Composite Steel Concrete Structures

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Composite Steel Concrete Structures

Prof. A. Plumier

Oslo, 7th November 2007



Structural Types

● Moment resisting frames

● Frames with concentric bracing

● Frames with eccentric bracings

● Composite wall structures Type 1 and 2

● Mixed systems Type 3 = Concrete walls/columns. Steel or composite beams

● Composite steel plate shear walls

Steel or composite

moment frame withconcrete infill panels.

Concrete shear walls

coupled by steel orcomposite beams.

Concrete walls

reinforced by encasedvertical steel sections.

TYPE I TYPE 2 TYPE 3



Dissipative composite structural elements ?

Steel Ductile, elongation at failure > 15 % or 150.10-3

Ductility ε y, max / ε y > 15

Concrete Limited deformation capacity ε cu2 at failure: 3,5x10-3

ε cu2 ≈ 2 ε c2 elastic range => ductility ≈ 2

εcu2 can be raised by 2 to 4 by confining transverse reinforcement

Ductil ity in composite elements => Steel yields: ε > ε yConcrete elastic ε << ε cu2

=> a condition on the position of the neutral axis: x / d < cu2/ ( cu2+ a)

x distance from top concrete compression fibre to plastic neutral axis

d depth of composite section

ε a total strain in steel at ULS x

d

s,steel

s,composite

s,composite

Limiting values of x / d for ductility of composite beams with slab

Ductility

class

q f y (N/mm2)

x / d upper limit

1,5 < q ≤ 4 355 0,27DCM

1,5 < q ≤ 4 235 0,36

q > 4 355 0,20DCHq > 4 235 0,27



Comment on ductil ity: Composite Pure Steel

Composite beam with slab

Neutral axis raised towards the upper part of the section

ε s,composite bottom flange of steel section > ε s,steel of symmetrical steel section

⇒ faster strength degradation due to buckling, reduced ductility

⇒ c /t of webs in compression are more restrictive

c /t : Eurocode 3 EN1993-1-1 : 2004, Table 5.2

c /t f flanges: unchanged

x

d

s,steel

s,composite

s,composite



A choice in the design: the degree of composite ‘character’

Two design options►1. Ductile composite elements/connections

►2. Rely on steel sections, ignore concrete in the resistance of dissipative zones

Option 2 ease analysis & execution but an effective disconnection of concrete from stee

in potential dissipative zones is required => correspondence between model and reality

Underestimating stiffness: T ↑ => smaller action effects (response spectrum)Underestimating resistance: elements sized using capacity design may be incorrect

=> Risk of creating plasticity in the wrong places.

Design concepts and behaviour factors q

Concept a): low-dissipative structural behaviour DCL => only Eurocode 4Concept b): dissipative with composite dissipative zones => Eurocodes 4 and 8

Concept c): dissipative with steel dissipative zones => Eurocodes 3, 4 & 8

Design concept

Structural

Ductility Class

Range of the reference

values of the behaviourfactor q

Concept a)

Low dissipative structural

behaviour

DCL q ≤ 1,5 (2*)

DCMq ≤ 4

+ Limits of Table 12

Concepts b) or c)

Medium or High Dissipativestructural behaviour DCH Limits of Table 12

*the National Annex can allow q = 2 in class DCL.



Upper limit reference values of behaviour factor q

Ductility ClassSTRUCTURAL TYPE

DCM DCH

Moment resisting frames

Frames with concentric or eccentric bracing

Inverted pendulum

As for steel structures.

See Table 3.

Composite structural systems

Default value:α

u/α

1 = 1,1Composite walls (Type 1 and Type 2) 3αu/α1 4αu/α1

Composite or concrete walls coupled by steel or

composite beams (Type 3)3αu/α1 4,5αu/α1

Composite steel plate shear walls

Default value: αu/α1 = 1,23αu/α1 4αu/α1

Behaviour factors q



Materials.

