DE4039335A1 - Rolled steel section beam - bridge piece and two flanges with concrete layer - Google Patents

Abstract

The rolled-steel section in the form of a double-T support has a bridge-piece (2) and two flanges (3). The ratio of the thickness (s) of the bridge-piece to the thickness (t) of the flanges is greater than 0.9, preferably greater than 1. At least one concrete layer (4) is fixed to the double-T support (1) and is subjected to a pressure load mainly, and the double-T support subjected mainly to tension. The concrete layer (4) on the top of the top flange (3) is joined to the double-T support (1) by means of headed-bolt plugs (5). USE/ADVANTAGE - The double-T support is safe as regards overload and fire risk without it being oversized.

Description

The present invention relates to double-T beams, preferably to Rolled profiles with a double T-shaped cross-section, such as as Composite beams and used as composite columns with chamber concrete, with two flanges and a web.

Such double-T beams have long been from steel construction and from Composite construction, in which steel and concrete are firmly connected become known. There are different profile shapes, e.g. B. Wide flange beam or medium-wide carriers, so-called European carriers, which are essentially by the ratio of flange width to flange distance, d. H. Web height, differentiate. Profiles with different flange widths are also known. Such double-T beams are mostly used as rolled profiles in the continuous casting process blocks produced. It is initially carried in a scaffold fixed calibres produced a pre-blocked profile, which is already a Has double-T profile, but the double-T shape only partially is present and flanges and webs are still relative to their width are fat.

It is understood that according to the purpose of such double-T beams the profile is made as possible in such a way that a maximum Bending rigidity and maximum bending load capacity with the lowest possible Receives material input. This is best when looking at a concrete example to explain. Is a double T-beam with its two ends on two spaced supports and if he is loaded in the space in between, so depending on the size of the load, it will bend more or less. Doing so the upper flange under pressure, the lower flange under tension. The so-called Zero line, d. H. an imaginary line along which the material of the double-T beam One is not subjected to pressure or tension symmetrical double-T beam exactly in the middle of the web. The footbridge itself is therefore only a little near the zero line and only in the immediate vicinity the flanges are loaded almost like the flanges.  

So essentially the web only has the function, the two flanges to connect with each other and to keep the desired distance, whereby only it must be ensured that the web does not bulge or deform in any way. As a result, this has resulted in such double T-beams having a ratio the web thickness to the flange thickness in the range of about 0.52 to 0.66. Man So if possible saves material in the web area in favor of the flange area a, since the compressive and tensile strength of the flange areas ultimately the Determine the bending strength and bending stiffness of the double-T beam.

These standards have also become established for composite construction. It are also solid in composite construction, in which steel components be connected with concrete, such beams with maximum bending rigidity and Bending load capacity used. In the case of double-T beams, for example So-called head bolts or dowel bolts are provided, which differ from the extend the upper flange from up and so a firm connection of the upper Secure the flange with the poured and hardened ceiling concrete. When using double-T beams as supports, the Cavities formed between the flanges with so-called chamber concrete poured out, with corresponding head pin dowels, which are then preferred are attached to the bridge, the connection between concrete and steel girder to ensure.

The aim of the composite construction in the construction and arrangement of the individual components is to ensure that the composite concrete is subjected to pressure and the steel, here specifically the double-T beam, is subjected to tension. The corresponding stress relationships for a double-T beam with ceiling concrete arranged on the upper flange are shown in FIG. 1b. In the static equilibrium, the total compressive force that occurs is equal to the total tensile force that occurs, although the stresses in the more resilient steel are significantly higher than in the concrete, since the corresponding forces in the concrete are distributed over a substantially larger cross-sectional area.

Such composite elements are in accordance with the relevant regulations calculated so that, in addition to the nominal load for which they are calculated, one  Have a safety reserve of around 70%. That means the actual burden may are up to 70% higher than the nominal load before the load-bearing composite elements yield in a more or less irreversible manner.

