The basic objective in structural analysis and design is to produce a structure capable of resisting all applied loads without failure during its intended life. The primary purpose of a structure is to transmit or support loads. If the structure is improperly designed or fabricated, or if the actual applied loads exceed the design specifications, the system will probably fail to perform its intended function, with possible serious consequences. A well-engineered structure greatly minimizes the possibility of costly failures.
Structural design process
The design process is divided into three, planning, design and construction.
Planning: This phase involves consideration of the various requirements and factors affecting the general layout and dimensions of the structure and results in the choice of one or perhaps several alternative types of structure, which offer the best general solution.
Design: This phase involves detailed consideration of the alternative solutions defined in the planning phase and results in the determination of the most suitable proportion dimensions and details of the structural elements and connection for constructing each alternative structural arrangement being considered.
Construction: This phase involves mobilization of personnel; procurement of material and equipment including their transportation to the site, and actual on-site erection. During this phase some redesign may be required if unforeseen difficulties occur, such as unavailability of specified materials or foundation problems.
Design aids
The design of any structure requires many detailed computations. Some of these are of a routine nature. An example is the computation of allowable bending moments for standard sized, species and grades of dimension timber. The rapid development of technology in the last decade has resulted in rapid adoption of Computer Structural design software that has now replaced the manual computations. This has greatly reduced the complexity of analysis and design process as well as reducing the amount of time required to finish a project. Standard construction and assembly methods have evolved through experience and need for uniformity.
Design Codes
Many countries have their own structural design codes, codes of practice or technical documents which perform a similar function. It is necessary for designer to become familiar with local requirements or recommendations in regard to correct practice.
Design of members in Direct Tension and Compression
Tensile Systems
Tensile systems allow maximum use of the material because every fibre of the cross-section can be extended to resist the applied loads up to any allowable stress.
As with other structural systems, tensile systems require depth to transfer loads economically across a span. As the sag (h) decreases, the tension in the cable (T1 and T2) increases. Further decreases in the sag would again increase the magnitudes of T1 and T2 until the ultimate condition, an infinite force, would be required to transfer a vertical load across a cable that is horizontal (obviously an impossibility).
A distinguishing feature of tensile systems is that vertical loads produce both vertical and horizontal reactions. As cables cannot resist bending or shear, they transfer all loads in tension along their lengths. The connection of a cable to its supports acts as a pin joint (hinge), with the result that the reaction (R) must be exactly equal and opposite to the tension in the cable (T). The R can be resolved into the vertical and horizontal directions (H) is known as the thrust. The values of the components of the reactions can be obtained by using the conditions of static equilibrium and resolving the cable tensions into vertical and horizontal components at the support points.
Short Columns
A column which is short (i.e. the height is small compared with the cross-section area) is likely to fail because of crushing of the material.
Note, however, that slender columns, which are tall compared with the cross-section area, are more likely to fail from buckling under a load much smaller than that needed to cause failure from crushing.
Design of simple beams
Bending Stresses
When a sponge is put across two supports and gently pressed downwards between the supports, the pores at the top will close, indicating compression, and the pores at the bottom will open wider, indicating tension. Similarly, a beam of any elastic material , such as wood or steel , will produce a change in shape when external loads are acting on it.
The stresses will vary from maximum compression at the top to maximum tension at the bottom. Where the stress changes from compressive to tensile, there will be one layer that remains unstressed and this is called the neutral layer or the neutral axis (NA).
This is why beams with an I- section are so effective. The main part of the material is concentrated in the flanges, away from the neutral axis. Hence, the maximum stresses occur where there is maximum material resists them.
If the material is assumed to be elastic, then the stress distribution can be represented by two triangular shapes with the line of action of the resultant force of each triangle of stress at its centroid.
The couple produced by the compression and tension triangles of stress is the internal-reaction couple of the beam section.
The moment caused by the external loads acting on the beam will be resisted by the moment of this internal couple. Therefore:
M = MR = C (or T) * h
Where,
M = the external moment
MR = the internal resisting moment
C = resultant of all compressive forces on the cross-section of the beam
T = resultant of all tensile forces on the cross-section of the beam
h = lever arm of the reaction couple
Horizontal Shear
The horizontal shear force (Q) at a given cross-section in a beam induces a shearing stress that acts tangentially to the horizontal cross-sectional plane. The average value of this shear stress is:
T = Q\A
Where A is the transverse cross-sectional area.
This average value is used when designing rivets, bolts and welded joints.
The existence of such a horizontal stress can be illustrated by bending a paper pad. The papers will slide relative to each other, but in a beam this is prevented by the developed shear stress.
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