ICT Role in 21st Century Education & its Challenges.pptx
Manual on building engineering homereading part II
1. Ufa State Petroleum Technological University
Manual on
Building
Engineering
Part II
Арсланова Айгуль Ринатовна
2. Федеральное агентство по образованию
Государственное образовательное учреждение высшего
профессионального образования
«УФИМСКИЙ ГОСУДАРСТВЕННЫЙ НЕФТЯНОЙ
ТЕХНИЧЕСКИЙ УНИВЕРСИТЕТ»
Кафедра иностранных языков
УЧЕБНО-МЕТОДИЧЕСКОЕ ПОСОБИЕ
для внеаудиторного чтения по английскому языку
специальности БПГ
Уфа 2013
4
4. Text 3 for written translation (5 т. зн.)
1. Translate the text paying attention to the following points:
1. The types of construction projects should be considered.
2. Construction processes.
3. What do we mean by the term “Authority having jurisdiction”.
4. Construction careers you would prefer.
Types of construction projects
Each type of construction project requires a unique team to plan, design, construct,
and maintain the project1
. In general, there are three types of construction:
1. Building construction
2. Heavy/civil construction
3. Industrial construction
Building construction
In the fields of architecture and civil engineering, construction is a process that
consists of the building or assembling of infrastructure.
Building construction for several apartment blocks.
The blue material is insulation cladding, which will
be covered later.
Far from being a single activity, large scale
construction is a feat of multitasking2
.
Normally the job is managed by the project
manager and supervised by the construction
manager, design engineer, construction engineer or project architect. For the
1
«содержать проект»
2
«мастерство выполнять множество различных задач»
6
5. successful execution of a project, effective planning is essential. Those involved
with the design and execution of the infrastructure in question must consider the
environmental impact of the job, the successful scheduling, budgeting, site safety,
availability of materials, logistics, inconvenience to the public caused by
construction delays, preparing tender documents, etc.
Building construction is the process of adding structure to real property. The vast
majority of building construction projects are small renovations, such as addition
of a room, or renovation of a bathroom.
A large unfinished building
Often, the owner of the property acts as
labourer, paymaster, and design team for the
entire project. However, all building
construction projects include some elements in
common – design, financial, and legal
considerations. Many projects of varying sizes
reach undesirable end results, such as structural collapse, cost overruns, and/or
litigation reason, those with experience in the field3
make detailed plans and
maintain careful oversight during the project to ensure a positive outcome.
Building construction is procured privately or publicly utilizing various delivery
methodologies, including hard bid, negotiated price, traditional, management
contracting, construction management-at-risk, design & build and design-build
bridging.
Trump International Hotel and Tower (Chicago4
)
3
field experience – производственный опыт; производственная практика
4
[ʃɪ'kɑ:gəu]
7
6. May, 23, 2006
Residential construction practices, technologies, and resources must conform to
local building authority regulations and codes of practice. Materials readily
available in the area generally dictate the construction materials used (e.g. brick
versus stone, versus timber). Cost of construction on a per square metre5
(or per
square foot) basis for houses can vary dramatically based on site conditions, local
regulations, economies of scale (custom designed homes are always more
expensive to build) and the availability of skilled tradespeople.
As residential (as well as all other types of construction) can generate a lot of
waste, careful planning again is needed here.
September 14, 2007
The most popular method of residential construction in
the United States is wood framed construction6
. As
efficiency codes have come into effect in recent years,
new construction technologies and methods have
emerged. University Construction Management
departments are on the cutting edge of the newest
methods of construction intended to improve efficiency,
performance and reduce construction waste.
Industrial construction
Construction of the Havelock City Project in Sri
Lanka.
Industrial construction, though a relatively
small part of the entire construction industry, is
5
on a per square metre – за (один) квадратный метр
6 6
деревянная фахверковая конструкция (фахверк – деревянный брусчатый остов
малоэтажного здания, см.также framework, half-timbering, trelliswork)
8
7. a very important component. Owners of these projects are usually large, for-profit,
industrial corporations. These corporations can be found in such industries as
medicine, petroleum, chemical, power generation, manufacturing, etc. Processes
in these industries require highly specialized expertise in planning, design, and
construction. As in building and heavy/highway construction, this type of
construction requires a team of individuals to ensure a successful project.
Construction processes
Design team
In the modern industrialized world, construction usually involves the translation of
paper or computer based designs7
into reality.