With such an overload, the yielding occurs in such a way that the individual components of the composite carrier begin to flow. In the case of an optimized composite girder, the concrete and steel cross-sections and qualities are matched to one another in such a way that both the steel elements and the concrete change into the so-called "fully plastic load-bearing state" with the same overload. In this state, too, the sum of the compressive forces that occur is the same as the sum of the tensile forces that occur. However, as shown in Figure 1c, the stress diagram has now been changed in such a way that the maximum statically possible tensile stress acts over the entire steel cross section, while the maximum possible compressive stress acts essentially over the entire concrete cross section. As already mentioned, this condition should occur at the earliest at 70% overload, ie 1.7 times the nominal load.

In addition, such composite components must also be used depending on the intended use meet different fire protection criteria. For this, the constructions divided into so-called fire resistance classes F30, F60, F90 and F120. Here the number of the aforementioned classes indicates the time in minutes, the one Corresponding composite girder with a thermal load of one (standardized) fire must withstand before the element becomes irreversible yields.

In order to be able to meet the relevant fire safety regulations, the Double-T beams also filled with chamber concrete. So in the event of a fire at least initially only the lower, exposed flange of the double-T beam heat loaded. The tensile strength of the steel decreases with increasing heating gradually, so that the double-T beam after a certain time the Heat of the tensile load occurring at nominal load no longer can withstand.  

It is understood that the lower flange of the double-T beam, in which by default concentrates a significant part of the mass of the carrier is of essential importance. As a result, for example for classification in fire resistance class F90 is to dimension the double-T beam much larger than this for the Compliance with the other safety regulations at normal temperature (Achieving the fully plastic moment only with 70% overload) required would. For classification in the corresponding fire resistance classes So beams used that have a larger cross section and therefore in general also have larger external dimensions, which makes the construction more expensive.

Against this background, the present invention is based on the object to create a double-T beam, on the one hand, in a material-saving manner can be produced and at the same time with composite construction both the Security requirements regarding possible overloads as well as tightened Fire protection regulations met without this being overdimensioned Double-T beam is required.

This object is achieved in that the ratio of web thickness to Flange thickness of the double T-beam is greater than 0.9. So in the event of a fire the lower flange, which may be directly exposed to the fire flames is and thus almost completely loses its tensile strength, an essential less importance, since a large part of the tensile load is now from the dock is recorded, which is much better in front of the exterior thanks to chamber concrete Heat is protected.

Surprisingly, this change in cross-section of the double-T beam with the same use of material, ie with the same overall cross-section of the double-T beam, does not have a disadvantageous effect on the load-bearing capacity of the composite beam at normal temperature. While the practice of composite construction so far was such that it was believed that maximum load-bearing capacity could only be achieved by using steel girders with maximum bending strength and bending load-bearing capacity per se and combining it with concrete in the manner described, it has been found that other beam cross-sections can in any case achieve the same effect in the composite, even if their inherent rigidity and load-bearing capacity are lower than that of a double-T beam optimized for this in cross-section. This can be explained from the fact that, according to FIG. 1c, in the state of the maximum overload, the entire cross-section of the girder is subjected to a maximum of tension, so that it does not matter at which point in the cross-section of the girder the tensile load is absorbed. In this respect, the web contributes to the tensile strength of the overall beam in accordance with its cross-section, as does the flanges. However, since a larger part of the carrier mass is concentrated in the web than in the carrier cross-sections that are usually used, and since this area is also protected from the effects of heat, the carriers according to the invention in composite construction have a higher load-bearing capacity in the event of a fire than the elements previously used conventionally. However, the total cross section of the double-T beam and therefore also the total material use need not be changed at normal temperature, since in the event of the maximum tensile load, the corresponding tensile forces are absorbed by the entire beam cross section.

