The Precast Concrete Bridge Beam: The first 50 years

Paper: Taylor
Ordinary Meeting
A paper to be presented
and discussed at the Institution
of Structural Engineerson Thursday 19 November 1998 at 6.00pm.
The precast concrete bridge beam:
the first 50 vears
J
H. P. J. Taylor, BSc(Tech),PhD, FEng, FIStructE, FICE
Tarmac Precast ConcreteLtd
-e
Howard Taylor is Technical Director at Tarmac
Precast Concrete. His career spans a wide range of
experience in design, research and construction. AI the
C&CA he became technical manager of a national
research programme into ‘Concrete and the oceans’.
He moved
back
into
industry, as Chief
Engineer
and
then a Director of Dow Mac Concrete Ltd, remaining
with rhe company when ir was acquired by Costain
Building Products. He has written more
than
70
oublications. He was Presidenr of the Institution of
2in (560mm) and spans from llft (3.35m) to 45ft (13.72m). More willbe
said about the similarity with Y-beam
the
later in this paper.
The WR range
is illustrated in Table1.
At the same time Abeles
had designed a beam witha shape not unlike the
m
M-beam‘. These beams were made mainly
by Dow Mac and used in various
also introduced a standard range, the
places in the LNER network. Dow Mac
f..
%
SBB (small bridge beam), at that time. The SBB range, again with
a shape
reminiscent of the Y-beam, was produced with three different mould sizes,
designated SBB1,2 and 3, which gave bottom flange depths of2X, 3%and
4Xin (63.5, 89, 114.3mm). TheSBB2 design is still in production today
by
Tarmac Precast Concrete Ltd and is probably
the oldest bridge beam design
still being manufactured.This section is also illustratedin Table 1.
Introduction
The author has copies
of the original brochures and some
calculationsof
In the last 50 years, prestressed concrete, pretensioned bridge beams have
these andother beam sectionswhi.ch provide a greatdeal of material thatis
developed froma few experimentalapplicationsto one of the key solutions
of use to the assessment engineer. Unfortunately, the space available
for this
to bridging problems in the short- to medium-span range, IO-40m. This
paper does not allow it to be reproduced, but
enquiries are welcomed from
paper is written to chartthat progress and to put in context the current and any bridge beam manufacturer’s design office.
likely future developments in precastconcrete bridges.
The influence of Government was very great in the early days, with
This paper concentrateson UK development, althoughsome attention is
encouragement of the use of prestressed concrete because of shortages of
given to the progress of precast bridge construction other
in countries. Itis
steel, and Government technicalinput was used to progress design, testing
impossible to pin down thestarting point of a 50-yearinterval that would
and the planningof prototypes.
allow a Golden Jubilee year to be celebrated with total precision, but, for
The public and private sectors combined in the Prestressed Concrete
reasons describedin this paper, the years 1948-1998 have
a good claimfor
1998 to be considered thejubilee year.
The 50 years have been characterisedby constant progress, with developments of standard beams, in analysis, in the development of standard
bridges - a process which fell by the wayside - and, in the last decade, a
reengineering round, leading tcr a new bridge beam family and integral
bridges.
In the future we will see the impact of the European Codes,
of new
the use
materials, and the building of intelligence into our bridges. The bridge beam
story demonstrates the power
of both incremental innovation and more radical change in what has been,overall, a success
story.
2R 3in
It is hoped that this paper is more than a historical review in thatit will
give the engineer involved in bridge assessment a better understanding of
the many precast bridge beam types, particularly those of the early years
Bridge over LMS Railway
when non-current beam profiles were used.
Bridge beam development1948-1973
This period saw the introduction of the first manufacturer-produced ranges
of standard beams which later were overtaken by the national standard
range. The year1973 saw the introductionof the U-beam, putting in place
the last standard design until the introduction they-beam
of
rangein 1990.
Pretensioned, precast bridge beams werecast before 1948 but that year
marked the introduction of the first manufacturers
advertisingand producing rangesof proprietary standard beams.
A number of beams were cast before World War 2 and stored by the
Ministry of War Transport. Paul’describes theconstruction of two bridges
of box beam and I-beam
construction,respectively, for theLMS and LNER
railway companies.The timeof construction was 1943.
