Structural_Evaluation_of_30_year_old_Pre

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1 
STRUCTURAL EVALUATION OF 30-YEAR-OLD PRESTRESSED CONCRETE GIRDERS 
 
 
 
Daia Leonid Zwicky, dipl. Bauing. ETH 
Swiss Federal Institute of Technology ETH Zurich 
Institute of Structural Engineering IBK 
HIL E 34.1 
CH – 8093 Zurich, Switzerland 
Tel. +41 / (0)1 / 633 31 44, Fax +41 / (0)1 / 633 10 64 
e-mail: zwicky@ibk.baug.ethz.ch 
Supervisor: Prof. Thomas Vogel, IBK / ETH Zurich 
Abstract 
Due to the replacement of a 30-year-old bridge deck, prestressed concrete beams of 21 m length 
and a weight of approx. 35 tons are available for load tests. For these full-scale tests, an outdoor 
testing facility was built, where girders with spans of 15 – 25 m can be loaded. The girders were 
designed for shear according to the concept of inclined principal stresses and were highly pre-
stressed. The webs were reinforced with secondary reinforcement only. According to current stan-
dards, the beams exhibit insufficient shear strength. The paper reports on three full-scale tests, 
focussing on failure predictions, test set-ups and the discussion of selected results. Conclusions 
about the prediction of bending and shear resistance are drawn and recommendations for full-scale 
tests are given. 
Keywords 
Full-scale tests, prestressed concrete structures, shear strength, structural analysis. 
1 Introduction 
During the 1960's and 1970's prefabricated concrete beams were often used for road bridges with 
spans up to approx. 30 m in Switzerland. Due to prefabrication, the structures were mostly stati-
cally determinate. The beams were frequently prestressed with wires and post-tensioned with ten-
dons. The road deck was extended with cast-in-situ concrete. The girders were often fully prestres-
sed for dead and traffic loads. 
At that time, the beams were designed for shear according to the concept of inclined principal 
stresses. The design was conducted for service loads for uncracked or cracked cross-sections, 
depending on the valid standards. In addition, high prestressing ratios were applied in order to re-
duce tensile stresses to an admissible value. Therefore, the shear reinforcement consisted of sec-
ondary reinforcement only. 
Actual standards are usually based on general truss models with restricted inclination of the com-
pression diagonals [1], [2]. Applying these models, the aforementioned girders exhibit an insuffi-
cient shear strength. 
Due to the replacement of a 30-year-old bridge deck, several prestressed concrete beams – 21 m 
long and with a weight of approx. 35 tons – are available for load tests. This gives the opportunity 
to carry out full-scale tests after 30 years of service life. 
2 Aim of research project 
The shear strength of the available beams is obviously higher than the value calculated with actual 
standards. The research project aims at predicting the effective shear strength of this kind of con-
crete girder. 
Higher shear strength in beams can be calculated by accepting flatter inclinations of the compres-
sion diagonals than allowed by standards. Also, it is worth considering the shear resistance of the 
flexural compression zone in beams with wide flanges. This shear resistance depends on the ten-
sile strength of the concrete, similar to the punching shear strength of concrete slabs. 
Structural Evaluation of 30-year-old Prestressed Concrete Girders 2 
Applying a methodology relying on established basic principles and being confirmed by full-scale 
tests, enables one to confirm sufficient structural safety for this kind of structure and prevent ex-
pensive strengthening or premature replacement. 
3 Test procedure 
3.1 Experimental program 
Full-scale tests up to failure have been carried out. Table 1 shows the relevant structural data of 
three tests. The fourth test will not be discussed because it was similar to the third one and re-
vealed few additional information. 
 Structural system 
Internal forces 
Geometry 
Maximum loads and internal forces 
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1 
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6.36 m 8.00 m 20.72 m 0.12 m 
h 
1.49 m 
QF1 QF2 QA1 QA2 q Mmax Vextr VQF1 VQF2 
[kN] [kN] [kN] [kN] [kN/m] [kNm] [kN] [kN] [kN] 
633 625 – – 17.12 4916 808 699 696 
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3.20 m 8.00 m 14.40 m 3.25 m 
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QF1 QF2 QA1 QA2 q Mmax Vextr VQF1 VQF2 
[kN] [kN] [kN] [kN] [kN/m] [kNm] [kN] [kN] [kN] 
2071 2039 234 234 16.05 6192 2180 2129 2111 
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QF1 QF2 QA1 QA2 q Mmax Vextr VQF1 VQF2 
[kN] [kN] [kN] [kN] [kN/m] [kNm] [kN] [kN] [kN] 
1336 – – – 16.42 4968 1172 1100 – 
Table 1 Structural data of the reported tests. 