Shasta Dam under construction
The design usually consists of drawings
and specifications, usually prepared by a
design team including the client
architects8
, interior designers, surveyors,
civil engineers, cost engineers (or quantity
surveyors), mechanical engineers, electrical engineers, structural engineers, and
fire protection engineers. The design team is most commonly employed by (i.e. in
contract with) the property owner. Under this system, once the design is completed
by the design team, a number of construction companies or construction
management companies may then be asked to make a bid for the work, either
based directly on the design, or on the basis of drawings and a bill of quantities
7
компьютерная система проектирования (система программ, позволяющая осуществлять
большую часть проектирования в электронном режиме) (cf: CAD (Computer-Aided Design)
система автоматизированного проектирования, САПР)
8
архитектор, работающий с клиентами (сотрудник компании, отвечающий за
индивидуальную работу с клиентами; находится в постоянном контакте с клиентом,
отвечает на его вопросы или консультирует по проблемам, связанным с проектированием)
9
8. provided by a quantity surveyor. Following evaluation of bids, the owner will
typically award a contract to the lowest responsible bidder. The modern trend in
design is toward integration of previously separated specialties, especially among
large firms. In the past, architects, interior designers, engineers, developers,
construction managers, and general contractors were more likely to be entirely
separate companies, even in the larger firms.
Apartment is under construction in Daegu, South Korea.
Presently, a firm that is nominally an "architecture" or "construction management"
firm may have experts from all related fields as employees, or to have an
associated company that provides each necessary skill. Thus, each such firm may
offer itself as "one-stop shopping" for a construction project, from beginning to
end. This is designated as a "design Build9
" contract where the contractor is given
a performance specification, and must
undertake the project from design to
construction, while adhering to the
performance specifications.
Construction of a prefabricated house
Several project structures can
assist the owner in this
integration, including design-
build, partnering, and
construction management. In
general, each of these project structures allows the owner to integrate the services
of architects, interior designers, engineers, and constructors throughout design and
construction. In response, many companies are growing beyond traditional
9
«от проекта до строительства»
10
9. offerings of design or construction services alone, and are placing more emphasis
on establishing relationships with other necessary participants through the design-
build process.
The increasing complexity of construction projects creates the need for design
professionals trained in all phases of the project's life-cycle and develop an
appreciation of the building as an advanced technological system requiring close
integration of many sub-systems and their individual components, including
sustainability. Building engineering is an emerging discipline that attempts to meet
this new challenge.
Construction on a building in Kansas City
During the planning of a building, the
zoning and planning boards of the
governmental agency or sub-agency
which regulates the construction process
will review the overall compliance of the
proposed building with the municipal
General Plan and zoning regulations.
Once the proposed building has been
approved, detailed civil, architectural, and structural plans must be submitted to the
municipal building department (and sometimes the public works department) to
determine compliance with the building code and sometimes for fit with existing
infrastructure. Often, the municipal fire department will review the plans for
compliance with fire-safety ordinances and regulations.
Before the foundation can be dug, contractors are typically required to notify
utility companies, either directly or through a company such as Dig Safe to ensure
that underground utility lines can be marked. This lessens the likelihood of damage
to the existing electrical, water, sewage, phone, and cable facilities, which could
cause outages and potentially hazardous situations. During the construction of a
building, the municipal building inspector inspects the building periodically to
ensure that the construction adheres to the approved plans and the local building
code. Once construction is complete and a final inspection has been passed, an
occupancy permit may be issued. Changes made to a building that affect safety,
11
10. including its use, expansion, structural integrity, and fire protection items, usually
require approval of the AHJ for review concerning the building code.
Construction careers
Ironworkers erecting the steel frame of a new building,
at the Massachusetts General Hospital, USA
There are many routes to the different careers
within the construction industry which vary by
country. However, there are three main tiers of
careers based on educational background which
are common internationally:
Unskilled and Semi-Skilled – General site
labour with little or no construction
qualifications.
Skilled On-site managers whom possess
extensive knowledge and experience in their craft or profession.
Technical and Management – Personnel with the greatest educational
qualifications, usually graduate degree10
s, trained to design, manage and
instruct the construction process.
Skilled occupations in the UK require Further Education qualifications, often in
vocational subject areas. These qualifications are either obtained directly after the
completion of compulsory education or through "on the job" apprenticeship
training. In the UK, 8500 construction-related apprenticeships were commenced in
2007.
Technical and specialised occupations require more training as a greater technical
knowledge is required. These professions also hold more legal responsibility. A
short list of the main careers with an outline of the educational requirements are
given below:
Architect – Typically holds at least a 4-year degree in architecture. To use
the title "architect" the individual must hold chartered status with the Royal
Institute of British Architects and be on the Architects Registration Board.
Civil Engineer – Typically holds a degree in a related subject. The
Chartered Engineer qualification is controlled by the Institution of Civil
Engineers. A new university graduate must hold a masters degree to
10
степень выпускника учебного заведения
12
11. become chartered, persons with bachelors degrees may become an
Incorporated Engineer.
Building Services Engineer – Often referred to as an "M&E Engineer"
typically holds a degree in mechanical or electrical engineering. Chartered
Engineer status is governed by the Chartered Institution of Building Services
Engineers.
