1

Steel Structures

2

Introduction

Structures may be classified on the basis ofmaterials used for construction, as follows:

• Steel structures

• Aluminium structures

• Concrete structures

• Composite structures

3

Steel

• Steel, as a building

material has been

extensively used in

various structures in

Singapore.

• To facilitate safe and

economical design,

standards and codes

are prepared.

4

Advantages of Steel

Steel has many advantages as a structuralmaterial.

• Steel members have high strength per unitweight.

• Being light, steel members can be convenientlyhandled and transported.

• Properly maintained steel structures have a longlife.

5

Advantages of Steel (Contd)

• The properties of steel mostly do notchange with time.

• Steel, being a ductile material, does notfail suddenly, but gives visible evidence ofimpending failure by large deflections.

• Additions and alterations can be madeeasily to steel structures.

6

Advantages of Steel (Contd)

• They can be erected at a faster rate.

• Steel has the highest scrap value amongst all

building materials.

7

Disadvantages of Steel

Steel has the following disadvantages as a

structural material.

• Steel structures when placed in exposed

conditions are subjected to corrosion. Therefore,

they require frequent painting.

• Steel structures need fire proof treatment, which

increases cost.

8

Uses of Structural Steel

• Use of steelwork in building construction is

gaining popularity in view of:

- speed of construction

- less labour intensive

- cleaner work site

- economy in terms of construction cost

- allow design flexibility due to excellent strength

and predictable behaviour under loading

9

Projects in Singapore

• Singapore National Library

• The Esplanade

• Capital Tower

• Gardens by the Bay

• Singapore Sports Hub

10

Elements Present in Steel

• Carbon

• Silicon

• Phosphorus

• Sulphur

• Manganese

11

Mild Steel

Composition:

• Carbon 0.23 – 0.25%

• Sulphur 0.055%

• Phosphorus 0.055%

12

Properties of Mild Steel

• Its structure is fibrous with dark bluish colour.

• It is malleable and ductile

• It is difficult to harden and temper.

• Its specific gravity is 7.8.

• It can easily be welded, riveted and forged.

13

Properties of Mild Steel (Contd)

• It is more tough and elastic than cast iron and

wrought iron.

• It is equally strong in compression, tension and

in shear.

14

Uses of Mild Steel

• As rolled structural sections like I-section, T-section, channel section, angle iron, plates,round and square rods in construction works.

• Mild steel round bars are extensively used asreinforcements in Reinforced CementConcrete (RCC).

• Mild steel tubes are finding much use instructures.

15

Uses of Mild Steel (Contd)

• Plain and corrugated sheets of mild steel are

being used as roof coverings.

• Mild steel is also used in the manufacture of

various tools and equipments, machine parts,

towers and industrial buildings.

16

High Tensile Steel

Composition

• Carbon 0.8%

• Manganese 0.6%

• Silicon 0.2%

• Sulphur 0.05%

• Phosphorus 0.05%

17

Uses of High Tensile Steel

• It is essentially a low carbon steel and the

percentage of carbon is kept less than 0.15%. It

is also known as high strength steel.

• High tensile steel is normally used in pre-

stressed concrete

18

Types of Steel

Steel is given different names based on itsalloyed composition:

• Carbon Steel

• Alloy Steel

• Structural Steel

• Sheet Steel

19

Mechanical Properties of Steel

The most important mechanical properties of

steel are its

• strength,

• elasticity and

• plasticity,

• characterised by stresses and elongations as

well as its tendency to brittle failure.

20

Behaviour of Steel under Tension

• Stress-strain diagram for mild steel and high

strength steel is shown below:

21

Stress-Strain Diagram

The load-deformation or conventional stress-strain diagram is very important, as it is thebasis for accepting the steel for most of thestructural requirements.

The total curve can be divided into threedistinct regions:

• Elastic Region

• Region of Pure Plasticity

• Region of Plasticity with Work Hardening

22

Design Objectives for Steelwork

Structural Steel work can be

• single member

• an assembly of a number of sections

connected together & perform a specified

function

Three Main Design Criteria:

- Safety

- Economy

- Appearance

23

Factors Influencing Design of Steel

• The spans involved

• The vertical loading

• The horizontal loading

• The services required

• The ground conditions

24

Design Methods

1. Elastic (allowable stress) Design

The process involves:

• Determine the stress of members subject to

service load on the structure & thereafter.

