floor design

8/9/2019 Floor Design

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1purpose and scope

Concrete structures have for many years dominated the Australian commercial building scene, frommodest suburban offices to hi-rise city office buildings.Landmark projects such as Sydney's Governor Phillip building and Melbourne's Rialto Tower rankamong the tallest reinforced concrete buildings in the world and testify to the skills of Australiandesigners, builders and tradesmen.

This design guide has been developed to permit the rapid selection of economical solutions for long-span concrete floors.

It will assist: Students

Designers Quantity surveyors Builders Developers

It examines the types of floor that are feasible for typical office and carparking loadings (3-5 KPa) for a range of medium to long spans.

To assist the cost comparison of these floor types, information is provided which relates thequantities of concrete, reinforcement and formwork to the selected span and floor type.

A checklist for design procedure is provided.

Cost saving information is provided to assist practical design and detailing for construction.


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2advantages of concrete

Concrete floors have inherent advantages over other types of flooring solutions. Concrete floors are economical. All the necessary information for their design and construction is

well understood and covered by Australian Standards. Concrete floors are strong and durable. Concrete floors are quiet. Concrete is a very dense material, which limits the transmission of

sound. Concrete floors are energy efficient. Concrete floors are suitable for a wide range of floor coverings. Concrete floors are inherently fire resistant. Concrete floors allow design flexibility. Concrete floors lend themselves to fast construction.

Speed of construction is of greatest interest to builders and developers. Reinforced concrete floorsare delivering floor construction cycles as low as 3-4 days per floor on high-rise buildings.

Penetrations for services are easily accommodated. Steel reinforced concrete floors are amenableto later cutting for penetrations as may be required over the life of a building. There is a growingview that designers need to build in future flexibility for the buildings they design. For this reason thepartial prestress solution is finding favour for large spans as it ensures that the majority of the floor area is conventionally reinforced. This allows later service penetrations to be cut without the risk of cutting through prestressing wires.


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3concrete floor systems

There are many feasible concrete floor systems from which the designer can select an economical

and technically satisfactory solution. To provide satisfactory performance a concrete floor must haveadequate strength to safely resist the applied loads, and sufficient stiffness to limit deflections under both transient and long term loads. With the trend toward longer spans, the criterion of stiffness hasbecome more important, so that in practice the principal dimensions of the floor are determined bystiffness considerations rather than strength.

Concrete floors are reinforced using either reinforcing bars or fabric to form a normal reinforcedconcrete structure, or using high-strength wire strand, which is stressed to form a prestressedconcrete structure. The action of prestressing a draped cable in concrete enables the applied loadsto be balanced by the uplift force so that deflection is largely counterbalanced. This is a significantbenefit in long-span floors as it eliminates the need to camber formwork or to provide deeper concrete sections. Prestressing brings with it additional complexity on site over conventionalreinforcement, but allows concrete to compete with structural steel framed floors for long spans.

In recent times the difference between normal reinforced concrete and fully prestressed concretehas become less clear-cut with the increasing popularity of partially prestressed concrete for largespans. This approach combines the prestressing benefits of controlling deflection and cracking withthe economy of reinforced concrete.

A combination of partially prestressed beams (with both strand and reinforcement) together withconventional steel reinforced slab panels make an economical floor system for large spans. It hasthe advantage that the conventionally reinforced slab panels allow future services penetrations, or the possible provision as a later addition, of a stair between floor levels


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4design considerations

Traditionally, column spacings were selected to provide the most economical structure, and slab

spans were often in the order of 8 -9 metres. However, recently there has been a trend to larger floor areas in city office buildings, where the economics of smaller spans have been disregarded toobtain spans as great as 16 metres or more. Large spanning floors incur penalties in structuraldepth, self weight, bounce, deflection, and cost.

It behoves the designers to look carefully at the need for large spans as the cost penalty increasesin a logarithmic proportion to the span.

