STRUCTURAL CE 382 ANALYSIS Struct Structural Analysis ural - PDF document

STRUCTURAL CE 382 ANALYSIS Struct Structural Analysis ural Analysis As a structural engineer, you will • Blue text is important course be required to make many material material technical decisions about technical decisions about structural systems. These • Green text is important decisions include: definition material for the (1) selecting an efficient, econo- course mical and attractive structural • Red text is important conclu- form; sion or caution material for the sion or caution material for the (2) evaluating structural safety (2) evaluating structural safety, course that is, its strength and stiffness; and Text in bold is more signi- (3) planning the erection of the ficant than non-bolded text structure under temporary 1 2 construction loads. In order to perform structural Force is used to represent design, you need to learn to carry stresses or stress resultants , out structural analyses. e.g., axial forces, shear forces, bending moments, and torsional Structural analysis involves the moments along with their prediction of the performance p p associated stresses associated stresses. of a given structure under prescribed loads and/or other Designers determine the internal external effects, such as forces in key members in order to support movements and size both members and the temperature changes. connection between members. The fundamental purpose of a The fundamental purpose of a Deflection calculations are carried Deflection calculations are carried structural analysis is to deter- out to ensure a serviceable mine the magnitudes of force structure – one that does not and displacement for each deflect or vibrate excessively element of a design system for under load such that structural a given set of design loads. function is impaired. 3 4 1

See also pages 2 - 4 in the supplemental notes. Structural Stated differently, structural Analysis/Design engineering is the science and Flowchart art of planning, designing, and constructing economical structures that can safely resist the forces to which they i t th f t hi h th are subjected to with elegance. 5 6 A. Understand the environment, DESIGN OBJECTIVES use of the structure, behavior of the construction materials, structural behavior, appropriate Factors which influence design allowances for the uncertainty include: safety, serviceability, include: safety, serviceability, involved in the prediction of feasibility, and esthetics . load effects and structural Safety – Normally, structural response, and any other engineers design safely in case influences that may influence something unforeseen or the design or design unknown happens. To properly performance. assess safety, the structural assess safety the structural engineer must: B. Determine the internal forces and failure load. 7 8 2

A number of approaches to These limit state procedures structural safety are possible. One incorporate safety by multiplying method is referred to as Allowable dead loads and live loads by load Stress Design (ASD). factors greater than one but less than two. The live load factor is ASD is based on the assumption p typically larger than the dead load t picall larger than the dead load that if the stresses under working factor since dead loads are more loads are limited to values accurately assessed. substantially smaller than stresses corresponding to failure, then Furthermore, all design procedures safety is assured. typically reduce the load carrying capacity or strength parameters Other approaches include: Other approaches include: associated with the structure. Ultimate Strength Design , Typically, the structural strength Plastic Design , Load and parameters are reduced by 0.60 to Resistance Factor Design and 0.90. others. 9 10 Feasibility – Construction of a Serviceability – All aspects of proposed structure must be performance must be acceptable economical as well as plausible for the intended use. Some (feasible). A basic rule in design specific performance consider- is that the designer should ations that the structural engineer g contemplate at least one method contemplate at least one method must consider include: deflection of construction. The structure and cracking must be limited, i.e., may not be built in exactly the not visible; vibration and noise manner anticipated, but still the should be controlled; liquid and feasibility of the structure is gas containers should not leak; assured. foundations must not settle improperly; etc. Esthetics (or Aesthetics) – The looks of the structure are of great concern to the community and the designer. 11 12 3

STRUCTURES Arch : The arch can be defined Truss: A truss is a geometrically as a curved structural shape that stable arrangement of slender is usually configured to support members which primarily support gravity loads in a manner that axial load. results in uniform compressive resistance resistance. Gravity loading tends Gravity loading tends Frame : A frame is a stable to flatten the arch and push its structural form consisting of two or supports outward. more flexural members that can resist bending moment, shear, Cable : Cable supported struc- and axial forces. A frame is tures are very common and are classified as a rigid frame when often expressed in the form of its members are joined together it b j i d t th suspension and cable-stayed i d bl t d by moment resisting connections; bridge construction. i.e., where joint translations and rotations occur without relative rotations between the connected 13 14 members. Computer Analysis Membranes are thin-walled structures such as air supported Most structural computer stadium roofs and weather programs perform a first-order balloons which provide tensile analysis : ( 1 ) linear elastic resistance in two directions. behavior, ( 2 ) member forces are behavior ( 2 ) member forces are unaffected by deformations Flat plate structures can (change of geometry) of the provide bending, tensile, and structure, and ( 3 ) no reduction in compressive force resistance, flexural stiffness. e.g., floor slabs. Computer programs have tremen- p p g Shells are often defined as Shells are often defined as dously reduced the labor of per- curved plates. Roof domes, forming structural analysis and water and fuel storage tanks, and design. grain silos typify some forms of shell structures. 15 16 4

CE 382 Structural Calculations The designer must still possess • Prepare a clear sketch of the basic insight into all potential structure, showing all loads and failure modes in order to assess dimensions. computer solution reliability . computer solution reliability • Include all steps of your computations. Furthermore, the engineer must • Check your results . prepare a mathematical model that adequately represents the For computer generated results: structure and the engineer must Verify that the deflection direc- interpret the results correctly . p y tions are consistent with the tions are consistent with the applied forces – computer programs typically include post- processing features that include scaled displacement plots of the structure. 17 18 5