Project Manager – Typically holds a 2-year or greater higher education
qualification, but are often also qualified in another field such as quantity
surveying or civil engineering.
Quantity Surveyor – Typically holds a masters degree in quantity surveying.
Chartered status is gained from the Royal Institute of Chartered Surveyors.
Structural Engineer – Typically holds a bachelors or masters degree in
structural engineering, new university graduates must hold a masters degree
to gain chartered status from the Institution of Structural Engineers.
The construction industry in the United States provides employment to millions
with all types and levels of education. Construction contributes 14% of the United
States Gross National Product. Construction engineering provides much of the
design aspect used both in the construction office and in the field on project sites.
To complete projects construction engineers rely on plans and specifications
created by architects, engineers and other constructors. During most of the 20th
century structures have been first designed then engineering staff ensure it is built
to plans and specifications by testing and overseeing the construction. Previous to
the 20th century and more commonly since the start of the 21st century structures
are designed and built in combination, allowing for site considerations and
construction methods to influence the design process.
Construction engineers have a wide range of responsibilities. Typically entry level
construction engineers analyze reports and estimate project costs both in the office
and in the field. Other tasks may include: analyzing maps, drawings, blueprints,
aerial photography and other topographical information. Construction engineers
also have to use computer software to design hydraulic systems and structures
while following construction codes. Keeping a workplace safe is key to having a
successful construction company. It is the construction engineer's job to make
sure11
that everything is conducted correctly. In addition to safety, the construction
engineer has to make sure that the site stays clean and sanitary. They have to make
sure that there are no impediments in the way of the structure's planned location
and must move any that exist. Finally, more seasoned construction engineers will
11
«именно работой инженера-строителя является убедиться, что…»
13
12. assume the role of project management on a construction site and are involved
heavily with the construction schedule and document control as well as budget and
cost control. Their role on site is to provide construction information, including
repairs, requests for information, change orders and payment applications to the
managers and/or the owner's representatives.
Construction engineers should have strong understanding for math and science, but
many other skills are required, including critical thinking, listening, learning,
problem solving, monitoring and decision making. Construction engineers have to
be able to think about all aspects of a problem and listen to other’s ideas so that
they can learn everything about a project before it begins. During project
construction they must solve the problems that they encounter using math and
science. Construction Engineers must maintain project control of labor and
equipment for safety, to ensure the project is on schedule and monitor quality
control. When a problem occurs it is the construction engineer who will create and
enact a solution.
Construction engineers need different abilities to do their job. They must have the
ability to reason, convey instructions to others, comprehend multi variables,
anticipate problems, comprehend verbal, written and graphic instructions, organize
data sets, speak clearly, visualize in 4D time-space and understand Virtual Design
and Construction methods.
Text 4 (20 т.зн.)
1. Read the text and answer the questions:
1. What is a structural engineer?
2. Which structural elements are usually used in construction?
3. What does the structural engineering theory perceives?
4. What are common building materials?
1. Structural engineer
Structural engineers are responsible for engineering design and analysis. Entry-
level structural engineers may design the individual structural elements of a
structure, for example the beams, columns, and floors of a building. More
experienced engineers would be responsible for the structural design and integrity
of an entire system, such as a building. Structural engineers often specialise in
particular fields, such as bridge engineering, building engineering, pipeline
engineering, industrial structures or special structures such as vehicles or aircraft.
14
13. Structural engineering has existed since humans first started to construct their own
structures. It became a more defined and formalised profession with the emergence
of the architecture profession as distinct from the engineering profession during the
industrial revolution in the late 19th Century. Until then, the architect and the
structural engineer were often one and the same – the master builder. Only with the
understanding of structural theories that emerged during the 19th and 20th century
did the professional structural engineer come into existence. The role of a
structural engineer today involves a significant understanding of both static and
dynamic loading, and the structures that are available to resist them. The
complexity of modern structures often requires a great deal of creativity from the
engineer in order to ensure the structures support and resist the loads they are
subjected to. A structural engineer will typically have a four or five year
undergraduate degree, followed by a minimum of three years of professional
practice before being considered fully qualified. Structural engineers are licensed
or accredited by different learned societies and regulatory bodies around the world
(for example, the Institution of Structural Engineers in the UK). Depending on the
degree course they have studied and/or the jurisdiction they are seeking licensure
in, they may be accredited (or licensed) as just structural engineers, or as civil
engineers, or as both civil and structural engineers.
Civil engineering structures
The Millau Viaduct in France, designed by Michel
Virlogeux with Foster & Partners
Civil structural engineering includes all structural
engineering related to the built environment. It
includes:
Bridges
Dams
Earthworks
Foundations
Offshore structures
Pipelines
Power stations
Railways
Retaining structures and walls
Roads
Tunnels
Waterways
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14. The structural engineer is the lead designer on these structures, and often the sole
designer. In the design of structures such as these, structural safety is of paramount
importance (in the UK, designs for dams, nuclear power stations and bridges must
be signed off by a chartered engineer).