Thomas2, incoma
prehensive paper to an ICE conference in 1940, recorded that the beams
were designed by Dr Mautner for the Prestressed Concrete Development
Company. Brief details of these two early bridges are given in Fig 1, and
further detailsmay be found in a more accessible formin the recent ICE
Proceedings volume dedicatedto historicconcrete’. It is of interest tonote
that these bridges wereof integral construction.
The year 1948can also be takenas the start of the jubilee years as, at this
time, Francis Walley designed a range of pretensioned sections for the
Costain Concrete Company.This range,the WR range, hasa section similar to themodem Y-beam and covereda depth range fromloin (250mm) to
The Structural Engineer
Volume 76/NO 21
3 November 1998
1943
Span 42ft
Length 44ft
Bridge over LNER
1943
Span 50ft
Length 52ft
Fig 1. Bridge beam sections, 1940
407
Paper: Taylor
TABLE l -Bridge beam sections
Beam
SBB
Sections
I xI
Depth
225mm-600mm
Width
Notes
17%n
1948 developedby Francis Walley for Costain Concrete Ltdfor infill & voided
decks
600mm
Circa 1948 developedby Dow Mac Concrete Ltd. Still in production. Original
design had lower flange depths of 2Xin, 3Xin & 4Xin
495mm
1951 PCDG section still in production
380mm - 8 15mm
T
Eagle
I
I I
I 1I
I 01
19Xin
1965-1970 developed by Dow Mac Concrete Ltdfor infill decks
to 30in
Early 1960s PCDG designto long span beam& slab bridges,usually
constructed within span diaphragms
410mm710mm- 1980mm
510mm- 1510mm
970mm
Early 1960s PCDG design. Many variants, 1200 wide box by Dow Mac.
Shear
connector beam by Maunsell etc
Cheshire
762mm-13711~11
850mm
Late 1960s use on a varietyof bridge jobs in the county
Lancashire
- 1300mm
Box
M
U
I 11
I I
600mm 750mmLate 1960s. Used on M62
720mm - 1360mm
970mm
800mm - 1600mm
970mm
1
Late 1960s. Acceptedas a standard bridge beam. Originally designedto accept an
in situ long flange
1973.Standardbridgebeam.
Development Group (PCDG) which was founded also in 1948 and was
subsumed into the Concrete Society when
it was founded in 1966.
The first
Chairman of the PCDG wasDr A. R. Collins and the SecretaryP. Gooding,
both of the Cement & Concrete Association. By 1958 the PCDG had 650
members andhad as the next target lOOO!
The PCDG drove the research and dissemination phase of prestressing
technology. In recognition of the difficulty of getting a take up of prestressed bridge beams in a commercial environment that required specified
goods to be obtainable froma number of suppliers by competitive tendering, the concept of national standard bridge beams was born. The first
PCDG beam, the inverted T-beam,is still in production after its introduction in 19615.The full PCDG range was completed withthe introduction of
the box and I-beams by1964"','".
It is worthwhile to cover developments in analysisat this stage, as these
of the success of the bridge beam.Rowel', in hisbook
were an integral part
in 1962, definesthe analysisproblem of bridge
a
plate or grillagehaving
as
been approached inthree ways:
(1) By dividing the bridge deckinto individual longitudinaland transverse
members and developing
slope deflection simultaneousequationsthat were
solved by hand or by relaxation method.
(2) By separating longitudinal and transverse members and representing
loading and deflection by sine series. These methods were developed by
Hendry & Jaeger as harmonic analysis to providefor general cases.
(3) Thedeck is considered to obey isotropic or orthotropicplate theory and
from this a series of load distribution factors can be derived. Guyon and
Massonnet were early pioneers, but Rowe, Morice,
Little and others developed the process to the stage of being a working tool.
Voided construction
The load distribution method was validatedby the Cement & Concrete
Association with microconcrete models of in situ and prestressed beam
bridges in the laboratory and
in tests on real bridges. Rowel' described tests
on a bridge St
atMartins, Stamford, in 1952 (FigThis
2).bridge, which takes
the oldA1 over the railway, was renovatedafter reassessment in theearly
1990s when a few
of the original 32 beams
were replaced.
By 1960, therefore, bridge deck analysis methods were in general use,
Morice & Little having first published in The Structural Engineerin 1954.