In many cases the addition of just a couple of internal columns at the rear of a lift core can transformthe economics of a structure by markedly reducing slab span.

In this case the columns at the rear of the lift core form an area that would be ideal for a meetingroom or compactus storage, whilst saving structural cost.

Sensible positioning of columns need not detract from the flexibility of floor areas.

The irony of large expenditure to provide long-span 'column free' space is that the tenants of thecolumn-free space often install dummy columns to contain vertical cabling and services.

There are trends in design of high-rise buildings now to separate the core of the building intoseparate vertical elements which provides the benefit of reduced spans and shorter air conditioningduct runs. This adds up to reduced structural weight, cost and depth; and, reduced ceiling depthneeded for smaller ducts, giving savings on floor to floor height.


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5MaterialsDesigners are taking advantage of the high strength materials now available, with the advent of 500MPa steel reinforcement and high strength concrete mixes. These high strength materials have led

to review of the Australian Standard AS 3600 - Concrete Structures. Under the recommendations,the use of 500 MPa steel reinforcement - classes N (Normal Ductility) and L (Low Ductility) - ispermitted in the design of reinforced concrete structures. While these can strongly influence theductility and load carrying capacity of reinforced concrete beams and slabs, other details are alsoimportant. The ratio of moment capacity to cracking moment in critical regions is a vital factor.Particular consideration is directed to the effects of moment redistribution when predicting the fireresistance period for structural adequacy of continuous beams and slabs.

High strength concretes are being used in the construction industry predominantly in columns andcore walls. This trend is extending to the use of higher strength concretes in slabs and beams. Atthis stage many engineers limit the maximum design strength of concrete to 50 MPa because mostcodes for concrete structures do not provide for higher concrete strengths. It is to be expected thatthere will be change in time to much higher strengths as knowledge of their behaviour is gained,particularly in regard to brittleness and confinement.

It is no longer possible to simple expect the designer’s specifications will ensure that the materialsare fit for purpose, unless Test Certificates for those materials are submitted and approved. This isvital to the performance of the structure.

Designers should be aware that direct substitution of non Australian produced steel reinforcementfor Australian made product may not achieve design intent. There are some significant differencesin metallurgical properties of reinforcing steels. Australian site practices require that the reinforcingsteel have a low carbon equivalent to suit the prevalence of on site welding of steel cages. Weldingis also used in high-speed factory made reinforcing cages, produced by member companies of theSteel Reinforcement Institute of Australia, where reliability of performance and delivery are assured.It is of real concern, that decisions may be made to circumvent the assured nature of a known

Australian product allied to known welding practices that suit the metallurgical nature of the localproduct, in favour of uncertain results. A further factor influencing caution in the selection of steelsupply is the “rebend” characteristics of the reinforcement to suit Australian site practices whererebending to 90 degrees then complete straightening is common.

The vital significance of the ductility of reinforced concrete structures is gaining increasedrecognition. Materials and ductility are no logner of passing concern.


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6floors for apartment buildings

Apartment buildings, home units, townhouses and other forms of residential building make particular

demands on floor systems. These demands differ from those that serve office buildings, schools or warehouses.

The different demands on floor systems for apartment buildings may be summarised, as follows:

COLUMN LAYOUTThe columns supporting the floor slab of apartment buildings are invariably of a narrow bladetype set in the walls. The walls are placed to suit the apartment layout not to suit a regular structural grid. As a result, there may be no regular grid type layout for columns, andadjacent slab panel spans may differ markedly. The design flexibility of a concrete flat platefloor answers this demand better than any other flooring type.

FLOOR SLAB DESIGNThe floor slab design for apartment buildings is usually driven by the demand that the depthof the structure be an absolute minimum, with the soffit of the slab being the ceiling. Theshallow structural depth of a concrete flat plate floor answers this demand better than anyother flooring type. A flat plate floor without beams is the usual design selected, to achievethe minimum structural depth.On low height apartment buildings the floor slab may be supported on load bearing internalwalls, or on beams, and be designed as a hinged one-way or two-way slab.On apartment buildings of medium height and above, flat plate slabs without beams prevail,to minimise structural depth.