Civil engineering structures are often subjected to very extreme forces, such as
large variations in temperature, dynamic loads such as waves or traffic, or high
pressures from water or compressed gases. They are also often constructed in
corrosive environments, such as at sea, in industrial facilities or below ground.
An Airbus A380, the world's largest passenger
airliner
The design of static structures assumes they
always have the same geometry (in fact, so-
called static structures can move
significantly, and structural engineering
design must take this into account where necessary), but the design of moveable or
moving structures must account for fatigue, variation in the method in which load
is resisted and significant deflections of structures.
The forces which parts of a machine are subjected to can vary significantly, and
can do so at a great rate. The forces which a boat or aircraft are subjected to vary
enormously and will do so thousands of times over the structure's lifetime. The
structural design must ensure that such structures are able to endure such loading
for their entire design life without failing.
These works can require mechanical structural engineering:
Airframes and fuselages
Boilers and pressure vessels
Coachworks and carriages
Cranes
Elevators
Escalators
Marine vessels and hulls
2. Structural elements
16
15. A statically
determinate simply
supported beam,
bending under an
evenly distributed
load.
Any
structure is essentially made up of only a small
number of different types of elements:
Columns
Beams
Plates
Arches
Shells
Catenaries
Many of these elements can be classified according to form (straight, plane / curve)
and dimensionality (one-dimensional / two-dimensional):
One-dimensional Two-dimensional
straight curve plane curve
(predominantly) bending beam
continuous
arch
plate, concrete
slab
lamina,
dome
(predominant) tensile
stress
rope Catenary shell
(predominant)
compression
pier, column Load-bearing wall
Columns
Columns are elements that carry only axial force – either tension or compression –
or both axial force and bending (which is technically called a beam-column but
practically, just a column). The design of a column must check the axial capacity
of the element, and the buckling capacity. The buckling capacity is the capacity of
the element to withstand the propensity to buckle. Its capacity depends upon its
geometry, material, and the effective length of the column, which depends upon
the restraint conditions at the top and bottom of the column. The effective length is
K * l *where K is the real length of the column/. The capacity of a column to carry
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16. axial load depends on the degree of bending it is subjected to, and vice versa. This
is represented on an interaction chart and is a complex non-linear relationship.
Beams
A beam may be:
cantilevered (supported at one end only with a fixed connection)
simply supported (supported vertically at each end; horizontally on only one
to withstand friction, and able to rotate at the supports)
continuous (supported by three or more supports)
a combination of the above (ex. supported at one end and in the middle)
Beams are elements which carry pure bending only. Bending causes one section of
a beam (divided along its length) to go into compression and the other section into
tension. The compression section must be designed to resist buckling and crushing,
while the tension section must be able to adequately resist the tension.
Struts and ties
Little Belt: a truss bridge in Denmark
A truss is a structure comprising two types of
structural element, ie struts and ties. A strut is a
relatively lightweight column and a tie is a
slender element designed to withstand tension
forces. In a pin-jointed truss (where all joints
are essentially hinges), the individual elements
of a truss theoretically carry only axial load. From experiments it can be shown
that even trusses with rigid joints will behave as though the joints are
pinned.Trusses are usually utilised to span large distances, where it would be
uneconomical and unattractive to use solid beams.
The McDonnell Planetarium by Gyo Obata in St
Louis, Missouri, USA, a concrete shell structure
18
17. A masonry arch
1. Keystone
2. Voussoir
3. Extrados
4. Impost
5. Intrados
6. Rise
7. Clear span
8. Abutment
Plates
Plates carry bending in two directions. A concrete flat slab is an example of a plate.
Plates are understood by using continuum mechanics, but due to the complexity
involved they are most often designed using a codified empirical approach, or
computer analysis. They can also be designed with yield line theory, where an
assumed collapse mechanism is analysed to give an upper bound on the collapse
load (see Plasticity). This is rarely used in practice.
Shells
Shells derive their strength from their form, and carry forces in compression in two
directions. A dome is an example of a shell. They can be designed by making a
hanging-chain model, which will act as a catenary in pure tension, and inverting
the form to achieve pure compression.
Arches
Arches carry forces in compression in one direction only, which is why it is
appropriate to build arches out of masonry. They are designed by ensuring that the
line of thrust of the force remains within the depth of the arch.
Catenaries
Catenaries derive their strength from their form, and carry transverse forces in
pure tension by deflecting (just as a tightrope will sag when someone walks on it).
They are almost always cable or fabric structures. A fabric structure acts as a
catenary in two directions.
19
18. 3. Structural engineering theory
Figure of a bolt in shear. Top figure illustrates single shear,
bottom figure illustrates double shear.
