WO1993011314A1

WO1993011314A1 - Method of increasing towards an upper limit the carrying ability in constructions by optimization of the degree of utilization of reinforcement and similar strengthening elements

Info

Publication number: WO1993011314A1
Application number: PCT/SE1992/000840
Authority: WO
Inventors: Lars Svensson
Original assignee: Lars Svensson
Priority date: 1991-12-04
Filing date: 1992-12-04
Publication date: 1993-06-10
Also published as: FI915726A; FI915726A0; AU3121193A

Abstract

Method for, by optimizing of the degree of utilization of supporting elements added to a supporting structure, giving such structure an increased load carrying capacity. The method is characterized by the steps to add to such a structure, besides existing or calculated means for achieving a cooperation within the structure, one or more supporting elongated elements, e.g. beams or girders (5, 12), which before and during the mounting are loaded to deformation and connected to the structure in such position.

Description

METHOD OFINCREASINGTOWARDSAN UPPERLIMITTHE CARRYINGABILITYINCONSTRUC¬ TIONSBYOPTIMIZATIONOFTHEDEGREEOFUΗLIZAΗONOFREINFORCEMENTANDSIMILAR STRENGTHENINGELEMENTS

Technical field of the invention

This invention relates to a method for by optimization of the degree of utilization of reinforcement and the like load carrying elements such as flanges of supporting beams maximize the load carrying capacity of structures where on utilization of conventional reinforcement systems the load carrying ability would be insufficient if not the amount of reinforcement and consequently the volume of the structure was allowed to increase in an undesirable amount. As examples on structures may be mentioned decks, framing of joists, entry and exit ramps, drive bridges preferably in new production and preferably existing bridges, quays, viaducts and the like where the load carrying capacity because of changed conditions have become insufficient.

Background of the invention

Calculations of the reinforcement for say a vault or a floor structure are normally based on so called slack reinforcement, i.e. that the reinforcement is put in and connected together into a continuous system but it is left passive and is not put under tension. After the casting of the concrete, the concrete and the reinforcement will cooperate and form a monolitic body. If in some cases the demands on a floor structure are higher than usual, the reinforcement is increased, sometimes there are reasons to arrange underslung stiffenings but reinforcement is normally achieved by addition, the bigger the load, the bigger amount of reinforcement and, consequently, a more voluminous structure, greater dead weight and also increased stress acting against the foundations or the like.

Because of continuously increased demand on the carrying or supporting capacity a great number of existing bridges are in need of strengthening alternatively replacement. These bridges are normally built as conventional reinforced concrete structures.

On strengthening of existing bridges uptil now the solution has been selected including adding of new structural details which details are intended to cooperate with and thus take part in the supporting of both dead and movable weights. Alternatively, bridges are strengthened by tension cables added giving a tension force in its most stiff directions, that is longitudinally relatively to the said unit to be put under tension. Such strengthening can also include casting of a supplementary structure to a concrete bridge or adding to steel lattice work supplementing strengthening plates.

The methods already known imply high costs, are demanding concerning space and often result in a reduction of headway below the bridge and also that extra weight will bias the foundation.

One aspect of this invention is to on new production by using or utilizing the qualities of the reinforcement and/or strengthening material make it possible to use those up to an optimum in order to get less voluminous casting constructions but retaining or increasing the carrying ability and this without essentially increasing the bias on adjoining structural details. Another aspect with this invention is to make it possible to by a minimum of work effort and material make it possible to strengthen existing bridges, quays, floors and the like where present structures already have been made use of to full extent.

Summary of the invention

This invention is based on the idea of adding to a structure or a system of structures in order to strengthen or increase the supporting ability, strengthening elements without incurring true statical cooperation and in such a way that the strengthening element not directly participates in supporting load, i.e. with minimum activity.

In a preferred embodiment may be used preferably weak or slender beams or girders of high tensile strength steel which are put under tension transversally towards the object which is to be or is the basic structure, alternatively supported by stable abutments or other stationary supports . Beams or girders put under tension in such a way do not participate in supporting loads, but stay with essentially constant distribution of tensions. The risk for fatigue stress in the material is thereby eliminated.

In a preferred embodiment there are used one or more beams or structures or similar elements, which are put under tension between stationary supports and loaded system biassed thereby with equally heavy force needed to bring about the deformation in this case straighten the beams or the like from their unbiassed position to final tension position at the supporting structure.

