The struts in the construction of the concrete structure
The struts are used to transmit the loads of the Concrete structure while it is setting. They are necessary during in-situ formwork, since the concrete takes at least 3 days to harden, as to remove the formwork. Although humidity and temperature influence the hardening process. The total setting time takes approximately one month and that is when the concrete fully acquires its hardness.
Classification and use of struts in concrete structures
An adjustable telescopic steel prop is a temporary support that works in compression and is normally used as a temporary vertical support in construction sites or to perform similar functions such as preventing collapse in unstable structures.
A prop consists of two tubes that can be telescopically moved one inside the other. It has an adjustment system with a pin, inserted in the holes of the inner tube and a means of fine adjustment through a threaded collar. The main parts of an adjustable telescopic steel prop are:
- Base plate: Plate that is fixed perpendicular to the axis at each end of the inner tube and the outer tube.
- Outer tube: Tube of greater diameter with one end threaded.
- Inner tube: tube of smaller diameter provided with holes for the approximate adjustment of the prop. It slides into the outer tube.
- Device for adjusting the length: Device consisting of a stud (bolt, dowel or pin), adjusting nut and holes in both tubes, exterior and interior.
- The stud is inserted through the holes in the inner tube, and marks the approximate length.
- The adjustment force has at least one handle and has a face that supports the stud to hold the pin or the quick recovery mechanism in which they have it, it is used to make fine adjustments to the height of the prop.
Telescopic props for concrete structures
Adjustable steel telescopic struts are classified according to their nominal characteristic strength and their maximum extension length; in the market there are many models based on these two variables. Their lengths vary between 1 and 6 meters and with variable resistance capacities. For higher heights, formwork type structures should be used.
Struts are used on construction sites to temporarily support the formwork and part of the reinforced concrete structure. So that it can be sustained until it acquires adequate resistance.
Their safe use must be carried out under the professional direction provided that they comply with the maximum permissible loads for the different heights at which they are to be used. Following the load diagrams and safety factors provided by the manufacturer.
On the right a manufacturer table (GAM) in which the exhaustion loads in kilograms and the height of the struts are specified. In this commercial house, the struts are differentiated between "normal" and "strong". Referring to the struts. In this commercial house, the struts are differentiated between "normal" and "strong". Referring to their differences in bearing capacity.
Critical load concrete struts
The struts, whatever their material, work in buckling, the origin of which is as follows: When studying straight pieces subjected to simultaneous compression and bending, it is seen that there is a certain value of the compressive force, called critical load, for which a large deflection may occur, even if the transverse load is very small. For a prismatic part with hinged ends, the critical load is:
Pc=(π2 x E xl) / l2
Experimentally, it is seen that when the compressive force of a slender part approaches that value, part Y begins to bend. This lateral deformation increases so rapidly with increasing compression that a load equal to the critical one is sufficient to produce ruin. complete structure.
Up to now, everything that has been said and calculated for tubular telescopic struts refers to the determination of the critical buckling loads. But in reality you have to work with security charges, obtained from Pc, dividing this by the safety factor v to be adopted. This is somewhat the responsibility of the user, who may reduce v as much as you pay attention to the correct and safe mounting of the struts.
But to use props with the lowest possible safety coefficient, not only those should be carefully placed and secured to the formwork they support, but also duly entertained, since in principle tubular telescopic props can be used countless times, provided they are given the proper ones. care, especially in the threads, greasing them periodically and painting them when the state of their surface requires it.
- Royal Decree 1627/1997, of October 24, which establishes minimum health and safety provisions in construction works.
- RD 1215/1997, of July 18, which establishes the minimum health and safety provisions for the use of work equipment by workers.
- Royal Decree 773/1997, of May 30, on minimum health and safety provisions relating to the use by workers of personal protective equipment.
- RD 1407/1992, of November 20, which regulates the conditions for the commercialization and free intra-community circulation of personal protective equipment.
- Royal Decree 2661/1998, of December 11, approving the Structural Concrete Instruction (EHE). Partially modified by RD 996/1999, of June 11.