Designing concrete bridges to EN 1992-2

Dr Stephen Salim

Structure of Eurocodes

EN 1990

EN 1991

EN 1992 EN 1993 EN 1994

EN 1995 EN 1996 EN 1999

EN 1997 EN 1998

Design & Detailing

Actions on structures

Geotechnical & seismic design

Structural safety, serviceability & durability

Eurocodes have National Annexes

Gives values/approaches where National Determination is allowed

Eurocodes required for concrete bridge design

EN 1990 EN 1991

EN 1992 EN 1997

Design

Analysis and section design, partial factors

Design approach, partial factors, foundations, earth pressures etc.

Limit states, combination and partial factors

Actions, inc. load groups, application etc.

Comparisons with current practice Concrete design

Uses cylinder strength ( 0.8fcu ) More rooted in plasticity theory Consistent approach for reinforced concrete and

prestressed concrete Greater coverage of non-linear analysis and time

dependent effects

What effect will change of code have on designs?

ULS Flexure

Stress BlocksStrain Stress

cc * fck / c cc * fck / c0,0035 (for fck< 55)0.8

or

Design Concrete Strength

fcd = cc * fck / c Wherecc = 0,85 (From UK NA)c = 1,5fck = 0,8 * fcu (approx)

cc * fck / c = 0,453 * fcu Close to BS 5400!

Reinforcement

= 1.15 as BS 5400Stress/strain relationship very similar (actually closer to 8110).

Overall effect on Flexural design at ULS

Compression steel more advantage than in BS 5400 but otherwise Very Similar

Some change due to loading (e.g. switching from the old HB abnormal loads to new loads similar to the loads already used in BD 86 assessment

ULS Shear

Case 1: No designed links

For RC approach and results fairly similar to BS 5400 except more benefit for compression

For Prestressed approach same as for RC with axial load. Tends to be more conservative than BS 5400, except for external prestress.

Case 2: Designed Links

Unlike BS 5400 which uses the addition principle ( V = Vconcrete + Vlinks)

In EN 1992 - shear is taken by the links once the shear strength without links are exceeded and the strength is calculated using the varying angle truss approach

Case 2: Designed LinksVariable Angle Truss Analogy

Steel Ties

Concrete Struts

Strength limited by Links

VRd,S = (Asw / s) *z * fywd * cot whereAsw / s = Link Area / Spacingz = Lever Arm (normally 0,9d for RC)fywd = Design yield strength of links

(i.e. with factor of 1,15) = Angle of struts (1< cot

Strength limited by Concrete

VRd,max = cw * b * w * z * 1 * fcd / (cot + tan )

where cw = Coefficient taking account of

compression stress (1.0 for R.C. can be up to 1.25)

bw = Web width (after reductions for ducts)

fcd = Design concrete strength(As in flexure but cc can be 1.0)

Choice of

For minimum links cot = 2.5But, for maximum shear cot = 1.0 (45o truss)If shear too great for cot of 2.5 but within limit, optimum is with:

VRd,s = VRd.maxfor higher shear, pays to use 80% yield

Link Design Comparison

0

100

200

300

400

500

600

0 1 2 3 4 5

Links

S

t

e

n

g

t

h

(

k

N

)

Shear Strength of

300 wide 400 deep RC beam

with 25/30 Concrete

(1% steel)

EN 1992

BS 5400

Link Design Comparison

Shear Strength of

300 wide 400 deep RC beam

with 50/60 Concrete

(1% steel)

10 links = T16-60A lot but possible!

EN 1992

BS 5400

0

200

400

600

800

1000

1200

0 2 4 6 8 10

Links

S

t

e

n

g

t

h

(

k

N

)

Varying Angel Truss Analogy

Can give significant link savingBut Affects curtailment and EN 1992 already tends to be more

conservative for these Design calculations more complicated (can optimise for

link design with simple excel spreadsheet)

Shear in Prestressed Concrete

Same general approach as RCBut Strength without links enhanced

VRd,max = cw * b * w * z * 1 * fcd / (cot + tan ) + k1 * cp Concrete crush strength increased for

( 0 < cp < 0.5fcd )

Effect of Compressive Stress on cw

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1

Stress/fcd

Link Design Comparison (Prestressed)

0

500

1000

1500

2000

2500

3000

0 2 4 6 8 10

Links

S

t

r

e

n

g

t

h

(

k

N

)

For

250 x 1100 beam50/60 concrete

7N/mm2 prestressBS 5400

(uncracked in flexure)