The cross-sectional design according to the invention has an additional advantage, which relates to the production of such rolled sections. When producing the known double-T profiles was a problem in that compared to the Flanges of much thinner bars are correspondingly faster after the rolling process cooled and solidified faster than the flanges. According to the The temperature decrease of the web, however, extends in the longitudinal direction together, the flanges being lighter because of their higher temperature are deformable, undergo this shrinkage. As the Flanges in turn contract further, but now the in the meantime the web of further shrinkage in the longitudinal direction solidified one opposed considerable resistance. This leads to undesirable residual stresses in the flanges and in the web and thus also causes a inhomogeneous formation of the internal crystal structure of the double-T support.

Due to the larger web thickness in relation to the flange thickness, the Carrier according to the invention largely resolves this problem.  

Ratios of the web thickness to the flange thickness have proven particularly useful proven in the range of about 1.1 to 1.8. It is obvious that for the Compliance with stricter fire protection regulations the ratio of web thickness to Flange thickness is chosen larger than with lower fire protection requirements. In practice, however, an optimized value of the ratio s / t (= web thickness / flange thickness) in the range of 1.1 to 1.5 than for application purposes best suited. Very good results were obtained with a Ratio s / t of 1.3 achieved for the usual fire resistance class F90.

It is understood that the double T-beam according to the invention for any Purpose can be used, however, due to its in practice its lower inherent rigidity and bending capacity less in the pure Steel construction, but primarily in the aforementioned composite construction Should be used, since corresponding composite elements conventional composite elements with double-T beams with conventional Profile, but the same overall cross-section are produced, the same maximum Have resilience, but at the same time in higher fire resistance classes are classified. In addition, the manufacture of double-T beams simplified by the fact that no complex measures for uniform Cooling and thus avoiding tension are necessary because of the carrier with the cross-section according to the invention in the flange and web area relative cool evenly.

Further advantages, features and possible uses of the present Invention will become apparent from the following description of a preferred Embodiment and the associated figures. It shows

Fig. 1a the cross-section of a steel beam in composite construction with a top cover made of concrete,

Fig. 1b, a composite support in the Fig. 1a corresponding voltage diagram,

FIG. 1c, the corresponding voltage diagram in a fully plastic state load,

Fig. 2 is a schematic representation of a composite beam cross-section in the event of fire and

Fig. 3 shows the beam cross section according to the invention with chamber concrete filling.

Fig. 1a shows in cross section a conventional steel beam 1 , which consists of two flanges 3 , which are connected to each other via a web 2 . The ratio of the web thickness s to the flange thickness t is typically in the range from about 0.5 to 0.7.

Head bolt dowels 5 are fixedly attached to the upper flange 3 , e.g. B. welded, and extend into a concrete layer 4 , which is poured onto the top of the upper flange 3 with the aid of an appropriate formwork. After the concrete has hardened, the concrete layer 4 and the steel girder 1 form a solid composite girder, since the steel girder 1 is anchored firmly and force-transmitting in the concrete layer 4 via the headed bolt dowels 5 .

FIG. 1b shows the voltage waveform along the vertical section through the double-T-carrier 1 and the overlying concrete layer 4. A tensile stress σ⁺ is plotted on the left, which is generally marked with a plus sign, while the corresponding compressive stress σ is marked on the right.

The load represented arises when for example the two ends of the beam 1 rest on corresponding supports and is charged to the existing concrete layer 4 and the double support 1 composite beam between the supports from above. The composite girder bends slightly downwards, whereby, as can be clearly seen in FIG. 1b, the girder 1 is essentially completely under tensile stress, while the concrete layer is subjected to compressive stress. 6 denotes a (force-free) zero line, which in such constructions is always in the area of the upper flange 3 . Ideally, the zero line 6 is located exactly in the boundary layer between the concrete layer 4 and the upper flange 3 . In the example identified here, however, a thin layer of the upper flange 3 also comes under compressive stress.

The different gradient of the stress curve in the area of the steel girder 1 and the concrete layer 4 in FIG. 1b is due to the different material properties, although these differences are not shown here quantitatively but only schematically. In fact, the compressive stress in the concrete in relation to the tensile stress in the steel girder is even smaller than shown. Nevertheless, the total pressure force is the same as the total pull force, since the composite beam is not moved as a whole. This is to be reconciled with the stress diagram shown insofar as the compressive stress in the concrete acts over a much larger cross-sectional area than the tensile stress in the steel, so that the stresses integrated over the existing cross-sectional areas precisely balance the compressive and tensile forces.

