Introduction to Structures A

Introduction

The fundamental purpose of a structure is to transmit loads from the point of application to the
point of support and, ultimately, through the foundations to the ground.

The structural design itself is a combination of art and science in that it consists of the creation of
a structural form that will accommodate the often-conflicting aspects of cost, function, and
services, not forgetting aesthetics, and be capable of being quantified to produce dimensioned
details for the purpose of erection. There are, therefore, two distinct stages in the process. In the
first, the structural engineer draws on his experience, intuition and knowledge to make an
imaginative choice of a preliminary scheme in terms of layout, materials and erection methods. In
the second stage, the chosen scheme is subjected to detailed analysis based on the principles of
structural mechanics. The resulting scheme must be consistent with the engineer’s basic aims to
provide a structure that satisfies the criteria of SAFETY and SERVICEABILITY at a reasonable
cost.

Structural design

Structural design is the methodical investigation of the stability, strength and rigidity of structures.
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 device 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.

A structural design project may be divided into three phases, i.e., 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. The primary consideration is
the function of the structure. Secondary considerations such as aesthetics, sociology, law,
economics and the environment may also be taken into account. In addition, there are structural
and constructional requirements and limitations, which may affect the type of structure to be
designed.
Design: This phase involves a detailed consideration of the alternative solutions defined in the
planning phase and results in the determination of the most suitable proportions, dimensions and
details of the structural elements and connections for constructing each alternative structural
arrangement being considered. These include structural analysis, structural element design,
structural detailing, costing and specifications of materials and workmanship.
Construction: This phase involves the mobilization of personnel; procurement of materials 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. The key aspects during this stage is the supervision by qualified
personnel.

Philosophy of designing

The structural design of any structure first involves establishing the loading and other design
conditions, which must be supported by the structure and therefore must be considered in its
design. This is followed by the analysis and computation of internal gross forces, (i.e., thrust,
shear, bending moments and twisting moments), as well as stress intensities, strain, deflection
and reactions produced by loads, changes in temperature, shrinkage, creep and other design
conditions. Finally comes the proportioning and selection of materials for the members and
connections to respond adequately to the effects produced by the design conditions.

The criteria used to judge whether particular proportions will result in the desired behaviour
reflect accumulated knowledge based on field and model tests, and practical experience. Intuition
and judgment are also important to this process.
The following design philosophies are identified and used to achieve a safe and workable
structure.

• Permissible stress design

• load factor design
• Limit state design

a) Load Factor Design or Plastic Design. This was developed to take account of the
behaviour of the structure once the yield point of the materials has been reached. This
involves calculating the collapse load of the structure. The working load is obtained by
dividing the collapse load by a load factor.
b) Permissible stress method or elastic design. In this design, the stresses in the structure at
working loads are not allowed to exceed a certain proportion of the yield stress of the
material. The stress levels are limited within the elastic range. By assuming that the stress-
strain relationship over this range is linear, it is possible to calculate the actual stress in the
material concerned.
Drawbacks:
• The design process tends to complicate the design process and leads to conservation
solutions.
• For materials such as concrete the stress-strain relationship may not be linear thus, it is
difficult to determine the stress within the elastic range.
c) Limit state design. This could be seen as a compromise between the permissible and load
factor methods. Most modern structural codes of practice are now based on the limit-state
approach. Principal exceptions are the code of practice for design in timber, BS 5268, and
the structural steel code, BS 449, both of which are permissible stress codes. The Euro code
is based on limit state principles.
The limit state design has two subsections:
Ultimate and serviceability limit states.
The aim of the limit state design is to achieve acceptable probabilities that a structure will not
become unfit for its intended use during its design life, that is the structure will not reach the limit
state.
Ways in which the structure may become unfit for use:
• excessive bending, shear, compression, deflection and cracking.
Each of these mechanisms is a limit state whose effect on the structure must be individually
assessed.
Deflection and cracking affect the appearance of the structure. Others such as bending, shear and
compression may lead to partial or complete collapse of the structure.
Those limit states which can cause the failure of a structure are termed ultimate limit states. The
others are categorized as serviceability limit states.
The ultimate limit states enable the designer to calculate the strength of the structure. Serviceability
limit states model the behaviour of the structure at working loads.
Other limit states are durability and fire resistance.
It is important to base the design on the most critical limit state and then check for the other limit
states.

