What is a composite slab?

A composite slab is a unidirectional slab in which concrete is poured over a corrugated sheet profile that functions as both formwork and a positive framework. Once the concrete has hardened, this profile works with it to absorb tensile stresses. Therefore, this slab can be built without the need for support props.
Composite slabs are commonly used for steel framed buildings, although they may also be used on concrete, wood, and brick structures.

What is a composite slab of ?

Composite slabs consist of a number of materials which include concrete, steel rebar or profiled steel sheets and shear connectors. In case of steel rebars are used, this reinforcement can be connected to the supports (beams), and in such case the slab may required some temporary supports during casting. ‘

When profiled metal decking is used, the steel rebars are not required, the metal decking acts as permanent shuttering eliminating the need for support supports during the concrete casting. Profiled steel decking is normally galvanized for durability, and this can be achieved through application of 275g/m² of zinc coating.

A composite slab also requires shear stud connectors to transfer shear from the slab to supports (beams). According to BS EN ISO 13918 , this is requirement ,and shear stud shank sizes for solid concrete slabs may range between 16 to 25mm , and can not exceed 19 mm for studs through deck welded inside the ribs of the profiled steel decking.

Advantages of composite slabs

Composite slabs can offer a number of advantages such as:

• They are adaptable to a wide range of practical situations and solutions.
• They also have good resistance/weight ratio which allows for a global weight reduction of the structure.
• Use of composite slabs can also reduce on construction time as no props are required and multiple floors can concreted at once.
• Composite slabs are also cost effective both in terms of labour and material cost.

Disadvantages of Composite slabs

Composite slabs can offer be unfavorable for use in certain situations such as:

• When compared to wood, a composite decking is significantly heavier which makes them difficult to handle.

• Compared to wood decks which can be sanded and repainted, composite decking is limited in its repainting which affects its long term use.

Design for composite slab

A number of design considerations need to be taken in the design of composite slabs such as; flexure, vertical shear , and fire resistance as well as point and line loads. The importance of these design considerations is to ensure that the slab does not experience any of its possible failure modes such as ;shear failure at the end support, moment failure near the mid span , and de-bonding between the interface of the slab and decking as a result of longitudinal shear.

To account for the effect of bending of the slab during loading, the flexural capacity of the slab needs to be checked and designed for. The shear resistance of a composite slab is highly dependent on the effective depth of its cross section. With regard to fire resistance, the insulation ,integrity and load bearing capacity of the slab need to be considered.

Because of factors such as; temporary supports during construction and wheel load, concentrated point and line loads are common in structures. At the locations of these loads, the effect of the reduced effective slab width available for bending and vertical shear resistance should therefore be assessed.

To account for longitudinal shear in the composite slab, mechanical interlocking provided through indentations or embossments in the profiled sheeting. This also helps to ensure composite behaviour between the profiled sheet and the concrete.

The design of composite slabs with profile sheets is normally based on the values (such as the load per span and fire resistance data) from the supplier. The profile sheet generally has enough capacity to provide the slab with required tension resistance for the composite bending. However, the composite can be further enhanced to placing rebars in the ribs with sufficient cover to provide tensile resistance in case of fire. Mesh reinforcement can also be provided in the composite slab together with the profile sheet to enable crack control especially during fire conditions.

For the purpose of design , minimum depth of 80 mm is required for a composite slab, and in case the slab is to act as a diaphragm , this value can be taken as 90mm. The minimum area of reinforcements in any direction of the slab is 80 mm² per metre , and this reinforcement has to be provided within the depth of the slab above main flat surface of the top of the ribs of the sheeting.

Example on design of a composite slab

Consider the design of a 2.5 m span composite slab supported below:

The slab is to be constructed using a profile sheet with the following information:

Profiled sheeting:

Material properties:

• Concrete strength, f_ck = 25 N/mm²
• Strength of steel sheeting, f_y.p.k = 320 N/mm²
• Modulus of elasticity for concrete,E_c =30kN/mm²

Loading:

Live load = 2.5 kN/m2

Dead load =1.2kN/m2

Solution:

Assuming the composite slab has overall depth, h of 130 mm.

Then from the table 1 below, the dry and wet weights are 3.02 kN/m2 and 3.08 kN/m2 respectively.

Table 1: Concrete Volume and weight values adopted from Metal Decking Specialists Technical Manual and Guidance Notes (2013)

Assuming the slab is designed for a fire rating of 1 hour, then mesh of A142 will be provided for crack control from table 2 below.

Table 2: Load span values adopted from Metal Decking Specialists Technical Manual and Guidance Notes (2013)

Total unfactored applied load = 1.2+2.5 = 3.7 kN/m2

Assuming a profiled sheet of 1.0 gauge is provided, then the sheet is adequate for our design span since the span of 2.5m is less than maximum permissible of 3m.

Design of shear connectors

 Assuming shear connectors with a diameter of 19 mm , and an overall welded height of 90 mm. The ultimate tensile strength of connectors can be taken as 450 N/mm² ,and they are welded according to BS EN 14555.

f_ut = 450 N/mm²

n= 1.0

diameter of the shear connector, d=19

α=1.0 for h_sc/d = 90/19 = 4.7 >4

Assuming γ = 1.3

For a single headed shear connector, the design resistance is given by:

P_rd= (0.8*f_ut * π * 0.25d²)/(γ*1000) = 78.5 kN

or

P_rd = (0.29*α*d² *√(f_ck*Ec))/(γ*1000) =69.7 kN

The design resistance is therefore 69.7kN.

For the shear connectors in profiled sheeting:

K_t=(0.7/√n_r)*(b₀/h_p)*(h_sc/h_p-1)

h_sc=90 mm

number of shear connectors per rib, n_r =1

width of trapezoidal rib at mid height, b₀ = (133+175)/2 =154 mm

height of the profile sheeting dentation, h_p = 60 mm

K_t = 0.898

Design resistance = K_t*P_rd = 0.898 * 69.7 =62.6 kN

Design resistance per rib = n_r * 62.6 = 62.6 kN

Compressive resistance of the concrete flange :

Compressive resistance of the concrete flange is given by, N_c = 0.85*f_cd*b_eff*h_c

Overall height of profile sheet, h_d = h_p+12 =60+12 =72 mm

The depth of concrete above , h_c = 130-72=58 mm

design strength of concrete, f_cd=25/1.5 =16.7 N/mm2

effective breadth of the concrete flange, b_eff =b₀ t 2*b_e1

For a single connector, the spacing b₀ =0

Assuming the span of the supporting beams , L_e = 8 m.

b_e1 =L_e/8= 1000 mm

b_eff = 0 +2*1000 =2000 mm

N_c = 0.85*16.7*2000*58 =1646.62 kN

Assuming 356x171x67 UB steel beams are used to support the slab, then area of the beam can taken as 85.5 cm²

Tensile resistance of the steel member :

The strength of the steel, fy = 275 N/mm²

N_pl,a = f_y * A = 275*85.5*10² = 2351 kN

Since 1646.62 is less than 2351 kN, Nc,f = 1646.62 kN.

Reduced compressive strength of the concrete flange, N_c = n * Prd =

number of shear connectors, n = 8000/(2*(200+133)) +1 =12

N_c = 12 * 69.7 =836.4 kN

The degree of shear connection , η = N_c/N_c,f= 836.4/1646.62 = 0.51

1- ((355/f_y)*(0.75-0.03*L_e))=1-((355/275)*(0.75-0.03*8)) =0.34

Since 0.34 <0.4, then η has to be greater than 0.4

Hence since η =0.51 > 0.4, the degree of shear connection is adequate.

References