Fig. 1c is also a stress diagram for the same cross section, in which the so-called "fully plastic load condition" is shown. This means that in steel the tensile stress σ⁺ has reached the value at which the material begins to flow, just as in the concrete the compressive stress σ has reached a value that cannot be increased further, since the concrete then more or less gives way plastically.

In addition, the position of the zero line 6 in FIG. 1 c has been assumed to lie exactly in the border area between the upper flange 3 and the concrete layer 4 . Also in Fig. 1c is of course only a schematic representation of the actual conditions, particularly the scale is different in Fig. 1b. The transition from FIG. 1b to FIG. 1c results, for example, by successively increasing the tension, the lower flange initially taking up the maximum possible tension value at which the material begins to flow. The tension then no longer increases in the lower flange, but the flange begins to expand more, whereupon the upper areas are also gradually covered by the increasing tension values. The same applies in reverse for the compressive stress in the concrete, so that the idealized course of the stress curve shown in FIG. 1c finally results. Since the stress is now constant over the entire cross-section, the bending stiffness and thus also the entire bending load-bearing capacity of the composite system can be easily calculated for this state. All you have to do is assume that the tensile force acts as a whole in the center of gravity of the (symmetrical) double-T beam, while the compressive force acts in the center of gravity above or in the central plane of the concrete layer. The maximum acceptable bending moment is then only the product of the tensile force and the distance a from the center of gravity P of the double-T beam 1 to the central plane of the concrete layer 4 .

As can be seen from Fig. 1c, the exact cross-sectional shape of the double-T beam is not included in these considerations, so that regardless of the exact shape and the exact cross-section of the double-T beam, the composite beam always has the same maximum bending stiffness and thus also the maximum total load capacity, as long as only the total cross-sectional area of the double-T beam 1 (and thus its entire tensile capacity) and the position of its center of gravity P are not changed.

FIG. 2 shows a double T-beam 1 again, similarly to FIG. 1a, the FIG. 2 being used to discuss the change in strength that results from heating the double T-beam 1 from below in the event of a fire. For this purpose, the carrier has been schematically divided into several zones, the lower three of which are designated I, II and III. The cavities of the double-T beam 1 between the flanges 3 are filled with so-called chamber concrete 4 '. In the event of a fire, the lower flange 3 in particular is subjected to heat, so that it can be assumed for a principle view that the strength in zone I after a certain period of exposure to heat is only about 5% of the strength when the carrier 1 is cold . For zone II it is assumed that the strength is still about 15% of the original strength, while in zone III about 90% of the tensile strength should still be preserved. The chamber concrete 4 'prevents the heating of the upper flange 3 and the upper part of the web 2nd Only the outermost edges of the upper flange 3 can be affected more or less. In the case of loading, that even under normal load, a significant portion of the tensile force acting on the support 1 is taken up by the lower flange 3 of the carrier 1, as is also apparent from Fig. 1b. This flange also already accounts for a considerable part of the total cross-sectional area of the beam 1 , so that the drastic reduction in the tensile strength in this area also considerably reduces the overall strength of the composite beam. With a roughly simplified view, one could neglect the contribution of the lower flange 3 when absorbing the tensile forces that occur and assume the full tensile capacity for the remaining beam cross section. As can then easily be seen, both the total tensile force that can be absorbed by the carrier 1 is reduced by the cross-sectional portion of the lower carrier 3 and, moreover, the center of gravity of the remaining carrier cross-section, ie the upper flange 3 and the web, is much higher, so that for the limit load capacity shown in FIG. 1c, not only has the maximum tensile force F _Z reduced by 30 to 40%, but also the distance a has been drastically shortened, which as a result leads to a considerably reduced maximum bending moment, which is a product from F _Z and a results.

