application of ec2 to office buildings

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Paper: Albrecht

~~ ~~~ ~ ~ ~

Paper

Adica t ion of the structural Eurocode EC2 to

an office building

U. Albrecht, Professor Dr k g .

Fachhochschule Nordostniedersachsen, Buxtehude, Germany

Synopsis

EC2: ‘Design of concrete structures: Part I ’has been

published by CEN as an ENV.

This paper reports

on

the application of EC2: Part

I

to the

design of the major structural elements in an office building

which has also,

in

comparison, been designed to BS 81

I O .

The

exercise was undertaken in collaboration with Andrews, Kent

& Stone, consulting engineers, London. The purpose was to

provide an indication of the use of EC2 in the design of

‘everyday’ tructures.

Typical structural concrete elements ibbed slab, frat

slab, beam, column, raft foundation were analysed and

reinforced, and as far as relevant deflections and crack widths

were controlled. The paper identifies aspects where the use

of

EC2 may change current UK design practice. The EC2 design

was based on reinforcement with a yield stress of 500N/mm2

(as compared to 460N/mm2 forhe

BS

81

10

design) to

demonstrate the advantage for the ultimate limit states and the

limitation which may be required to control defections.

It is hoped this paper will encourage practising engineers to

use the ENV Eurocode EC2.

Introduction

In many discussionsabout the Eurocodes amongngineers in he Member

States of the EuropeanCommunity and EFTA oncern has been expressed

about the extent f changes from current national esign practice and the

complexity of the Eurocodes. This paper is intended to assist practising

engineers to become familiar with EC2.

An impression of the writing of EC2l has been given by Somerville2 nd

acomparison of the design requirements inEC2 and

BS

8110 by

Narayanan3.

Eurocode EC2: ‘Design ofconcrete structures: Part General rules and

rules for buildings’ has been published by CEN as an ENV. Its use by

engineers will then be optional for 3 years with a possible extension of 2

or perhaps more years before it becomes an EN, replacing the national

Codes such as

BS

8110.

During this time application to various structures is necessary in all

Member States to check and improve the resent ENV version. Practising

engineers have an opportunity during this time to influence the content

of the final version of EC2.

Only a few

of

the comparisons maden the separateMember States on

the use of EC2 have been published. The European Committee of the

Consulting Engineers of the Common Market (CEDIC) applied EC2 to

the design of three different structures which had also, in parallel, been

designed according to the respective national Code4.

This paper reports on another comparativeesign exercise,undertaken

during a 6-montheriod which he authorspent witha consulting engineer

in London. It describes the applicationof EC2: Part

1

(revised final draft,

December 1989)to the design of the major structural lements inan office

building (the EC2 design) which have also been designedo BS 81

lo5

(the

BS 8110design). In addition, a comparisonwith German design practice

and DIN

1045

was made and will be published in Germany. The exercise

was undertaken with the following questions in mind:

(1)

Will EC2 introduce a severe change in design practice compared to

BS 81lo?

(2) Is EC2 too complicated for the design of ‘everyday’ structures?

In addition, thebrief outline of the EC2design of the major structural

elements of the building given in his paper provides an indication of the

use of EC2 in design. Where alternative design methods aregiven in EC2

the more simple method was applied in this design exercise. The figures

in brackets

[

refer to the relevantclauseof EC2

- (

) indicating

‘principles’ for which there is no alternative, ) indicating ‘application rules’

for which alternative rules may be permit ted.

It should be noted that theanalysis and the calculationf reinforcement

described in this paper is only a part of the design of concrete structures.

Other important aspects, not reported here, relateo durability and detailing,

taking into account standards of workmanship.

General arrangement of the office building

The main part of the four-storey well-equipped office building, recently

built in the London area, haseen arranged arounda central atriumwith

stairs and lifts. In addition, he building has wing with a regular structural

system (Fig

1).

This

part

of the building contains typicalstructural concrete

elements:

-

ibbed slabs in the upper floors

-

hallow beams

-

olumns

-

lat slab in the ground floor

aft foundation

Fig 3 shows the cross-section of this part of the building: gridline D to

N and to

8;

Figs 1 and 2 how the spans and the arrangementf columns.

The structure contains everal shear walls,

so

that it can be assumed to

be fully braced. The foundation consists of a raft bearing onto clay.

Design information

Actions

ECl: ‘Basis of design and actions on structures’6 is at a draft stage of

development. Therefore the exercise was undertaken by reference to the

relevant British Standard Code, ut introducing he new CEN terminology.

12000 500

I

l

In

c-

In

CI

5:

r-

In

c-

In

550 dp

ribbed floor

-

l

Fig

1.

