Page 27 - ICJ Jan 2023

TECHNICAL PAPER COLLECTOR’S EDITION

It would appear that there is a great divergence
between the values of girder secondary moments
in stage construction compared with those of
the bridge built and stressed as a single unit.
The property of concrete creep, however, is such
that the secondary moments obtained in stage
construction tend to creep towards those values
which would be obtained through construction
as a unit. These will approach within (1—e —ɸ ) of
each other. For design it was impossible to know
the time between the construction of each stage
and therefore, a constant creep factor ɸ = 1.75 was Fig 5 Moment distribution
used.

In checking the mathematical behaviour at various
stages, several criteria were followed :
(i) separate stage girder + secondary + initial
cable moments

(ii) stage 6 girder + secondary + final cable
moments

(iii ) stage 6 girder + secondary + final cable +
creep correction to secondary moments
(iv) stage 6 girder + secondary + final cable
+ superimposed dead load + positive live
load + creep correction
Fig 6 Typical cable layout
(v) condition (iv) above but with negative live
load moments This application of dead load factors in ultimate
(vi) conditions (iv) and (v) above without the strength analysis tends to produce slightly larger
creep correction moments than would normally be obtained.
Inelastic redistribution of moments was not
(vii) ultimate moments were based on stage
5 girder + restraint + superimposed dead considered.
load + 0.5 (dead load patch loadings
on influence lines) + 2.5 (live + impact) LOADING
moments. The standard highway loading for major bridges
in New Brunswick is the H25-S20 wheel loading,
or the equivalent lane loading, Fig 8. Because of
the small curvature of the bridge, influence lines
were constructed for a straight member and the
moment envelope shown in Fig 7(a) was obtained.
The effect of torsion resulting from the curvature

28 The Indian Concrete Journal | November 2018