• Design the member so as not to exceed the

allowable strength of material used.

• In view of possible inaccuracies in the loading

and material behaviour, the material factor

used is normally high.

25

Design Methods (Contd)

2. Limit State (load factor) Design

The process involves:

• Ultimate limit state which assess the strength

of material, stability against overturning, sway

and fatigue.

• Serviceability limits states which assess the

deflection, durability, corrosion & vibration.

• Determine the stress of member subject to the

ultimate load.

26

Limit State Approach

Two limit states considered in the design ofstructural steel work:

1. Ultimate Limit State

- strength

- stability against overturning

- fracture due to fatigue

- brittle fracture

27

Limit State Approach (Contd)

2. Serviceability Limit State

- deflection

- vibration

- durability

- corrosion

28

Load Factors

• For the limit state, the service or actual loads are

multiplied by load factors and the products are

referred to as ultimate loads.

• For the serviceability limit state, load factor is

unity (1).

• Partial safety factor for dead load = 1.4

• Partial safety factor for live load = 1.6

29

Categories of Steel Building

Construction

The majority of steel building fits within one of

the following categories:

Type Main Use

Bearing Wall Low rise, lightly loaded

Steel Frame Wide variety of types and sizes of building

Long Span Coverage of long column-free areas

High Rise Tall buildings i.e. more than 20 storeys

30

Types of Load

Dead Load (DL)

- Point load (kN)

- Uniformly distributed load, UDL (kN/m)

Refers to self-weight of steel member and other

permanent parts of the building.

Dead Load of a structure can be computed

based on the density of the material.

For eg: Density of Steel is 7850 kg/m3

31

Types of Load (Contd)

Imposed Load (Live Load)

- Point load (kN)

- Uniformly distributed imposed load UDL (kN/m)

Refers to temporary load. eg: people, furniture

Imposed load varies with functional usage of space / room

BS 6399 Part 1 – Code of Practice for DL & LL

32

Types of Load (Contd)

Wind Load (WL)

BS 6399 Part 2

Code of Practice for Wind Load

• Wind speed is selected based on the location of

building, topography, surrounding buildings,

height above the ground level, component size

and period of exposure.

33

Load Combination

In design of structural steel building, the

following principal combination of loads should

be taken into account:

• Load Combination 1: 1.4 Dead Load + 1.6 Imposed Load

• Load Combination 2: 1.4 Dead Load + 1.4 Wind Load

• Load Combination 3: 1.2 Dead Load + 1.2 Imposed Load + 1.2 WL

34

Design Procedure

Design problems start with:

• Selection of type and layout of structure

• Estimate the loading

• Conduct analysis to determine themaximum moments, forces and shear

35

Design Procedure (Contd)

• Select the material and proportioning of

members and connection

• Check the performance of the structure

under service load such as deflection,

vibration, corrosion, etc.

• Production of detailed drawing

36

Fabrication & Erection

• Simple connection

• Quality welding should be done at the factory

• Bolting is preferred at the site

• Avoid expensive weld – fillet weld is cheaper

than butt weld

• Design joints based on calculated forces rather

than section capacity

• Consider installation sequence in design

• Design for simple construction

37

Material Economy

• Use material only when necessary

• High strength & lightweight steel

• Cost per ton for SHS is 30% more than H or I-section

• Be reasonable in design

38

Buildability

• Consider an integrated system i.e. mechanical

services, superstructure and foundation

• Use high strength lightweight design to reduce

load on foundation

• Use fast track construction for early return of

investment

• Use complete design to enhance strength and

stiffness as well as for fire protection

39

Conclusion

• Steel is buildable compared to precast

construction

• Materials can be recycled, reused and reduced

• Light, dry and fast construction

• Cost effective for:

- commercial buildings

- long span bridges

- industrial buildings

- sport and leisure buildings