sc ena rios 1, 2 and 3 are

beyon d the scop e o f th i s paper .

For a l l co lumn to base p la te welded connec t ions ,

the same i ssues re la t ive to beam to co lumn

c onne c t i ons i n duc t il e mom e n t f r a me s s hou l d be

cons idered to prec lude a fa i lu re in or near the

weld , par t icu la r ly i f a fu l l p las t ic h inge in the

column above th e base p la te i s the des i red des ign

goa l . T he r e a de r shou l d r e f e r t o t he doc ume n t s

and research cur ren t ly be ing done by the SAC

J o i n t Ve n t u r e on m ome n t f r a me s ( Se e Re f e r e nc e

8).

13


16/19

..c o M . j / \ .BAs E

~ m m

~ " , . , . ,:...... : : . i. . : ~. , :,: ~ ( : . . ~ . : i ' ~ ' ~ . :: ~E ~ L I N G ~:;~: :~ ~ - '~ '

[ I i t I t k P T 11

A N C H O R

B O L T

Y I E L D

Z O N E

a HINGE IN

COLUMN

b B A SE P ~ T E

YIELDING

c ANCHO R BOLT

YIELDING BELOW

B A SE P ~ T E

F IG U R E 7 - Y I E L D IN G M E C H A N I S M S

~ t ~ ] . ~ ~ A N C H O R

I I I I I I I I I | B o LT

, , , , , ,

I . Y . 2

~::::

~ ~ ~~f w~

NO

/ I ' - L E V E L IN

6 ~NCNORBOkT

Y k ~IN G ~ O V ~

B AS E P ~ T E

F o r . a p a r t i a l l y f i x e d c o l u m n b a s e a s d e s c r i b e d i n

s c e n a r i o 4 , t h e r e a r e t w o m e c h a n i s m s t o a c h i e v e

the duc t i l i t y in the base p la t e as sembly :

a . D e s i g n t h e b a s e p l a t e t o y i el d i n b e n d i n g b y

d e s ig n i ng t h e a n c h o r b o l t s t o b e s t r o n g e n o u g h t o

f o r c e p l a s t i c h i n g e s i n t h e b a s e p l a t e ( S e e F i g u r e

7 b ). T h e p l a t e m u s t b e la r g e e n o u g h , b u t n o t to o

t h i c k s o t h a t a p l a s t i c h i n g e r e g i o n c a n f o r m

b e t w e e n t h e c o l u m n f l a n g e s a n d t h e a n c h o r b o l t s

wi thou t induc ing a shear f a i lu r e in the base p la t e .

A l eve ling p la t e shou ld be p rov ided und er the b ase

p l a t e t o p r o t e c t t h e g r o u t w h i l e t h e b a s e p l a t e

u n d e r g o e s d e f o r m a t i o n s d u r i n g t h e c y c l i c a l

b e n d i n g e x c u r s i o n s .

b . D e s i g n t h e b a s e p l a te s tr o n g e n o u g h t o f o r c e

y i e l d i n g i n t h e a n c h o r b o l t s ( S e e F i g u r e s 7 c a n d

7 d ) . N u t s a n d w a s h e r s m u s t b e u s e d a b o v e a n d

b e l o w t h e b a s e p l a t e ( F i g u r e 7 c ) o r t h e t o p

h o r i z o n t a l p l a t e ( F i g u r e 7 d ) t o f o r c e t h e a n c h o r

bo l t s to r es i s t ax ia l fo rces in bo th t ens ion and

c o m p r e s s i o n s o t h a t t h e r e w i l l b e c y c l i c c a p a c i t y

a n d d u c t i l it y in t h e a s se m b l y . T h e r e m u s t b e

suff icient unbonded length in the bol ts to al low for

t h e r e q u i r e d e l o n g a t i o n w i t h o u t o v e r s t r a i n i n g t h e

b o l t s . T h e u l t i m a t e s t r e n g t h o f t h e b o l t m u s t b e

h i g h e n o u g h t o p r e c l u d e f a i l u r e a t t h e r e d u c e d

s e c t i o n in t h e t h r e a d e d p o r t i o n o f t h e b o l t b e f o r e

t h e b o l t y i el d s. T h i s c a n b e c o m e a p r o b l e m w h e n

the u l t imate s tr eng th o f the bo l t (F , ) i s too c lose to

t h e y i e ld s t r e n g t h ( Fy ). S o m e a c c o m m o d a t i o n

m u s t b e m a d e s o t h a t t h e b o l t s w i l l n o t b u c k l e

w h e n t h e y a r e in c o m p r e s s i o n . D e - b o n d i n g a n d

b u c k l in g r e s i s t a n c e c a n b e p r o v i d e d b y u s i n g p i p e

s l e e v e s w i t h i n t h e f o o t i n g ( F i g u r e 7 c ). I f t h e

y i e l d i n g p o r t i o n o f t h e a n c h o r b o l t s i s a b o v e t h e

b a s e p la te , s l e e v e s o r g u i d e s c a n b e p r o v i d e d t o

res i s t bo l t buck l ing (F igure 7d) .

