Budapest University of Technology and Economics
Department of Mechanics, Materials and Structures English courses Reinforced Concrete Structures Code: BMEEPSTK601
Lecture no. 14:
COMPOSITE STRUCTURES
Reinforced Concrete 2012
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Content: Introduction, suppositions, definitions 1. RC columns with rigid steel perfils, and steel columns filled with concrete 2. Hollow concrete blocks filled with rc 3. Composite slabs with profiled steel sheeting 4. Steel beams with monolithic rc slab 5. Steel beams with partial concrete encasement 6. Rc floors with ceramic blocks 7. Tinber and concrete floors
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Introduction, suppositions, definitions Concrete-steel composite member a structural member with components of concrete and of structural or cold-formed steel, interconnected by shear connection so as to limit the longitudinal slip between concrete and steel and the separation of one component from the other. Suppositions Plane sections remain plane after deformations; idealized steel and concrete σ-ε relationships are accepted, the concrete′s tensile strength is neglected; the co-action of concrete and steel – if not otherwise supposed – is perfect Propped structure or member a structure or member where the weight of concrete elements is applied to the steel elements which are supported in the span, or is carried independently until the concrete elements are able to resist stresses. Reinforced Concrete 2012
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For composite structures, relevant stages in the sequence of construction shall be considered: -Phase 1: investigation of the propped structure. Weight of the fresh concrete is applied to the steel structure – provisory supported or not. -Phase 2: the total loading is applied to the composite structure. No load increment is allowed during hardening of the concrete!
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1. RC columns with rigid steel perfils, and steel columns filled with concrete
Design of composite columns and composite compression members with -concrete encased sections, -partially encased sections and -concrete filled rectangular and circular tubes Members of doubly symmetrical and uniform cross-section over the member length with rolled, cold-formed or welded steel sections are considered
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N pl, Rd = A a f yd + 0,85A c f cd + A s f sd
MR-NR capacity diagram of steel-concrete encased section Reinforced Concrete 2012
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Typical cross-sections of composite columns with concrete encased section and partially encased sections
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Simplified MR-NR capacity diagram and the corresponding stress-states Reinforced Concrete 2012
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For concrete filled tubes of circular cross-section, the capacity of confined concrete can be increased: t fy N pl, Rd = ηa A a f yd + A c f cd (1 + ηc ) + A s f sd d f ck For members with e = 0 the values ηa = ηao and ηc = ηco are given by the _
following expressions:
ηao = 0,25 (3 + 2 λ ) ≤ 1,0 _
_2
ηco = 4,9 – 18,5 λ + 17 λ ≥ For members in combined compression and bending with 0 < e/d ≤ 0,1, the values ηa and ηc should be determined from: ηa = ηao + (1 – ηao) (10 e/d) ηc = ηco (1 – 10 e/d) _ N pl, Rk For e/d > 0,1, ηa = 1,0 and ηc = 0 λ= N cr Ncr is the elastic critical normal force for the relevant buckling mode Reinforced Concrete 2012
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Coinstructional rules: Steel tubes filled with concrete: max(d/t) = 90 Partially encased I-sections: max ( b/tfl) = 44 Improvement of co-action by transverse steel inserts: screw bolts and other perfils Importance of fire and corrosion protection Reinforced Concrete 2012
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2. Hollow concrete blocks filled with rc Only the monolithic - plain or reinforced - concrete section can be considered as loadbearing, because of the gaps between concrete blocks.
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3. Composite slabs with profiled steel sheeting
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The shear connection can be full or partial The thickness of concrete hc above the main flat surface of the top of the ribs of the sheeting shall be not less than 40 mm.
The sagging bending resistance of a cross-section with the neutral axis above the sheeting:
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The sagging bending resistance of a cross-section with the neutral axis in the sheeting:
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the hogging bending resistance of a cross-section:
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4. Steel beams with monolithic rc slab
Headed stud shear connectors A connector may be taken as ductile if the characteristic slip capacity δuk is at least 6 mm. To prevent separation of the slab, shear connectors should be designed to resist a nominal ultimate tensile force, perpendicular to the plane of the steel flange, of at least 0,1 times the design ultimate shear resistance of the connectors.
Full shear connection: when increase in the number of shear connectors would not increase the design bending resistance of the member. Otherwise, the shear connection is partial. Reinforced Concrete 2012
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Spacing of connectors where the slab is in contact over the full length
(e.g. solid slab): 22 tf
235 / f y .
The design shear resistance of a headed stud: 0,8f y πd 2 / 4 0,29αd 2 f ck E cm PRd = min( , ) γV γV h where: α = 0,2( sc + 1) ≤ 1 d d is the diameter of the shank of the stud, 16 mm ≤ d ≤ 25 mm; fy is the specified ultimate tensile strength of the material of the stud but not greater than 500 N/mm2; fck is the characteristic cylinder compressive strength of the concrete at the age considered, of density not less than 1750 kg/m3; hsc is the overall nominal height of the stud. γ V partial safety factor, recommended value: 1,25 Reinforced Concrete 2012
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Typical cross-sections of composite beams:
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Plastic resistance moment Mpl,Rd of typical composite cross-sections
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In case of ductile shear connectors, the compression force Ncf of the concrete flange should be reduced by the factor η, the degree of shear connection
See the 2nd plastic neutral axis on the figure!
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Relation between MRd and Nc for ductile shear connectors Non-linear theory can also be used to determine the rsistance moment Reinforced Concrete 2012
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Constructional rules
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5. Steel beams with partial concrete encasement Class 1 and 2 steel sections with d/tw ≤ 124ε are allowed
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Plastic bending resistance of partially encased beams Reinforced Concrete 2012
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Use of stirrups when concrete encasement is also respected by resistance to vertical shear Reinforced Concrete 2012
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6. Rc floors with ceramic blocks Ceramic blocks are used to reduce the selfweight of the structure When concrete topping is used, its minimum thickness is 30 mm Ceramic and concrete floors were used during the 1st part of the 20th cent. with ceramic blocks type Bohn. Here the mean compression strength of concrete and ceramics was considered in the compression zone Porotherm floor: Hollow ceramic blocks are also used in modern floor constructions too, but they are not respected as part of the loadbearing section Reinforced Concrete 2012
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7.Tinber and concrete floors
Strengthening of traditional timber beam floor constructions with rc. concrete slab topping connected to timber by steel studs and Gangnail steel plates respectively. Effectiveness of the connectors are verified by tests ducumented by the producer. Polyethilene foil to protect the timber from constant humidity and by perforated plastic tube for ventillation are important parts of the constuction above. Provisory supporting of timber during construction is necessary. Reinforced Concrete 2012
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