Office building: general arrangement,first hird floors

The

Structural

Engineer/Volume 70/No.13/7 uly 1992

229



Paper: Albrecht

TABLE

I Nominal cover to reinforcement

EC2 design

BS

8 110 design

Exposure

Nominal Nominal

Conditions

coverover

(mm)

Offices environment

.l

ry 20 mild 20 (C35)

262x874 600

x

6

Carpark, 2aumid

25

aft environment moderate 35

founda tion without frost

l

Column drop

45 x 45

I

NOTE: Nominal cover includes 5 mm allowance for tolerances

cube strengthf,,,,, is given in addition i3.1.2.41. The EC2 design exercise

was based on C30/37 which corresponds approximately to concrete grade

C35 used for the

BS

8110 design.

Specifications for reinforcement will eventually be included inN 10080.

A

provisional guide in EC2 specifies reinforcement with a yield stress

yk

=

500N/mm2. EC2 defines two classes of reinforcement ductility L3.2.4.21

which limit he redistribution of moments i2.5.3.4.2 (3)l. The EC2 design

was based on high ductility steel with

yk

=

500N/mm2 to demonstrate,

on the one hand, the advantage of high yield stress nd, on he other hand,

the limitations dueo serviceabilityequirements. The amount of

reinforcement cannot directly be compared in the two designs because he

BS

81 10 design was based on steel

f =

460N/mm2.

I

9 _

l

Fig

2.

Office building; general arrangement, ground floor

Considerationof durability

To enable design ofn adequately durable structure EC2ists the interrelated

factors affecting durability

12.41

and gives further provisions r4.11. The

concrete cover i4.1.3.31 is related to the environmental conditions [Table

4.11. To allow more direct comparisons ofhe EC2 design with he

BS

81 10

design the nominal covers given in Table 1 were used with a tolerance

allowance of only

h

=

5mm. It should be noted hat, in practice, a greater

tolerance allowance

Ah =

lOmm for

in situ

concrete structures is advisable

14.1.3.3

8)l.

Q

. 45 75

-

- 45

I I l

Fire protection

Provisions concerning fire protection re not included in EC2: ar t 1. They

will eventually be given in a separate Part. The provisions of

BS

81 10 or

a fire resistance of

l

h for all floors above ground and 4h for the lower

ground-floor used as a carpark were therefore applied to both the

BS

81

10

and the EC2 design.

3rd fr

2nd fir

Ribbed slab

Basis o design

The ribbed slab forminghe first, second and third floors covers two unequal

spans of 12.00m and 4.50m. Examination of the cross-section given in Fig

4 indicates the floor is rather slender.

1st flr

Grd

f

Ir

II

Lower grd

f

Ir

I I I I

W

Fig

3.

Office building; cross-section

l

125

Ribbed slab

The permanent action (EC2)

-

ead load (BS 81 10)

-

as not changed

for the EC2 design, althoughEC2 specifies the density of reinforced concrete

as 25 KN/m3 i4.2.1.21.

The variable action (EC2)

-

mposed load

(BS

81 10)

-

sed for the

design of the floors was the same for both designs:

5.0

kN/m2 third floor to ground loor, offices.

This valueexceeds the specifiedvalueof the national Code and he

Eurocode. For the column design a reduction in total mposed f loor load

was used as given in BS 6399:

Part

l and

AMD

4949.

l

12

Main beam

Material properties

EC2 relates the concrete strength class to the cylinder strengthf,, but the

Fig

4 .

Cross-section ribbed slab and main beam

The

Structural

Engineer/Volume

70/No.13

/ 7 July 1992

30



Paper: Albrecht

TABLE 2 ibbed slab: design values

of

actions/loads

I EC2 design I BS

8

110esign

TABLE 3 - ibbed slab: design moments and tensile reinforcement

5

7

8

A A

A

12000500

Location

EC2 design

As

M

f y k= 500

(kNm) N/mm2edistr.

(mm2)

Support 7

176 0.9

829

Span 5-7

175 0.9

828

BS 8110 design

P

redistr.

As

f y = 460

N/mm2

(mm2)

847

193

I

0.8

I

1001

For the EC2 design, he basis is outlined in the following Eurocode clauses

ltimate limit states i2.3.21

-

artial safety factors for actions [2.3.3.11

-

artial safety factors for material properties i2.3.3.21

erviceability limit states 12.3.41

-

nalysis 12.51

The design values for actions are given in Table

2.

There are only small

differences between the EC2 and

BS

81 10 designs.

EC2 defines and EC l provides combination factors

g )

for additional

variable actions. Theyo not apply o the EC2 design of this office building.

The EC2 analysis for the ultimate limit states i2.5.3.2.21 may be linear

elastic with or without redistribution, non-linear or plastic. For the EC2

design the moments were calculated by linear analysis and redistributed

-

ee Table 3 .

An

explicit check on the rotation capacity may beomitted

i2.5.3.4.21 provided tha t

6

2 0.44 + 1.25 x/d), concrete grades not greater than C35145

6

2 0.7 , highuctilityteel

where

6

is the ratio of edistributedmoment to he moment before

redistribution and

x/d

is the ratio of neutral axis depth.

