Who is a Structural Engineer?
Any element, component, structure, building, monument, equipment must be held from free fall, gravity, seismic, wind, hurricane, heights, tsunamis, man-made or natural disasters. Otherwise that element will fall.
Of course the famous Hammurabi’s building code states that if the building fails, and kills the son or daughter, the building owner can kill equivalent son or daughter of the designer.
When component is placed on 3rd floor of a building, the forces of the loads transfer from one member to another finally through columns; they are transferred to ground soil. This transfer requires a foundation. Foundation is like a shoe that permits a stable building. Eskimos wear shoes not to sink in snow in complete contrast to mountain hikers. The soil below the building is classified from A to F, from rock to very unstable soil. Foundation design is an art itself and correlates to geotechnical engineers’ science.
If you begin designing and accommodating the load on that level, basic components of structural design are as follows:
o Basic Theories: Very simple static and strength of materials can provide necessary knowledge to understand the concept, even design a system. More advanced theories get involved with micro issues, joints, connections, curves, more elaborate structures. However, one can break down any structure to simpler and through superposition method, one can provide complete understanding of the forces acting on the structure and each component.
o Modeling: Modeling a structure comprises of geometric shape, coordinates, segments, angles, and properties of material. The statistical quantities are loads, reactions, and stresses. The resultant deformations must be checked with allowed deformation; hence the material or material size is altered till full code compliance is met.
o Assumptions: The materials are homogenous, continuous, isotropic and act as spring (Hooke’s Law). The deformations do not alter the original geometry significantly. Since the loads applied are gradual, the superposition concept is valid. In superposition, one can apply each load independently and add the results. Therefore sum of effect of system of loads is equal to sum of effect of individual loads. Materials are constant and independent of time, therefore the system is assumed to be in static equilibrium.
o Type of loads:
• Single loads:
o concentrated load,
o moment applied by single load,
o uniformly distributed load,
o regularly varying load,
o irregularly varying load,
• Systems of loads:
o the combination of single loads,
o symmetrical loads,
o asymmetrical loads,
o cyclo symmetrical,
o cyclo antisymmetrical, and
o unsymmetrical loads.
Forces acting on members can be complex and number of equilibrium conditions required to solve these forces vary from one type of force to the other.
Coplanar force system or single concentrated load in two dimensional (2D) and three dimensional (3D) requires 3 and 3 equations, respectively.
Collinear force system where all the forces share same line of action in 2D and 3D require 1 and 1 equilibrium conditions, respectively.
Parallel forces where all forces are in parallel (both directions) in 2D and 3D require 2 and 3 equilibrium conditions, respectively.
General three dimensional forces where all forces are in one plane (all directions) in 2D and 3D require 3 and 3 equilibrium conditions, respectively.
o Effects of the loads: Loads produce reaction forces on members, the produce stresses on members, and they deform the structural members.
Effect of loads produce reactions on the connections.
o Connections: Connections are movable hinged end, immovable hinged end, guided end, and rigidly fixed end. These connections carry moments and forces. If the member is free and able to move in a direction, reaction forces in that direction does not exist.
o Static equilibrium: Since the body under load is assumed to be in equilibrium, algebraic sum of all forces are zero, and sum of al moments at any point is zero. The vector form of these forces can be broken into any coordinate systems, ie. Cartesian, cylindrical, spherical coordinates.
o Stresses: Three stresses at center of any materials are: Normal forces, shear forces, and bending moments.
Procedure to perform a design:
One begins with the statement of the problem. A single or multiple or complex load is on top of the structure. Is the structure fit to support the component, say in Los Angeles. As the steps was described, first, you model the building in simple lines and provide all elements that support this component from top to bottom, to foundation, and to the characteristics of the soil.
For example, a battery rack is on 6th floor. The load of the battery rack is on several floor joists. Each floor joist must carry tributary load of the component. Each joist acts as a beam supported by sub-purlines. sub-purlines can be supported by beam, beams sit on columns, columns sit of foundation. The floor joist acts as beam. Beams designs are checked by insuring the can carry the stress, shear, and deflection. Floor joists transmit the load to sub-purlines. The sub-purlines act as uniformly distributed loads plus the contribution of the batteries as concentrated load. Using the superposition, the resulting parameters will be added. Same pattern must be devoted to the purlines, then to main beams. Finally the load is transferred to the column. Then the column is checked for stress, deflection, and buckling. The load on the column is transferred to the foundation. The soil has a capacity, for stable soil, worst case is 1000 lbs per square feet. If the load is 5000 lbs, the base of the foundation area is 5 square feet. There are other issues such as bending of the building and its impact on the soil is also defined.
What we did not discuss was materials, localities, and equations. Materials vary from steel, wood, concrete, timber fiber composites, and non-traditional materials such as bamboo. Location of the building is also critical. The wind, hurricane, earthquake, seaside, and mountain sites make quite difference.
There are two methods of calculations: LRFD and ASD.
Allowable stress Design or ASD is very simple. The allowable stress of the materials is reduced by a safety factor and then the load effects are compared to this reduced value.
Load and Resistance Factor Design or LRFD simply increases the load effects introduced by components and then it compares them to allowable material capacities. There are also resistance factors as multipliers on load effects that play a role in final design.
In eater cased, ASD where material capacity is reduced, or LRFD where the load effects are increased, the safety factors built in varies and has created two separate philosophies in structural engineering.
Capacity is the capability of the building to carry demand, the load. The C to D ratio is a common terminology used by structural engineer in evaluating existing buildings.
Equilibrium or Static
An object, i.e. a beam, a building is at equilibrium when sum of all forces in all directions (algebraic sum), moments and torques are at rest (or is zero) and are not moving, otherwise we would be in a mechanical engineering world called dynamics. Therefore, sum of all the (a) moments acting on the body, (b) vertical forces acting on the body, and (c) horizontal forces acting on the body must equal zero.
Example:
Assuming the beam’s weight is negligible, the forces in the vertical direction algebraically are zero. Summation of all vertical forces are
Forces
The unit of force is pounds, lb or # (Newton (SI), N) or thousand pounds, Kips (KiloNewtons, KN). The issue is the seesaw created is stable and not move? To respond to this question, one must take a moment.
Moment at any point is similar to the seesaw, the hammer, the wedge science class in 5th grade. Moment is the force time the arm distance. Arm distance is from the pivot to the position where the force is applied. This is the reason for monumental constructions of the pyramids, temples, and castles nearly 3 thousand years ago.
Moment at point b, or any other point, must be zero.
Therefore,
A.Rb= C.Rc+D.Rd
where the A, C, or D are arms or distances creating the torque.
Algebraic sum is the sum of components of vectors.
Force is divided into X component and y component. Applying the equilibrium, sum of all forces in x direction must be zero. Then apply this to y directions.
Please note that Pythagorean theorem and trigonometric sine and cosines can also be used in these calculations.
Any load can be converted into simple diagrams, namely, free body diagram, where vectors correspond to forces acting on a body. Fx is the force pushed by guy, Ff is the surface friction opposing the Fx, W is the gravity force applied on the box, and N is the normal force acting against the box by earth. (You can omit the picture of the guy in structural engineering).
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