DROP PANELSDrop panels may be used to assist the slab span and to control shear at the column head;however, the recent trend is to use shear mat reinforcement or stud rails at column heads toobviate the need for drop panels. It is much simpler to build a plain flat plate floor slabwithout drop panels or band beams, as the formwork deck is totally flat, and as a result mucheasier to construct. Shear mat reinforcement, by eliminating drop panels, removes the droppanel protrusion below the slab, which is often in the way of services. Additionally, droppanels are usually unpopular where exposed to view.

LARGER SPANSFlat plate slab conventionally steel reinforced construction is now capable of longer spans in

relation to slab depth for apartment buildings. Considerable savings in depth and cost aredue to recent advances in technology and code requirements.Even in shorter spans, the advances bring savings in slab depth and cost.

The advances that allow increased flat plate slab spans include:

The introduction of high strength 500 MPa steel reinforcement. The now common use of higher strength (32 MPa plus) concrete in slabs. The use of negative top reinforcement to limit deflections in multiple bay spans. The acceptance of a range of serviceability responses (revisions to AS 1170). The opportunity for reductions in cover for fire (for apartments) due to the performance

based Building Code of AustraliaCombine the advances (with the lesser floor loadings allowable for apartments compared tooffices), and the flat plate slab becomes very efficient.


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New formwork systems and new prefabricated steel reinforcement systems add to theeconomic effectiveness of the flat plate floor while maintaining it's inherent flexibility in beingsuitable for any shape in plan. Electrical conduits are easily located within the floor slab, andplumbing and other services are readily accommodated. Future services can generally beincorporated later due to the forgiving nature of conventional steel reinforced concrete.


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7serviceability considerations for floorsConcrete floors are noted for their inherent advantages in regard to serviceability. However regardmust be addressed to serviceability issues which arise with any floor system, of any type. The

design of any floor system is governed by serviceability limit states.

SERVICEABILITY LIMIT STATESThese are states beyond which the specified service requirements of the flooring system isno longer met, and the functionality is impaired.

Serviceability limit states (ref AS 1170.1-1989) include:

Deformations or deflections affecting the appearance or effective use of the structure. Vibrations causing discomfort to people or damage to finishes and fixtures. Cracking of the concrete likely to adversely affect the appearance, durability, or water-

tightness of the structure.These states are satisfied for most flooring systems by limiting the span to depth ratio and bysatisfying detailing requirements.

EFFECTS ON APPEARANCE AND FUNCTIONALITYShort term deflectionsDeflection limited relative to span and relative to proximity of reference sight lines (eg. facebrickwork) and dependent upon available lines of sight.Long term deflectionsExcessive long term deflections can lead to loss of function and amenity. Slab deflectionscan cause unintended load transference to partitions. Suitable movement allowance shouldbe provided for partitions both below and above slabs. This is particularly important in thecase of masonry walls.Differential deflection of longer span floor beams can result in end rotation. This can besignificant in respect of appearance, function or damage. A 500mm deep beam with aspan/300 deflection has an end rotation of 0.1 radians corresponding to a 5mm relativehorizontal displacement.Dynamic responseThe dynamics (bounce) of flooring systems is a limiting state in design. For lightly loadedfloors in particular, the application of static live load deflection limits does not necessarilyensure satisfactory dynamic performance. The response can result in shaking, rattling, andhorizontal movement of furniture.

Cracking and damage to ceilings and linings can also result.Camber Camber may be used in some cases to reduce the visual impact of deflections, or to preventponding. However, camber does not reduce the actual deflections due to load. The use of camber to allow for larger deflections than usual for non-cambered floors, can lead toproblems with end rotations, and misalignments of associated building elements. The use of camber needs to be carefully considered as it impacts on the cost of formwork.


