68402: Structural Design of Buildings II 61420: Design of Steel Structures 62323: Architectural Structures II

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68402 Structural Design of Buildings II61420
Design of Steel Structures62323 Architectural
Structures II
Introduction to Structural Design of Steel

• Monther Dwaikat
• Assistant Professor
• Department of Building Engineering
• An-Najah National University

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Contents

• Structural Design
• Design Loads
• Structural Steel - Properties
• Design philosophies
• Determining load and resistance factors
• Load and resistance factors

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Introduction to Design of Steel Structures

• General Introduction
• Structural design is a systematic iterative
  process that involves
• Identification of intended use occupancy of a
  structure by owner
• Development of architectural plans layout by
  architect
• Identification of structural framework by
  engineer
• Estimation of structural loads depending on use
  occupancy
• Analysis of the structure to determine member
  connection design forces
• Design of structural members connections
• Verification of design
• Fabrication Erection by steel fabricator
  contractor
• Inspection Approval by state building
  official

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Primary Responsibilities

• The primary responsibilities are
• Owner - primary responsibility is deciding the
  use occupancy, approving the arch. plans of
  the building.
• Architect - primary responsibility is ensuring
  that the architectural plan of the building
  interior is appropriate for the intended use
  the overall building is aesthetically pleasing.
• Engineer primary responsibility is ensuring the
  safety serviceability of the structure, i.e.,
  designing the building to carry the loads safely.

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Primary Responsibilities

• Fabricator primary responsibility is ensuring
  that the designed members connections are
  fabricated economically in the shop or field as
  required.
• Contractor/Erector - primary responsibility is
  ensuring that the members connections are
  economically assembled in the field to build the
  structure.
• State Building Official primary responsibility
  is ensuring that the built structure satisfies
  the appropriate building codes accepted by the
  Govt.

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Structural Design

• Conceptually, from an engineering standpoint, the
  parameters that can be varied (somewhat) are
• The material of construction
• The structural framing plan.
• The choices for material include
• Steel
• Reinforced concrete
• Steel-concrete composite construction.
• The choices for structural framing plan include
• Moment resisting frames.
• Braced frames.
• Dual frames
• Shear wall frames, and so on.
• The engineer can also innovate a new structural
  framing plan for a particular structure if
  required.

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Structural Design

• All viable material framing plan alternatives
  must be considered designed to compare the
  individual material fabrication / erection
  costs to identify the most efficient economical
  design for the structure.
• For each material framing plan alternative
  considered, designing the structure consists of
  designing the individual structural components,
  i.e., the members the connections, of the
  framing plan.

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Structural Design

• Determination of dimensions and selection of
  cross sections.
• The design process is a loop

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Structural Design

• Optimal structural design shall achieve balance
  between the following requirements

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Roles and responsibilities of the structural
steel designer

• Arrange and proportion the members of the
  structures, using engineers intuition and sound
  engineering principles, so that they can be
  practically erected, have sufficient strength
  (safe), and are economical.
• Practicality Ensure structures can be fabricated
  and erected without problems
• Safety Ensure structures can safely support the
  loads. Ensure deflections and vibrations are
  controlled for occupants comfort.
• Cost Minimize costs without sacrifice of
  strength (consider labor costs in fabrication
  and erection, not just material costs)

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Basic Structural Shapes

• Trusses
• Frames ( Beam-Column)
• Beams
• Girders
• Columns
• Space trusses/frames

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Steel Structures
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Steel Structures
Industrial/Parking structures Frames
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Steel Structures
Joists/Trusses
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Steel Structures
High rise buildings
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Steel Structures

• Girder bridges

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Steel Structures

• Truss bridges

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Steel Structures

• Cable stayed suspended bridges

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Structural Members

• Structural members are categorized based up on
  the internal forces in them. For example
• Tension member subjected to tensile axial force
  only
• Column or compression member subjected to
  compressive axial force only
• Tension/Compression member subjected to
  tensile/compressive axial forces
• Beam member subjected to flexural loads, i.e.,
  shear force bending moment only. The
• axial force in a beam member is negligible.
• Beam-column member member subjected to combined
  axial force flexural loads (shear
• force, bending moments)

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Structural Members

• In trusses
• All the members are connected using pin/hinge
  connections.
• All external forces are applied at the
  pins/hinges.
• All truss members are subjected to axial forces
  (tension or compression) only.
• In frames
• The horizontal members (beams) are subjected to
  flexural loads only.
• In braced frames
• The vertical members (columns) are subjected to
  compressive axial forces only.
• The diagonal members (braces) are subjected to
  tension/compression axial forces only.
• In moment frames
• The vertical members (beam-columns) are subjected
  to combined axial flexural loads.

