Describe (with the drawings) the stabilization methods of prefabricated concrete structures - conceptual design.

One of the biggest advantages of precast concrete technology is the speed of construction - fixing rates of up to 1000 m2 floor area per week are common.

But to achieve this it?s essential that simple and easy to handle solutions are pursued at all stages of the construction process, from design to manufacture, transportation and erection.

The main structural difference between cast in-situand precast structures lies in their structural continuity. The connections act as bridging (pont) links between the elements, forming together structural chains linking every element to the stabilising elements.

For example in the braced frames shown in Fig. 19 between them the elements and connections form a chain of horizontal forces and reactions to transmit the horizontal load to the ground. The reactions at each floor level are determined as shown in Fig. 20 such that the entire design method for the stability of a precast structure is described in these four diagrams.

Fig. 19. Braced frame. The stability in precast concrete technology requires continuous attention:
a) shear forces, b) tensile and compressive forces

Fig. 20. Reactions in shear walls due to horizontal load:

a) when the resultant H of the horizontal load passes through the shear centre (S.C.), there is only translation,
b) rotation is due to eccentric positioning of the stabilising elements (the horizontal load resultant does not pass through the shear centre), the total deformation is translation + rotation

Precast structure must be robust and designed against progressive collapse, structural failure, cracking and unacceptable deformations. The stability of the system has to be ensured at all stages during the erection as during its service life. Design process, the steps are iterative and cyclic, starting with rough conceptual lines and decisions and ending up with design of details and fine tuning (réglages) adjustments.

First part is the choice of structural concept as envisaged by the architect (functional aspects, form, mass, appearance). At this stage the important decisions are:

o position and the approximate requirements for the stabilising elements

o the need for expansion joints

o grid distances

o span directions of slabs and beams

o positions of columns and walls

o use of load bearing walls and/or facades

The positions of shear walls and cores should be according to Fig. 20a, rather than Fig. 20b, in order to avoid torsional sway (balancement), which will eventually lead to large connection forces around the walls. In this way evenly distributed stresses in the stabilising elements can be achieved resulting in :

? balanced design and repetition of connections in the stabilising elements

? equal horizontal sway

? equal angle of rotation of the structural elements such as columns, walls etc. following the horizontal deformation of the stabilising elements

? uniform detailing.

It is the choice of the right force paths (chemin) and the main structural scheme, which makes the further development of the precast concrete system and details a success or failure. The structural engineer still has the freedom to position shear walls and cores and to choose spans of floors and beams in such a way that the gravity load acting on the cores and shear walls is large enough to eliminate uplifting forces and tensile stresses due to bending (Fig. 21).

Tensile forces require more complicated and time-consuming connections, using for example reinforcing bars passing from one element to another, welding steel plates, bolted connections, posttensioning, etc. Tensile stresses will cause opening of the joints or even cracking of the concrete in stabilising elements before the reinforcing steel bars will be activated to take the resulting force. Also the second moment of area of the cracked sections will be less than in the uncracked state (Fig. 22). This leads to larger deformations of the stabilising elements and of the whole building. Compression forces on the other hand can be easily transferred from one element to another through, for instance insitu mortar joints, which are easy and cheap to make.

Fig. 21. Position of cores/shear walls in the plan of the building: a) good, b) good shear wall

Fig. 21. Position of cores/shear walls in the plan of the building: c) good,
d) satisfactory, two transversal walls have already moments due to eccentricity of the vertical load,

Fig. 21. Position of cores/shear walls in the plan of the building:

e) bad, almost no vertical load, and/or eccentric vertical load,

f) bad, almost no vertical load on the longitudinal shear wall

Fig. 22. In the case of a small vertical load on shear wall or core the concrete section will crack resulting in larger deformations or more reinforcement needed

Date: 2016-06-12; view: 51