Shear Nib Column Base

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NCCI: Design of simple column bases with shear nibs

SN021a-EN-EU


This NCCI provides the rules for the design of shear nibs for column bases. The rules given

are complementary to those given in NCCIs SN037 and SN043 for the design of simple andfixed base plate joints respectively.

Contents

1. Introduction 2

2. Types of shear nib 3

3. Parameters 5

4. Design model 6

5. Design situation 1: Dimension a base plate with a shear nib to resist the shear force 8

6. Design situation 2: Determine the shear resistance of a column base joint with a

shear nib 12

7. References 13

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1. Introduction

The types of column bases concerned by the present NCCI are the simple column basesdescribed in SN037, and the fixed column bases described in SN043.

The shear resistance developed by friction between the column base plate in compression and

the joint material (grout), as calculated in SN037, is often adequate for most typical simple

base plate joints and fixed base plate joints.

For simple base plate joints, if there is axial tension acting shear resistance by friction cannot

be developed. For fixed base plates, shear resistance by friction alone may not suffice when

high shear is combined with a low moment and either low axial compression or axial tension.

In the latter situations other means are required to transfer the shear force.

Means other than friction for transferring shear force to the foundation are as follows:

Shear / bearing of the anchor bolts (see 6.2.2(7) of EN 1993-1-8).

Setting the column end with its base plate within a pocket in the foundation pad. Thepocket depth is usually 300 mm or more and is filled with non-shrink concrete once the

column is in place. This type is suitable for fixed column base plate joints. The shear

force is transferred by lateral bearing of the embedded column part on the pocket infill

concrete. The concrete surround of the pocket may require reinforcement in accordance

with EN 1992-1 to transfer the column end forces and moments.

Setting the column end with its base plate in a shallow pocket, usually not more than 100mm. The behaviour of the joint can be assimilated to that of a shear nib mentioned below.

The shallow pocket is not usually recommended for simple base plate joints because the

column end rotations are likely to produce local damage to the concrete above and around

the base plate.

Providing a tie from the column end into an adjacent ground floor slab. This may requireensuring that there is appropriate reinforcement in the slab to anchor the horizontal tie

force.

Providing a shear nib (key) welded to the underside of the base plate which isaccommodated in a foundation pocket of sufficient depth and size. The pocket is filled

with non-shrink concrete after the column and the anchor bolts are positioned.

It is not common practice to use anchor bolts in shear. To do so, one must take precautions to

ensure that the shear force transfer to the foundation through the anchor bolts is possible

without causing excessive lateral movement at the column base (see 6.2.2(5) of

EN 1993-1-8). If anchor bolts are grouted in sleeves they may not be dependable in

shear/bearing. Oversized holes are often used in base plates in order to account for the usual

tolerances in the positioning of anchor bolts set in concrete. In the latter case the plate-

washers used under the anchor bolt nuts would need to be welded to the base plates so as to

allow transferring the shear force to the anchor bolts. It is recommended that hole sizes in

these plate washers may be reduced to a minimum, for instance d+ 1,5 mm (where d is the

nominal anchor bolt diameter). With these precautions, the design resistance of anchor boltsin shear/bearing, which is given in 6.2.2(7) of EN 1993-1-8, can be added to the friction

resistance when relevant.

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Neither the design of foundation pockets (but see remark below for the shallow pocket

type) for fixed base plate joints nor that of ties to the floor slab is considered in this NCCI.

The subject of the present NCCI is the design of a shear nib under the base plate for

transferring shear forces to the foundation.

A shear nib (or shear key) typically consists of a short length of steel section welded to the

underside of the base plate. Once the concrete is poured into the reserve hole for the anchor

bolts and the column grouted in its final position, the nib is embedded in the foundation. The

shear force acting on the column base can be transmitted to the foundation by the nib acting

horizontally leading to compression over the vertical surface of the nib against the concrete

foundation.

In practice, the following two design situations are encountered:

1. The column section and the design forces are known. The dimensions of the requiredbase plate and shear nib are to be determined.

2. The column section, base plate, shear nib and foundation dimensions are known. Thedesign compressive resistance of the column base is required to be determined, including

that of the shear nib.

