Title of Invention	COMPOSITE STRUCTURES
Abstract	The present invention relates to n asymmetric rolled cast steel beam having a central web and upper and lower flanges, the width of the lower flange being greater than the width of the upper flange, wherein the thickness of the web of the asymmetric beam is greater than the thickness of its lower flange.

Full Text

This invention relates to composite structures and more especially to composite floor and ceiling structures of concrete and steel having enhanced resistance to fire. The invention also relates to steel beams for use with such composite structures. Steel beams are used extensively in buildings to support floors, ceilings and like structures. For fire resistance, these beams require to be clad or coated with suitable fire protection materials. Composite floor and ceiling structures which comprise a profiled steel deck supported on the lower flange of steel beams and covered in situ with a concrete layer are also well known. Advantages of such structures include reductions in floor thickness and weight, ease and speed of construction and savings in labour and cranage costs during assembly. Also, encasement of the beams in concrete provides enhanced resistance to fire. A composite structure is disclosed^ in our co-pending Patent Application 9603165.3. The steel beams jjisclosed in this application are rolled cast and are of "1" section having a central web and upper and lower flange plates. In one arrangement the width of the lower flange plate of each beam is greater than the width of the upper flange plate of the respective beam to provide support for steel decking of the structure. Such a beam is known as an asymmetric steel beam (ASB). Even with conventional composite structures, some surfaces of the steel beams are exposed and therefore require to be protected by passive barriers to prevent heat from a fire impinging directly onto the steel section. Alternatively, in some cases, the lower flange plate of a steel beam is designed to be sacrificial in the event of a fire. In these cases, it is necessary to increase the bottom flange appropriately to provide the necessary resistance to fire. In other systems, the concrete surrounding the steel section is relied upon to absorb heat from a fire to provide the required level of fire resistance. The present invention sets out to provide an improved asymmetric rolled cast steel beam having enhanced fire resistance. An asymmetric composite beam is disclosed in GB-PS-1372095. This beam is, however, fabricated from a plurality of suitably bent sheets. Such a beam will not have the required physical properties of a rolled cast asymmetric beam. Fabricated beams are also inherently expensive to produce. According to the present invention in one aspect, there is provided an asymmetric rolled cast steel beam whose web is thicker than at least one of its flanges. In another aspect, there is provided a composite structure comprising a profiled steel deck supported on the lower flange of an asymmetric rolled cast steel beam covered in situ with a concrete layer, the web of the beam being thicker than its lower flange. The thickness of the web may be greater than the thickness of each flange. The thickness of the web may be between 2mm and 5mm greater than the thickness of one or each flange. For a beam having a flange thickness of between 15 and 17mm, the web thickness may be between 18 and 20mm. For a beam having a flange thickness of between 21 and 23mm, the web thickness may be between 24 and 26mm. For a beam having a flange thickness of between 23 and 25mm, the web thickness by be between 26 and 28mm. The width of the upper flange is typically between 180mm and 200mm and the width of the lower flange is typically between 290mm and 310mm. The height of the web is typically between 200mm and 350mm. The invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which:- Figure 1 is an isometric view in section of a composite structure in accordance with the invention; and Figure 2 is a section taken through an asymmetric steel beam of the composite structure illustrated in Figure 1. The composite structure shown in Figure 1 includes a plurality of asymmetric I-section cast rolled steel beams 1 (only one of which is shown) having a lower flange plate 2, a central web 3 and an upper flange plate 4. The width of the lower flange plate 2 is greater than that of the upper flange plate 4 and defines a support platform for one end of a profiled steel deck 5 and steel diaphragms 6 on which the individual deck members locate. The diaphragms 6 are secured to the flange plate 2 before the deck members are offered to the beams. Typically, the deck 5 is fixed at 600mm centres using either shot fired pins or self drilling/tapping fasteners. The diaphragms