Concrete and reinforcement of concrete● C20/25 ≤ Concrete ≤ C40/50

● Reinforcing steel, bars and welded meshes considered to contribute to the plastic

resistance of dissipative zones => ratio f u/f y and available elongation

Class B or C in DCM EN1992-1-1:2004, Table C.1

Class C in DCH● Only ribbed bars except for closed stirrups or cross ties

● In slabs flanges of composite beams:

welded mesh that does not comply with the ductility requirements may be used

in dissipative zones provided thaty ductile reinforcing bars are present to duplicate

the mesh

Justification: a reliable negative plastic moment resistance in the connection zones

requires the presence of ductile reinforcement

Plastic resistance of dissipative zones: 2 different plastic resistances● Lower bound plastic resistance (pl, Rd) in design checks concerning the sections

of dissipative elements, calculated considering concrete and ductile steel components

● Upper bound plastic resistance (U, Rd) in the capacity design of the elements

that are adjacent to the dissipative zones, established considering the concrete

and all the steel components present in the section



Composite connections in dissipative zonesPanel Zone Resistance

If web panels of beam/column connections are ful ly encased

⇒panel zone resistance can be computed as

the sum of contributions of concrete and steel shear panel

Conditions: 0,6 < hb/hc < 1,4 V wp,Ed < 0,8 V wp,Rd

V wp,Ed design shear in web panel

(from capacity design ref plastic resistance of adjacent composite dissipative zonesin beams or connections)

V wp,Rd shear resistance of the composite steel-concrete web panel- Eurocode 4

Partially encased sti ffened web panels

Same condition, if straight links are provided at a maximum spacing s1 = c in the panel

Links oriented perpendicularly to the longer side of web panel

no other reinforcement of panel requiredLinks not required if hb/bb < 1,2 and hc/bc < 1,2

A

C

B

s1 c<

c

b b

b p = hc

s1s1 s1 s1 s1 h b



Composite connections in dissipative zonesTransfer of bending moment and shear from beam to column

Realised by a couple of vertical reaction forces into the concrete ↑ ↓

Should be checked:

►Capacity of column to bear locally those forces without crushing

=> confining (transverse) reinforcement + “face bearing plates”

►Capacity of the column to resist locally tension mobilised by vertical forces=> vertical reinforcements with a design strength equal to the shear strength in beam

Part or total of reinforcement present in the column for other reasons

= part or total of the reinforcements so required

Vertical reinforcing bars: confined by transverse reinforcement already mentioned

+ face bearing plates B

A

C

B

A steel beam

B face bearing plates

C reinforced concrete column



Favourable influence of concrete encasement on local ductil ity.

Concrete prevents inward local buckling of the steel walls&reduces strength degradation

=> Limits for wall slenderness of composite sections > those for pure steel sections

increase up to 50% if:

■ confining hoops

for fully encased sections

■ additional straight bars

welded to inside of flangesfor partially encased sections

Limits of wall slenderness for steel and encased H and I sections

for different design details and behaviour factors q.

Ductility Class of StructureDCM DCH

Reference value of behaviour factor q 1,5 < q ≤ 2 2 < q ≤ 4 q > 4

FLANGE outstand limits c/t f Reference: H or I Section in steel only

EN1993-1-1:2004 Table 5.2 14 ε 10 ε 9 ε FLANGE outstand limits c/t f

H or I Section, partially encased,with connection of concrete to web

as in Figure 57 b) or by welded studs.EN1994-1-1:2004 Table 5.2 20 ε 14 ε 9 ε

FLANGE outstand limits c/t f H or I Section, partially encased

+ straight links as in Figure 57 a) placedwith s/c ≤ 0,5EN1998-1-1:2004 30 ε 21 ε 13,5 ε FLANGE outstand limits c/t f H or I Section, fully encased+ hoops placed with s/c ≤ 0,5EN1998-1-1:2004 30 ε 21 ε 13,5 ε