For example, for the design of reinforced concrete beams, ultimate limit states of bending and
shear are used to size the beam. The design is then checked for deflection and cracking. The
serviceability limit state of deflection is normally critical in the design of concrete slabs.

Terminologies.

Structure – refers to a system of connected parts used to support a load (e.g., self-weight,
occupants/furniture for building, traffic for a bridge)

Structural design – follows a series of successive approximations in which every cycle requires a
structural analysis.

Structural analysis – prediction of the performance of a given structure under prescribed load, or
other external effects, such as support movement and temperature changes.

Or defined as Structural analysis is the prediction of the response of structures to specified

arbitrary external loads.
Structural engineering – science and art of planning, designing and constructing safe and
economical structures that will serve their intended purpose.

Factor to consider in the analysis

• Safety (Most important)

• Serviceability (e.g., extensive deflection or vibration is not allowed)
• Esthetics
• Economic and environmental constraints (Cost to build and use, energy consumption)

Structural members and their characteristics

There are several types of civil engineering structures, including buildings, bridges, towers,
arches, and cables. Members of components that make up a structure can have different forms or
shapes depending on their functional requirements. Structural members can be classified as beams,
columns/Struts, tension structures/tie rods, frames, and trusses.

For ease of design and construction, structures are often composed of a number of structural
elements or members.

The following members shall be used;

• Tie rods
• Struts
• Beams
• Columns

Beams

Beams are structural members whose longitudinal dimensions are appreciably greater than their
lateral dimensions. For example, the length of the beam, as shown below is significantly greater
than its breadth and depth. The cross-section of a beam can be rectangular, circular, triangular, or
(solid or hollow) it can be of what is referred to as standard sections, such as channels, tees, angles,
and I-sections. Beams are always loaded in the longitudinal direction. Beams are usually straight
horizontal members used primarily to carry vertical loads.

• They are classified according to how they are supported.

• Design to resist bending moment and shear forces.
• In steel beams, wide flange cross sections are commonly used.

Beam.

Compression and Tension Structures.

Compression members (Columns) are vertical structural members that are subjected to axial
compression, as shown below. They are also referred to as struts (compression member that is
inclined) or stanchions (made of steel). Columns can be circular, square, or rectangular, (solid or
hollow) in their cross sections, and they can also be of standard sections. In some engineering
applications, where a single-member strength may not be adequate to sustain a given load, built-
up columns are used. A built-up column is composed of two or more standard sections, as shown
below. Tension structures are similar to columns, with the exception that they are subjected to
axial tension.

Columns.

• Columns usually have straight vertical members to resist vertical compression.

• Tube and wide flange cross sections are often used.
• A column can also be subjected to both vertical compression and bending moment.

Tie rod and struts.

• Tie rods normally refer to structural members subjected to tensile force only.
• Normally tie rods are rather slender
• Struts are members that could be subject to both tensile and compressive forces.
• They are commonly used in trusses and bridges.
Frames

Frames are structures composed of vertical and horizontal members, as shown below. The vertical
members are called columns, and the horizontal members are called beams. Frames are classified
as sway or non-sway. A sway frame allows a lateral or sideward movement, while a non-sway
frame does not allow movement in the horizontal direction. The lateral movement of the sway
frames is accounted for in their analysis. Frames can also be classified as rigid or flexible. The
joints of a rigid frame are fixed, whereas those of a flexible frame are moveable, as shown below.

Frame.

Trusses

Trusses are structural frameworks composed of straight members connected at the joints, as shown
below. In the analysis of trusses, loads are applied at the joints, and members are assumed to be
connected at the joints using frictionless pins.
 Truss.

