Precast concrete framing

Precast concrete framing are well traditional for the construction of low- rise and multi

storey building. There are also have a lot of examples of precast concrete framing in

retail, industrial and warehousing developments.

The skeletal framed system contains of main structural components of beams, columns,

connections and bases. The layout of floor system consists of precast beams in both

transverse and longitudinal directions of the building. Precast beam –column

subassemblages have the advantages that the connecting faces between the

subassemblages can be placed away from the critical frame regions.

However, most of the linear elements are mostly favored because of the hardness

associated with forming, handling and erecting spatial elements. Placing the connecting

faces at the beam-column junctions is use by linear elements. It can be placed on

corbels at the columns, for easiness of construction and to help the shear transfer from

the beam to the column. The beam-column joints accomplished in this way are hinged.

A precast reinforced concrete space frame considered as load – bearing structure. It is

built by using two main modular elements which is cruciform element and linear beam

element. The cruciform element consists of the transverse frame joint with half of the

adjacent beam and column lengths. Installing the precast beam elements in between the

transverse frame joints is always considered as longitudinal frames.

The precast elements are connected by welding the projected reinforcement bars and

casting the concrete in place. Joints between the cruciform elements are located at the

mid-span of beams and columns, whereas the longitudinal precast beam-column

connections are located close to the columns.

Advantages of precast concrete framing

Fire resistance – concrete itself consisted fire resistance which is present during

all construction phases. It is typically reached without any application of

additional sprays or linings. This is an important inherent benefit over steel and

timber types. Precise frames are commonly considered for one hour inbuilt fire

rating. This will deeply reduces the need for additional fire protection and the

associated costs.

Reduced construction time– Time- consuming works which is formwork,

scaffoldings and curing are needed to produce a structural element. Most of the

precast elements are made in factory, when the structural elements are need

they will send to the site immediately and assembled to forming the structural

frame and build up the building.

Reduced construction cost – In precast concrete machineries plants are utilized

with some technicians attending to certain production process. This deeply

reduced the number of unskilled labor requirements.

Increased quality of structural elements – most of the precast elements is

constructed in plants by used of modern techniques and machineries. Raw

materials such as concrete, sand, reinforcement bars and prestressing strands

are under high level of quality control. Precast components have higher density

and better crack control and provide better protection from harsh weathers and

sound insulation.

Flexibility of forms and shapes – architects and designers can full use of their

imagination to construct and allowing all sorts of decorative pattern and shapes

for all the precast structural components.

Less wastage – precast systems significantly reduce the amount of waste

materials produced on site.

Plug and play – precast concrete can carry preinstalled utility services and

fixtures, such as communications, plumbing or even windows. Value – added

components can be cast within a precast components can include connection

plates ready to receive heating and lighting fittings on site. This will make easy

for construction and maintenance.

Air tightness – air infiltration in precast buildings is minimal because of the

relatively small number of joints in the construction. This factor combined with the

thermal mass of concrete gives excellent thermal performance.

Disadvantages of precast concrete framing

High capital cost – a large amount of resources need to be invested in order to

set up a precast concrete plant. Such as sophisticated machineries which are

expensive and require heavy investment. Precast concrete framing are utilized in

construction of housing estates where the design of houses is uniform. Such as

large stadiums, halls, factories, warehouses, airports and hangars are suitable to

use precast concrete. Therefore, the scale of the construction projects using

precast concrete components should have large enough to ensure sufficient cost

to offset the initial capital cost,

Sophisticated connection works – the performance of connections determines

the behavior of precast concrete structures. When erection of precast concrete

structures, connections between precast components have to be checked and

done correctly. Therefore, a skilled and well- trained labors are required to

ensure proper connection is produced during erection stages, in order to avoid

additional cost happen.

Transportation, handling difficulties and modification limitation – installer or

workers must handle the precast component carefully to avoid damage. Precast

concrete component must be transport from the plants to the sites using trailers.

Usually difficulties in transportation because of large and heavy components.

Upon arrival at the sites. Tower cranes or portable will lift the precast

components into place for erection. This is increase the speed of construction.

Proper construction planning and site management is a must in order to provide

large space for several cranes. All the workers must be trained to ensure that

precast components are placed and connected correctly to avoid cases where

the columns, beams, walls or slabs are not well aligned, dislocated or out of

plane.

Limitation - Due to panel size is limited, precast concrete can not be used for

two-way structural systems.

Types of frames and components

Beam

Beams served as the main horizontal load carrying members in skeletal structures.

The specified loadings and support conditions are designed for prestressed or reinforced

concrete beams in a precast concrete frame. The most common shape is the inverted

tee where part of the beam section fall within the slab so that the whole depth of the

available slab area is used structurally.

External beams are more often made of reinforced concrete due to have an

asymmetrical cross section, which makes them difficult to prestress without causing

unwanted deflections. The beam may be composite or non - composite. The floor,

column and screed to form a monolithic structure are act by composite beams. Beams

generally span from column to column. It can formed few connections such as beam to

column and beam to beam. Alternatively, the beam can cantilever over the end of the

column to facilitate a drop- in beam.

Precast beam members are generally inverted T, I and L beams desgined to

facilitate the support of the precast concrete floor units. L shape beams normally provide

support non-symmetrical floor loads. The design of beams is based on ordinary

reinforced or prestressed concrete principles for specified loads and support conditions.

it may be simple or continuous.

Beams with steel reinforcement bar

Columns

A column in structural engineering is a vertical structural element that transmits,

through compression, the weight of the structure above to other structural elements

below.