Structural engineering depends upon a detailed
knowledge of loads, physics and materials to
understand and predict how structures support and
resist self-weight and imposed loads. To apply the
knowledge successfully a structural engineer will
need a detailed knowledge of mathematics and of
relevant empirical and theoretical design codes. He
will also need to know about the corrosion resistance of the materials and
structures, especially when those structures are exposed to the external
environment. The criteria which govern the design of a structure are either
serviceability (criteria which define whether the structure is able to adequately
fulfill its function) or strength (criteria which define whether a structure is able to
safely support and resist its design loads). A structural engineer designs a structure
to have sufficient strength and stiffness to meet these criteria.
Loads imposed on structures are supported by means of forces transmitted through
structural elements. These forces can manifest themselves as:
tension (axial force)
compression (axial force)
shear
bending, or flexure (a bending moment is a force multiplied by a distance, or
lever arm, hence producing a turning effect or torque)
Loads
Some Structural loads on structures can be classified as live (imposed) loads, dead
loads, earthquake (seismic) loads, wind loads, soil pressure loads, fluid pressure
loads, impact loads, and vibratory loads. Live loads are transitory or temporary
loads, and are relatively unpredictable in magnitude. They may include the weight
of a building's occupants and furniture, and temporary loads the structure is
subjected to during construction. Dead loads are permanent, and may include the
weight of the structure itself and all major permanent components. Dead load may
also include the weight of the structure itself supported in a way it wouldn't
normally be supported, for example during construction.
20
19. Strength
Strength depends upon material properties. The strength of a material depends on
its capacity to withstand axial stress, shear stress, bending, and torsion. The
strength of a material is measured in force per unit area (newtons per square
millimetre or N/mm², or the equivalent megapascals or MPa in the SI system and
often pounds per square inch psi in the United States Customary Units system).
A structure fails the strength criterion when the stress (force divided by area of
material) induced by the loading is greater than the capacity of the structural
material to resist the load without breaking, or when the strain (percentage
extension) is so great that the element no longer fulfills its function (yield).
Stiffness
Stiffness depends upon material properties and geometry. The stiffness of a
structural element of a given material is the product of the material's Young's
modulus and the element's second moment of area. Stiffness is measured in force
per unit length (newtons per millimetre or N/mm), and is equivalent to the 'force
constant' in Hooke's Law. The deflection of a structure under loading is dependent
on its stiffness. The dynamic response of a structure to dynamic loads (the natural
frequency of a structure) is also dependent on its stiffness. In a structure made up
of multiple structural elements where the surface distributing the forces to the
elements is rigid, the elements will carry loads in proportion to their relative
stiffness - the stiffer an element, the more load it will attract. In a structure where
the surface distributing the forces to the elements is flexible (like a wood framed
structure), the elements will carry loads in proportion to their relative tributary
areas. A structure is considered to fail the chosen serviceability criteria if it is
insufficiently stiff to have acceptably small deflection or dynamic response under
loading. The inverse of stiffness is the flexibility.
Safety factors
The safe design of structures requires a design approach which takes account of the
statistical likelihood of the failure of the structure. Structural design codes are
based upon the assumption that both the loads and the material strengths vary with
a normal distribution. The job of the structural engineer is to ensure that the chance
of overlap between the distribution of loads on a structure and the distribution of
material strength of a structure is acceptably small (it is impossible to reduce that
chance to zero). It is normal to apply a partial safety factor to the loads and to the
21
20. material strengths, to design using 95th percentiles (two standard deviations from
the mean). The safety factor applied to the load will typically ensure that in 95% of
times the actual load will be smaller than the design load, while the factor applied
to the strength ensures that 95% of times the actual strength will be higher than the
design strength. The safety factors for material strength vary depending on the
material and the use it is being put to and on the design codes applicable in the
country or region.
Load cases
A load case is a combination of different types of loads with safety factors applied
to them. A structure is checked for strength and serviceability against all the load
cases it is likely to experience during its lifetime.
Typical load cases for design for strength (ultimate load cases; ULS) are:
1.4 x Dead Load + 1.6 x Live Load
1.2 x Dead Load + 1.2 x Live Load + 1.2 x Wind Load
A typical load case for design for serviceability (characteristic load cases; SLS) is:
1.0 x Dead Load + 1.0 x Live Load
Different load cases would be used for different loading conditions. For example,
in the case of design for fire a load case of 1.0 x Dead Load + 0.8 x Live Load may
be used, as it is reasonable to assume everyone has left the building if there is a
fire. In multi-story buildings it is normal to reduce the total live load depending on
the number of stories being supported, as the probability of maximum load being
applied to all floors simultaneously is negligibly small. It is not uncommon for
large buildings to require hundreds of different load cases to be considered in the
design.