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8selection of floor typeCommon types of floor systems.

8.1 Flat Plate

The principal feature of the flat-plate floor is its flush soffit which requires only simpleformwork and easy construction. The overall depth of this floor is a minimum and it allowsgreat flexibility for locating horizontal services.

The economical span of a flat plate is limited, however, by the need to control long-termdeflection.

The span 'L' of a reinforced concrete flat-plate is approximately D x 30 to D x 32. Theeconomical span of a flat Plate can be extended by prestressing to approximately D x 35

for a single span and D x 42 for a multi-span, where D is the depth of slab.

This guide allows a quick overview of suitable floor systems for a range of spans.

Advantages:• Simple formwork• No beams - suits services• Minimum structural depth

Disadvantages:• Smaller economical spans• Long term deflection may be controlling factor

8.2 Flat Slab

A flat-slab floor maintains many of the advantages of the flat plate: flat soff it, simpleformwork and easy construction. By adding a drop panel at the column, which increasesthe stiffness of the floor, the economical span range is increased. The economical span 'L'of a reinforced concrete flat slab is approximately D x 32 for an end span and D x 36 for an


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interior span. Prestressing increases the economical span to D x 45 for an end span and Dx 50 for an interior span. D is the depth of the slab excluding the drop panel, in each case

Advantages:• Simple formwork• No beams - suits services• Minimum structural depth

Disadvantages:• Smaller economical spans• Economical span not as great as beam and slab

8.3 Waffle Slab

Introducing waffles to the soffit of the slab reduces the quantity of concrete andreinforcement and also the weight of the floor. The saving of materials tends tobe offset by increasing structural depth. Formwork complication is minimised by use of standard, modular, reusable formwork. The deeper, stiffer floor permits longer spans to beused. The economical reinforced concrete floor span 'L' is approximately D x 20 for asingle span and D x 25 for a multi-span, where D is the depth of the slab and waffle.

Advantages:• Reusable formwork pans• Savings on weight and materials• Long span possible• Attractive soffit appearance• Economical

Disadvantages:• Structural depth• Penetrations controlled by waffle ribs


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8.4 Slab and Joist

Frequently the slab thickness between the joists/ribs is controlled by requirements for fire-resistance rating. For example a 2-hour fire resistance rating requires a 120-mm slabthickness, which is capable of spanning approximately 4 m. For this widely spaced rib or

joist floor the economical span 'L' is D x 20 for a single span and D x 25 for a multi-span.Prestressing the joists/ribs permits the economical span 'L' to be increased to D x 24 in asingle span. D is the depth of slab plus the joist/rib in each example.

Advantages:• Thin slab panels possible• Suits industrial structures• Suits long spans

Disadvantages:• More formwork• Joists and beams intrude on services• Depth of floor


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8.5 Beam and Slab

The traditional beam-and-slab floor comprises beams framing into columns and supportingslabs spanning between the beams. The relatively deep beams provide a stiff floor capableof long spans and able to resist lateral loads.

The traditional reinforced concrete beam-and-slab floor has an economical span 'L' of D x15 for a single span and D x 20 for a multi-span, where D is the depth of the slab plusbeam.

Advantages:• Traditional effectiveness• Good cost/time solution• Suits long spans

Disadvantages:• May need service penetrations through beams• Depth of floor


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8.6 Band Beam and Slab

The traditional beam-and-slab floor comprises beams framing into columns and supportingslabs spanning between the beams. The relatively deep beams provide a stiff floor capableof long spans and able to resist lateral loads.

The traditional reinforced concrete beam-and-slab floor has an economical span 'L' of D x15 for a single span and D x 20 for a multi-span, where D is the depth of the slab plusbeam.

