e-mosty December 2017 MSS and Formwork Travellers | Page 9

With the competition of autonomous overhead MSS
that can auto assemble and disassemble the pier
supports as well as carry or lift the rebar, the
underslung manufacturers were forced to make their
machines also capable of auto install the pier supports
and the rebar.
Nowadays there are also autonomous underslung
MSS. These machines use auxiliary travelling gantries
to carry rebar from the previous
deck to the one under construction.
They also use a travelling gantry
running along their nose that can
carry the support brackets to the
next pier.
Even though, if the front pier
supporting brackets can only be
released after pre-stressing the
previous deck, the time for moving the brackets to the
front pier and its assembly will be on the critical path
of the construction schedule.
The most efficient way to install front pier brackets off
the critical path is to use a set of brackets not
necessary for pouring the concrete of the previous
span.
Underslung MSS are very popular with some
contractors because their use is very similar to
traditional scaffolding, enabling similar methods for
the subcontractors for rebar and pouring the concrete
with truck pumps placed along the bridge side or over
the previous decks.
The most current underslung MSS are formed by a
pair of main girders each one under one side of the
bridge (left-right).
Each main girder is composed by a central load body
made of closed box girders and two noses, one front
nose and one rear nose. Noses are normally made by
trusses, as in Figure 6 and Figure 7.
Using large cranes these MSS are easy to assembly
because they are made of rigid blocks, around 12 m
long that are connected by plate cover joints unions.
Using the main girders under the formwork, and
below the bridge deck, the height of the Main Girders
is an important limitation when bridges are close to
the ground or when they cross over existing roads,
railways or other facilities due to the lack of space for
main girder plus the height of the pier supports.
Therefore, for the underslung MSS there is always a
compromising situation to define the optimal height
in order to have enough inertia (I=Sh 2 ), to take in
account the limitations due to MSS transportation and
to have space for the MSS to fit below the deck.
For these reasons underslung MSS main girders are
normally much heavier than overhead MSS that can
be higher and therefore can reach the same inertia
using larger webs and less material.
Underslung
MSS front
– first noses
span using
the MSS without
the
The Figure
use 7: of
rear and
sometimes
carries
rear nose (left) and abutment allowing whole MSS to pass
problems
for the first span such as those seen in
Figure
7
-
left, the that
can be MSS
avoided
Therefore, for
underslung
there by is designing
always a
abutments
that
allows
a
full
assembly
the rear height
nose
compromising situation to define the of optimal
behind
and the
use of
the 2 ), MSS
for the
in order the
to abutment
have enough
inertia
(I=Sh
to take
in
first
span
of
the
bridge
as
in
Figure
7
-
right.
account the limitations due to MSS transportation and
to have space for the MSS to fit below the deck.
For these reasons underslung MSS main girders are
normally much heavier than overhead MSS that can
be higher and therefore can reach the same inertia
using larger webs and less material.
The use of rear and front noses sometimes carries
problems for the first span such as those seen in
Figure 7 - left, that can be avoided by designing
abutments that allows a full assembly of the rear nose
behind the abutment and the use of the MSS for the
first span of the bridge as in Figure 7 - right.
Figure 8: Underslung MSS – pier supports for pile-piers and double T deck
4/2017