EP2586927A1

EP2586927A1 - Non-self-supporting steel truss for mixed steel-concrete truss systems

Info

Publication number: EP2586927A1
Application number: EP11425264.6A
Authority: EP
Inventors: Giuseppe Carlo Marano; Giovanni Bufi; Nicola Ruggiero; Floriana Petrone; Francesco Trentadue
Original assignee: MetalRi SNC
Current assignee: MetalRi SNC
Priority date: 2011-10-28
Filing date: 2011-10-28
Publication date: 2013-05-01

Abstract

Metal non-self-supporting lattice for the realization of mixed steel-concrete reticular structures comprising at least a couple of upper longitudinal reinforcements and at least a couple of lower longitudinal reinforcements, connecting rods of said upper and lower longitudinal reinforcements characterized in that said rods are configured such that they are stressed only by tractive efforts in conditions of downwards vertical load, uniformly distributed on the lattice.

Description

The present patent application relates to a steel lattice, of the non-self-supporting kind, to be used in the realization of mixed steel-concrete reticular systems. In these systems, the non-self-supporting steel lattices can be used when it is not needed that the beams are self-supporting in phase 1. With phase 1 it is intended the placement phase of the mixed steel-concrete structure in which the concrete resistance cannot be relied on because the casting has not yet been provided or in the first phases of the casting, in which it is not yet cured.
For the placement of mixed reticular structures with non-self-supporting lattices it is needed to arrange supporting shoring of the metal structures and respective formworks; to position the single core lattices as shown in fig. 1 with suitable spacers; to position the stirrups and the additional bars and in the following to carry out the concrete casting. When concrete curing is ended, the beams and the orientations have become supporting to all the loads of the working phase and the form removal can be provided.
With reference to the technological aspects, what currently distinguishes the ordinary concrete structures from the mixed reticular ones with non-self-supporting lattices is the provision of shear reinforcements which arrive in the building yard in the form of single core lattices, pre-welded in bar couples, two upper bars and two lower bars, with the function of upper and lower longitudinal reinforcement respectively, as it is shown in figure 2 and figure 3.
This feature is a considerable advantage in the placement phase as firstly it reduces the time significantly, which is a fundamental aspect from the technical point of view, and it reduces the possibilities of error in the reinforcement positioning phase. As for the structural computing, the non-self-supporting lattices are to be tested in the whole mixed steel-concrete structure, obviously referring to the continuous beam model. The models of the non-self-supporting metal lattices known at the state of the art have some drawbacks linked to the placement phase. In fact, as said, the lattices arrive as singles pieces in the building yard, where then they have to be correctly positioned on the formworks and constrained by means of bindings to the stirrups so that during the casting they maintain their original position. This implies that, even if the longitudinal reinforcements are yet pre-welded to the lattices, a series of expensive and uncertain operations continue to be needed.
From the point of the view of the structural behavior, according to the current constructive modes, the steel lattices have to resist when the concrete casting is cured and so they have to interact substantially therewith. Given the morphology of the entire structure and the natural vocation of the steel to resist to traction and of the concrete to resist to compression, the compressed steel bars can reasonably, and with suitable constructive measures, be eliminated from the lattice, thus allowing material to be spared and lighter structures to be obtained which are to be maneuvered in the placement phase.
There are also other drawbacks relating to the structural behavior of the currently used models, linked in particular to the lack of transversal holding reinforcements along the development of the beam, which are needed to allow the balance of the lower transversal thrusts, generating at the concrete connecting rod. The lack of transversal reinforcements, as for example stirrups, along the whole development of the beam, which is also prescribed by the national and international rules, exposes the structural element to the risk of flaw and of structural behaviors, which are not easily predictable in the projecting phase.
Aim of the lattice object of the present invention is therefore to provide a non-self-supporting metal lattice to be used in the realization of mixed steel-concrete reticular structures, which solves the drawbacks linked to the embodiments known at the state of the art, and allows a faster and more correct placement, a material sparing and a better predictability in the projecting phase of the structural behavior.
The detailed description of the invention will refer to the appended drawings 1 to 10.

