CT4 Lecture Notes PDF

Preview

Summary

Notes for quiz 1 CT4...

Description

CT4 (Steel Construction) Lecture Notes (BUILDING SUPERSTRUCTURE) Module 1 - Steel Construction and types of structural members Why do we use steel? • • •

Steel is the most preferred metal for the construction of large structures such as bridges, single storey industrial buildings, and residential buildings. There are three main types of steel construction – conventional steel fabrication, bolted steel structure construction and light gauge steel structure construction. Steel structure construction has many unique advantages over concrete construction.

Advantages -

Highly ductile metal – better earthquake performance Excellent load carrying capacity compared to concrete High strength to weight ratio (weighs 60% less then conc.) Light and small foundation in comparison with concrete construction Faster and easier to erect Has good recycling and scrapping value (eco-friendly) Prefabrication and Accuracy Suitable for complex projects and easy to transport Standardisation

Disadvantages (compared with concrete – the main competitor) -

Higher cost due to requirement of skilled labour force and large investment Susceptibility to corrosion and buckling Maintenance costs / thinned walled structures Loss of strength at elevated temperatures (fire) Fire protection costs Fatigue and brittle failure High resource consumption during production and manufacturing (energy and water)

History of Steel Products • • • • • •

•

Use of iron ore around 4000 years BC in Asia and Africa for making farming tools Around 1400 BC we learned to heat with charcoal to produce crude steel (stronger) Eighth century, cast iron and wrought iron were introduced to the field of civil engineering. The earliest known application of steel on record was on a suspension bridge in China. Fifteenth century, Britain was manufacturing large quantities of iron Iron is the basic element in steel; therefore Steel is iron with a controlled level of carbon. Steel is produced by adding iron ore pallets and limestone to alloy.

Steel Production – Comparison

Steel Skeleton •

Assembly of individual stickframe - Simple linear member - 2D frame - 3D frame/space frame - Shell and plate – tanks, ships etc. - The connection is important

Basic Structural Systems -

-

Basic portal frame system (Also known as a Post and Beam construction) o Wind forces has an effect on the behaviour on the deflection, bending, buckling and compression of the individual and connected members o C= Column o B= Beam o CorT = Compression Tension Spine beam can be thought of as bearers i.e. the main load bearing member which transfer the floor loads onto the upright columns

-

-

Ribs can be thought of as the joists of the structure which transfer the floor loads onto the Spine beams Service ducts are used to transfer services throughout the building without being damaged from crushing or fire etc… A rigid frame in structural engineering is the load-resisting skeleton constructed with straight or curved members interconnected by mostly rigid connections which resist movements induced at the joints of members. Its members can take bending moment, shear, and axial loads A cantilever is a a long projecting beam or girder fixed at only one end.

Classification of Structural Systems

Design Process • • • • • •

Conceptual design-Architect/Engineer Structural systems Loading analysis Structural Analysis Sizing Design documentation

Fabrication Process • • • • • •

Conceptual designArchitect/Engineer Shop detail drawing Member process, cut/mill/dill CNC equipment Welding Post-fabrication – Cleaning – Hot-dip gal – prime

Commonly used Structural Steel Section

SHS = Square hollow section RHS = Rectangular hollow section CHS = Circular hollow section EA = Equal angle UA = Unequal angle Structural Failures Steel is a very strong material and very reliable in structural construction of buildings. Its effectiveness, however, is only guaranteed when the steel is properly designed to withstand the imposed forces. Poor design can lead to the above-mentioned failures of steel structures. Common failure modes of steel structural failures include: - Shear failures

- Compression failures - Tensile failures - Heat induced failures -

Steel degradation / corrosion – A material failure rather than structural Flange local buckling Tear Out - Starts of as bearing failure but ends up as a tear out where the member is ripped off the connection Inadequate weld of bracing Steel fracture due to fatigue Fatigue failure

Performance of Steel vs Timer in fire -

In this American Society for Testing and Materials (ASTM) regulated fire test the steel beam collapsed after 30mins, while the Glulam retained over 70% of its strength. Large wood members have greater resistance to fire than unprotected steel Steel, due to its high thermal conductivity quickly heats up and loses strength during fires Abbreviations in Steel Structures

Tutorial 1 Questions 2. What is cast iron and what is wrought iron? Define broadly their tensile and compressive strengths. How does the carbon content affect these values? -

-

-

Cast iron - iron or a ferrous alloy which has been heated until it liquefies, and is then poured into a mould to solidify. It is usually made from pig iron (intermediate product of smelting iron ore with a high-carbon fuel). Wrought iron - an iron alloy with a very low carbon (0.02% to 0.05%) content in contrast to cast iron (2.25% to 4%), and has fibrous inclusions. The tensile strength of cast iron range from 100MPa to 150MPa) as compared with wrought iron which range from 250MPa to 400MPa. The compressive strength of cast iron range from 400MPa to 1000MPa as compared with wrought iron which range from 250MPa to 400MPa.  he lesser the carbon the better for tensile strength but not in compressive strength.