The carrier 1 according to the invention is shown in cross section in FIG. 3. In the example shown, the ratio of web thickness s to flange thickness t is approximately 1.3. If one places such a girder in place of the steel girder 1 in Fig. 1a, there is no change for the corresponding representation in Fig. 1c, that is, the composite system has the same bending capacity corresponding to the maximum possible bending moment F _Z × a, with the distance a remains unchanged since the position of the center of gravity in the carrier has not changed, and F _{Z is} also large since the total cross-sectional area of the carrier 1 has not changed.

However, if one makes the same considerations for the carrier shown in FIG. 3 as in connection with the carrier shown in FIG. 2, it immediately becomes clear that a decrease in the tensile strength of the lower flange 3 has a significantly smaller impact on the overall tensile strength of the carrier 1 has. On the one hand, the lower flange 3 contributes less to the overall tensile capacity due to its smaller cross-section, and on the other hand, if the lower flange 3 is neglected, the center of gravity of the remaining support parts moves upwards less because a larger part of the mass is concentrated in the web 2 .

The maximum possible bending moment F _Z × a is therefore reduced less, since each of the two factors F _Z and a decrease to a lesser extent than is the case with conventional beam cross sections.

So although the (inherent) bending stiffness and (inherent) bending load-bearing capacity of one Carrier according to the present invention may be slightly less than that conventional double T-beam, the present invention has succeeded in just by changing the cross-sectional shape of double-T beams Load capacity of corresponding composite elements or their maximum load capacity fully preserved and at the same time a lower sensitivity to achieve typical heat effects in the event of fire. The invention Double-T beams can have the same external dimensions as typical ones conventional carriers and replace conventional carriers with the same Total cross-sectional area. They are in the same as normal composite beams Suitable and resilient as conventional carriers, but have additional benefits such as. B. suitability for higher fire resistance classes, when the cavities of the girder are filled with reinforced concrete, whereby they can also be produced without slight stress, as the cooling in the web and takes place more evenly in the flanges.

For the rest, it is clear to the person skilled in the art that the individual optimized s / t ratio can also depend on the flange width. In the event of a double-T beam with a relatively large flange width can therefore be used choose a larger s / t ratio than the narrower one compared to the web height Flanges.

Claims (9)

1. Double T-beam, preferably made of rolled steel, with a web ( 2 ) and two flanges ( 3 ), characterized in that the ratio of web thickness (s) to flange thickness (t) of the double-T beam ( 1 ) is greater than 0.9, preferably greater than 1. 2. Double T-beam according to claim 1, characterized in that the Ratio s / t is between 1.1 and 1.8.   3. Double T-beam according to claim 2, characterized in that the Ratio s / t is between 1.1 and 1.5. 4. Double T-beam according to claim 3, characterized in that the Ratio s / t is about 1.3. 5. Composite element for receiving loads, consisting of at least one double-T beam ( 1 ) with a web ( 2 ) and two flanges ( 3 ) and at least one concrete layer firmly connected to the double-T beam ( 1 ) 4 ), which in the ready-to-use state is arranged relative to the double-T beam ( 1 ) in such a way that the concrete layer ( 4 ) is predominantly subjected to pressure and the double-T beam ( 1 ) is predominantly subjected to tension, characterized by Ratio of the web thickness (s) to the flange thickness (t) of the double T-beam ( 1 ) according to one of claims 1 to 4. 6. Composite element according to claim 5, characterized in that for the connection of the double T-beam ( 1 ) with the concrete layer ( 4 ) on the top of the upper flange ( 3 ) head bolt dowels ( 5 ) are fixedly attached. 7. Composite element according to claim 5 or 6, characterized in that the cavities formed between the flanges and the web Chamber concrete are filled. 8. Composite element according to claim 7, characterized in that projecting head bolt dowels are provided on the web ( 2 ) in the chamber concrete ( 4 '). 9. Composite element according to claim 7 or 8, characterized in that the chamber concrete ( 4 ') has an additional reinforcement in the longitudinal direction of the beam ( 1 ).