V e r y f e w t e s t r e s u l t s a r e a v a i l ab l e to v a l i d a t e t h e

b e h a v i o r o f e i t h e r m e c h a n i s m d e s c r ib e d i n a o r b .

M o r e r e s e a r c h a n d d e v e l o p m e n t i n t o b a s e p l a t e

b e h a v i o r a n d d e s i g n g u i d e l i n e s a r e n e e d e d f o r

a n c h o r b o l t / b a s e p l a t e a s s e m b l i e s t h a t a r e

s u b je c te d to v e r y h ig h m o m e n t s w h e r e y i e ld i n g is

n e c e s s a r y t o a c h i e v e t h e d e s i r e d p e r f o r m a n c e .

4 7 Arch itectural Issues

A r c h i t e c t u r a l i s s u e s s h o u l d b e c o n s i d e r e d w h e n

des ign ing and de ta i l ing base p la t e as sembl ies .

A n c h o r b o l t a s s e m b l i e s n e e d t o f i t w i t h i n s l a b

th icknesses . Th ere nee ds to be su f f ic i en t d i s t ance

b e t w e e n t h e t o p o f f o u n d a t i o n a n d t h e t o p o f sl ab

t o a c c o m m o d a t e g r o u t , l e v e l i n g n u t a n d w a s h e r

14


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e , nu t and bol t p ro jec t ion p lus co ncre te

c ove r a bove t he t op o f bo l t. Us ua l ly , t h is

d imens ion i s a t l eas t 12 inches . Fo r pro jec t s wi th

la rge c o lum ns and th ick base p la tes , 12 inches is

not enough . This i s an imp or tan t d imens ion to

es tab l i sh ear ly because i t a f fec t s the founda t ion

depth.

Any s t i f fener p la tes added above the base p la te

mu st f i t wi th in the a rch i tec tura l f in ish a round th e

colum n. I f s t i ffeners a re needed , the d imens ions

should be c o-ord in a ted wi th the a rch i tec t ea r ly in

the design since i t ma y be necessary to increa se the

f in ish d imens ions , s ince th i s d im ens ion wi l l a f fec t

useable f loor space . Also , the d imens ions of any

vert ical s ti ffener plates should be check ed to insure

that the s t iffener plates will not p rotru de abo ve th e

s lab ou ts ide o f the co lum n f in i sh d imens ions .

5 . 0 C O N C L U S I O N S

Base plates serve a cr i t ical role in t ransferr ing

c o l um n l oa ds t o the founda t i on . T h i s

Stee l Tips

discusses des ign , fabr ica t ion and e rec t ion i ssues

re la ted to base p la tes and anchor bo l t s . Sugges ted

de tai ls a re presented and de ta i l s to be avoided a re

s hown . T he e ng i ne e r ne e ds t o be a wa r e o f

mater ia l s ava i lab le and should recognize tha t

spec ia l a t ten t ion to base p la te and anchor bo l t

de ta i l s can resu l t in reduced cos t s dur ing

fabr ica t ion and e rec t ion . Base p la te assembl ies

mus t be de s i gne d t o a c c ommoda t e t e mpor a r y

erect ion loads unt i l the colum n is t ied in with oth er

s t ruc tura l mem bers . Spec ia l a t ten t ion by

c on t ra c t o rs whe n p l a c ing a nc h o r bo l t s c a n r e duc e

field problems and delays due to mislocated an cho r

bolts .

6 0 R E F E R E N C E S

1. De s i gn o f We l de d S t r uc t u r e s by Ome r W.

Blodge t t , James F . L incoln Arc Weld ing

Fo und ation , 15 ~h Print ing , 1996

2 . AI SC M a nua l o f S t e el Cons t r uc t ion , L FRD

Design , Volumes I and I I , Second Edi t ion ,

1994

3 . AI SC M a nua l o f S t e el Cons t r uc t i on , ASD

Design , Nin th Edi t ion , 1989

4 . AST M S t a nda r ds i n Bu i l d i ng Code s , Vo l ume

1, 35 h Ed it ion, 1998

5. Column Base P la tes , S tee l Des ign Guide

Se r i e s 1 by J ohn T . D e W ol f a nd Da v i d T .

Ricker , AISC 1990

6 . AI SC E ng i ne e r i ng J ou r na l Be a m - Co l um n

Ba s e P l a te De s i gn - L RF D Me t hod by

Richard M. Drake and Sharon J . E lk in , F i r s t

Qua r t e r 1999 , Vo l ume 36 , Numb e r 1 .

7. Stee l Tips

Com mo n S t e el E r e c t i on P r ob le ms

and Suggested Solu t ion s by Jam es J . Putk ey,

SSEC publ ica t ion , Dec em ber 1993.

