walls and building structure
TRANSCRIPT
ARC 625: ARCHITECTURAL ENGINEERING
A TERM PAPER
ON
WALLS AND BUILDING SKELETON
BY
OJEBOLA AYOBAMI OYEBAMIJI
M.TECH/SET/2010/2702
MENTOR
DR. R.E. OLAGUNJU
22ND AUGUST 2011
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WALLS AND BUILDING STRUCTURE
Building walls frequently become works of art externally and internally, such as when
featuring mosaic work or when murals are painted on them; or as design foci when they
exhibit textures or painted finishes for effect. On a ship, the walls separating compartments
are termed "bulkheads", whilst the thinner walls separating cabins are termed "partitions".
In architecture and civil engineering, the term curtain wall refers to the facade of a building
which is not load-bearing but functions as decoration, finish, front, face, or history
preservation. A partition wall is a wall for the purpose of separating rooms, or dividing a
room. Partition walls are usually not load-bearing. Partition walls may be constructed with
bricks or blocks from clay, terra-cotta or concrete, reinforced, or hollow. Glass blocks may
also be used. They may also be constructed from sheet glass. Glass partition walls are a series
of individual toughened glass panels, which are suspended from or slide along a robust
aluminium ceiling track. The system does not require the use of a floor guide, which allows
easy operation and an uninterrupted threshold. Timber may be used. This type of partition
consists of a wooden framework either supported on the floor below or by side walls. Metal
lath and plaster, properly laid, forms a reinforced partition wall. Partition walls constructed
from fibre cement sheeting are popular as bases for tiling in kitchens or in wet areas like
bathrooms. Galvanized sheet fixed to wooden or steel members are mostly adopted in works
of temporary character. Plain or reinforced partition walls may also be constructed from
concrete, including pre-cast concrete blocks. Metal framed partitioning is also available. This
partition consists of track (used primarily at the base and head of the partition) and stud
(vertical sections fixed at 600mm centres).Internal wall partitions also known as office
partitioning are made using plasterboard (drywall), or varieties of glass. Toughened glass is a
common option as it is feasible however there is also low iron glass better known as opti-
white glass which increases light and solar heat transmission. Wall partitions are constructed
using beads and tracking which are either hung from the ceiling or fixed into the ground. The
panels are inserted into the tracking and fixed. There are variations of wall partitions which
include the level of fire resistance they have, and their acoustic performance rating
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The external walls of a structure are vital to the integrity safety and aesthetics of the building.
They are more often then not the load bearing walls and must also face the harsh conditions
that the weather can throw at a building. External walls normally consist of two different
designs in conventional style buildings cavity and solid.
CAVITY WALLS
Cavity walls are normally made from brickwork and were introduced to the UK in the 19th
century, becoming a commonly accepted building practice by the 1920s. Commonly a cavity
wall is constructed with a 102mm half brick outer skin with a 100mm dense brick inner skin.
The size of a cavity will vary. Traditionally sizes of between 50mm and 100mm were used
but this is now being increased in modern builds to allow for extra insulation. The two walls
are joined in places by wall ties to help spread lateral loads.
BRICK
Brick can also be clad onto a modern eco timber structure. Some more attractive bricks such
as older London Reds, used to match other street brickwork, are recycled material and so
attractive to an eco builder; but the motive to use them is to create a uniform blandness of
appearance. The air in a cavity wall acts as a poor conductor of heat. This has the effect of
insulating a building, as the heat does not easily pass into the air of the cavity. The air in a
cavity wall will also heat up during the day releasing heat at a slow rate helping to keep a
building warm. A damp proof course laid through the cavity will prevent moisture from
entering the building through the walls. Whilst the air in a cavity provides greater insulation
than a comparable solid wall would there are ways in which it can be further enhanced. The
most common of these is to fill the cavity with insulating foam. This traps in even more heat
and makes it easier to regulate the temperature inside a building. Cavity wall insulation can
be injected into any building with a cavity wall;
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SOLID WALLS
Solid load bearing external walls can be made using a variety of different techniques. The
most common is to use masonry but other alternative construction methods are available such
as rammed earth and cob building. Solid walls lose heat much more rapidly then cavity walls
but they can be insulated from both the inside and outside.
INSULATION
Outside insulation is called rendering and is often done decoratively. This type of insulation
is not cheap but it does have the added bonus of helping to weatherproof a wall. Insulating a
solid wall internally is a cheaper option.
INSULATING CONCRETE FRAMEWORK
Insulating Concrete Framework (ICF) walling is a construction method beginning to get a lot
of popularity for its ease of construction and insulating properties. In an ICF system a
framework made from insulating material (usually expanded polystyrene) designed to lock
into each other similar to LEGO bricks allowing the build to not require binder materials such
as mortar. Once the framework is in place concrete is poured in. When this sets the structure
is strong and has a high insulation value. The method of which ICF forms are created allow
them to be moulded into various shapes and designs as well as requiring less labour and time
to build. The use of thick concrete also provides exceptional sound insulation and prevents air
leakage into a property.
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CURTAIN WALL
A curtain wall is an outer covering of a building in which the outer walls are non-structural,
but merely keep out the weather. As the curtain wall is non-structural it can be made of a
lightweight material reducing construction costs. When glass is used as the curtain wall, a
great advantage is that natural light can penetrate deeper within the building. The curtain wall
façade does not carry any dead load weight from the building other than its own dead load
weight. The wall transfers horizontal wind loads that are incident upon it to the main building
structure through connections at floors or columns of the building. A curtain wall is designed
to resist air and water infiltration, sway induced by wind and seismic forces acting on the
building and its own dead load weight forces. Curtain walls are typically designed with
extruded aluminum members, although the first curtain walls were made of steel. The
aluminium frame is typically infilled with glass, which provides an architecturally pleasing
building, as well as benefits such as daylighting. However, parameters related to solar gain
control such as thermal comfort and visual comfort are more difficult to control when using
highly-glazed curtain walls. Other common infills include: stone veneer, metal panels,
louvers, and operable windows or vents. Curtain walls differ from store-front systems in that
they are designed to span multiple floors, and take into consideration design requirements
such as: thermal expansion and contraction; building sway and movement; water diversion;
and thermal efficiency for cost-effective heating, cooling, and lighting.
