17-08-2012, 03:23 PM
The Use of Geodetic Methodology in Precast Concrete Building Assembling
The Use of Geodetic.pdf (Size: 87.93 KB / Downloads: 72)
INTRODUCTION
In concrete construction, and especially in precast concrete construction, discrepancies
inevitably occur between the nominal values of the building’s geometric features, given in the
construction plans, and those finally realized. Apart from the aesthetic point of view, if the
magnitude of discrepancies (otherwise called divergences) exceeds an upper limit, specified
by the precast construction rules as tolerance, deterioration of the building’s bearing ability is
possible.
In precast concrete constructions, the effect of the divergences in the construction’s behavior
is of prominent importance, since it is a combination of divergences concerning the
fabrication quality and the precast elements’ dimensions and divergences occurring during
the consecutive stages of the building’s assembling (Georgopoulos – Telioni, 2003).
MONITORING THE GEOMETRY OF A PRECAST CONSTRUCTION
The Geodetic Control Network
In order to ensure the correct positioning of the various elements throughout the precast
assembling, a geodetic control network is established in the vicinity of the construction,
before the beginning of the works.
Since the network is going to be used throughout the consecutive stages of the assembling, as
well as for the determination of the horizontal divergence or displacement vectors after the
completion of the construction, the optimum reference system is a local geodetic coordinate
system. The network’s geodetic datum is defined by the minimum constraints, i.e. one point
and one azimuth fixed, making the assumption that the coordinate system is centered to the
fixed point (Zero Order Design). The fixed point should be located to the most stable area of
the surroundings of the construction, and its stability must be checked at regular intervals.
Setting Out the Foundation Grid of the Construction
The setting out of the foundation grid consists of the field and office works, as a whole, in
order to determine accurately the position of the centers of the precast concrete foundation
elements (cones). The coordinates of the centers of the cones are determined in the office
from the construction plans, in the reference system of the network, and the corresponding
polar coordinates from the network points are calculated. The cones centers’ setting out is
carried using total stations of high precision. Experience shows that an accuracy of ±1cm (or
even better) can be achieved if special care in the instrument centering is given.
Monitoring of the Precast Elements’ Positioning in Place
The monitoring of the positioning in place concerns primarily the control of the vertical
positioning of the precast columns during the erection. Two control points are established, on the upper and lower part of the symmetry axis of at least one side of each column. Their
position is determined through angular and length observations from arbitrarily chosen
points, which need not belong to the network’s reference frame. The observations are carried
out using total stations of high precision, with the capability of length measurements without
prisms (reflector less). The corresponding polar coordinates of the control points are,
consecutively, determined in field from the instrument’s software. Thus, the column’s
vertical position is directly controlled in site through the comparison of the control points’
coordinates. It must be pointed out that it would be very useful if the marking of the control
points is done during the column’s fabrication in the factory.
Estimation of the Elements’ Final Divergences
After the building’s erection the discrepancies between the “as built” and the “as designed”
situation can be determined, through the estimation of the elements’ final divergences and the
comparison of their magnitude against the given tolerances. For this purpose, control points
are established on the construction elements. The control points are usually established on the
precast columns: a pair of points on each column’s symmetry axis of at least one side, on the
upper and lower part of it, respectively. These points, if possible, are the same with those
used during the building’s assembling (§ 2.3). Thus, it is possible to determine various kinds
of divergences, such as declinations from vertical position, dimension differences from the
construction plans.
Monitoring the Kinematic Behavior of the Construction
Displacements of the precast elements may also occur due to accidental causes (earthquake
tremors, differential subsidence of the foundations etc), with a serious effect on the bearing
ability of the construction.
The magnitude of the above mentioned displacements can be estimated through a reobservation
of the geodetic control network elements. The estimated coordinates of the
network’s control points are then compared to the former ones, and the corresponding vectors
of displacement are estimated and tested for their statistical significance. From the
displacement vectors the deformation parameters of the precast elements are also estimated.
Moreover, by estimating the displacement vectors, in successive, especially selected, time
intervals, the kinematic behavior of the construction is monitored.
The Precast Assembling Using the Geodetic Methodology
The coordinates of the centers of the 54 foundation cones were determined from the
construction plans, in the reference system of the network. By applying the law of error
propagation it was realized that the coordinates of all points had an accuracy of the range of
±1cm . The centers’ setting out was carried out using the total station TC1600, Wild. The
differences between the nominal distances of the foundation cones centers (as given from the
construction plans) and those realized in site were of the range of ±1cm
CONCLUSIONS - SUGGESTIONS
From the methodology proposed to be used throughout the precast assembling, as applied in
the above case study, as well as from the estimated final divergences, the following
conclusions-suggestions are withdrawn:
− Geodetic methodology, used through out the precast assembling, is a reliable tool for the
minimization of divergences of the precast elements. According the experience obtained
through its application in the above case study, an overall accuracy of ±1cm in the
positioning of the various precast elements can be easily achieved.