Md. Zafar

7/30/2019 Md. Zafar

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A

Seminar

On

By Zafar Iqbal

Under the guidanceof

Prof. T. Valsa Ipe

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Introduction

A composite slab is one in which profiled steel sheets are used

as permanent shuttering capable of supporting the wetconcrete, reinforcement and construction loads. Subsequently,

the profiled steel sheets combine structurally with the

hardened concrete and act as part or all of the tensile

reinforcement in the finished floor.

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Steel beam bonded to concrete slab

with shear bond connectors

Traditional steel - concrete composite slab consist of rolled or

built-up structural steel beams and cast in-situ concrete slabsconnected together using shear connectors in such a mannerthat they would act monolithically

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The composite slabs formed using profiled sheeting as a

permanent formwork and tensile reinforcement to a concrete

slab, The main economy in using profiled sheeting is achieved

due to speed in construction. Care has to be taken in the construction of composite slabs

with profiled sheeting to prevent excessive 'ponding',especially in the case of long spans

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A composite slab with profiled sheeting consists of the following

structural elements along with in-situ concrete and steel beams:

Profiled sheeting

Shear connectors

Reinforcement for shrinkage and temperature

stresses.

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Profiled sheetingThe sheeting is very thin, for economic reason.

It has to be galvanised to resist corrosion.

The sheets are pressed or cold rolled, and are typically

about 1m wide and up to 6m long.

Design of composite slabs is still often governed by a

limit on deflection. So most sheeting in the UK is of mildsteel.

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There are two well-known generic types of profiles

Dovetail profile

Trapezoidal profile

with web indentations

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Shear connectors

Self drilling and tappingscrews

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Reinforcement for shrinkage and

temperature stresses

In buildings, temperature difference in the slabs is negligible;

thus there is no need to provide reinforcement to account for

temperature stresses. The effect of shrinkage is considered

and the total shrinkage strain for design may be taken as 0.003

in the absence of test data.

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The width of the slab ‘b’ is one typical wavelength of profiled

sheeting.

The overall thickness is ht is not less than 80 mm and hc is notless than 40 mm from sound and fire insulation considerations.

Eurocode assumes the equivalent ultimate stress of concrete in

compression as 0.85(fck)cy/ gc where (fck)cy is thecharacteristic cylinder compression strength of concrete. IS

456: 2000 uses an average stress of 0.36 (fck)cu

accommodating the value of gc

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Neutral axis above the sheeting [Fig. 5(b)] Full shear connection is assumed. Hence, compressive force Ncf in

concrete is equal to steel yield force N pa

.

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Neutral axis within sheeting and

full shear connection [Fig. 5(c)]

The tensile force in sheeting is split into Na (equal to compressive

force Ncf ) plus Nac. Na = Ncf and the remaining force Nac such that the

total tensile force is Nac

+ Na.

The equal and opposite force Nac provide resisting moment M pr .

The moment of resistance is given by

The lever arm z can be found by examining the two extreme cases.

For case (i)where Ncf = N pa or Ncf /N pa = 1.0, Nac = 0 and hence M pr

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For case (ii), Ncf

@ 0; Na

= 0.M pr

= M pa

.

The neutral axis is at a height e p above the bottom

Thus the equation to the line EF is

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Partial shear connection (N c <N cf )

The depth of the stress block is

In this case, equation 5, 6 and 10 get modified by substituting

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The shear resistance of composite slab largely depends on

connection between profiled

Sheet and concrete. The following three types of mechanismsare mobilised:

•Natural bond between concrete and steel due to adhesion

• Mechanical interlock provided by dimples on sheet and shear

connectors• Provision of end anchorage by shot fired pins or by welding

studs when sheeting is made to rest on steel beams.

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Natural bond is difficult to quantify and unreliable, unless

separation at the interface between the sheeting and

concrete is prevented.

Dimples or ribs are incorporated in the sheets to ensure

satisfactory mechanical interlock.

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If the shear connection is partial, slip occurs between sheeting

and concrete. The effectiveness of the shear connection istested using an m-k shear bond test. The test is describedbelow. The failure of the beam is initiated by one of thefollowing three modes

Flexure

Shear at supportShear bond mode

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The expected mode of failure in a test depends on the ratio

of (ℓs

) to the effective depth (d p

) of the slab.