HISTORY OF CURTAIN WALL
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Oriel Chambers, Liverpool, England,1864. The world's
first glass curtain walled building. The stone mullions are decorative.
16 Cook Street, Liverpool, England,1866. Extensive use of
floor to ceiling glass is used, enabling light penetration deeper into the building maximizing
floor space.
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A building project in Wuhan China, the difference in progress between the two towers
illustrates the relationship between the inner load bearing structure and the exterior glass
curtain.
Prior to the middle of the nineteenth century, buildings were constructed with the exterior
walls of the building (bearing walls, typically masonry) supporting the load of the entire
structure. The development and widespread use of structural steel and later reinforced
concrete allowed relatively small columns to support large loads and the exterior walls of
buildings were no longer required for structural support. The exterior walls could be non-load
bearing and thus much lighter and more open than the masonry load bearing walls of the past.
This gave way to increased use of glass as an exterior façade and the modern day curtain wall
was born.
Oriel Chambers in Liverpool, England, was the world's first metal framed glass curtain
walled building in 1864, followed by 16 Cook Street, Liverpool, in 1866. Both buildings
were designed and built by local architect Peter Ellis. The extensive glass walls allowed light
to penetrate further into the building utilising more floor space and reducing lighting costs in
short winter months. Oriel Chambers comprises 43,000 sq ft (4,000 m2) set over a maximum
of five floors as the elevator had not been invented.
Some of the first curtain walls were made with steel mullions and the plate glass was attached
to the mullions with asbestos or fibreglass modified glazing compound. Eventually silicone
sealants or glazing tape were substituted. Some designs included an outer cap to hold the
glass in place and to protect the integrity of the seals. The first curtain wall installed in New
York City was this type of construction. Earlier modernist examples are the Bauhaus in
Dessau and the Hallidie Building in San Francisco.
The 1970s began the widespread use of aluminum extrusions for mullions. Aluminum offers
the unique advantage of being able to be easily extruded into nearly any shape required for
design and aesthetic purposes. Today, the design complexity and shapes available are nearly
limitless. Custom shapes can be designed and manufactured with relative ease.Similarly,
sealing methods and types have evolved over the years, and as a result, today’s curtain walls
are high performance systems which require little maintenance.
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SYSTEMS AND PRINCIPLES OF CURTAIN WALLS
STICK SYSTEMS
The vast majority of curtain walls are installed long pieces (referred to as sticks) between
floors vertically and between vertical members horizontally. Framing members may be
fabricated in a shop, but all installation and glazing is typically performed at the jobsite.
UNITIZED SYSTEMS
Unitized curtain walls entail factory fabrication and assembly of panels and may include
factory glazing. These completed units are hung on the building structure to form the building
enclosure. Unitized curtain wall has the advantages of: speed; lower field installation costs;
and quality control within an interior climate controlled environment. The economic benefits
are typically realized on large projects or in areas of high field labour rates.
RAINSCREEN PRINCIPLE
A common feature in curtain wall technology, the rainscreen principle theorizes that
equilibrium of air pressure between the outside and inside of the "rainscreen" prevents water
penetration into the building itself. For example the glass is captured between an inner and an
outer gasket in a space called the glazing rebate. The glazing rebate is ventilated to the
exterior so that the pressure on the inner and outer sides of the exterior gasket is the same.
When the pressure is equal across this gasket water cannot be drawn through joints or defects
in the gasket.
DUAL AIRLOOP SYSTEM/ TINGWALL
The dual airloop system improves on the performance of earlier unitized curtain walls
employing the rainscreen principle of separating rain from wind to lessen reliance on
"perfect" seals. The joints around glass and metal panels contain both outer (wet, drained)
and inner (dry, protecting the panel seals) air loops, to prevent water leakage no matter where
the inevitable seal imperfections occur.
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DESIGN OF CURTAIN WALLS
Curtain wall systems must be designed to handle all loads imposed on it as well as keep air
and water from penetrating the building envelope.
LOADS
The loads imposed on the curtain wall are transferred to the building structure through the
anchors which attach the mullions to the building. The building structure design must account
for these loads.
DEAD LOAD
Dead load is defined as the weight of structural elements and the permanent features on the
structure. In the case of curtain walls, this load is made up of the weight of the mullions,
anchors and other structural components of the curtain wall, as well as the weight of the infill
material. Additional dead loads imposed on the curtain wall, such as sunshades, must be
accounted for in the design of the curtain wall components and anchors.
WIND LOAD
Wind load acting on the building is the result of wind blowing on the building. This wind
pressure must be resisted by the curtain wall system since it envelops and protects the
building. Wind loads vary greatly throughout the world, with the largest wind loads being
near the coast in hurricane-prone regions. For each project location, building codes specify
the required design wind loads. Often, a wind tunnel study is performed on large or unusually
shaped buildings. A scale model of the building and the surrounding vicinity is built and
placed in a wind tunnel to determine the wind pressures acting on the structure in question.
These studies take into account vortex shedding around corners and the effects of
surrounding area
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SEISMIC LOAD
Seismic loads need to be addressed in the design of curtain wall components and anchors. In
most situations, the curtain wall is able to naturally withstand seismic and wind induced
building sway because of the space provided between the glazing infill and the mullion. In
tests, standard curtain wall systems are able to withstand three inches (75 mm) of relative
floor movement without glass breakage or water leakage. Anchor design needs to be
reviewed, however, since a large floor-to-floor displacement can place high forces on
anchors. (Additional structure must be provided within the primary structure of the building
to resist seismic forces from the building itself.)
SNOW LOAD
Snow loads and live loads are not typically an issue in curtain walls, since curtain walls are
designed to be vertical or slightly inclined. If the slope of a wall exceeds 20 degrees or so,
these loads may need to be considered.
THERMAL LOAD
Thermal loads are induced in a curtain wall system because aluminium has a relatively high
coefficient of thermal expansion. This means that over the span of a couple of floors, the
curtain wall will expand and contract some distance, relative to its length and the temperature
differential. This expansion and contraction is accounted for by cutting horizontal mullions
slightly short and allowing a space between the horizontal and vertical mullions. In unitized
curtain wall, a gap is left between units, which are sealed from air and water penetration by
wiper gaskets. Vertically, anchors carrying wind load only (not dead load) are slotted to
account for movement. Incidentally, this slot also accounts for live load deflection and creep
in the floor slabs of the building structure.