The Cement & Concrete Associationin the 1960s actedas the disseminator, with its training courses and many backup publications. At the
time of
writing of Rowe's book, Lightfoot & Sawko had just published theresults
of computer analysis of grillage equations". The method is easily comprehended, andonce thepower ofcomputers increased, the
use of load distribution theory reduced. West in 1993" published the results of a further
validation of analysis with respect to practical tests and gives simple
Fig 2. St Martins Bridge, Stamford, I952
The many referencesfrom this work are not reproduced in this paper, as it
is not part of the central theme. Rowe's book gives anexcellent reference
list and starting point for furtherreading.
This was clearly atime of development ofanalysis - indeed, the author
remembers in 1961 his
final-year degreeproject, making and testing grila
lage and attempting to solve the equationsby relaxation and by using the
computer at Manchester. The relaxation analysis was tedious and did not
come to a stable solution!
408
The Structural Engineer Volume 76/No 21
3 November 1998
Paper: Taylor
The full range
of standard beams was therefore
in place by 1973, halfway
through the 25-year period.
The longest beam of which the author has evidence of manufacture in that period was 41.5m.The 1973standard range
comprised the inverted T-box, M- and U-beams, and
it was this range that
endured, with no additions untilthe new Y-beam was introduced in 1990.
This range and some of the earlier sections mentioned in this paper are
shown in Table 1.
Finally, 1973 was marked with the founding of the Prestressed Concrete
Association (PCA), the product association of the bridge beam manufacturers.
The standard bridge beam range: 1973-present
The second halfof the jubileeyears saw development from a periodof stability of both product and
concept to one
of more rapid change that we have
experienced inthe 1990s. The extent
of development of design andof backup research, however, in
no way approaches the achievements of the 1950s
and 1960s.
I
With the range of beam types set, at first the impetuswas to encourage
Fig 3. Westway,London, 1965
more standardisation, first in design anddetailing of beams and secondly
in the design of standard bridges.
Somerville& Tiller” and Green2’ worked with users and industry
in disrecommendations for the use of grillage analysis. Finally, Hambly’s book,
seminating material that was
helpful in encouraging the
use of robustecofirst published in 1976”and now in a second edition in 1991,provides an
nomical details.The practical advice givenby Green holds goodto this day
excellent up-to-date, physical interpretation of the bridge deck analysis
and is still neglected by some engineers. This area isbeing addressed curproblem.
reminder of the benefit of
The advances inanalysis were mirrored with those in Codes, except that, rently by CIlUA, and this will provide a valuable
using standard, proven
details interms of economy
of design effort in mansuch was the quality of the early recommendations for the design of preufacture and inresultant performance.
stressed concrete, few significant changes were made. The Institution of
The largest standardisation projectin the 1970s was the
development of
Structural Engineers First Report14was the first generally available set of
standard bridges. The 1979 brochure of the standard bridges2’ describes
design rules, and it is reasonable to assume that, from 1948 onwards, the
this as a ‘not inconsiderable’project which went as far asproducing comdesign process in the IStructE report, or a process very like it, was used.
plete designs, manufacturing, construction and specification material for
CP 115 was first published in 1959, and later bridge beam design would
in situ concrete, four using precast bridge
eight bridges. Of these, three were
BS 5400, and the Department of Transport
have followedCP 116, CP 110,
beams and one with steel beams. The precast designs wereof single-span
regulations issued over the last
30 years. The IStructEFirst Reportof 1951
four spans),and M- and U-beam
required designersto consider the working load and ultimate load conditionsinverted T, multispan inverted T (two and
deck, both withtwo and four spans. The designs were carried out by teams
- a limitstate approach.
from Atkins, Gifford, Maunsell and CONSTRADO, each with a RCU part
Materials data for the early years are contained in the text of a lecture
ner. Even now, designershave to give reasons
on an AIP form not
for using
given by Walley to the PCDG in1962”.
a standard bridge.
