Transcript 6. Buildings - Mr. Leidl`s Webpage

Foundations
A foundation supports the weight of the building. The
foundation may be called the substructure.
The part of the building above the ground is called the
superstructure. Without a substructure, the superstructure
would sink into the ground. Foundations are usually made of
steel reinforced concrete.
There are three types of foundations: raft, box, and pile.
The raft or floating is a slab of concrete on the surface
of the ground. Houses and factories may be built using
a raft foundation.
A box foundation is built into the ground and is usually
made of concrete. The hollow space may be used for
a basement. Most houses and tall buildings are built
with this type of foundation.
Buildings require a pile foundation when built in soft
soil. In a pile foundation, steel tubes are driven into
the ground and filled with concrete.
Roofs
The appearance of a building is often
dictated by its roof. There are three types
of roofs: flat, sloped, and domed.
Roofs are heavy and the walls must be built strong enough to
support them. Wind, rain,, and snow also place loads on the
roof which must be supported by the walls.
Flat roofs are useful because they do not push sideways on
the walls. However, they are less attractive and do not shed
the rain and snow.
Sloped or domed roofs pose additional problems because
they push out on the walls. Historically, the solution to this
problem was to resist the outward thrust with thick,
battered, or buttressed walls.
The walls of the Gothic
Cathedrals had to be
very thick to hold up the
huge domed roofs.
Modern construction methods tie the lower ends of the
sloping roof together instead of making the walls thicker.
Two sloped members with a tie across the bottom was the
earliest type of truss. The tie is only subject to tension and
could be made from a rope, a cable, or a wood brace.
The truss pushes down like a horizontal beam and not
outward like a sloped roof.
Once a series of trusses are placed to form a roof they
must be braced to prevent them from falling over. In a
house, the roof itself can act as the cross bracing for the
trusses.
When trusses are needed to span large areas, such as a
hockey rink or aircraft hanger, a space grid is used.
Trusses of equal strength and span are used in both
directions.
The engineer has many designs of trusses to choose from
when fabricating the roof of a building.
Walls
Walls support the roof and keep the occupants
sheltered from the weather. There are two common
ways of constructing the walls for buildings: bearing
and framed.
Framed Walls
Load bearing walls transfer roof loads to the foundation
and may be of the bearing or the framed type.
Non load bearing walls keep the weather out or serve
as room dividers. Some buildings use concrete, stone,
or brick walls to support the weight of the roof.
The bearing wall superstructure is used in factories and
schools. Until recently, this was the common method of
building.
Framed superstructures are built with a wood, steel, or
concrete frame. A skin is applied to the frame to form
the walls.
Skyscrapers are often built
with a steel frame. Houses are
built with a 2 x 4 or 2 x 6 wood
frame. The outside walls do
not support anything but their
own weight.
The load bearing walls are
subject to very high levels of
stress. The load increases in
the lower part of the wall as
the weight of the stories above
push down on the ones below.
One way of dealing with stress
in a structure is to spread the
stress over a larger area.
Battered walls are wider at the
bottom than at the top. This
spreads the weight of force
over a greater area.
Battering also leads to greater
stability because the bulk of the
weight is at the bottom, making
the wall difficult to push over.
Buttressing is common in tall buildings with high
open roofs (such as churches and cathedrals).
This prevents the outward thrust of the roof from
pushing the wall over.
As the buildings become bigger and taller the buttresses
became so large that they blocked most of the light from
the windows. Holes were designed in the buttress to make
a flying buttress.
Castle walls and dams are battered, but modern walls of
brick or concrete are capable of taking the increased
stress in the lower parts of the wall without battering.
Modern walls are built with footings under the walls to
spread the load over the soil.
Tall Buildings
TALL BUILDINGS WOULD REQUIRE
VERY THICK LOADBEARING WALLS
It would seem that the tall narrow buildings used today
would require very thick walls at the base.
However, this would conflict with the need for large
openings on the ground floors for retail or office space.
Therefore, other systems must be employed.
The continuous walls can be replaced by four posts and two
beams. The posts must be very strong and have a very firm
foundation.
This is the post and beam system. The columns or posts
take up less space than the continuous load bearing walls.
Post and beam construction systems predominate in the
design of modern skyscrapers. One of the problems with the
post and beam system shown above is that the columns must
be larger at the bottom than at the top.
The cost of construction is high because every floor has
columns and windows of different sizes.
Also, the amount of unobstructed space on the lower floors is
reduced as well.
Place columns in
the floor area
Place columns
around the periphery
The problem of different sized columns can be solved by
placing additional columns inside the floor area.
Using major columns at the center or periphery and
suspending the floors from cables eliminates this problem.
These buildings use tension as well as compression to hold
the loads.
There are so many variations in the methods that can
be used to build high rise buildings that no two buildings
look alike.
Arches and Domes
Arches do not cover much
space and are not stable in a
lateral direction. Arches can be
placed together to form what is
called a barrel vault.
A barrel vault can be modified a
number of ways to form many
interesting shapes.
By joining four barrel vaults and
removing the excess, a groin
vault is created. It touches at
four points and is joined with
ties to prevent outward thrust.
An arch rotated around a vertical axis will form a dome. A
dome, too, is a very stable structure.
Like all arches, the dome has a tendency to push
outwards at its feet. This can be prevented by adding
either buttresses or a tension hoop placed around its
circumference.
There are many variations
of the arch in architecture.
Last card on the Buildings tour.
You have completed the Buildings tour. You can go
over the information again and study it some more or
if you feel you know the information well enough you
can go and get the Buildings Quiz from Mr. Leidl.