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BLAST LOAD
Accidental explosions and terrorist threats have brought on increased concern for the fragility
of a curtain wall system in relation to blast loads. The bombing of the Alfred P. Murrah
Federal Building in Oklahoma City, Oklahoma, has spawned much of the current research
and mandates in regards to building response to blast loads. Currently, all new federal
buildings in the U.S. and all U.S. embassies built on foreign soil, must have some provision
for resistance to bomb blasts.
Since the curtain wall is at the exterior of the building, it becomes the first line of defense in a
bomb attack. As such, blast resistant curtain walls must be designed to withstand such forces
without compromising the interior of the building to protect its occupants. Since blast loads
are very high loads with short durations, the curtain wall response should be analyzed in a
dynamic load analysis, with full-scale mock-up testing performed prior to design completion
and installation.
Blast resistant glazing consists of laminated glass, which is meant to break but not separate
from the mullions. Similar technology is used in hurricane-prone areas for the protection
from wind-borne debris.
INFILTRATION
Air infiltration is the air which passes through the curtain wall from the exterior to the interior
of the building. The air is infiltrated through the gaskets, through imperfect joinery between
the horizontal and vertical mullions, through weep holes, and through imperfect sealing. The
American Architectural Manufacturers Association (AAMA) is an industry trade group in the
U.S. that has developed voluntary specifications regarding acceptable levels of air infiltration
through a curtain wall . This limit is expressed (in the USA) in cubic feet per minute per
square foot of wall area at a given test pressure. (Currently, most standards cite less than 0.6
CFM/sq ft as acceptable.) Testing is typically conducted by an independent third party
agency using the ASTM E-783 standard.
Water penetration is defined as any water passing from the exterior of the building through to
the interior of the curtain wall system. Sometimes, depending on the building specifications, a
small amount of controlled water on the interior is deemed acceptable. AAMA Voluntary
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Specifications allow for water on the interior, while the underlying ASTM E - 1105 test
standard would disqualify a test subject if any water is seen inside. To test the ability of a
curtain wall to withstand water penetration, a water rack is placed in front a mock-up of the
wall with a positive air pressure applied to the wall. This represents a wind driven heavy rain
on the wall. Field quality control checks are also performed on installed curtain walls, in
which a calibrated spray nozzle is used to spray water on the curtain wall for a specified time
in order to investigate known leaks or leading up to a validation test like the ASTM E-1105.
DEFLECTION
One of the disadvantages of using aluminum for mullions is that its modulus of elasticity is
about one-third that of steel. This translates to three times more deflection in an aluminum
mullion compared to the same steel section under a given a load. Building specifications set
deflection limits for perpendicular (wind-induced) and in-plane (dead load-induced)
deflections. It is important to note that these deflection limits are not imposed due to strength
capacities of the mullions. Rather, they are designed to limit deflection of the glass (which
may break under excessive deflection), and to ensure that the glass does not come out of its
pocket in the mullion. Deflection limits are also necessary to control movement at the interior
of the curtain wall. Building construction may be such that there is a wall located near the
mullion, and excessive deflection can cause the mullion to contact the wall and cause
damage. Also, if deflection of a wall is quite noticeable, public perception may raise undue
concern that the wall is not strong enough.
Deflection limits are typically expressed as the distance between anchor points divided by a
constant number. A deflection limit of L/175 is common in curtain wall specifications, based
on experience with deflection limits that are unlikely to cause damage to the glass held by the
mullion. Say a given curtain wall is anchored at 12 foot (144 in) floor heights. The allowable
deflection would then be 144/175 = 0.823 inches, which means the wall is allowed to deflect
inward or outward a maximum of 0.823 inches at the maximum wind pressure. HOWEVER,
some panels require stricter movement restrictions, or certainly those that prohibit a torque-
like motion.
Deflection in mullions is controlled by different shapes and depths of curtain wall members.
The depth of a given curtain wall system is usually controlled by the area moment of inertia
required to keep deflection limits under the specification. Another way to limit deflections in
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a given section is to add steel reinforcement to the inside tube of the mullion. Since steel
deflects at 1/3 the rate of aluminium, the steel will resist much of the load at a lower cost or
smaller depth.
STRENGTH
Strength (or maximum usable stress) available to a particular material is not related to its
material stiffness (the material property governing deflection); it is a separate criterion in
curtain wall design and analysis. This often affects the selection of materials and sizes for
design of the system. For instance, a particular shape in aluminum will deflect almost three
times as much as the same steel shape for an equivalent load (see above), though its strength
(i.e. the maximum load it can sustain) may be equivalent or even slightly higher, depending
on the grade of aluminium. Because aluminum is often the material of choice, given its lower
unit weight and better weathering capability as compared with steel, deflection is usually the
governing criteria in curtain wall design.
THERMAL CRITERIA
Relative to other building components, aluminum has a high heat transfer coefficient,
meaning that aluminum is a very good conductor of heat. This translates into high heat loss
through aluminum curtain wall mullions. There are several ways to compensate for this heat
loss, the most common way being the addition of thermal breaks. Thermal breaks are barriers
between exterior metal and interior metal, usually made of polyvinyl chloride (PVC). These
breaks provide a significant decrease in the thermal conductivity of the curtain wall.
However, since the thermal break interrupts the aluminium mullion, the overall moment of
inertia of the mullion is reduced and must be accounted for in the structural analysis of the
system.
Thermal conductivity of the curtain wall system is important because of heat loss through the
wall, which affects the heating and cooling costs of the building. On a poorly performing
curtain wall, condensation may form on the interior of the mullions. This could cause damage
to adjacent interior trim and walls.
Rigid insulation is provided in spandrel areas to provide a higher R-value at these locations.
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INFILLS
Infill refers to the large panels that are inserted into the curtain wall between mullions. Infills
are typically glass but may be made up of nearly any exterior building element.
Regardless of the material, infills are typically referred to as glazing, and the installer of the
infill is referred to as a glazier. More commonly this trade is now known as Fenestration.
Glass
The Mexican hothouse at the Jardin des Plantes, built by Charles Rohault de Fleury from
1834 to 1836, is an early example of metal and glass curtain wall architecture.