3.2 Testing facility 
For the purpose of testing the available concrete girders, an outdoor testing facility was built. 
Ground anchors and transfer beams enable the loading of the beams. The minimum distance bet-
ween load and support is restricted to 2.85 m. When applying two loads, their spacing is set to 
8.0 m. Fig. 1 shows a schematic overview. 
The facility's capacity is limited by the four ground anchors. Each anchor was tested up to a load of 
940 kN, exhibiting only minor creeping. The nominal yield strength of an anchor rod is 1100 kN. 
Thus a short time maximum load of about 1000 kN is possible. 
3.3 Test specimen 
In Fig. 2 the documented geometry and the web reinforcement of the prefabricated beam are 
shown. The concrete deck cast-in-situ was planned to have a thickness of 150 mm and was longi-
tudinally and transversally reinforced with mild steel. Table 2 shows the material properties of the 
concrete of all test specimens and of the mild steel web reinforcement of test specimen PV1. 
The geometry of the specimens could be confirmed to a great extent. The width of the flanges var-
ied because the beams were cut out of the bridge deck. The heights h also varied slightly, see Ta-
ble 1, owing to execution inaccuracies. 
Structural Evaluation of 30-year-old Prestressed Concrete Girders 3 
The prestressed reinforcement consisted of wires, 44· Ø6 mm, concentrated in the lower part of the 
beam, and 14· Ø4 mm distributed over the rest of the cross-section. The post-tensioned tendon 
consisted of wires, 32· Ø6 mm, in a steel duct with Øa = 56 mm. The arrangement of the preten-
sioning wires and of the tendon as well as the material properties of the prestressed reinforcement 
is given in Fig. 3. 
 
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Fig. 1 Outdoor testing facility, [m] – (a) longitudinal view and (b) cross-section. 
 
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Fig. 2 Geometry and web reinforcement of the test specimen, [mm] – (a) geometry of the pre-
fabricated beam; (b) and (c) mild steel reinforcement of the web and (d) longitudinal ar-
rangement of the reinforcing meshes. 
 
 fcw
S 
[MPa] 
fc
'S 
[MPa] 
fct
S 
[MPa] 
Ec
S 
[GPa] 
εcu
S 
[‰] 
fcw
P 
[MPa] 
fsy 
[MPa] 
fsu 
[MPa] 
εsu 
[‰] 
εsg 
[‰] 
Es 
[GPa] 
PV1 73.6 50.0 2.64 34.5 2.17 56.5 526 571 12.3 6.9 185 
PV2 73.0 56.9 – 40.9 1.68 68.7 – – – – – 
PV3 82.8 65.4 – 36.9 2.03 87.0 – – – – – 
Table 2 Material properties of concreteand mild steel reinforcement. 
Structural Evaluation of 30-year-old Prestressed Concrete Girders 4 
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Fig. 3 Prestressed reinforcement – (a) pretensioning wires; (b) post-tensioned tendon; (c) mate-
rial properties of the pretensioning wires and (d) material properties of the tendon. 
4 Experiments 
4.1 Specimen PV1 
The first full-scale test aimed at checking the functionality of the outdoor testing facility and at gain-
ing preliminary information on the response of the test specimens. 
4.1.1 Test set-up and failure predictions 
An overview of PV1 during the test and the load assembly is given in Fig. 4. Fig. 5 shows a selec-
tion of the provided measurements. 
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Fig. 4 Specimen PV1 – (a) overview of the test set-up and (b) load assembly. 
Flexural failure was expected in the middle of the beam. The maximum load was predicted by sec-
tional analysis and resulted in failure loads in the range of QF = 627…658 kN, depending on the 
assumed material properties. The deviation of the predicted load from the observed failure load 
was very small, see Table 1. 
Structural Evaluation of 30-year-old Prestressed Concrete Girders 5 
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Fig. 5 Test set-up PV1, [mm] – (a) instrumentation for continuous measurements and (b) net for 
periodic strain measurements. 