Newton's laws of motion
The most important natural laws for structural engineering are Newton's Laws of
Motion. Newton's first law states that every body perseveres in its state of being at
rest or of moving uniformly straight forward, except insofar as it is compelled to
change its state by force impressed. Newton's second law states that the rate of
change of momentum of a body is proportional to the resultant force acting on the
22
21. body and is in the same direction. Mathematically, F=ma (force = mass x
acceleration). Newton's third law states that all forces occur in pairs, and these two
forces are equal in magnitude and opposite in direction. With these laws it is
possible to understand the forces on a structure and how that structure will resist
them. The Third Law requires that for a structure to be stable all the internal and
external forces must be in equilibrium. This means that the sum of all internal and
external forces on a free-body diagram must be zero:
: the vectorial sum of the forces acting on the body equals zero.
This translates to
Σ H = 0: the sum of the horizontal components of the forces equals zero;
Σ V = 0: the sum of the vertical components of forces equals zero;
: the sum of the moments (about an arbitrary point) of all forces
equals zero.
Statical determinacy
A structural engineer must understand the internal and external forces of a
structural system consisting of structural elements and nodes at their intersections.
A statically determinate structure can be fully analysed using only consideration of
equilibrium, from Newton's Laws of Motion. A statically indeterminate structure
has more unknowns than equilibrium considerations can supply equations for (see
simultaneous equations). Such a system can be solved using consideration of
equations of compatibility between geometry and deflections in addition to
equilibrium equations, or by using virtual work. If a system is made up of b bars, j
pin joints and r support reactions, then it cannot be statically determinate if the
following relationship does not hold:
r + b = 2j
It should be noted that even if this relationship does hold, a structure can be
arranged in such a way as to be statically indeterminate.
Elasticity
Much engineering design is based on the assumption that materials behave
elastically. For most materials this assumption is incorrect, but empirical evidence
23
22. has shown that design using this assumption can be safe. Materials that are elastic
obey Hooke's Law, and plasticity does not occur.
For systems that obey Hooke's Law, the extension produced is directly
proportional to the load:
where
x is the distance that the spring has been stretched or compressed away from the
equilibrium position, which is the position where the spring would naturally come
to rest [usually in meters],
F is the restoring force exerted by the material [usually in newtons], and
k is the force constant (or spring constant). This is the stiffness of the spring. The
constant has units of force per unit length (usually in newtons per metre)
Plasticity
Comparison of Tresca and Von Mises Criteria
Some design is based on the assumption that
materials will behave plastically.[27]
A plastic
material is one which does not obey Hooke's
Law, and therefore deformation is not
proportional to the applied load. Plastic
materials are ductile materials. Plasticity theory
can be used for some reinforced concrete
structures assuming they are underreinforced,
meaning that the steel reinforcement fails before
the concrete does. Plasticity theory states that the point at which a structure
collapses (reaches yield) lies between an upper and a lower bound on the load,
defined as follows:
If, for a given external load, it is possible to find a distribution of moments
that satisfies equilibrium requirements, with the moment not exceeding the
yield moment at any location, and if the boundary conditions are satisfied,
then the given load is a lower bound on the collapse load.
24
23. If, for a small increment of displacement, the internal work done by the
structure, assuming that the moment at every plastic hinge is equal to the
yield moment and that the boundary conditions are satisfied, is equal to the
external work done by the given load for that same small increment of
displacement, then that load is an upper bound on the collapse load.
If the correct collapse load is found, the two methods will give the same result for
the collapse load.
Plasticity theory depends upon a correct understanding of when yield will occur. A
number of different models for stress distribution and approximations to the yield
surface of plastic materials exist
Mohr's circle
Von Mises yield criterion
Henri Tresca
The Euler-Bernoulli beam equation
The Euler-Bernoulli beam equation defines the behaviour of a beam element (see
below). It is based on five assumptions:
(1) continuum mechanics is valid for a bending beam
(2) the stress at a cross section varies linearly in the direction of bending, and is
zero at the centroid of every cross section.
(3) the bending moment at a particular cross section varies linearly with the second
derivative of the deflected shape at that location.
(4) the beam is composed of an isotropic material
(5) the applied load is orthogonal to the beam's neutral axis and acts in a unique
plane.
A simplified version of Euler-Bernoulli beam equation is:
Here u is the deflection and w(x) is a load per unit length. E is the elastic modulus
and I is the second moment of area, the product of these giving the stiffness of the
beam.
This equation is very common in engineering practice: it describes the deflection
of a uniform, static beam.
25
24. Successive derivatives of u have important meaning:
is the deflection.
is the slope of the beam.
is the bending moment in the beam.
is the shear force in the beam.
