Department of Civil Engineering
University of Engineering & Technology, Taxila
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Design of Steel Structures
Course Name: Design of Steel Structures
Course ID: 306
Lecture Days: Section - B Monday (3-7)
Section - A Tuesday (3-7)
Text Books: • STEEL STRUCTURES by Prof. Dr. Zahid Ahmad Siddiqi
• LRFD Steel Design Aids by Prof. Dr. Zahid Ahmad Siddiqi
• AISC 2005 Specifications (free on web site www.aisc.org
)
Instructor:
Reference Books:
STRUCTURAL STEEL DESIGN LRFD Method by Jack C. McCormac
STEEL STRUCTURES Design & Behavior by Charles G. Salmon & John E. Johnson
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Assignment
Quiz
Sessional
Marks
Attendance
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Design of Steel Structures
Chapter – 1
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1.1 Introduction to Steel Structures
•
Steel structures are assembly of structural steel shapes joined together by means of riveted / bolted or welded connections.
•
Majority of concrete structures are cast in-situ but in steel structures, we have to select out of those available in the market.
•
Joints are monolithic in concrete structures whereas in steel structures special methods are required to join individual members.
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Steel Shapes
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Connections
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Bolted Connections
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Riveted Connections
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Simple & Moment
Resisting Joints
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In steel structures, connections including the details are to be designed for expected forces.
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Steel construction is being used for almost every type of structure including high-rise buildings, bridges, industrial building, towers etc.
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There are main two categories of steel structures:-
–
Framework or Skeletal Systems
– Shell Systems
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Framework or Skeletal Systems
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The main load carrying elements are one-dimensional or line elements (such as beams, columns, etc.) forming two-dimensional or three-dimensional frames.
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Examples are:-
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The frameworks of industrial buildings with their internal members such as crane girders, platforms, etc.
–
Highway and railways large span bridges.
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Industrial Building
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•
Multi-storey buildings, large halls, domes etc.
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Towers, poles, structural components of hydraulic works.
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All other trusses and rigidly connected frame structures.
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Shell Systems
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The main load carrying elements in this category of structures are plates and sheets.
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Examples are:-
–
Gas tanks for storage and distribution of gases.
–
Tanks and reservoirs for storage of liquids.
–
Bins and bunkers for storage of loose material.
–
Special structures such as blast furnaces, air heaters, etc.
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–
Large diameter pipes.
–
All other plate and shell structures.
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Large Diameter Pipe
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Shell Structures
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Frame & Shell Structures
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Merits of Steel Construction
1. Reliability
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Consistency and uniformity in properties.
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Better quality control due to factory manufacture.
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Large elasticity and ductility.
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Truly homogeneous.
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Satisfies most of the analysis & design assumptions.
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2. Industrial Behavior
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Rolled steel sections are manufactured in factories.
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Also, the members may be cut & prepared for assembly in factories while only joining is carried out at site.
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Sometimes parts are also assembled in the factories, that is, there is great adaptation to prefabrication.
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Erection
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Hot Rolled Steel
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Hot Rolled Steel
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Hot Rolled Steel
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Assembling in factory
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3. Lesser Construction Time
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Because of its industrial nature, progress of work is very fast resulting in economical structures.
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The reason is that these structures can be put to use earlier.
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The reduction in labor cost & overhead charges and the benefits obtained from the early use of the building contribute to economy.
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4. High Strength & Light Weight Nature
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It means that the dead load will be smaller.
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Dead loads are bigger part of the total structure load.
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If dead load reduces, the resulting member will be smaller.
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This fact is important for long span bridges, tall buildings & for structures having poor foundations.
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•
A factor (C) defined as the ratio of the density of the material ( γ) to the stress it can carry (ƒ) is one of the least for steel.
Material
Aluminum
Steel
Wood
Concrete
C= γ/ƒ (m -1 )
1.1 x 10 -4
3.2 x 10 -4
4.5 x 10 -4
24.0 x 10 -4
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5. Uniformity, Durability & Performance
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Durability means long life of structures.
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Steel is a homogeneous & uniform material.
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Satisfies the basic assumptions of analysis & design.
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If properly maintained with painting, etc., the properties of steel do not change appreciably with time.
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6. Elasticity
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Steel behaves closer to design assumptions than most of the other materials because it follows Hook’s
Law up to fairly high stresses.
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The stress produced remains proportional to the strain applied or stress-strain diagram remains a straight line.
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The steel sections do not crack or tear before ultimate load and hence the moment of inertia of a steel structure can be definitely calculated.
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7. Ductility & Warning before Failure
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The property of a material by which it can withstand extensive deformation without failure under high tensile stresses is said to be its ductility .
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Mild steel is a very ductile material. The percentage elongation of a standard tension test specimen after fracture can be as high as 25 to 30%.
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This gives visible deflections or evidence of impending failure in case of overloads.
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The extra loads may be removed to prevent collapse.
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Even if collapse occurs, time is available for occupants to vacate the building.
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In structural members under normal loads, high stress concentrations develop at various points.
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The ductile nature of the structural steels enable them to yield locally at those points, thus redistributing the stresses and preventing premature failure.
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8. Additions to Existing Structures
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Additions to existing steel structures are very easy to be made.
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Connections between new and existing structures can be employed very effectively.
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9. Possible Reuse
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Steel sections can be reused after a structure is disassembled.
10. Scrap Value
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Steel has a scrap value even though it is not reuseable in its existing form.
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Steel Scrap
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11. Water-Tight & Air-Tight
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Steel structures provide completely impervious construction.
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Structures like reservoirs, oil pipes, gas pipes etc., are preferably made from structural steel.
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12. Long Span Construction
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High-rise buildings, long span bridges and tall transmission towers are made up of structural steel.
