Calculation models for shoring in buildings
As we know, the construction phase of a structure is where a good share of the accidents in the sector take place and in this phase the control of the shoring is fundamental.
It is important to carry out a detailed study of the shoring for each building, especially when the structures begin to be of significant weight (solid slabs). To avoid this study, oversizing the shoring is often preferred, with no guarantee of a better construction process.
Performing the calculation allows you to optimize the shoring, making significant cost savings in terms of material, production and execution time. So it is important for any company to be able to calculate the most optimized shoring, choosing the most suitable calculation model.
The purpose of this calculation is to determine the actual load that will be supported by the props and slabs of the structure during the different construction phases of the building.
Shoring is a very important component of the formwork used to build concrete structures. The formwork is auxiliary equipment composed of a formwork skin, supported by a resistant structure of beams and props. When constructing a building with successive storeys, the shoring goes through three successive loading phases:
- Complete equipment: This is the first phase and it concerns the shoring necessary to be able to concrete a slab: the amount of props is the maximum and therefore we call this loading situation ‘complete equipment’.
- Shoring equipment: The formwork currently used is formwork that can be recovered on the third day, where after recovery there is a reduction in the number of props that remain supporting the slab (“clearing”). In this second phase, the load situation is different because the structure is partially loaded and there are fewer props, and the team must evaluate the actual load it is supporting. We call this second loading phase ‘shoring equipment’.
- Equipment at height: on an already built slab, there is a third loading phase, when the slab above it is going to receive the load of other slabs that are placed on top of it. Every time a new slab is added, loads are transferred from the new slabs to the lower ones through the shoring and the structure that we have built. What we are going to do in this third loading phase, whatever the concreting situation on top of it, is to control the worst of the load on this shoring, and by controlling it we will know that the execution of the building in vertical will have no problems. We call this third loading phase ‘equipment at height’.
The load distribution that occurs between props and slabs can have different calculation models; in fact, eight theoretical models and nine experimental models have been produced and developed over time. Let’s take a look at the most important ones.
The ’60s: the Grundy and Kabaila model
The most established and best-known theoretical calculation model dates back to 1963 and is the Grundy and Kabaila method. The main design assumption of this model is that the struts are infinitely rigid.
This hypothesis makes sense because we are talking about the year ’63 in the United States, where the props were made of wood and were nailed, acquiring great rigidity, far from the metal props used today that act as “springs”. On the other hand, the structures were thinner and less rigid than the structures built today.
Therefore, this calculation model, which is very functional and easy to apply, does not represent the real situation of the equipment and structures that are actually executed.
The ’90s: change of models
Since 1990, other models have appeared (EFM, Duan and Chen, Fang…) where the hypothesis varies, in the sense of considering the props as having finite rigidity, and on the other hand a compatibility of deformations between the slabs and props.
From these hypotheses, a formulation based on the relationship between slab rigidity and strut rigidity has been developed. Evidently this allows the calculation models to be brought closer to reality, but not yet in a sufficiently significant way.
At the same time, experimental models are being developed. In 1992 Moragues, at the Polytechnic University of Valencia, made a real measurement in two buildings in Alicante with recoverable formwork and found that the loads that actually occur have little to do with the theoretical assumed loads, and therefore what is determined in these first tests is that a more thorough study must be done to really know what is the load that is distributed throughout the structure.
2005: A new calculation model
As an evolution of the previous models and continuation of the research started by Moragues, in 2005 the University of Valencia (UPV) and its ICITECH institute started a research project, together with Encofrados Alsina, which will last several years and will go through different phases.
In the first phase, a real building is constructed experimentally, with its whole structure fully monitored, developing the doctoral thesis of Yezid A. Alvarado Vargas.
The building to be constructed is a 25 cm solid slab building, consisting of a span with two lateral cantilevers, with fully instrumented props, and with an execution process concerned with the different actual construction phases of the structure, with a recoverable formwork.
First the concreting of a first slab, the clearing, then the concreting of a second one on top, the clearing and when the third one was concreted, the shoring of the first slab was released from the ground, isolating it from the effect on the foundation, which is infinitely rigid. Finally, the execution of a fourth slab with water basins was simulated, reproducing a standard construction with 3 sets of formwork storeys.
The monitoring of the concrete was intense, and the tests were carried out in the laboratory building annexed to the construction site.
Strain gages were introduced to measure deformations, as well as probes to measure ambient temperatures and probes to measure internal concrete temperatures. In this way, the type of concrete and its characteristics were perfectly characterized.
It should be noted that each of the props was equipped with three gages arranged at 120 degrees, protected with Armaflex shells, to monitor the load exerted on the whole prop.
For data acquisition and storage, 40 data acquisition modules and more than 3,700 meters of cable were used, with 17,000 measurements taken. The measurements were continuous, and in each of the work phases of concreting, clearing, or stripping, measurements were taken every five seconds, and when there was no construction operation, measurements were taken every five minutes.
At the end of the experiment, 4,500,000 load records were obtained, making it possible to have strict control of the entire construction process, far beyond any previous experimentation.
The NPS model
In parallel to the experimental study and in accordance with it, the new NPS calculation method has been developed, which considers that the struts have finite rigidity as its main calculation hypothesis. In addition to its experimental validation, the method is validated on the basis of 3D finite element numerical models, using the Ansys program.
In order to further validate the method, a second doctoral thesis was produced in 2008, by Isabel Gasch Molina, to check whether or not the on-site measurements would correspond to the results of the model.
Three different works were measured with the three types of bidirectional slabs applicable: solid slab, lost cassette, recoverable cassette, and the results were compared with different calculation models, proving that the new calculation method is the most consistent of all, with a coincidence in the loads of 90-95% and a standard deviation of between 0.14 and 0.16.
The proof that this computational model represents a new scientific contribution is given by its publication in the Engineering Structures journal, (Vol 33, 2011- Pag. 1565 to 1575).
A new recognition and validation was obtained in 2014, with its inclusion in the EHE-08 Application Guide.
In summary, for all the above, we can conclude that the NPS method is the closest to reality, being the one with the highest level of numerical and experimental validation of all the calculation models.
Why is it important? Because with the application of this model (through the use of STC Software) companies will be able to optimize shoring equipment and execution times, with a safe execution of the structure. In this way, costs and productivity losses will be minimized during construction. Competitive factors that are essential today.