2.05 Common Structural Elements

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Nov 25, 2013 (3 years and 9 months ago)

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2.05 Common Structural Elements

Common structural elements.




Rigid Structural Elements

Truss and Trussing

Arches

Gothic arch

Domes

Plates

Vault

Bearing walls

Barrel Vault

Beam

Column

Frame

Space Frame

Mass Structure




Non rigid elements

Cable

Membrane systems


Structural loads

on Elements

Properties structures must have



Stability



Strength



Ridigity




Sources of loads

Weight of structure

Occupants, users and interior components

Environmental impacts




Load classifications

Static

Dynamic


Permanence of load



Dead load



Live

load



Loads from Natural Elements

2.05 Common Structural Elements




Design loads

o

Building codes

o

Impacts of live loads



Rigid Structural Elements


Trussing
: To support or brace with a truss.

Truss
:

a supporting structure or framework composed of girders, rods, or
beams commonly of wood or steel lying in a single plane.

Trusses usually
take the form of a triangle or combination of many triangles, this design

makes certain the greatest rigidity.


Trusses are used for heavy loads and
large spans, especially in roofs and bridges.

Their open construction is
lighter than, yet just as sturdy as, a beam.

The members are known as struts,
tie
-
beams, rafters, and pos
ts; the space over which the truss extends is called
the span.


The upper and lower beams are connected by web members. X
-
bracing is a way to help resist wind and earthquakes.


Below is an example of a simple truss.



Truss systems
: Is combination of many single trusses matched together to
form a complete system. An example of this is either a roof system or a
flooring system.

Below is an example of an arch.

Arch
:

A structure forming
the curved, pointed, or flat
upper edge of an open space
and supporting the weight
above it.

It is composed of
voussoirs
,

which
are wedge
shaped masonry blocks.


A
structure, such as the Roman
Aqua ducts, is shaped like an
upturned U.

As the arch’s
2.05 Common Structural Elements

span increases in relationship to the rise, thrust also increases.

Thrust is
controlled by the use of buttresses, ties, and span
-
to
-
r
ise ratio modification.
More on arches
,
http://www.civilzone.com/arch.html


Gothic arch
:

Arch with a pointed top instead of a curved
top.


Dome
:

A vaulted roof structure having a
circular, polygonal, or elliptical base and a
generally hemispherical or semispherical
shaped top.













Plates
:

(slabs) are the
structural
elements that span areas
between
beams, columns etc. They
are used to
enclose framed structures
and create
surfaces (floors, roofs, and
walls).
Plates require support that
may be
supplied

through a one
-
way or
two
-
way support system.
The
required support is determined by the depth span relationship (as span
increases, depth must also increase to support the same weight).









2.05 Common Structural Elements




Vault
: A structure that uses
arches usually made of masonry
or concrete, serving to cover a
space










Bearing wall
: Are solid walls that hold loads,
provide support for e
ach other, and a roof system
are much like plates except that they are vertically positioned. They support
weight, produce height and enclose space (create rooms). Uneven load can
cause a bearing wall to buckle or collapse.










Barrel

vault: Is a roof shaped
like ½
cylinder and
resembles
a series of
connected
arches.







2.05 Common Structural Elements


Post

and

Lintel
:
is a horizontal beam (lintel)
across two posts.









Beam
:
The
b
eam

is a rigid,
linear, horizontal structural
element used to
span distance
.
Beams can sag when supporting
weight.

Sag can be prevented
through understanding and
application of the principal of the
depth
-
span ratio.


Types of
beams include the girder, joi
st,
rafter, header, and lintel.


Column
:

The
column

is a rigid,
vertical, linear structural element
that
supports the weight
of

beams and other elements.
Crushing

and

buckling

are two potential problems to avoid
through

application of the
diameter
-
to
-
length ratio

and
deliberate “over design”, the practice of usi
ng

stronger columns
than are needed.

The
pier
,

pile
and

pillar
are types of columns.



Identify and apply significant proportional relationships


a. Depth to

span

ratio otherwise known as Hook’s’ Law (Calculating
deflection in beams): Is a beam’s t
endency to bend while supporting weight
and is directly related to the length of the span.






