EP2484833A1

EP2484833A1 - A reinforced concrete slab for bridge floors with two bearing beams and relevant floor fabrication method.

Info

Publication number: EP2484833A1
Application number: EP12152918A
Authority: EP
Inventors: Stefano Niccolini
Original assignee: Dsd-Dezi Steel Design Srl
Current assignee: Dsd-Dezi Steel Design Srl
Priority date: 2011-02-02
Filing date: 2012-01-27
Publication date: 2012-08-08
Also published as: ITAN20110010A1; IT1404293B1

Abstract

A fabrication method of reinforced concrete slabs (100) of bridges with two bearing beams, using prefabricated predalles (110) formed of a reinforced concrete slab (110b) stiffened by electrowelded lattice girders (110a), said reinforced concrete slab (110b) having length equal to the total width of the slab and longitudinal section with variable thickness.

Description

The present patent application for industrial invention relates to a reinforced concrete slab, normally known as "predalle", used to make bridge floors with two bearing beams.
Fig. 1 shows the fabrication of a bridge according to the prior art.
The bridge comprises a plurality of vertical pillars (200) arranged along two rows extending for the entire development of the bridge. A longitudinal bearing beam (2) with overturned-H section is disposed on the pillars (200) of each row, in such manner that each beam extends in the longitudinal direction (Y) of the bridge. So, the two beams (2) are parallel and extend longitudinally for the length of the bridge.
A reinforced concrete slab (1) is made on the two beams (2), defining the width of the bridge and extending for the entire length of the bridge. As shown in the figure, the slab is made of two lateral sections that project outwards with respect to the bearing beams (2).
Today, two techniques are commonly used to make the slab (1):

A - Cast in mobile formwork laid on the longitudinal bearing beams (2).
B - Cast on concrete slabs, technically known as "predalles", with constant thickness, disposed on the beams (2) and self-bearing by means of electrowelded lattice girders during casting.

For a better understanding the description of the aforementioned two techniques continues with reference to the enclosed drawings, wherein Figs. 2 and 3 refer to the first technique A); and Figs. 4-8 refer to the second technique B).
Referring to Fig. 2, according to the first technique, a formwork (3) for concrete cast is held from above and handled by means of a special metal framework (4) laid on the beams (2) of the bridge, with the possibility to be translated forward in the direction of the length of the bridge.
The implementation of this method involves the following sequence of operating steps, which is repeated for each single section of slab (1) for the entire length of the bridge:

removal of formwork from previous cast;
forward movement of formwork (3) with framework (4);
laying of reinforcements of the slab (1) inside the formwork and concrete cast inside the formwork (3) to make the next section of the slab.

Fig. 3 is a cross-sectional view of a finished slab (1) disposed on the two longitudinal bearing beams (2).
However, said technology is impaired by the fact that it requires the use of an apparatus (consisting in said formwork (3) supported by said mobile framework (4)) that is very burdensome in terms of cost, transportation and assembly in the building site.
Precisely because of said high costs, the first aforementioned technique is not suitable to fabricate the slab of bridge with modest length.
A further disadvantage of said first technique is that the formwork must be removed within 24-36 hours from cast, in order for the work to be completed in a reasonable time. This results in high complexity in concrete management, both for the difficulties encountered in preparing a suitable concrete mix and also for the inspections that must be carried out in the building site in order to guarantee the necessary minimum resistance at any time of the year upon removal.
Finally, it must be noted that the total construction time of the work cannot be reduced as desired. In fact, although multiple formworks can be used, construction time is affected by the fact that the fabrication of the slab (1) can only start after completely laying the main beams (2) whereon said mobile framework (4) is laid.
This means that the construction of the slab (1) either entirely or partially, cannot be anticipated until the understructures (i.e. pillars (200) and main bearing beams (2)) are built, and the main bearing beams (2) are mounted.
Now referring to Figs. 4 to 8, the second technique B) is described.
Referring to Fig. 8, the second construction technique of a slab (10) provides for concrete cast (12) on the prefabricated slabs (11) commonly known as "predalles". Predalles (11) are made of reinforced concrete with constant thickness (normally 4-6 cm) and are self-bearing by means of electrowelded lattice girders (11a).
Fig. 5 shows the disposition of a predalle (11) on the beams (2) of the bridge. As shown, the predalle (11) is shaped as a rectangular, long narrow slab and is disposed on the beams (2), in such manner that the longitudinal direction of the predalle (11) extends along axis (X), that is transversally and orthogonally to the length of the bridge. Instead, the transversal direction of the predalle extends along axis (Y), that is along the longitudinal direction of the bridge. The length of the predalle (11) is equal to the width of the bridge and referred to as (L), whereas the width of the predalle (11) is referred to as (H). For illustrative purpose, the length (L) of the predalle is approximately 5-7 times its width (H). As shown in Fig. 5, the predalle (11) is made of two lateral sections projecting outwards with respect to the bearing beams (2) of the bridge.
The typical construction sequence is as follows:

prefabrication - either in building site or workshop - of the predalles (11) with concrete cast, reinforced with electrowelded lattice girders (11a);
mounting of predalles (11) on main bearing beams (2) with a crane truck (or other lifting and handling equipment);
laying of reinforcements (13) on predalles (11);
final concrete cast (12) on reinforcements (13) and predalles (11) to obtain the configuration of the slab (10).

It can be said that the total thickness of the slab (10) is equal to the sum of the thickness of the prefabricated predalles (11) and the final concrete cast (12).
The advantage of the second construction technique is that it reduces the total construction time of the work, since predalles can be made during the construction of substructures (pillars (200) and beams (2)) and the mounting of the main bearing beams (2).
So, when the main beams (2) are ready, the construction time of the slab (10) can be considerably reduced using a suitable number of equipment and workers to lay the prefabricated predalles (11), mount the reinforcements (13) and make the final concrete cast (12).
In spite of such an advantage, a drawback consists in the fact that slabs (10) can only be made with constant thickness, since the market only offers electrowelded lattice girders (11a and 13) with constant height.
In view of the above, the thickness of the slab (10) is determined by the thickness of the section with maximum stress and is maintained unchanged for the entire length.
GB 1 303 858 discloses a predalle with variable thickness cross-section; nevertheless, the longitudinal section of said predalle has constant thickness.
JP 2001 011821 relates to bridges with more than two beams and discloses a predalle with constant thickness with projections formed by thickening the ends of the predalle to provide planking with transversal inclination, laying the predalle on the bearing beams that are disposed horizontally. Moreover, the predalle does not permit to make the entire width of the slab (projections included) with a single element.
The purpose of the present invention is to devise a predalle for construction of reinforced concrete slabs for bridges with two bearing beams that is not impaired by the drawbacks of the prior art and is practical, inexpensive, reliable and simple to make and install.
Such a result is achieved by the predalle disclosed in the first claim.
The predalle of the invention provides for variable thickness along the longitudinal direction (referred to as (X) in Fig. 5), according to the designer's technical, static and architectural requirements.
The method of the invention is similar to the aforementioned second construction technique B), introducing the new idea of giving a variable thickness to the predalles in such manner that - in spite of the traditional final cast with constant thickness - the cross-section of the slab has a variable thickness.
Because of this innovative idea, the method of the invention can offer the advantages of the two constructions systems, without being impaired by the corresponding drawbacks, and more precisely:

possibility to fabricate variable thickness slabs, thus minimizing the weight of the slab, and optimizing both the quantity of concrete and the dimensions and cost of the main beams.
possibility to make a partial prefabrication of the slab, thus reducing execution time of the entire work.

The method according to the invention provides for the following sequence of operating steps:

realization in workshop, by means of formworks, of variable thickness predalles reinforced with electrowelded lattice girders;
mounting of the prefabricated predalles on the main beams of the bridge;
laying of reinforcements for final cast;
final concrete casting to reach the desired total thickness of the slab.