4.1.2 Test procedure and results 
Due to the large longitudinal deformations at higher loads, the ground anchors of QF2 were con-
strained by the transfer beam. Additionally, the bearing plate of the sliding support was pushed out 
of the lateral guide as well, squeezing out the sliding layer. Owing to this and other minor problems 
the test had to be interrupted after two days. Due to their limited range, several measuring instru-
ments had to be adjusted during the test and the unloading was not completely recorded. For this 
reason the data of the third day has to be interpreted separately. 
0 100 200 300 400 500
wm [mm]
0
100
200
300
400
500
600
700
QF1 [kN]
load stage
(a)
0 100 200 300 400 500
wm [mm]
0
100
200
300
400
500
600
700
QF1 [kN]
max. load
(b)
failure
0·10-3 2·10-3 4·10-3 6·10-3 8·10-3 10·10-3
�m [m
-1]
0
1000
2000
3000
4000
5000
Mm [kNm]
(c)
0·10-3 1·10-3 2·10-3 3·10-3 4·10-3
�m [m
-1]
Mm [kNm]
0
1000
2000
3000
4000
5000
(d)
0 1 2 3 4 5 6 7 8
x [m]
-3
-2
-1
0
1
�2 [‰]
(e)
0 1 2 3 4 5 6 7 8
x [m]
�2 [°]
-90
-60
-30
0
30
60
90
(f)
Fig. 6 Results of PV1 – (a) load - deflection behaviour at midspan for test days 1 & 2 and (b) for 
day 3; (c) moment - curvature relationship at midspan for test days 1 & 2 and (d) for day 
3; (e) minor principal strains in selected load stages and (f) inclination of minor principal 
strains. 
Structural Evaluation of 30-year-old Prestressed Concrete Girders 6 
Due to the separation of the test data, the deflection at midspan at failure – Fig. 6 (a) and (b) – 
could only be estimated to about wm = 625 mm. This corresponds to li /33 and points to a rather 
high deformation capacity of the specimen. 
The curvature χm in Fig. 6 (c) and (d) was calculated using the average strain from the strain 
gauges DMSm at midspan and the measured elongation of the tension chord ZDm at midspan, ac-
cording to Fig. 5 (c) and assuming the cross-section to remain plane. The bending moment Mm was 
calculated considering dead loads as well. 
The minor principal strains ε2 in the middle of the web in Fig. 6 (e) indicate a concentration of the 
spread cracks at the bottom of the beam into single cracks towards the top of the web, showing 
rather large differences and changes in sign. The inclinations θ2 in Fig. 6 (f) of the minor principal 
strains correspond well with the expected changes according to stress fields, e.g. [3]. 
4.2 Specimen PV2 
The second test aimed at obtaining a web crushing failure mechanism according to limit analysis, 
i.e. yielding of the shear reinforcement and crushing of the web concrete, e.g. [4]. 
4.2.1 Test set-up and failure predictions 
The load assemblies for QF were the same as in PV1. The forces QA were applied with the help of 
transfer beams and prestressing rods which were anchored in the foundations by means of glued 
dowels, see Fig. 7. The continuous measurements and the nets for the periodic strain measure-
ments were similar to those in PV1. 
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Fig. 7 Load assembly for QA – (a) longitudinal view; (b) cross-section with legend and (c) QA2 at 
the right end of test specimen PV2. 
The experimental set-up for this specimen was optimised using failure mechanisms. The most criti-
cal mechanism is shown in Fig. 8 (a). The failure loads were predicted to be in the range of QF = 
1210…1410 kN. They depend on the effective compressive strength fce of the web concrete and on 
the applied forces QA = 60…290 kN at the cantilevers. As shown in Table 1, the maximum loads 
were far higher than the predicted loads. 
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Fig. 8 Failure predictions using limit analysis – (a) upper-bound approach and (b) lower-bound 
approach. 
4.2.2 Test procedure and results 
At loads of approx. QF = 1270 kN and QA = 117 kN stability problems in the load assemblies of the 
cantilever loads QA occurred, owing to the lower transfer beams acting as short compression 
members – see Fig. 7 (c) –, and the assemblies had to be restructured as shown in Fig. 7 (a) and 
(b). At loads of approx. QF = 1450 kN and QA = 288 kN a prestressing rod in the load assembly of 
QA1 failed spontaneously due to a nut shearing off the prestressing rod. After restructuring of the 
load assembly and re-increasing of the load, the bars holding the lower transfer beam down to the 
foundation failed and the test had to be interrupted after three days. During unloading of the speci-
men the instrumentation for the continuous measurements was removed and the unloading was 
not completely recorded. The data of the fourth day has to be interpreted separately as well. 