D = PL
3
/48EI

2.05 Common Structural Elements

D = Deflection, P = force in center of span, L = length of beam, E =
modulus of elasticity, I = moment of inertia.


b. Diameter to length ratio (C
alculating

deflection

in columns) is a columns
tendency to bend under load and is directly related to the length and
diameter of the column.







F = P/A


F = compressive stress, P = co
mpressive force, and A = cross
-
section area


Frame
:
A frame is a three
-
dimension assembly of components used to
construct a structural skeleton. Any frame has structural tendencies to bend
an
d twist. Stabilization is achieved through reinforced
joints
, trussing and
rigid plates.











Space frame systems
:
Consists of three dimens
ional truss beams extended
to cover a large area. They are strong, lightweight, economical and able to
cover large areas of open space with very few supporting columns.


Mass structure
:

Achieves s
trength and stability through tremendous weight.
Dams, roads, bridges, and buttresses
are mass structures. Mass structures
have little or no useable internal space.








2.05 Common Structural Elements

N
onrigid Structural Elements


Cable
:

The cable is a nonrigid
rope like, element used to span
distance. Major types of cable

include:
guys
,
used to brace rigid
vertical structures like radio
towers. Then ther
e are

hangers


which are suspended vertically to
support weight; and
spanning
cables,
which are draped

steel cables use
d in cable
-
stayed
bridges and suspension bridges.

As a span increases in relation to
sag

the thrust increases.

(
Thrust

is the

force that pulls on the ends of the
c
able)



Membranes
: Membranes
are thin
flexible materials used to enclose or
cover an area. There are two
methods of supporting structural
membranes:

pneumatic

(air)
pressure and
stretching
. Pneumatic
structures are of two types: the air

inflated

structure and the air
supported

structure. Air inflated
structures are support by pressurized air between enclosed membrane layers.
Air s
upported structures consist of one membrane layer anchored and
supported by internal air pressure. Stretching the membrane on a pole and
post is the non
-
pneumatic way. An example is a circus tent
.


Structural Loads

A. To resist loads structures must poss
ess:






1.
Stability

(
equilibrium
):

The state or quality of being stable, especially


r
esistance to change, det
erioration, or displacement.

2.
Strength
:
The state, property, or quality of being strong.


3.
Rigidity
:
The quality or st
ate of being rigid.


B. Loads come from three main sources:

Weight of structure: Dead load.

2.05 Common Structural Elements

Occupants, users and interior components: Live load.

Environmental impacts: Natural loads.


C.

Loads are classified by:

1. The effects of the load on a structure






Static
: Not in motion; at rest; quiescent. Fixed; stationary. Static and
dead load are the same. This is the total weight of all attached parts of
a structure.

Dynamic
: Of or relating to energy or to objects in motion. Dynamic
and live load are the same. This is all additional weight from traffic
and non
-
attached objects inside or on a structure.


D. Permanence of loa
d

A. Dead load: The weight of the entire structure and all connected parts.

B. Live load: Live load varies with the coming and going of people, interior


components, and vehic
les if the structure is transportation oriented.

C. Loads from natural elements: These include rain, hail, snow, earthquakes,


hurricanes, tornados, and soil types.


E. Building codes specify design loads and safety factors

Building codes are written

to reflect the different conditions in different
locales. Codes written for California would not be appropriate for the
coastal area of North Carolina. Codes for California must consider the
frequent earthquakes while the coastal North Carolina codes mu
st consider
the yearly hurricane season.


F. Impacts of live loads from natural elements.

Structures must be built to handle natural events along with the need for the
structure. Examples of t
hese are the
roof pitch
es

on houses in the north and
south of

the United States of America. In the north, the roof pitches are
steeper to keep snow and ice from accumulating and causing compression
failure. In the south, roofs have a much lower pitch angle, except in the
mountain areas that have large snow accumul
ation. Expansion and
contraction due to heat is thermal load. Most concrete structures contain
expansion joints to counter this condition.


Resources

Dictionary.com

http://www.pbs.org/wgbh/buildingbig/dam/

http://www.gossamersf.com/