By far the most common glazing type, glass can be of an almost infinite combination of
color, thickness, and opacity. For commercial construction, the two most common
thicknesses are 1/4 inch (6 mm) monolithic and 1 inch (25 mm) insulating glass. Presently,
1/4 inch glass is typically used only in spandrel areas, while insulating glass is used for the
rest of the building (sometimes spandrel glass is specified as insulating glass as well). The
1 inch insulation glass is typically made up of two 1/4-inch lites of glass with a 1/2 inch
(12 mm) airspace. The air inside is usually atmospheric air, but some inert gases, such as
argon or krypton may be used to offer better thermal transmittance values. In residential
construction, thicknesses commonly used are 1/8 inch (3 mm) monolithic and 5/8 inch
(16 mm) insulating glass. Larger thicknesses are typically employed for buildings or areas
with higher thermal, relative humidity, or sound transmission requirements, such as
laboratory areas or recording studios.
Glass may be used which is transparent, translucent, or opaque, or in varying degrees thereof.
Transparent glass usually refers to vision glass in a curtain wall. Spandrel or vision glass may
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also contain translucent glass, which could be for security or aesthetic purposes. Opaque
glass is used in areas to hide a column or spandrel beam or shear wall behind the curtain wall.
Another method of hiding spandrel areas is through shadow box construction (providing a
dark enclosed space behind the transparent or translucent glass). Shadow box construction
creates a perception of depth behind the glass that is sometimes desired.
Fabric veneer
Fabric is another type of material which is common for curtain walls. Fabric is often much
less expensive and serves as a less permanent solution. Unlike glass or stone, fabric is much
faster to install, less expensive, and often much easier to modify after it is installed.
Stone veneer
Thin blocks (3 to 4 inches (75–100 mm)) of stone can be inset within a curtain wall system to
provide architectural flavour. The type of stone used is limited only by the strength of the
stone and the ability to manufacture it in the proper shape and size. Common stone types used
are: Arriscraft(calcium silicate);granite; marble; travertine; and limestone. To reduce weight
and improve strength, the natural stone may be attached to an aluminium honeycomb backing
as with the StonePly system.
Panels
Metal panels can take various forms including aluminum plate; thin composite panels
consisting of two thin aluminum sheets sandwiching a thin plastic interlayer; and panels
consisting of metal sheets bonded to rigid insulation, with or without an inner metal sheet to
create a sandwich panel. Other opaque panel materials include fiber-reinforced plastic (FRP),
stainless steel, and terracotta. Terracotta curtain wall panels were first used in Europe, but
only a few manufacturers produce high quality modern terracotta curtain wall panels.
Louvers
A louver is provided in an area where mechanical equipment located inside the building
requires ventilation or fresh air to operate. They can also serve as a means of allowing outside
air to filter into the building to take advantage of favourable climatic conditions and minimize
the usage of energy-consuming HVAC systems. Curtain wall systems can be adapted to
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accept most types of louver systems to maintain the same architectural sightlines and style
while providing the necessary functionality.
Windows and vents
Most curtain wall glazing is fixed, meaning there is no access to the exterior of the building
except through doors. However, windows or vents can be glazed into the curtain wall system
as well, to provide required ventilation or operable windows. Nearly any window type can be
made to fit into a curtain wall system.
Fire safety
Combustible Polystyrene insulation in point contact with sheet metal backban. Incomplete
firestop in the perimeter slab edge, made of Rockwool without top caulking.
Fire stopping at the "perimeter slab edge", which is a gap between the floor and the backpan
of the curtain wall is essential to slow the passage of fire and combustion gases between
floors. Spandrel areas must have non-combustible insulation at the interior face of the curtain
wall. Some building codes require the mullion to be wrapped in heat-retarding insulation near
the ceiling to prevent the mullions from melting and spreading the fire to the floor above. It is
important to note that the fire stop at the perimeter slab edge is considered a continuation of
the fire-resistance rating of the floor slab. The curtain wall itself, however, is not ordinarily
required to have a rating. This causes a quandary as Compartmentalization (fire protection) is
typically based upon closed compartments to avoid fire and smoke migrations beyond each
engaged compartment. A curtain wall by its very nature prevents the completion of the
compartment (or envelope). The use of fire sprinklers has been shown to mitigate this matter.
As such, unless the building is sprinklered, fire may still travel up the curtain wall, if the glass
on the exposed floor is shattered due to fire influence, causing flames to lick up the outside of
the building. Falling glass can endanger pedestrians, fire-fighters and firehoses below. An
example of this is the First Interstate Tower fire in Los Angeles, California. The fire here
leapfrogged up the tower by shattering the glass and then consuming the aluminium skeleton
holding the glass. Aluminium's melting temperature is 660°C, whereas building fires can
reach 1,100°C. The melting point of aluminium is typically reached within minutes of the
start of a fire. Firestops for such building joints can be qualified to UL 2079 -- Tests for Fire
Resistance of Building Joint Systems. Sprinklering of each floor has a profoundly positive
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effect on the fire safety of buildings with curtain walls. In the case of the aforementioned fire,
it was specifically the activation of the newly installed sprinkler system, which halted the
advance of the fire and allowed effective suppression. Had this not occurred, the tower would
have collapsed onto fire crews and into an adjacent building, while on fire. Exceptionally
sound cementitious spray fireproofing also helped to delay and ultimately to avoid the
possible collapse of the building, due to having the structural steel skeleton of the building
reach the critical temperature, as the post-mortem fire investigation report indicated. This fire
proved the positive collective effect of both active fire protection (sprinklers) and passive fire
protection (fireproofing).
Fireman knock-out glazing panels are often required for venting and emergency access from
the exterior. Knock-out panels are generally fully tempered glass to allow full fracturing of
the panel into small pieces and relatively safe removal from the opening.
Maintenance and repair
Curtain walls and perimeter sealants require maintenance to maximize service life. Perimeter
sealants, properly designed and installed, have a typical service life of 10 to 15 years.
Removal and replacement of perimeter sealants require meticulous surface preparation and
proper detailing.
Aluminum frames are generally painted or anodized. Factory applied fluoropolymer
thermoset coatings have good resistance to environmental degradation and require only
periodic cleaning. Recoating with an air-dry fluoropolymer coating is possible but requires
special surface preparation and is not as durable as the baked-on original coating.
