3. Marco teórico
3.2. Storytelling
3.2.4 Los Argumentos Universales de Balló y Pérez
3.2.4.1 Argumentos universales sobre el amo r
Where protruding reinforcement bars or loops out of precast elements interlock or overlap with other reinforcement bars, care should be taken to ensure that the bars do not interfere with each other and that there is enough space (including the necessary tolerances) to place the elements in their final position in one single operation.
This aspect should be especially looked at where precast concrete elements are connected to or integrated in cast insitu concrete structures, since it is not a common use in the cast insitu concrete technology to position every reinforcement bar in plan to exact measurement. See example in Fig. 4-3 where a cast insitu loop connection of precast parapet panel to cast insitu concrete floor is shown.
Fig. 4-3: Loop connection between precast concrete parapet and cast insitu concrete floor. Care should be taken that correct position of loops is ensured
DETAIL SECTION cast insitu floor
B B
HORIZONTAL SECTION B-B
position loops element
position loops floor
SECTION A-A
A
A
precast concrete parapet panel cast insitu loop connection
(11) Accessibility
The connection detail must be accessible when the element is in its final position, see Fig. 4-4. It is often necessary to gain access to a connection after erection; to adjust a bolt, do some welding, put in some shims, check alignment, etc. The place where this is most often forgotten is the connection detail that connects a façade panel to the outside of a column. It must always be possible to enter bolts, see Fig. 4-5.
Fig. 4-4: The connection detail must be accessible when the element is in its final position
Fig. 4-5: Allow room for placing of loose components
4.1.4 Modular co-ordination
It is important to keep the modular system in mind when the connection detail is designed. If the connection details are adjusted for the best individual solutions in all joints, this may not be beneficial for the overall system. It may create variations on elements that otherwise could have been the same, and the benefit of repetition is lost. The advantages of repetition are obvious in the drafting and production process, but it is also of importance not to loose the freedom to interchange elements at the site. A typical example is erection of coloured facade panels, where it is sometimes necessary to be able to sort the panels according to the shade of colour in order to achieve the best possible result.
It used to be rather common to make the corner solution for horizontal facade panels as shown in Fig. 4-6 a. However, this solution makes it impossible to use a corner element at any other location in the facade. The solution shown in Fig. 4-6 b can in many cases be recommended to be used instead.
This solution will necessitate one extra corner element, or some other facade material around the column, to cover it and to insulate. However, this solution will make all facade panels of the same length, and this advantage will probably more than outweigh the cost of the extra corner element.
a) b)
Fig. 4-6: Corner solution for façade panels, a) traditional solution with one special façade element, b) alternative solution with specially designed corner piece and otherwise identical elements
Another example concerns the same facade used in the previous example, in which the columns may have concrete corbels to support the panels. In order to permit the corner columns to be cast in the same mould as the other columns, it may be advantageous to have the panels in the gable supported by corbels made after the column is cast. These corbels can be made of steel, welded to an embedded
anchorage rail
difficult to enter hammerheaded maximum bolt
length of bolt
steel plate, or concrete corbel cast as an extra operation. The important thing is to achieve a corbel without having to do any extra formwork on the mould, and to have the extra corbel fixed in the factory and not on site. In this way the corner columns can be cast in the same form as the rest of the columns.
4.1.5 Tolerances
Tolerances can be defined as the maximum allowable deviation. Deviations have basically two reasons:
− Inaccuracies caused by humans, e.g. inaccurate reading of the tape measure, or a column erected slightly at an angle.
−
Physical reasons, like the tape measure’s change of length due to temperature, or a column’s change of length due to axial loads, temperature, or expansion of wood due to moisture.Deviations must not be confused with outright errors, or with a planned erection clearance. The total deviation in a prefabricated structure is the result of the following part deviations:
− product deviation,
− erection deviation,
−
deviation of the work done at the site (foundations).The reasons for the total deviation are the same for any structure where the units are produced off site, regardless of material. However, if something is not quite right, concrete is a little more difficult to alter or adjust compared to for instance steel or wood. Therefore one has to pay more attention to tolerances in prefabricated concrete structures, and to develop connection details that have the necessary room for adjustments.
A key element when designing a connection detail should always be an emphasis on making it as adjustable as possible. Preferably the connection detail should be adjustable in three directions. This may be difficult to combine with a requirement for simplicity, so the engineer has to use his judgement to decide on the priorities in each case. The easiest way to make a connection adjustable is by welding;
a welded connection is automatically adjustable in two directions, limited only by the size of the steel units. However, welding is not a desirable solution due to the reasons mentioned before: necessity of skilled labour, different levels of quality control etc.
Fig. 4-7 shows an example of how bolted connections can be made adjustable.
Fig. 4-7: Adjustable bolted connection for façades
slotted hole in embedded plate anchorage rail for
hammerheaded bolts
washer(s) between wall panel and column
4.1.6 Quality control
For precast concrete structures, connections are very important. The ability of the connections to transfer the acting forces is vital to the stability and load carrying capacity of the entire structure.
Consequently, emphasis has to be placed upon the quality control of the connection details, and at the execution of the connections at the site.
All precasters should have a quality control scheme, and for connection details this quality control should focus on the following items.