Advantages:• Traditional effectiveness• Good cost/time solution• Suits long spans

Disadvantages:• May need service penetrations through beams• Depth of floor


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Single Span

Multi Span

8.7 Precast and Composite Floors

Precasting offers the advantages of off-site manufacture under factory conditions and fasterection on site. When combined with prestressing, additional benefits of long span andhigh load-capacity can be obtained.

One popular type of precast floor unit is the hollow-core slab. Such slabs are cast bymachine in a long line bed and the units are cut to required length. The relativelylightweight units are erected to form a flush soffit, finished with a composite topping and ashear key between units to ensure load sharing between the units

The economical typical span for a precast hollow core unit is approximately D x 35 to D x40 where D is the depth of the precast unit plus topping. A typical multi-span arrangementincludes precast soffit beams spanning between the columns


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Advantages:• Speed when properly set up• Elimination of formwork• Structural efficiency

Disadvantages:• Propping may be required• Careful detailing needed• Limited penetrations• cranage may prove critical

8.8 Soffit Slabs

Known in Australia as “Transfloor” or “Humeslab”

Precast soffit slabs are an economical method of combining formwork and part of thestructural floor. These comprise thin concrete sections either reinforced, prestressed or incorporating partially exposed trussed reinforcement. These floors may be referred to as‘Composite lattice girder soffit slab’

To reduce dead weight and increase the effective span, a voided slab can be formed by

using polystyrene blocks.

For long spans, soffit slabs require temporary props for support until the composite sectionis able to carry the construction loads

The economical span 'L' between band beams is approximately D x 30 for a single spanand D x 35 for a composite multi-span, where D is the depth of the soffit slab plus topping.

A band beam formed up using a similar precast system typically increases depth by 50 mm.

Advantages:• Speed when properly set up•

Elimination of formwork• Structural efficiency

Disadvantages:• Propping may be required


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• Careful detailing needed• Limited penetrations• cranage may prove critical

8.9 Single and Double T-Beams

Precast prestressed T-beams of standard profile are relatively light units of high loadcapacity capable of long spans. Either double-tee or single-tee sections are used, typicallytied together by welded plate inserts and insitu topping to ensure adequate load transfer.

The economical span 'L' between supporting beams is approximately D x 25 where D is theoverall depth of the T-beam plus topping.

Advantages:• Working platform• Speed when properly set up• Long spans

Disadvantages:• cranage may prove critical• Limited penetrations


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9Selection of Floor types

Preliminary Selection Guide and Ready Reckoner

A convenient and quick selection guide indicating the economical span range for variousslab and beam floor types is given in the Preliminary Selection Guide (below) and theReady Reckoner (on the following page)

6 7 8 9 10 11 12 13 14 15 16SPAN , L (m)

Flat Plate

Flat Slab

Pan Floor

Band Beamand Slab

Hollow Core

Soffit Slab

T - Beam

Generally uneconomical for single spans greater than 8 m

Reinforced ConcretePrestressed Concrete

*

*

*

*

span depth ready reckoner This guide allows a quick overview

Using the 'depth-multipliers' from the floor types shown in section 8(Clauses 8.1-8.9), the Ready Reckoner provides a simple basis for a quickcomparison of economical spans for different floor types.

For example:

(A) If the multiplier is 25 and the depth must be restricted to 550 mm, then theeconomical span can be read off as 13.8m.

Alternatively:

(B) If the required span is 11.5m and the system has a multiplier of 30, thedepth can be read off as 380 mm.


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18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

SPAN, L (m) DEPTH MULTIPLIER

50 45 40 35 30 25 2018

16

14

12

10

DEPTH, D (mm)

0 100 200 300 400 500 600 700 800 900 1000


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10cost comparisonsThe choice of Boor system is usually made by comparing the cost of alternative proposalswhich are technically feasible and which satisfy the constraints imposed by the planning

and construction of the building. In practically every case the final decision is made on thebasis of least cost.