3. What are the typical carbon content ranges in structural steel or so called “mild steel”? -

Carbon content in structural steel ranges from 0.05 to 0.32%

4. Why is structural steel construction, in particular for building project, defined as “stick” construction? -

It is because of its steel skeleton which is assembled using individual steel frame.

5. Summarise the typical members/components in a regular patterned steel frame building. -

Column Beam Purlins (also type of beam) Tension rods

6. Discuss the importance of lateral bracing in steel frame construction. What are the typical ways of providing the lateral bracing? - Lateral bracing provide support for horizontal movement caused by wind or earthquake. Example of lateral bracing: diagonal tension rods and shear walls (as shown in the figure). 8. What is so called shop drawing? Discuss the importance of shop drawing in the context of structural steel work. - A shop drawing is a drawing or set of drawings produced by the contractor, supplier, manufacturer, subcontractor, or fabricator. Shop drawings are typically required for prefabricated components. Examples of these include elevators, structural steel, trusses, pre-cast, etc. For structural steel works: shop drawings show detailed measurements of members and connections of each component.

9. Summarise the typical construction process in a low-rise structural steel framed building.

Module 2 – Loading on Steel Structures Loads on Steel Structures • • • • • • • •

Dead load Live load Wind load Earthquake load Snow load Impact/explosive load Thermal load Settlement load

Australian Standards 1170 -

AS/NZS 1170.0: 2002 - Structural design actions Part 0 – General Principles Dead and Live load definition Weight of construction system-dead load (DL) Floor UDL live load (as a kPa load) Concentrated live load (as a kN load) for localised effect kPa is a measure of pressure 1 Pascal = 1 newton/ m 2 or 0.1kg/ m 2 1kPa = 100kg/ m 2

 

kPa is a measure of pressure, 1 Pascal = 1 newton/m2 or 0.1kg/m2 One pascal is equivalent to one newton (1 N) of force applied over an area of one meter squared (1 m2). That is, 1 Pa = 1 N. m-2. Reduced to base units in SI, one pascal is 0.1 kilogram per meter per second squared; that is, 1 Pa = 0.1 kg/m2. and 1kPa = 100kg/m2 which is 1000 Pascals

 

Weight of Common Materials • • • • • •

Water has a mass of 1000 kilograms (kg) or one tonne per cubic metre ( m 3 ) Water at 0 °C is 999.972 kg ¿ m 3 Ice = 917 kg ¿ m 3 Wind packed snow = 350 – 400 kg ¿ m 3 Very wet to firm = 600 – 700 kg ¿ m 3 Dry Sand = 1602 kg ¿ m3

• • • • • •

Wet Sand = 1905 kg ¿ m3 Soil = 1200 – 1700 kg ¿ m 3 Unreinforced concrete = 2400 kg ¿ m 3 Reinforced concrete = 2500 kg ¿ m 3 Mild Steel = 7850 kg ¿ m3 Aluminium = 2712 kg ¿ m 3

Dead Loads (DL) Dead load is defined as: • Due to weight (self-weight) of structural system or components which are fixed or permanent on the structure • Concrete, steel, glass, awnings, overhangs, cantilevers etc. Can be evaluated accurately in terms of: • Magnitude and Distribution mass m or short form d ¿ • density ¿ Volume V • To calculate dead load, we need volume and density (unit weight) of structural element Typical Dead Load Values • • • • • •

Unreinforced concrete = 24 kN/m3 Reinforced concrete = 25 kN/m3 Timber = 5 kN/m3 for softwood and 10 kN/m3 for hardwood typical Tiled Roof = 0.9 kPa (kN/m2) Sheet Roof = 0.5 kPa Timber floor system = 0.5 kPa