The main changes in design thatare of interest to assessment engineers
The brochure remarks that
the bridgeelevations wereall accepted by the
are in the provision
of reinforcement for shear. The First Report and
CP 115
Royal FineArt Commission. The M-beam deckis of particular interest, as
did not require nominal link reinforcement
for shear. CP 115 suggested the
this had spaced M-beams, notcontiguous, and no special treatment of the
use of linksin thin webs, mentioning beams
of 2ftin depth and 30ftin span
edge beam; aUM edge beam with a vertical externalface was not used.
as a limit beyond which links should be provided. In practice, therefore,
Without knowing the numbers of bridges that were constructed to the
designers and manufacturers did not put nominal links
into inverted T-type
standard designs itis impossible to be precise about thesuccess of the probeams until compelled to do so by CP 110 in 1972. The change in shear
ject. In the period after their introduction,the writer noted a number
of Udesign came from the
deliberations of the ShearStudy Group16 inthe midbeam contracts that had a common detail to produce an in situ part depth
1950s. The Group, after comparing design rules with a large population
of
tests, noted thatlinks, when requiredby Codes, were put in at an excessive end diaphragm. These were probably influenced by the standard U-beam
design which incorporated such a detail.
level. At the same time,it was recommended that nominal links should
be
At some time between 1975 and 1985
seaa changetook place in the way
provided at low shear stress levels as a matter
of prudence. It is ironic that
in which bridge design was considered. From a period when concern was
this measure has causedso much trouble in assessment,
when it was instiprimarily with the economy of new build, there was achange toan appregated as a measure to improve upon past
practice, not to condemn it. The
use of grillage analysiswhich could consider torsion effects and the development of design rulesfor torsion also acted as a driverfor change,in this
TABLE 2 - The Y beam: Reasons for develoument
case forthe provision of longitudinal and vertical torsionsteel that previTo replace M-beam which hadsmall flangeswhich could
ously was notconsidered.
not accept links bent to new standards
The discussionof the period 1948-1973, the first half of 50
theyears, saw
the completion of the standard bridge beam ranges from the first PCDG
To eliminate need for link in top flange
required by
Design
beams.
torsion design
The PCDG inverted T-beam endured, but the I-beam
fell into disuse. The
To allow the topof the beam to be easilyprofiled in
box beam was next in 1963. The box beam was used extensively in the
elevation without loss of the complete top flange
1960s, and a varietyof transversely post-tensioned and in situ reinforced
I
To allow all the beams in the range to be cast from a
stitched versions were made. Notable was the Maunsell ‘top hat’ shear-consingle mould
nected box beam, used in the Westway in 1965
(Fig 3) and elsewhere.The
Manufacture
box beam was liked, particularly by the railway authorities, asit provided
To speed steel fixing and mouldset-up
an instant access deck and
is still used in boxor solid ‘log’ formto this day.
To have a shape which is optimum for good mould filling
Box beam construction is less popular with highway agenciesand, in the
TO allow greater covers than the M-beam
writer’s view, is best avoided,as theinside surfaces are uninspectable.
The M-beam was introduced in the
late 1960s”. This beam, conceivedto
To have a steep slopeat the topof the bottomflange,
giving a finishappropriate forspaced beam construction
situ infill above the bottom flange and beam
provide a voided deck withinan
or slabdeck, became a mainstayfor spans longerthan the PCDG inverted
To have a deeper bottom flange giving room for
T could reach.
The reassessment of the M-beam and the introduction
of the
continuity steel in continuoushntegral decks
Y-beam will be covered later in this paper. The U-beam, a true voided beam,
To
have a narrow bottomflange toallow beams tobe
was developed and introduced in 1973, althoughthe first beams were not
sDaced
apart, giving access for inspection
used from standard steel moulds until 1975.
The Structural Engineer
Volume 76/No 21
3 November 1998
409
Paper: Taylor
TABLE 3 - The Y beam rc
Section
Beam
Span range
7.5m - 17.5m
Y
SY
E
1
Notes
Introduction in 1994. Usingthe same750mm pallet asthe.Y-beam,
allows infill and beam& slab construction at the short-span range
14m - 31m
Introduced in 1990. Designed to replace the M-beam
24m - 45m
Introduced in 1992. Usingthe same shutterpallet as theY-beam,
extends the conceptto the long-span range
I
----
'-l l
\.
c,
P
800
10%kxceedance line
from designs between
1971 - 1976 (see ref.25)
400
Fig 4. Norwich Southern Bypass, rhePrsr Y-beambridge
l
I
15
I
I
10
I
20
I
25
I
30
ciation of the importance of lifetime performance. The increase in road
Span (m)
saltingfrom 1955 onwards, and
its detrimental effecton highway structures
(a) M beams 1996 - 1997
of all materials,is well
As the highway bridge stock grew, the introduction of bridge management
systems to drive maintenance and to respond
to the introductionof heavier
vehicles resultedin a further emphasis on performance.As far as concrete
bridges are concerned the report by Wallbank of Maunsellfor the Department of Transport2' was a turning point.