Structural Evaluation of 30-year-old Prestressed Concrete Girders 7 
At the fourth day the loads were increased to the values given in Table 1 without reaching shear or 
bending failure and using the full capacitiesof the ground anchors. The specimen was unloaded 
again and several static check calculations were conducted. The flow of forces could be modelled 
as combined strut and fan action, but the longitudinal reinforcement was too weak to fulfil equilib-
rium. Hence, it could be concluded, that friction in the sliding support had occurred. It was decided 
to load the specimen in a 4-point-loading test like PV1 with a = 6.345 m in order to check the cross-
section of the longitudinal reinforcement. The severe damage from the previous loading hardly in-
fluenced the failure behaviour and the failure loads amounted to QF1 = 658 kN and QF2 = 644 kN. 
QA1 [kN]
0 50 100 150 200 250 300 350
0
500
1000
1500
2000
QF1 [kN]
(a)
load stage
QA1 [kN]
0 50 100 150 200 250 300 350
QF1 [kN]
0
500
1000
1500
2000 (b)
0 20 40 60 80 100 120
wm [mm]
0
500
1000
1500
2000
QF1 [kN]
(c)
0 20 40 60 80 100
wm [mm]
0
500
1000
1500
2000
QF1 [kN]
(d)
ZDm [‰]
0 1 2 3 4
0
1000
2000
3000
4000
5000
6000
Mm [kNm]
(e)
ZDm [‰]
0 1 2 3 4
Mm [kNm]
0
1000
2000
3000
4000
5000
6000
(f)
13 14 15 16 17 18 19 20 21
�1 [‰]
0
2
4
6
8
10
(h)
x [m]
13 14 15 16 17 18 19 20 21
0
2
4
6
8
10
�z [‰]
(g)
x [m]
Fig. 9 Results of PV2 – (a) anchor load - cantilever load relationship for days 1 through 3 and 
(b) for day 4; (c) load - deflection behaviour at midspan for test days 1 through 3 and (d) 
for day 4; (e) moment - tension chord elongation relationship at midspan for days 1 
through 3 and (f) for day 4; (g) average vertical strains in selected load stages and (h) 
first principal strains in the middle of the web. 
Structural Evaluation of 30-year-old Prestressed Concrete Girders 8 
Fig. 9 (a) and (b) show the complex relationship between QF1 and QA1, owing to the fact that the 
loads were hydraulically independent but related through the deformations of the specimen. 
The deflection at midspan wm – Fig. 9 (c) and (d) – indicates that there were additional forces act-
ing on the specimen from a load of approx. QF1 = 1400 kN. This is attributed to the aforementioned 
friction in the sliding support. 
The relationship between the bending moment Mm and the average elongation of the tension chord 
ZDm at midspan in Fig. 9 (e) and (f) confirms the existence of additional forces. 
The average vertical strains εz at about the middle of the web in Fig. 9 (g) were higher than the 
yield strain of the shear reinforcement, but did not reach its rupture strain. Although only average 
strains could be measured, it can be stated that the strain in cracks is always higher than the aver-
age strain. The first principal strains ε1 in Fig. 9 (h) and the average vertical strains εz in Fig. 9 (g) 
confirmed, that there were very small changes only in the state of strain in the last two load stages, 
indicating the acting of additional forces again. 
4.3 Specimen PV3 
This test also aimed at obtaining a web crushing failure mechanism. Due to the experience gained 
from specimen PV2, the distance between load and support should be longer and cantilever loads 
should not be applied. 
4.3.1 Test set-up and failure predictions 
An asymmetric load arrangement using only one anchor load was chosen. The continuous meas-
urements consisted of deflection measurements and of vertical elongation measurements in the 
web. A net for the periodic strain measurements, similar to Fig. 5 (d), was only applied at the 
shorter section c of the span between load and support. 