WEB depth to thickness limit c w / t w

c w / t w = h – 2t f Reference: H or I Section, in steel only,web completely in compressionEN1993-1-1:2004 Table 5.2 42ε 38 ε 33 ε

WEB depth to thickness limit c w / t w

H or I Section, web completely incompression, section partially encasedwith connection of concrete to web or fully

encased with hoops.EN1993-1-1:2004 Table 5.2 ,

EN1994-1-1, cl.5.5.3(3) 38ε 38 ε 33 ε

note: ε = (f y/235)0.5 with f y in MPa

c

b = bc

h

= h

c

t w

t f

c

b = bc

t w

t f

h = h

c

sssssss



General rules for the design of elements

● ‘Critical zones’: confining reinforcements for dissipative & non dissipative columns

● The resistance in bending of the steel section may be steel alone or composite

● When concrete is assumed to contribute to axial and/or flexural resistance of a

non dissipative column, the design rules for dissipative columns to ensure full shear

transfer between concrete and steel parts should be applied

● Because of the cyclic character of seismic action effects: reduced design shear

resistances in the transmission of forces (/2 of EC4 shear resist.)

● When, for capacity design purposes, the full composite resistance of a column

is employed, complete shear transfer between the steel and reinforced concrete

parts should be ensured.

● If insufficient shear transfer through bond and friction=> shear connectors● In essentially axially loaded non dissipative members: shear transfer to ensure

that steel and concrete share the loads applied to the column at connections

● In non dissipative composite columns, the resistance in shear of the steel section

may be considered either alone or combined with the resistance in shear of

the concrete section ( Eurocode )● In dissipative members, the shear resistance should be determined considering

the steel section alone (or special details to mobilise the shear resistance

of the concrete encasement)

● Fully encased columns assumed to act compositely:

min dimensions b and h ≥ 250 mm



hC

s

bo bc

ho

hc

1 0 d b

w

Anchorage and splices of reinforcement barsSame as for earthquake resistant reinforced concrete structures

Transverse reinforcement

Closed stirrups with 135° hooks and extensions ≥ 10d bw in length

Length lcr

of critical regions (in metres) are:

for ductility class M

for ductility class H

hc is the largest cross-sectional dimension of the column

l cl is the ‘clear length’ of the column

{ }m450, ;6/;max clccr lhl =

{ }m60, ;6/;5,1max clccr lhl =



Steel beams composite with a slab

● Beams may be designed for full or partial shear connectionMinimum degree of connection η ≥ 0,8

● Total resistance of shear connectors within any hogging moment region

≥ than the plastic resistance of the reinforcement

● Reduced design strength for the connectors in dissipative zones: (Eurocode 4)x

0,75

● Full shear connection required when non-ductile connectors are used

● Minimum thickness of concrete poured on site assumed as a diaphragm: 70 mm

● Profiled steel sheeting with ribs transverse to the supporting beam with the “waves”

characterised by angle α =>the EC4 reduction factor k t for the design shear resistance of connectors

reduced by a rib shape efficiency factor k r

k r = 1 k r = 1 k r = 0,8

α 10°<α<80°




‘Seismic Re-bars’In the dissipative zones of beams, specific ductile steel reinforcement of the slab:

in the connection zone

Detailed design guidance: Annex C of Eurocode 8

A Exterior Node B Interior Node A Exterior Node

C Steel beam D Façade steel beam

E Reinforced concrete cantilever edge strip

AT

}

}

}

}

}

} }

AT

}

AT

}

AT

}

C CC

C

DE

A AB




Effective width of slab

Effective width beff of concrete flange: be1 + be2

Partial effective widths be in Tables, not ≥ available widths b1

and b2

2 Tables: determination of the elastic properties

determination of the plastic properties of the composite beamMoments inducing compression in slab: +

tension -




The bearing width of the slab concrete on the column in the horizontal direction:perpendicular to the beam for which the effective width is determined

may include additional details aimed at increasing the bearing capacity

A: Exterior column

B: Interior column

C: Longitudinal beam

D: Transverse beam

or steel façade beam

E: Cantilever concrete edge strip

F: Extended bearing

G: Concrete slab



Partial effective width be

of slab for evaluation of

plastic moment resistance

be Transverse element be for I (Elastic Analysis)