This limitation is more severe than the BS 81 10 requirements.

Reinforcement

The cross-sectiondesign sbased on stress-strain diagrams which are

parabolic-rectangular for concrete and bilinear for reinforcing teel

i4.2.2.3.21.

Although the EC2 partial safety factors for material properties are the

same as in

BS

81 10

y

= 1.5 for concrete

y =

1.15 for reinforcement

design charts are not yet available. For the present, the design aids in the

CEB manual* may be used. he resulting areas of reinforcementare given

in Table 3. The lower amount of steel in the EC2 design results mainly

from the higher yield stress used.

Limiting defections

Serviceability limit states include limitation of deflections. Generally, this

requirement may be met by limiting the span/depth ratio rather than on

the basis of an explicit calculation of deflection. EC2 gives in Table 4.13

basic ratios of span/depth for lightly and highly stressed concrete. These

ratios havebeen derived on he assumption that the steel stress under design

service load a t midspan is 250N/mm2, corresponding roughly t o

f y k

=

400N/mm2 k4.4.3.21. For sections with wide flanges he values should be

The Structural EngineerNolume 70/No.13 7 July 1992

multiplied by

0.8.

If there are partitions liable to be damaged by excessive

vertical deflections the values should also be reduced for spans exceeding

7m. Applying these actors in the EC2 design he spanldepth ratio amounts

to:

= 32 x

Om8 4 0 0 / f y k )

(As,prov/As,~eq)

For direct comparison with the

BS

81 10 design values

fyk =

460N/mm2,

As,prov As,req

ere taken giving:

I/d =

32

X 0.8

(400/460)

=

22.3

This result is reduced further when

fyk

=

500N/mm2, i.e.

l /d =

32

X

0.8

(400/500)=

20.5

These values are much lower than found in the BS 81 10 design where

combining Table 3.10 and 3.1 1 of BS 81 10 gave:

I/d =

20.8

X

1.46

=

30.4

without taking into account the reduction for spans exceeding 10m. In his

exercise, the 12m-long span ofhe 55Omm-deep ribbed slab could be justified

without further calculation

of

deflectionbecauseof the surplus of

reinforcement given by 2425 [982mm21bars.

The exercise illustrates that EC2 is rather conservative for elements such

as ribbed and solid slabs where

P

= AJbd < O S Yo.

For the range

0.5

Vo

< P < 1.5

Yo, the spanldepth atios of EC2 and BS 8110 correspond more

closely. However, the EC2 design ofhe shallow floor reinforced with high

yield steel shows tha t deflections may govern the design rather than the

ultimate limit states.

Main beam

Bending

To simplify analysis the beam in gridline 7 (Fig

1 )

was reduced to four

equal spans of 7.50m. EC2 provides no table for bending moments and

shear forces. They have o be calculated for different load cases i2.5.1.21:

(a) alternate spans carrying the design permanent and variable load

?G YQ Q,,

and other spans carrying only the design permanent load

Y G

(b) two adjacent spans carrying

yG G + yQ

Qk

nd all other spans

carrying only

yG

G,.

The calculated moments are given in Table 4. They are based on linear

analysis and have been redistributed. n addition EC2 offers simplifications

for beams and slabs i2.5.3.3 (4)l cast monolithically into their supports,

where the negative moment may be aken as that at theace of the support

(Fig

5 .

For the 600mm-wide columnadjacent to the pan of 7500mm the

reduction of the hogging moments is about 20%.

However, the design moment at the faces of the supports should not

be less than 65

Yo

of the support moment calculated assuming full fixity

at the faces of all continuous supports i2.5.3.4.2 (7)l as shown in Fig 5 .

The results derived from applying these rules are also given inTable 4.

Moments are about5-20

070

lower in the spans and about 30

070

lower over

the supports in the EC2 design compared to the

BS

81 10 design. The

redistribution was limited to

6 = 0,85

to provide minimum moments at

supports or to avoid increase of span moments.

Shear

The EC2 method for shear ssimilar to BS8110. Because of direct

transmission of loads close to supports the design shear force

V,

may be

calculated at a distance d from the face of the support i4.3.2.2 (lo)], at

the expense, however, of curtailment within

. 5d

from the support i4.3.2.2

231



Paper: Albrecht

I

l I

I

I

I I In I I

Moments at

face

of

support

I

,

I

l

I

,

MI

I

l

M

nin

Mmin

Minimum

moments

I

1

Mmin=0.65 GdtQd)ln 2

I

Mmin=0.65GdtQd) n2

1

8 12

Fig 5 . Moments at face of support, minimum moments

(1 l)]. There are three values of design shear resistance l4.3.2.2 (l )]:

V,,,

is the shear resistance without shear reinforcement

V,,,

is the maximum shear force that can be carried by the notional

concrete struts

V,,,

is the shear force that can be carried with shear reinforcement.

