STRUCTURAL ELEMENTS OF A BUILDING
To understand the structural elements of a building it is important to break it down into
the following elements.
Roof:
This is either a horizontal plate inclined whose main purpose
is to protect the building against aggressive weather conditions.
Slab: A horizontal plate that is mainly subjected to direct vertical loads
arising from the use of the building. They generally have high
span/thickness ratios. They may be designed to carry horizontal loads.
They may be supported by beams or not. They are generally flexural
members that is they resist loads by bending.
Beams:
Beams are mainly horizontal members that carry the slabs
together with their loads and horizontal loads that are transferred to the
columns and then to the foundations. They generally have low
depth/length ratios compared to slabs. They are generally flexural
members that is they resist loads by bending and transfers the loads to
the columns.
Columns: Columns are vertical members that mainly carry vertical loads.
They are predominantly axial compression members but may also be
subjected to bending moments arising from horizontal loads. They carry
the loads from beams, floor slabs and walls. They have very low width to
height ratios to reduce buckling effect.
Walls:
These are vertical plate elements subjected to axial
compression loading and lateral loading. They have low height to width
ratios to reduce buckling effect.
Foundations: This is the sub-structure of the entire engineering
structure. It anchors the entire structure into the ground and form the main
transfer mechanism of all loads from the structure into the ground safely
without any settlement.
Other Structural Elements include Cables and Arches.
The load transfer mechanism in most engineering structures is that loads
are transferred in the following order:
Slab
Beam
Column /
Wall
Foundation
/ Wall
Ground
RESOLUTION OF FORCES
Force is a vector meaning it has direction and magnitude.
Magnitude describes the size of force in terms of Newton (N) or Kilo Newton
(KN) (1 KN = 1000N).
For a force acting at particular point on a structure, it is necessary to know the
direction it is being applied.
In the analysis of structures, it is necessary to simplify the force into its vertical
and horizontal components. The process of this simplification is referred to as
resolution of force on the conventional X-Y coordinate system.
EXAMPLES:
1. Resolution of forces done in class
2. Resolution of forces and determination of the resultant force by at
least two forces
Given a set of concurrent co-planar forces, it is possible to determine the
resultant force of that set through resolution of forces into their
respective horizontal and vertical components.
EXAMPLES:
Figures of examples of resolution of coplanar forces in systems of
concurrent coplanar forces for determination of the magnitude of
resultant force and direction.
STATIC EQUILIBRIUM
1.
Most engineering structures are in a state of rest that is they do not
move nor rotate.
2.
3.
4.
In this state, the energy of the structure is evenly distributed and all
forces on the engineering structure balance out
We say they are in Static Equilibrium.
The whole structure as well as parts of it is in equilibrium (static).
LAWS OF STATIC EQUILBRIUM OF ENGINEERING STRUCTURES
1.
The algebraic sum of all forces and their components in the
horizontal direction is equal to ZERO.
That is ∑Fx = O
2.
The algebraic sum of all forces and their components in the vertical
direction is equal to ZERO.
That is ∑Fy = O
3.
For engineering structures, the sum of all moment about any fixed
point is equal to ZERO.
That is ∑Mi = O
Note:
1.
The three laws of static equilibrium enables us to determine
unknown forces in engineering structurers by generating
simultaneous equations during the analysis.
2.
For engineering structures, where we can determine the unknown
forces using the three equations of static equilibrium, the structure is
statically determinate.
3.
For engineering structures, where we CANNOT determine the
unknown forces using the three equations of static equilibrium, the
structure is statically indeterminate or redundant or hyper static.
TYPES OF LOADS ON ENGINEERING STRUCTURERS FOR
STRUCTURAL ANALYSIS
1. Concentrated loads or Point Loads
2. Uniformly distributed loads
SUPPORTS
These are the points or locations where the structural element is
anchored or sitting.
By virtue of loading on the structural element, some action is exerted by
the structural element onto the supports
From Newton’s 3rd Law, these actions are countered by forces from the
support called reactions.
There are three main types of supports namely:
1. Roller – Does not take horizontal forces nor moments
2. Hinged or Pin – Takes vertical and horizontal forces but does not
take moment
3. Fixed Support – Takes Moment, Horizontal and Vertical forces.
FREE BODY DIAGRAM
Free body diagram as applied in structural analysis that show the whole
or part of an engineering structure with all the forces acting on it. It is an
idealisation of the real structure in the real world.
These forces include:
Reactions:
Internal forces, horizontal forces, vertical forces,
moments and external forces (loads).
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