The structural component of a multi-storey commercial/office building accounts for approximately 17-25% of the total direct building cost; of this the floors account for approximately half. The floor structure thus represents a significant component of the costof a commercial/office building. For a simple structure with few services, such as a parkingstation or storage building, the floor structure represents an even greater proportion of thetotal cost

The direct cost of a floor structure comprises the cost of materials, plant labour formworkand consumable items directly associated with the floor. The normal sub-contract systemused on a building project permits a reasonably accurate assessment of the direct cost of alternative structures since the costs of the main items - concrete, reinforcement andformwork - are known. Thus a useful guide to comparative direct costs and the costsensitivity of different floor systems to increasing span is obtained from the quantities of materials required.

These quantities determined for various floor systems in popular current use are set out inthe nomographs which follow. They may be used directly to assist in the assessment of different floor systems for a building proposal.

Clearly there is a cost premium for increased span, since quantities of materials per squaremetre increase as the span increases. This is as expected because the principal factors of bending, shear and deflection which affect the design of the floor, increase with increasingspan. The premium paid for increasing span can be kept to a minimum by selecting a floor system that is highly efficient for the required span and thus is less sensitive to the effectsof the increased span.

Any cost comparison of alternatives should include also the effect of consequential costsarising from associated elements such as the facade and the mechanical services, eg adeeper floor zone increases the cost of the facade, core and services. Beam penetrationsand complicated ductwork required with some floor systems also increase costs. Suchcosts can also be assessed fairly accurately.

Indirect costs arising from overheads related to the estimated time of construction shouldbe considered also. However, an important distinction should be made between directcosts that can be assessed and controlled by appropriate contractual arrangements andtime-related overhead costs that can be estimated for a construction floor cycle which maybe difficult to achieve. A builder assessing the relative merits of different floor systems willconsider the risk exposure and apply what is considered to be an appropriate weighting todirect and indirect costs.


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FLOOR SYSTEMS FOR LIGHTER LOADING (Live Load range 2-3 kPa)

REINFORCED CONCRETE FLAT SLAB MULTI SPAN(Column Support)

400

380

360

340

320

300

280

260

240

220

200

180160

140

120

1006 7 8 9 10 11 12

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

SPAN, L (m)

AVERAGE CONCRETETHICKNESS (mm)

REINFORCEMENT(Kg/m 2)

CONCRETE

REINFORCEMENT

1

1

2

2

NUMBERS REFER TO DIAGRAM BELOW

For typical square gridinterior span only

Case 1Self weight plus 2 kPa Live Load

Case 2Self weight plus 0.5 kPa Dead Loadplus 3 kPa Live Load


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REINFORCED CONCRETE FLAT SLAB MULTI SPAN(Wall Support)

400

380

360

340

320

300

280

260

240

220

200

180

160

140

120

1006 7 8 9 10 11 12

15

14

13

12

11

10

9

8

7

6

5

4

32

1

0

SPAN, L (m)



CONCRETE

REINFORCEMENT

1

1

2

2



Case 1Self weight plus 2 kPa LiveLoad

Case 2Self weight plus 0.5 kPaDead Load plus 3 kPa LiveLoad


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FLOOR SYSTEMS FOR HEAVIER LOADING (Live Load range 3-5 kPa)

Notes on Nomographs

1. Diagrams are suitable for park stations, offices and normal commercial buildings, ietotal loadings of 3-5 kPa. Heavy weight partitions or storage loads will requireincreased quantities of concrete and/or reinforcement.

2. For single span floors, quantities are given for a typical bay assuming minimalstructural edge beams. Deep architectural beams should be considered separately

For multi-span floors quantities are for a typical internal bay. Allowance should bemade for approximately 5-10% additional concrete and/or reinforcement for endspans and non-typical areas.

3. For convenience, multi span beam layouts assume a transverse spacing of 8.4 m tosuit carparking. Longer spans are possible eg 9.6m but quantities will increase.