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Structural Connections

• Members of a structural frame are connected
  together using connections. Prominent connection
  types include
• Truss / bracing member connections are used to
  connect two or more truss members together. Only
  the axial forces in the members have to be
  transferred through the connection for
  continuity.
• Simple shear connections are the pin connections
  used to connect beam to column members. Only the
  shear forces are transferred through the
  connection for continuity. The bending moments
  are not transferred through the connection.
• Moment connections are fix connections used to
  connect beam to column members. Both the shear
  forces bending moments are transferred through
  the connections with very small deformations
  (full restraint).

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Structural Connections
Truss connection
Simple Shear connection
Moment resisting connection
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Structural Loads

• The building structure must be designed to carry
  or resist the loads that are applied to it over
  its design-life. The building structure will be
  subjected to loads that have been categorized as
  follows
• Dead Loads (D) are permanent loads acting on the
  structure. These include the self-weight of
  structural non-structural components. They are
  usually gravity loads.
• Live Loads (L) are non-permanent loads acting on
  the structure due to its use occupancy. The
  magnitude location of live loads changes
  frequently over the design life. Hence, they
  cannot be estimated with the same accuracy as
  dead loads.
• Wind Loads (W) are in the form of pressure or
  suction on the exterior surfaces of the building.
  They cause horizontal lateral loads (forces) on
  the structure, which can be critical for tall
  buildings. Wind loads also cause uplift of light
  roof systems.

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Structural Loads

• Snow Loads (S) are vertical gravity loads due to
  snow, which are subjected to variability due to
  seasons drift.
• Roof Live Load (Lr) are live loads on the roof
  caused during the design life by planters,
  people, or by workers, equipment, materials
  during maintenance.
• Values of structural loads can be computed based
  on the design code.

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Dead Loads (D)

• Dead loads consist of the weight of all materials
  of construction incorporated into the building
  including but not limited to walls, floors,
  roofs, ceilings, stairways, built-in partitions,
  finishes, cladding other similarly incorporated
  architectural structural items, fixed service
  equipment such as plumbing stacks risers,
  electrical feeders, heating, ventilating, air
  conditioning systems.
• In some cases, the structural dead load can be
  estimated satisfactorily from simple formulas
  based in the weights sizes of similar
  structures. For example, the average weight of
  steel framed buildings is 3 - 3.6 kPa, the
  average weight for reinforced concrete buildings
  is 5 - 6 kPa.

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Dead Loads (D)

• From an engineering standpoint, once the
  materials and sizes of the various components of
  the structure are determined, their weights can
  be found from tables that list their densities.
  See Tables 1.2 1.3, which are taken from
  Hibbeler, R.C. (1999), Structural Analysis, 4th
  Edition.

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Dead Loads (D)
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Live Loads Summary Table

• Building floors are usually subjected to uniform
  live loads or concentrated live loads. They have
  to be designed to safely support these loads.

Type of occupancy kPa
Offices 2.5 - 5
Corridors 5
Residential 2
Stairs and exit ways 5
Stadiums 5
Sidewalks 12
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Wind Loads

• Design wind loads for buildings can be based on
  (a) simplified procedure (b) analytical
  procedure (c) wind tunnel or small-scale
  procedure.
• Refer to ASCE 7-05 for the simplified procedure.
  This simplified procedure is applicable only to
  buildings with mean roof height less than 18 m or
  the least dimension of the building.
• The wind tunnel procedure consists of developing
  a small-scale model of the building testing it
  in a wind tunnel to determine the expected wind
  pressures etc. It is expensive may be utilized
  for difficult or special situations.
• The analytical procedure is used in most design
  offices. It is fairly systematic but somewhat
  complicated to account for the various situations
  that can occur