Anodized aluminum frames cannot be "re-anodized" in place, but can be cleaned and
protected by proprietary clear coatings to improve appearance and durability.
Exposed glazing seals and gaskets require inspection and maintenance to minimize water
penetration, and to limit exposure of frame seals and insulating glass seals to wetting
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FRAME CONSTRUCTION
Framed construction methods allow different materials to be used for external walls. In a
framed construction the frame bears the load of the building instead of the walls. Frames can
be made from various materials including wood and steel.
Timber framed
A wood framed building can be made from sustainably forested materials such as FSC
sourced timber instead of energy intensive man-made materials such as concrete. Wood is not
as good as an insulator as masonry and concrete. Wood's low density also stops it from acting
as a heat store, requiring faster acting heating systems to maintain a constant temperature.
Timber framed buildings can be premade in sections in a factory. Design drawings are
engineered to take account of load bearing sections etc, then the kit of parts is made and
transported to site in a series of staged deliveries. The timber builders, not always from the
manufacturing company, then assemble the building according to the instructions.
Another method is so-called stick building, where the timber builders make up the building
from the engineered drawings, but the wood is measured and cut on site. This is a cheaper
method.
Concrete frame construction
Above is a high rise under construction in London. This is using a 1960s method which is
called RC. This is reinforced concrete framing. The concrete is set in situ, using ply boards
with the metal reinforcing elements pre-positioned. Concrete is then poured into the mould. It
sets to give the large panels and blocks. Anything else is made of reinforced concrete beams.
Steel framed construction
Steel-framed construction is often coupled with a heavy use of glass. This is common in high
rise buildings and in such a construction the frame bears all the weight. These structures can
be coupled with Passive Solar Design (PSD) allowing a lot of heat to enter the building
through solar radiation.
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The external walls of a structure are vital to the integrity safety and aesthetics of the building.
They are more often then not the load bearing walls and must also face the harsh conditions
that the weather can throw at a building. External walls normally consist of two different
designs in conventional style buildings cavity and solid
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The internal walls of a building often do not support any load. This allows for a greater range
of materials to be used with design and sound insulation becoming more important than
overall strength.
Timber stud walls are entirely of timber simply nailed together, with only the head and sole
plate being screwed or nailed to the ceiling and floor.
The studs (verticals) need to be accurately cut to length, and horizontal noggins need to be
fitted between studs for rigidity and for the fixing of heavy wall mounted items such as basins
or cupboards. Where services like water pipes or electric cables need to be run inside the
framework, the studs must be drilled out to accommodate them.
When using timber in any way it is essential to first try to source recycled timber and if none
is available always ensure it comes from a sustainable forest ideally one that has been FSC
certified. Insulation can be added to a timber stud wall. However care should be taken when
choosing which type, whilst foams are more effective, they create problems in disposal.
Ideally natural wool should be used providing thermal and acoustic insulation.
Earth based internal walls can be constructed by using a fine wire mesh. Clay plaster with a
higher aggregate mixture than normal can then be placed onto the mesh and, when dry, will
form a breathable durable wall.
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Plasterboard walls, or dry walls, as they are sometimes known consist of a core of gypsum
plaster wrapped in a liner of paper. Dry walls are supplied in panel lengths and as such cannot
be made on site. This means that they will require transportation, which will add to the
carbon footprint of the build.
Dry wall manufacture is also a very heat intensive process with over 25% of the total cost of
a dry wall going on natural gas used in its construction. Dry walls cut to size often lead to a
large amount of offcuts.
These cannot be disposed of in normal landfills and specialist disposal will be required.
Solid wood walls are one of the few carbon negative building materials. When sourced from
a sustainable forest the wood used can be of a benefit to the environment having taken CO2
out of the air before being turned into a building material. A solid wood wall is built from
wood slab boards and is often created off site, being bought in as one unit. This has the
advantage of speeding up construction.
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When in place a solid wood wall will provide a durable wall which can take heavy
mountings. The breathability of wood allows it to store some of the moisture from a building
without being damaged, reducing condensation.
Internal walls are also a major part of controlling noise within a build. To effectively control
noise, it is important to understand the difference between sound absorption and sound
insulation, as the two are often confused.
Sound absorption relates to the amount of reverberation within a room and its effect on sound
quality and intelligibility. Sound insulation, on the other hand, relates to the actual reduction
in sound travelling from one area to another through a wall, floor or ceiling. Sound
transmission in buildings is a result of both airborne and impact noise.
Solid wooden walls will provide a good form of sound insulation as does plaster board due to
there thick natures, thin skinned walls such as stud based timber walls and thin earth walls
will require more insulation in order to cut noise to a lower level. The easiest way is to add
insulation materials to a wall, these can be in the form of fibre boards or foam insulation
which provide a trap for heat and sound (although sound specific insulation does not block
much heat).
Insulation materials used on internal walls are similar to those used around other areas of the
home. Mineral wool can be used although its use should be kept to a minimum if possible as
the small fibres involved can have a negative effect on human health. Natural wool is an ideal
choice as it not irritating to human health however it does require more thickness to meet the
same specifications of man made insulation. Foam insulation will work the best however this
can cause some problems if a wall is required to be remodelled adding to waste that cannot be
easily recycled.
The choice of an internal wall is a difficult one for a green builder and there is no one
solution.
Solid wood walls provide a good degree of heat and sound insulation.
Stud timber framed walls allow for lighter and fewer materials to be used.
Earth walls allow the use of nature's most available and sustainable resource, but have
difficulties in modern building practices.
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Dry walls require an intensive construction and transportation regime.
Brickwork and blockwork is traditionally costly, heavy and non-recyclable.
Insulated concrete formwork (ICF) is concrete but has green credentials as very
insulating, and quick and simple to build.
Suitable for larger and more repetitive builds where more esoteric materials like cob and
earth are not useful.
STEEL FRAMED
It usually refers to a building technique with a "skeleton frame" of vertical steel columns and
horizontal I-beams, constructed in a rectangular grid to support the floors, roof and walls of a
building which are all attached to the frame. The development of this technique made the
construction of the skyscraper possible.
The rolled steel "profile" or cross section of steel columns takes the shape of the letter "H".
