voided and ribbed slabs

8/9/2019 Voided and Ribbed Slabs

1/17


2/17

RIBBED SLABSIntroduction

Ribbed slabs are made up of wide band beams

running between columns with equal depth narrow

ribs spanning the orthogonal direction. A thick top

slab completes the system.

The term ribbed slab in this sub-clause refers to in-

situ slabs constructed in one of the following ways.

a) Where topping is considered to contribute to

structural strength

1) as a series of concrete ribs cast in-situ between

blocks which remain part of the completedstructure; the tops of the ribs are connected by a

topping of concrete of the same strength as that

used in the ribs;

2) as a series of concrete ribs with topping cast on

forms which may be removed after the concrete has

set;

3) with a continuous top and bottom face but

containing voids of rectangular, oval or other shape.

b) Where topping is not considered to contribute to

structural strength: as a series of concrete ribs cast

in-situ between blocks which remain part of the

completed structure; the tops of the ribs may be

connected by a topping of concrete (not necessarily

of the same strength as that used in the ribs).


3/17

Working

Providing ribs to the soffit of the floor

slab can reduce the quantity of

concrete and reinforcement, and

thus the weight of the floor. The

deeper, stiffer floor permits longer

spans to be used. Formwork

complexity can be minimized by the

use of standard modular, re-usable

formwork. When flying form panels

are used, the ribs should be

positioned away from the columnlines. Ribbed slab floors are very

adaptable for accommodating a

range of service openings.

Economic in the range 8 to 12

m.The saving of materials tends to

be offset by some complication in

formwork. The advent of expanded

polystyrene moulds has made the

choice of trough profile infinite and

largely superseded the use of

standard T moulds. Ribs should be

at least 125 mm wide to suit

reinforcement detailing.


4/17

ADVANTAGES

Medium to long spans

Lightweight

Holes in topping easily

accommodated

Large holes can be accommodated

Profile may be expressed

architecturally, or used for

heat transfer in passive cooling

Electrical and mechanical

installations can be placed between

voids

Good resistance to vibrations

DISADVANTAGES

Higher formwork costs than for

other slab systems

Slightly greater floor thicknesses

Lower span

Only moderate and uniformly

distributed load can be

accommodated


5/17


6/17

SIZING OF SLAB AND RIBS

The thickness of the concrete

slab or topping should not

be less than

30mm for slab with permanent

blocks contributing to

structural strength and

where there is a clear

distance between ribs not

more than 500mm.

25mm when blocks mentioned

in 1) are jointed with a

cement-sand mortar.

40mm or 1/10th of the clear

distance between ribs,

whichever is greater, for all

other slabs with permanent

blocks.

50mm or 1/10th of the clear

distance between ribs,

whichever is greater, for

slabs without permanent

blocks.

Reinforcement in topping or slab

shall constitute of a wire

mesh.


7/17

SINGLE AND DOUBLE TEE SLABS

Single Tee Beam ST)-

Combination beam and slab

Spans up to 120-0"

Typical width = 8-0"

Typical depths of 36" and 48"

Designation = 8ST36+2 (8 = width in feet, 24

= depth, +2 = 2" topping)

Double Tee Beam DT)

-

Combination beam and slab

Spans up to 100-0"

Typical width = 8-0"

Depths of 12", 18", 24" and 32"

Designation = 8DT24+2 (8 = width in feet, 24 =

depth, +2 = 2" topping)


8/17

DOUBLE TEE SLABSDESCRIPTION

Double Tee flooring units consist of two prestressed

ribs and a connecting top slab. The ribs can vary

in depth from 200 to 500mm.The connecting

slab is 2400mm wide x 50mm thick.

Double Tees are ideally suited for larger spanning

floors with a wide variety of services suspended

from the flooring

system.Double Tees can easily accommodate large

floor voids/penetrations through the slab region.

SOUND TRANSMISSION

A major practical benefit of a concrete floor is its

ability to reduce noise transmission. Double Tee

concrete floors are quiet and do not creak with

temperature and moisture changes. The table

below shows sound transmission ratings

achieved by Double Tees.

MATERIAL

Double Tee concrete strength =42 MPa.

Topping concrete strength = 20 MPa.