For explanatory reasons the description of the construction method of the invention continues with reference to attached drawings, which only have an illustrative, not limitative value, wherein:

Fig. 1 is a partially interrupted diagrammatic perspective view of a bridge fabricated according to the prior art;
Fig. 2 is a diagrammatic cross-sectional view of a mobile formwork for fabrication of slabs on bridges;
Fig. 3 is a cross-sectional view of a slab obtained with the mobile formwork of Fig. 2.
Fig. 4 is a longitudinal view of a concrete slab (predalle) according to the prior art;
Fig. 5 is a perspective diagrammatic view, showing the positioning of the predalle of Fig. 4 on the beams of a bridge;
Fig. 6 is a cross-sectional view with respect to the slab, showing a reinforcement to be disposed on the predalles of Fig. 5
Fig. 7 is a cross-sectional view with respect to the slab, showing concrete casting on the reinforcement of Fig. 6 to fabricate a slab;
Fig. 8 is a cross-sectional view of the assembled slab obtained with the predalles of Fig. 4, the reinforcement of Fig. 6 and concrete cast of Fig. 7.
Fig. 9 is a cross-sectional view of a concrete slab (predalle) according to the invention;
Fig. 9A is an enlarged view of a section of the predalle of Fig. 9.
Fig. 10 is a perspective diagrammatic view, showing the positioning of the predalle of Fig. 9 on the beams of a bridge;
Fig. 11 is a cross-sectional view with respect to the slab, showing a reinforcement to be disposed on the predalles of Fig. 10
Fig. 12 is a cross-sectional view with respect to the slab, showing concrete cast on the reinforcement of Fig. 11 to fabricate a slab;
Fig. 13 is a cross-sectional view of the assembled slab obtained with the predalles of Fig. 9, the reinforcement of Fig. 11 and concrete cast of Fig. 12.

Referring to Fig. 9, 9A and 10, a predalle (110) according to the invention is disclosed.
The predalle (110) comprises a monolithic reinforced concrete slab (110b) stiffened with electrowelded lattice girders (110a). The reinforced concrete slab (110b) has a longitudinal section (i.e. along axis X of Fig. 10) with variable thickness. In view of the above, said slab (110b) has higher thickness in sections (P) subjected to higher stress. As shown in Fig. 10, the predalle (110) is made of two lateral sections projecting outwards with respect to the bearing beams (2) of the bridge.
Considering that said predalle (110) is disposed on the two bearing beams (2) of the bridge, the sections (P) subjected to higher stress are two in number and are situated in intermediate positions of the predalle, in correspondence of the contact surface with the beams (2).
Therefore, the concrete slab (110b) comprises two intermediate sections (P) with higher thickness, a central section (C) with lower thickness between the two intermediate sections (P) with higher thickness and two end sections (E) with lower thickness at the ends of the predalle. The end sections (E) are connected to the intermediate sections (P) by means of tapered sections (R) with increasing thickness. Instead, the intermediate sections (P) are connected to the central section (C) by means of tapered sections (V) with decreasing thickness.
For illustrative purpose, the intermediate sections with higher thickness (P) have thickness approximately equal to three times the thickness of the central sections (C) with lower thickness and end sections (E).
Referring to Fig. 9A, the intermediate sections (P) with higher thickness have length (L1) approximately equal to the width of the upper surface of each longitudinal bearing beam (2) of the bridge. It must be considered that the length (L1) of the two sections with higher thickness (P) is approximately equal to 1 /10 of the length of the predalle, therefore with considerable saving on material.
To obtain a concrete slab (110b) with variable thickness, a formwork with inclined, instead of planar, bottom can be used.
Through holes (F) are provided in the intermediate sections with higher thickness (P).
Referring to Figs. 11 - 13, the fabrication of a slab (100) comprises the following sequence of operating steps:

prefabrication of predalles (110) by means of formworks;
mounting of said predalles (110) on the main beams (2) of the bridge;
laying of reinforcements (130) on said predalles (110) for final concrete cast (120),
final concrete cast (120) on said predalles (110) and said reinforcements (130) in order to achieve the desired total thickness of the slab (100).