Where

V,,

> VRdl i.e. shear einforcements required, twodesign

methods are given:

-

he standard method [4.3.2.4.31

-

he variable truss angle method l4.3.2.4.41.

TABLE 4 Main beam: design moments

L J

G E D

A

A

A

A

A

7500

Location

BS 81 10C2 design

MI, MII

(kNm)

(kNm)

support

edistr.

M

M min

ace of

M

Table 3.6

(kNm)

(kNm)

Support

J

811

.85

35

pan

J-G

1043

.85

89

pan L-J

927

19

08

*)

.85

39upport

G

1275 779

21

*)

.85 1069

NOTE:

*)

design moment

TABLE

5

Main beam: shear resistance and shear reinforcement

The first method assumes the notionalconcrete struts to be a t 45

,

similar

to

BS

81 10. The second method permits an inclination between 22 and

68 which reduces the shear reinforcement but affects the curtailment of

the tensile reinforcement.

The shear resistance without shear reinforcement depends on concrete

strength, effective depth, and reinforcement ratio 14.3.2.31.

Except for beams with low percentages of reinforcement, EC2 gives lower

values of shear resistance compared with BS 8110. When comparing the

values in Table

5 ,

it should be noted that the shear resistance is increased

because of the higher tensile reinforcement in the

BS

81 10 design.

For he EC2 design, the shear reinforcement required for he most

unfavourable section at the first interior support was calculated by the

standard method. The minimum shear reinforcement, which isbout 10

Yo

more compared with

BS

81 10, is almost sufficient for all other sections.

This suggests hat the simple standard method of EC2 will be suitableor

typical structural elements.

In general the design shear force

Vs,

will be less than the maximum

shear capacity V . The relative value V /

V,,,

governs the spacing of

links. In most cases the longitudinal and transverse spacing of links will

be limited to 300mm.

Flat

slab

Bending

An additional row of columns on gridline 5(a) could be provided in the

basement which is to be used for carparking (Fig 2). Thus the maximum

span is reduced to 7.50m which makes flat slab economical. The thickness

of 300mm is increased to 400mm by column drops.

The basic analysis for slabs in EC2 is the same as for beams. Different

load cases are considered as described above in oth directions. EC2 gives

no details about moment distribution for flat slabs. In the absence of

supporting documents providing more information, the divisionf moments

between column and middle strip as given in

BS

81 10 was applied or the

EC2 design.

Except fromhe interior span, EC2 ives ighermoments.The

reinforcement required in the EC2 design

to

resist the negative moment

over the first nterior support is illustrated n Fig 6. It hould be noted hat

P = As,,,/bd

in Fig 6 is almost constant within the column strip because

the higher amount of reinforcement in he central half of he column strip

is related t o the increased depth of the column drops.

However, EC2 requires that the tensile reinforcement over supports

should be greater han P

= 0.5

070 [4.3.4.1 (9)l to avoid punching. As shown

in Fig 6 this rule demands more tensile reinforcement within the critical

perimeter than that required to resist bending.

Punching

EC2 gives detailed principles nd rules for design against punching 4.3.41.

The critical perimeter for the EC2 design is at a distance 1.5d from both

the faceof the column and the column head. Because of rounded corners

the perimeter is less compared with the BS 81 10 design.

The method for punching shear design is characterised by

VR d l , V,,,,

VRd3 ,

f4.3.4.3. (l )] , as explained above, but related to the unit length. The

effects of eccentricity of loading are taken into account with the same facto

as

in

BS

8110, i.e. 1.15 for an internal column i4.3.4.3 4)l. The total design

shear force at the most unfavourable column J5(a) (Fig 2)

V = 1.15

X

982

=

1129kN

is almost identical with that for the BS 8110 design.

L J

G E

D

A iA

A

A

l 4 7500

)tc 7500

*

7500500

Location

EC2 design l BS 8 10esign

7q F

in. reinf.

Shear resistance

f y k

=

5yJ

without

N/mm

f

reinf.

( W

W

reinf.

335

41

1

2126

276

1320 258

min

06

A W

(mm2/m)

wd

764

532

Shear resistance

f,

=

460

without

( W

W

(mm2/m)

V

-

, bd

,

bd

reinf.

einf.

N/mm2

f

455

309 1559

325

207

min

236 1199

AS

232 Structural Engineer/Volume 70/No.

13

7 July992



Paper: Albrecht

methods. This difference may affecthe design of flat slabs without column

drops and, as shown later, the thickness of raft foundations.

Critical Derimeter

.

F

I

Tensile reinforcement

I

r p =0.5

Central cd strip

I

Middle stri p Column str ip

Middle strip

--

ly

=

4500

ly

=

7500

Fig 6. Flat slab, EC2 design: critical perimeter, tensile reinforcement

Limiting deflections

The valuesof spanldepth atio given in

EC214.4.3.21

and BS

8110

correspond for .5 To reinforcement, a enerally representative mount for

flat slabs:

EC2 : l /d = 30400/460)

=

26.1

fyk

=

460N/mm2

Column drop

<

U3

Table 3.1

1:f = 5/8 (460), M/bd = 1.25

BS8110 : l/d = 26 X 0.9 X 1 .13 = 26.4

However, the

EC2

design is based on a yield stress of 500N/mm2 which

consequently reduces

/d.