4. A required fire rating of two hours is assumed. Higher ratings will require anincrease in concrete quantities and possibly reinforcement.


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REINFORCED CONCRETE FLAT SLAB MULTI SPAN

ECONOMIC SPAN RANGE

NOT ECONOMIC

AT SPANSEXCEEDING 10m

400

380

360

340

320

300

280

260240

220

200

180

160

140

120

1006 7 8 9 10 11 12

40

38

36

34

32

30

28

26

24

22

20

18

16

14

12

10

SPAN, L (m)



CONCRETE

REINFORCEMENT



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PRESTRESSED CONCRETE FLAT SLAB MULTI SPAN

DASH LINESINDICATEUNECONOMIC

SPAN LENGTHS

320

300

280

260

240

220

200

180

160

140

120

100

80

14

13

12

11

10

9

8

7

6

5

4

3

2

SPAN, L (m)


TOTAL REINFORCEMENT ANDPRESTRESSING (Kg/m 2)

CONCRETE

PRESTRESSING

6 7 8 9 10 11 12 13 14

REINFORCEMENT



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PRESTRESSED CONCRETE FLAT SLAB SINGLE SPAN

400

380

360

340

320

300

280

260

240

220

200

180

160

140120

100

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

ECONOMICSPAN RANGE

NOT ECONOMIC

AT SPANSEXCEEDING 10m

6 7 8 9 10 11 12SPAN, L (m)



CONCRETE

REINFORCEMENT

PRESTRESSING

NOT ECONOMIC AT

SPANS SHORTERTHAN 6m


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REINFORCED CONCRETE BAND BEAM AND SLAB MULTI SPAN

300

280

260

240

220

200

180

160

140

120

100

38

36

34

32

30

28

26

24

22

20

18

ECONOMIC

SPAN RANGE

NOT ECONOMICAT SPANS

EXCEEDING 12m

6 7 8 9 10 11 12 13 14SPAN, L (m)



CONCRETE

REINFORCEMENT

NOT

ECONOMIC


For typical interior span only

Case B w S D s

1 1200 8400 2002 2400 8400 170


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REINFORCED CONCRETE BAND BEAM AND SLAB SINGLE SPAN


SPAN LENGTHS

340

320

300

280

260

240

220

200

180

160

140

120

100

44

42

40

38

36

34

32

30

28

26

24

22

20

SPAN, L (m)



CONCRETE

6 7 8 9 10 11 12 13 14

REINFORCEMENT


Case B w S D s

1 600 4200 1202 1200 4800 1203 2400 6000 120


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PRESTRESSED CONCRETE BAND BEAM (4m centres) SINGLE SPAN


SPAN LENGTHS

280

260240

220

200

180

160

140

120

100

80

60

40

20

0

9.0

8.5

8.0

7.5

7.0

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

SPAN, L (m)



CONCRETE

7 8 9 10 11 12 13 14


REINFORCEMENT

PRESTRESSING

Case B w S D s

1 600 4000 1202 1200 4000 120


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PRESTRESSED CONCRETE BAND BEAM (8.4m centres)MULTI SPAN

DASH LINESINDICATE

UNECONOMICSPAN LENGTHS

300

280

260

240

220

200

180

160

140

120

100

8.0

7.5

7.0

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

SPAN, L (m)



CONCRETE

7 8 9 10 11 12 13 14 15 16


REINFORCEMENT

PRESTRESSING

Case B w S D s

1 1800 8400 1602 2400 8400 150


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PRESTRESSED CONCRETE BAND BEAM (8.4m centres)SINGLE SPAN

DASH LINESINDICATE

UNECONOMICSPAN LENGTHS

280

260

240

220

200

180

160

140

120

100

80

60

40

20

0

9.0

8.5

8.0

7.5

7.0

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

SPAN, L (m)



CONCRETE

7 8 9 10 11 12 13 14


REINFORCEMENT

PRESTRESSING

Case B w S D s

1 1800 8400 1602 2400 8400 150


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11Practical Design and Detailing for ConstructionHaving selected the type of concrete floor system and established the principal dimensionsof the slabs and any beams to meet the overall criteria for economy the designer should be

confident that he has taken the correct decision. However, the following steps of detaildesign and documentation are equally important in achieving the desired overall economyand speed of construction. Poor detailing or complex details can add large cost and timepenalties to an otherwise economical solution.