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Industrial buildings up to a span of 90 m can be designed by plate girders or trusses.
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Bridge spans up to 260 m are made with plate girders.
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For truss bridges, spans of 300 m have been used.
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Long Span
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Long Span
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Long Span
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Long Span
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Long Span
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Long Span
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Long Span
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Long Span
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13. Temporary Construction
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Steel structure is always preferred for temporary construction.
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Army constructions during war are mostly made out of structural steel.
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The structures may be disassembled by opening few bolts, component parts are carried to new places and the structure is easily reassembled.
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Creativity
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Demerits of Steel Construction
1. High Maintenance Costs & More Corrosion
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Most steels are susceptible to corrosion when freely exposed to air and water.
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They must be periodically painted.
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This requires extra cost and special care.
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The use of weathering steels, in suitable design applications, tends to eliminate this cost.
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•
If not properly maintained, steel members can loose
1 to 1.5 mm of their thickness each year.
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Such constructions can loose weight up to 35% during their specified life and can fail under the external loads.
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Corrosion
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Corrosion
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2. Fireproofing Costs
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Although steel members are incombustible, their strength is tremendously reduced at temperatures prevailing in fire.
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At about 400 °C, creep becomes much more pronounced.
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Creep is defined as plastic deformation under a constant load for a long period of time .
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This produces large deflections/deformations of main members forcing the other members to higher stresses or even to collapse.
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Steel is an excellent conductor of heat and may transmit enough heat from a burning compartment of a building to start fire in other parts of the building.
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Extra cost is required to properly fire proof the building.
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Steel in Fire
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3. Susceptibility to Buckling
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Steel sections usually consists of a combination of thin plates.
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The overall steel member dimensions are also smaller than reinforced concrete members.
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If these slender members are subjected to compression, there are greater chances of buckling.
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Buckling is a type of collapse of the members due to sudden large bending caused by a critical compressive load.
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Steel when used for columns is sometimes not very economical because considerable material has to be used to stiffen the column against buckling.
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Buckling in a
Composite column
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4. Higher Initial Cost / Less Availability
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In few countries, Pakistan is one such example, steel is not available in abundance and its initial cost is very high compared with the other structural material.
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This is the most significant factor that has resulted in the decline of steel structures in these countries.
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5. Aesthetics
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For certain types of buildings, the steel form is architecturally preferred.
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However, for majority of residential & office buildings, steel structures without the use of false ceiling and cladding are considered to have poor aesthetic appearance.
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A considerable cost is to be spent on such structures to improve their appearance.
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Cladding is a covering of metal, concrete, plastic or timber put on the surface of a structural member to completely encase it.
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The cladding not only protects the member but also improves its appearance.
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False Ceiling
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Cladding
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Specifications
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The adequacy of a structural member is determined by a set of design rules, called specifications, which include formulas that guide the designer in checking strength, stiffness, proportions and other criteria that may govern the acceptability of the member.
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There are a variety of specifications that have been developed for both materials and structures.
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Each is based on years of research and experience gained through actual structural usage.
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Following Specification will be used in this class quite often:
AISC
AWS
American Institute of Steel Construction
American Welding Society
AASHTO American Association of State Highway &
Transportation Officials
AREMA American Railway Engineering & Maintenanceof-way Association
ASTM American Society for Testing & Material
AISI
ASCE
American Iron & Steel Institute
American Society of Civil Engineers
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1. Dead Load
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It almost retains its magnitude and point of application throughout the life of the structure and is denoted by D .
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This is estimated by multiplying volume of a member with the standard density of the material.
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This load constitute a bigger part of the total load on a structure.
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2. Live Load
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The load due to persons occupying the structure and their belongings, denoted by L .
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Its magnitude and point of application changes with time.
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In case of bridges, live load consists of weight of vehicles moving over the bridge.
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Typical values for common occupancy types are given in next slide.
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Occupancy or Use
Private apartments, school class rooms
Offices
Fixed-seats, assembly halls, library reading rooms
Corridors
Moveable seats assembly halls
Wholesales stores, light storage warehouses
Library stack rooms
Heavy manufacturing, heavy storage ware houses, sidewalks and driveways subject to trucking
Live Load
(kg/m 2 )
200
250
300
400
500
600
750
1200
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3. Self Load
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This is type of load which is due to self weight of the member to be designed.
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For design, a reasonable value of self load depending on the past experience is assumed in the start which is then compared with the actual self weight at the end.
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Corrections in design are made if necessary.
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4. Imposed Load
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This term is used for all external loads, leaving the self weight, acting on the member to be designed.
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This include live load, wind load, earthquake load etc.
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5. Service Load
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The maximum intensity of load expected during the life of the structure depending upon a certain probability of occurrence is called service load.
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No additional factor of safety or overload factor is included in the service load.
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6. Factored Load
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Service loads increased by some factor of safety or overload factor are called factored loads.
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Snow Load
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Mechanism of Load Transfer
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The gravity load passes from top to bottom through all the members of the structure until it reaches the underneath soil.
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The load acts at the floor finish, goes to the underneath slab and transfers to the beams and walls.
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This is then accumulated in the columns, moves to the foundations and then finally dissipates in the soil.
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The terms member and support are defined relative to each other.
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There are no separate supports in the structure as in normally seen in the structural analysis books.
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For the roof slabs, beams and walls are supports.
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For beams, columns are acting like supports, and for the columns, foundations are acting as supports.
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Similarly, the underneath soil acts as support for the foundations.
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This load path is only in one direction.
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The load of roof slab may acts on the beams, columns and foundations, but the load of column is not acting on the beams.
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Similarly, the load of foundation can not act on the column.
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Load Path
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End of File
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