Steel and Timber Design

• Structural design


  A mixture of art and science, combining the engineer's judgment (experienced engineer's intuitive feeling for the behavior of a structure) with a sound knowledge of the mechanics (principles of statics, dynamics, mechanics of materials, and structural analysis), to produce a safe, economical structure that will serve its intended purpose
• Structural analysis


  The process of determining the "response" of a structure due to external "actions"
• Response of the structure


  • Changes in the geometry (displacements or deformations) and/or changes in the state of stress of the structure
• Actions


  • The anticipated forces or loads during the lifetime of the structure
• Design criteria


  • Minimum cost
  • Minimum weight
  • Minimum construction time
  • Minimum labor
  • Minimum cost of manufacture of owner's products
  • Maximum efficiency of operation to owner
• Design procedure


  1. Functional design

  2. Structural framework design
• Structural design procedure


  1. Planning

  2. Preliminary structural configuration

  3. Establish the loads to be carried

  4. Preliminary member selection

  5. Analysis

  6. Design

  7. Evaluation

  8. Redesign

  9. Final decision
• Capacity ≥ Demand
• Resistance ≥ Load
• R ≥ Q
• Structural loadings


  • Dead Load
  • Live Load
  • Wind Load
  • Earthquake Load
  • Snow Loads
  • Hydrostatic Loads
  • Special Loads
• Limit states


  A condition of a structure at which it ceases to fulfill its intended function
• Limit state categories


  • Strength Limit State
  • Serviceability Limit State
• Codes and design standards in the Philippines


  National Structural Code of the Philippines 2015 (NSCP) and Steel Construction Manual by American Institute of Steel Construction
• Structural steel


  refers to a number of steels that, because of their economy and desirable mechanical properties, are suitable for load-carrying members in structures
• Groups of hot-rolled structural steels

  • Carbon steels
  • High-strength low-alloy steels (HSLA)
  • Quenched and tempered alloy steels
• Carbon steels


  • Uses carbon as chief strengthening element with minimum yield stresses ranging from 220 MPa to 290 MPa
  • An increase in carbon content raises the yield stress but reduces ductility, making welding more difficult
• High-strength low-alloy steels (HSLA)


  • Have yield stresses from 480 MPa to 840 MPa
  • In addition to carbon and manganese, these steels contain one or more alloying elements such as columbium, vanadium, chromium, silicon, copper, and nickel
• Quenched and tempered alloy steels

  • Have yield stresses of 480 MPa to 690 MPa
  • These steels of higher strength are obtained by heat-treating low-alloy steels
  • The heat treatment consists of quenching (rapid cooling) and tempering (reheating)
• Yield stress, Fy


  That unit tensile stress at which the stress-strain curve exhibits a well-defined increase in deformation without an increase in stress
• Tensile stress, Fu


  The largest unit stress that the material achieves in a tension test
• Modulus of elasticity, E


  The slope of the initial straight-line portion of the stress-strain diagram and is usually taken as 200,000 MPa for design calculation for all structural steel

  
• Ductility


  The ability of the material to undergo large inelastic deformations without fracture
• Toughness


  The ability of the material to absorb energy and is characterized by the area under a stress-strain curve
• Weldability


  The ability of steel to be welded without changing its basic mechanical properties
• Poisson's ratio


  The ratio of the transverse strain to the longitudinal strain, is essentially the same for all structural steels, and has a value of 0 in the elastic range
• Shear modulus


  The ratio of the shearing stress to shearing strain during the initial elastic behavior
• Structural shapes


  Structural steel sections used in construction

  
• Hot-rolled sections


  • Manufacturing involves casting molten steel and then rolling into desired shapes while preventing it from cooling completely
  • Geometric properties of these sections may be looked up at ASEP Steel Handbook Volume 1 or AISC Construction Manual
• Toughness

  The ability of a material to absorb energy and is characterized by the area under a stress-strain curve
• Structural steel sections used in construction are classified as hot-rolled sections and cold-rolled sections
• Hot-rolled sections


  • Manufacturing involves casting molten steel and then rolling into desired shapes while preventing it from cooling completely
  • Geometric properties can be looked up in ASEP Steel Handbook Volume 1 or AISC Construction Manual
• Common hot-rolled steel sections

  • Structural beams
  • Structural columns
  • Structural beams / Girders
  • Deep foundation system
  • Purlins
  • Support for Framing System
  • Bridge Girders
  • Deck support
  • Poles, transmission towers, Pipelines, HVAC
  • Panels, road platforms, decks, Construction of storage tanks
• Cold-rolled sections


  • Manufacturing involves bending thin sheets of steel into desired shape without heating
  • Advantages: versatility and increased yield strength
  • Disadvantages: limited applications and reduced ductility (In the Philippines, it is only used for light structures and for walls and roof purlins)
  • Geometric properties can be looked up at 2004 ASEP Steel Handbook
• LRFD (Load Resistance Factor Design)


  Load factors are applied to the service loads, and a member is selected that will have enough strength to resist the factored loads. The theoretical strength of the member is reduced by the application of a resistance factor.
• Resistance factor (φ)


  A factor less than 1.0 that accounts for uncertainties in resistance
• Load factor (γ)


  A factor greater than 1.0 that accounts for uncertainties in loads
• Types of loads


  • Dead load (D)
  • Fluid load (F)
  • Live load due to occupancy (L)
  • Roof live load (Lr)
  • Earth load (H)
  • Rain load (R)
  • Wind load (W)
  • Earthquake load (E)
• LRFD


  • Uncertainties are handled through nominal capacities and resistance factors, and load factors
• Allowable Stress Design (ASD)

  Uses a single factor of safety, typically applied to the allowable stress or strength of materials, to ensure the structure remains within safe limits under all anticipated conditions