minimise concrete leakage and provide precise alignment of the deck profile. Each support beam 1 is rolled from a cast "I" section as a single piece with the lower and upper flange plates 2, 4 formed integrally with the central web section 3 of the beam. Preferably, the beams are formed from S355 steel. A pattern of grooves 7 is formed in the upper surface of the upper flange plate 2 of each beam to aid keying of the concrete layer 8 of the structure to the support beams and to produce an effective composite structure. The grooves 7 extend across the full width of the flange and define a diamond-like pattern. Typically, the depth of each groove approximates to 1mm to 2mm, the grooves being rolled into the upper surface of the upper beam flanges during production of the same. The steel deck 5 comprises a plurality of side-by-side elongate profiled deck members 9 each having a ribbed upper surface bordered by downwardly and outwardly extending ribbed side surfaces. The upper surfaces of the side surfaces define troughs for receiving concrete. As mentioned previously, to provide the required fire resistance in composite structures, it has hitherto been the practice to increase the thickness of the lower flange 2. Applicants have surprisingly established that enhanced fire protection is achieved by increasing the thickness of the beam web 3 instead of the lower flange. Thickening of the web takes less material thereby providing a lighter structure and provides improved fire resistance. An asymmetric cast rolled steel beam in accordance with the invention is shown to an enlarged scale in Figure 2. As will be seen from this Figure, the thickness of the upper and lower flanges of the beam (t, and t3) are less than the thickness of the web (t2). Thus, t, and T3 are typically 16, 22 or 24mm and t2 is typically 19, 25 or 27mm respectively. Typically the width of the upper flange is 190mm and the width of the lower flange is typically 300mm. The height of the web is typically 244 or 262mm. The ^/eight/metre of a beam having a web thickness of 19mm is typically COO, ' that of a beam having a web thickness of 25mm(1 35Jand that of a beam having a web thickness of 27mrrif 50. i\s mentioned above, the increased web thickness significantly improves the fire rating of the beam. Thickening of the web is more effective than thickening the exposed bottom flange in fire conditions. A 3mm increase in web thickness over flange thickness achieves a 60 minute fire resistance period without any need for additional fire protection material. The lower flange plate loses strength as temperatures are elevated throughout the beam in a fire situation and the web accordingly becomes critical in retaining the beams ability to continue to carry loads in the presence of fire. The thicker web is therefore specifically designed to be capable of retaining sufficient strength at specific load ratio (load ratio = ratio of cold movement capacity/applied movement in fire situation) for given fire rating periods. The moment resistance of the composite ASB section takes into account the compression transferred to the concrete slab and is calculated using conventional plastic analysis principles, according to BS 5950 Part 3. The plastic neutral axis of the composite section generally falls into the lower part of the web so that all of the slab is in compression. Therefore, equilibrium is satisfied when the compressive resistance of the concrete slab is equal to the net tensile force in the steel section. The degree of composite action that can be developed depends on shear bond action between the steel section and the concrete encasement, and the amount of reinforcement placed transverse to the beam. The shear bond action is enhanced by the rib pattern on the top flange. The factored applied moment acting on the asymmetric steel beam section is established at the construction stage. The highest moment occurs when the adjacent deck spans are fully loaded. In the construction stage, the top flange is laterally unrestrained and may buckle laterally, so that it does not reach its plastic bending resistance. The effect of local bending on the bottom flange should also be considered but, in most practical cases, its effect on major axis bending may be neglected. The moment resistance of the asymmetric steel section is calculated using conventional plastic analysis principles. The degree of asymmetry is such that the plastic neutral axis of the section lies in the lower part of the beam web. The moment resistance may be obtained by multiplying the section modulus, Sx, in Table A below by the design yield strength of the steel used in these sections (py = 345 N/mm2). Typical ASB section sizes selected to achieve required load and span characteristics are identified in Table B