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Wind Loads

• Wind velocity will cause pressure on any surface
  in its path. The wind velocity hence the
  velocity pressure depend on the height from the
  ground level. Equation 1.3 is recommended by ASCE
  7-05 for calculating the velocity pressure (qz)
  in SI
• qz 0.613 Kz KztKd V2 I (N/m2)

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Wind Loads

• qz Static wind pressure
• V - the wind velocity in m/s
• Kd - a directionality factor ( 0.85 see Table
  6.4 page 80)
• Kzt - a topographic factor ( 1.0)
• I - the importance factor (1.0)
• Kz - varies with height z above the ground level
  (see Table 6.3 page 79)
• exposure B structure surrounded by
  buildings/forests/ at least 6m height
• exposure C open terrain

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Wind Loads

• A significant portion of Palestine has V 100
  km/h. At these location
• qz 402 Kz (N/m2)

The velocity pressure qz is used to calculate the
design wind pressure (p) for the building
structure conservatively as follows p q GCp
(N/m2)
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ASCE 7-05 pg. 79
Kz - varies with height z above the ground
level A large city centers B urban/ suburban
area C open terrain with scattered
obstructions D Flat unobstructed surface
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Wind Loads

• G - gust effect factor ( 0.85)
• Cp - external pressure coefficient from Figure
  6-6 page 48-49 in ASCE 7-05 or
• Cp 0.8 windward
• Cp -0.5 leeward
• Cp -0.7 sidewalls
• Cp -0.7 slopelt0.75

(1.5)

• Note that
• A positive sign indicates pressure acting
  towards a surface.
• Negative sign indicates pressure away from the
  surface

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Example 1.1 Wind Load

• Consider the building structure with the
  structural floor plan elevation shown below.
  Estimate the wind loads acting on the structure
  when the wind blows in the east-west direction.
  The structure is located in Nablus.

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Example 1.1 Wind Load
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Example 1.1 Wind Load

• Velocity pressure (qz)
• Kd - directionality factor 0.85
• Kzt - topographic factor 1.0
• I - importance factor 1.0
• V 100 kph in Nablus
• qz 402 Kz (N/m2)
• Kz - varies with height z above the ground level
• Kz values for Exposure B, Case 2

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Example 1.1 Wind Load

• Wind pressure (p)
• Gust factor G 0.85 for rigid structures
• External pressure coefficient Cp 0.8 for
  windward walls
• Cp -0.5 for leeward walls
• Cp -0.7 for side walls
• External pressure q G Cp
• External pressure on windward wall qz GCp 402
  Kz x 0.85 x 0.8 273.4 Kz Pa toward surface
• External pressure on leeward wall qh GCp 402
  K18 x 0.85 x (-0.5) 145.2 Pa away from surface
• External pressure on side wall qh GCp 402 K18
  x 0.85 x (-0.7) 203.3 Pa away from surface
• The external pressures on the structure are shown
  in the following two figures.

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Example 1.1 Wind Load
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Example 1.1 Wind Load
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Background of Structural Steel

• Economical production in large volume not
  available until mid 19th century and the
  introduction of the Bessemer process. Steel
  became the principal metallic structural material
  by 1890.
• Steels consists almost entirely of iron (over
  98) and small quantities of carbon, silicon,
  manganese, sulfur, phosphorus, and other
  elements.
• The quantities of carbon affect properties of
  steel the most.
• Increase of carbon content increases hardness and
  strength
• Alloy steel has additional amounts of alloy
  elements such chronium, vanadium, nickel,
  manganese, copper, or zirconium.
• The American Society for Testing of Materials
  (ASTM) specifies exact maximum percentages of
  carbon content and other additions for a number
  of structural steels. Consult Manual, Part 2,
  Table 2-1 to 2-3 for availability of steel in
  structural shapes, plate products, and structural
  fasteners.