A bending moment manifests itself as a tension and a compression force, acting as
a couple in a beam. The stresses caused by these forces can be represented by:
where σ is the stress, M is the bending moment, y is the distance from the neutral
axis of the beam to the point under consideration and I is the second moment of
area. Often the equation is simplified to the moment divided by the section
modulus (S), which is I/y. This equation allows a structural engineer to assess the
stress in a structural element when subjected to a bending moment.
Buckling
A column under a centric axial load exhibiting the characteristic
deformation of buckling.
When subjected to compressive forces it is possible for
structural elements to deform significantly due to the
destabilising effect of that load. The effect can be initiated
or exacerbated by possible inaccuracies in manufacture or
construction.
Lateral-torsional buckling of an aluminium alloy plate girder
designed and built by students at Imperial College London.
The Euler buckling formula defines the axial
compression force which will cause a strut (or
column) to fail in buckling.
26
25. where
F = maximum or critical force (vertical load on column),
E = modulus of elasticity,
I = area moment of inertia, or second moment of area
l = unsupported length of column,
K = column effective length factor, whose value depends on the conditions of end
support of the column, as follows.
For both ends pinned (hinged, free to rotate), K = 1.0.
For both ends fixed, K = 0.50.
For one end fixed and the other end pinned, 0.70.
For one end fixed and the other end free to move laterally, K = 2.0. This value is
sometimes expressed for design purposes as a critical buckling stress.
where
σ = maximum or critical stress
r = the least radius of gyration of the cross section
Other forms of buckling include lateral torsional buckling, where the compression
flange of a beam in bending will buckle, and buckling of plate elements in plate
girders due to compression in the plane of the plate.
4. Materials
Stress-strain curve for low-
carbon steel. Hooke's law
27
26. (see above) is only valid for the portion of the curve between
the origin and the yield point.
1. Ultimate strength
2. Yield strength-corresponds to yield point.
3. Rupture
4. Strain hardening region
5. Necking region.
A: Apparent stress (F/A0)
B: Actual stress (F/A)
Structural engineering depends on the knowledge of materials and their properties,
in order to understand how different materials support and resist loads.
Common structural materials are:
Wrought iron
Wrought iron is the simplest form of iron, and is almost pure iron (typically less
than 0.15% carbon). It usually contains some slag. Its uses are almost entirely
obsolete, and it is no longer commercially produced. Wrought iron is very poor in
fires. It is ductile, malleable and tough. It does not corrode as easily as steel.
Cast iron
Cast iron is a brittle form of iron which is weaker in tension than in compression. It
has a relatively low melting point, good fluidity, castability, excellent
machinability and wear resistance. Though almost entirely replaced by steel in
building structures, cast irons have become an engineering material with a wide
range of applications, including pipes, machine and car parts. Cast iron retains high
strength in fires, despite its low melting point. It is usually around 95% iron, with
between 2.1-4% carbon and between 1-3% silicon. It does not corrode as easily as
steel.
Steel
The 630 foot (192 m) high, stainless-clad (type 304) Gateway Arch
in Saint Louis, Missouri
28
27. Steel is a iron alloy with between 0.2 and 1.7% carbon. Steel is used extremely
widely in all types of structures, due to its relatively low cost, high strength to
weight ratio and speed of construction. Steel is a ductile material, which will
behave elastically until it reaches yield (point 2 on the stress-strain curve), when it
becomes plastic and will fail in a ductile manner (large strains, or extensions,
before fracture at point 3 on the curve). Steel is equally strong in tension and
compression.
Steel is weak in fires, and must be protected in most buildings. Because of its high
strength to weight ratio, steel buildings typically have low thermal mass, and
require more energy to heat (or cool) than similar concrete buildings. The elastic
modulus of steel is approximately 205 GPa. Steel is very prone to corrosion (rust).
Stainless steel
Stainless steel is an iron-carbon alloy with a minimum of 10.5% chromium
content. There are different types of stainless steel, containing different proportions
of iron, carbon, molybdenum, nickel. It has similar structural properties to steel,
although its strength varies significantly.
It is rarely used for primary structure, and more for architectural finishes and
building cladding.
It is highly resistant to corrosion and staining.
Concrete
The interior of the Sagrada Familia, constructed of
reinforced concrete to a design by Gaudi
Concrete is used extremely widely in
building and civil engineering structures,
due to its low cost, flexibility, durability,
and high strength. It also has high
resistance to fire.
A "cage" of reinforcing steel
29
28. Concrete is a brittle material and it is strong in compression and very weak in
tension. It behaves non-linearly at all times. Because it has essentially zero strength
in tension, it is almost always used as reinforced concrete, a composite material. It
is a mixture of sand, aggregate, cement and water. It is placed in a mould, or form,
as a liquid, and then it sets (goes off), due to a chemical reaction between the water
and cement. The hardening of the concrete is called curing. The reaction is
exothermic (gives off heat).