below. + in addition to a partition load of 1 kN/m2 The root radius between the web and flanges is 24m in the 280 ASB, and 27m in the 300 ASB sections. As will be seen from Table B, the 300 ASB section has longer spanning capabilities. This section is designed so that the slab surface may be cast with 30m top cover, or alternatively, level with the top of the section. In this second case, additional bars are passed through punched holes in the web to develop the necessary tying action in the floor slab. The choice between the two methods depends on the required depth of the slab for fire resistance. The minimum slab depth for insulation purposes in fire determines the self weight of the slab, and hence the maximum span capabilities of the steel decking. Lightweight concrete is preferred because of its better insulating properties and lower density (approximately 1900 kg/m3, or 3/4 of that of a normal weight concrete slab), which leads to thinner and longer span slabs. The minimum slab depth is also controlled by the depth of concrete over the top of the beam to permit placing of the mesh reinforcement. It is considered that 30mm is a sensible minimum, making a total slab depth of 290 to 315mm, depending on the ASB section size. This slab depth satisfies the insulation requirement for 60 minutes fire resistance for both concrete types. The ASB 300 section may be designed to be level with the top of the slab for 30/60 minutes fire resistance. Typical slab depth (excluding the bottom flange thickness) are set out in Table C. Table C (LWC = Light weight concrete; NWC = Normal weight concrete) All slab depths exclude bottom flange of section All slab depth include 30mm top cover to section except x which is flush with top of section. The steel beam is designed to act compositely with the concrete slab at the ultimate limit stage, partly due to a raised rib pattern rolled into the top flange of the section. The beam resists the bending and shear due to the factored loads applied over the entire supported area. Out of balance loads do not cause additional stresses in the composite beam because of the restraint provided by the torsional and bending stiffness of the slab at the ultimate limit state. It will be appreciated that the foregoing is merely exemplary of composite structure and asymmetric steel beams in accordance with the invention and that modifications can readily be made thereto without departing from the true scope of the invention as set out in the appended claims. WE CLAIM: 1. An asymmetric rolled cast steel beam having a central web and upper and lower flanges, the width of the lower flange being greater than the width of the upper flange, wherein the thickness of the web of the asymmetric beam is greater than the thickness of its lower flange. 2. The beam as claimed in Claim 1, wherein the thickness of the web is greater than the thickness of each flange. 3. The beam as claimed in Claim 1 or Claim 2, wherein the thickness of the web is between 2mm and 5mm greater than the thickness of one or each flange. 4. The beam as claimed in any one of the preceding Claims having a flange thickness of between 15 and 17mm, and a web thickness of between 18 and 20mm. 5. The beam as claimed in any one of Claims 1 to 3 having a flange thickness of between 21 and 23mm, and a web thickness of between 24 and 26mm. 6. The beam as claimed in any one of Claims 1 to 3 having a flange thickness of between 23 and 25mm, and a web thickness of between 26 and 28mm. 7. The beam as claimed in any one of the preceding Claims in which the width of its upper flange is between 180mm and 200mm and the width of its lower flange is between 290mm and 310mm. 8. The beam as claimed in. any one of the preceding Claims wherein the height of the web is between 200mm and 350mm. 9. A composite structure comprising a profiled steel deck and an asymmetric rolled cast steel beam having a central web and upper and lower flanges, the width of the lower flange being greater than the width of the upper flange, the profiled steel deck being supported on the lower flange of the asymmetric rolled cast steel beam and being covered in situ with a concrete layer, wherein the thickness of the web of the asymmetric beam is greater than the thickness of its lower flange.

Documents:

325-mas-1998 abstract-duplicate.pdf

325-mas-1998 abstract.pdf

325-mas-1998 claims-duplicate.pdf

325-mas-1998 claims.pdf

325-mas-1998 correspondence-others.pdf

325-mas-1998 correspondence-po.pdf

325-mas-1998 description (complete)-duplicate.pdf

325-mas-1998 description (complete).pdf

325-mas-1998 drawings-duplicate.pdf

325-mas-1998 drawings.pdf

325-mas-1998 form-1.pdf

325-mas-1998 form-19.pdf

325-mas-1998 form-2.pdf

325-mas-1998 form-26.pdf

325-mas-1998 form-4.pdf

325-mas-1998 form-6.pdf