The two wide flanges of a column are thicker and wider than the flanges on a beam, to better
withstand compressive stress in the structure. Square and round tubular sections of steel can
also be used, often filled with concrete. Steel beams are connected to the columns with bolts
and threaded fasteners, and historically connected by rivets. The central "web" of the steel "I
"-beam is often wider than a column web to resist the higher bending moments that occur in
beams.
Wide sheets of steel deck can be used to cover the top of the steel frame as a "form" or
corrugated mold, below a thick layer of concrete and steel reinforcing bars. Another popular
alternative is a floor of precast concrete flooring units with some form of concrete topping.
Often in office buildings the final floor surface is provided by some form of raised flooring
system with the void between the walking surface and the structural floor being used for
cables and air handling ducts.
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The frame needs to be protected from fire because steel softens at high temperature and this
can cause the building to partially collapse. In the case of the columns this is usually done by
encasing it in some form of fire resistant structure such as masonry, concrete or plasterboard.
The beams may be cased in concrete, plasterboard or sprayed with a coating to insulate it
from the heat of the fire or it can be protected by a fire resistant ceiling construction.
The exterior "skin" of the building is anchored to the frame using a variety of construction
techniques and following a huge variety of architectural styles. Bricks, stone, reinforced
concrete, architectural glass, sheet metal and simply paint have been used to cover the frame
to protect the steel from the weather.
STEEL FRAME CONSTRUCTION
Thin sheets of galvanized steel can be formed into steel studs for use as a building material
for rough-framing in commercial or residential construction (pictured), and many other
applications. The dimension of the room is established with horizontal track that is anchored
to the floor and ceiling to outline each room. The vertical studs are arranged in the tracks,
usually spaced 16" apart, and fastened at the top and bottom.
The primary shapes used in residential construction are the C-shape stud and the U-shaped
track, and a variety of other shapes. Framing members are generally produced in a thickness
of 12 to 25 gauges. The wall finish is anchored to the two flange sides of the stud, which
varies from 1-1/4" to 3" thick, and the width of web ranges from 1-5/8" to 14". Rectangular
sections are removed from the web to provide access for electrical wiring.
Steel mills produce galvanized sheet steel, the base material for light-gauge steel. Sheet steel
is then roll-formed into the final profiles used for framing. The sheets are zinc coated
(galvanized) to prevent oxidation and corrosion. Steel framing provides excellent design
flexibility due to the inherent strength of steel, which allows it to span over a longer distance
than wood, and also resist wind and earthquake loads.
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Light Steel Framing has been extensively used in cold climate countries due to its good
thermal and structural behaviour. Heat loss reduction and tenement thermal comfort have
been the main driving forces defining the design of these frames. The main issue to be
addressed is how striving for thermal efficiency can lead to structural weakening and poor
fire performance.
Interior wall studs made with light-gauge steel
Steel framed housing development
The spacing between studs is 16 inches on center for homes exterior and interior walls. In
office suites the spacing is 24 inches on center for all walls except for elevator and stair wells
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DRY STONE WALLS
A dry stone wall, also known as a dry stone dyke, drystane dyke, dry stone hedge,
or rock fence is a wall that is constructed from stones without any mortar to bind them
together. As with other dry stone structures, the wall is held up by the interlocking of the
stones. Such walls are used in building construction, as field boundaries, and on steep slopes
as retaining walls for terracing.
LOCATION AND TERMINOLOGY
Terminology varies regionally. When used as field boundaries, dry stone structures often are
known as dykes, particularly in Scotland. Dry stone walls are characteristic of upland areas
of Britain and Ireland where rock outcrops naturally or large stones exist in quantity in the
soil. They are especially abundant in the West of Ireland, particularly Connemara. They also
may be found throughout the Mediterranean, as in Majorca, Catalonia, Languedoc, Provence,
Liguria, the Apulia region of Italy, Cyprus, and in the Canary Islands, including retaining
walls used for terracing. Such constructions are common where large stones are plentiful (for
example, in The Burren) or conditions are too harsh for hedges capable of retaining livestock
to be grown as reliable field boundaries. Many thousands of miles of such walls exist, most of
them centuries old.
In the United States they are common in areas with rocky soils, such as New England, New
York, New Jersey, and Pennsylvania and are a notable characteristic of the bluegrass
region of central Kentucky, where they are usually referred to as rock fences, and the Napa
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Valley in north central California. The technique of construction was brought to America
primarily by Scots-Irish immigrants. The technique was also taken to Australia (principally
western Victoria and some parts of Tasmania and New South Wales) and New Zealand
(especially Otago).
Mosaic embedded in a dry stone wall in Italian Switzerland
Similar walls also are found in the Swiss-Italian border region, where they are often used to
enclose the open space under large natural boulders or outcrops.
The higher-lying rock-rich fields and pastures in Bohemia's South-Western border range
of Šumava (e.g. around the mountain river of Vydra) are often lined by dry stone walls built
of field-stones removed from the arable or cultural land, serving both as cattle/sheep fences
and the lot's borders; sometimes also the dry stone terracing is apparent, often combined with
parts of stone masonry (house foundations and shed walls) held together by a clay-cum-
needles "composite" mortar.
Dry stone wall construction was known to Bantu tribes in south-eastern Africa as early at
1350 to 1500 AD. When some of the Zulu migrated west into the Waterberg region of present
day South Africa, they imparted their building skills to Iron Age Bantu peoples who used dry
stone walls to improve their fortifications.
In Peru in the 15th century AD, the Inca made use of otherwise unusable slopes by building
dry stone walls to create terraces. They also employed this mode of construction for
freestanding walls. Their ashlar type construction in Machu Picchu uses the classic Inca
architectural style of polished dry-stone walls of regular shape. The Incas were masters of
this technique, in which blocks of stone are cut to fit together tightly without mortar. Many
junctions are so perfect that not even a knife fits between the stones. The structures have
persisted in the high earthquake region because of the flexibility of the walls and that in their
double wall architecture, the two portions of the walls incline into each other.
Construction
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Using a batter-frame and guidelines to rebuild a dry stone wall in South Wales UK
Newly rebuilt dry stone wall in South Wales UK
There are several methods of constructing dry stone walls, depending on the quantity and
type of stones available. Older walls are constructed from stones and boulders cleared from
the fields during preparation for agriculture (field stones) but many also from stone quarried
nearby. For modern walls, quarried stone is almost always used. The type of wall built will
depend on the nature of the stones available.
