Building Technology 5
Alternative Building Construction Systems
1. Cast-in-place and Pre-cast
Cast-in-place
Cast-in-place construction, also known as in-situ concrete construction or cast-insitu construction, refers to the process of pouring concrete on site at the location
where a structure will be built. This method involves casting concrete into molds
or formwork, allowing it to cure and harden into the desired shape.
Below is the brief overview of the methodology:
Formwork: Temporary molds or formwork are set up on site according to the
design specifications of the structure. These formwork systems can be made of
wood, steel, or other materials and are used to contain the poured concrete until
it sets.
Concrete Placement: Once the formwork is in place, concrete is poured into the
molds. The concrete may be transported to the site in trucks and then poured
into place using pumps, chutes, or buckets.
Compaction: After pouring, the concrete must be compacted to remove air voids
and ensure proper consolidation. This can be done using vibration techniques or
by manually tamping the concrete.
Curing: After compaction, the concrete is left to cure and harden. Curing is a
critical step in the process and typically involves keeping the concrete moist and
at the appropriate temperature to facilitate hydration and strength development.
Stripping: Once the concrete has sufficiently cured and gained strength, the
formwork is removed, revealing the finished concrete structure.
Pre-cast
Precast construction involves the manufacturing of structural elements off-site,
typically in a controlled factory environment, and then transporting these
elements to the construction site for assembly. These precast elements are
manufactured using molds or forms and are cured to achieve the desired
strength before being transported.
Below is the brief overview of the methodology:
Design and Mold Preparation: The design of the precast elements is finalized,
and molds or forms are prepared according to the specifications. These molds
can be made of steel, fiberglass, or other materials and are used to shape the
concrete into the desired form.
Concrete Casting: Concrete is poured into the prepared molds, and
reinforcement such as steel bars or fibers may be added to provide additional
strength. The concrete is allowed to cure and harden within the molds.
Demolding and Finishing: Once the concrete has sufficiently cured, the precast
elements are removed from the molds. They may undergo additional finishing
processes such as surface treatment, painting, or application of architectural
details.
Transportation to Site: The finished precast elements are transported to the
construction site using trucks or other means of transportation. Care is taken
during transportation to prevent damage to the precast elements.
Installation: At the construction site, the precast elements are lifted into place
using cranes or other lifting equipment and assembled to form the structure.
Connections between precast elements are typically made using bolts, welding,
or grouting.
1.1 Floor system
A floor system refers to the structural elements that support the floor loads
and provide a stable platform for the occupants of a building. The floor system
typically consists of several components working together to distribute the loads
from the building's occupants, furniture, and equipment to the underlying
structure.
Floor systems are the horizontal planes that must support both live loads
people, furnishings, and movable equipment and dead loads the weight of the
floor construction itself. Floor systems must transfer their loads horizontally
across space to either beams and columns or to loadbearing walls. Rigid floor
planes can also be designed to serve as horizontal diaphragms that act as thin,
wide beams in transferring lateral forces to shear walls.
A floor system may be composed of a series of linear beams and joists
overlaid with a plane of sheathing or decking, or consist of a nearly
homogeneous slab of reinforced concrete. The depth of a floor system is directly
related to the size and proportion of the structural bays it must span and the
strength of the materials used. The size and placement of any cantilevers and
openings within the floor plane should also be considered in the layout of the
structural supports for the floor. The edge conditions of the floor structure and its
connection to supporting foundation and wall systems affect both the structural
integrity of a building and its physical appearance.
Because it must safely support moving loads, a floor system should be
relatively stiff while maintaining its elasticity. Due to the detrimental effects that
excessive deflection and vibration would have on finish flooring and ceiling
materials, as well as concern for human comfort, deflection rather than bending
becomes the critical controlling factor. The depth of the floor construction and the
cavities within it should be considered if it is necessary to accommodate runs of
mechanical or electrical lines within the floor system. For floor systems between
habitable spaces stacked one above another, additional factors to consider are
the blockage of both airborne and structure-borne sound and the fire-resistance
rating of the assembly. Except for exterior decks, floor systems are not normally
exposed to weather. Because they all must support traffic, however, durability,
resistance to wear, and maintenance requirements are factors to consider in the
selection of a floor finish and the system required to support it.
