Chapter 10 - Structural Steel

10S TRUCTURAL S TEEL10.1S COPEThis chapter covers the design of steel components,splices and connections for beam and girder structures,frames,trusses and arches,cable-stayed and suspension systems,and metal deck systems,as applicable.Curved girder structures are not included.A worked example for the design of a steel girder bridge is presented in the appendix GB.Most of the provisions for proportioning main elements are grouped by structural action:•Tension and combined tension and flexure.•Compression and combined compression and flexure.•Flexure and flexural shear:•I-sections•Box sections•Miscellaneous sectionsThis chapter also contains provisions for connections and splices,and provisions specific to particular assemblages or structural types,e.g.,through-girder spans,trusses,orthotropic deck systems,and arches.10.2S TRUCTURAL S TEELSG ENERALStructural steel shall be conform to the requirements of the specifications as summarized in Table10-1below.If European steel is used,the European standard EN10025,EN10113-2,EN10113-3and EN137-2or later,shall apply.The modulus of elasticity shall assumed as200000MPa.The thermal coefficient of expansion of all grades of structural steel shall be assumed as 11.7*10-6mm/mm/°C.Structural steel tubing shall be either cold-formed,welded or seamless tubing conforming to ASTM A500,Grade B;or the European standard EN10219-1;EN10219-2or hot-formed, welded or seamless tubing conforming to ASTM A501or the European standard EN10210-1;EN10210-2.Steel for rollers,pins and expansion rockers shall conform to the requirements in Table10-1 above.Expansion rollers shall not be less than100mm in diameter.For those parts of design not covered by these Specifications,the Ethiopian Building Code Standard EBCS–3“Design of Steel Structures”shall be used(Ref.1).Structural Steel Bridge Design Manual-2002Structural Steel High-Strength Low-Alloy SteelHigh Yield Strength,Quenched andTempered Alloy SteelAASHTO Designation M270MGrade250M270MGrade345M270MGrade345WM270MGrades690/690WEquivalent ASTM Designation A709MGrade250A709MGrade345A709MGrade345WA709MGrades690/690WThickness of Plates,mm t≤100t≤100t≤100t≤6565≤t≤100 Shapes All Groups All Groups All Groups N/A N/A Minimum Tensile Strength,F u MPa400450485760690 Minimum Yield point or MinimumYield Strength F y Mpa250345345690620 Equiv.European steel(EN10025)S275S355S355S460M/Q S460M/Q Equiv.British steel(BS4360)BS43B BS50C BS50D--Equiv.German steel(DIN17100)St44-2St52-3N St52-3U--Table10-1Minimum Mechanical Properties of Structural Steel(Shape,Strength andThickness)10.3S TEEL D ESIGN D ETAILS10.3.1G ENERALSteel structures should be cambered during fabrication to compensate for dead load deflection and to reach the specified vertical alignment.Selective changes to component length,as appropriate,shall be used for truss systems to:•adjust the dead load deflection to comply with the final geometric position,•adjust the dead load moment diagram in indeterminate structures.10.3.2M INIMUM T HICKNESS OF S TEEL:Structural steel,including bracing,cross-frames and all types of gusset plates,except for parts below,shall be not less than5mm in thickness.The web thickness of rolled beams or channels and of closed ribs in orthotropic decks shall not be less than6.0mm.Where the metal is expected to be exposed to severe corrosive influences,it shall be specially protected against corrosion,or sacrificial metal thickness shall be specified(10percent of the total thickness)Closed pipes and railings shall have a minimum thickness of4mm.Bridge decks for pedestrian bridges shall be at least8mm thick with stiffeners welded as per design,but with a minimum distance of30times the thickness of the plate and with a maximum distance of50times the thickness of the plate.Inspection platforms shall have a minimum thickness of3mm.They shall be at least0.8m wide and1.6m high with1.0m high railings on both sides.The deck shall be made of woodor steel grating.The platform shall be approachable from one of the abutments or a pier.Connections and splices for main members shall be designed at the strength limit state for not less than the larger of:•the average of the flexural moment,shear or axial force due to the factored loadings at the point of splice or connection and the factored flexural,shear or axial resistance of the member at the same point,or•75%of the factored flexural,shear or axial resistance of the member.End connections for diaphragms,cross-frames,lateral bracing or floorbeams for straight flexural members shall be designed for the factored member loads.10.3.3L IMIT S TATESThe structural behavior of components made of either steel,or steel in combination with other materials shall be investigated for each stage that shall be critical during construction, handling,transportation and erection,as well as during the service life of the structure of which they are part.Structural components shall be proportioned to satisfy the requirements at strength,extreme event,service and fatigue limit states.Service Limit State:The provisions of subchapter2:General Provisions:Deflection shall apply as applicable.Steel structures shall satisfy the load combination requirements for Service II Load Combination in Table3-2.Fatigue and Fracture Limit State:Steel bridge components and details shall be investigated for fatigue.The force effect considered for the load induced fatigue design of a steel bridge shall be the live load stress range.Residual stresses shall not be considered in investigating fatigue.The fatigue load combination,specified in Table3-2and the fatigue live load specified in subchapter3.10:Fatigue Load,shall apply.Distortion induced fatigue does not apply for small and medium concrete bridges,but shall be considered for large and/or slender special bridges.10.3.4S TRENGTH L IMIT S TATEStrength Limit State Strength and stability shall be considered using the applicable strength load combinations specified in Table3-2.Webs of plate girders shall satisfy the Strength Limit State flexural Resistance.Resistance factors[ϕ]for the strength limit state shall be as shown in Table10-2.Structural Steel Bridge Design Manual-2002 for flexureϕf=1.00for shearϕv=1.00for axial compression,steel onlyϕc=0.90for axial compression,compositeϕc=0.90for tension,fracture in net sectionϕu=0.80for tension,yielding in gross sectionϕu=0.95for bearing on pins,in reamed,drilled or bolted holes andmilled surfacesϕb=1.00for bolts bearing on materialϕbb=0.80for shear connectorsϕsc=0.85for A307,A325M and A490M bolts in tensionϕt=0.80for A307bolts in shearϕs=0.65for A325M and A490M bolts in shearϕs=0.80for block shearϕbs=0.80for weld metal in complete penetration welds:shear on effective areaϕe1=0.85•tension or compression normal to effective areaϕ=base metalϕ•tension or compression parallel to axis of the weldϕ=base metalϕfor weld metal in partial penetration welds:•shear parallel to axis of weldϕe2=0.80•tension or compression parallel to axis of weldϕ=base metalϕ•compression normal to the effective areaϕ=base metalϕ•tension normal to the effective areaϕe1=0.80for weld metal in fillet welds:•tension or compression parallel to the weldϕ=base metalϕ•shear in throat of weld metalϕe2=0.80•for resistance during pile drivingϕp=1.00Table10-2:Resistance factors[ϕ]for the strength limit stateR EFERENCES1.Ethiopian Building Code Standard EBCS–3“Design of Steel Structures.”。