Brief description of the drawings

The invention will explained more in detail in the following with references to the attached drawings, on which Figures 1 and 2 schematically illustrate how the method according to the invention is utilized on casting a reinforced concrete deck or floor,

Figures 3a, 3b and 4 schematically illustrate the method on strengthening a frame girder bridge shown as it is before and after a strengthening work,

Figure 5 schematically and in a side view, shows a bridge arranged with strengthenings according to "the invention,

Figure 6 is a cross section through the bridge according to Figure 5 along the line A-A in Figure 5,

Figure 7 is a cross section through the bridge according to Figure 5 along the cross girder and the line B-B,

Figures 8, 9, 10 and 11 schematically and in a broken-out cross section show the edge portions of a bridge deck and ho a pre-bent beam is clamped and mounted in place.

Steel materials available today as well as other available materials can thanks to an improved material control and more secure methods on manufacturing and the determination of tensile strength be loaded far higher than earlier known materials and up towards figures close to a point of maximum load or breaking point. The material will thus be loaded, normally under large deformation, up to a remote or far limit of inherent elasticity but on this side of the limit where yielding portending fracturing or'rupturing occurs. As statical cooperation is avoided this can be done without risk for fatigue. This applies especially to so called high tensile steel which may be permanently loaded up to deformation limit without resulting in any load variation.

By utilizing the properties of such materials it is also possible - compared with rebuilding or new building - at very modest costs to strengthen a majority of bridges and other building constructions which do not meet the norms concerning load carrying capacity. In order to reach such an utilization the strengthening elements will have to be arranged in a specific earlier not used way.

On building of structures - prestressed concrete structures disregarded - the constructional elements will be introduced and connected together in neutral position, i.e. only biassed by their own deadweight. Before the final joining up, in case of need, a stress relieving takes place in order to compensate sagging and make details fit together in a desired way.

One of the basic thoughts behind this invention is to utilize the passive action of added or subsequently added elements and allow those to excert a counterforce or back pressure brought to by bias existing or intended loaded or supporting elements and act in a direction opposite to the load loading the structure without having the added elements biassed by further loads other than marginal ones under payload or working load.

Various applications of the idea according to the invention will be illustrated in the accompanying drawings .

Figures 1 and 2 show schematically a mould device 1 or casting a concrete floor to be strenthen by using the method according to this invention. On a form work arranged in an ordinary way on ledgers carried by posts 2 there is a essentially conventional reinforcement 3. According to the invention slender high tensile steel beams 5 in advance bent are carried by appropriate end abutments 4 and the beams are intended to be loaded or tightened by means of drawing device 6 so that they take up an approximately straight position, whereon the far limit of the inherent elasticity may be approached. By means of the drawing devices anchored at a structure below or the like the beams are kept in their straight position during the mounting of completing reinforcement and the casting of the concrete into the form. Not until the concrete has reached a predetermined hardening, the drawing device will be slackended and removed as well as the form work and the posts wherein, the force necessary to keep the beams straight and pressed down instead is given by the reinforced concrete casting itself. Without real cooperation with the reinforced concrete casting the beams will bias the same with an upward force corresponding to the force necessary to deform, i.e. bend the beams down to straight position. It is also possible to adjust this biassing force so that the beams essentially support the whole weight of the concrete floor, which in turn in this case will be biassed by moving added loads only.

Another application is at existing constructions and in Figures 3a, 3b and 4 a bridge of frame girder type is designated 10. The bridge is presumed to carry a load of 20 tons, whereas the desired requirement on carrying ability according to the new norms is 35 tons. Insufficient load carrying capacity of that extent, today normally results in demolition of the existing bridge and the building of a new built one. Thanks to this invention the existing bridge may be strengthen in a simple and far less expensive way. In Figure 3b is shown from the one side a strengthening girder 12 which together with one or more such ones is intended to be mounted at the bridge 10 in order to increase its load carrying capacity.

The girder 12 like the girders 5 above is in its non-mounted position faintly curved but the surfaces 13 arranged along the underside of the bridge and against which the girder is to be mounted is straight or level . In order to make the girder 12 and further such girders fit against surfaces 13 they have to be straightened out and this is preferably done by means of drawing devices attached at the ends of the girders and arranged to pull the same upwardly. By doing this the girders will be charged with a load corresponding to the force by means of which they subsequently will bias the bridge. The girders 12 are so constructed and dimensioned that for the straightening out of the same from the curved shape a force of predeterminable value is needed and the combined forces biassing the beams are to correspond with the necessary increase of load carrying capacity. By fixing the ends 14 of the girders 12 lengthwise and laterally relatively to the ends of the bridge, abutments, foundations or the like simultaneously as the middle portion of the straightened out beam is allowed to press corresponding portion of the bridge between the ends thereof upwardly with a force of the same range as the one necessary to straighten out the curved beams the load carrying capacity of the bridge will be increased with the same value. If the force is 15 tons the total load carrying capacity will be increased to 20 + 15 = 35 tons.