Concrete
a. Cast-in-place floor slabs are classified
according to their span and cast form.
b. Precast concrete planks may be
supported by beams or loading walls
Steel
a. Steel beams support steel decking
or precast concrete planks.
b. Beams may be supported by girders
columns, or loadbearing walls
c. Beam framing is typically an integral part of
a steel skeleton frame system.
d. Closely space light-gauge or open-wed
joints may be supported by beams or
loadbearing walls.
e. Steel decking or wood plans have relatively
short spans.
Wood
a. Wood beams support structural planking or
decking.
b. Beams may be supported by girders, post, or
loadbearing walls.
c. Concentrated loads and floor openings may
require additional framing
d. Underside or floor structure may be left exposed
an applied ceiling is optional.
e. Relatively small, closely joints may be supported
by beams or loadbearing walls.
f. Subfloor underlayment, and applied ceiling finishes
have relatively short spans.
g. Join framing are flexible in shape and form.
Roof slab system
A roof system is an assembly of interacting roof components designed to
weatherproof and, normally, to insulate a building’s top surface. The roof
assembly includes the roof deck, vapor retarder and roof insulation (if they
occur), and the roof covering.
A roof slab system typically refers to a structural design used in building
construction for the roof of a building. It consists of a horizontal layer or slab of
concrete or other suitable material that is supported by beams, columns, or walls.
This system provides a solid, durable, and often fire-resistant surface for
covering and protecting the building's interior spaces from weather elements.
Roof slab systems can vary in design and construction method depending
on factors such as building type, size, structural requirements, and local building
codes. Common types of roof slab systems include flat slabs, waffle slabs, ribbed
slabs, and post-tensioned slabs. These systems are often reinforced with steel
bars or mesh to increase their strength and durability, especially in areas prone
to seismic activity or heavy loads. Proper design and construction of roof slab
systems are crucial for ensuring structural integrity and longevity of the building.
The methodology for designing and constructing a roof slab system
involves several steps to ensure structural integrity, durability, and compliance
with building codes and standards. Below is a generalized methodology for
designing and constructing a roof slab system:
Initial Assessment and Planning:
Evaluate the building's architectural and structural requirements. Consider the
building's location, climate, and intended use. Determine the load-bearing
capacity of the structure.
Selection of Roof Slab Type:
Choose the appropriate type of roof slab system based on structural
requirements, aesthetic preferences, and budget constraints. Common options
include flat slabs, waffle slabs, ribbed slabs, and post-tensioned slabs.
Structural Analysis and Design:
Perform structural analysis using engineering principles and software to
determine loads, stresses, and deflections. Design the roof slab system to
withstand dead loads (self-weight of the slab and roofing materials), live loads
(occupant loads, snow loads, etc.), wind loads, and seismic loads. Determine the
required thickness, reinforcement layout, and detailing of the slab.
Reinforcement Detailing:
Specify the type, size, and spacing of reinforcement bars or mesh.
Design reinforcement detailing to distribute loads evenly and resist cracking and
deformation.
Construction Preparation:
Prepare the site by clearing and leveling the area where the roof slab will be
constructed. Install formwork to define the shape and dimensions of the slab.
Ensure proper access for construction equipment and materials.
Concrete Pouring and Curing:
Pour concrete into the formwork, ensuring proper consolidation and compaction
to eliminate voids and air pockets. Monitor concrete placement to prevent
segregation and achieve uniformity.
Cure the concrete using appropriate methods such as wet curing or curing
compounds to promote strength development and minimize cracking.
Reinforcement Installation:
Place reinforcement bars or mesh within the concrete as per the design
specifications. Ensure proper positioning and alignment of reinforcement to
provide structural support and enhance durability.
Finishing and Surface Treatment:
Finish the surface of the roof slab to achieve the desired texture and appearance.
Apply surface treatments such as waterproofing membranes or coatings to
protect the slab from moisture ingress and weathering.
Quality Control and Inspection:
Conduct quality control checks throughout the construction process to ensure
compliance with design specifications and standards. Perform inspections of the
completed roof slab system to verify structural integrity and identify any defects
or deficiencies.