The following is the requirement to column design:

i. It is designed for joined axial compression and bending.

ii. Column bending moments at beam level are determined from the eccentric

loading at the connection.

iii. In the case of slender unbraced columns, the additional moments due to

slenderness effects are added to the total design moment at each floor level.

There have 2 type of precast concrete column which are:

i. Continuous Columns

ii. Single Storey Columns

Type of column Description

Continuous column

Have a corbel at each floor level to support the

surrounding beams.

It can be made up to 24m lengths.

Its main advantage is speed of installation

Single storey column

Usually located inside the perimeter of the building

Spine beams are normally connected over these

columns.

Slabs

It is a flat structural member of a house or building in construction that are made using

concrete. The importance of a slab cannot be overstated as it is one part of the home

that will bear the weight and dead load and live loads of everything that is placed on top

of it.

In recent years many larger projects are using precast concrete slabs as the

building’s frames. Precast concrete slabs are separated into 3 categories which are:

i. Prestressed hollow-core slabs

ii. Prestressed double-tee slabs

iii. Precast composite solid slabs

Type of slab Description

Prestressed hollow-

core slabs

Using a long line extrusion to manufacture it, in the

prestress degree, strand pattern and the depth of slab

are the main design parameter.

It is cut to lengths to suit the building design in factory

so that resulting in safe and speedy construction and

time efficiency.

Low construction cost and low wastage

It can be easily transportable as it is a lightweight

concrete

It is durability, corrosion resistance, impact resistance,

fire resistance, security and long lasting maintenance

Prestressed double-tee

slabs

It has greater span and load carrying capacity

compared to hollow-core slabs

Its ends can be notched to form a halving joint thus

reducing the overall structural depth.

Double-tees can be cantilevered over supports to form

balconies by combining top reinforcement to alter

structural behavior.

Have a minimum fire resistance of two hours.

Precast composite

solid slabs

It is combined with an in-situ reinforced concrete

topping to construct a powerful solid composite slab

Have a smooth surface and can be fixed rapidly

The fire resistance is depends on the thickness of

precast planks, span and load capacity

Staircases

Staircases is a compulsory structural element for a building that more than one storey

high. It was also acts as an escape way in case of fire happen in a building. Precast

concrete staircases are normally used with precast concrete frame. It is costly but

shorter construction times are needed. The most important factor when using precast

concrete stair is the plan configuration of the staircase and its compatibility with the

structural frame. This can makes optimum use of the frame to prevent the need for extra

components to support the staircase.

Precast concrete connections

Beam to column

Beam to column connections are the most important connections in precast skeletal

frames. It is also one of the basic structures in the design of multi-storey precast

structures. The structural performance of the connection such as beam-to-column

connections will affect the strength, stability and constructability as well as load

distribution of the structure under load. In beam and column construction, the vertical

member is continuous and horizontal members are framed into it at several levels.

Another for low-rise unbraced structures is to upright columns floor-by-floor and in situ

connections are made between precast components. There is an even split between the

use of concrete corbels with twin rib or rectangular beams and cast-in steel inserts

forming the invincible corbel.

In our site, they are using the hybrid beam to column connection. It has consisted of mild

steel and post-tensioning steel, both of it contributed to the moment resistance. The mild

steel contains as a greater energy dissipater and post-tensioning clamps the beam

against the column, allowing beam trim at the interface to be resisted by friction.

They beam usually in rectangular and with corbel in our site, the size always depending

to the design and calculation by the engineer and architecture.

Columns to foundation

In our site, there were using column shoe to connect a precast column to a foundation.

The column shoes are standardized base connections for precast columns available in

different capacities (bolt sizes) and finishes (black steel or hot dip galvanized). A precast

column is fixed to bolts which are cast into the base of the structure. Fixing is achieved

with nuts and washers attached to the anchor bolts. It is also possible to set the column

at the correct height level and vertical position using the leveling nuts. The joint between

column base and structure below should be grouted as soon as possible after

installation. After that the connection parts and grouting will work as reinforced concrete

structure.

Column to column

Column to column connections are continuity of vertical element is essential for

structural stability. It is connected either by coupling, welding or bolting together

mechanical connectors anchored into the separate precast components or by continuity

of reinforcement through a grouted joint.

In our site, there were using grouted sleeve connections method to connect the

column to another. Grouted sleeve connections are the most popuar and economical

column connections. These connections may be made at almost any level in the column,

but they must be made carefully.

Precast concrete columns with Holes for connecting another

projecting bars column

Before connecting another After connected

column on top of it

Slabs to beam

For connecting one precast concrete component to another, there are two types of joints

methods, which is the dry and the wet connection. The dry connection is a method

makes use of an ancillary steel plate as connector between the precast concrete

components and the connection between the respective steel plates is made with a bolt

or by welding. The second method of joints is the wet connection.

In our site, second method is being used as its joint method. It is a method used

to emulate cast in-place detailing by pouring concrete on the joint parts on the spot.

Temporary supports are used to keep the slabs in the position until the connection with

the columns has been achieved. In the connections, the steel bars that project from the

edges of the slabs are welded to the dowels of the adjacent components. The concrete

quality used for joining the precast elements need not be of equal concrete grade to the

precast concrete components.

Staircases to beam

Staircase to beam connections are usually using scarf or halving joints. There are

designed as short cantilever where it is important to position the reinforcement

accurately.

In our site, there was using intermittent scarf joint method to connect the stairs to

the beam. The intermittent scarf joints enable the joints to connect the structural

continuity. Welded connections made between fully anchored plates also may be

specified. Concrete cover to the welding is provided for fire protection and appearance.