UNIT 1 - INTRODUCTION Why Structural Design Courses? Anyone managing the construction process needs a basic understanding of the engineer’s environment and the basic understanding of how a structure behaves. Constructors must be able to address a number of technical questions at the project site including structural issues that sometimes are not addressed by the design professionals. Since the safety of construction workers as well as the strength and stability of structures during the construction phase is of paramount importance, construction mangers need this knowledge. Structural Design • • • • • Definition: Determination of overall proportions and dimensions of the supporting framework and the selection of individual members. Responsibility:The structural engineer, within the constraints imposed by the architect (number of stories, floor plan,..) is responsible for structural design Safety (the structure doesn’t fall down) Serviceability (how well the structure performs in term of appearance and deflection) Economy (an efficient use of materials and labor) Alternatives • Several alternative designs should be prepared and their costs compared
Types of Load • • • Dead Loads (permanent; including self-weight, floor covering, suspended ceiling, partitions,..) Live Loads (not permanent; the location is not fixed; including furniture, equipment, and occupants of buildings) Wind Load (exerts a pressure or suction on the exterior of a building) Types of Load Continued • • • Earthquake Loads (the effects of ground motion are simulated by a system of horizontal forces) Snow Load (varies with geographical location and drift) Other Loads (hydrostatic pressure, soil pressure) Types of Load Continued • • If the load is applied suddenly, the effects of IMPACT must be accounted for. If the load is applied and removed many times over the life of the structure, FATIGUE stress must be accounted for Design Specifications • • • • Provide guidance for the design of structural members and their connections. They have no legal standing on their own, but they can easily be adopted, by reference, as part of a building code. American Concrete Institute (ACI 318-99) Building Code Requirements for Structural Concrete National Design Specifications for Wood Construction by American Forest and Paper Association. Structural Steel • • Steel is an alloy of primarily iron, carbon (1 to 2%) and small amount of other components (manganese, nickel, …) Carbon contributes to strength but reduces ductility. Steel Properties • The important characteristics of steel for design purposes are: y o yield stress (F ) u o ultimate stress (F ) o modulus of elasticity (E) o percent elongation (ε) o coefficient of thermal expansion (α)
Standard Cross-Sectional Shapes Refer steel table Design Philosophies • • Allowable Stress Design Method (ASD) Load and Resistance Factor Design (LRFD) A member is selected such that the max stress due to working loads does not exceed an allowable stress. • It is also called elastic design or working stress design. o allowable stress=yield stress/factor of safety o actual stress ⊆ allowable stress LRFD –Load and Resistance Factor Design • • A member is selected such that its factored strength is more than the factored loads. o Σ(loads x L factors) ⊆ resistance x R factor Each load effect (DL, LL, ..)has a different load factor which its value depends on the combination of loads under consideration. Load Factors • The values are based on extensive statistical studies o DL only 1.4D o DL+LL+SL (LL domin.) 1.2D+1.6L+0.5S o DL+LL+SL (SL domin.) 1.2D+0.5L+1.6S o In each combination, one of the effects is considered to be at its “lifetime” max value and the others at their “arbitrary point in time “ values. Resistance Factor • • The resistance factors range in value from 0.75 to 1.0 depending on the type of resistance (tension, bending, compression, ..) These factors account for uncertainties in material properties, design theory, and fabrication and construction practices. History • • • ASD has been the primary method used for steel design since the first AISC specifications was issued in 1923. In 1986, AISC issued the first specification for LRFD. The trend today is toward LRFD method, but ASD is still in use.
Advantages of LRFD • It provides a more uniform reliability in all structures subjected to many types of loading conditions. It does not treat DL and LL as equivalent, thereby leading to a more rational approach. • It provides better economy as the DL make up a greater percentage on a given structure. Because DLs are less variable by nature than live loads, a lower load factor is used. This may lead to a reduction in member size and therefore better economy STEEL AS A STRUCTURAL MATERIAL 1.1General Structural steel is a material used for steel construction, which is formed with a specific shape following certain standards of chemical composition and strength. They can also be defined as hot rolled products, with a cross section of special form like angles, channels and beams/joints. There has been an increasing demand for structural steel for construction purposes in the United States and India. Measures are been taken by the structural steel authority for ready availability of structural steel on time for the various projects. The people at every level are working hard to realize the purpose of producing steel on time, like, service centers, producers, fabricators and erectors along with the general contractors, engineers and architects are all working hand in hand. Steel has always been more preferred to concrete because steel offers better tension and compression thus resulting in lighter construction. Usually structural steel uses three dimensional trusses hence making it larger than its concrete counterpart. There are different new techniques which