Some general principles should be followed:

• Avoid unnecessary complications and refinements of detail. Use simple details thatreduce construction complications and problems. There is a tendency for somedesigners to be blinded by the power of computers to generate voluminous calculationsand refined analysis of forces with consequent refined details of reinforcement to suit.

Such refinements are wasted if the practical conditions on site are not considered.Rationalise the sizes of members to simplify formwork consistent with structural economyUse standard plywood sheets or multiple thereof to reduce waste, eg band-beam widths of 1200, 1800 and 2400 mm. Detail band/beam and beam/column intersections to simplifyformwork. Adopt standard dimensions for drop panels in flat slabs to suit plywood sheetsand timber sizes.

Co-ordinate the requirements of other trades for holding-down bolts and block-outs for electrical and plumbing work.

Simplify reinforcement details to suit fixing in the field, reduce the risk of errors and easechecking on site. The sizes and spacings of reinforcement should be rationalised so thatdifferences between structural bays are kept to a minimum. There is a trade off betweenthe cost of the material and labour which usually results in an additional cost for complexity.Refer to the Concrete Institute of Australia’s Reinforcement Detailing Handbook for specificexamples.


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12 FORMWORK FACTORS(For minimum structural edge details)

1.30

1.25

1.20

1.15

1.10

1.05

1.00

SPAN, L (m)

6 7 8 9 10 11 12 13 14

FORMED FACEPLAN AREA

Formwork factors are based on minimal structural edge beams. Deep architecturalbeams increase these factors significantly and may inhibit the use of flying tableforms.


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13checklist for design procedureThe Australian Standard AS 3600 Concrete Structures Code is a significant documentdefining the design requirements.

It specifies design criteria for both:serviceabi l i ty - by limiting deflections to tolerable values and designing for durability/exposure conditionsand st rength - by defining appropriate section details to resist applied loads

The following check list sets out the principal steps in designing a concrete floor to meetthe requirements of AS 3600.

1. Member ArrangementDetermine a feasible arrangement for columns, walls, beams. Notepreferred options for structural efficiency (Section 7)

2. Establish the Basic Design Criteriaa) Occupancy of the structureb) Fire rating (Building regulations)c) Sound transmission class (Building regulations)d) Exposure classification and durability requirements (AS 3600)

3. Floor Slab SizingDetermine the minimum slab thickness from step 2 above and the concretestrength and cover

4. Floor DepthSelect a suitable overall depth of floor to satisfy deflection control from theguideline values

5. Determine the Dead and Live Loads (AS1170)6. Calculate Design Bending Moments and Shear Forces

Determine the design actions at the critical sections in accordance with thestrength requirements of AS3600

7 Calculate Flexural and Shear Reinforcement for StrengthDesign

8. Calculate DeflectionCheck the calculated deflection or the span to depth ratio using thesimplified method in accordance with AS 3600. Estimate required camber if any or recycle design from step 4 if deflection is excessive.

9. Prestress DesignFor a prestressed design, proceed as above to step 5, then select a load tobe balanced - typically 0.8 to 1.0 times selfweight and a combination of prestress force to drape and balance this load. Calculate design bendingmoments and shear forces as in step 6 above and additional reinforcementfor strength design as in step 7. Check deflection if required. This will notbe critical for the usual combination of balanced live and dead load.

10. Complete Detailed Design of Flexural and ShearReinforcement

floor design

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