This work, andthe previous inves1400
tigations carried out by DTp referenced by Wallbank, was thestart of the
move to continuity and integral construction that
is so important today.
A parallel development, thatof the Y-beam, also took place at the same
1200
time. The introductionof the Y-beam and the thinking behindit is covered
in the papers writtenfor its l a u n ~ h ~ The
~ ~ ~Y-beam
. * ~ . used a cross-sectional shape which wassimilar to those used by the first pioneers, theWR and
4
the SBB beams. The reasons why this reversal in practice took place are
E
1000
E
important, and the rationalebehind the Y-beam is briefly reestablished in
3
/
Table 2. The first Y-beam bridge at Norwich Southern Bypass
by Maunsell,
5
Q
with deck designby Rowland Structural Design, is shown in Fig 4.The Yd 800
beam was followed in 1992by the long-span SY-beam and the short-span
version for infill decks, the TY-beam.There always was a span range that
was too great
for theinverted T-beam buttoo short for the M-beam.The new
600
TY-beam solves this problemby being suitable forboth beam andslab and
infill deck construction. The current TY-beam orders have been divided
evenly into these two forms
of construction.The fully-beam family is illus400
trated in Table 3.
In the paper which introduced the
Y-beam in 199025,
the resultof a parameter study of M-beams
in two periods, 1971-76 and 1981-86, was shown
in figure form.
The information for M-beams and Y-beams in the period 1996-97 is
shown in Figs 5 and 6. There was a tendency, between the 1971 and 1981
periods, for depths and prestress to become greater at
all spans. This does
(b) Y beams 1996 - 1997
not seemto have continued in the next 15 years.
The graphs showthe 10%
Fig 5.1996-1997parameter study based on span VS depth
exceedance lines from 1981, and it can be seen that, despite the greater
h
410
The Structural Engineer
Volume 76/NO 21
3 November 1998
Paper: Taylor
/
0
0
10% exceedance line
- from designs between
1971 - 1976 (see ref.25)
I
I
I
200
400
600
I
800
1
1000
PS (tonnes)
(a) M beams 1996 - 1997
30
25
20
Fig 7. SY-beam, lateral torsional displacement test
E
v
mac
15
roboration with the resultof a tilt test (Fig9), and extensive further analyThis has
sis has produced a general solutionthetolateral buckling problem.
shown that the SY-beam, made with normal construction imperfections, has
10
an adequate factor of safety against lateral buckling
in handling and that the
transport frame provides a useful further margin
to dealwith dynamic and
10% M beam exceedance
static forces during transportation.
line 1971 - 1976
The Wallbank survey work previously referred
to demonstrated the need
for continuity in bridge decksto remove deckjoints which, when theyfail,
This requirement for conallow salt water to reach the bridge substructure.
tinuous decks at inner supports led to the logical conclusion of requiring
integral construction. Theinterest in these developments from the point
of
I
I
I
I
l
view of bridge beamsis two-fold.
200 600 400
800
1000
The Wallbank work covered a range of bridge types, including32 steel
PS (tonnes)
decks. Of the 200 bridges, 74 used prestressed concrete.There seem to be
no particular problems with precast concrete bridge beams, although in
(b) Y beams 1996 - 1997
some bridges leakage between the
outer and second beamof the deck was
noted and was probably
due to failure
of the waterproofing layer
in the deck
Fig 6. 19961997parameter study based on load VS span
at the kerb or in a service duct. The prestressed concrete bridge beam
appears tobe a survivor! Further research to look
at concrete mix and proweight of they-beam over the M-beam and
its relative inefficiency, in prac- duction issues particularto bridge beam manufacture may
be of more than
tice the resulting structuresare broadly similar.
narrow interest.