The empirical constants m and k are determined from

prototype slab tests to failure and are calculated from the

slope and intercept of a regression line

Physically "m" is a broad measure of the mechanical interlock

and k represents the friction load.

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At high values of ℓs /d p, flexural failure occurs.

The maximum bending moment (Mu) is given byMu = V.ℓs (15)

Mu is proportional to A p f ypd p

At low values of (ℓs /d p), vertical shear failure occurs.

The mean vertical shear stress on the concrete is roughly

(V/bd p).

Longitudinal shear failure occurs at intermediate values

(ℓs /d p) and be on the line

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One of these has ls/dp values chosen in such a manner

that the results be near the point ASecond group is chosen with a lower ls/dp values such that

the results lie near the point B.

Values of m and k are found for a line

drawn below the lowest result in eachgroup, at a distance that allows for the

scatter of test data. The behaviour is

controlled by the two parameters of the

straight line, namely•m - the slope of the line

•k - the intercept of y -axis.

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The composite slab is checked for the following serviceability

criteria:

• Cracking

• Deflection

• Fire endurance

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Cracking of concrete Lower surface surface of the slab is protected by the sheeting

Cracking will occur in the top surface where the slab is

contineous over a supporting beam

Normally crack width should not exceed 3 mm.

If environment is corrosive it is advisable to design the slab as

continuous and take advantage of steel provided for negative

bending moment for resisting cracking during service loads.

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Cracking pattern of 1mx1mtrapezoidal profiled composite slab

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Tensile membrane action can occur in slabs with fixed or simply

supported edge conditions and occurs when the slabs undergolarge deflections. If a simply supported slab undergoes

relatively large deflections, the regions of the slab on the

supports start to move inwards but are restrained by the

adjacent outer regions.

Deflection

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Eurocode 4 gives the following guidance:

•The deflection of sheeting due to its own weight and the wet

concrete slab should not exceed ℓ/180 or 20 mm,

•25 for simply supported slabs

•32 for spans one end continuous

• 35 for internal spans

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Fire Endurance

A series of full-scale fire tests conducted at Cardington on aneight storey building have shown that unprotected reinforced

concrete floor slabs on steel decking do not collapse after a

compartment burnout, despite very high measured steel

temperatures, and suffering considerable deformations .

It is postulated that the load capacity is sustained by means of

tensile membrane action within the floor slab.

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The design of deck composite floors must ensure adequate

strength and stiffness both during construction and in service.

Hence the following must be checked

• Bending strength of steel sheeting to ensure that weight of

concrete can be supported during construction, before

composite action is achieved

• Flexural stiffness of the sheeting to prevent excessive

deflection during construction

• Tensile strength of sheeting to provide the necessary

reinforcement to the slab in its final composite form

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•

Strength of concrete in composite for which thefloor is considered as an equivalent R.C.C slab

• Stiffness of the composite slab to prevent the

excessive deflection under normal working loads

• Bond between concrete and steel in order to

achieve the composite action

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1. R.P. Johnson “ Composite Structures of Steel and Concrete” Volume 1, Blackwell Scientific

Publications, UK, 1994.2. R. Narayanan “Composite Steel Structures” Advances, Design and Construction, Elsevier,

Applied science, UK, 1987.

3. R.M. Lawson, D.L Mullett and FPD Ward “Good practice in Composite floor Construction”.

The Steel Construction Institute, 1990.

4. EUROCODE 4; (1992), Design of Composite Steel and Concrete Structures – Part 1-1: General

rules and rules for buildings. ENV 1994-1-1:5. Mark Lawson and Peter Wickens “Composite Deck Slab”, Steel Designers Manual (Fifth

edition), The Steel Construction Institute, UK, 1992.

6. Bryan E.R. and Leach. P “Design of Profiled sheeting as Permanent Formwok”, Construction

Industry Research and Information Association (CIRIA), Technical Note 116, 1984.

7. Data Sheet: Fire resistance of Composite Slabs with Steel Decking, Construction Industry

Research and Information Association (CIRIA), Special Publication 428. Zoltan V. Nagy and Istvan Szatmari “COMPOSITE SLAB DESIGN” Technical University of

Budapest, Dept. of Steel Structures

9. WWW.insdag.org

http://www.insdag.org/

http://www.insdag.org/

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