• Used when a long span is needed with not much depth constraints.
• Consists of slender struts/ties arranged in triangular or other fashions.
• Planer trusses (all members lie in same plane) are used in bridges and roofs.
• Space trusses (members lie in different planes) are used in derricks and towers.

Structural forms and classifications

The combination of structural elements (rods, struts, beams, columns) and materials (concrete,
steel, timber) generate different structural forms or structural systems

1.3 Fundamental Concepts and Principles of Structural Analysis

1.3.1 Equilibrium Conditions

Civil engineering structures are designed to be at rest when acted upon by external forces. A
structure at rest must satisfy the equilibrium conditions, which require that the resultant force
and the resultant moment acting on a structure be equal to zero. The equilibrium conditions of a
structure can be expressed mathematically as follows:

1.3.2 Compatibility of Displacement

The compatibility of displacement concept implies that when a structure deforms, members of the
structure that are connected at a point remain connected at that point without void or hole. In other
words, two parts of a structure are said to be compatible in displacements if the parts remain fitted
together when the structure deforms due to the applied load. Compatibility of displacement is a
powerful concept used in the analysis of indeterminate structures with unknown redundant forces
in excess of the three equations of equilibrium. For an illustration of the concept, consider the
propped cantilever beam shown in Figure 1.5a. There are four unknown reactions in the beam: the
reactive moment, a vertical and horizontal reaction at the fixed end, and another vertical reaction
at the prop at point B. To determine the unknown reactions in the beam, one more equation must
be added to the three equations of equilibrium. The additional equation can be obtained as follows,
considering the compatibility of the structure:

Figure 1.5c). Students should always remember that the first subscript of the displacement
indicates the location where the displacement occurs, while the second subscript indicates the load
causing the displacement.
 Fig. 1.5. Propped cantilever beam.

1.3.6 Structural Idealization

Structural idealization is a process in which an actual structure and the loads acting on it are
replaced by simpler models for the purpose of analysis. Civil engineering structures and their
loads are most often complex and thus require rigorous analysis. To make analysis less
cumbersome, structures are represented in simplified forms. The choice of an appropriate
simplified model is a very important aspect of the analysis process since the predictive response
of such idealization must be the same as that of the actual structure. Figure 1.9a shows a simply
supported wide-flange beam structure and its load. The plan of the same beam is shown in Figure
1.9b, and the idealization of the beam is shown in Figure 1.9c. In the idealized form, the beam is
represented as a line along the beam’s neutral axis, and the load acting on the beam is shown as a
point or concentrated load because the load occupies an area that is significantly less than the total
area of the structure’s surface in the plane of its application. Figures 1.10a and 1.10b depict a
frame and its idealization, respectively. In the idealized form, the two columns and the beam of
the frame are represented by lines passing through their respective neutral axes. Figures
1.11a and 1.11b show a truss and its idealization. Members of the truss are represented by lines
passing through their respective neutral axes, and the connection of members at the joints are
assumed to be by frictionless pins.

Fig. 1.9. Wide – flange beam idealization.

Fig. 1.10. Frame idealization.

Fig. 1.11. Truss idealization.

1.3.8 Free-Body Diagram

A free-body diagram is a diagram showing all the forces and moments acting on the whole or a
portion of a structure. A free-body diagram must also be in equilibrium with the actual
structure. The free-body diagram of the entire beam shown in Figure 1.13a is depicted in Figure
1.13b. If the free-body diagram of a segment of the beam is desired, the segment will be isolated
from the entire beam using the method of sections. Then, all the external forces on the segment
and the internal forces from the adjoining part of the structure will be applied to the isolated part.
Fig. 1.13 Freebody diagram of a beam.

Tutorials
1. Define structural design.
2. Briefly describe the structural design process.
3. Define the following terms;
a. Structure
b. Structural design.
c. Structural analysis
4. Describe methods (the design philosophy) employed to achieve a safe and workable
structure.
5. With aid of sketches, describe the following structural elements and the characterization.
a. Tie rods
b. Struts
c. Beams
d. Columns
e. Frames
f. trusses