EN 1992

Shear in Prestressed Concrete

Can have thinner webs May require more links Bigger reduction for plastic ducts

Short Shear Spans

av

Load multipliedby

= av/2d(av 0.5d)

Short Shear Span Enhancement

Altering loads is inconvenient and conservative for multiple loads and impractical for envelope load cases

So EN 1992-2 NA has changed it back to an enhancement

factor to the resistance

Serviceability Limit state (SLS)

SLS Stress Limits

Steel: 0.80fyk(0.75fy in BS 5400)

Concrete: 0.6fck for both RC & PSC(0.50fcu for RC and 0.40fcu for PSC in BS 5400)

but calculated on cracked section

SLS Cracking

Crack Width Check(As for RC)

3

None2

Decompression1

EN 1992 EquivalentBS 5400 Class

Decompression VS Class 1

Cracked

Tendons

OK to either OK for decompression, not class 1

Table 7.101N Recommended values for wmax & relevant combination rules

a For X0, XC1 exposure classes, crack width has no influence on durability and this limit is set to guarantee acceptable appearance. In the absence of appearance conditions this limit may be relaxed.b For these exposure classes, in addition, decompression should be checked under the quasi-permanent combination of loads.c For the crack width checks under combinations which include temperature distribution, the resulting member forces should be calculated using gross section concrete properties and self-equilibrating stresses may be ignored. d 0.2 applies to the parts of the member that do not have to be checked for decompression

0.20.3aX0, XC1

0,2d and Decompression0.3XD1, XD2, XD3 XS1, XS2, XS3

0.2b0.3XC2, XC3, XC4

Frequent load combinationcQuasi-permanent load combinationc

Prestressed members with bonded tendons

Reinforced members and prestressed members without bonded tendonsExposure Class

Cracking in RC

Only checked under quasi-permanent Unlikely to be critical Result: despite apparently radical treating of RC and

prestressed together, still tends to give:

RC: designed at ULSPrestressed: designed at SLS

Cracking in PSC

More sensitive to damage from corrosion than normal reinforcement due to smaller diameter and higher level of stress under which they normally operate

Therefore more onerous ruler and reflects in stricter crack control criteria for bonded tendons

Checked under frequent OR quasi-permanent load combination depending on exposure class

Determination of crack widths

Calculateor Comply with max bar spacingor Comply with max bar diameter table

Same basic approach used for PSC

Design Examples

1. Concrete composite construction with precast, pre-tensioned beams and a cast in-situ slab

2. In-situ post-tensioned box girder bridge

Precast Beam & Slab Bridge

Section

Elevation (half in Section)

Comparison of moments - BS 5400 and EN 1992

1230(Quasi-permanent LC)

2460(Frequent LC)

1160(LC 1, no LL)

Decompression / Class 1

2460(Frequent LC)

3090(Characteristic LC)

EN 1992XC exposure

3090(Characteristic LC)

3000(LC 1-5)

Compression

Design moment (kNm) for checking

3090(Characteristic LC)

EN 1992XD exposure

2900(LC 3)

BS 5400

Cracking / tensile stress

Design code & exposure

class

Precast Beam & Slab Bridge

In-Situ Post-tensioned Box Girder Bridge

Mid-span section

Spans 70m - 100m 70m

Support section

Comparison of moments - BS 5400 and EN 1992

115-2.29

(Quasi-permanent LC)

199-346

(Frequent LC)

109-224

(LC 1, no LL)

Decompression / Class 1

199346

(Frequent LC)

235-397

(Characteristic LC)

EN 1992XC exposure

235-397

(Characteristic LC)

235-402

(LC 1-5)

Compression

Design moment (MNm) for checking

235-397

(Characteristic LC)

EN 1992XD exposure

231-386(LC 3)

BS 5400

Cracking / tensile stress

Design code & exposure

class

Post-tensioned Box Girder Bridge

First positive value represents the sagging moment at mid-span, second negative is the hogging moment at the piers

Comparison of prestress requirement

45000

66700

70800

Prestressing force (kN)

Mid-span

22.0

18.7

18.5

Peak concrete

compressive stress

(N/mm2)

50500

76400

73000

Prestressing force (kN)

Pier

26.0EN 1992XC exposure

22.7

23.3

Peak concrete

compressive stress

(N/mm2)

EN 1992XD exposure

BS 5400

Design code & exposure

class

Post-tensioned Box Girder Bridge

Challenges for UK bridge designers and clients

To be ready for the introduction of the Eurocodes Minimise increases in design costs due to unfamiliarity of

documents Manage the risks associated with this magnitude of

change