For the

EC2

design an increase of reinforcement

amount obtained simply by reducing the spacing in the critical endspan

avoids the need for a detailed calculation of deflection.

Control of cracking

The serviceability limit states in

EC2

require the control of cracking for

structural elements for exposure classes -4 r4.1.3.3Table 4.11.This control

of cracking is required in the carpark n the lower ground-floor where the

exposure is a humid environment,while the office floors, which are exposed

t o

a dry environment, need

no

consideration of control of cracking.

EC2 provides simple detailing rules i4.4.2.31for ensuring crack control

and does not require xplicit calculation of crack widths.he bardiameter

or the bar spacing has to be limited, depending on the steel stress under

the quasi-permanent combination of loads. The draft

ECl:

'Basis of design

and actions on structures' gives combination factor 2 = 0.3 for he

quasi-permanent combination for offices.

The design value is:

Gd

+

Q d = 1.35 X 8.7

+ 1.5

X 5.0 = 19.3kN/m2

and is associated with a quasi-permanent value:

Gk 2

Qk = 8.7

-l-

0.3

X

5.0 =

10.2kN/m2

A simplified approach for he steelstress under he quasi-permanent

combination, assuming

As prov As req

as used, giving

us

=

(10.2/19.3) (500/1.15) = 230 N/mm2

The maximum bar diameter and the maximum bar spacing are given in

Table 7; either requirement has to be met.

The bar spacing rules of BS

8110

are based on the service stress in the

The hear resistances without hear einforcement for both designs are

reinforcement which

may be assumed to be: = 5/8 (460) = 288

in

~ ~ b l ~

.

Because

of

the increased tensileeinforcement,he

Because Of

the advantages

for

'labs reinforced he

EC2 design gave hi.er

values

for the shear

stress.

But

when clear spacing is, in this ase,

320

mm for

0.5 To

reinforcement which

multiplied by

the shorter

perimeter the advantage

in

was reduced.

corresponds to acentre-to-centre spacing ofabout

340

mm

-

ee Table

7 .

However, shear reinforcement may be avoided in this case. The shear

Generally,

EC 2

design will be more conservative than BS

81 10

design

resistance at the

outside

the

drop

is

sufficientwithoutnelation to the maximumpacing of barss shownn the lastolumn

reinforcement for both the EC2 and BS 8110 designs.

of Table

7.

EC2 limits

the maximum shear resistance in flat slabs containing shear

reinforcement

14.3.4.5.2 (l)]

by

Interrelations

The design of flat slabs is characterised by the interrelation of

=

1.4

VRdl

-

ending, punching, deflections

This

procedure is ifferent to that required

in BS 81 10

where the shear stress

depending

on

at the face f the column s limited. The alues in the ast column of Table

-

epth

6 demonstrate the substantial difference resultingrom theEC2 and BS 81 10

-

mount and stress of reinforcement

TABLE

6

- lat slab: punching shear

Design Shear resistance

I . I I

Tensile

I

concrete

I

I

~~ ~ ~~~ ~ ~

EC2

design:

V, = 1129

kN

1570.64

.5

.2964.5d

from column drop

1572

123.59

.5

.23

64.5d from column face

Rd2

Rd 1

BS 8110 design: V,, = 1140kN

column

face of

VC

v, ud

1.5d

from column face

1290.49

.24

.97 264.5d

from column drop

3001

033.46.26.17

64

The Structural Engineer/Volume 70/No.13/7 July 1992

233



Paper: Albrecht

TABLE 7- lat slab: control of cracking and detailing

Control of cracking General

e

=

0.5

070

rules

EC2

230 2o

200

<

1.5hesign

<

350

288

-

340

< 750

design

<

3d

The differentapproach of EC2 o assure shear resistance requiresn amount

of tensile reinforcementof 0.5

070

within the critical perimeter which may

be enough to avoid shear reinforcement in someases. On the other hand,

this surplus of reinforcement in flat slabs with column drops does not

produce an increase of the spanldepth ratio. The simple approach of

controlling deflections by the spanldepth ratio related to the steel stress

in the middle of the span may reduce the advantage of high yield steel.

Table8 demonstrates these interrelationsor the lat slab

of

300

mm

depth

with column drops and for alat slab without olumn drops butwith depth

increased to 350 mm. Though the atter requires shear reinforcement,his

different design may be an alternative with advantages for the provision

of formwork and services.

Columns

Classifcation of structure and slenderness

In compression members the influence f second-order effects should e

considered n EC2 design if the ncrease n bending moments due to

deflections exceeds 10 070. This may be assumed to be the case where the

slenderness of the st ructure or structural members exceeds certain limits

i4.3.5.1 (5)l.

