o_19jdnovcg1eh21852116govv1lgra.pdf

permissible MEch = approx. 20-70
permissible Mtruss (experience)
The principle is a “deviation” of the
bending as compressive and tensile
forces via the diagonal brace (see
below).
couplers with force transmission by
friction (slipping). As a makeshift approved
values of
scaffolding are used (e.g. Layher), but
this approach has to be agreed with
the responsible authority
e. Transverse bending of the main
chords of the truss. In this case it
should be tried to keep the distance to
the bracings of the truss short. From
the compressive force in the diagonal
pipe and the eccentricity A (see illustration
below) results local transverse
bending in the main chords of the column,
which reduces the load bearing
capacity greatly. (Caution: this transverse
bending (the resulting stress)
alone may lead to a failure of the truss)
D. Cross bracing with ropes
In this case it is important that there
are three independent wall bracings
with crossed ropes as it is done at
stage
constructions, and the roof level is designed
as a plate with bracings of crossed
ropes and possibly with pressure
tubes. Alternatively the girder grid can
be braced by bending resistant horizontal
corners.
GROUND SUPPORT
2. ‘Welded corners’ – unbraced corners
These are usually ‘decoration’-trusses
with somehow placed diagonal bracings.
If the construction is unfavorable
as shown in the illustration the absorbable
moment declines to the comparatively
small bending load bearing
capacity of the main chords.
MEch = approx. 5 - 10 Mtruss
(experience)
It has to be noted that due to the
frame effect horizontal forces at the
base point and corner moments result
from
vertical loadings at the rig. This also
applies for the following system (B).
B. Corners braced by coupled pipes
This version can be used for example if
the girder grid is hoisted with ‘sleeve
blocks’.
The resisting moment is dependent on:
a. The lever arm of the diagonal pipe
(to the corner)
b. The permissible compressive force
of the pipes (the longer the pipe the
lower the permissible force due to
buckling)
c. A symmetrical installation of the
pipes (on both sides)
d. The permissible connection forces
for example of a swivel coupler. The
problem in this regard is that there is
no official approval in Germany for
Furthermore of importance is that the
sleeveblock is interlocked, for example
with a socket at the column that is interlocked
with a pipe or hanging in a
chain attached above with couplers
and pipes.
C. Bending resistant connection to the
basements and/or the extension arms
The stability of the column fixed at the
base is dependent on the ratio tilting
moment - stability moment. Figuratively
speaking this means, that the H-forces
want to tip the column over the basis.
That means the column needs a basement
with sufficient width and ballast
and/or additional extension arms.
Decisive for the stability of the column
are therefore usually the dimensions of
the basement (L), how solid the basement
is built and the self weight of the
total construction including the ballast.
Only for very solid basements the
weak point is the tower itself.
E. Example for mixed forms
Outdoor rigg with pipe bracings in the
corners in transverse direction, in longitudinal
direction with two columns
with
bending resistant connection to the
basements (in front B) and crossed
rope bracings (in the back A).
Local application of loads to
trusses
The local application of loads to trusses
is often underestimated. In addition
to the tensile and compression forces
in the main chords there are local bending
moments resulting from the total
loading of the truss. These bending
moments greatly reduce the load bearing
capacity of the truss.
As an example the suspension of a LED
display from a truss. This is realized
e.g. by several suspension points and
motors.
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