8 . SAC In te r im Guide l ines : Eva lua t ion , Repa i r ,

Mod i f i c a t i on a nd De s i gn o f We l de d S t e e l

Mome n t F r a me S t r uc t u r e s , FE MA 267 ,

Augus t 1995

More research and des ign guide l ines a re needed

for base plate assemblies subjected to high ben ding

mom ents , such as in mom ent f rames subjec ted to

earthquak e forces. Fo r part ia l ly f ixed colum n base

assembl ies , mechanisms tha t mus t behave in a

duc t i le ma nner a re needed . Som e a l te rna t ive

s t ra teg ies and con cepts a re presented .

15


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bout the uthors

Bi ll H one ck a P r inc ipa l o f Fore l l /E l s es se r

E n g i n e e r s I n c . h a s 3 4 y e a r s o f e x p e r i e n c e in

s t r u c t u r a l e n g i n e e r i n g 9 o f w h i c h w e r e i n

s t ruc tu ra l s t ee l des ign f abr i ca t ion and e rec t ion .

Th i s a l so inc luded 2 year s des ign ing e lec t r i c

t r a n s m i s s i o n t o w e r s a n d s u b s t a ti o n fr a m i n g . I n

a d d i t io n s e v e r a l o f h is la r g e - s c a l e p r o j e c t s h a v e

b e e n p r o d u c e d o n f a s t - t r a c k .

F r o m 1 9 6 5 t o 1 9 7 4 B i l l H o n e c k w a s p r o j e c t

m a n a g e r / e n g i n e e r f o r B e t h l e h e m S t e el in t h e

c o n s t ru c t io n o f n u m e r o u s h i g h r is e a n d l a r g e s c a le

s t ru c t u r e s . D u r i n g t h e s e 9 y e a r s B ill H o n e c k h a d

prac t i ca l exper i ence in s t ruc tu ra l s t ee l h ighr i s e

bu i ld ings an d l a rge b r idges . Hi s respons ib i li t ies

inc luded s t ruc tu ra l s tee l e r ec t ion cos t es t imat ing

a n d e r e c t io n e n g i n e e r in g c o o r d i n a t i n g j o b s

s c h e d u l in g r e v i e w i n g c o s t s a n d i m p l e m e n t i n g

s a v i n g s w h e r e p o s s ib l e .

H e w o r k e d i n t h e f i e l d a n d o f f i c e i n c o n n e c t i o n

wi th s t ee l e r ec t ion as a f i e ld eng ineer and wa s in

c h a r g e o f e r e c t i o n e n g i n e e r i n g f o r th e W e s t e r n

D i s t r ic t f r o m 1 9 6 7 t o 1 9 7 4 . H e w a s a l s o

r e s p o n s i b l e f o r d e s i g n i n g f a l s e w o r k a n d r e l a t e d

s t r u c t u r e s e r e c t i o n s c h e m i n g a n d c h e c k i n g

s t ruc tu ra l in t egr ity o f s t ee l f r am ew ork fo r e r ec t ion

re la t ed loads .

D e r e k W e s t p h a l a p r o j e c t e n g in e e r a n d a n a l y s t

wi th Fore l l/E l ses ser began h i s ca ree r wi th the f i rm

i n e a r l y 1 9 9 6 . I n h i s e x p e r i e n c e t o d a t e h e h a s

d e v e l o p e d a st r o n g b a c k g r o u n d in t h e s e is m i c

re t ro f i t o f h i s to r i c bu i ld ings as wel l as the new

c o n s t r u c t i o n o f o f f ic e b u i ld i n g s l a b o r a t o r y a n d

univers i ty faci l i t ies .

16


19/19

S T R U C T U R L S T E E L E D U C T I O N L C O U N C I L

470 Fernwood Dr i ve

Moraga , CA 94556

925) 631 :9570

S P O N S O R S

A d a m s S m i t h

A l l i e d S t e e l C o . , I n c .

B a n n i s t e r S t e e l , In c .

B a r e se l C o r p .

B e t h l e h e m S t e e l C o r p o r a t i o n

C . A . B u c h e n C o r p o r a t i o n

B u t l e r M a n u f a c t u r i n g C o .

G . M . I r o n W o r k s C o .

T h e H e r r i c k C o r p o r a t i o n

H o e r t i g I r o n W o r k s

H o g a n M f g . , In c .

Ju n io r S t e e l C o .

L e e D a n i e l

M c L e a n S t e el , In c .

M a r t i n I r o n W o r k s , I n c .

M i d W e s t S t ee l E r e c ti o n

N e l s o n S t u d W e l d i n g C o .

O r e g o n S t e e l M i l l s

P D M S t r o ca l , In c .

R e n o I r o n W o r k s

H . H . R o b e r t s o n C o .

S M E I n d u s t r i e s

S o u t h l a n d I r o n W o r k s

S to c k to n S t e e l

V e r c o M a n u f a c t u r i n g , I n c .

V u l c r a f t S a l e s C o r p .

The local structural steel industry (above sponsors) stands ready to assist you in determining the mo st

econom ical solution for your products. Our assistance can range from budg et pr ices and estimated tonnage

to cos t com parison s, fabrication details and delive ry schedules.

Funding for this publication provided by the C alifornia Iron Workers A dministrative Trust.

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