Wallbank also noted the deck
joint as a primarycause of bridge deterioThe SY-beam reintroduced an interest in stability that required new
ration. The move towards integral construction
is a sensiblefurther step.
research. I-beams inthe 1960s must havehad high top flange slenderness
for their intended span range, but
little research or design guidance appears
Integral constructionis not justa prestressedbridge beam issue. Bridges
of all construction materials benefit. Prestressed concrete brings some adde
to have been produced in that era. Some work was available
from America,
complications in design, with respect particularly
to long-term effects. These
but more assurance was sought when
the SY-beam wasfirst used. The SYbeam can havetop flange slendemesses
in the orderof 125 at the long-span
were pointed out and partly addressed
they-beam
in
launch paper^^.'^.''. The
of
end of the range. Initially, simple
a
checkof the lateral torsional stiffness
Prestressed Concrete Association (PCA) has commissioned
further work in
a SY-beam was carried out, and a stiffening transport frame was devised
this area, tothe extent that design advice
is now a ~ a i l a b l e ~ ~ ' ~ ~ ' ~ ' .
(Figs 7,8). Subsequent researchby Stratfordz8 has provided an excellent cor- There .is a considerable body of researchfrom theUSA on these issues
v)
The StructuralEngineer
Volume 76/No 21
3 November 1998
411
Paper: Taylor
Apart from the UK, the European countries have developed standard
bridge beams to some extent. Most
countrieshave some form of
I- and box
beam, with some developinglarge
a top flanged bulb
T. The notable exception is Germany wherebridgebeams used areoftenpost-tensioned.
Belgium and France have recently published design guides, with the Fren
guide being up-to-date in addressing secondary continuity moments for
precast bridge beams made
The future
Eurocodes
A suite of Eurocodes must be used to design prestressed
concrete bridges:
the basicconcreteEurocode EC2, EN 1992-1-1,
its section on precast concrete, ENV 1992-1-3, the section on concrete bridges, EN 1992-2, andthe
one on loading, EN 1991-2, will be the minimum requirement.
There will
also be a product standard
for precast bridge beams. All
of these documents
exist currently either in ENV form or as committee drafts and, in most
cases, NADs exist.
NADs (National Application Documents) tend
to be written to give design
results similar to current designs
to UK Codes and have been devised
to simplify trial design. The final form of the Eurocodes, which may not have
accompanying NAD material, is as yet unclear, andit is the author’s view
that it is better, fromthe point of view ofunderstanding the implications
for
bridge design, rather than understanding
the design approach,to study the
Codes withas little NAD intervention as possible.
Fig 8. SY-beam transportframe
60
l
Prediction for beam without frame
0
ow
New materials
Despite the excellent performance
of steel reinforced bridge beams,
it would
clearly be beneficial if non-metallic reinforcement could be used both
for
longitudinal stressing and for shear. Advantages would come from an ass
ance of chlorideresistance over the long term.
ff(
I
%
-L”
Fig 9. SY-beam: resultsof lateral torsional displacement analysis and test
L-J
Type I
4in
which are referenced by Clarkz9.The final and useful reference on integral
construction isthe report ofthe study tour of the Concrete Bridge Development Groupin thesummer of 19973z.
3ft 6in
I------
3ft 6in
Development overseas
Mag~~el’~
describes a number
of exciting early bridges constructed in France
using the Freyssinet system and in Belgium using his own, Magnel, stressing system. Construction was well advanced andofahead
practice in the UK
with respectto post-tensioned construction;Magnel acknowledges the pretensioned method and mentions the early
UK rail bridges.
Magnel designedthe first prestressed bridge
in the USA in 1949 -Walnut
Lane inPhiladelphia. This bridge had a 47m span and was made from
site
precast, post-tensioned I-beams. Unfortunately, the grouting was inadequate and the bridge was demolished some years later.
I-IV were adopted in1958-1959 (Fig 10). The
The first AASHTO girders
wide-top flanged typeV and V1 were introduced
in 1960-1961. AASHTO
girders tend to be used at greater spacings than equivalent M-or Y-beams
because US bridge loading
is much less than that in the UK and has less prestressed reinforcement. Further development in the USA has come from
rationalisations of changes tothe standard section that were generated
by a
number of State departments of transport.
In 1980 a second attempt
at deriving national standard shapes was made
by the Federal Highway Administration, and a bulb T design was finally
.-W
adopted (Fig 10). Rival versions
of the bulb Tare still developed, and were
S
compared recently”.The more recent changes were
justified on groundsof
efficiency of cross-sectional areaand minimum materialcost, not the more
holistic analysis used in the development
of the Y-beam in theUK. A further reasonfor change in the USAis to enable precast beams
to be post-ten2ft 2in
sioned onsiteto extend their spanrange beyond the transportation limit.A
BT-54
discussion of theBT-72
USbridge beam is given in the BT-63
PCDG studytour report”,
and an exampleof such abridge is illustrated.