The structure or members are classified 14.3.5.2 (4)l as

-

raced or unbraced and

-

way or non-sway

This office building contains substantial shearalls which are sufficiently

stiff to transmit at least 90

70

of all horizontal loads to the foundations.

It may be classified, therefore, as braced and non-sway.

The necessity to consider second-order effects dependsn the lenderness

ratio of the isolated column

X = / i

l,=

P x

l O l

where

l,

is the effective height

lcol

s the height measured

i

is the radius of gyration.

between centres of restraint

TABLE 8 - lat slab, EC2 design: interrelation of bending, punching and deflections

Isolated columns are considered slender if

X >

25 or 15/f

(N,/(A,

x

f,,))

which ever is the greater 14.3.5.3.5 (2)l.

But because of the advantage of restrained ends, isolated columns need

not be checked for second-order effects if

Xc t > 25 (2-eoJe02)

where eoland eO2re the eccentricities of the axial load at the endsf the

column. In this caseminimummoment should be considered i4.3.5.5.3 (2)l

The interior column gridline 7 and the erimeter column gridline

5

were

checked as described below.

Interior column:

M =

N

X

(h/20).

IC

=

3975mm

l, <

l ol ,

ake

l ,

=

3975mm

b

= h = 6 0 0 m m

X

=

3975/(0.289 X 600)

=

22.9 25

This column is not slender.

Perimeter column:

l ol 3975mm

h

=

262mm

,

b =

874 mm

Theeffective height

I

was determined by means of hemonogram

[4.3.5.3.5 (2)l based on the rigidity of restraints at the column ends:

P

=

0.67

X

= (0.67 X 3975) / (0.289 X 262) = 35.1

>25

The next step was to check the influence onuckling of bending moments

at the ends of the column. By subframe analysis, shown in Fig 7, the

eccentricity at the topf the column isopposite to that at the bottom,here

eo, = -0.5 eO2 I eo11 l eod

X

=

25

X

(2-

(-0.5)

/(l)

=

62.5

>

35

Thus no econd-order effect had to be taken intoaccount for theperimeter

column.

Compared with the BS 81 10design the range which omits second-order

effects is quite similar

EC2

:

Zo/0.289h

=

52 corresponds to

BS 81 10:Jh

=

15.

Axial forces and moments

EC2 allows the simplification that the loads transmittedo a column may

be calculated on the ssumption that beams and slabs are simply supported.

Continuity should, however, be taken into account at the first internal

support and a t other internal supports if the spans on either side of the

support differ by more than 30

070

i2.5.3.3 (6)l.

I Over support

Bending Punching

AS

reinforcement

070)

mm2/m)

070)

mm2/m)

Shear

s x = As,

With column drops

I

1250 0.34 1820

0.5

not

h

=

400mm

Without drops I 1570 0.5 I 1570 I 0.5 I

required

h = 350mm

I

atpan

I

Bending

I

Deflections

A s

I

I/d

s

k

(mm2/m)

7.42

0.0

(m)

mm2/m) (N/mm2)

h

=

300mm

d =

247 mm

[

11491

918

500

16

@

175

h

=

350mm

d

= 297 mm

10051

806

500

16 @ 200

29.9

8.89

234

The Structural Engineer /Volume 70

/No.

13

7

July

1992



Papers:

Albrecht

hcrit

t

+ l -1

Critlcal dQndQrnQSSatio A crit

02

JQ0’

I

Subframe, perimeter column bending moments

Q01

Q02

Fig 7 Column with restrained ends

This clear statement draws attention to the unfavourable conditions at

the first internal support which, in this case of two rather unequal spans,

increase the force by 46

070

compared with simple support .

The total imposed floor load for both designs was reduced according

to BS 6399 and AMD 5881.

EC 2 gives no rules concerning the moments to be considered for the

design of columns withhe exception of he above minimum moment. The

following assumptions were met:

-

eglect moments at interior columns

-

onsider moments at perimeter columns

This corresponds to the German practice for braced structures.

When comparing he

axial

forces for both designs

in

Table 9, the different

amount of redistribution should be remembered. The bending moments

were calculated by subframes, but M

=

143 kNm for column E7 in the

lower ground-floor designed by BS 81 10 corresponds to the minimum

moment. In theEC2 design the same provision of a minimum eccentricity

TABLE 9

- esign

of

columns

applies only to the slenderness ratio 25

<

h ,< hcrit. enerally, EC2 is less

rigorous inaccounting for bending of columns nbracednon-sway

structures.

Rein orcement

In columns, highly stressed by axial forces, the strength of the concrete

is a key figure for design. Small differences inhe design concretestrength

may result in large differences in the amount of reinforcement required.

