Designing with Eurocodes.
The mandatory pan-European switch from
local design codes to Eurocodes is now
very close. The availability of
software for the new design codes has
always been acknowledged as a key
factor in the implementation of the
change.
Bestech has been working for over four
years adapting its popular and unique
SAM bridge loading, analysis and
design software, and most of the
modules are now available for
Eurocodes. This case study looks at the
design of a typical simple bridge and
gives some insight into the differences
that bridge engineers will encounter.
The Bridge
A simple concrete bridge deck was
designed to the British design code BS
5400, and then to the Eurocodes using
the UK National Annexes, and the
differences evaluated, both in terms of
the resulting design, and the design
process.
The deck comprises two 20m spans with a
25° skew, made continuous over its
central support carrying a single
carriageway, and constructed with UK
standard Y3 beams at 1m centres. For
simplicity and brevity this short
review considers only the
Serviceability Limit State bending
design.
The differences summarised.
The Actions
The BS 5400 beam is designed for a live
load sagging moment of 384kNm and
hogging moment of 328kNm. The Eurocodes
beam is designed for a variable load
characteristic sagging moment of 511kNm
(383kNm frequent) and characteristic
hogging moment of 387kNm ( 289kNm
frequent).
The Design process.
There are two principal differences in
the design process for this
bridge.
Firstly, using BS 5400 it is necessary
to check the design both for
combination 1 and for combination 3
(thermal effects included) whereas to
the Eurocodes the traffic and thermal
effects are checked in a single
process. Although the process itself is
more complex, the use of software makes
it simpler overall.
The second principal difference is that
the concrete properties (strength at
transfer, creep, shrinkage etc) are all
calculated within the program from
expressions (given by the Eurocodes),
rather than estimated and specified as
data (by BS 5400). This introduces a
significant additional amount of
calculation, but again, with software
tools the only impact for the engineer
is the time to review the calculation
output. In this case there are 420
lines of calculations for the BS
design, and 550 lines for the Eurocodes
design.
Using the software tools therefore, the
design process is very similar, taking
a similar amount of time despite the
added complexity of the
calculations.
As may be expected, resulting from the
fact that the physics are independent
of the design code, the serviceability
limit state still controls the bending
design for this type of bridge deck.
However, whereas for BS 5400 the
tension limit is controlled by stress,
in the Eurocodes the tension limit is
controlled by either decompression or
crack width, and here an additional
complication is introduced by the fact
that a lower level of loading is
applied for these checks than for the
compression limit.
The Structure
Fig 1. SLS Stress Results for BS 5400
Design for Load Combination 1
Fig 2. SLS Stress Results for Eurocode
Design with Frequent combination of
actions
In both cases the traffic loading
controls the design rather than thermal
effects. For the BS 5400 design, 19
tendons are required, and for the
Eurocode design, 17 tendons are
required. Each tendon contributes
approximately 0.65MPa to the average
concrete stress in this example. The
difference in the number of tendons
arises from the increased jacking force
allowed by the Eurocodes, and from the
differences in default values for creep
and shrinkage suggested by BS 5400.
When these parameters are matched, both
designs give the same number of
tendons. This suggests, as might be
expected, that the BS 5400 default
values are conservative.
Recommendations.
The principal area where engineers new
to the Eurocodes will find most
unfamiliarity in use is in the
combinations of actions using EN 1990
and EN 1991-2. Most experienced
engineers coming to this fresh will
most likely spend a week getting to
understand it, and then making
mistakes. Even just a half day of
training can result in a far better and
more reliable understanding, and cannot
be recommended highly enough.
The second main recommendation is to
make responsible use of the embedded
knowledge and training help that is
available in software. Whilst becoming
familiar, every single item of data
should be appraised, using on-line help
which gives references to the relevant
code clauses, and similarly every line
of calculations printout (which also
makes use of the copious references to
the code clauses) should be critically
reviewed.
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