theory of structures

1. Theory & Design of Structures III
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2. Structural Members
Structural members: are the primary load bearing
components of a building, and each have their own structural
properties which need to be considered. such mmbers include:
Beams: horizontal members which transfer loads to supports.
Columns: vertical members which transfer compressive loads
to the ground.
Bracing: members that interconnect stiffen columns and
beams.
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3. Structural Members
.
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4. Cont’d . . .
Roof trusses: load-bearing frames constructed of connected
triangular shapes.
Retaining walls: support soil where a sloping site requires
excavation.
Concrete slabs: span horizontally between supportts, used as
floors and sometimes as roof systems.
Footings: transfer load from the structure to the foundations.
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5. Cont’d . . .
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6. Cont’d . . .
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7. Cont’d . . .
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8. Cont’d . . .
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9. Structural Systems
Structural system: in structural engineering refers to the
load-resisting sub-system of a building or object. The structural
system transfers loads through interconnected elements or
members.
For instance, a steel frame is a structural system that supports
the building and everything on it and in it. A space frame is a
structural system that typically supports the roof.
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10. Types of structural system
 Continuous structures: These comprise continuous supporting
walls through which the combined loads and forces in a building are
transferred, mainly by direct compression, into the subsoil through
the foundations.
 Framed structures: Timber, reinforced concrete and steel can all
be used to create regular frameworks comprising beams and
columns. The beams transfer loads from roof, floors and walls to the
columns. The columns transfer the beam loads to the sub-soil
through the foundations. The dead and imposed loads from roofs or
floor slabs will be transferred to the floor beams and then to the
structural frame.
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11. Cont’d . . .
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12. Cont’d . . .
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13. Cont’d . . .
 Shell structures: Shell structures are made from structural
‘skins’ where the shell material is thin in section relative to the other
dimensions of the roof and undergoes relatively little deformation
under load.
 They are commonly used where a building interior needs to be free
from intermediate walls or columns that might support a more
conventional flat or pitched roof, such as; libraries, theatres, leisure
centres, airport and railway terminals, and so on.
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14. Cont’d . . .
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15. Cont’d . . .
 Tensile structures:Conventional structures tend to be
stabilised by the action of gravity on their mass holding them in
compression.
 A tensile structure is a structure that is stabilised by tension rather
than compression. In practice, structures tend to carry both tension
and compression, and it is the degree to which a structure is
intentionally tensioned to stabilise it that determines whether it is
considered a tensile structure. A suspension bridge is an example of
a tensile structure.
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16. Cont’d . . .
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17. Cont’d . . .
 Membrane structures: Membrane structures (or fabric
structures) create spaces that are enclosed by tensioned membranes.
At its simplest, a tent may be regarded as a membrane structure
given its steel or fibreglass poles support a canvas or plastic
membrane covering.
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18. Cont’d . . .
.
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19. Support Systems
A structural support: is a part of a building or structure that
provides the necessary stiffness and strength in order to resist the
internal forces (vertical forces of gravity and lateral forces due to wind
and earthquakes) and guide them safely to the ground.
 Supports can be either at the end or at any intermediate point along a
structural member or a constituent part of a building and they are
referred to as connections, joints or restraints.
 Building support structures, no matter what materials are used, have
to give accurate and safe results.
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20. Cont’d . . .
Structures can be either Horizontal-span support systems (floor and
roof structures) or Vertical building structure systems (walls, frames,
cores . . .
There are five basic idealized support structure types, categorized by
the types of deflection they constrain: roller, pinned, fixed, hanger and
simple support.
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21. Cont’d . . .
Roller supports: A roller support allows thermal expansion and
contraction of the span and prevents damage on other structural
members such as a pinned support. The typical application of roller
supports is in large bridges.
In civil engineering, roller supports can be seen at one end of a bridge.
A roller support cannot prevent translational movements in horizontal
or lateral directions and any rotational movement but prevents vertical
translations.This support type is assumed to be capable of resisting
normal displacement.
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22. Cont’d . . .
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23. Cont’d . . .
Pinned support: A pinned support attaches the only web of a beam to a
girder called a shear connection. The support can exert a force on a member
acting in any direction and prevent translational movements, or relative
displacement of the member-ends in all directions but cannot prevent any
rotational movements.
 Its reaction forces are single linear forces of unknown direction or horizontal
and vertical forces which are components of the single force of unknown
direction. A pinned support is just like a human elbow. It can be extended and
flexed (rotation), but you cannot move your forearm left to right (translation).
 A single pinned support cannot completely restrain a structure. At least two
supports are needed to resist the moment.
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24. Cont’d . . .
.
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25. Cont’d . . .
Fixed support: Rigid or fixed supports maintain the angular relationship
between the joined elements and provide both force and moment resistance.
It exerts forces acting in any direction and prevents all translational movements
(horizontal and vertical) as well as all rotational movements of a member.
The application of the fixed support is beneficial when we can only use single
support, and people most widely used this type as the only support for a
cantilever. They are common in beam-to-column connections of moment-resisting
steel frames and beam, column and slab connections in concrete frames.
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26. Cont’d . . .
.
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27. Cont’d . . .
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28. Example -1
Find reactions of simply supported beam when a point load of 5KN is
acting on it. As shown in figure below
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29. Example -2
Determine the reaction forces of the cantilever beam loaded with
uniformly distributeed load as shown below.
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30. Excercise
Calculate the reaction forces for the simply supported beam loaded as
shown in the figure below.
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