The concrete strength class in the EC2 design C30137 may be stressed

under ultimate loads up to

d

=

0.85

(30A.5)

=

17 N/mm2

compared with the BS 81 10 design related to the concrete grade C35 the

stress may be up to

0.67f,,/yc

=

0.67 (35/1.5)

=

15.6N/mm2.

This difference, due to classification of concrete, increases the capacity

of an EC2 column by 9

70

or rather decreases the amount of reinforcement

required.

Furthermore, the bending moments due to subframe analysis require

additional reinforcement for the interior column E7 inhe

BS

8110 design.

Reinforcement for the perimetercolumnE5 sneeded to increase ts

compression strength in both the EC2 and BS 81 10 design.

For less stressed columnshe reinforcement in EC2 design ill not differ

that much f rom BS 8110 design.

In the EC2 design the minimum amount of longitudinal reinforcement

should be 15.4.1.2.1

(2)l

As i =

0.15 NSd/f,d 0.003

A .

This means hat the reinforcement of highly stressed columns has

o

carry

about 15

‘70

of the axial force. For concrete C30/37 and steel grade

500

the minimum reinforcement may be up to 0.6%.

The maximum amount of reinforcement is limited o

8 To,

even at lapped

joints 15.4.1.2.1 (3)1, which is slightly less than required in BS 81 10design.

The transverse reinforcement is similar inboth designs. However, EC2

reduces the spacing of links to 60 Yo on the top and at the bottom of the

column and at lapped joints L5.4.1.2.23.

Raft foundation

Scope

Part 1 of EC2 gives a general basis of the design of concrete structures

and provides general rules applicable mainly to buildings. Further parts

to be developed in the future will complement or adapt it for particular

aspects 1 l .31, one of which is

At present, however, EC2: Par t 1 covers punching shear in foundations

14.3.4.11, and these rules were applied. This was the only aspect of the

foundat ion design considered in the exercise.

The bending moments and the soil pressure were analysedby computer

for the BS 81 10 design. he EC2/BS81 10 omparison of the shear resistance

was based on the tensile reinforcement and forces as calculated in the

BS 81 10 design.

Part 3: ‘Concrete foundations and piling’.

Punching

Punching was checked for a column load of

V

= 5095 kN applied to the

600mm-thick raft, reinforced at the bottom in both directions by 432

Q250 mm. This force may be reduced byA

V

to allow for the soil reaction

of 171 kN/m2 within the critical perimeter f4.3.4.1 (5)l. The result

V,,,,

s

given in Table 10.

EC2 design BS

81 10 design

Location

C30/37,

fyk =

500N/mm2 C35,

f =

460 N/mm2

h x b

As

le

S

h

(mm)

(mm2)

kNm)kN)mm2)

kNm)kN)

-

Interior E7

gr flr

5400 143 7164.0

420*)

004

17

00

x 6

grflr

5760

75 6378.4

160*)

251

23

00

x

600

Perimeter E5

gr flr

3890 166515

2950 9.8

58 32755

62

x

874

NOTE:

*) min. reinforcement

The Structural Engineer/Volume 70/No.13/ July 1992

2 3 5



Paper: Albrecht

TABLE I O aft foundation: punching shear

Soil

(kN)

Shear resistance

(kW

(N/mm2) I (kN)

reinforcement

einforcement

V

d

Tensile

reinf.

reaction

max. with

ithout

,,

(mm)

Vo)

m)

EC2 design

‘Rd2

1.5d from column face

4323 3088

.47 3969

126 0.45 9.16 718

ncreased depth

2813 2009

.53 4390

05 0.62 7.28

18

BS 81 10 design

face of column

c ud

,

1 t perimeter

1 5d

from column face

518

5884

680

.60

302

93

.62 8.62

2nd perimeter

3646

.60626 1469 0.62 11.72

18

As described earlier for flat slabs, EC2 stipulates the perimeter with

rounded corners which allows less for the soil reaction and is shorter. In

addition, the basic shear given by EC2 is less for all but low percentages

of tensile reinforcement. Thus the concrete shearesistance is reduced by

about 25

Yo.

The main difference oncerns the maximum shear resistance with shear

reinforcement which is limited by EC2 i4.3.4.5.2 (l )]

to

= .4 ‘Rd

V =

2813 kN compared with

V,,, =

4390 kN in Table 10 for the EC2

design means that the thickness of the raft foundationhas to be increased

regardless of the amount of shear reinforcement. As shown in the last

column of Table 10, the maximum shear resistance given by BS 8110 at

the face of the column is more than twice the value given by EC2.

By an increase of thickness of 200 mm this severe requirement was met

in the EC2 design, though shear reinforcement was still required

-

ee

Table 10:

V,, > V .

However, the given tensile reinforcement now being

reduced to less than

p =

0.5 Vo does not any longer meet the requirement

of EC2 14.3.4.1 (9)l concerning punching, although there s an advantage

concerning bending. The question o be asked is whether the minimum of

0.5 9’0 tensile reinforcement is required for thick slabs like raft foundations

as well as for comparatively shallow floorslabs. The German Code,DIN

10459, for instance, makes this allowance for foundations to encourage

the designer to provide the shear capacity more by the thickness of concrete

than by shear reinforcement.