The usual procedure is to begin with the design of the base plate using the design procedures

given in sections 4 or 5 of SN037or SN043 as relevant. The design of the nib is then

undertaken using procedures given in Sections 5 and 6 respectively of the present NCCI.

2. Types of shear nib

Figure 2.1 shows two types of shear nib in common use, one being a short length of angle

capable of resisting relatively modest shear forces and the other a short length of I section

used if the shear forces to be transmitted are relatively high.

Note: Figure 2.1 shows typical simple base plates details with nibs. For fixed base plates (see

figure 1.1 of SN043) the anchor bolt rows are not on the column major axis as shown here,

but usually beyond the column flanges on projected parts of the base plates.

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8

4

dn

7

1

2

5

910

33

4

dn

6

1

2

5

9

Key :

1. I section column

2. Base plate

3. Joint space to be filled with grout

4. Anchor bolt

5. Concrete foundation

6. Angle section shear nib

7. I section shear nib

8. Steel positioning/levelling plate

9. Pocket reservation to be filled with non shrink concreteor grout after column positioning

10. Foundation reinforcing bar

Figure 2.1 Typical column bases with shear nibs

Other types of shear nibs than those shown in Figure 2.1 are :

a vertical plate welded to the base plate, which plays the role described below for thevertical leg of the angle nib.

A horizontal plate of sufficient dimensions (thickness embedded in the concrete, weldedperimeter to the base plate) to develop the necessary resistances of the concrete in bearing

and of the welds.

While the design rules given below specifically cover the nib types shown in Figure 2.1, they

may easily be adapted to the design of the latter types as well as to the shallow pocket type

mentioned above in Section 1.

Ideally, shear nibs are welded to the base plate in a central position relative to the column

axes. In the case of an angle nib on a simple base plate, while the angle length (nib width) can

be is centred about the column minor axis, the angle leg protruding down into the foundation

must be slightly off the major column axis in order to avoid the anchor bolts. If the angle

length is greater than that of the anchor bolt spacing, the horizontal leg of the angle section

requires holes to allow the anchor bolts on the minor axis to pass through. For a nib of an

unequal angle it is usual to weld the smaller angle leg to the base plate.

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3. Parameters

The following table provides the parameters referred to in this NCCI:

Table 3.1

Parameters (includes those for SN037)

Parameter Definition

Ratio of the base plate width or depthof the design distribution area withinthe foundation to the width or heightof the base plate.

cc Coefficient taking account of longterm effects and unfavourable effectsdue to the manner of loading on thecompressive strength of concrete(see EN 1992-1-1)

j Foundation joint material coefficient.

c Partial factor on the concretecompressive strength (see EN 1992-1-1).

M0 Partial factor on the bendingresistance of the base plate.

ba Angle nib leg plan height (leg lengthwelded to the base plate).

bp Width of the base plate.

bf Width of the foundation(corresponding to the column width).

bfc Width of the column section (width ofthe I section column flange).

beff Effective width of a base plate T-stubin compression.

bn Plan width of a shear nib.

c Additional bearing width (outside thecolumn section perimeter).

df Depth of the foundation.

fyb Yield strength of the anchor bolt.

fyp Yield strength of the base plate.

fjd Design bearing strength of thefoundation joint.

fcd Design compressive strength of theconcrete according to EN 1992-1-1.

fun Tensile strength of the nib steel.

Parameter Definition

hf Angle nib leg length embedded inthe foundation

hc Depth (height) of the columnsection.

hn Plan height of an I section shearnib.

hp Depth of the base plate.

tfc Column flange thickness.

leff Effective length of a base plate T-stub in compression.

deff,n Effective depth of a shear nib.

dn Total depth of a shear nib.

twc Column web thickness.

tan Leg thickness of an angle shearnib.

tfn Flange thickness of an I sectionshear nib

tp Base plate thickness.

Ac0 Compression area under the baseplate of dimensions bpand hp.

Ff,Rd Design friction shear resistance.

Fv,Rd Design shear resistance of thecolumn base plate joint.

Nsec,Ed Secondary axial force in the nibfoundation.

Nj,Rd Design compressive resistance ofthe column base.