At interior column Present or not present For negative M : 0,05 l

At exterior column Present For positive M : 0,0375 l

At exterior column Not present,

or re-bars not anchored

For negative M : 0

For positive M : 0,025 l

Sign of bendingmoment M

Location Transverse element be for M Rd (Plastic

resistance)

Negative M Interior

column

Seismic re-bars 0,1 l

Negative M Exterior

column

All layouts with re-bars anchored to façade

beam or to concrete cantilever edge strip

0,1 l

Negative M Exterior

column

All layouts with re-bars not anchored to

façade beam or to concrete cantilever edge

strip

0,0

Positive M Interior

column

Seismic re-bars 0,075 l

Positive M Exterior

column

Steel transverse beam with connectors.

Concrete slab up to exterior face of column

of H section with strong axis oriented as in

Figure 63 or beyond (concrete edge strip).Seismic re-bars

0,075 l

Positive M Exterior

column

No steel transverse beam or steel transverse

beam without connectors.

Concrete slab up to exterior face of column

of H section with strong axis oriented as in

Figure 63, or beyond (edge strip).

Seismic re-bars

b b/2 +0,7 hc/2

Positive M Exterior

column

All other layouts. Seismic re-bars b b/2 ≤ be,max

be,max =0,05l

Partial effective width be of slabfor computation of

second moment of area I

used in the elastic analysis

of the structure




Composite steel concrete moment resisting frames.

Design objective● Plastic hinges in beams or their connections, not in the columns

‘Weak Beam-Strong Column’ WBSC

● Plastic rotation capacity at beam ends: 25 mrad DCM 35 mrad DCH.

Composite character ● Dissipative zones are at beam ends

● For a design to be “steel only”:no contact between the slabs and any vertical side

of a steel element (columns, connectors, connecting plates, corrugated flanges, etc)

within a circular zone around each column of diameter 2beff

Analysis

● 2 flexural stiffness:EI 1 for parts of spans subjected to + bending un-cracked section

EI 2 - bending cracked section

● An equivalent I eq

constant over span may be used: I eq

= 0,6 I 1

+ 0,4 I 2● For composite columns: (EI )c = 0,9( EI a + r E cm I c + E I s )

E and E cm :modulus of elasticity for the steel and concrete respectively

r a reduction factor r = 0,5.

I a, I c and I s : I of the steel section, the concrete and the re-bars respectively

● Composite trusses: not as dissipative beams.● In columns where plastic hinges will form: N Ed/N pl,Rd < 0,30



Composite steel-concrete braced frames

Composite frames with concentric bracings

The beams and columns can be steel alone

composite steel-concrete

The dissipative elements are the bracings

They can be structural steel alone, not composite

2 reasons:

►prior to buckling, composite braces would tend to overload beams and columns►composite braces: no research => uncertainties with regard to their cyclic behaviour

Design procedure for the braces: identical to steel frames with concentric bracing




Composite frames with eccentric bracings

► Uncertainties associated with:

■ composite elements capacity at large deformations (rotations up to 80 mrad)

■ ‘disconnection’ of the slab■ contribution of slab in bending at rotations up to 80 mrad

► Design: dissipative behaviour through yielding in shear of the links

The contribution of the slab to the shear resistance is negligible

=> Links should be short or intermediate lengthMaximum length e:

● when plastic hinges form at both ends: e = 2Mp, link/ Vp, link.

● when a plastic hinge form at only one end: e < Mp, link/ Vp, link

►Links may not be encased steel sectionsbecause of uncertainties about the contribution of the concrete to shear resistance

► Analysis: 2 ≠ stiffness for zones under sagging and hogging moments.