The test set-up was optimised using a lower-bound approach, shown in Fig. 8 (b). The inclinations α1 and α2 were varied in order to maximise the reaction R. The failure loads were predicted to be in 
the range of QF = 1110…1350 kN. They depended on the assumed material properties of the web 
concrete and the mild steel reinforcement as well as on the assumed forces FT and FP in the 
prestressed reinforcement. The failure mode consisted of yielding of the stirrups and crushing of 
the web concrete in the stress field above the tendon. Flexural failure was expected to occur at 
about QF = 1290 kN. As shown in Table 1, the effective failure load was in the predicted range al-
though the beam failed in bending, see 4.3.2. 
4.3.2 Test procedure and results 
The deflection wQ under the loading point was 191 mm at failure – Fig. 10 (a) –, corresponding to 
li /98, it being difficult to judge for deformation capacity of the test specimen. If it is compared to a 
beam with l = 2·c = 8.8 m, the deflection corresponds to l/46 and lies within a customary range of 
deflections at failure. 
Fig. 10 (b) shows the vertical elongation BDQ close to the load against the shear force VQF next to 
the loading point. Before cracking in the observed region occurred, compressive strains were 
measured. At a load of approx. QF = 1200 kN the average strain exceeded the yield strain of the 
shear reinforcement. Because cold-deformed steel was used, it can not be regarded as real yiel-
ding. After maximum load, the deflection wQ could still be slightly increased. At the same time, the 
vertical strains BDQ decreased, and it is assumed that strains were localised in the failure crack. 
The location of the maximum deflection – Fig. 10 (c) – at x = 7.11 m already formed at a load of 
approx. QF = 1200 kN. The deflection line is confirmed by the fact, that significant curvatures χ 
shown in Fig. 10 (d) were concentrated over a length of approx. 13 m of the specimen. The failure 
cross-section did not correspond with the location of the largest curvature, but was between the 
two peaks. 
The vector plot of the minor principal strains ε2 for the last load stage in Fig. 10 (e), with only nega-
tive strains, shows the separation of the load as well as a maximum value just beneath the loading 
point. From the variation of the minor principal strains ε2 it can be concluded, that the part of the 
load transferred to the closer support was hung up once by the stirrups. From there it seems, that 
parts of the load were supported behind the bearing and parts in front of the bearing, the latter 
pointing to a dowel action of the pretensioned reinforcement. 
In a first interpretation, the flow of forces was analysed using stress fields. A stress-free zone bet-
ween load and support was found, corresponding to the region with very low compressive strains 
and showing no cracks, see Fig. 10 (e) and (f). 
Structural Evaluation of 30-year-old Prestressed Concrete Girders 9 
This allowed to shorten the distance c in the test set-up of the fourth full-scale test without risking 
direct load transfer between load and support. The reduction of c however was not sufficient to 
provoke shear failure and this fourth specimen also failed in bending. 
0 50 100 150 200
wQ [mm]
0
200
400
600
800
1000
1200
1400
QF [kN]
max. load
failure
load stage
(a)
0 1 2 3 4
BDQ [mm]
VQF [kN]
0
200
400
600
800
1000
1200
1400
(b)
0 2 4 6 8 10 12 14 16 18 20
x [m]150
100
50
0
w [mm]
(c)
x [m]
0 2 4 6 8 10 12 14 16 18 20
12·10-3
10·10-3
8·10-3
6·10-3
4·10-3
2·10-3
0·10-3
-2·10-3
��[m-1]
(d)
0 2 4 6 8
x [m]
(e) � �� � �
Fig. 10 Results of PV3 – (a) load - deflection behaviour; (b) shear force - vertical elongation be-
haviour; (c) deflection lines for all load stages; (d) curvatures for selected load stages; (e) 
vector plot of the minor principal strains of the last load stage and (f) crack pattern in the 
shorter section of the span of the last load stage. 
5 Conclusions 
The reported tests reveal clearly that bending failure can be predicted very accurately with devia-
tions smaller than 5%. Shear failure is much more difficult to predict, owing to the dependency on 
the scatter in material properties of the concrete and on simplifying assumptions in the modelling, 
like assigning the shear forces to the web only and neglecting the shear strength of the flanges. 
Further research is necessary to determine the effective shear strength of existing structures.Size does matter, regarding the test set-ups. To install the specimens in the testing facility, a mo-
bile crane with a capacity of approx. 350 m·t was needed as well as a heavy weight transport vehi-
cle. The large size of the specimen also implies high forces, e.g. almost 5000 kN of total force in 
PV2. If accidents occur, the damage to persons and equipment would be much more severe due to 
the high forces. 