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ASTM classifications of structural steels

• Carbon steels A36, A53, A500, A501, A529, A570.
  Have well-defined yield point. Divided into
  four categories
• Low-carbon steel (lt 0.15)
• Mild steel (0.15 to 0.29, structural carbon
  steels)
• Medium-carbon steel (0.3 to 0.59)
• High-carbon steel (0.6 to 1.7)
• High-Strength Low-Alloy steels A242, A572,
  A588, A606, A607, A618, A709
• Well-defined yield point
• Higher strengths and other properties
• Alloy Steels A514, A709, A852, A913.
• Yield point defined as the stress at 0.2 offset
  strain
• Low-alloy steels quenched and tempered ? 550 to
  760 MPa yield strengths

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Advantages and disadvantages of steel as a
structural material

• Advantages
• High strength per unit of weight ? smaller weight
  of structures
• Uniformity
• Elasticity
• Long lasting
• Ductility
• Toughness
• Easy connection
• Speed of erection
• Ability to be rolled into various sizes and
  shapes
• Possible reuse and recyclable

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Advantages and disadvantages of steel as a
structural material

• Disadvantages
• Maintenance costs
• Fire protection/Fireproofing costs
• Susceptibility to buckling failure
• Fatigue
• Brittle fracture

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Types of Steel

• Three basic types of steel used for structural
  steel
• Plain Carbon Steel
• Low-alloy steel
• High-alloy specialty steel
• The most commonly used is mild steel - ASTM A36
• Typical high strength steel
• The higher the steel strength, the higher the
  carbon content and the less ductile it is.

ASTM A242
ASTM A992
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Stress-strain curve

• Standard Plain Carbon Steel

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What is a Limit State

• When a structure or structural element becomes
  unfit for its intended purpose it has reached or
  exceeded a limit state
• Two categories of limit states
• Strength limit states
• Serviceability limit states

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Limit States

• Strength Limit States
• a) Loss of Equilibrium
• b) Loss of load bearing capacity
• c) Spread of local failure
• d) Very large deformations
• Serviceability Limit States
• a) Excessive deflection
• b) Excessive local damage
• c) Unwanted vibration

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Design Philosophies

• Allowable Stress Design (ASD)
• Plastic Design (PD)
• Load and Resistance Factor Design (LRFD)

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Allowable Stress Design

• Service loads are calculated as expected during
  service life.
• Linear elastic analysis is performed.
• A factor of safety (FOS) of the material strength
  is assumed (usually 3-4)
• Design is satisfactory if (maximum stress lt
  allowable stress)
• Limitations
• Case specific, no guarantee that our design
  covers all cases
• Arbitrary choice of FOS?!

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Plastic Design

• Service loads are factored by a load factor.
• The structure is assumed to fail under these
  loads, thus, plastic hinges will form under these
  loads Plastic Analysis.
• The cross section is designed to resist bending
  moments and shear forces from the plastic
  analysis.
• Members are safe as they are designed to fail
  under these factored loads while they will only
  experience service loads.
• Limitations
• No FOS of the material is considered, neglecting
  the uncertainty in material strength!
• Arbitrary choice of overall FOS?!

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Load and Resistance Factor Design (LRFD)

• LRFD is similar to plastic design in that it
  performs design with the assumption of failure! -
  Reliability Based Design
• Service loads are multiplied by load factors (g)
  and linear elastic analysis is performed.
• Material strength is reduced by multiplying the
  nominal material strength by a resistance factor
  (f)
• The design rule is Load Effect lt Resistance
• Where Rn is the nominal strength and Q is the
  load effect for the ith limit state

This rule shall be attained for all limit
states!!
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Load and Resistance Factor Design (LRFD)

• Resistance Shear, Bending, Axial Forces
• Advantages of LRFD
• Non-case specific, statistical calculations
  guarantee population behavior.
• Uniform factor of safety as both load and
  material factors are tied by reliability analysis

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Probabilistic Basis for LRFD

• If we have the probability distribution of the
  load effect (Q) and the material resistance (R)
  then
• The probability of failure can be represented by
  observing the probability of the function (R-Q)
• The probability of failure PF can be represented
  as the probability that Q R

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AISC Load combinations

• AISC considers the following load combinations in
  design

• Dead loads (D)
• Live loads (LL)
• Occupancy load (L)
• Roof load (Lr)
• Snow load (S)
• Rain loads (R)
• Trucks and pedestrians
• Wind Loads (W)
• Earthquakes (E)

e.g. ? for yield is 0.9 and for bolt shear is 0.75