Design shear force at the columnbase.V

Ed

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4. Design model

The mechanical model adopted for the nib is shown schematically in Figure 4.1. The columnbase shear force is resisted by pressure developed over the vertical face (or faces) of the nib

embedded in sound foundation concrete. The eccentricity between the horizontal reaction on

the nib and the applied column base shear causes a secondary moment creating a couple of

additional vertical forces (Nsec,Ed) at the base plate joint, a compressive force and a tensile

force. The tensile force may be resisted either by the anchor bolts or by the nib itself. In the

present NCCI, it is conservatively assumed that the tensile force is resisted by the nib. The

additional compression force between the base plate and the joint material (grout) is often

neglected in design, although it could be added to that in the column flange compressive

T-stub when doing the final check on the design of the base plate joint.

deff,n

maxfc,d

VE,d

NsecE,d NsecE,d

VE,d

4

1

2

53

hc/2

maxfc,d

6

Key :

4. Nib1. I section column

2. Base plate 5. Concrete foundation

6 Triangular distribution of pressure on the nib3. Joint material ( grout )

Figure 4.1 Shear nib model showing the forces and stresses induced: distribution of compressive

stresses over shear nib and secondary forces

The following simplifying assumptions are made in the design model [1]:

Both embedded flanges of an I section nib provide equal horizontal resistance to theapplied column base shear force.

For the full width of an angle leg or flange within the concrete foundation, there is atriangular distribution of compressive stresses over the effective depth of the nib (see

Figures 4.1 and 4.2).

The effective nib depth, deff,n, is taken as equal to the full height of the nib , dn, below thebase plate minus a thickness at the top surface to allow for the possible inadequacy of the

packing of the joint material (grout) beneath the base plate. It is usual to assume that thelatter thickness is equal to that of the grout layer, which is typically 30 mm and rarely

over 50 mm thick. In the following it is taken as 30 mm thick.

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The secondary moment is considered to be resisted by a couple of forces acting on thecolumn base, one a normal tension force in the base plate over the shear nib and one a

compressive force between the base plate and the grout which is centred under one of the

column flanges. Assuming the shear nib to be centred at the column centroid and a grout

layer thickness of 30 mm, one obtains the following axial tension design forces:

o I section nib : Axial tension in a nib flange:

)11

(3032

1)

2(30

3)

1(30

3 cfnn

neff,

Ed

c

neff,

Ed

fnn

neff,

EdEdhth

dV

h

dV

th

dVN +

+=

++

+=

o Angle nib: Axial tension in the vertical leg:c

neff,

EdEd

230

3 h

dVN

+=

In order to ensure against pull-out of the nib from the concrete foundation and to have an

efficient shear nib, the following limits are placed on the nib dimensions:

o Height of an I section nib section: hn0,4 hc

o Effective depth in the foundation of an I section nib: 60 mm deff,n1,5hn

o Effective depth in the foundation of an angle nib: 60 mm deff,n1,5bn

In the case if a simple base plate, the respect of the latter limits on the nib dimensions

is recommended so as to avoid creating a fixed column base condition.

Being embedded in the concrete, angle legs or I section flanges are considered to besubjected to negligible local bending. To support this assumption, the following

maximum slenderness criteria are imposed:o I section nib: Maximum flange slenderness: ( bfn/ tfn) 20

(a criterion which all IPE and HE sections meet except HEA 260, 280 and 300)

o Angle nib: Maximum leg slenderness: ( d,n/ tan) 10

(not all standard hot rolled angle sections meet the latter requirement)

For an I section shear nib, the shear force is transferred from the base through the web.The moment at the underside of the base plate level is resisted by a force couple in the

flanges. Rather than assume the anchor bolts to be active, the secondary normal tensile

force is considered to be shared by the flange sections. The flange in tension the most

loaded. The column web opposite the flange also resists the total force thus obtained.

For the leg of an angle section shear nib, both the shear force and the secondary normalforce are taken by the vertical leg section. Bending at the top of the vertical angle leg is

neglected.

The basic design approach is to ensure that the compressive stresses over the vertical surface

of the nib in contact with the foundation neither exceed the design compressive strength of the

concrete nor lead to excessive stresses in the nib member (leg, flange or web).

The supplementary design checks required are as follows:

The column web is checked for the concentrated force corresponding to the secondary

tensile force in a nib angle leg or nib flange,

The base plate to nib fillet welds resistances are checked for both the horizontal shear andfor the secondary tensile forces.