According to a traditional technique, the reinforcement (130) is tied to the lattice girders (110a) of the predalles (110) and final concrete cast (120) is made to drawn both reinforcement (130) and electrowelded lattice girders (110a). As shown in Fig. 13, concrete cast (120) penetrates the holes (F) of the predalle (110) and adheres on the upper surface of the beam (2) in such manner to firmly anchor the slab (110) to the beams (2).
Evidently, in view of the fact that said predalles (110) are fabricated with shaped formworks, the lower surface of the predalles - and consequently the slab - can be given any shape according to the specific technical, static or architectural requirements.

Claims

A predalle (110) for fabrication of reinforced concrete slabs (100) for bridges with two bearing beams (2), comprising a monolithic prefabricated reinforced concrete slab (110b) stiffened by electrowelded lattice girders (110a), said predalle (110) having a longitudinal axis (X) and being suitable to be laid on the two bearing beams (2) extending for the entire development of the bridge, said predalle having length equal to the total width of the slab of the bridge
characterized by the fact that
said reinforced concrete slab (110b) has variable thickness along the longitudinal axis (X) of said predalle, in such manner to define sections with higher thickness (P) subjected to higher stress, sections with lower thickness (E, C) subjected to lower stress and sections with variable thickness (R, V) to connect sections with higher thickness (P) and sections with lower thickness (E,C).
A predalle (110) according to claim 1, characterized in that said sections with higher thickness (P) of the reinforced concrete slab are disposed in correspondence of the contact surface between the predalle (110) and the longitudinal bearing beams (2) of the bridge.
A predalle (110) according to claim 2, characterized in that said sections (P) with higher thickness have length (L1) approximately equal to the width of the upper surface of the longitudinal bearing beam (2) of the bridge whereon they are laid.
A predalle (110) according to claim 3, characterized in that said length (L1) of the sections (P) with higher thickness is approximately equal to 1 /10 of the total length of the predalle.
A predalle (110) according to any one of claims 2 to 4, characterized in that said reinforced concrete slab (110) comprises two sections with higher thickness (P) in intermediate positions with respect to the length of said predalle.
A predalle (110) according to claim 5, characterized in that said reinforced concrete slab (110) comprises two sections with lower thickness (E) at the two ends of the predalle and one section with lower thickness (C) in the center of the predalle between said two intermediate sections with higher thickness (P).
A predalle (110) according to claim 6, characterized in that said end sections (E) are connected with said intermediate sections (P) by means of tapered sections (R) with increasing thickness, and said intermediate sections (P) are connected with the central section (C) by means of tapered sections (V) with decreasing thickness.
A predalle (110) according to any one of the previous claims, characterized in that said sections with higher section (P) of the reinforced concrete slab have thickness approximately equal to three times the thickness of the sections with lower thickness (E, C).
A fabrication method of reinforced concrete slabs of bridges with two bearing beams (2) comprising the following sequence of operating steps:
- prefabrication by means of formworks of predalles (110) with length equal to the total width of the bridge slab, each predalle (110) being composed of a monolithic reinforced concrete slab (110b) stiffened with electrowelded lattice girders (110a);

- mounting of said predalles (110) on the main longitudinal beams (2) of the bridge;

- laying of reinforcements (130) on said predalles (110) for final concrete cast (120), and

- final concrete cast (120) on said predalles (110) and said reinforcements (130) in order to achieve the desired total thickness of the slab (100);
characterized in that fabrication of said reinforced concrete slab (110b of the predalle is made with shaped formworks in order to give a variable thickness to the section along longitudinal axis (X) of said slab (110b), in such manner to define sections with higher thickness (P) subjected to higher stress, sections with lower thickness (E, C) subjected to lower stress and sections with variable thickness (R, V) for connection between sections with higher thickness (P) and sections with lower thickness (E,C).