► Vertical steel links: OK




Composite frames with eccentric bracings

C

A

B

B

E

D

T

A : seismic link

B : face bearing plate

C : concrete

D : additional longitudinal rebars

E : confining ties Specific construction details

►face bearing plates for links framinginto reinforced concrete columns

►transverse reinforcement

in ‘critical regions’ of fully encased

composite columns adjacent to links



Composite steel-concrete walls and systems with walls

Composite wall systems

►Shear strength and stiffness comparable to those of RC walls

►The structural steel sections or boundary members

increase the flexural resistance

delay the onset of flexural plastic hinges in tall walls

Structural Type 1 and 2: designed to behave as shear walls

dissipate energy in the vertical steel sections

in the vertical reinforcing bars

Structural Type 3 dissipate energy in the shear walls and in the coupling beams

Steel or composite

moment frame with

concrete infill panels.

Concrete shear walls

coupled by steel or

composite beams.

Concrete walls

reinforced by encased

vertical steel sections.

TYPE I TYPE 2 TYPE 3




Mechanical behaviour

of

shear walls Type 1 and 2

Analysis

Type 1 or Type 2 vertical fully or partially encased

Structural steel sections act as boundary members of reinforced concrete infill panels

Analysis assumes: ● seismic action effects in boundary members are axial forces only

● shear forces are carried by the reinforced concrete wall● the entire gravity and overturning forces are carried by

concrete shear wall acting compositely

with the vertical boundary members

Type 3

Composite coupling beams: 2 different flexural stiffness in the analysis




Detailing rules for composite walls of ductil ity class DCM

The reinforced concrete infill panels in Type 1 systems: RC design

The reinforced concrete walls in Types 2 and 3: RC wall of class DCM

Partially encased steel sections used as boundary members: class of cross-section

related to the behaviour factor of the structureHeaded shear studs or tie reinforcement should be

welded to the steel member

or anchored through holes in the steel member

or anchored around the steel member

Headed shear studs or tie reinforcement should be provided to transfer vertical andhorizontal shear forces between the boundary elements and the reinforced concrete

B A

min = 2h h

A: bars welded to column C: shear connectorsB: transverse reinforcement D: cross tie

hmin = 2h

C

D



Detailing rules for coupl ing beams of ducti lity class DCM

Transfer of bending moment & shear at beam end: a couple of vertical reaction forces

Wall capacity to bear locally those forces without crushing: confining reinforcement

Sufficient embedment length of the beam into the wall

Applied forces: M pl,Rd and the shear V Ed of the beam

Embedment length l e begin in 1st layer of confining reinforcement in the wall

not less than 1,5 x depth of coupling beam

Confining hoops: not compulsory in DCMmay be required over l e by design checks

Vertical wall reinforcement

design axial strength equal to the shear strength of the coupling beam

placed over the embedment length of the beam

2/3 of the steel located over the first 1/2 length l eextend a distance ≥ 1 anchorage length above & below the flanges of the coupling bea

Stiffeners “face bearing plates” contribute to the confinement of concrete

˜ 2/3 le

V M

C

B

D A

le

A: Additional wall confining ties

at embedment of steel beamB: Steel coupling beam;

C: Face bearing plates.



Composite steel plate shear walls

► Designed to yield through shear of the plate► Plate stiffened by concrete encasement on one or both sides

Concrete thickness not less than 200 mm when on one side

100 mm when on both sides

Minimum reinforcement ratio: 0,25% in 2 directions

► Encasement attached to prevent buckling of steel► Checks : V Rd ≥ V Ed

f yd : design yield strength of the plate

Apl : horizontal area of the plate.

► The connections between the plate and the boundary columns and beams

as well as the connections between the plate and its concrete encasement

must be designed such that the full yield strength of the plate can be developed

► The steel plate should be continuously connected on all its edgesto structural steel boundary members with welds and/or bolts

to develop the yield strength of the plate in shear

► Openings in the steel plate should be stiffened

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Composite Steel Concrete Structures