Live Loads (LL) Live load is defined as: • Result of function of the building or structure • Result of occupants, movable furniture and stacked materials • More uncertainty in terms of value and distribution • Has to be defined by design code and standards • E.g. Australian Standards and NCC/BCA Typical Live Loads • • • • • •

1.5 kPa (kN/m2) for house floor 3 kPa for apartment floor 5 kPa for industrial concrete floor with forklift 10 kPa for industrial floor with pallet racking and forklift Up to 20 kPa for some of the industrial warehouse floor Refer to AS1170.1 for detailed information

Australian Standards 4055 -

AS 4055 – 2012 Wind loads for housing Simplified version of AS1170.2 and easy to use

-

For single dwellings Scale of building is limited Scope of AS/NZS 1170.2 o Structural design actions - Part 2: Wind actions Limited to Heights up to 200 m and Spans up to 100 m i.e. transmission towers, bridges etc.

Wind Load Evaluation Process Need to know the following: 1. Regional wind speed 2. Design wind speed for the site 3. Converting to Design wind pressure 4. Work out wind forces from wind pressure Regional Wind Speed *refer to slide 16, week 2 • •

• • • •

Wind regions are derived from the AS/NZS 1170.2: 2011 Structural Design Actions The region refers to the location within Australia & the terrain category refers to the description of the terrain at the location Classified as A, B, C and D which related to the wind speed in meters per second (m/s) For example: Dubbo is located in the Western Plains area of NSW and has Terrain Category 2 Dubbo is within the regional wind speed category of A 1 Therefore, Dubbo has a Ultimate Design Wind speed of 41m/s or 147.6km/h

Design Wind Speed Design Wind Speed Considerations (>30m/s): • Wind direction • Terrain/height multiplier • Shielding multiplier • Topographic multiplier

Normal Pressure • • •

Wind direction and speed has a positive or negative pressure effect on the building as well as generating uplift forces External pressure is due to wind acting against building due to nature or funnelling effects from other buildings Internal pressure is due to H.V.A.C. systems, fans, air flow within building etc..

Frictional Drag • • • •

Drag or air resistance is a type of friction This is a force acting opposite to the relative motion of any object Drag forces depend on velocity or speed (measured in m/s) in a particular direction The shape of a building or structure determines how much frictional drag is produced

Local Pressure • • • •

Pressure forces on building and structures can be concentrated

This is known as local pressure Influenced by height, width and overall geometry For example, re-entrant corners trap wind forces rather than funnelling away

Determining Wind Classification • • • • • • •

Typical suburban house in Sydney, simplified example: Located in Sydney: Wind region A (AS 4055 Fig 2.1) Typical suburban housing: Terrain Category 3 –TC 3 (AS 4055 Section 2.3) Bottom of a hill: Topographic class T0 (AS 4055 Section 2.4) House is in the middle of the suburb and surrounded by houses: Fully shielded (AS 4055 Section 2.5) Once all the required parameters have been determined using the site conditions AS 4055 Table 2.2 can be utilised to determine the wind classification as N1 (Design speed at ultimate 34 m/s).

Structural Design Limit States • • •

Strength Limit state Serviceability limit state Stability Limit state

Module 3 – Properties of Structural Steel Material properties – Mechanical Properties Mechanical properties are also used to help classify and identify material. The most common properties considered are strength, ductility, hardness, impact resistance, and fracture toughness. Most structural materials are anisotropic, which means that their material properties vary with orientation. -

Strength: Ability to withstand a load without plastic/permanent deformation Stiffness: Resistance to global deformation from a load Toughness: Resistance to fracturing, very closely related/a subset of strength relating to ductility Hardness: Resistance to local plastic deformation from a load Elasticity: Ability for a material to maintain/return to original shape after deformation from a load Ductility: Ability to deform plastically under tensile stress without critical failure such as fracturing Plasticity: The behaviour of a material in deformation beyond elastic limit, i.e deformation that is nonreversible, and will no longer return to its original shape after a load is removed. Weldability: ability for material to be welded without losing integrity – must be metallic obviously Fire Resistance: material resistance to heat caused by chemical fire i.e will not undergo detrimental changes to mechanical properties as a result of intense heat, Corrosion: determination of a material due to chemical reaction with surrounding environment, often leads to inclusion of highly stable i.e non reactive chemical layers such as corrosion resistant paint

-

Fatigue: weakening of a material i.e detrimental changes to material properties, due to cyclic or repeated loading, this can be constant loading and unloading or vibration as rapid loading

