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Writer's pictureIng. Oscar Ramírez

STAGED CONSTRUCTION

- English version -


We get across some ideas including a practical example to illustrate the concept of staged construction analysis and its influence on a structure with a discontinuous concrete column


During construction, all structures pass through loading conditions different than those obtained when construction is completed. If the engineer wanted to faithfully represent everything that happens during the execution of a building, he would have to take into account the geometric imperfections typical of the site work, concrete with different f’c values, imperfections in the formwork and imprecisions placing the steel bars as well such as the loss of vertical alignments, differential settlements resulting from the different allowable stresses of the soil in different areas of the foundation, the stockpiling of construction materials inside the unfinished structure, movements of equipment and wind actions, or seismic events , are situations that could occur before completing the construction of a building.


However, conventional calculation procedures developed to date allows to calculate and build safe structures despite the occurrence of those factors inherent to the execution of work, when we make a calculation model we do not take into account any of these factors , the resistance of construction materials is a deterministic value for the entire structure, the beam dimensions and steel bar placement are invariable in our structural project, even so, the structures we build behave in a similar way to the hypotheses raised when we are designing.


Leaving aside these factors, there are other situations that do influence in a decisive way what we calculate when we design, and what is executed on site. Next, we will develop some ideas that illustrate these considerations and can be used to train engineering criteria in order to identify if we are facing a structure that might be susceptible to this type of analysis.

 

COLUMN DISCONTINUITY

The structures located in non-seismic zones, are designed to resist permanent loads, variable loads, wind loads, snow or special loads depending on their use and geographical location. As far as residential buildings are concerned, the structural geometry depends -in many cases- to what is defined on the architectural project, this means that internal spaces distribution at different levels can vary significantly, which affects the location of structural elements.


A typical architectural typology in Buenos Aires, is to have a ground floor height much larger than the upper levels height, as well as to have a first story for offices while the upper levels can be used as residential apartments.


Let’s suppose we have a building with this geometry where it is necessary to interrupt a central column which transfers vertical loads to a primary beam in the first level, therefore this element will have large values ​​of bending, as well as vertical deformations.


The usual constructive sequence of a building it’s typically carried out level by level, therefore, the set: columns, beams and slabs, can considered to be built at the same time for each stage. When ground floor is finished and the formwork is removed, the construction continues to the second level, and this is where the differences between the calculated structure and the built structure begin to appear.


The second level is built on a structure that already has a vertical displacement due to self-weight after removing the formwork, therefore when the second floor formwork is stripped, the self-weight load it’s transferred to the primary beam that already has an initial deformation, as well as tensions product of the previous step. The same happens for each level that is added to the structure, stresses and strains build up progressively as self-weight loads appear.

 

Deflection comparison

(for móbil, see full screen)

 

If this structure is analyzed with the conventional calculation procedure (linear static), the loads all occur simultaneously on a completely constructed structure. In a figurative sense it would be as if we built a structure with zero gravity, and at the end of the construction we suddenly activate it, producing all the gravitational effects simultaneously over the whole building.

In any structure, staged construction induces different types of tensions depending on how the building has been built, so why does the construction sequence influence certain structural configurations and not others? If we analyze what happens in this case where we have an interrupted column, we will notice that the answer to this question lies on the deformations, the cumulative stresses over a building with regular geometry, are mainly absorbed by the columns, these elements have enough axial stiffness to absorb self-weight loads regardless the staged construction used, thus the final deformation its very similar to the one obtained with a static linear load case without considering the constructive sequence.


Consequently, when we have elements that do not offer enough stiffness to absorb vertical deformations (like an interrupted column above a primary beam), we will be facing a situation where the design can be influenced by the staged construction.


What happens when we design a structure susceptible to the constructive sequence, without taking these effects into account?


Let's take as an example a concrete building with 3 spans of 3 meters in the X direction and 2 spans in the Y direction, with 4 floors with a high ground floor and a discontinuous central column as shown in the image. The slab will be 12 cm thick, beams of 30x55 cm, columns with 40x40 cm for the first two levels and 30x30 cm for the last two.


Figure 1 - Structure Geometry

The primary beam supporting the discontinuous column will have a 35x80 cm cross section as well as the same beam for the upper levels. Finally, we will apply permanent loads of 3 kN/m2 on each level.


 

Results


For this case, the results indicate that the structural analysis of the primary beam shows an increase in stresses due to permanent loads of around 30% when it’s calculated using a staged construction analysis. Similarly, deflections show a 23% increase.

TABLE 1 - Results comparison

The self-weight tensional state of a built structure, shows lower deflections because the stiffness of the entire structure is present, the discontinuous column is supported by the primary beams that intersect it and the set participates trying to prevent rotations and displacements, while in the step-by-step analysis, the only structure present is the one involved on the current analysis stage.

Table 2 - Deflection comparison

As we have seen, the design of a structure with discontinuous column is prone to the construction sequence, designing a structure without this analysis and building it without taking these effects into account, can lead to deformations in site larger than those calculated, if these are concrete elements, the cracking is something that certainly will occur, and depending on the loading sequence before the service condition, these cracks will induce additional deformations that might affect the none structural elements below the primary beam. Remember there are other factors that we are not considering, such as the cracked section deflection, as well as long-term creep.


Having said that, have you wondered:

  • · How does underpinning influence this type of analysis?

  • · How to make the discontinuous column not receive compression and work as a tension element?

  • · How to make the column hang from the ceiling beam to decrease the size of the lower level beams?

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