Monday, April 2, 2012

Structural Theory: Introduction

Overall, engineers design bridges, buildings, ships, machine parts, equipment and other structural installations. To design such it is a must to determine first the layout of the structural, its dictated future shape and constituent members. Estimation or determination of loads that the structure will carry will then proceed.

  • The theory of structure concerns about direct stress, shear area and bending moment and deflection at any section of the structure's constituents. After acquiring such, it is vital to design each members proportion to the allowable working stresses of the materials while complying to other requirements and limits for the proper function of the engineered structure.
Four Stages of Design:
  1. layout of the structure
  2. loadings that consists of dead load (own weight) and live load (which may include snow load, wind load, etc.)
  3. direct stresses that the members will carry
  4. sizes of the members that will bear the loads and stresses most costly efficiently within the bound limit
  • Layout and Classification. Structures layout largely depends on its function, loading conditions and the properties of the materials that will be used. The determination of the layout requires field experience and expertise, judgment and vast knowledge. After considering all the necessary elements in the preliminary design it will be test to determine the preferred design that meet even the unanticipated conditions that the structure might encounter during the stages of creation. Basic structure classification are inclusive but not limited to beams, rigid frames, trusses or combination of two or more of these elements. Generally, a beam is a structural member subjected to transverse loads only that bears shear and bending moment, hence, horizontally oriented. Rigid frames on the other hand is a structure composed of several members connected by rigid joints whether welded or bolted that is completely analyzed when all variations in direct stress, shear and bending moment along  the entire lengths of all members is acquired. Finally, a truss is a structure consists of several members connected by frictionless hinges that is completely analyzed with determined direct stresses. Other structures with members or machine parts are also subjected to react on direct stresses, shear and bending and twisting moments.
  • Loads. Structures are subjected to dead loads, live loads and dynamic outside force like nature and impact of live loads. Dead Load is the weight of the structure itself, it has to be first assumed since most of its part cannot be determined until the members have actually been design, then checked after the sizes of the members are finalized although changes are at minimum that in routine design it is seldom modified. Live Load also known as the moving loads are maximum loads to be carried by the structure. Since it is moving, the Impact or its Dynamic Effect is also considered because it is usually more critical than stagnant live loads only since there are times when the live load comes on rather suddenly as a moving, passing or rolling load.
  • Methods. Theory of Structures deals with the principles and methods wherein direct stress, shear and bending moment at any section of the member may be found under the conditions of loading. It is then assumed that the forces acting on each structural elements are on the same plane and in equilibrium.
    • General coplanar-force system:
      • Summation of Forces on X-axis = 0
      • Summation of Forces on Y-axis = 0
      • Summation of Moment at a certain joint or point = 0