Figures 1 and 2 show views of the non-self-supporting lattice according to an embodiment known at the state of the art,
Figures 3, 4 and 5 show views, respectively side axial and section views, of the lattice known at the state of the art,
Figures 6 and 7 show views of a preferred embodiment of the non-self-supporting lattice according to the present invention,
Figures 8, 9 and 10 show views, respectively side, axial and section views, of a preferred embodiment of the non-self-supporting lattice according to the present invention.

Fundamental feature of the metal non-self-supporting lattice is the provision of the sole rods (21) which, in a standard uniformly distributed load configuration, are stretched.
This is clear from the comparison of figures 2 and 7. In figure 2, where it is shown a lattice (1) according to an embodiment known at the state of the art, there are provided rods (11, 12), which in uniformly distributed load configuration are both stretched and compressed. In the lattice according to the present invention (2), shown in figure 6 and more detailed in figure 7, there are instead provided only rods (21), which in uniformly distributed load configuration are stretched. As it is well visible in section view of figure 8, they are diagonal rods constrained to the upper reinforcement in the closest end to the support (23) of the lattice (2) on the pillar (3), and to the lower reinforcement in the closest end (212) to the middle (24) of the lattice (2). This choice comes from the fact that in the working phase the concrete connecting rods, with suitable constructive measures, are able to face by themselves the compression stresses generating along the shear paths.
As it is clear from figure 8, the structure of the beam is symmetrical with respect to a vertical axis passing through the middle. By analyzing the load path relative to a load concentrated in the middle, it is to be noticed how the diagonal rods are positioned at the traction stressed areas, since the steel reinforcement has to react to the tractive efforts.
The lattice according to the present invention (2) comprises also closed stirrups (25) at the lower knots of the core reinforcement, such that the balance of the transversal thrusts is guaranteed, which generate at the concrete connecting rods at the soffit of the beam (2).
Such stirrups (25) can be suitably comprised in the computing of the whole shear resistance of the structural element, thus providing another resistant resource. Moreover the introduction of the stirrups (25) allows to conform to what prescribed by the national and international rules for similar structures.
Another advantage of the lattice (2) according to the present invention is that two or more lattices (2) and the respective stirrups (25) can arrive in the building yard yet welded according to the different constructive needs. In this manner, the placement operations are reduced to the simple positioning of the pre-assembled lattice and to the following casting. This allows not only to speed significantly the building yard operations, but also to reduce the possibilities of error while positioning the reinforcement before and during the casting.
What described represents a preferred embodiment of the metal lattice defined in the following claims.

Claims

Metal non-self-supporting lattice for the realization of mixed steel-concrete reticular structures comprising:
- at least a couple of upper longitudinal reinforcements (26) and at least a couple of lower longitudinal reinforcements (27),

- connecting rods (21) of said upper and lower longitudinal reinforcements
characterized in that
said rods (21) are configured such that they are stressed only by tractive efforts in conditions of downwards vertical load, uniformly distributed on the lattice (2).
Metal non-self-supporting lattice for the realization of mixed steel-concrete reticular structures according to claim 1, characterized in that said rods (21) are constrained diagonally in a vertical plane to said upper (26) and lower (27) longitudinal reinforcements, with the closest end (211) to the end portion (23) of the lattice (2) constrained to the upper longitudinal reinforcements (26), and the other end (212) constrained to the lower longitudinal reinforcements.
Metal non-self-supporting lattice according to claim 2, characterized in that the inclination of said rods (21) with respect to the vertical is between 30° and 60°.
Metal non-self-supporting lattice for the realization of mixed steel-concrete reticular structures according to any one of claims 1 to 3, further comprising closed stirrups (25) constrained to said supper and lower reinforcements.
Metal non-self-supporting lattice for the realization of mixed steel-concrete reticular structures according to claim 4, characterized in that said closed stirrups (25) are positioned along the whole length of the lattice (2) at the lower ends (212) of said rods (21).
Metal non-self-supporting lattice for the realization of mixed steel-concrete reticular structures according to claim 5, characterized in that said closed stirrups (25) are positioned at close axial distance in the end portions of the lattice.
Mixed steel-concrete reticular structure comprising a metal lattice according to any one of the preceding claims.