1

Gregory MacRae

ENCI425:

STEEL STRUCTURES

ENCI425 – Steel Structures University of Canterbury

T4_L1 – Composite Beams 1

ENCI 425 – STEEL STRUCTURES

Low-Rise Frame Design

- member design

- second order analysis

- plastic analysis

- construction details

Seismic Frames + external speakers …

Beams - torsion

- composite action

- fatigue

- floor vibrations

- plate girders

COMPOSITE BEAMS

(a) Slab on beam (b) Concrete-Filled Tube (CFT)

(c) Reinforced Concrete Steel (RCS) (d) Steel Reinforced Concrete (SRC)

Common Types of Composite Construction:

COMPOSITE BEAMS

SRC – Steel Reinforced Concrete columns are steel columns surrounded by reinforced concrete.

1.1 INTRODUCTION

Columns are generally:

- square (or circular)

- less susceptible to buckling that the steel

sections alone

- have some protection against corrosion

- increased stiffness/strength

- increased fire rating

These are addressed in NZS 3404 Ch 13. We will consider gravity loading only in this class.

COMPOSITE BEAMS

RCS – Reinforced Concrete Steel columns are reinforced concrete columns through which hot-formed beams pass.

CFT – Concrete Filled Tubular columns have

- high construction speed (no formwork)

- high stiffness/strength/ductility

1.1 INTRODUCTION

COMPOSITE BEAMS

Composite beams are generally made from steel I-shapes which support a concrete slab, or a concrete deck floor system. Because the concrete is there, it may be used to increase the strength of the beam.

These are addressed in NZS 3404 Ch 13. We will consider gravity loading only in this class.

1.1 INTRODUCTION

2

Concrete cast in situ Welded mesh reinforcement for crack control, transverse load distribution and

fire resistance

Headed stud connectors for shear connection to the composite beam and,

when required, end anchorage to the slab

(from Hicks, HERA)

COMPOSITE BEAMS

Conventional composite construction

COMPOSITE BEAMS

(from Hicks, HERA)

1.1 INTRODUCTION

Typical composite beams:

Reinforced concrete Concrete slab steel

slab on beam decking on beam

Steel Decking Air

Shear studs

Concrete

I beam

Shear studs

Reinforced Concrete

I beam

COMPOSITE BEAMS

Types of profiled steel sheeting defined in EN 1994-1-1 (from Hicks)

Re-entrant profiled steel sheet

Open trough profiled steel sheet

COMPOSITE BEAMS

1.1 INTRODUCTION

Advantages of composite beams

- Safety - sheeting acts as working platform

- Fast – formwork

- unpropped

- Flexibility for irregular structures

- Sheeting stabilizes beams

- Sheeting can provide all main reinforcement

- erection

- higher flexural strength (1.5-2.5 times)

- higher flexural stiffness

(3-4.5 times)

- economy

- Increased span length (span:depth ≈ 25)

- Shallower steel beams (easy

accommodation of building services)

- total weight savings of 20-30% typically

- steel weight savings of 30-50% typically

- ease of modification

COMPOSITE BEAMS

(based on Hicks, HERA)

1.2 COMPOSITE ACTION

Two identical rectangular beams

sitting on top of each other

Displaced shape:

Second moment of area, I:

Displacement, d:

Strength, My:

INC = 2 x (bd3)/12

P

L

Non-Composite Composite

P

L

IC = (b(2d)3)/12 = 4INC

dNC = PL3

48EINC

dC = dNC

4

My,NC = 2 x (bd2)/6.fy

Each beam has depth d

and breadth b, strength fy

My,C = b(2d)2/6.fy = 2My,NC

COMPOSITE BEAMS

3

Where are the best locations for shear studs?

To make a beam fully composite, shear connectors should have sufficient

strength and stiffness to ensure that there is no slip between the beams.

The maximum slip occurs at the beam ends, so this is the best place to make

connection.

1.2 COMPOSITE ACTION - ELASTIC

COMPOSITE BEAMS

For infinitely stiff shear connectors, what strength must they have?

Normal stress, f, is: f = My/I

Shear stress, v, is: v = VQ/Ib

Shear flow, q, is: q = VQ/I

1.2 COMPOSITE ACTION - ELASTIC

COMPOSITE BEAMS

P

L

SFD

BMD

5. FLEXURAL MEMBERS

Cross-Section

dy

d

dx P x

Slice along length

dx

M(x) M(x)+ dM

ds = dM.y

I

y

Net Stresses

ds = dM.y

I t

tbdx = sdA 𝑑/2

𝑦

COMPOSITE BEAMS

1.2 COMPOSITE ACTION – ELASTIC – Shear Review 5. FLEXURAL MEMBERS

Cross-Section

dy

d

dx P

Slice along length

ds = dM.y

I

dx

x

M(x) M(x)+ dM y y

Net Stresses

ds = dM.y

I t

At the centre of the section, y = 0

COMPOSITE BEAMS

1.2 COMPOSITE ACTION – ELASTIC – Shear Review

Rectangular Section

At the centre of the section:

5. FLEXURAL MEMBERS

Cross-Section

dy

d

dx P

Slice along length

ds = dM.y

I

dx

x

M(x) M(x)+ dM y y

Net Stresses

ds = dM.y

I t

COMPOSITE BEAMS

1.2 COMPOSITE ACTION – ELASTIC – Shear Review

The shear force per unit length of beam (or shear flow), q, is:

q = tb = VQ/(Ib) * b = VQ/I If the studs are stiff enough to provide no slip, then the required strength per stud, Vss,

is equal to the shear flow at the stud, q, multiplied by the spacing between studs, s.

That is: fVss > q.s

In reality, shear studs are not stiff and their strength is obtained at significant

deformations. Plastic, rather than elastic, analysis is used to find the ultimate strength.