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deff,n

max fc,d

VE,d

NsecE,d NsecE,d

VE,d

t a

bn

NsecE,d

deff,n

max fc,dhn

NsecE,d

VE,d

M secE,d/(hn- tfn )

hc /2

t fn

bn

hn

VE,d

Figure 4.2 Dimensions of shear nibs, distribution of compressive stresses and secondary forces

5. Design situation 1: Dimension a base plate witha shear nib to resist the shear force

If the column forces are given, the following procedure can be followed to dimension the base

plate and the shear nib. It is conservatively assumed that the shear nib provides all of the shear

resistance required, i.e. both the friction resistance when the column is in compression is

ignored as well as the resistance of the anchor bolts to shear.

While it is usual to have a shear nib of the same steel (fyn) as that of the base plate, they maybe of different steel grades.

The rules given cover the case of a column base shear force acting in the plane of the column

web i.e. a shear force parallel to the column section minor axis. The design method can be

adapted for cases of when the shear force is parallel to the principal column axis or for when

there are components of shear force along both axes.

Step 1: Dimension the base plate by referring to SN037or to SN043

The values of the base plate dimensions (hp, bp, tp) are established for the column section (hc,

bc, twc, tfc) load and the concrete (fcd) to be used in the foundation is identified.

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Step 2: Dimension the shear nib if required

Note: It is not usual to have to make a choice between the two types of shear nib.

Assume the joint material (grout) layer to be 30 mm thick

Adopt a practical shear nib width, bn, within the following limits, min bn bn.max bn:

Angle nib : mm)30

:90max(mincd

Edn

f

Vb and max bn bp 2tfc

I section nib : mm)15

:90max(mincd

Edn

f

Vb and max bn bp 2tfc

Angle shear nib:

The suitable and available angle sections are identified (ha,ba, ta). Noting that it is usual to useunequal angles, equal angle legs can be used also. The suitability of a given angle section

requires that:

taha/10

where hais the length of the longer leg, the leg to be embedded in the concrete foundation.

a) Estimate the minimum required depth of angle nib:

mm)2

:60max(min

cdn

Edneff,

fb

Vd

b) Check the maximum practical limits on the nib depth:

):8,0min(mm30min afnefff, hdd + .

If the latter condition is not met, restart using a greater nib width bn(length of angle

section).

c) Choose an angle size such that:

mm30)(min neff,a + dh ; 8,0 fa dh ; 6,0 ca hh 6,0 ca hb andtaha/10

Take mm30aneff, = hd

Estimate the secondary tensile force in the vertical angle leg:

c

neff,

EdEdsec

230

3 h

dVN

+=

Check the leg thickness under combined shear and tension using the Von Mises

criteria:

3)303/(23

2

c

neff,

nyn

Ed

2

ynn

Ed

2

ynn

sec +

+=

+

h

d

bf

V

fb

V

fb

Nt Eda

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If it is not possible to complete the checks by modifications of the shear nib width

and/or depth, change to an I section shear nib.

I section shear nib:

Take the following steps in order.

a) Choose an I section: the nib width, bn=bf, nib ,within the maximum and minimumlimits given above.

b) Check that the nib section height hnib0,4 hc,.

If satisfied, the nib height becomes hn=hnib.

If the condition is not met restart the procedure with a shallower I section for the nib.

c) Check the flange slenderness of the nib section:(hn/tf, )nib20

d) Make an estimate of the required minimum nib depth:

mm):60max(mincdn

Edneff,

fb

Vd

e) Check the maximum recommended limits on the effective nib depth (in theconcrete):

)5,1:8,0min(mm30min nfneff, hdd + .

If the latter conditions cannot be met, restart using a different I section of greater

width (bf, hc)nib section.

f) Confirm the suitability of the section choice: hn0,4 hc; tfnbfn/10 ;

g) Check the nib section web shear resistance:

Vpl,Rd = Avnfyn/(M03 ) VEd

If necessary, restart the process with another section providing adequate web shear

resistance.

h) Adopt the value for the nib depth: mm):60max(cdn

Edneff,

fb

Vd

For shear nib depth chosen, estimate the secondary normal force in nib flange:

)11

)(303

(cfnn

neff,

EdsecEdhth

dVN +

+=

i) Check the nib flange resistance in tension: Afnfyn/M0Nsec Ed

If all the all checks above are satisfied, the nib section chosen is adequate.