Concrete increases in strength continually from the day it is cast. Assuming it is
not cast under water or in constantly 100% relative humididy, it shrinks over time
as it dries out, and it deforms over time due to a phenomenon called creep. Its
strength depends highly on how it is mixed, poured, cast, compacted, cured (kept
wet while setting), and whether or not any admixtures were used in the mix. It can
be cast into any shape that a form can be made for. Its colour, quality, and finish
depend upon the complexity of the structure, the material used for the form, and
the skill of the worker.
Concrete is a non-linear, non-elastic material, and will fail suddenly, with a brittle
failure, unless adequate reinforced with steel. An "under-reinforced" concrete
element will fail with a ductile manner, as the steel will fail before the concrete. An
"over-reinforced" element will fail suddenly, as the concrete will fail first.
Reinforced concrete elements should be designed to be under-reinforced so users
of the structure will receive warning of impending collapse. This is a technical
term. Reinforced concrete can be designed without enough reinforcing. A better
term would be properly reinforced where the member can resist all the design loads
adequately and it is not over-reinforced. The elastic modulus of concrete can vary
widely and depends on the concrete mix, age, and quality, as well as on the type
and duration of loading applied to it. It is usually taken as approximately 25 GPa
for long-term loads once it has attained its full strength (usually considered to be at
28 days after casting). It is taken as approximately 38 GPa for very short-term
loading, such as footfalls. Concrete has very favourable properties in fire - it is not
adversely affected by fire until it reaches very high temperatures. It also has very
high mass, so it is good for providing sound insulation and heat retention (leading
to lower energy requirements for the heating
of concrete buildings). This is offset by the
fact that producing and transporting concrete
is very energy intensive.
Aluminium
30
29. Stress vs. Strain curve typical of aluminum
1. Ultimate strength
2. Yield strength
3. Proportional Limit Stress
4. Rupture
5. Offset strain (typically 0.002).
Aluminium is a soft, lightweight, malleable metal. The yield strength of pure
aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from
200 MPa to 600 MPa. Aluminium has about one-third the density and stiffness of
steel. It is ductile, and easily machined, cast, and extruded. Corrosion resistance is
excellent due to a thin surface layer of aluminium oxide that forms when the metal
is exposed to air, effectively preventing further oxidation. The strongest aluminium
alloys are less corrosion resistant due to galvanic reactions with alloyed copper.
Aluminium is used in some building structures (mainly in facades) and very widely
in aircraft engineering because of its good strength to weight ratio. It is a relatively
expensive material.
In aircraft it is gradually being replaced by carbon composite materials.
Composites
Light composite aircraft
Composite materials are used
increasingly in vehicles and aircraft
structures, and to some extent in other
structures. They are increasingly used
in bridges, especially for conservation
of old structures such as Coalport cast
iron bridge built in 1818. Composites
are often anisotropic (they have
different material properties in different directions) as they can be laminar
materials. They most often behave non-linearly and will fail in a brittle manner
when overloaded. They provide extremely good strength to weight ratios, but are
also very expensive. The manufacturing processes, which are often extrusion, do
not currently provide the economical flexibility that concrete or steel provide. The
most commonly used in structural applications are glass-reinforced plastics.
31
30. Masonry
A brick wall built using Flemish Bond
Masonry has been used in structures
for hundreds of years, and can take
the form of stone, brick or
blockwork. Masonry is very strong
in compression but cannot carry
tension (because the mortar between
bricks or blocks is unable to carry
tension). Because it cannot carry structural tension, it also cannot carry bending, so
masonry walls become unstable at relatively small heights. High masonry
structures require stabilisation against lateral loads from buttresses (as with the
flying buttresses seen in many European medieval churches) or from windposts.
Historically masonry was constructed with no mortar or with lime mortar. In
modern times cement based mortars are used. The mortar glues the blocks
together, and also smoothes out the interface between the blocks, avoiding
localised point loads that might have led to cracking.
Since the widespread use of concrete, stone is rarely used as a primary structural
material, often only appearing as a cladding, because of its cost and the high skills
needed to produce it. Brick and concrete blockwork have taken its place.
Masonry, like concrete, has good sound insulation properties and high thermal
mass, but is generally less energy intensive to produce. It is just as energy intensive
as concrete to transport.
Timber
The reconstructed Globe Theatre, London,
by Buro Happold
Timber is the oldest of structural
materials, and though mainly
supplanted by steel, masonry and
concrete, it is still used in a
significant number of buildings. The
properties of timber are non-linear
and very variable, depending on the
32
31. quality, treatment of wood, and type of wood supplied. The design of wooden
structures is based strongly on empirical evidence. Wood is strong in tension and
compression, but can be weak in bending due to its fibrous structure. Wood is
relatively good in fire as it chars, which provides the wood in the centre of the
element with some protection and allows the structure to retain some strength for a
reasonable length of time.
Bamboo scaffolding can reach great heights.
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