Frame girder bridges of the kind illustrated in Figures 5, 6 and 7 are very common along the roads and such bridges the building of which started more than 50 years ago do not have the load carrying capacity necessary today. Bridges of the kind just mentioned can by means of the method according to this invention in a simple way be strengthened well up to the figures relevant today and this while the traffic on the bridge may be allowed to continue during the works without being disturbed to any mentionable exten .

According to the invention there are arranged along the under side of the bridge, preferably inside the existing concrete beams a couple of slender high tensile girders 12, which in unbiassed position have an upwardly bent shape but which after mounting followed by pressing down of the curved shape will be straight. The mounting is facilitated if the ends of the girders are allowed to rest against spacer bodies 15 while between the middle portion of the girders and the corresponding portion of the bridge there are inserted so called flat jacks 16, i.e. jacks with a minimum height which after the pressing down of the girders and straightening out of the same may be left in the structure filled with concrete in order to together with conventional pads or spacers 15, keep the girders in intended positions relatively to the bridge.

Figures 5-7 show the girders in mounted position, i.e. straightened out, and Figure 6 illustrates how the girders engage the under side of the bridge between the longitudinal beams while Figure 7 illustrates how recesses 17 have been arranged in existing casted lateral beams T or the beams 12. The recesses 17 may be refilled with concrete after the mounting of the girders.

From Figure 5 a further application of the method according to this invention can be seen. Basement plates P supporting a bridge are in spite of the fact that they are dimensioned for the original capacity of the bridge normally greatly over-dimensioned. In certain cases it might appear that the plates and especially the outwardly extending portions thereof can be calculated to be excerted to so big loads that there exists a risk for damages. As plates of this kind for space reasons often are unsymmetrical, it is the outer end risking to be broken upwardly.

By, as illustrated with broken lines to the left in Figure 3, inserting slender high tensile rods 19 through drilled or otherwise arranged bores or channels 18 and load these axially to above the breaking limit it is possible to achieve an active concentration of power assisting the passive power being inherent in the decking structure and prevent this one from bending upwardly. The number of rods necessary is determined by means of calculated necessary support power.

Many bridges include monolitic concrete structures with or without underslung stiffening beams resting on appropriate abutments and have either along the edges arranged stiffeners or abutments along the edges alternatively and abutments along the edges and stiffening girders further inwardly.

As can be seen from Figures 8-11 that sort of structures are easily strengthened as the high tensile girders 12, here shown as a hollow square steel, may be mounted inside the normally existing edge recess at the concrete structure and by means of bolts 20 pulled upwardly into each recess to be finally anchored there by means through-bolts 21.

In the cases no esthetical demands are made concerning the strengthening, the slender added components may be initially straight and be deformed during their mounting.

Claims

C A I M S

1. Method for by optimizing the degree of exploitation of supporting elements added to a supporting structure give such structure an increased load carrying capacity, c h a r a c t e r i z e d by the measures of adding to the structure beyond existing or calculated means for achieving cooperation within the structure one or more supporting elongated elements, e.g. beams or girders (5,12), which before or on mounting are loaded under deformation at least to the vicinity of the far limit of inherent elasticity and in such a position be anchored to the structure essentially without causing any cooperation.

2. Method according to claim 1, c h a r a c t e r i z e d in that at a structure, e.g. a floor structure, a roof or the like structure under construction, the supporting elements (5) are mounted bent in advance and that the elements before the casting of the structure are put under tension to reach essentially straight shape and by means of temporary external drawing devices (6) kept thus loaded until the cast concrete has hardened allowing the drawing devices to be deactivated.

3. Method according to claim 1, c h a r a c t e r i z e d in that at an earlier built structure the supporting elements (12) are added in a shape deviating from the shape of spaces intended to receive the same elements and form abutments therefore, in that during the mounting the elements are deformed from the said shape utilizing the structure to be stengthened as an abutment for temporarily acting forces and in that the elements in their thus deformed shape adapted to the receiving spaces are anchored to the structure.