Topping thickness = 65 mm

FIRE RESISTANCE RATING


9/17


10/17

Working

Introducing voids to the soffit reduces dead

weight and these deeper, stiffer floors

permit longer spans which are economic

for spans between 9 and 14 m. The saving

of materials tends to be offset bycomplication in site operations. Standard

moulds are 225, 325 and 425 mm deep

and are used to make ribs 125 mm wide

on a 900 mm grid. Toppings are between

50 and 150 mm thick. The chart and data

assume surrounding and supporting down

stand beams, which should be subject to

separate consideration, and solidmargins. Both waffles and down stand

beams complicate formwork.

ADVANTAGES

Medium to long spans

Lightweight

Profiles may be expressed architecturally, orused for

heat transfer

DISADVANTAGES

Higher formwork costs than for other slab

systems

Slightly deeper members result in greaterfloor


11/17

POST-TENSIONED WAFFLE/JOIST SLAB

CONSTRICTIONIntroduction

Where concrete is relatively expensive, spans

are generous, and it is not critical to select

the smallest floor thickness, a post-

tensioned waffle slab construction is likely

to be the economical alternative.

CONSTRUCTION

Waffles are generally limited to the interior of a

slab, leaving one or two of the forms out to

create a solid fill around the supports. The

solid fills provide the strength required for

shear transfer to the supports. The fills

also reduce the compression stresses at

the soffit of the floor around the supports,

thus avoiding the necessity of bottom

reinforcement in this region. Figures 1 -1

and 1-2 illustrate typical waffle

constructions using unbonded tendons. A

light top mesh over the waffles is generally

the only top reinforcement at the interior

of the floor panels.


12/17

ANALYSIS MODEL OF A WAFFLE SLAB WITH

MODERATE SIZE SOLID SLAB BANDS ALONG

SELECTED LINES OF SUPPORT

With larger loads and longer spans, such as is

common in department stores a heavier solid

slab band between the supports accommodates

the overage of reinforcement from the

individual waffle stems in each direction

ANALYSIS OF A LONG-SPAN WAFFLE SLAB WITH STOUT

SOLID SLAB BANDS ALONG THE LINES OF SUPPORTS

Where design requirements demand more

reinforcement that is

generally assigned to a typical interior waffle

stem, solid strips along the lines of supports is

used to accommodate the excess of

reinforcement.


13/17

HOLLOW CORE SLABIntroduction

Hollow core slabs are precast, pre-stressed concrete elements that are

generally used for flooring. Some of their advantages are as follows:

long spans, no propping; flexible in design; fast construction; light

weight structures. The slabs have longitudinal cores running through

them, the primary purpose of the cores being to decrease the weight,

and material within the floor, yet maintain maximal strength. To

further increase the strength, the slabs are reinforced with steel

strand, running longitudinally.

Hollow core slabs derive their name from the voids or cores which run

through the units. The cores can function as service ducts and

significantly reduce the self-weight of the slabs, maximising structural

efficiency. The cores also have a benefit in sustainability terms inreducing the volume of material used. Units are generally available in

standard 1200mm widths and in depths from 110mm to 400mm.

There is total freedom in length of units and splays and notches can

readily be accommodated.

Hollowcore slabs have excellent span capabilities, achieving a capacity of

2.5 kN/m2 over a 16m span. The long-span capability is ideal for

offices, retail or car park developments. Units are installed with or

without a structural screed, depending on requirements. Slabs arriveon-site with a smooth pre-finished soffit. In car parks and other open

structures, pre-finished soffits offer a maintenance free solution.

Prestressed units will have an upward camber dependent upon the span,

level of prestress, etc. This will be reduced when screeds/toppings or

other dead loads are applied.

Hollow ore slab Details

Thicknesses of 4", 6", 8", 10" and 12"

Spans up to 40-0"

Standard panel width = 4-0"

Typical designations = 4HC6 (4 = panel

width in feet, HC = Hollow Core, 6 = slab

thickness in inches)

= 4HC6+2 (2 = 2" of concrete topping

added)


14/17

ADVANTAGES

SMALL WEIGHT:

The hollow core slabs are lighter than custom prestressed slabs from 37 to 54 . Thus, the cost of

construction is lower, and pillars and beams are having less load. Also, we have smaller dimensions for

bearing constructions and for foundations.

LARGE SPANS:

Hollow core slabs can bridge the spans of 16 m without support, and as a result it lowers the number of

supports. These slabs transfer the load in one direction.

ADAPTIVE FOR ANY SYSTEM OF CONSTRUCTION:

These slabs are not only used in prestressed concrete structures but in masonry and steel structures,

regardless of whether it is prefabrication or traditional construction.