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Step 3: Determine the nib to base plate fillet weld sizes

Fillet welds are usually adopted. The minimum throat size is 3mm.

Angle shear nib:

An all round perimeter fillet weld is adopted. The shear force is assumed to be taken by the

two side welds and the toe weld, all of equal throat size aV. The normal force is assumed to be

taken by the weld at the angle heel of weld size aN. The weld steel strength is takenfu= min

(fup:fun)

The minimum required weld sizes are then:

)2(

3

nnu

EdM2wV

bhf

Va

+

single fillet around the angle leg perimeter

nu

EdsecM2wN

2

bf

Na

single fillet at the end of the vertical leg


The nib web is assumed to take the column base shear force and the nib flange is assumed to

take the secondary normal force. Double fillet welds are usually used.

Web double fillet welds :

)2(

3

nibf,nibc,u

EdM2wV

thf

Va

Flange double fillet welds :)2(

2

wnfnu

EdsecM2wN

tbf

Na

Step 4 : Check of the local resistance of the column web

The column web is subjected to the concentrated secondary tensile forceNsecEd. The following

local resistance check is made:

Nsec Ed(twcbeff)fyc/M0

The force is assumed to be distributed over the following effective width in the column web:

Angle shear nib: beff= ta+ 2tp+ 5 (2 awc)

I section shear nib: beff= tfn+ 2tp+ 5 (2 awc).

where awcis the throat size of the column web to base plate double fillet weld.

If the local column web resistance is not adequate the web should be reinforced locally, either

by a vertical stiffener or by a doubler plate.

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6. Design situation 2: Determine the shearresistance of a column base joint with a shear nib

Step 1: Determine the shear resistance of the nib based on the concrete

Angle shear nib:2

cdneff,n

Rd

fdbV =

I section shear nib:cdneff,nRd fdbV =

Step 2: Determine the shear resistance of the nib based on the welds

The weld steel strength is takenfu= min (fup:fun).

Angle shear nib:wM2

nnVu

Rd3

)2(

bhaf

V

+

=

)90(22

3

neff,wM2

cnNuRd

+=

d

hbafV

I section shear nib:wM2

nibf,nibc,Vu

Rd3

)22(

thafV

=

)(

)(

)90(

)2(

2

3

fnnc

fnnc

neff,

wnn

wM2

Vu

Rd thh

thh

d

thafV

+

+

=

Step 3: Determine the shear resistance of the nib based on the angle leg or flangeand web resistances

Angle shear nib:

Resistance of the leg section under shear and axial forces:

33

)90(2

2

c

neff,

an

M0

yn

Rd

+

+

=

hd

tbfV


)90)((

)(3

neff,fnnc

fnnc

M0

ynfn

Rd ++

=dthh

thhfAV

(nib flange in tension)

3M0

ynvn

Rd

fAV = (nib web in shear)

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Step 4: Determine the shear resistance of the nib based on column web resistance

Angle shear nib:)90(

)252(

2

3

neff,

wcpaca

M0

yn

Rd +

++=

d

atthtfV

I section shear nib:)90)((

)252)((3

neff,fnnc

wcpafnncwc

M0

ynfn

Rd ++

++=

dthh

attthhtfAV

Step 5: The design resistance is taken as least value for the shear resistance VR,dgiven by steps 1 to 4

7. References

1 Lescouarch, Y.

Pinned column bases, CTICM collection, 1982 (in French).

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Quality Record

RESOURCE TITLE NCCI: Design of simple column bases with shear nibs

Reference(s)

ORIGINAL DOCUMENT

Name Company Date

Created by Ivor Ryan CTICM 20/12/2005

Technical content checked by Alain Bureau CTICM 20/12/2005

Editorial content checked by

Technical content endorsed by thefollowing STEEL Partners:

1. UK G W Owens SCI 07/04/06

2. France A Bureau CTICM 07/04/06

3. Sweden B Uppfeldt SBI 07/04/06

4. Germany C Mller RWTH 07/04/06

5. Spain J Chica Labein 07/04/06

Resource approved by TechnicalCoordinator

G W Owens SCI 31/07/06

TRANSLATED DOCUMENT

This Translation made and checked by:

Translated resource approved by:

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