Conclusions

(1) At present several parts of the Eurocodes are still at a draft stage of

development. Therefore, reference to the relevant national Codes is

necessary to get all the information needed to complete a design.

(2) Because of its wide scope only parts of EC2 Par t

l

are required for

typical buildings. Once knowledge and familiaritywith the layoutof EC2

is gained, it wasound not to

be

too complicated for the design of ‘everyday’

structures. However, it took considerable time to learn the implications

of EC2 and to cope with the cross-references.

(3) A manualon the use ofEC2 for the design of ypical concrete buildings

comparable to the IStructE manual for the design of reinforced concrete

building struc tures” would assist ngineers to gain the necessary

knowledge more quickly. In addition, the publication of typical designs

to the new EC2 would greatly facilitate therocess and assist the build-up

of confidence.

4)

The basic analysis of EC2 does not distinguish between beams, ribbed

slabs, and flat labs. Depending on redistribution,which is more restrictive

in theEC2 design, the mgments calculated by linear analysis may be both

higher and lower than those based on BS 81 10.

( 5 )

The results concerninghe tensile reinforcement

can

hardly

be

generalised

except for the considerable reduction ver supports in theEC2 design for

beams and slabs cast monolithically into their supports.

(6) EC2 generally gives lower basic values for shear resistance but the

minimum shear reinforcement required will easily satisfy many cases.

(7) EC2 requires substantialchanges of design practice for flat slabs both

concerning the minimum tensile reinforcement and the shear capacity. EC2

may require an increased depth for hallow flat slabs and raft foundations.

(8)

The EC2 design was based on reinforcement with a yield stress of

500 N/mm2. The results cannot therefore be compared directly with the

BS

8110 design. The higher steel stresses result in higher deflections.

236

Generally, the span/depth ratios given in EC2 are similar to the BS 8110

values, but lightly reinforced members, such as ribbed or solid slabs, may

have to have an increase of depth.

(9) EC2 provides simple ruleso control cracking without urther calculation.

Generally, the EC2 design will require smaller maximum spacing of bars

for slabs.

(10) This paper refers to the final draf t of EC2, dated December 1989.

Alterations of some clauses in the ENV version may affect some findings

given in this paper.

Outlook

Many of the factors ha t affect the design of a building are traditional or

related to the different means provided locally in eachountry. For example,

in comparison to practice in the United Kingdom, mesh reinforcement is

common inGermany, where the steel manufacturers provide standard mesh

or curtailed mesh for slabs and mesh cages as links or beams and columns.

By means of partially prefabricated elements for slabs and beams German

consultants and contractors reduce formwork and expedite the construction

of

concrete buildings. The floors are completed with

in situ

concrete to

provide continuity and stiffness.

Standard details in design and simplified construction methods are means

to reduce costs and to assure quality. Although buildings will remain

individual products, many of their tructural elements could be tandardised.

Such standardisation is not a matter of Codes but one of he future

challenges for structural engineers in Europe. Albert Einstein expressed

it this way:

‘Everything should be made as simple as possible, but not simpler’.

Acknowledgements

The work described in the paper was carried out during the author’s

sabbatical with Andrews, Kent & Stone, London. The author would like

to thank Mr D.

W .

Lazenby, Dr J . B. Menzies and other Partners and staff

for assistance.

References

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

Eurocode EC2: ‘Design of concrete structures: Part 1 General rules

and rules for buildings’, revised final draft, December 1989

Somerville,

G:

‘The writing of Eurocode 2,

The Structural Engineer,

67 No. 11, 6 June, 1989, p216-218

Narayanan, R.

S.:

‘Comparison of design requirements in EC2 and

BS 8110’,

The StructuralEngineer, 7

No. 1 1,6 June, 1989, p218-227

CED1C:‘Comparative design Eurocodes

-

ational Codes: Eurocode

No. 2: Part 1: Design of concrete structures

-

eneral rules’, final

report, Part 1 Conclusions, revised draft, April 1990

BS81 10

Structuraluse of concrete,

London, British Standards

Institution, 1985

Eurocode EC1: ‘Basis of designnd actions on structures’,Task Group

document (1): draft: June 1990

BS

6399

Loading fo r buildings, Part : Code of practice for dead and

imposed loads,

London, British Standards Institution, 1984

CEBIFIP

Manual on bending and compression,

London,

Construction Press, 1982

DIN 1045:

Beton und Stahlbeton, Bemessung and Ausfuehrung,

uly

1988

Manual for the design of reinforced concrete building structures,

London, Institution

of

Structural Engineers, 1985

The

Structural

Engineer/Volume 70/No.

13

/ 7 July

1992

application of ec2 to office buildings

Documents