rclect14_composite_12.pdf

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Reinforced Concrete 2012 lecture 14/1

Budapest University of Technology and Economics

Department of Mechanics, Materials and Structures

English coursesReinforced Concrete StructuresCode: BMEEPSTK601

Lecture no. 14:

COMPOSITE STRUCTURES


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Content:

Introduction, suppositions, definitions1. RC columns with rigid steel perfils, and steel columns filled with

concrete2. Hollow concrete blocks filled with rc

3. Composite slabs with profiled steel sheeting4. Steel beams with monolithic rc slab5. Steel beams with partial concrete encasement6. Rc floors with ceramic blocks

7. Tinber and concrete floors


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Introduction, suppositions, definitions

Concrete-steel composite membera structural member with components of concrete and of structural orcold-formed steel, interconnected by shear connection so as to limit the

longitudinal slip between concrete and steel and the separation of onecomponent from the other.SuppositionsPlane sections remain plane after deformations; idealized steel and

concrete σ-ε relationships are accepted, the concrete′s tensile strengthis neglected; the co-action of concrete and steel – if not otherwisesupposed – is perfectPropped structure or member

a structure or member where the weight of concrete elements is appliedto the steel elements which are supported in the span, or is carriedindependently until the concrete elements are able to resist stresses.


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For composite structures, relevant stages in the sequence of construc-

tion shall be considered:

-Phase 1: investigation of the propped structure. Weight of the freshconcrete is applied to the steel structure – provisory supported or not.

-Phase 2: the total loading is applied to the composite structure. Noload increment is allowed during hardening of the concrete!


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1. RC columns with rigid steel perfils, and steel columns filledwith concrete

Design of composite columns and composite compression memberswith

-concrete encased sections,-partially encased sections and

-concrete filled rectangular and circular tubes

Members of doubly symmetrical and uniform cross-section over themember length with rolled, cold-formed or welded steel sections are

considered


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sdscdcydaRd,pl f Af A85,0f AN ++=

M R-N R capacity diagram of steel-concrete encased section


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Typical cross-sections of composite columns with concrete encased

section and partially encased sections


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Simplified M R-N R capacity diagram and the corresponding stress-states


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For concrete filled tubes of circular cross-section, the capacity of

confined concrete can be increased:

sdsck

yccdcydaaRd,pl f A)

f

f

d

t1(f Af AN +η++η=

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

ηco = 4,9 – 18,5_

λ + 172_

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 )

For e / d > 0,1, ηa = 1,0 and ηc = 0cr

Rk ,pl_

NN=λ

N cr is the elastic critical normal force for the relevant buckling mode


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.

Coinstructional rules:

Steel tubes filled with concrete: max(d/t ) = 90

Partially encased I-sections: max ( b/t fl) = 44Improvement of co-action by transverse steel inserts: screw bolts andother perfilsImportance of fire and corrosion protection


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2. Hollow concrete blocks filled with rc

Only the monolithic - plain or reinforced - concrete section can beconsidered 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 h c above the main flat surface of the top of the ribs ofthe sheeting shall be not less than 40 mm.

The sagging bending resistance of a cross-section with the neutral axisabove 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 thecharacteristic slip capacity δuk is at least 6mm.To prevent separation of the slab, shearconnectors should be designed to resist anominal ultimate tensile force, perpendicularto the plane of the steel flange, of at least 0,1times the design ultimate shear resistanceof the connectors.

Full shear connection : when increase in the number of shear connec-tors would not increase the design bending resistance of the member.Otherwise, the shear connection is partial.


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Spacing of connectors where the slab is in contact over the full length

(e.g. solid slab): 22 t f yf / 235 .The design shear resistance of a headed stud:

)Ef d29,0

,4 / df 8,0

min(P

V

cmck 2

V

2y

Rd

γ

α

γ

π=

where: )1d

h(2,0 sc +=α ≤ 1

d is the diameter of the shank of the stud, 16 mm ≤ d ≤ 25 mm;

f y is the specified ultimate tensile strength of the material of thestud but not greater than 500 N/mm2;

f ck is the characteristic cylinder compressive strength of theconcrete at the age considered, of density not less than 1750 kg/m3;

h sc is the overall nominal height of the stud.V partial safety factor, recommended value: 1,25


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Typical cross-sections of composite beams:


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Plastic resistance moment M pl,Rd of typical composite cross-sections


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In case of ductile shear connectors, the compression force N cf of the

concrete flange should be reduced by the factor η, the degree of shearconnection

See the 2nd plastic neutral axis on the figure!


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Relation between M Rd and N c for ductile shear connectors

Non-linear theory can also be used to determine the rsistance moment


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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 / t w ≤ 124ε are allowed


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Plastic bending resistance of partially encased beams


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Use of stirrups when concrete encasement is also respected byresistance to vertical shear


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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 wereused during the 1st part of the 20th

cent. with ceramic blocks type Bohn.Here the mean compression strengthof concrete and ceramics was consi-dered in the compression zone

Porotherm floor:Hollow ceramic blocks are also usedin modern floor constructions too,but they are not respected as part of the loadbearing section


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7.Tinber and concrete floors

Strengthening of traditional timberbeam floor constructions with rc.

concrete slab topping connectedto timber by steel studs and Gang-nail steel plates respectively.Effectiveness of the connectors are

verified by tests ducumented by theproducer.Polyethilene foil to protect the timberfrom constant humidity and by perfo-

rated plastic tube for ventillation areimportant parts of the constuctionabove.Provisory supporting of timber during construction is necessary.

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