Scaffolding lecture notes 2016
Scaffolding
Scaffolding has been used for 5000 years to provide access areas for
building and decorating structures taller than people who work on them.
The word “scaffolding” refers to any raised platform or ramp used for
ingress and egress for pedestrian movement and/or the passage of building
materials.
Since the mid-1920s the concept of using steel pipes fastened together with
metal-form or cast clamps (couplers) instead of poles and ropes was
introduced.
Aluminum alloy pipes and couplers were developed for their lighter weight
and speedier construction.
Aluminum alloy is only two-thirds as strong as steel, but it is only one-third
to one-half its weight. Because of the higher initial cost, aluminum is
restricted mostly to building maintenance scaffolds and suspended
platforms.
General Design Considerations
All types of scaffold have incorporated in their design a minimum safety
factor of 4. This means that scaffolds and their components shall be capable
of supporting without failure at least 4 times the maximum intended load.
Scaffolding lecture notes 2016
To comply with this requirement, multiply the design load by 4 and derive
the limiting strength of the component from the yield stress of the metal in
accordance with acceptable engineering criteria and practices.
Scaffolding - Design Loads
In accordance with OSHA and ANSI criteria and common practice for many
years, design load ratings for scaffold platforms are as follows:
Light-Duty Loading. 25 lb/ft2 maximum working load for support of people
and tools (no equipment or material storage on the platform).
Medium-Duty Loading. 50 lb/ft2 maximum working load for people and
material restricted not to exceed this rating, often described as applying to
bricklayers’ and plasters’ work.
Heavy-Duty Loading. 75 lb/ft2 maximum working load for people and stored
material often described as applying to stone masonry work.
Nb OSHA: Occupational Safety & Health Administration
ANSI: American National Standards Institute
These ratings assume uniform load distribution.
With the exception of the weight of stored materials, scaffold loads most
often consist of personnel, both stationary and transitory.
It is important to remember that the OSHA and ANSI load-rating system
is intended for guidance of field personnel in the construction and use of
nonspecifically engineered scaffolding applications.
Tube and Coupler Scaffolds
Tube and coupler scaffolds are assembled from three basic structural elements:
the uprights, or posts, which rise from ground or other solid support
the bearer, which supports the work platforms and / or provide transverse
horizontal connections between the posts;
the runners, which attach to the posts directly below the bearers and provide
longitudinal connections along the length of the scaffold.
These three elements are usually connected with standard or fixed couplers
which provide a 90°
connection in two places.
The three elements (uprights,bearer, and runners) form the basic structure
Scaffolding lecture notes 2016
The basic assembly and components of tube and coupler scaffolds.
Diagonal bracing is used to stiffen the structure as necessary − most important
in the longitudinal direction.
Bracing is generally connected to the posts with “adjustable” or “swivel”
couplers which have the facility of adjusting a full 360°.
Diagonal bracing should always be attached to the posts as closely as practical
to the “node” points formed by the runner-bearer connections.
Another important structural element is the building tie which connects the
scaffold to the wall or structure and is needed to provide rigidity and
anchorage of the scaffold in the transverse direction.
Scaffolds need to be laterally supported; otherwise, they are unstable because
of their height-to-width ratio and have low strength to resist wind and other
lateral forces.
Methods of stabilizing against a building
Scaffolding lecture notes 2016
a) Wall tie and anchorage
b) Window reveal tube
c) Reveal between pilasters
Application
Tube and coupler scaffolds can be assembled in numerous ways because of
the flexibility of their assembly dimensions in the horizontal and vertical
planes.
Scaffolding lecture notes 2016
Unlike sectional frame scaffolds they are not restricted by frame width in
the transverse direction, by brace length in the longitudinal direction, or by
frame height in the vertical direction.
Consequently, they are preferred for access to workplaces having irregular
dimensions and contours, e.g., churches, old auditoriums, etc.
Basic Configuration
The basic configurations are as follows:
1. Double Pole. Also called “independent” wall scaffolds, these are used for
access to vertical surfaces for construction, alterations, or surface finishing and
repair. They consist of repetitive pairs of posts along the length, connected by
bearers and runners.
2. Single pole. Also called “putlog” wall scaffolds, these are used for the
construction of masonry walls. They consist of single posts 3 to 5 feet away from
the wall surface spaced at regular or varying intervals along the wall. The different
feature of this type of scaffold is that the inside ends of the bearer are
supported at joints or courses in the wall being built instead of the inside posts.
3. Tower Scaffolds. These consist of one or few bays in either horizontal
plane, constructed to required height for access to ceilings or for specialized load
support requirements not conveniently achievable with sectional frames. They may
be mounted on casters and become mobile scaffolds or rolling towers.
� An application of tower scaffolds is to provide stair access to to unusual
structures such as cooling towers.
Scaffolding lecture notes 2016
Sectional Scaffolding
The construction principle of sectional scaffolding is shown below.
The most common material used in the fabrication of steel frames is 1 5/8-in.−
OD tubing with a wall thickness between 0.086 and 0.105 in.
The most common grade of steel used for this purpose is AISI designation
A1050, a high-carbon alloy having a minimum yield stress of 50,000 psi with a
corresponding ultimate stress of over 75,000 psi.
The higher carbon steel is generally preferred because its lower ductility and
greater rigidity make it more resistant to damaging and bending of the
members and because it has greater strength.