Owing to the size of the specimens, displacements become very large. Regarding their limited 
range, measurement instruments have to be observed carefully during loading and unloading. At-
tention also has to be paid to the limited range of movement of elements of the test set-up like 
transfer beams and bearing plates. 
The testing facility showed good behaviour, though as expected it was too soft to counteract brittle 
failure after exceeding the maximum loading. In such an outdoor testing facility with a low stiffness, 
the use of different hydraulically independent load groups like in PV2 can not be recommended, as 
the magnitudes of the loads are sensitive to the changing ratio of stiffness of test specimen and 
testing facility. As the strokes of jacks are restricted and the prediction of the effective deformation 
behaviour of test specimens implies many assumptions, the exact planning of a loading path is dif-
ficult to establish and to follow. If the test set-up has a sufficient stiffness, the application of several 
load groups is simplified, e.g. [5]. 
Structural Evaluation of 30-year-old Prestressed Concrete Girders 10 
Acknowledgements 
Financial support from the Swiss Federal Highway Administration (ASTRA) and StahlTon AG is 
gratefully acknowledged. The high accuracy of the installation of the specimens by the employees 
of Richi AG contributed greatly to the execution of the full-scale tests. Further, the author would like 
to thank all the people giving a helping hand during the full-scale tests, M. Baumann, C. Gisler and 
M. Ingold in the first. H. Stempfle's work, carrying out the third test PV3 as a diploma thesis [6], is 
also gratefully acknowledged. The author wishes to thank S. Köppel for the constructive discus-
sions and comments on the work in progress. 
Notation 
The following notations are used in this paper: 
 Capital Latin letters χ curvature 
BD vertical elongation Ø diameter 
D compression chord force 
DMS strain gauge Superscripts 
E modulus of elasticity P cast-in-situ concrete 
F force in reinforcement S web concrete 
M bending moment ' cylinder 
Q force 
R reaction subscripts 
V shear force, initial prestress force A side-span 
ZD elongation in tension chord F midspan, field 
 P pretensioning steel 
 Small Latin letters T tendon 
a distance of ground anchors, part of main span QF load point in midspan 
b distance of cantilever load from end a side-span, external 
c shear lever arm, length of bearing plate c concrete 
d lever arm e effective 
f strength extr extreme 
h height of test specimen g permanent strain at ultimate 
l length i internal, midspan 
n number m midspan 
s spacing max maximum 
q distributed load p prestressing steel 
w deflection s mild reinforcing steel 
x longitudinal co-ordinate t tensile 
 u ultimate 
 Greek letters / symbols v shear α inclination w web, cube 
ε strain y yield 
θ inclination z vertical co-ordinate 
σ stress 
References 
 
[1] SIA 162, Betonbauten (Concrete Structures), code, edition 1989, partly revised 1993, Swiss 
Society of Engineers and Architects (SIA) Zurich, 86 pp. 
[2] Eurocode 2, Design of Concrete Structures, European Committee for Standardisation, ENV 
1992-1-1 and ENV 1992-2, 1992, 173 pp. (part 1) and 48 pp. (part 2). 
[3] Muttoni, A., Schwartz, J. and Thürlimann, B.: Design of Concrete Structures with Stress 
Fields; Birkhäuser Basel Boston Berlin, 1996, 145 pp. 
[4] Sigrist, V., Alvarez, M. and Kaufmann, W.: Shear and Flexure in Structural Concrete, Bulletin 
d'Information 223, Comité International du Béton, Lausanne 1995, pp. 7 – 49. 
[5] Kaufmann, W. and Marti, P.: Versuche an Stahlbetonträgern unter Normal- und Querkraft 
(Tests on Structural Concrete Girders Subjected to In-plane Shear and Normal Forces), IBK 
report 226, ETH Zurich, Institute of Structural Engineering, November 1996, 131 pp. 
[6] Stempfle, H.: Schubversuch an einem Spannbetonträger (Shear Strength Test on a 
Prestressed concrete Girder), diploma thesis, ETH Zurich / University of Stuttgart, Institute of 
Structural Engineering Zurich, August 1998, 63 pp., not published.