Steel – Advantages and Disadvantages Advantages - High strength - Excellent quality control - Predictability - Elasticity - Ductility - Speed of erection - Lightweight - Easily modifiable - Easily recyclable Disadvantages - Needs fireproofing protection - Needs corrosion protection - skilled labour (riggers, welders, CNC) - Temperature effects – thermal effect - Fatigue - Heat loss-good conductor Stress and Strain Definition of Stress and Strain • Stress = force/cross section area

• • • • • •

Has unit of N/mm2 = MPa Indicate force intensity in a local spot Strain = change in length/original length Has no unit (it’s a ratio) Measure the deformation due to load/force/stress Stress and strain are results of force/loads

Grades of Structural Steel • •

• • • • •

Defined by Strength and Ductility Strength is the capacity or resistance ability of a material or a member to support the load/force/stress Grade 250 (MPa) Grade 300 (e.g., One steel’s 300PLUS) Grade 350 Grade 400, 450, 500 for cold formed Grade 500-800 for high strength steel

High Strength Steel Typically defined as yield strength > 500 MPa Between 500-800 MPa For high loading application Weldability is reduced due to the thicker steel sections, therefore takes longer to heat up and may require pre heating to get temperature to weld point - Reduced ductility: The increase in strength and usually sectional sizes leads to a reduction in ductility - Fracture resistance: The increased size leads to a reduction in fracturing Tensile Testing -

• • • •

• •

Typically, Tensile testing measures the tensile (stretchability) strength of a piece of steel. This piece of steel is referred to as a steel specimen (coupon). The specimen is usually a round or flat bar (bottom left image). The specimen is gradually pulled in a testing machine until the specimen breaks. The test measures tensile strength and other properties (necking, elongation, brittleness etc.) Two points, called gauge points, are marked on the central portion.

Typical Mechanical Properties • • • •

Modulus of elasticity, E = 200,000 MPa Yield strength = 250-500 MPa typical Density = 7850 kg ¿ m3 Shear modulus = 80,000 MPa

• • • • • • •

Thermal expansion C = 117.x10-6 per deg Stress (N/mm2=MPa) Strain (non-dimension value) Stress = E times Strain In the linear region only Hooke’s law E is the Young's modulus (=200,000 N/mm2)

Modulus of Elasticity – Relates to the stiffness of the material which defines the relationship between stress and strain (Hooke’s Law). On the stress-strain curve, the stiffness is represented by the slope of the first stage of the curve before the material yields. The slope of this line is defined as the young’s modulus or modulus of elasticity, it contributes to the stiffness of a material. Typically, sharper slope = larger stiffness. E.g. compared with timber, concrete displays a sharper slope and compared with concrete steel is even sharper. Young’s modulus for steel is almost a constant parameter, i.e. no matter how strong or weak the steel, the Young’s modulus does not change much, typically in the order of 200 x10 3 MPa (200 GPa). -

-

-

Sigma p(lower case Greek rho) = Proportionality stress limit – When stress and strain are increasing by the same gradient, i.e. linear region. Yield Strength (sigma subscript y)- The yield point is the point on a stress–strain curve that indicates the limit of elastic behaviour and the beginning of plastic behaviour. Yielding means the start of breaking of fibres. Shear Modulus -In materials science, shear modulus or modulus of rigidity, denoted by G, or sometimes S or μ, is defined as the ratio of shear stress to the shear strain Ultimate Tensile Stress at point of ultimate tensile strength (Sigma subscript T) = maximum stress material can handle before total failure leading to fracture Thermal Expansion - Thermal expansion is the tendency of matter to change in shape, area, and volume in response to a change in temperature. Temperature is a monotonic function of the average molecular kinetic energy of a substance. When a substance is heated, the kinetic energy of its molecules increases Hooke’s Law: Force = elastic constant of material * elastic deformation Epsilon (lower case green e) on x axis: Yield Strain, initial point of Strain hardening, strain at ultimate tensile stress and fracture stress.

Mechanical Properties of High Strength Steel -

-

High-strength low-alloy steel (HSLA) is a type of alloy steel that provides better mechanical properties or greater resistance to corrosion than carbon steel. HSLA steels vary from other steels in that they are not made to meet a specific chemical composition but rather to specific mechanical properties. They have a carbon content between 0.05–0.25% to retain formability and Weldability. Other ...