One type of wall is called a “Double” wall and is constructed by placing two rows of stones
along the boundary to be walled. The rows are composed of large flattish stones. Smaller
stones may be used as chocks in areas where the natural stone shape is more rounded. The
walls are built up to the desired height layer-by-layer (course by course), and at intervals,
large tie-stones or through stones are placed which span both faces of the wall. These have
the effect of bonding what would otherwise be two thin walls leaning against each other,
greatly increasing the strength of the wall. The final layer on the top of the wall also consists
of large stones, called capstones, coping stones or copes. As with the tie stones, the cap
stones span the entire width of the wall and prevent it breaking apart. In addition to gates a
wall may contain smaller purposely built gaps for the passage or control of wildlife
and livestock such as sheep. The smaller holes usually no more than 8 inches in height are
called 'Bolt Holes' or 'Smoots'. Larger ones may be between eighteen and 24 inches in
height, these are called a 'Cripple Hole'.
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Boulder walls are a type of single wall in which the wall consists primarily of large boulders,
around which smaller stones are placed. Single walls work best with large, flatter stones.
Ideally, the largest stones are being placed at the bottom and the whole wall tapers toward the
top. Sometimes a row of capstones completes the top of a wall, with the long rectangular side
of each capstone perpendicular to the wall alignment.
Another variation is the “Cornish hedge” or Welsh clawdd, which is a stone-clad earth bank
topped by turf, scrub, or trees and characterised by a strict inward-curved batter (the slope of
the “hedge”). As with many other varieties of wall, the height is the same as the width of the
base, and the top is half the base width.
Different regions have made minor modifications to the general method of construction —
sometimes because of limitations of building material available, but also to create a look that
is distinctive for that area. Whichever method is used to build a dry stone wall, considerable
skill is required. Selection of the correct stone for every position in the wall makes an
enormous difference to the lifetime of the finished product, and a skilled waller will take time
making the selection.
As with many older crafts, skilled wallers, today, are few in number. With the advent of
modern wire fencing, fields can be fenced with much less time and expense using wire than
using stone walls; however, the initial expense of building dykes is offset by their sturdiness
and consequent long, low-maintenance lifetimes. As a result of the increasing appreciation of
the landscape and heritage value of dry stone walls, wallers remain in demand, as do the
walls themselves. A nationally recognised certification scheme is operated in the UK by the
Dry Stone Walling Association, with four grades from Initial to Master Craftsman.
NOTABLE DRY STONE WALLS
Mourne Wall - twenty-two mile long wall in the Mourne Mountains location in County
Down, Northern Ireland
Ottenby Nature Preserve, built by Charles X Gustav in mid 16th century, Öland, Sweden
DRY STONE BUILDINGS
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Intihuatana ritual buildings of dry stone at Machu Picchu, Peru
While the dry-stone technique is generally used for field enclosures, it also was used for
buildings. The traditional turf-roofed Highland Black house was constructed using the double
wall dry stone method. When buildings are constructed using this method, the middle of the
wall is generally filled with earth or sand in order to eliminate draughts. During the Iron Age,
and perhaps earlier, the technique also was used to build fortifications such as the walls
of Eketorp Castle (Oland,Sweden), Maiden Castle, North Yorkshire, Reeth, Dunlough
Castle in southwest Ireland and the rampart of the Long Scar Dyke. Many of the dry-stone
walls that exist today in Scotland can be dated to the 14th century or earlier when they were
built to divide fields and retain livestock. Some extremely well built examples are found on
the lands of Muchalls Castle.
Dry stone bridges
Medieval dry stone bridge in Alby, Sweden
Since at least the Middle Ages some bridges capable of carrying horse or carriage traffic have
been constructed using drystone techniques. An example of a well preserved bridge of this
type is a double arched limestone bridge in Alby, Sweden on the island of Öland.
Dry-stone markings
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Dry stone marking
In the UK and Switzerland, it is possible to find dry stone constructions without any obvious
function. The largest and oldest of them, such as Stonehenge, are likely related to ancient
pagan rituals. However, the smaller structures may be built just as signs, marking the
mountain paths or boundaries of owned land. (Some stand on the boundary between Italy and
Switzerland; see photo). In many countries, cairns are used as road and mountain top
markers.
Ancient walls
The Lion Gate of the Mycenae acropolis is dry stone
Some dry-stone wall constructions in north-west Europe have been dated back to
the Neolithic Age. Some Cornish hedges are believed by the Guild of Cornish Hedgers to
date from 5000 BC,[2]although there appears to be little dating evidence. In County Mayo,
Ireland, an entire field system made from dry-stone walls, since covered in peat, have been
carbon-dated to 3800 BC. The cyclopean walls of the acropolis of Mycenae have been dated
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to 1350 BC and those of Tiryns slightly earlier. In Belize, the Mayan ruins
at Lubaantun illustrate use of dry stone construction in architecture of the 8th and 9th century
AD.
RETAINING WALL
A gravity-type stone retaining wall
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Retaining walls are built in order to hold back earth which would otherwise move
downwards. Their purpose is to stabilise slopes and provide useful areas at different
elevations, e.g. terraces for agriculture, buildings, roads and railways.
A retaining wall is a structure designed and constructed to resist the lateral pressure of soil
when there is a desired change in ground elevation that exceeds the angle of repose of the
soil.
Every retaining wall supports a “wedge” of soil. The wedge is defined as the soil which
extends beyond the failure plane of the soil type present at the wall site, and can be calculated
once the soil friction angle is known. As the setback of the wall increases, the size of the
sliding wedge is reduced. This reduction lowers the pressure on the retaining wall.
The basement wall is thus one form of retaining wall.
However, the term is most often used to refer to a cantilever retaining wall, which is a
freestanding structure without lateral support at its top.
Typically retaining walls are cantilevered from a footing extending up beyond the grade on
one side and retaining a higher level grade on the opposite side. The walls must resist the
lateral pressures generated by loose soils or, in some cases, water pressures.
The most important consideration in proper design and installation of retaining walls is to
recognize and counteract the fact that the retained material is attempting to move forward and
downslope due to gravity. This creates lateral earth pressure behind the wall which depends
on the angle of internal friction (phi) and the cohesive strength (c) of the retained material, as
well as the direction and magnitude of movement the retaining structure undergoes.