MATERIAL SAVINGS:

Application of hollow core slabs saves up to 50 of concrete and 50 of armature, all compared to

traditional slabs. It means that in structures of 1000 m2 35 tons of concrete and 7,5 tons of armature is

saved.

SIMPLE PRODUCTION:

Using the same amount of materials, workforce and energy, you can produce 1 m2 of traditional slabs and

2,5 m2 of hollow core slabs. Production of hollow core slabs is completely automated.

FAST PRODUCTION:

In 24 hours we are producing around 500 m2 of hollow core slabs.

LOAD:

Hollow core slabs can hold up to 2000 kg/ m2, typical for production plants and warehouses.

HIGH QUALITY:

Production is highly equipped with machinery and performed in strictly controlled conditions.

DISADVANTAGES

Camber in beams and slabs

Very small margin for error

Connections may be difficult

Need bracing during on-site erection of structure

Somewhat limited building design flexibility


15/17

VOIDED SLABS

Introduction

A relatively new technology developed in Europe has taken the

efficiency of cast-in-place flat plate slabs to new heights.Voided slabs have been used in the construction of office

buildings in Switzerland, Germany, Austria and the United

Kingdom, with floor spans up to 17 meters (~56 feet) and

overall slab thicknesses up to 60 cm (~24 inches).

These slabs are more efficient than traditional structural floor

systems commonly used in the construction of office

buildings in the United States. The main effect of the voided

slab system is to decrease the overall weight by as much as35% when compared to a solid slab of the same capacity.

From a sustainability standpoint, the reduced slab weight also

allows the quantity and dimensions of vertical bearing

elements, such as columns, to be reduced by as much as

40%. Reduced dead weight also means a smaller deflection

of the slab, and provides scope for potential savings in

foundation design, including fewer piles and/or reduced

length of piles. While the design lowers overall weight, thevoided two-way slabs offer very high load-carrying capacity

and considerable flexibility.

From the developers and contractors viewpoint, this technology

can offer other potential benefits, including direct and

indirect cost savings due to reduced volume, lower

transportation requirements and easier lifting.


16/17

Design Principle

The concept centres on removing the non-working concrete dead load while maintaining biaxial strength

throughout the slab. This is an essential feature found in the wings of birds. A hard shell with struts

formed by multiple cavities, appropriately located, gives the bones a stability that is equivalent to

solid bones. The result is a highly efficient structure that has less mass and requires less force to lift.The design principle of these slabs is based on industrially produced spherical hollow shells made from

recycled plastic that are inserted into the positioning cage to create modules of several lengths,

depending on the application. These cage modules are placed on the lower reinforcing mat, and the

upper reinforcing mat is then placed on top of them. The voids in the slab displace non-working

concrete with the aim of saving material where it is not required for structural reasons.

The voided slab system has the same bearing capacity as conventional concrete solid slabs, and

standard design and detailing techniques can be directly applied. However, research performed at auniversity in Germany has produced several numeric factors that have to be considered to reflect the

presence of the void formers. This affects:

Dead load

Stiffness of the slab

Maximum shear stress

Also, the positioning cages have a compensating positive effect

on the slabs shear strength, which is impacted by the presence of

the voids. In the vicinity of the column, the slab is designed to

resist punching shear stresses using a solid cross-section, with

additional shear reinforcement as required to maintain a flat

soffit throughout the slab.


17/17

Advantages

Biaxial capacity

Larger spans without beams

Larger open floor areas

Lower floor to floor heights

Earthquake resistance

Resource efficiency

The shallow profile of the voided slabs is another

attribute that offers the opportunity to reduce

floor to floor heights. The implication is the

potential addition of rentable floor space, or

conversely a reduction in energy requirements

along with cost savings to the structure and

associated building systems such as cladding,elevators, fire protection systems, heating and

air conditioning requirements.

Earthquake resistance is another major benefit of this

system. During an earthquake event, the

accelerated mass of the building creates seismic

forces that have to be absorbed by the vertical

elements of the structure. The reduced dead

weight results in lower force demands on thestructure, with associated savings in detailing

and constructability requirements.

Voided slabs can also be coupled with post-tensioning

to minimize dead load deflections further, while

still maintaining the same light weight and

biaxial attributes. One of the main benefits of

post-tensioning is to obtain a slab that is

"almost" free of cracks and deflection at the

service load level. As a result, the slab is stiffer,

since the full cross-section rigidity is available to

resist the applied loads.

voided and ribbed slabs

Documents