Maintenance and Monitoring:
Implement a maintenance plan to preserve the condition and performance of the
roof slab system. Monitor the roof slab periodically for signs of deterioration or
damage, and undertake repairs or reinforcement as necessary.
1.1.1 Flat slab
A flat slab is a structural system used in building construction that consists
of a reinforced concrete slab directly supported by columns or load-bearing walls,
without the inclusion of beams or girders between the supports. This type of slab
system offers a simple and efficient solution for spanning horizontal distances
within a building, providing a flat and unobstructed soffit (underside) that
enhances architectural flexibility and interior aesthetics.
A two-way concrete flat slab is a structural system used in building
construction, composed of a horizontal, reinforced concrete slab that spans in
both directions, transferring loads to supporting columns.
Key features of a two-way flat slab include:
1. Bidirectional Spanning: Unlike one-way slabs, which primarily span in one
direction (such as beams or ribs spanning between supports), two-way flat slabs
distribute loads in both the longitudinal and transverse directions. This allows for
square or rectangular floor plans with evenly distributed loads.
2. Reinforcement Layout: The reinforcement in a two-way flat slab is typically
arranged in a grid pattern, with reinforcing bars placed in both the longitudinal
and transverse directions. This grid of reinforcement helps to resist bending
moments and shear forces in both directions.
3. Column Supports: Two-way flat slabs are directly supported by columns or loadbearing walls at their corners or along their edges. The columns provide vertical
support and transfer the loads from the slab to the foundation or structural
elements below.
4. Formwork and Construction: During construction, formwork is erected to define
the shape and dimensions of the slab. Concrete is poured into the formwork, and
reinforcement is placed within the slab according to the design specifications.
After the concrete has cured, the formwork is removed, and the finished slab is
ready for further construction activities.
The methodology for designing and constructing a two-way flat slab involves
initial assessment and planning to evaluate structural and architectural
requirements, followed by structural analysis and design to determine loads,
stresses, and reinforcement detailing. Formwork is erected to define the slab's
shape, and concrete is poured and cured to ensure proper consolidation and
strength development. Quality control checks are conducted throughout
construction, and measures such as deflection control and punching shear
reinforcement are implemented if necessary. Finally, finishing and surface treatments
are applied, and documentation is prepared to accurately reflect the constructed
condition of the two-way flat slab system, ensuring compliance with standards and
performance criteria.
1.1.2 Flat plate
A flat plate in the context of building construction involves a structural floor
system where a concrete slab, typically of uniform thickness, is directly supported
by columns or walls without the inclusion of beams or girders. The term "flat
plate" refers to the simplicity of the system, with the floor slab appearing as a flat,
uninterrupted surface.
Designing and constructing a flat plate involves initial assessment of
structural and architectural requirements, followed by structural analysis and
design to determine slab thickness, reinforcement layout, and detailing.
Formwork is erected to define the slab's shape, and concrete is poured and
cured to ensure proper consolidation and strength development. Quality control
checks are conducted throughout construction to verify compliance with design
specifications. Flat plates offer advantages such as simplicity, flexibility in
architectural design, and reduced construction time. However, considerations
such as deflection control and punching shear reinforcement may be necessary
depending on structural requirements and loading conditions. Finally, finishing
and surface treatments are applied, and documentation is prepared to accurately
reflect the constructed condition of the flat plate system.
1.1.3 Ribbed floor slab
A ribbed floor slab, also known as a ribbed slab or a waffle slab, is a type
of reinforced concrete slab that contains a series of ribs or joists within the slab
thickness, creating a grid-like pattern. These ribs are typically formed by thicker
bands of concrete or by prefabricated elements such as hollow clay blocks or
polystyrene void formers. The ribs are arranged in a grid pattern, usually with
equal spacing in both directions, resulting in a waffle-like appearance when
viewed from below. The flat portions between the ribs are referred to as the "slab
bands" or "slab panels." Ribbed floor slabs are commonly used in building
construction to provide efficient structural performance, reduce dead weight, and
enhance the strength-to-weight ratio of the slab. They offer advantages such as
reduced material usage, longer spans, and improved load-carrying capacity
compared to solid slab systems. Additionally, ribbed slabs can accommodate
services such as ductwork, electrical wiring, and plumbing within the voids
between the ribs, allowing for easier installation and maintenance.