Fig 10. AASHTO sections
C
.m
E
E
c[x{x
412
Type v
’ - 2ft 4in
-’
Type
3ft 6in
I
I
-
%‘
W
2ft 2in
typesI-Nand PCA bulb Ts
TheStructuralEngineer
Volume 76/No 21
3 November 1998
Paper: Taylor
e
High performance
polymer - Aramid
I
I
I
II
Unsuitable
for prestressing
andinefficient
for
reinforcing
I
I
Mild steel
e
High yield
steel
Stabilised and
e . deformed strand
I
I
3000
1000
2000
Ultimate strength(N/mm2)
Fig 11. Cost of reinforcing and prestressing on a pellformance basis
I
for crack control and too
low for efficient stressing. Glass and aramids have
low creep-to-failure limits and, when used for prestressing, the design
process and formof structure must take account
of the initial prestress levels that would be lower than the70-75% commonly used for steel. In the
author’s view, the reasons for using non-metallic reinforcement in bridge
beams must be particularly persuasive, much more than in abutments, deck
and parapets.
Intelligent bridges
The microelectronics revolution has resulted in of
costs
processing data from
sensors being significantly reduced. The development
of durable sensors is
continuing, and it may soon be possible to fit a transducer to a bridge or
bridge beam that will
be suffrcientlyreliable to produce data over the extended lives of real bridge structures. A more important issue is to consider w
such instrumentation should measure: thereis little point in instrumentising a bridge beam and monitoring it50
foryears or more in the hope that a
critical bridge-life-endangering event is recorded that would otherwise not
be obvious from the road below.
The use of an alternative approach may be worthwhile, similar to that use
in offshore structures which may be analysed
by spectral methods in which
the loading spectrum
is transferred into a spectrum
of response of the structure at a particular location- e.g. the sensor site. The sensor could measure
the specified responseby having intelligence that could enable it to act as
a datalogger. This spectrum could ultimately tell the bridge owner about t
loading experienced by the bridge and,if a damage rule could be devised
for the structure with respect to fatigue, for example, it may be possible to
make predictions about design life.
A number of materials have been considered, including glass, carbon
Conclusion
fibre, and high performance aramid polymers. Glass has well-known alka- This paper has traced the development
of the bridge beam from its first trili-resistance problems and even in its alkali-resistant form
is unsuitable for
als to being the major contributor to short- and medium-span bridging ove
prestressing. Carbon and aramid tendons can bebutused
would require carethe first50 years of its use.The power of incremental innovation has been
ful design to take advantage
of their very different properties from steel and demonstrated in the many improvements in reaction to success and failure
of designs and details. The basic premise that pretensioned beams are an
their cost. B u r g ~ y n has
e ~ ~given an excellent reviewof the need for anew
inexpensive and durable form
of bridging still applies. There will, no doubt,
approach to design using advanced high performance materials.The new
be future changes,but it has to be accepted that these will be from a posimaterials have lowerE values and higher strength than prestressing steel,
tion of strength.
pronounced creep rupture properties, and tend to fail abruptly without yield.
In some cases, bond can be too good, not allowing slip at a crack and not,
therefore, producing a ductile failure. Unbonded systems have been used Acknowledgements
but
The author is not trained as an historian and in describing developments
these have the disadvantages of no gain of strength from first crack to
failure and the cost and difficulty of anchorages. It has been shown by
attempted to present a brief review
of a process to which many engineers,
Lees3’ that intermittent bond be
can
a suitable form
of design for beams using
some well known and some less prominent, have all contributed. The inclu
pretensioned aramid reinforcement. This, and some additional confinement sion or omission of contributions and contributors is not intended to be
of the compression zone, can produce a load displacement curve not unlike
indicative of critical prominence.
The author would nevertheless like to acknowledge valuable discussions
that for a beam prestressed with steel with large deflections and warning
of
with Dr F. Walley, Mr M. Burke, Dr G. Somerville, Mr G. Tootle, Mr K.
failure.
Wilson, MrS. Chakrabarti, and colleagues
now retired, Mr E. Harby and Mr
Shear also requires anew approach if non-metallic links are to be used,
J. SnasdelI.
perhaps with novel reinforcement layouts.
Finally, there is the problem of cost. A bridge beam using an advanced
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