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Lateral earth pressures are zero at the top of the wall and - in homogenous ground - increase
proportionally to a maximum value at the lowest depth. Earth pressures will push the wall
forward or overturn it if not properly addressed. Also, any groundwater behind the wall that
is not dissipated by a drainage system causes hydrostatic pressure on the wall. The total
pressure or thrust may be assumed to act at one-third from the lowest depth for lengthwise
stretches of uniform height.
Unless the wall is designed to retain water, It is important to have proper drainage behind the
wall in order to limit the pressure to the wall's design value. Drainage materials will reduce or
eliminate the hydrostatic pressure and improve the stability of the material behind the
wall. Drystone retaining walls are normally self-draining.
As an example, the International Building Code requires retaining walls to be designed to
ensure stability against overturning, sliding, excessive foundation pressure and water uplift;
and that they be designed for a safety factor of 1.5 against lateral sliding and overturning.
Types
VARIOUS TYPES OF RETAINING WALLS
GRAVITY
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Construction types of gravity retaining walls
Gravity walls depend on the weight of their mass (stone, concrete or other heavy material) to
resist pressures from behind and will often have a slight 'batter' setback, to improve stability
by leaning back into the retained soil. For short landscaping walls, they are often made
from mortarless stone or segmental concrete units (masonry units). Dry-stacked gravity walls
are somewhat flexible and do not require a rigid footing in frost areas. Home owners who
build larger gravity walls that do require a rigid concrete footing can make use of the services
of a professional excavator, which will make digging a trench for the base of the gravity wall
much easier.
Earlier in the 20th century, taller retaining walls were often gravity walls made from large
masses of concrete or stone. Today, taller retaining walls are increasingly built as composite
gravity walls such as: geosynthetic or with precast facing; gabions (stacked steel wire baskets
filled with rocks); crib walls (cells built up log cabin style from precast concrete or timber
and filled with soil); or soil-nailed walls (soil reinforced in place with steel and concrete
rods).
CANTILEVERED
Conterfort/Buttress on Cantilevered Wall
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Cantilevered retaining walls are made from an internal stem of steel-reinforced, cast-in-place
concrete or mortared masonry (often in the shape of an inverted T). These walls cantilever
loads (like a beam) to a large, structural footing, converting horizontal pressures from behind
the wall to vertical pressures on the ground below. Sometimes cantilevered walls are
butressed on the front, or include a counterfort on the back, to improve their strength resisting
high loads. Buttresses are short wing walls at right angles to the main trend of the wall. These
walls require rigid concrete footings below seasonal frost depth. This type of wall uses much
less material than a traditional gravity wall.
SHEET PILING
Sheet pile wall
Sheet pile retaining walls are usually used in soft soils and tight spaces. Sheet pile walls are
made out of steel, vinyl or wood planks which are driven into the ground. For a quick
estimate the material is usually driven 1/3 above ground, 2/3 below ground, but this may be
altered depending on the environment. Taller sheet pile walls will need a tie-back anchor, or
"dead-man" placed in the soil a distance behind the face of the wall, that is tied to the wall,
usually by a cable or a rod. Anchors are placed behind the potential failure plane in the soil.
ANCHORED
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An anchored retaining wall can be constructed in any of the aforementioned styles but also
includes additional strength using cables or other stays anchored in the rock or soil behind it.
Usually driven into the material with boring, anchors are then expanded at the end of the
cable, either by mechanical means or often by injecting pressurized concrete, which expands
to form a bulb in the soil. Technically complex, this method is very useful where high loads
are expected, or where the wall itself has to be slender and would otherwise be too weak.
ALTERNATIVE RETAINING TECHNIQUES
SOIL NAILING
Soil nailing is a technique in which soil slopes, excavations or retaining walls are reinforced
by the insertion of relatively slender elements - normally steel reinforcing bars. The bars are
usually installed into a pre-drilled hole and then grouted into place or drilled and grouted
simultaneously. They are usually installed untensioned at a slight downward inclination. A
rigid or flexible facing (often sprayed concrete) or isolated soil nail heads may be used at the
surface.
SOIL-STRENGTHENED
A number of systems exist that do not simply consist of the wall itself, but reduce the earth
pressure acting on the wall itself. These are usually used in combination with one of the other
wall types, though some may only use it as facing (i.e. for visual purposes).
GABION MESHES
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This type of soil strengthening, often also used without an outside wall, consists
of wire mesh 'boxes' into which roughly cut stone or other material is filled. The mesh cages
reduce some internal movement/forces, and also reduce erosive forces.
MECHANICAL STABILIZATION
Mechanically stabilized earth, also called MSE, is soil constructed with artificial reinforcing
via layered horizontal mats (geosynthetics) fixed at their ends. These mats provide added
internal shear resistance beyond that of simple gravity wall structures. Other options include
steel straps, also layered. This type of soil strengthening usually needs outer facing walls
(S.R.W.'s - Segmental Retaining Walls) to affix the layers to and vice versa.
The wall face is often of precast concrete units that can tolerate some differential movement.
The reinforced soil's mass, along with the facing, then acts as an improved gravity wall. The
reinforced mass must be built large enough to retain the pressures from the soil behind it.
Gravity walls usually must be a minimum of 50 to 60 percent as deep or thick as the height of
the wall, and may have to be larger if there is a slope or surcharge on the wall.
REFERENCES
Patrick McAfee, Irish Stone Walls: History, Building, Conservation, The O'Brien Press,
2011
Alan Brooks and Sean Adcock, Walling, a practical handbook. BTCV. 1999
The Dry Stone Walling Association, Dry Stone Walling, Techniques and Traditions. 2004
Murray-Wooley, Carolyn and Karl Raitz, Rock Fences of the Bluegrass, University Press
of Kentucky. 1992.
Francis Pryor, Britain BC, Harper Perennial. 2003.
Colonel F. Rainsford-Hannay, Dry Stone Walling, Faber & Faber. 1957
Louis Cagin & Laetitia Nicolas, Construire en pierre sèche, editions Eyrolles, 2008
http://www.era.lib.ed.ac.uk/handle/1842/1409
Ruggiero, S. S., and Myers, J. C. 1991. “Design and Construction of Watertight Exterior
Building Walls.” Water in Exterior Building Walls:
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