1.1.4 Waffle Slab
A two-way waffle slab is a type of reinforced concrete slab designed to span in
two directions, both longitudinally and transversely, forming a grid-like pattern of
deepened beams (ribs) and thin slab segments between them. Similar to a conventional
waffle slab, a two-way waffle slab consists of a series of intersecting deepened beams
that create a waffle-like appearance when viewed from below.
Constructing a two-way waffle slab involves initial assessment of structural and
architectural requirements, followed by structural analysis and design to determine slab
band thickness, rib depth, and reinforcement layout. Formwork is erected to define the
shape and dimensions of the slab bands and ribs, and concrete is poured and cured to
ensure proper consolidation and strength development. Quality control checks are
conducted throughout construction to verify compliance with design specifications, and
measures such as deflection control and punching shear reinforcement are
implemented as necessary. Finally, finishing and surface treatments are applied, and
documentation is prepared to accurately reflect the constructed condition of the two-way
waffle slab system, ensuring compliance with standards and performance criteria.
1.1.5 Lift Slab
Lift slabs are precast-concrete floor and roof panels that are cast on a
base slab at ground level, one on top of the other, with a bond-breaking
membrane between them. Steel collars are embedded in the slabs and fit loosely
around the columns. After the slabs have cured, they are lifted to their final
position by a patented jack system supported on the columns. The embedded
steel collars then are welded to the steel columns to hold the lift slabs in place.
This method of construction eliminates practically all formwork.
• proprietary systems are used to
form joist and waffle slabs.
• For economy, standard
forms should be used in a
repetitive
manner
whenever possible.
• Knee brace
• Ledger
• Blocking
• Kicker
• Braced T- and L-heads provide
support for beam forms.
• Using columns and
beams of a constant
section and varying the
amount of steel
reinforcement to carry
the imposed loads
results in greater
economy.
• Shoring must be braced in
both the vertical and
horizontal planes to
stiffen and prevent
buckling of individual
members of the
formwork.
• Sills may be required
to distribute the
shoring load over green
concrete.
Fresh concrete must be shaped and supported by formwork until
it cures and can support itself. This formwork is often designed
as a separate structural system by an engineer because of the
considerable weight and fluid pressure a concrete mass can exert
on it.
• Slab sheathing of plywood, hardboard, or boards
• Metal or wood joists
• Stringers
To support beam and slab forms until the placed concrete can cure
and support itself, temporary supports called shoring are used.
• Adjustable shores are metal or wood-and-metal shores available
with jacks or screw-type devices for adjusting the elevations
of the shores once they are placed; various fittings can be
interchanged at the top for vertical extensions, U-heads, and
T-heads.
• Single-post wood shores are cut slightly short of the
desired elevation and adjusted by driving wooden wedges
under the shore or at its top.
• Double-post shores may be assembled with cross bracing for
relatively heavy loads.
• Horizontal shoring consists of adjustable metal members
used to support slab forms over comparatively long spans
without intervening vertical shores. Horizontal shoring
requires fewer vertical shores, each carrying a comparatively
greater load, and leaves open spaces clear for work, but
each vertical support carries a greater concentration of
load.
• After a concrete slab or beam has cured sufficiently to
carry its own weight, the original formwork is removed and the
slab or beam is reshored until the concrete reaches it full
strength.
• See 5.07–5.08 for the formwork required
for concrete columns and walls.
• Flying forms are large sections of
formwork, including supporting trusses,
beams, or scaffolding, that can be moved
by a crane in constructing the concrete
floors and roofs of multistory buildings.
1.1.6 Span stress Floor System
A construction method which utilizes span-stress prestressed T-Joist that
can be used with filler blocks or collapsible steel forms, or plywood forms.
The system has the stiffness of a conventional slab since the concrete was
poured monolithic and is connected with the structure by providing connection
reinforcement that will transfer the lateral forces to the lateral load resisting
members. A system that is light yet capable of heavy loads and long spans.
Much economical compared to conventional cast-in-place concrete slab. Length
goes from 3.00 meters to 9.00 meters.